The authors gratefully acknowledge the following for review of this bulletin: Charles Averre, Professor Emeritus, Dept. of Plant Pathology, N.C. State University; William Branch, Professor Emeritus, Dept. of Biological and Agricultural Engineering, Louisiana State University AgCenter; Abner Hammond, Professor, Dept. of Entomology, Louisiana State University AgCenter; George Kennedy, Professor, Dept. of Entomology, N.C. State University; and Jonathan Schultheis, Professor, Dept. of Horticulture, N.C. State University.

On the cover: The process of harvesting sweetpotatoes involves digging the roots. Here the soil is turned over using large disks that bring sweetpotato roots to the surface. (Bottom) High quality, fresh-market sweetpotatoes (cultivar Covington) cured, washed and ready for sale.

Sweetpotato as ONE WORD. Throughout this book, sweetpotato is deliberately spelled as one word unless directly quoting a source where it is spelled as two words (i.e., sweet potato). The one-word spelling was officially adopted by the National Sweetpotato Collaborators in 1989. Sweetpotato (Ipomoea batatas) must not be confused in the minds of shippers, distributors, warehouse workers, and above all consumers with the equally unique and distinctive potato (Solanum tuberosum) or the yam (Dioscorea sp.) which are also grown and marketed commercially in the United States.

• Proper Handling Pays
• Growing for Improved Postharvest Quality
• Harvesting for Quality
• The Curing and Storage Facility
• Curing for Quality
  • Benefits of Curing
  • Problems Associated with Improper Curing
• Storing for Quality
  • Problems Associated with Improper Storage Conditions
• Packing for Quality
• Reducing Packing Damage
• Packing Line Cleaning and Sanitation
• Product Safety and Certification Programs
• Packaging and Shipping for Quality
  • Packaging for Quality
  • Shipping for Quality
  • Market Life of Packed Sweetpotatoes
• Key References

• Appendix 1: Guide to Common Postharvest Diseases, Abiotic Damage and Insects
• Appendix 2: U.S. Standards for Grades of Sweetpotatoes
• Appendix 3: Construction Guidelines for Negative Horizontal Ventilation (NHV) Curing and Storage Facilities

*Bold text indicates items that differ from medium packing lines. ↲

*Cumulative impacts measured on all drops and turns on a packing line. ↲

Average of five runs with the impact recording device.

* Several packing lines used different products one time to another. ↲

Recommendations to reduce damage on packing lines

• Dump roots slowly into water (not onto roots) in the dump tank.
• Use high-quality padding on all impact surfaces.
• Use long inclines to reduce drop heights between components (Figure 33a-c).
• Reduce the number of drops and turns (Figure 34).
• Reduce the overall length of the packing line.
• Remove belt supports (if feasible) to reduce impact.
• Use deceleration flaps and blankets to reduce the speed over drops.
• Instruct workers to handle roots with care, and monitor handling frequently.
• Avoid abrupt changes in direction and speed of belts. Add padding if turns are unavoidable.
• Reduce packing line speed.

*Average of five passes with the instrumented impact recording device ↲

**EPR=expanding pitch rollers ↲

Cleaning vs. Sanitation

Cleaning: the removal of debris from packing line surfaces using physical methods such as high-pressure water sprays or a detergent.

Sanitation: the reduction of the number of microorganisms on packing line surfaces to levels that do not cause disease problems.

* Two packing lines have movable sets of rollers that are used only when roots are muddy; otherwise, roots are dumped directly into the dump tank.

**EPR=Expanding pitch rollers

*** One line hand picks sometimes and lets them fall from end of conveyor other times.

Some pathogens that cause significant postharvest diseases of sweetpotato, such as Rhizopus, are common everywhere, and it is not possible to eliminate them completely. However, the amount of contamination is often important in determining whether disease develops. Thus, reducing the amount of contamination is the general aim of cleaning and sanitation efforts.

Reducing the introduction of pathogens onto the packing line. On most sweetpotato packing lines, roots are dumped into a tank of water and then fed onto the rest of the line. This process often spreads disease-causing microorganisms. Thus, reducing the amount of disease that develops in storage will reduce the number of diseased roots that enter the dump tank and contaminate the packing line. The following are a few important considerations for reducing disease development in storage:

• Extreme field conditions at harvest: Extreme environmental conditions at harvest can increase disease development in storage. Very dry soil increases skinning of the roots, a problem in itself, and can also favor Fusarium root rot. The other extreme, flooded soils, can contribute to the complex condition of souring that leads to simultaneous increases in many diseases, including Rhizopus soft rot, bacterial soft rot, Fusarium root rot, and sour rot. Flooding also results in mud adhering to the sweetpotatoes and being carried into storage. Anything that can be done to avoid extremely dry or wet conditions at harvest will reduce problems in storage.
• Sanitation of storage bins: Bins that can hold in excess of 20 bushels of sweetpotatoes are likely to have at least a few roots that develop disease during a long storage period. Assume that they are all contaminated after use. Cleaning and sanitizing the bins after each use should be a regular practice. Spotts and Cervantes (1994) studied disinfestation of wood and plastic surfaces used in bins for storing pears. They found that steam was the most effective in reducing populations of fungi that infect pears. Chlorine compounds, sodium orthophenylphenate (SOPP), and quaternary ammonium compounds were also effective but were less consistent than steam. Most of these agents were similarly effective on wood and plastic except sodium hypochlorite, which was less effective on wood. Leaving the bins outside in the summer, where the surfaces that contact sweetpotatoes are exposed to direct sunlight, may also help reduce microbial contamination.
• Reducing disease development in storage: Following the recommendations for proper curing and storage of roots remains one of the most effective ways of reducing diseases in storage. See the “Curing for Quality” and “Storing for Quality” sections for specific guidelines.

A single root affected with Rhizopus soft rot can produce millions of fungal spores. Likewise, a single root affected with bacterial soft rot can release millions of bacterial cells. Since both Rhizopus soft rot and bacterial soft rot can completely rot a sweetpotato in a few days, it makes sense to remove all roots that fall off the packing line from the packing area at the end of each day. Otherwise, they will quickly become a source of inoculum that can infect healthy roots.

Cleaning and Sanitation. No matter how rigorous your disease-control efforts are, some diseased roots will go into the packing operation, and there will be a need to clean and sanitize the packing line. Regardless of what surface is being sanitized or what agent is used for sanitizing, it will be more effective if the surface is cleaned first. Some microorganisms form biofilms (a microscopic layer of bacteria) that are particularly difficult to remove. Removal of biofilms requires detergent and pressure, either from scrubbing or from a pressure washer. In order to properly sanitize surfaces, the following sequence should be followed:

Rinse ➔ Clean ➔ Rinse ➔ Sanitize

Nonporous and exposed surfaces may be relatively easy to clean and sanitize, but other components of the packing line may require special attention. Of particular concern are the brush beds. Small bits of debris and tissue broken from sweetpotato roots may become lodged at the base of the brush bristles and support the growth of microorganisms. High-pressure hoses or a pressure spray will flush debris out of the brushes.

There are no uniform standards in the sweetpotato industry for sanitizing packing lines, and the effect of sanitation on postpacking disease development is not known. Some packers have relied on chlorine, primarily sodium or calcium hypochlorite, without understanding the factors that influence its efficacy. In water, the most commonly used forms of chlorine that are used for sanitizing (sodium hypochlorite, calcium hypochlorite, or chlorine gas) all form hypochlorous acid. Hypochlorous acid is the component that kills microorganisms and is referred to as available or active chlorine. At high pH, hypochlorous acid dissociates to form a hypochlorite ion, which is not effective as a sanitizer, and chlorine solutions with a pH above 8 are not effective in killing pathogens. Below pH 6, chlorine solutions are highly corrosive, and the activity is rapidly lost due to volatility of elemental chlorine. The volatile chlorine can also irritate workers. Thus, in order to have an effective sanitizing solution, it is necessary to measure both chlorine content and pH and maintain the pH between 6.5 and 7.5.

Organic matter inactivates hypochlorous acid and reduces the amount of available chlorine. Even though the chlorine combined with the organic matter is inactivated, it is still measured by “total chlorine” test kits. This makes it difficult to maintain an effective chlorine solution in a dump tank where large volumes of soil and organic material enter. Chlorine must be added frequently to replace the chlorine lost. For produce harvested above ground, checking and replenishing the chlorine supply at least once an hour is often recommended. But a root crop introduces much more soil into the system, so the chlorine must be replenished more frequently. Generally, chlorine should be maintained at 100 to 150 parts per million (ppm) active chlorine with pH maintained between 6.5 and 7.5. Thus, chlorine test kits that measure active or available chlorine should be used. You may need to dilute the source product in distilled water for proper readings. Commercial systems that automatically monitor and adjust pH and chlorine are likely to be more effective in maintaining effective sanitizing conditions. Always follow the label directions for any product used.

Water temperature and water quality can also affect the efficacy of chlorine. For example, chlorine solutions are more effective at higher temperatures, but the chlorine is lost faster. Registered formulations of chlorinators for use on produce contain either 5.25 percent or 12.75 percent sodium hypochlorite (NaOCl), or 65 percent calcium hypochlorite [Ca(OCl)2]. To estimate the amount of these formulations to use in making chlorine solutions with various concentrations of free chlorine, use Table 6. Current research does not suggest that chlorine can be used for disease control. It should only be considered a sanitizer of water and equipment. Also, check current guidelines, as some products may not be acceptable by certain markets.

*From “Chlorine Use In Produce Packing Lines” Document HS-761 Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, and used with the permission of the authors. ↲

One additional problem with these chlorine products is that they produce trihalomethanes when they react with organic matter. These compounds are carcinogens and pose a potential health risk. Chlorine dioxide gas circumvents this problem because it does not react with organic matter to form trihalomethanes. Chlorine dioxide is also active over a wider range of conditions than other forms of chlorine and is an effective sanitizer. However, its use is not without problems, as the gas must be generated on site. When used indoors, adequate ventilation is essential because some of the gas can come out of solution.

As indicated earlier, there has been little research on sanitizing sweetpotato packing lines. However, several classes of chemical sanitizing agents have been considered for other perishable produce. There are many factors to consider when choosing a sanitizer, in addition to whether it will effectively reduce the number of pathogens. Many sweetpotato packers have experience with the corrosive nature of chlorine, and several have had to rebuild their packing lines because of this problem. Table 7 compares the properties of some of the classes of chemical sanitizers.

*Comparisons made at approved ”no rinse” use levels. Adapted from B.R. Cords and G.R. Dychdala (1993) and R.H. Schmidt (2003), and used with the permission of the authors. ↲

General recommendations for packinghouse sanitation: Fresh sanitizing solutions should be prepared and used daily. For sanitizing wash water, it is recommended that active (not total) chlorine levels be kept between 100 to 150 ppm, and the water pH should be kept between 6.5 to 7.5. The wash water should be replaced as often as possible during the day, or at least when it becomes obviously dirty. The frequency that wash water is replaced depends on the soil load, packing line configuration, and regulations regarding disposal, and it is specific to each packing line. Storage bins and packing line components should be cleaned and sanitized after each use, and sweetpotato roots and debris should be removed from the packinghouse floor daily.

Diseases That Develop in Storage

Bacterial root rot, caused by Dickeya didantii (Pectobacterium (Erwinia) chrysanthemi), can occasionally cause heavy losses in storage or after packing. Infection results in mostly internal, very wet, soft rot that usually occurs during warm humid conditions (Figure 38). Occasionally, bacterial soft rot has developed after sweetpotatoes coming from the field were wetted to help set the skin, or when pallets were wrapped too heavily in plastic for stability during shipping, depriving them of adequate gas exchange. Bacterial soft rot can be internal and without symptom in roots, developing only under the proper conditions. The disease is managed by using planting stocks that are free of disease, avoiding wounding during harvest and handling, using clean water on packing lines, and preventing high carbon dioxide/low oxygen environments with high temperatures.

Black rot infections caused by the fungus Ceratocystis fimbriata occur in the field. However, the disease can be spread during harvest from diseased to healthy roots. The presence of even small black rot lesions in the crop during harvest may lead to large losses in storage (Figure 39a and Figure 39b). If a small amount of black rot is present at harvest, the lesions will expand in storage. Black rot was once the most important postharvest disease of sweetpotato, but using disease-free seed roots, cutting slips above the soil line, and rotating crops have dramatically reduced its occurrence.

Fusarium surface rot and Fusarium root rot decay caused by Fusarium is commonly seen in storage. There are two species of Fusarium that cause two distinct symptoms on roots. Fusarium root rot, caused by Fusarium solani, is a decay that extends into the flesh of the root. Externally, lesions are dark tan and often have a distinct ring pattern of light and dark brown bands (Figure 40). The rot is firm, dry, and dark brown, with internal cavities often filled with white mycelium of the fungus (Figure 41). Fusarium surface rot, caused by Fusarium oxysporum, has surface lesions that do not extend into the flesh. They are generally circular, light to dark brown, firm, and dry. Both diseases can be managed by minimizing injuries during harvesting and handling, harvesting when soil moisture is optimal, curing promptly after harvest, and using cultivars resistant to Fusarium root rot. Fusarium surface and root rots develop slowly and, therefore, are not considered a post-packing disease because there is insufficient time for the diseases to develop between packing and consumption.

Java black rot, caused by the fungus Lasiodiplodia theobromae, is occasionally found during storage. This disease is easily identified by its distinct symptoms. Symptoms usually begin at the root end as a firm, moist decay that turns color from pale yellow, to brown, to black (Figure 42).

As the disease progresses, hard black masses called “stroma” break through the surface of the root (Figure 43). With time, these masses will break down to release powdery spores. The fungus enters through a wounded area and can be controlled with good packinghouse sanitation and with prompt curing at the proper temperature and humidity to heal wounds. Java black rot is more common in tropical areas of the world.

Rhizopus soft rot, caused by the fungus Rhizopus stolonifer, is the most common disease of stored and packed sweetpotatoes. After the fungus enters the tissues through wounds, it can cause a soft, wet decay of the entire root within three days. Rhizopus soft rot is characterized by white, whiskery fungal growth and prolific dusty black spores on the surface of infected roots (Figure 44). Wet soil and low temperature at harvest cause sweetpotatoes to be especially susceptible, and symptoms appear soon after storage. Rhizopus soft rot is most likely to occur after packing, because infections occur through wounds.

Rhizopus soft rot can be managed by properly curing roots to heal wounds incurred during harvest and by avoiding damage on packing lines. Most packers apply fungicide on the packing line. Less susceptible cultivars are available; however, none show complete resistance. For specific fungicide recommendations, refer to the latest edition of the North Carolina Agricultural Chemicals Manual, Louisiana Plant Disease Management Guide, or contact your local Extension center for guidelines in your state.

Diseases that occur in the field and are also seen in storage and packing

Circular spot is caused by the soilborne fungus Sclerotium rolfsii, which also causes sclerotial blight in plant beds. The circular, brown, shallow spots (Figure 45) develop on roots just before harvest but do not develop further after harvest unless there is free moisture on the roots during curing. Usually, the roots heal during storage, and the spots begin to peel away from the root.

Geotrichum sour rot begins in the field when flooding occurs and continues to develop in storage. A wet, soft rot develops that has a distinctive fruity-alcohol odor. Tufts of white mycelia develop on the outside of the root (Figure 46). Sour rot can also occur postharvest when roots are exposed to high temperatures and low oxygen environments.

Mottle necrosis is caused by at least two species of the soilborne fungus Pythium. Although it occasionally causes significant losses, it is not considered a common disease. External symptoms consist of slightly sunken, brown spots with irregular margins. When affected roots are sliced, a marbled decay is revealed (Figure 47). Avoid cool, wet conditions near harvest, and practice crop rotation.

Soil rot or pox is caused by a soilborne filamentous bacterium (Streptomyces ipomoea). Soil rot causes dark corky lesions that are usually indented (Figure 48 and Figure 49). The type of symptom produced depends largely on the timing of infection with respect to root development. Infections occurring prior to root enlargement are more likely to constrict and disfigure the root, but do not develop further in storage. Growing resistant varieties can greatly reduce the incidence of soil rot or pox. Lowering soil pH through applications of sulfur can reduce disease problems.

Russet crack is a disease caused by a strain of the sweet potato feathery mottle virus (SPFMV). In the field, brown bands develop on the skin and extend laterally around parts of the root. Within these brown bands there are usually shallow cracks that run perpendicular to the bands (Figure 50). The symptoms do not change in storage and are often missed until the roots are washed. Russet crack can be managed by a good seed program that utilizes virus-tested, Certified, or Foundation seed.

Scurf, caused by the fungus Monilochaetes infuscans, is a common sweetpotato disease that is transmitted primarily by infected planting stock, but it also may persist in soil for 1 to 2 years. Scurf is a disease of the skin (Figure 51). However, roots that are heavily infected with scurf may shrink more rapidly than normal roots. Research has shown that scurf does not spread to other roots during storage. Small, insignificant, infected areas may enlarge, especially in the presence of humidity higher than 90 percent.

Scurf can be managed by using scurf-free planting stock, treating seed with fungicide, avoiding problem fields, rotating land used for plant beds each year and, most importantly, cutting plants above the soil line instead of pulling to reduce the spread from seed plants (Figure 4). Scurf-infected roots should not be used for seed or disposed of in a field where sweetpotatoes will be planted later. USDA Standards (Appendix 2) list scurf covering more than 15 percent of the root surface as a defect.

Slime molds occasionally develop on the surface of sweetpotatoes. Little is known about the conditions that favor slime mold development on storage roots. Although they look superficial (Figure 52), it is difficult to wash slime mold off sweetpotatoes without permanently damaging the skin of the root.

Abiotic Damage

Chilling Injury is rare, but it can occur if roots are kept in common storage areas during the winter months (or during transit if temperatures are too low). Storage below 55°F (13°C) can result in chilling injury that may not be evident for several weeks. The roots may appear normal, but the flesh will be spongy and watery. When chilled roots are cooked, the central area of the root may remain hard. Chilled roots are often colonized secondarily by Penicillium species (Figure 9, Figure 10, and Figure 11).

Growth Cracks are generally caused by uneven growing conditions, usually when drought is followed by heavy rain (Figure 53). These cracks often serve as infection courts for secondary pathogens. Cracking may also be caused by root-knot nematode (Figure 54). Cracks caused by root growth and those caused by nematodes can be distinguished by the presence of nematodes near the surface of the root (Figure 55).

Mutations/Chimeras in flesh color are common in sweetpotatoes (Figure 56A and Figure 56B). Sprouts emerging from the mutated flesh color will produce roots of the same flesh color. A good seed program that uses Foundation or Certified seed that has been selected for freedom from mutations can minimize this problem.

Pithiness usually is a result of poor storage conditions (such as inadequate temperature or humidity control) and is characterized by white spongy flesh that may contain spaces or cavities and by roots that weigh much less than normal (Figure 10).

Skinning is common during rough handling at harvest and packing (Figure 5). Skinning injuries can be healed through the curing process, although the original appearance will never be regained.

Insects

In addition to diseases, potential harm from insects in stored sweetpotatoes is of significant economic concern. Among the insects found in sweetpotato storage facilities are fruit flies, soldier flies, corn earworm, beet armyworm, fall armyworm, southern armyworm, and the sweetpotato weevil. Specific recommendations for control of insects using chemicals are found in the North Carolina Agricultural Chemicals Manual and the Louisiana Insect Pest Management Guide, or you can contact your local Extension center for recommendations.

Fruit flies (Drosophila spp.) (Figure 57), also known as vinegar flies, pomace flies, sour flies, banana flies, or drunkards, are very common and are usually considered nuisance pests. They quickly breed into tremendous numbers, but they do not directly attack healthy roots. They lay their eggs and reproduce in diseased, soured, or mechanically damaged sweetpotatoes. Their presence in great numbers is a sure indication of a significant decay problem. Conditions that favor fruit flies are high temperatures, coupled with large quantities of sour, diseased, and mechanically bruised sweetpotatoes. Storing only the best quality, undamaged roots and maintaining that quality are the primary means of avoiding fruit flies. An insecticide spray may be justified when extremely high populations are present.

Soldier flies (Hermetia illucens) (Figure 58) are most often seen in the form of scavenging, thick-skinned, flattened larvae. Much like fruit flies, they favor warm, wet storage conditions with decaying roots. Other insects such as earworms and armyworms, which feed on the foliage and exposed sweetpotatoes in the field, are at times carried into storage facilities during harvesting operations. There, they continue to feed and cause large holes in roots. Managing these insects in the field prior to harvest will help avoid introducing worms into storage areas.

Sweetpotato weevil (Cylas formicarius,Fab.) (Figure 59, Figure 60, Figure 61) can be a serious pest of stored sweetpotatoes once it is established. Although they are not considered a problem in the majority of sweetpotato commercial production areas in the U.S., weevils can become a serious pest of stored sweetpotatoes in the South if not properly monitored and managed. Regulatory mandates differ, and growers should become familiar with their local regulations.

Prevention is the key to successful weevil management. Avoid bringing in sweetpotatoes or containers from areas where weevils are known to be a serious problem. Always insist on weevil-free roots when purchasing seed or roots for storage, and inspect shipments upon arrival to verify place of origin and proper tagging. If a threat of weevils exists, sex pheromone traps (Figure 62) that are specific to sweetpotato weevils can be used to monitor plant beds, storage areas, and production fields. If sweetpotato weevils are discovered outside the quarantine area, promptly alert regulatory officials. Certain insecticides are available and labeled for use in the field and in storage facilities. Contact your local Extension agent for more information on the potential risk of and management options available for the sweetpotato weevil in your area.

Summarized from the U.S. standards for grades of sweetpotatoes. Published by the USDA Agriculture Marketing Service and effective April 21, 2005. For complete information on grade standards.

Grades

U.S. Extra No. 1.

U.S. No. 1.

U.S. No. 1 Petite.

U.S. Commercial.

U.S. No. 2.

Tolerances

U.S. Extra No. 1, U.S. No. 1 and U.S. No. 1 Petite grades. 10 percent of the sweetpotatoes in any lot may fail to meet the requirements of these grades, but not more than one-half of this amount, or 5 percent, shall be allowed for sweetpotatoes which are seriously damaged, including therein not more than 2 percent for sweetpotatoes affected by soft rot or wet breakdown.

U.S. Commercial. 25 percent of the sweetpotatoes in any lot may fail to meet the

requirements of this grade, but not more than one-fifth of this amount, or 5 percent, shall be allowed for sweetpotatoes which are seriously damaged, including therein not more than 2 percent for sweetpotatoes affected by soft rot or wet breakdown.

U.S. No. 2. 10 percent of the sweetpotatoes in any lot may fail to meet the requirements of this grade, including therein not more than 2 percent for sweetpotatoes affected by soft rot or wet breakdown.

Off-size. 10 percent of the sweetpotatoes in any lot may fail to meet any specified size, but not more than one-half of this amount, or 5 percent, shall be allowed for sweetpotatoes which are below the minimum diameter and minimum length specified.

Definitions

Similar varietal characteristics: sweetpotatoes have the same character of flesh and practically the same skin color. For example, dry type shall not be mixed with semi-moist or moist type.

Firm: not more than slightly flabby or shriveled.

Smooth: the sweetpotato is free from veining or other defects causing roughness which more than slightly detract from the appearance of the individual sweetpotato or the general appearance of the lot.

Fairly clean: the individual sweetpotato is not caked with dirt and that dirt or other foreign matter does not materially detract from the general appearance of the lot.

Fairly well shaped: the sweetpotatoes are not so curved, crooked, constricted or otherwise misshapen as to materially detract from the appearance of the individual sweetpotato or the general appearance of the lot.

Damage: any defect which materially detracts from the appearance, or the edible or shipping quality of the sweetpotato or which cannot be removed without a loss of more than 5 percent of the total weight of the sweetpotato including peel covering the defective area. Specific defects:

(a) Sprouts over three-fourths inch in length

(b) Growth cracks which detract materially from appearance

(c) Scurf covering more than 15 percent of the surface

(d) Pox (Soil Rot) when detracting from appearance

(e) Wireworm, grass root or similar injury when any hole in a sweetpotato ranging in size from 6 to 8 ounces is more than three-fourths inch long or when the aggregate length of all holes is more than 1¼ inches, or correspondingly shorter or longer holes in smaller or larger sweetpotatoes.

Length: the dimension of the sweetpotato, measured in a straight line between points at or near each end of the sweetpotato where it is at least three-eighths inch in diameter.

Diameter: the greatest dimension of the sweetpotato, measured at right angles to the longitudinal axis.

One type: sweetpotatoes have the same character of flesh, and do not show an extreme range in skin color. For example, dry type shall not be mixed with semi-moist, or moist type, and deep red or purple skin color shall not be mixed with yellow or reddish copper skin color.

Fairly smooth: the sweetpotato is free from veining or other defects causing roughness which detract from appearance.

Serious damage: any defect which seriously detracts from the appearance or edible or shipping quality or which cannot be removed without a loss of more than 10 percent of the total weight of the sweetpotato including peel covering the defective area. The following specific defects shall be considered as serious damage:

(a) Dirt or other foreign matter when the individual sweetpotato is badly caked with dirt, or when seriously detracting from the appearance of the lot;

(b) Growth cracks when unhealed or when seriously detracting from the appearance of the individual sweetpotato or general appearance of the lot;

(c) Pox (Soil Rot) when seriously detracting from the appearance of the individual sweetpotato; and,

(d) Wireworm, grass root or similar injury when any hole in a sweetpotato ranging in size from 6 to 8 ounces, is more than 1¼ inches long, or when the aggregate length of all holes is more than 2 inches, or correspondingly shorter or longer holes in smaller or larger sweetpotatoes.

A grading board (Figure 63) can be used by growers, graders and packers to determine size classification. A grading board can be made from plywood, metal, Masonite, or plastic. Unless otherwise specified, sweetpotatoes that go through the large hole (left) but not through the small hole (right) and are between three and nine inches in length meet the requirements for U.S. No. 1 size. Additional holes can be made for sizing other grades.

Revised from The Postharvest Handling of Sweetpotatoes: with construction guidelines for negative horizontal ventilation curing and storage facilities. M.D. Boyette, E.A. Estes, A.R. Rubin, and K.A. Sorenson. 1997. North Carolina Extension Service publication AG-413-10-B

In the 1970s, the Southeast sweetpotato industry began to evolve as a reliable source of good-quality sweetpotatoes, not only in the fall and early winter, but essentially year-round. A growing concern about the quality of stored sweetpotatoes, combined with a move toward larger and more mechanized operations, eventually prompted a re-examination of then-current curing and storage facility design. In Europe, a design known as the “letter-box,” or negative horizontal ventilation (NHV) system, had long been employed for ventilation of (Irish) potato storage facilities. During the 1980s, a variation of this design produced excellent results in California and other regions for forced-air cooling of fruit and vegetables in pallet bins.

In the NHV system, the ventilation air is generally pulled horizontally past the pallet bins by a slight negative pressure. The negative pressure is created by fans mounted internally along the top of a plenum wall on one end of the room (Figure 12). The NHV system mixes the air, which results in little internal variation in temperature or humidity throughout the room. Further, because the air is in motion and passing through the mass of sweetpotatoes (no root is more than one-half the depth of a pallet bin from a moving stream of air), there is opportunity for heat transfer. Good heat transfer is important for warming the sweetpotatoes at the beginning of the curing cycle, or cooling at the end, and for the removal of heat generated by respiration throughout the storage period.

A sweetpotato curing and storage facility is an investment in quality maintenance. In general, the longer the storage time, the more sophisticated the facility and hence, the more expensive. Postharvest facilities, unlike many other types of large capital expenditures (such as tractors and harvesters), are frequently subject to alterations and additions. It is a major mistake to assume that any building design is “final.” Economic conditions, marketing opportunities, and many other forces tend to alter the requirements of these structures. Very few of these buildings remain as they were built for more than a few years. A facility with excessive and unused capacity makes little economic sense. However, planning for future expansion or alteration should begin with site selection. After a facility is built, all future expansions must necessarily “work around” that facility. A great many problems can be avoided with proper planning.

The following important points should be seriously considered during the planning process:

Function. The first step in planning is to determine the primary function of the facility. Will the facility be used only for curing and storing sweetpotatoes? Do immediate or long-term plans include provisions for a packing line and possible cold storage? Will the facility be used for other types of produce or for other purposes (for example, equipment storage or storage of other crops or supplies) during the off-season? Will activities at the facility be mostly seasonal, or will there be workers at the facility year-round? A useful and efficient facility can be built only when all the needs and functions have been thoroughly explored.

Location. The chain from sweetpotato field to the consumer’s table is essentially an exercise in material handling. Visualizing the movement of sweetpotatoes to the facility, through the facility, and away from the facility as a flow of material can significantly simplify the planning process. Land is expensive. Situating an 8,000-square-foot sweetpotato facility in the middle of a 10-acre field may seem a waste of valuable land. However, there are hundreds of postharvest facilities on sites that effectively eliminate any future expansion, even though the facility was originally conveniently located. There are no good guidelines to follow in projecting what future requirements might be, although it is often recommended that a site should have room for a facility to be doubled in size twice. The outlines of a generous expansion program should be included on the original site plan.

As you begin planning, it is a good idea to become familiar with any zoning regulations, local ordinances, or land-use plans that are applicable to the site. You should also become familiar with any laws or regulations pertaining to construction, electrical systems, worker health or safety, the proper use and storage of pesticides, and the proper handling and storage of food products. Although building contractors should be familiar with the laws governing construction, do not assume they are familiar with laws governing the use of the facility.

Before any construction activity, the owners or operators of a sweetpotato facility are urged to complete a comprehensive site evaluation to ensure that wetlands or other sensitive areas will not be affected by the proposed activity. Most county planning departments maintain a comprehensive list of rules and regulations that may impact small busi

nesses and industries. Early contact with regulatory agencies is recommended to ensure that the construction and use of the facility are in compliance with all applicable rules and regulations and that all necessary permits are acquired.

A level, elevated site with good natural drainage is ideal. Standing water is not only a nuisance, but it contributes to the early failure of floors, foundation walls, and column footings. Further, pumping water from loading docks—especially pumping wastewater from packing lines—can be expensive. Avoid sites too near property lines, creeks (and designated wetlands), and railroad, highway, or power rights-of-way. It is also prudent to avoid residential areas whenever possible. Although highway accessibility is essential both for the movement of sweetpotatoes and for those working in the facility, try to maintain a reasonable setback. Allow for adequate parking and truck turnaround space, provide room for future expansion, and avoid potential problems should the highway be altered or widened.

Power. All except the smallest sweetpotato curing and storage facilities should have access to three-phase power. Access may be problematic in some rural areas, but the amount of power required for fans, pumps, and refrigeration systems makes access to three-phase essential in most cases. Although the cost of three-phase power may be considerably less than single phase, connection fees may be quite high. However, the cost of running three-phase equipment may be considerably less than single phase. For example, a five-horsepower, three-phase electric motor may cost about the same as a one-horsepower, single-phase electric motor. Some electrical equipment, such as refrigeration systems with a capacity greater than 10 tons, may require three-phase power. Power stand-by charges or minimum monthly charges may be levied if power is not disconnected in the off-season, so check details with your utility company.

Water. Some water is required for all sweetpotato curing and storage facilities. For facilities intended for curing and storage only, 8 to 12 gallons per minute may be adequate for humidifiers, drinking, bathrooms, and miscellaneous cleaning. However, for facilities with packing lines, the water requirements may range from 1,000 gallons per hour for a small line to more than 3,000 gallons per hour for a large line. If well water is to be the source, follow local permit requirements. Back-flow protection should be incorporated into any well’s construction design.

Waste disposal. For a curing and storage facility that may include a packing line, you should closely examine proper waste disposal options. All such facilities must have means for proper management and disposal of both domestic and industrial process waste and may also require one or more regulatory permits, depending on state regulations.

Potential sites that are otherwise suitable may be rejected because they do not provide for the proper disposal of wastewater. Land application of the wastewater, through spray irrigation or other means, should be explored first. The guidelines for this option may require an assessment by a qualified irrigation engineer, depending on the state. This is definitely the best option for most operations. A second alternative is to discharge to a municipal wastewater treatment system, which may have industrial pretreatment requirements of its own.

A final option is to discharge wastewater into the surface waters (stream, canal, drainage ditch, road ditch, and so on). If you are classified as an industrial operation, (you pack any sweetpotatoes other than your own), you may also be required to obtain a wastewater discharge permit. You should try to avoid this step if at all possible. These permits are issued only after all options for land application or reuse are evaluated. Permits to discharge to surface waters are issued only after the surface water and flow regime has been modeled and discharge limits are established. This is a long and often costly process, resulting in only a temporary permit that is subject to review.

In addition to permits for wastewater management, additional permits may be required for managing the solid waste. The solid waste generated in a sweetpotato packing facility, such as culls, mud, sand, and plastic and paper packaging materials, must be disposed of in a permitted landfill or through some permitted on-site composting facility. The quality of the composted materials must be monitored and can be determined through testing by state regulatory agencies for a small fee. Detailed information on composting methods and equipment may be obtained from your county Cooperative Extension center. Land application or livestock feeding of cull potatoes may also be an option.

Capacity. Many of the details of a sweetpotato curing and storage facility are ultimately determined by the required capacity. The capacity requirements are not always so simple as the sum of your anticipated production, plus the amount you might be willing to store for others. The practices of segregating the roots from different growers, from different fields, of different grades or different varieties, and arranging them for easy access, can all have a significant effect on the working capacity of an individual facility. It makes no economic sense to build more capacity than can be efficiently utilized. A measure of excess capacity may be a wise investment when it will be needed to accommodate higher than normal yields or to provide adequate space to store sweetpotatoes from other sources. Remember that the cost per square foot decreases and energy efficiency increases with increasing facility size.

Curing, unlike storage, is a process that involves a definite period of time. Curing for too short or too long a time can have negative effects on the quality of the roots. For this reason, the size of the individual rooms in an NHV facility is customarily limited to no more than two days’ harvest. Two days’ harvest for some growers may be as few as 5,000 bushels, while for others it may be as many as 30,000 bushels. The typical capacity of an individual room in an NHV facility is about 15,000 bushels. Larger rooms may make slightly more efficient use of space than smaller rooms, but they can result in undercured or overcured roots if not filled on a timely basis. Smaller rooms, however, are more quickly emptied and taken out of service.

Although there have been some notable efforts in the handling and storage of sweetpotatoes in bulk, almost 100 percent of the domestic sweetpotato crop is still harvested into, cured, and stored in pallet bins. Although there are published standards for pallet bins (ASAE 337.1, American Plywood Association and the National Wooden Pallet and Container Association), the construction and dimensions of the bins have evolved through voluntary agreements between the manufacturers and the sweetpotato industry. In the Southeast, most pallet bins built since about 1985 for sweetpotatoes have generally measured 42 inches wide, 47 inches deep (in the direction of the fork slots), and 35 inches high. The level, full capacity of these bins is slightly more than 20 bushels. In practice, however, to prevent crushing and skinning the roots, the bins are not filled to capacity, but are assumed to hold 18 bushels. The gross weight of a full bin at harvest is approximately 1,250 pounds. In recent years, some growers have moved to double-pallet bins for increased efficiency. These double bins (84 inches wide, 47 deep, and 35 inches high) are designed to exactly replace two single bins but require heavier forklifts and specialized bin rotation equipment.

Pallet bins are designed to be stacked one on top of the other to make efficient use of floor space. It is customary to stack bins at least six high (171/2 feet). This allows only 21/2 feet of head space around the walls in buildings with 20-foot eave height. This should be considered a minimum in NHV facilities. Ample head space is necessary above the stack for proper air movement. Some recently built facilities have 22-, 24-, or even 30-foot eave heights. A 30-foot eave height allows pallet bins to be stacked eight high (23.3 feet). All stacked pallet bins should be perfectly aligned and in good condition. The collapse of one lower pallet bin can cause a stack—or even an entire row—to fall over, with catastrophic results.

As mentioned, the total capacity of a sweetpotato curing and storage facility depends on production plus some prudent excess, whereas the capacity of individual rooms is usually limited to two days’ harvest. Most sweetpotato curing and storage facilities consist of several individual rooms. Use the following formulas and guidelines to determine the actual dimensions of these rooms:

C = (1.2)(L)(W)(N) Where:

C = room capacity in bushels, normally limited to two days’ harvest

L = length of the room in feet

W = width of room in feet

N = number of bins in stack, (for example, if the bins are stacked six high, N = 6)

The length of the room (dimension parallel to the direction of air flow) is normally limited to 100 feet because of the difficulty of the fans to consistently move the air greater distances. Although there are no such limits on the width of the room, good planning suggests the selection of room widths that utilize standard building components (beams, girders, and bar joists) and limit the length of unsupported roof spans. It is also a good idea to select widths that match well with the combined width of the pallet bins, plus about two feet to take care of gaps between bins and clearance along both walls. Support columns in the middle of a room are a major inconvenience, a possible safety hazard, and should be avoided when possible.

Example: A room eight bins wide would require a room width of: W = (8)(42) + 24 = 360 inches = 30 feet. Assuming a room capacity of 12,000 bushels with pallet bins stacked six high, the formula above would yield a calculated length of: 12,000 = (1.2)(L)(30)(6) L= 55.6 feet.

When the width of the plenum is added, the total length of the room approaches 60 feet. Therefore, the capacity of a single room for an NHV facility with inside dimensions of 30 feet wide by 60 feet long, with bins stacked six high, is approximately 12,000 bushels. Table 11 provides estimated capacities for rooms of various dimensions.

If the pallet bins are carefully stacked near the door, almost all the floor space inside the room may be used. Often the pallets in the last row or pair of rows must be turned sideways (the forklift slots are perpendicular to the flow of air in the room) to allow the forklift maneuvering room. This arrangement will present no air movement problems, provided a free space of 6 inches to 1 foot is maintained around these bins.

Rarely will a curing and storage room be built without some type of paved (and often covered) staging area for

unloading and loading trucks. Carefully consider this aspect of the facility design. Such areas are seldom considered in initial designs but often become a central alleyway of an expanded facility. This area is important to the overall functioning of the facility because convenient access to each room from a central location is essential for ease of material handling. Access aisles and alleyways should be at least 16 feet wide to allow for safe operation of loaded forklifts. When loading docks are part of the design, they should be conveniently located to keep the trips short from truck to storage.

Construction Points and Equipment Selection

Building type. Many types of building construction have proved satisfactory for sweetpotato curing and storage facilities. Insulated concrete-block, curtain-wall buildings with bar joist roof supports were once popular with growers. Because of the labor costs, block construction has been almost replaced by engineered post-frame buildings or all steel buildings of various designs. These modern buildings are durable, functional, economical, and can be custom designed and constructed in a fraction of the time it took to build the older masonry buildings.

Foundation and floor. Although the cost of the foundation is relatively small, the foundation directly or indirectly affects the performance of all other parts of the building. The foundation distributes the weight of the building and its contents over an area sufficient to prevent excessive and uneven settling. A well-drained solid grade is essential to an acceptable foundation.

All sweetpotato curing and storage facilities are constructed on a slab floor of at least 4 inches of wire-mesh-reinforced concrete over a well-compacted grade. The floor should be capable of supporting at least three times the rated capacity of the forklift. Five to 8 inches of reinforced concrete may be necessary where loads are unusually heavy, such as around loading docks. The floor should be as smooth and level as possible. Even a slight unevenness of 1/4 inch between the two front wheels of the forklift may result in a 3-inch side movement of the pallet bin 16 feet off the floor (Figure 64). Because the recommended sweetpotato storage temperature is near the deep soil temperature in most growing areas of the U.S., under-floor insulation is not necessary. It is wise, however, to include a vapor barrier under the slab.

Insulation. Thermal energy flows naturally from warm objects to cold ones. All material, even good heat conductors like metals, offers some resistance to the flow of heat. Insulation, however, is any material that offers high resistance to the flow of energy. Hundreds of different

materials have been used at one time or another for thermal insulation. The characteristics of the insulation materials differ considerably. Suitability for the particular application, as well as cost, should be the deciding factors in choosing a material. Thermal resistance (R-factor) should be considered in addition to durability and the labor required to install the material. Some common insulation materials such as fiberglass bats are not recommended for sweetpotato curing and storage facilities because they perform poorly in a high-humidity environment and often harbor rodents and birds.

A measure of an insulation’s resistance to the movement of heat is its R-value. The R (for resistance) number is always associated with a thickness: the higher the R-value, the higher the resistance and the better the insulating properties of the material. The R-value can be given in terms of a 1-inch-thick layer or in terms of the total thickness of the material. The R-values of some common building materials are shown in Table 12.

The most popular insulation material now used in new sweetpotato curing and storage facilities is sprayed-on expanded polyurethane foam. Properly applied, this material has an R-factor of approximately 7 per inch, although this value may decrease somewhat with age. It is durable, moisture resistant, and effectively seals cracks and small openings. Its most important characteristic, however, is that it may be applied very rapidly with comparatively little labor. But this material is extremely flammable immediately after application and gives off a toxic smoke when burned. Some local fire codes or insurance companies may require a fire-resistant coating on this material. Fire-protective coating may provide some moisture resistance that is not permanent in urethane, causing the foam’s R-factor to decline with age.

Rigid boards of foil or plastic-backed polyisocyranuate foam have been occasionally used for insulating sweetpotato facilities. This material varies in thicknesses (1/2 inch to 3 inches) and in size (4 by 8 feet to as large as 8 by 30 feet). Certain formulations of this material may have R-factors as high as 8 per inch. In the past, similar materials have delaminated and otherwise performed poorly in the high humidity of a sweetpotato facility. This material also requires considerable installation labor. The main objection to this material in an NHV facility in the past was the difficulty in sealing the joints between boards and at roof and floor level. Because NHV facilities operate at slightly reduced or elevated air pressures, failure to adequately seal all openings to the outside can substantially reduce the performance of the system. Newer materials, joint sealing methods, and installation procedures have all but eliminated many of these objections.

Both sprayed-on expanded polyurethane and polyisocyranuate board materials are made of closed cell foams. Closed cell foams act as vapor barriers by retarding the movement of moisture out of the building. A good vapor barrier is desirable to maintain the humidity inside the facility, thereby preventing excessive evaporation and weight loss of the sweetpotatoes. Under high-humidity conditions, sufficient insulation will help prevent moisture from condensing on the walls and roof. When the relative humidity inside the building approaches saturation, moisture will begin to condense on any surface that is below the temperature of the air. Condensation is more likely to occur in the top of the building, is most prevalent at night, and indicates insufficient insulation rather than excessive humidity.

In the sweetpotato growing areas in the Southeast, the total R-factor should be a minimum of 12 for walls and a minimum of 16 for roofs. Roofs require greater insulation because their surfaces are more directly exposed to sunlight during the day and radiation cooling at night.

Doors. The door is another important part of the sweetpotato facility. The larger the door, the better the access to the room and the easier pallet bins may be moved in and out. However, well-designed doors can be expensive. Their price is roughly based on their total area, not width. For example, a 16-foot wide by 16-foot tall sliding or garage door may cost up to four times as much as an 8-foot door. Although small doors are less expensive, few sweetpotato facilities are built with doorways less than 12 feet wide. Doorways should exceed the height of a loaded forklift (about 10 to 11 feet). Improperly designed or maintained doors can waste large quantities of energy. Doors should have as much insulation as the walls and should always provide a good air seal when closed.

Plastic strip curtains are effective in reducing the energy loss when large doors must remain open for long periods. Because the doors of sweetpotato rooms are usually opened only when filling and unfilling, plastic strip curtains are probably not a good investment. Small access doors are a good investment, however, because they allow for inspection of the room and its contents without the bother and energy loss of opening the large access door (Figure 65).

Heating system. Those familiar with the heating requirements of the old trench floor system of curing are sometimes surprised at how much lower the heating requirement is for the NHV system. In general, the heating systems selected for the trench floor facilities were based not on the required heat capacity, but on the fan or air-moving capacity of the heating unit. It is not unusual to see a 12,000-bushel, trench-floor curing room with a heating system capacity of 1 million Btu per hour or more. The actual heating require

ment for an NHV room of the same capacity with the recommended minimum insulation is about 300,000 Btu per hour. Because in the NHV system, the fans that move the air about the room are separate from the heating system, the heating system may be rightly based on the heat requirement and not the fan capacity.

Sweetpotatoes delivered to the curing and storage facility usually arrive at a temperature near that of the outside air. Early in the harvest season, this may be near 85°F (30°C), the recommended curing temperature. On cold days late in the harvest season, however, the arriving roots may be significantly cooler. The amount of heat required to warm the sweetpotatoes to the recommended curing temperature depends on their pulp temperature at arrival, the quantity and rate delivered, and the rate of warm up. Generally, sufficient heat should be provided in the curing room to raise the temperature to 85°F (30°C) within 48 hours. This is an important point. In the past, heating systems were designed to raise the temperature of the roots as much as 40°F in 24 hours. This rate of heat input is excessive for two reasons. First, it often causes drying and other physiological problems associated with high temperatures in localized areas near the heater, even in facilities equipped with NHV systems. The fans simply cannot move the heated air away from the heater fast enough to prevent localized overheating. Second, there is some evidence that abrupt changes in temperature (from cool to warm or vice-versa) may negatively affect sweetpotato metabolism, decreasing quality and shelf life.

Excessively high curing temperatures contribute to excessive weight loss, sprouting, and other possible damage. Although 85°F (30°C) is the recommended curing temperature, it is prudent to stop or slow the input of heat well before that temperature is reached to prevent overshooting the target temperature. Excursions to 90°F (32°C) or even 100°F (38°C) are common problems that occur because of the production of respiration heat at high temperatures. For this reason, it is recommended that the curing thermostat be set no higher than 80°F to allow the pulp temperature to “coast” to 85°F (30°C).

Heating system load calculations are based primarily on two factors. The major factor is the heat required to raise the temperature of the sweetpotatoes from the ambient temperature to 85°F (30°C) at the beginning of the curing cycle. In a properly insulated building, this typically represents 90 to 95 percent of the total load. The rest of the load is composed of heat lost by conduction through the walls and heat lost with ventilation air.

The room from the previous example (30 feet wide by 60 feet long, 12,000 bushel capacity), insulated to R-12 in the walls and R-16 in the roof, would require a heater output of approximately 300,000 Btu per hour. About 94 percent of this total (282,000 Btu per hour) is needed to raise the sweetpotatoes to curing temperature in 48 hours. Only 6 percent of that capacity (18,000 Btu per hour) is needed to offset the conduction and ventilation losses on cold days.

As this example shows, in a properly insulated and operated curing facility, most of the heat load is used to raise the temperature of the sweetpotatoes. Only a small percentage of the heat load is used to offset infiltration losses. Even in very cold weather, heaters are seldom operated because heat from respiration of the sweetpotatoes generally maintains the correct storage temperature. For this reason, it is possible to accurately estimate the required heating system output based solely on the capacity of the facility, provided it is adequately insulated and properly operated. Operating experience with well-constructed NHV facilities in eastern North Carolina has shown that heating systems sized at 25 Btu per hour per bushel of capacity are adequate. Curing and storage facilities with as little as 10 Btu per hour per bushel have been successfully operated, but in cool weather may take four days or more to raise the curing temperature to 85°F (30°C).

Many different types of oil or gas heaters have been used successfully in sweetpotato curing and storage facilities. Most recently built NHV facilities have used unvented LP or natural gas heaters of the type used in animal confinement buildings or some greenhouses. These unvented gas heaters are simple, relatively inexpensive, and economical to operate. No matter what type of heater is used, it is important that the combustion air for the heater not come from inside the room. Unvented gas heaters in particular can quickly deplete the oxygen inside the room, which results in poor combustion, the production of carbon monoxide, and a potentially deadly situation. Ethylene is a by-product of heaters and can negatively affect sweetpotato quality. Heaters should always be mounted outside the room in some convenient location. Above and to the side of the roll-up or sliding door at the end of the room opposite the plenum wall has proven satisfactory. A duct can be put through the wall from the heater so the hot air enters the room above the stack of pallet bins. The heaters can also be located at the opposite end of the building and ducted into the plenum. Ideally, the thermostat used to control the heater should be located away from the heater in a position that has good air movement and allows easy access (but not directly on an exterior wall if possible).

Refrigeration system. The refrigeration system removes excess heat from the sweetpotato facility. It is possible to store sweetpotatoes for extended periods in both conventional and NHV facilities during cool weather without refrigeration. However, refrigeration allows the timely removal of heat at the end of the curing cycle, which eliminates excessive sprouting and weight loss. Additionally, refrigeration allows for much more precise control of the temperature during storage in warm weather and is essential for successful sweetpotato storage during late spring and summer. In late-season storage situations, the cost of refrigeration generally pays for itself in two years or less by reduced weight loss and increased quality maintenance.

Heat can be thought of as a form of energy that always flows “downhill” from warm objects to cool objects. For example, heat will flow from a warm sweetpotato to the cool surrounding air. A refrigeration system is functionally like a pump that pumps heat “uphill” from cool to warm objects. With a refrigeration system, it is possible to remove heat from a sweetpotato storage room at 65°F (18°C) to the outside air, which may be at 90°F (32°C).

A refrigeration system consists of three major parts: a motor/refrigerant compressor set, condenser coils, and evaporator (cooling) coils. These parts are connected by piping to form a closed loop. Sealed inside the piping is a liquid or vapor refrigerant. The motor/compressor acts as a pump that circulates the refrigerant through the system. Both the compressor and condenser coils are heat transfer devices and have fans to help move heat from the coils to or from the surrounding air. The evaporator coils are always located inside the room and are designed to transfer heat from the air to the refrigerant. The condenser coils are always

located outside the refrigerated room and are designed to transfer heat from the refrigerant to the surrounding air. The refrigeration system is controlled by a cooling thermostat. Like the heating thermostat, the cooling thermostat should be conveniently located, but not be directly in the air stream from the cooling coils or on an exterior wall.

The cooling coils of the refrigeration system must be cooler than the air inside the room if the air is to be cooled. The larger the temperature difference, the greater the rate of heat transfer and the smaller and less expensive the cooling coils for a given capacity. However, the colder the coil surface, the more water vapor from the air will condense on the coils. As water condenses on the coils, it gives up heat at the rate of approximately 1,000 Btu per pint. The cooling energy used to remove this latent heat is wasted because it contributes nothing to cooling the room. In addition to wasting refrigeration capacity and electrical power, excessive condensation on the cooling coils should be minimized because it reduces the relative humidity of the room.

To minimize condensation and maintain the optimum curing and storage humidity inside the facility, it is good practice to select a cooling coil that minimizes the temperature difference between the air and coil surfaces by selecting a larger coil. The temperature difference between the air and the coil surfaces (known in the industry as “delta T”) should be limited to 12 degrees or less to help maintain a relative humidity of 85 percent. Unfortunately, not all refrigeration contractors are aware of the special humidity requirements of sweetpotato storage facilities. In selecting a cooling coil, be sure to specify a 12-degree maximum delta T. The cooling coils of a refrigeration system are normally equipped with fans that blow or pull air past the heat transfer surfaces. It is advantageous to position the cooling coils inside the room in a way that allows the fans to move air in the same direction as the NHV fans in the plenum wall as shown in Figure 12. Allowing the cooling coils fans to move air across the room perpendicular to the direction of air from the NHV system, or even worse, opposite of that from the NHV fans, will disrupt the air flow patterns, and the result is poor performance of the NHV system.

Similar to the heating system, the refrigeration load calculations are based on two factors. The main factor is the refrigeration capacity required to remove the heat from the roots at the end of the curing cycle. In determining this part of the load, timely removal of heat to reduce weight loss must be balanced with the cost of the system. The longer the time allowed to reduce the temperature from 85°F (30°C) to 58°F (15°C), the smaller the refrigeration load and hence, the smaller and less costly the system. Experience has shown that a cool-down period of 96 hours (four days) strikes a good balance between quality maintenance and cost. In a well-insulated and operated NHV facility, this cool-down generally represents approximately 90 percent of the total required refrigeration load. Similar to heating, the other factor representing the remaining 10 percent is the refrigeration capacity required to overcome the conduction losses through the walls and roof and the ventilation losses.

Using the previous example—a 30-by-60-foot, 12,000-bushel capacity room—the facility would require a refrigeration capacity of approximately 185,000 Btu per hour (15.4 refrigeration tons). More than 90 percent of this amount is needed to lower the temperature at the end of curing. The other 10 percent is needed to offset the heat gained through conduction and ventilation. As with the heating system calculations, it is possible to accurately estimate the refrigeration system capacity required based solely on the capacity of the facility, provided it is adequately insulated and properly operated. Again, operating experience with well-constructed NHV facilities has shown that about 1.3 tons of refrigeration per 1,000 bushels of capacity is adequate.

Humidification equipment. For successful curing and storage of sweetpotatoes, maintaining the proper humidity should be considered almost as important as maintaining the proper temperature. In NHV facilities that are cooled with outside air only, low humidity and the resulting weight loss can be a serious problem without humidity control. Some water vapor is continually given up by sweetpotatoes, either by evaporation or by respiration. Under normal curing and storage conditions, however, this is not enough to maintain the humidity at 85 percent. To maintain optimum humidity, some provisions must be made to add additional water to the air. Traditional recommendations to wet the walls or floors are ineffective. The water must be evaporated directly into the air stream to raise the humidity quickly and uniformly throughout the room. A well-designed humidifier is the only way to efficiently accomplish this.

Many good humidifiers are available that are suitable for a sweetpotato curing and storage facility. Most consist of an electrically driven, high-speed fan and atomizer that produces a fine mist. Ideally, this mist is quickly evaporated, picked up by the circulating air, and distributed throughout the room. Properly adjusted, humidifiers in a sweetpotato facility will not moisten the floor, the sweetpotatoes, or any other surface. The purpose of humidification is to fill the sweetpotato facility with water vapor, not a fog of water droplets. For this reason, those humidifiers designed primarily for greenhouses should be avoided, because they often produce relatively large droplets that settle out of the air stream before they evaporate.

The selected humidifier should have a rated capacity of approximately one gallon per hour per 5,000 bushels of facility storage capacity. The rated capacity of the humidifier is indicative of its performance in low humidity. As the humidity in the facility increases to ideal levels, some of the water droplets may not completely evaporate and drip onto the floor or other surfaces. Additionally, water sources often contain lime or other chemicals that form deposits on nozzles, fan blades, and other parts of the humidifier. The

deposits can adversely affect the performance of the unit and should be removed during periodic maintenance. For this reason, humidifiers should be positioned conveniently for ease of servicing. Because the evaporation of moisture cools the air, it is advisable to mount the humidifier near the heat duct to help warm the air cooled by evaporation and further accelerate the evaporation of water droplets.

What cannot be measured, cannot be controlled. Humidity can be measured by hand using an inexpensive device called a sling psychrometer, using wet bulb and a dry bulb air temperature with a constant air speed passing over the thermometer (Figure 66). Other devices also can be incorporated into automated systems. They measure humidity electronically or by the shrinking or swelling of specially treated materials. A humidistat, which is an adjustable humidity sensor that activates a switch, should always be used to control the humidifier. Manual control of the humidifier by a simple on/off switch will invariably result in poor humidity maintenance. The humidistat should be properly positioned for best results. It should not be located too near the humidifier nor directly in the air stream from the cooling coils or heater.

Ventilation equipment. The fans mounted in the plenum wall are the essential elements of the NHV system. Along with the dampers and louvers, the fans regulate the movement of air inside the facility. Guidelines for the proper selection of this equipment are based on research and practical field experience in a variety of NHV facilities. The horizontal movement of air not only promotes uniform conditions of temperature and humidity throughout the mass of sweetpotatoes, but more important, it provides a means for heat transfer by forced convection. This forced convection allows the temperature of the sweetpotatoes to be raised uniformly at the beginning of curing and lowered at the end. It also allows for the timely removal of respiration heat and the maintenance of uniform conditions of temperature and humidity throughout the room.

Experimental data for heat transfer between the sweetpotatoes and the air forms the basis on which the required fan capacity is determined. If each of the pallet bins were exactly the same size and fitted perfectly together so that air leakage between bins was eliminated, approximately one cubic foot per minute (cfm) per bushel would be more than enough air to effect good heat transfer. Experimental evidence has shown, however, that about half the air passing through the fans leaks into the plenum through

gaps between the pallet bins. Therefore, to yield satisfactory results, the total fan capacity of the NHV facility must equal approximately 2 cfm per bushel based on a fan static pressure differential of 0.10 inches of water. Fan manufacturers normally supply charts giving volume flow rate as a function of static pressure.

Correct fan selection is essential to the proper operation of an NHV facility. Industrial ventilation axial flow fans similar to those shown in Figure 13 are offered in many different combinations of blade diameter, motor horsepower, and fan capacity. In general, where two fans of different blade diameter and horsepower have similar capacity, the one with the larger blade diameter is more energy efficient. Although it is possible that only one fan could provide sufficient capacity, experience has shown that multiple fans spaced 10 to 12 feet apart along the plenum wall will distribute air more uniformly. All the fans in a room must be operated simultaneously. If some fans are allowed to operate while others are switched off or otherwise inoperable, air will be drawn backwards into the plenum through the nonoperating fans. This will significantly reduce air movement through the sweetpotatoes, waste energy, and compromise environmental control. Always promptly repair inoperable fans.

When the fans are used to simply circulate the air inside the room, the motorized dampers across the plenum from the fans remain closed. When outside air is required, the dampers are opened. When correctly sized, the total damper area should equal approximately two-thirds of the total fan area. The total area of the gravity shutters at the end of the room opposite the plenum should equal the total area of the motorized dampers.

Using the example facility of a 30-by 60-foot, 12,000-bushel capacity room, the total required fan capacity would be approximately 2(12,000) = 24,000 cfm. Assuming that three fans spaced approximately 10 feet apart along the plenum wall would be sufficient, each fan would need a capacity of about 8,000 cfm at 0.10 inches of static pressure. From typical manufacturers’ specifications, three 30-inch

diameter, one-half horsepower fans should be sufficient. Based on the fan sizes, two 30-inch motorized dampers and two 30-inch gravity shutters would also be required per room.

Controls. Proper sweetpotato curing and long-term storage requires precise control temperature, humidity, and ventilation. The level of environmental control cannot exceed the precision of the control system. The electrical controls of the heater, refrigeration system, ventilation system, and humidifier form the brain of the NHV facility. Although there are many different possible designs for this equipment, each design essentially consists of sensors and associated switching equipment. There have been two broad categories of control systems utilized in NHV facilities.

Electromechanical controls consisting of thermostats, humidistats, timers, and associated relays are wired into a central panel box. All the necessary parts and wiring can be obtained from most electrical supply dealers, and the system can be built and serviced by most licensed electricians Generally, a separate system is built to control each room. Electromechanical control systems could be designed to control several rooms, but the cost of long runs of control wiring makes this option economically unfeasible. These controls, if correctly installed, are simple and reliable. With the addition of several remote thermostats and indicator lights, the control panel may be mounted just outside the room to provide the status of the room and equipment at a glance.

Programmable Logic Controllers (PLCs), Figure 67, are industrial computers that have also been used to successfully control NHV facilities. These sophisticated devices operate the control equipment (such as heaters and humidistats) through relays in the same manner as the electromechanical controls. However, unlike the electromechanical controls,

PLCs can be programmed to make various energy-saving decisions about the operation of fans, heaters, and refrigeration equipment. They also may be programmed to collect data, provide security, and sound alarms or activate automatic telephone dialing devices in emergency situations. One PLC-based system can easily control many rooms and, therefore, may be the least costly alternative for multi-room facilities. Field data suggest that PLC based systems can provide significantly more overall energy efficiency than electro-mechanical systems.

Although many parts of a PLC-based control system can be installed and serviced by a licensed electrician, the actual hardware selection and programming must be done by an experienced professional. These devices have a proven track record of reliable service in many different industries. Additionally, like many types of computer equipment, the power and versatility continue to increase as the price decreases.