Once a crop is harvested, it is almost impossible to improve its quality. Losses of horticultural crops due to improper storage and handling can range from 10 to 40 percent. Proper storage conditions—temperature and humidity—are needed to lengthen storage life and maintain quality once the crop has been cooled to the optimum storage temperature.
Fresh fruits and vegetables are living tissues, although they are no longer attached to the plant. They breathe, just as humans do, and their composition and physiology continue to change after harvest. They continue to ripen and, finally, they begin to die. Cellular breakdown and death (senescence) are inevitable, but can be slowed with optimal storage conditions. Fresh fruits and vegetables need low temperatures (32 to 55°F) and high relative humidities (80 to 95 percent) to lower respiration and to slow metabolic and transpiration rates. By slowing these processes, water loss is reduced and food value, quality and energy reserves are maintained.
Transpiration rates (water loss from produce) are determined by the moisture content of the air, which is usually expressed as relative humidity. At high relative humidity, produce maintains salable weight, appearance, nutritional quality and flavor, while wilting, softening and juiciness are reduced. Leafy vegetables with high surface-to-volume ratios; injured produce; and immature fruits and vegetables have higher transpiration rates. External factors affecting transpiration rates are temperature, relative humidity, air velocity and atmospheric pressure. High temperatures, low relative humidity and high air velocity increase transpiration rates.
Relative humidity needs to be monitored and controlled in storage. A hygrometer or a sling psychrometer, not the appearance of the produce, should be used to monitor humidity. Control can be achieved by a variety of methods:
- Operating a humidifier in the storage area.
- Regulating air movement and ventilation in relation to storage room load.
- Maintaining refrigeration coil temperature within 2°F of the storage room air temperature.
- Using moisture barriers in the insulation of the storage room or transport vehicle, and in the lining of the packing containers.
- Wetting the storage room floor.
- Using crushed ice to pack produce for shipment.
- Sprinkling leafy vegetables, cool-season root vegetables, and immature fruits and vegetables with water.
Respiration and metabolic rates are directly related to room temperatures within a given range. The higher the rate of respiration, the faster the produce deteriorates. Lower temperatures slow respiration rates and the ripening and senescence processes, which prolongs the storage life of fruits and vegetables. Low temperatures also slow the growth of pathogenic fungi which cause spoilage of fruits and vegetables in storage. Table 2 contains a list of fruits and vegetables classified by respiration rates. Producers should give special care and attention to proper storage conditions for produce with high to extremely high respiration rates—those crops will deteriorate much more quickly.
It is impossible to make a single recommendation for cool storage of all fruits and vegetables. Climate of the area where the crop originated, the plant part, the season of harvest and crop maturity at harvest are important factors in determining the optimum temperature. A general rule for vegetables is that cool-season crops should be stored at cooler temperatures (32 to 35°F), and warm-season crops should be stored at warmer temperatures (45 to 55°F). There are exceptions to this rule, though. Table 1 lists optimum storage temperatures for commonly grown Kansas fruits and vegetables.
Temperatures that are too low can be just as damaging as those too high. Freezing will occur in all commodities below 32°F. Whether injury occurs depends on the commodity. Some can be repeatedly frozen and thawed without damage, while others are ruined by one freezing. Table 1 shows the highest freezing point for most fruits and vegetables. Table 3 lists susceptibility to freezing injury. Produce that is likely to be injured by one freezing is classified as “most susceptible.” The “moderately susceptible” produce will recover from one or two freezings. Produce which is “least susceptible” can survive several freezings without injury.
Injury from freezing temperatures can appear in plant tissues as loss of rigidity, softening and water soaking. Injury can be reduced if the produce is allowed to warm up slowly to optimum storage temperatures, and if it is not handled during the thawing period. Injured produce should be marketed immediately, as freezing shortens its storage life.
Fruits and vegetables that require warmer storage temperatures (40 to 55°F) can be damaged if they are subjected to near freezing temperatures (32°F). Cooler temperatures interfere with normal metabolic processes. Injury symptoms are varied and often do not develop until the produce has been returned to warmer temperatures for several days. Besides physical damage, chilled produce is often more susceptible to disease infection. Table 4 lists susceptible fruits and vegetables, and characteristic symptoms of chilling injury.
Crops that require different storage conditions will need three different storage facilities.
- Cold storage (temperatures 32 to 36°F).
- Cool storage (temperatures 40 to 55°F).
- Warmer storage (temperatures 55 to 60°F for sweet potatoes, squash, pumpkins or similar crops).
A recording thermometer can be helpful in determining whether storage facilities are maintaining ideal conditions and are not fluctuating. A maximum/minimum thermometer could be substituted. The thermometer should not be the same as the thermostat controlling the refrigeration equipment. Relative humidity also should be monitored with a hygrometer or a sling psychrometer.
Controlling and monitoring temperature and relative humidity will enable a grower to maintain optimum conditions for maximum storage life of the crop, and to minimize crop damage from chilling, freezing and/or too-high temperatures and water loss from the crop. Close attention to storage conditions will yield returns through greater customer satisfaction, less waste and spoilage, and in the flexibility to hold a crop without significant storage losses to wait for better markets.
Table 1. Commonly grown fruits and vegetables with recommended storage conditions for temperature and relative humidity approximate storage life under optimum conditions and highest freezing points.
|Commodity||Temperature (°F)||Rel. humidity (percent)||Approximate
|Freezing point (°F)|
|— Blackberries||31–32||90–95||2–3 days||30.5|
|— Currants||31–32||90–95||1–4 weeks||30.2|
|— Elderberries||31–32||90–95||1–2 weeks||—|
|— Gooseberries||31–32||90–95||3–4 weeks||30.0|
|— Raspberries||31–32||90–95||2–3 days||30.0|
|— Strawberries||32||90–95||3–7 days||30.6|
|— Cherries, sour||32||90–95||3–7 days||29.0|
|Cherries, sweet||30–31||90–95||2–3 weeks||28.8|
|Grapes, American||31–32||85||2–8 weeks||29.7|
|Plums and prunes||31–32||90–95||2–5 weeks||30.5|
|Artichokes, Jerusalem||31–32||90–95||4–5 months||28.0|
|Beans, dry||40–50||40–50||6–10 months||—|
|Beans green or snap||40–45||95||7–10 days||30.7|
|Beans, lima||37–41||95||5–7 days||31.0|
|Beans, sprouts||32||95–100||7–9 days||—|
|Beets, bunched||32||98–100||10–14 days||31.3|
|Beets, topped||32||98–100||4–6 months||30.3|
|Brussels, sprouts||32||95–100||3–5 weeks||30.5|
|Cabbage, early||32||98–100||3–6 weeks||30.4|
|Cabbage, late||32||98–100||5–6 months||30.4|
|Cabbage, Chinese||32||95–100||2–3 months||—|
|Carrots, bunched||32||95–100||2 weeks||—|
|Carrots, mature||32||98–100||7–9 months||29.5|
|Carrots, immature||32||98–100||4–6 weeks||29.5|
|Chicory, witloof||32||95–100||2–4 weeks||—|
|Corn, sweet||32||95–98||5–8 days||30.9|
|Endive and escarole||32||95–100||2–3 weeks||31.9|
|Greens, leafy||32||95–100||10–14 days||—|
|— Cantaloupe (3/4 slip)||36–41||95||15 days||29.9|
|— Cantaloupe (full slip)||32–36||95||5–14 days||29.9|
|— Casaba||50||90–95||3 weeks||30.1|
|— Crenshaw||45||90–95||2 weeks||30.1|
|— Honey Dew||45||90–95||3 weeks||30.3|
|— Persian||45||90–95||2 weeks||30.5|
|— Watermelon||50–60||90||2–3 weeks||31.3|
|Onions, green||32||95–100||3–4 weeks||30.4|
|Onion, dry||32||65–70||1–8 months||30.6|
|Onion sets||32||65–70||6–8 months||30.6|
|Peas, green||32||95–98||1–2 weeks||30.9|
|Peas, southern||40–41||95||6–8 days||—|
|Peppers, chili (dry)||32–50||60–70||6 months||—|
|Peppers, sweet||45–55||90–95||2–3 weeks||30.7|
|Potatoes, early crop||40||90–95||4–5 months||30.9|
|Potatoes, late crop||38–40||90–95||5–10 months||30.9|
|Radishes, spring||32||95–100||3–4 weeks||30.7|
|Radishes, winter||32||95–100||2–4 months||—|
|Squashes, summer||41–50||95||1–2 weeks||31.1|
|Squashes, winter||50||50–70||1–6 months||30.5|
|Sweet potatoes||55–60||85–90||4–7 months||29.7|
|Tomatoes mature, green||55–70||90–95||1–3 weeks||31.0|
|Tomatoes firm, ripe||55–70||90–95||4–7 days||31.1|
|Turnip greens||32||95–100||10–14 days||31.7|
Table 2. Fruits and vegetables classified by their respiration rates (at 41°F).
|<5||Nuts, Dates, Dried Fruits and Vegetables|
|Low||5–10||Apple, Grape, Garlic, Onion, Potato (mature), Sweet Potato|
|Moderate||10–20||Apricot, Cherry, Peach, Pear, Nectarine, Plum, Cabbage, Carrot, Lettuce, Pepper, Tomato, Potato (immature)|
|High||20–40||Strawberry, Blackberry, Lima Bean, Raspberry, Cauliflower|
|Very high||40–60||Artichoke, Snap Bean, Green Onion, Brussels Sprouts|
|Extremely high||>60||Asparagus, Broccoli, Sweet Corn, Mushroom, Spinach, Pea|
Table 3. Fruits and vegetables classified by susceptibility to injury by temperatures below 32°F.
|Group 1, Most susceptible||Group 2, Moderately susceptible||Group 3, Least susceptible|
|Asparagus||Broccoli, sprouting||Brussels Sprouts|
|Beans, snap||Cabbage, new||Cabbage, mature and savoy|
|Berries (except cranberries)||Carrots1||Kale|
Table 4. Fruits and vegetables susceptible to chilling injury when exposed to temperatures below their optimum range and above 32°F.
|Commodity||Approx. lowest safe temperature (°F)||Symptoms of injury from below-optimum temperatures|
|Apples-certain cultivars||36–38||Internal browning, brown core, soggy breakdown, soft scald|
|Asparagus||32–36||Dull, gray-green, limp tips|
|Beans (lima)||34–40||Rusty brown specks, spots, or areas|
|Beans (snap)||45||Pitting and russeting|
|Cucumbers||45||Pitting, water-soaked spots, decay|
|Eggplants||45||Surface scald, alternaria rot, blackening of seeds|
|— Cantaloupe||36–41||Pitting, surface decay|
|— Honey Dew||45–50||Reddish-tan discoloration, pitting, surface decay, failure to ripen|
|— Casaba||45–50||Same as above, but no discoloration|
|— Crenshaw and Persian||45–50||Same as above, but no discoloration|
|— Watermelons||40||Pitting, objectionable flavor|
|Okra||45||Discoloration, water-soaked areas, pitting, decay|
|Peppers, sweet||45||Sheet pitting, alternaria rot on pods and calyxes, darkening of seed|
|Pumpkins and hardshell squashes||50||Decay, especially alternaria rot|
|Sweet Potatoes||55||Decay, pitting, internal discoloration; hard core when cooked|
|— Ripe||45–50||Water soaking and softening decay|
|— Mature-green||55||Poor color when ripe, alternaria rot|
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