Consistently producing high-quality produce that succeeds in a highly competitive market does not stop at the end of the row. The postharvest handling functions of grading, packing, cooling, storing, transporting, and marketing are dedicated to delivering the product to the consumer with as near-to-harvest quality as possible. Our ability to deliver a quality product depends on proper postharvest handling. Consumer satisfaction is the result of farming expertise and proper postharvest handling all the way to the consumer.

The quality and uniformity demanded by the market requires successful growers to view proper postharvest handling as an essential part of their overall production plan. Growers may take pride in their skill to grow a perfect crop, but unless that crop is protected on the way from the field or orchard to the consumer, the results will be disappointing. Recent estimates have shown that as much as 30% of some types of harvested produce may be lost from mishandling before it ever reaches the consumer. No postharvest treatment can correct inferior quality produce that is a result of poor production practices or improper handling. And, while first-quality produce can command premium prices, poor-quality produce often cannot be sold at any price. Proper postharvest handling is the important link between the producer and the consumer, and it requires special attention.

Understanding the postharvest physiology of fresh produce is key to the design and execution of proper postharvest engineering. The natural aging or ripening process of fresh produce is known as senescence. An item with living tissue will continue to age or senesce. Although the senescence of fresh produce cannot be halted completely, it can be slowed substantially by proper environmental control.

Produce is at peak quality at harvest. The purpose of proper postharvest handling is essentially the maintenance of this peak quality until the produce reaches the consumer. After harvest, fruits and vegetables remain alive and will maintain metabolic activity for an indefinite period. Harvested produce contains a finite amount of starches and sugars. As time passes and the produce respires, these starches and sugars are used to sustain the life processes and release carbon dioxide, water vapor, and heat. The higher the rate of respiration, the more quickly the product will lose its supply of starch and sugar. Different types of fresh produce have different rates of respiration, and the produce items with highest respiration rates will be more perishable. Produce with young, fast growing tissue has a high rate of respiration. Examples are sweet corn, green beans, strawberries, and broccoli. These items have an effective shelf life of only a few days even under optimal conditions. In contrast, root crops such as potatoes, carrots, and turnips have relatively low respiration rates and may be held for weeks or even months in a proper storage environment.

After harvest separates produce from its source of food and water, the produce must rely on what it contains internally to stay alive. As long as the produce is alive, it will continue to respire and use its internal resources. The respiration rate of a produce item is related to the type of produce, its temperature, gas concentrations in the environment, nature of the produce, whether it is climacteric/non-climacteric (the end of fruit maturation and the beginning of fruit senescence), physical stresses, mechanical damage, variety, and the stage at which it is harvested. The respiration may be slowed or sped up but it will continue. As respiration proceeds, stored food reserves are lost which result in lost food value, lost weight, and particularly lost flavor.

The process of senescence of freshly harvested produce is like a countdown clock with limited ticks left (see Figure 1-1). Eventually, the clock will run down. The decline in quality cannot be stopped or reversed and after a certain point, the produce is unmarketable.

Figure 1-1. The shelf-life clock is always ticking.

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Figure 1-1. The shelf-life clock is always ticking.

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Respiration involves a chemical reaction when starches and sugars are oxidized into CO₂, water, and heat. Like all chemical reactions, the rate of respiration reactions depends on the temperature. The Arrhenius equation is a simple, reasonably accurate model for the temperature dependence of reaction rates. The equation, proposed by Svante Arrhenius (see Figure 1-2) in 1889, was based on the work of Dutch chemist Jacobus H. van't Hoff. Arrhenius provided a physical justification and interpretation for the formula used to model the effects of temperature on diffusion coefficients, creep rates of solids, and especially the chemical reactions similar to those involved in plant respiration and ripening.

A useful application of Arrhenius' equation is that for many common chemical reactions that occur near room temperature, the reaction rate varies exponentially with rising temperature. It is important to be very careful about generalizing reaction rate changes as a function of temperature.

A general and often quoted rule is that the rate of respiration doubles for each 10 C° increase in temperature.

A subtle difference in notation: 10 C° indicates a quantity, for example, the number of Centigrade degrees between 45°C and 35°C. On the other hand, 10°C indicates a temperature.

The variation in respiration rates of different produce is primarily the result of the different chemicals involved. This explains the positive feedback loop and the reason the respiration rates and the rate of heat generation of fresh produce increases with increasing temperature. This also emphasizes why, in produce cooling, removing the first increment of heat (“top heat”) is so much more important than removing the last increment (“bottom heat”).

Arrhenius' equation below shows the dependence of the rate constant k of a chemical reaction on the absolute temperature T (in Kelvin), where A is a coefficient unique to the specific reaction, E is the activation energy, and R is the universal gas constant.

\(\mathrm{k}={\mathrm{Ae}}^{{-\mathrm{E}}/{\mathrm{RT}}}\)

The units of the coefficient A are identical to those of the rate constant and will vary depending on the order of the reaction. If the reaction is first order, it has the units s⁻¹, and for that reason, it is often called the frequency factor or attempt frequency of the reaction. Simply stated, k is the number of molecular collisions that result in a reaction per second, A is the total number of collisions (leading to a reaction or not) per second, and \(\mathrm{e}^{{-\mathrm{E}}/{\mathrm{RT}}}\) is the probability that any given collision will result in a reaction.

Either increasing the temperature or decreasing the activation energy (for example through the use of catalysts) will result in an increase in the rate of reaction. With fresh produce, the chemicals involved in active tissue growth yield lower activation energies than chemicals involved in simple cell respiration. This helps to explain why sweet corn (actively growing), for example, has a higher respiration rate than potatoes or apples (living but no longer growing).

Arrhenius argued that for reactants to transform into products, they must first acquire a minimum amount of energy, called the activation energy E_a. At an absolute temperature T, the fraction of molecules that have a kinetic energy greater than E_a can be calculated from statistical mechanics. The concept of activation energy explains the exponential nature of the relationship.

Figure 1-2. Svante Arrhenius, 1909.

Source: Lexico UK English Dictionary, Oxford University Press, archived from the original August 27, 2022.

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Figure 1-2. Svante Arrhenius, 1909.

Source: Lexico UK English Dictionary, Oxford University Press, archived from the original August 27, 2022.

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All living organisms respond to external stimuli. The environmental factors (stimuli) that influence the postharvest physiology of fresh fruit and vegetables include:

• Temperature (both below and above the optimum for a particular item)
• Moisture (the presence or absence of liquid water and relative humidity)
• Atmosphere (O₂, CO₂, ethylene)
• Light (levels, intensity, and wavelength)
• Time (affected by the four factors above, either separately or in combination).

The most important of the external stimuli that influence the quality of fresh fruit and vegetables is temperature. As stated, the respiration rate is dependent on the temperature of the reaction. In practical terms, if a certain item of fresh produce increases in temperature by 10 C° (18 F°), the respiration rate doubles and the shelf life will be effectively cut in half. For example, as shown in Table 1, the respiration rate of blueberries is:

• 6 mg/kg hr of CO₂ at 0°C, with the heat generation of (6) (220) = 1320 Btu/ton day or 0.028 Btu/lb. hr.
• 29 mg/kg hr. of CO₂ at 10°C, with the heat generation (29) (220) = 6380 Btu/ton day or 0.133 Btu/lb. hr.
• 70 mg/kg hr. of CO₂ at 20°C, for heat generation (70) (220) = 15,400 Btu/ton day or .320 Btu/lb. hr.

From 0°C to 10°C, the respiration rate increases 29/6 = 4.8 times, although from 10°C to 20°C it only increases 70/29 = 2.4 times. Thus, the rate of increase is decreasing as the temperature rises. The data indicate that the respiration rate increases 70/6 = 11.7 times from 0°C to 20°C.

Given that shelf life of an item of fresh produce is inversely related to respiration, the shelf life of blueberries picked and immediately cooled to 0°C is nearly 12 times longer than those picked and allowed to remain at 20°C. Table 1-1 classifies the respiration rates of some common produce. Since the respiration rate varies widely from item to item, each fresh produce item must be considered individually for its proper postharvest handling. No postharvest function has a greater influence on produce quality than temperature.

The effects of temperature go beyond raised respiration. For example, in storage, root vegetables such as Irish potatoes, sweetpotatoes, and turnips that experience prolonged episodes of warm temperatures will sprout. Storage roots store carbohydrates from one growing season to another. The second season’s growth is triggered when the root is exposed to the warm conditions that would usually occur in the spring. During sprouting, there is a very sharp rise in respiration as well as weight loss that results in loss of flavor.

Temperatures that are too low or too high can harm fresh produce. Many produce items suffer chill injury that should not be confused with freeze injury. Freeze injury occurs when the liquid in the cells of the produce freezes and forms ice crystals that puncture the cell walls and kill the cells. Many temperate warm season fruit and vegetables and almost all tropical fruit can be chill injured because in their natural state, they never needed to develop natural defenses against cool temperatures.

Chill injury in fresh produce is deceptive because the symptoms can be delayed by days or even weeks after the chill. Symptoms include:

• Water spotting. This causes watery, sunken lesions on the surface of the produce. These lesions may develop secondary infections such as Rhizopus, which hampers the discovery of the chill injury.
• “Off” color. Chill injury can cause lightening or darkening of the produce. A example is the blackening of bananas that have been refrigerated.
• “Off” taste and odor. Chilling disrupts the metabolism of the cells by altering the chemical pathways involved in respiration. This disruption causes the formation of organic compounds that have a bad smell and negatively affect the taste of the produce.
• Reduced resistance to rot pathogens. Plants produce natural chemical defenses that attack rot pathogens. Chilling reduces the plant’s ability to produce these defensive chemicals and makes the plant susceptible to attack. This loss of resistance may take weeks to develop, which makes it difficult to identify the chilling episode.

The severity of chill injury is a function of both time and temperature. For example, with sweetpotatoes that suffer chill injury below 50°F, a few hours held at 40°F may result in the same damage as a few days at 45°F.

Fresh produce can contain approximately 75% to 98% water. The exact percentages are specific to the type and variety of produce. For a type of produce that is alive and respiring, the percent of moisture does not vary over a wide range. Fresh produce includes living cells which to remain alive and healthy must keep their moisture level reasonably constant to maintain stability.

Loss of water from the plant cell tissue and into the atmosphere reduces the cell’s internal pressure and causes loss of turgor. When turgor is lost, the result is wilting, discoloration, weight loss, and ultimately, a loss of marketability. The produce has no way to regain the lost moisture and may respond to the loss with various physiological strategies such as closure of the plant stoma and increased respiration to produce more water.

Most types of produce cannot be sold after they have lost between 3% and 5% of their fresh weight. The rate of water loss depends on temperature, humidity, airflow, and the surface condition of the produce. Loss of turgor can be slowed by high humidity. A high humidity environment or surface misting will not restore the lost moisture. The misting and water sprays seen in grocery stores keep the humidity high only in the immediate area of the produce.

Water loss (evaporation) from the surface of the produce is much faster at high temperatures than low temperatures, when other factors such as relative humidity and air pressure are constant. The vapor pressure of water increases exponentially with increasing temperature as shown in Figure 1-3. The rate of evaporation is directly related to the temperature, with other factors being equal.

The water evaporation rate at 90°F is seven times faster than at 40°F. Water loss is also much more rapid in low humidity than high humidity because the amount of water vapor in the surrounding air affects the evaporation rate. No evaporation will occur if the air surrounding the produce is already fully saturated. As the humidity of the surrounding air approaches 100%, all water loss ceases, regardless of temperature. In addition, in still air, a thin layer of air surrounding the produce will become saturated, which inhibits further water loss. If air is kept moving past the produce, moisture moving from the surface will be continually carried away. Items such as lettuce and leafy greens, for example, are particularly susceptible to water loss because they have such large surface area to volume ratios.

Fortunately, many fruits and some vegetables have a natural wax coating that slows the loss of water. Water loss from warm products to warm relatively dry air is particularly serious under windy conditions or during transport in an open vehicle. In addition to reducing water loss, the wax coating on some fruits (such as bell peppers and apples) can reduce surface evaporation and the subsequent evaporative cooling to an extent that, in bright and direct sunshine, the surface cells are overheated and killed. This is known as “sunscald.”

When the air surrounding the produce item is at or near 100% relative humidity, little or no water can be lost from the surface by evaporation. Most produce items should be stored in areas with high relative humidity to maintain freshness. Care must be exercised as the humidity approaches saturation. Any surface below the dew point of the air such as cool floors or the north side of the building will collect condensation. This should never be allowed in produce storage because produce that comes in contact with water is especially susceptible to postharvest rots. In addition, liquid water can accelerate corrosion and decay of building materials. This is the reason that recommendations for optimum relative humidity for fresh produce are seldom above 90%.

Figure 1-3. Vapor pressure of water at various temperatures

Source: M. Boyette.

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Figure 1-3. Vapor pressure of water at various temperatures

Source: M. Boyette.

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Another factor with a profound effect on the postharvest physiology of fresh fruit and vegetables is atmosphere, which is the mix of gasses that surround the produce. Fresh produce respires by removing oxygen from the atmosphere to sustain its life processes. Some produce reacts negatively when the levels of oxygen (O₂ ) are too low or carbon dioxide (CO₂) levels too high as compared to ambient levels. When O₂ levels are too low, the cells of some types of produce become starved for oxygen and alter their respiration pathways in ways that produce alcohols and other aromatic organic compounds that have odors and strange tastes. Eventually this process results in cell death. Some produce reacts negatively to low levels of oxygen or high levels of carbon dioxide because such levels inhibit normal respiration. For example, lettuce can develop browning from high CO₂ levels.

In contrast, certain types of fresh produce such as apples and pears have a reduced respiration rate and slowed senescence at lower than ambient O₂ levels. This allows for a greatly extended shelf life. The process of storing produce is known as either modified atmosphere storage (MAS) when the gasses are passive controlled or controlled atmosphere storage (CAS) when the gasses are actively controlled. Both MAS and CAS have been shown to be beneficial in reducing certain pests and rots.

The CAS storage of apples is practiced widely and may yield excellent quality produce after six to eight months in storage. This is the reason we have apples in the grocery store year around with near “just picked” quality. Unfortunately, this process has been attempted with onions, sweetpotatoes, and other types of produce with disappointing results.

Modified atmosphere technology is now employed in consumer produce packaging. It is possible to engineer plastic packaging material that controls the entry and exit of gases including water vapor, O₂, CO₂ and ethylene in a way that enhances the shelf life of produce.

Another atmospheric gas of interest in postharvest handling is the plant hormone ethylene. Ethylene at trace levels (~50 ppm) can induce senescence and ripening in some fruit and vegetables. Not all produce will react to ethylene but many do (see Table 1-2). For example, bananas and mature green tomatoes are two produce items that are picked unripe and held for an indefinite period in refrigerated storage. When ready for market, these items are placed in ripening rooms where they are exposed to ethylene (~50 ppm) for a few hours that begins and hastens ripening.

It is good practice to be aware of ethylene production and sensitivity when storing different produce items in the same room. A small ethylene concentration and short exposure time can trigger ripening. As expected, ethylene production and sensitivity increase with warm temperatures, while timely cooling reduces the effects. Wood and paper smoke and other air contaminants also contain trace amounts of ethylene that may trigger ripening. Ethylene can also cause sprouting of potatoes, carrots, and onions and cause bitterness in some items. Since the green mold of citrus (Penicillium difitatum) and some other decay organisms also produce ethylene, all decayed produce should be removed promptly from storage rooms.

There are now approved chemicals for controlling ethylene in storage and cooling rooms. The chemical is 1-Methylcyclopropene (1MCP), which was developed at North Carolina State University in the late 1990s by teams led by Drs. Sylvia Blankenship and John Dole. The chemical, which is sold under the trade name of SmartFresh®, is used by the fresh produce industry to maintain the quality of fresh fruit and vegetables.

Many produce items may pass strong flavors or odors to other items if they are stored together. The strong odors of onions or cantaloupes are absorbed by many fruits and vegetables, as is the odor of citrus. Freshly dug potatoes can give an earthy taste to many fruits and vegetables. Produce has also been known to absorb odors from the floors and walls of coolers or trucks that were used to transport other produce items days or weeks before. Small growers, particularly those with roadside stands or retail establishments, may find it necessary to store non-produce items in a cooler with fresh produce. This should be done with caution because odors can be easily transferred to and from many of these items. Many fruits and vegetables will absorb odors from fresh flowers. Dairy products will absorb odors from a wide range of commodities. Onions and garlic cannot be stored or transported with grapes. Odors from green onions are absorbed by grapes, mushrooms, or sweet corn. Citrus absorbs odors from many strongly scented fruits and vegetables. Odors from apples and pears are absorbed by cabbage, celery, carrots, onions, and potatoes. Celery absorbs odors from onions, apples and carrots, while odors from peppers is absorbed by snap beans.

The intensity and wave length of light can affect fresh produce in several ways. One of the most common effects is heat generated by sunlight. Harvested produce should always be shielded from direct sunlight. Bright sunlight can heat the surface very rapidly and cause wilting. If hot enough, sunlight can cause sunscald as is seen in peppers, tomatoes, and other fruits. In addition, sunlight can cause the “greening” of potatoes by producing chlorophyll and inducing sprouting in a number of root crops. Some recent research has suggested that fruit and vegetables maintain their natural circadian rhythms in response to light and dark cycles and that this can influence their nutrient levels.

The effects of temperature, moisture, atmosphere, and light are influenced not only by level of exposure but also by the time of exposure. A long exposure at a low level can be just as detrimental as a high exposure for a shorter interval. The produce item that has been disconnected from the mother plant is now on its own with a finite supply of food and water. As long as it remains alive, the produce’s biological actions will be using their resources. Slowing the inevitable decline in quality from fresh picked produce is the goal of proper postharvest handling.

Ultimately, nothing can stop the decline in quality of fresh produce, although proper handling can slow the process substantially. Because fresh produce responds to environmental conditions of light, temperature, humidity, and the mix of surrounding atmospheric gases, the control of proper temperature, humidity, and ventilation is essential to the maintenance of quality. However, one set of conditions is not applicable to all types of produce. Each different type of produce has its own unique set of proper postharvest conditions. For example, strawberries and apples respond well to temperatures just above freezing. Tomatoes, peppers, and squash, however, may suffer chill injury if allowed to remain at these temperatures even for a relatively short time. Knowledge of the limitations of each item of produce will prevent unnecessary injury. Table 1-2 describes the specific handling requirements for many fresh fruits and vegetables.

For an engineer tasked with designing a postharvest cooling/storage facility, adhering strictly to the Fundamental Rule can result in a design with a separate room for each and every different produce item. This is not only costly to build and maintain but also very difficult to use. It is always important to be aware of the unique requirements of each produce item. The information in Tables 1-2 to 1-4 is based on years of research and shows the optimum requirements required for maximum shelf life. However, based on our experience and engineering judgment, we may sometimes bend the rules to our advantage with a minimum reduction in quality.

For example, if the optimum temperature of one produce item is 40°F and another is 50°F, it may make sense to cool and store them both in the same room in order to economize on refrigerated space. With this option, one question is the temperature to maintain in that room, which would be either 40°F, 50°F, or the average of 45°F. If chill damage is more of an issue with the item optimally held at 50°F than loss of shelf life for the item optimally held at 40°F, then 50°F might be better. The level of deviation from an item’s optimum temperature range for the sake of economy depends heavily on time. A few degrees above or below the optimum temperature range for a few hours may not have a significant effect. This is where judgment and experience matter.

The same is true for odor transfer and ethylene. Time of exposure is very important. One of the key rules of postharvest handling is keep the produce moving. Allowing produce to sit anywhere for an extended time, either in a cooler on the farm or in a refrigerated trailer on the road, is not a good idea. Cooling a mixed load of produce for a few hours in a postharvest facility or packing a mixed load of produce in a closed refrigerated trailer for a half-day trip to market will not cause a problem if the produce items are moved quickly to retail.

^[1] R: room cooling; F: forced air cooling; H: hydrocooling; I: icing, V: vacuum cooling. ↲

^[2] Caution — chilling injury may occur in some commodities at 10°F to 20°F above freezing. ↲

^[3] Optimum for most herbs; basil 48°F to 50°F, arugula 35°F to 37°F. ↲

^[4] Some commodities are sensitive to ethylene; some are both sensitive and produce ethylene. ↲

^[5] Respiration heat values are an estimated average of those at field ambient and optimum storage temperatures. ↲

^[1] Without tops ↲