|Preferred cooling method:||Forced air|
|Optimum temperature:||33 to 34°F|
|Optimum humidity:||90 to 95%|
|Storage life:||2 to 3 weeks|
Blueberries are a popular early summer small fruit. Successful blueberry marketing requires that the fruit not only be of very high quality but that it also be properly packaged and thoroughly cooled. This publication explains the postharvest handling procedures necessary to deliver North Carolina blueberries of the highest quality.
Most consumers purchase fresh blueberries on impulse and are prompted primarily by the perception of quality. Wholesale buyers also associate appearance and firmness with fruit quality and freshness. Successful blueberry marketing thus requires that the fruit be of the highest quality and appearance. However, blueberries are extremely perishable and easily damaged by rough handling and adverse temperatures. Because blueberries are often harvested and handled during hot, humid weather, attention to proper postharvest handling is essential to maintaining quality.
Blueberries of the highbush type (Vaccinium corymbosum L.) and the rabbiteye type (Vaccinium ashei) are grown in North Carolina. More than 90 percent of the blueberries produced commercially are of the highbush type. Harvesting of highbush blueberries begins in late May and often continues through late June. Rabbiteye varieties ripen in late June, and harvesting continues into August. Blueberries, even those growing on the same bush, do not all ripen at the same time. Under normal growing conditions, blueberries ripen over a period of three to four weeks. Ripe blueberries should have a completely uniform blue color. Fruit with a red tinge are less mature and will not be as sweet as more mature berries even though they ship well and will turn completely blue within several days. Depending upon the crop and the weather conditions, blueberry fields may be harvested as many as four times in intervals of five to seven days. For rabbiteye varieties the interval between harvests may be as long as nine days in cool dry weather. Harvesting more frequently may not make efficient use of labor, whereas less frequent harvesting may result in a high percentage of overripe fruit.
In general, high temperatures during blueberry harvest season promote ripening. Heavy rains during periods of high temperature can further hasten ripening, cause splitting, and greatly reduce storage quality. When picking labor is limited, prime fruit may easily become overripe. Overripe blueberries are extremely easy to damage. Blueberries that don t look too bad when picked may be totally unacceptable when they reach the consumer. Be careful! Price, and particularly reputations, depend largely on quality. Once damaged, your reputation and that of your marketing organization may be very difficult to repair. Consumers long remember experiences with a poor quality product.
Whenever possible, avoid harvesting wet blueberries. Wet berries are very susceptible to decay organisms and often resemble overripe berries leaking juice. Waiting a few hours to begin harvesting after a rain or heavy dew can significantly reduce the chance of infection by postharvest disease organisms and can improve the appearance of the fruit.
Proper instruction and careful supervision of the picking crew is essential to the success of any harvesting operation. Postharvest quality control begins in the field. Packing house problems and buyer complaints are often the result of poorly trained and supervised pickers. Have a responsible person supervise the pickers at all times. If the fruit is harvested directly into pint cups, it is a good idea to check one or two pints at random in each flat. This check can be made easily by emptying the contents of a pint cup into a shallow pan where each berry can be examined individually. Finding more than 6 to 8 percent poor-quality fruit per pint indicates a need for greater quality control.
Ripe blueberries are easily removed from the bush. Fruit should be removed with the thumb and forefinger, keeping the hand cupped under the berry to avoid dropping it. The whitish, dusty appearance (bloom) of the blueberry is highly desirable, and consumers regard it as a sign of quality. Overhandling the berries will remove the bloom. In addition, all workers should be reminded that they are handling a food product that is eaten fresh and often not washed. Clean hands and sanitary personal habits are required at all times.
Harvested blueberries should never be allowed to remain in the sun. The dark berries will readily absorb heat. If berries are picked into buckets, the buckets should be no larger than 5 quarts and of rigid construction to minimize mashing and the accumulation of respiration heat.
Rules for Blueberry Pickers
- Keep your hands clean. Remember that you are handling a food product. The law requires you to wash your hands after each visit to the rest station.
- Pick all the ripe blueberries on the bush before moving on to the next bush.
- Harvest only well-ripened fruit. Leave immature fruit for the next harvest.
- Place your hand under the clusters to avoid dropping the berries.
- Avoid overfilling your hands; do not squeeze or roll the fruit.
- Do not put trash or cull berries into the container.
- Never allow harvested fruit to remain in the sun.
Recurring labor problems have prompted some blueberry growers to investigate mechanical harvesting. The major advantage is a significant reduction in management and labor costs. Savings in labor costs of 30 to 45 percent have been reported. One mechanical harvester can replace as many as 100 manual pickers. Over-the-row mechanical blueberry harvesters have been available since the mid-sixties. These machines have been primarily of the shake-and-catch design, which detach the blueberries by shaking or vibrating the bush. The fruit drops onto a capturing device and conveyor, on which it is moved past an air-blast cleaner and finally into field lugs.
Even if carefully adjusted and operated, mechanical harvesters will harvest a significant amount of unacceptable fruit. These machines have no means of detecting quality and will harvest immature, overripe, diseased, and damaged fruit that would ordinarily be discarded by a human picker. The elimination of this unacceptable fruit and foreign matter is difficult and expensive, and it requires that the berries be handled excessively. A mechanical harvester and the required sorting machines often subject the fruit to excessive damage. Overripe fruit, in particular, is easily damaged by these machines. Any additional handling adversely affects the shelf life and quality of the blueberries.
Although a significant percentage of the blueberries grown worldwide are harvested by machine, most North Carolina growers strive to produce fruit for the higher-valued fresh market. Mechanically harvested blueberries are generally considered unfit for the fresh market and are therefore usually frozen for processing.
Rules for Minimizing Damage to Fruit by Mechanical Harvesting
- Harvest only dry fruit in the coolest part of the day.
- Minimize the drop heights on the machine and pad the impact surfaces.
- Eliminate as much vibration as possible.
- Limit the depth of harvested fruit in the field containers to 4 or 5 inches.
- Handle and dump field containers very gently.
- Cool the fruit as soon as possible after harvest.
Sorting harvested blueberries intended for the fresh market is time consuming, expensive, and damaging to the fruit. Therefore, this practice is generally not recommended. The best time to remove defective berries is during manual harvesting, thus minimizing handling. For machine-harvested blueberries, there may be no alternative but to remove trash and defects on a grading line. When a grading line must be used, it should be operated in such a way as to minimize mechanical damage.
Whether blueberries are mechanically or manually harvested, every effort should be made to keep the percentage of defective fruit to an absolute minimum. Seriously defective fruit may include those with insect damage, broken skin, decay or mold, attached stems, fruit remaining in clusters, and fruit that is overripe, underripe, shriveled, or off-color. U.S. Number 1 is the only specified grade for harvested blueberries. Blueberries grading U.S. Number 1 may have no more than 1 percent serious defects and no more than 13 percent total defects. However, in practice and especially with fresh-market blueberries buyers may require less than 5 percent total defective fruit.
Most buyers prefer large fruit over small fruit if they are of good quality. The U.S. Number 1 grade specifies that the count of berries per 1⁄2-pint container must not exceed 250. Within the U.S. Number 1 classification there are four size classifications, as listed in Table 1.
|Extra large||Fewer than 90 berries per 1⁄2-pint cup|
|Large||90 to 129 berries per 1⁄2-pint cup|
|Medium||130 to 189 berries per 1⁄2-pint cup|
|Small||90 to 250 berries per 1⁄2-pint cup|
|For the complete USDA standard classifications for blueberries, refer to 46 FR 63203 or the latest revision.|
Almost all North Carolina fresh-market blueberries are marketed in cellophane-covered pint containers shipped in 12-pint master containers known as flats. Flats may be conveniently stacked, six to a layer, on a standard 40-by-48-inch shipping pallet (Figure 1). A pallet of blueberries customarily consists of 96 flats (1,152 pints) with a gross weight of approximately 1,200 pounds.
At warm field temperatures, harvested blueberries are extremely perishable and decline rapidly in quality. Visible signs of decay organisms can develop on warm, wet blueberries in less than 12 hours. The most common blueberry postharvest decay organisms in North Carolina are Alternaria tenuissima (Alternaria rot) and Botrytis cinerea (gray mold). In situations where blueberries are wet and the risk of decay is high, a rinse in chlorinated water (75 to 125 ppm available chlorine) has been successful in controlling these diseases. However, chlorination is a preventive measure only. It will not salvage fruit already infected because it cannot kill microorganisms that are already below the surface. Moreover, chlorination cannot be used for fresh-market fruit because the chlorine completely removes the bloom and bleaches the fruit.
Decay organisms are very active when the fruit is warm but become much less active at lower temperatures. Warm, overripe blueberries, leaking juice from rough handling, provide an ideal site for various postharvest decays. Shelf-life experiments with packaged blueberries have demonstrated the dramatic reductions in decay possible with rapid and thorough cooling. Figure 2 illustrates the effect of cooling in reducing decay.
Harvested blueberries should be cooled as soon as possible but always within 4 hours of harvest, if possible. This may require more than one pickup or trip to the cooling facility during a day of harvesting. However, the increase in shelf life and overall quality will make it worthwhile. No fruit, particularly well-ripened fruit, should ever be held all day without cooling.
Blueberries remain alive after harvest, respiring and producing heat. At a temperature of 80°F, blueberries can produce as much as 22,000 Btu per ton per day from the heat of respiration. Unless this heat is removed by cooling, it can cause a rise in temperature of as much as 6°F. Cooling lowers the respiration rate, slowing the ripening process and the accompanying inevitable decline in quality. The respiration rate of blueberries at 80°F is nearly 20 times the rate at 40°F. In other words, blueberries held at 40°F have nearly 20 times the shelf life of those held at 80°F. Softening is the most visible physiological damage resulting from overripening.
Most North Carolina blueberries in wholesale channels receive some cooling. However, a portion may continue to reach the consumer in poor condition as a result of insufficient cooling. Often the effectiveness of cooling, not its availability, is the deciding factor in quality. Cooling must be thoroughly and consistently applied to be effective. The optimum temperature that will ensure the longest shelf life is just above freezing: 33 or 34°F.
A pallet of packaged blueberries, initially at 80°F, allowed to remain for an hour or so in a cooler without forced ventilation may appear to have cooled somewhat. However, only the blueberries on the outside will cool appreciably in that length of time. Temperature measurements taken inside cellophane-wrapped cups near the center of pallets standing in still air at 44°F have actually shown a slight temperature increase during the first hour of cooling. Calculations suggest that the center cups of pallets of 80°F blueberries would require more than 36 hours to cool to below 50°F. Pallets of hot blueberries allowed to stand in a cooler for several hours before shipment without forced ventilation will not be cooled significantly nor uniformly. Furthermore, numerous tests have shown that refrigerated transports provide little or no additional cooling.
In still air, the average cooling rate of pallets of blueberries is slow because heat is transferred from the interior only by conduction. The cellophane wrappers, the fiber cups, the paperboard flats, and especially the air gaps between the cups act as effective insulation, slowing the movement of heat. This fact has prompted shippers of many commodities to install equipment inside the cooler to force the cold air through the containers, greatly increasing the cooling rate. In existing refrigerated rooms not equipped with fans, the addition of fans (Figure 3) to move the air is the single most cost-effective change possible, provided that the refrigeration system has sufficient capacity to remove heat rapidly.
Blueberries cooled with forced air reach the desired storage temperature significantly faster than those in still air. In addition, they cool more uniformly. Figure 4 shows typical average pulp temperatures for a pallet of blueberries in still air and for a pallet in forced air. Forced-air cooling is clearly much more rapid than cooling in still air. Depending on the circumstances, the rate of cooling may be 16 to 20 times faster.
Forced-air cooling gives more consistent and thorough cooling through the entire pallet load. In situations where a considerable investment has already been made in refrigeration equipment and buildings, an additional nominal investment in one or more forced-air cooling fans can dramatically reduce the time required to cool blueberries satisfactorily, resulting in the delivery of a consistently higher quality product to the consumer. For additional information, see Extension publication AG 414-3, Forced-Air Cooling.
Although blueberries are grown commercially in about a dozen states, the major supply areas are Michigan, New Jersey, North Carolina, and the Pacific Northwest states. Over the last decade, domestic per capita consumption of blueberries has continued to expand at an average annual rate of 4 percent. In 1991, consumption of fresh blueberries reached 1⁄4 pound per person; consumption of processing blueberries (in canned and frozen form) was nearly 1⁄2 pound per person.
Domestic blueberry supplies are most readily available during June, July, and August, with July being the peak harvest month. In the past, North Carolina producers have enjoyed a timely marketing advantage because they often harvest their berries in mid-May and June, earlier than many other competitive supply regions. Recently, however, increased plantings of highbush varieties in Florida, Georgia, Mississippi and Texas have intensified early-season marketing pressures. In addition, more plentiful late-summer low-bush supplies are available from Canada.
Local grower prices for blueberries have varied considerably over the past decade, with sharp price declines observed in 1985 and 1990. As the supply of high-quality berries continues to increase, local growers should continue their efforts to provide high-quality, properly cooled blueberries to buyers in order to maintain and expand their market share.
The following publications in this series focus on the postharvest cooling and handling of North Carolina fresh produce.
- AG 414-1, Introduction to Proper Postharvest Cooling and Handling Methods
- AG 414-2, Design of Room Cooling Facilities: Structural and Energy Requirements
- AG 414-3, Forced-Air Cooling
- AG 414-4, Hydrocooling
- AG 414-5, Top and Liquid Ice Cooling
- AG 414-6, Chlorination and Postharvest Disease Control
Sponsored by the Energy Division, North Carolina Department of Economic and Community Development, with State Energy Conservation Program funds, in cooperation with North Carolina State University. However, any opinions, findings conclusions or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the Energy Division, North Carolina Department of Economic and Community Development.
Publication date: Aug. 1, 1993
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