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. 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).
Freezing Injury. 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.
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.
Chilling Injury. 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.
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.