Vegetable quality is generally at a peak at harvest, and decreases as the produce moves from packing to the final consumer. One of your goals as a vegetable producer should be to establish conditions and procedures that will help maximize the quality of your harvested produce. Vegetable produce items are living organisms that undergo physiological and pathological processes even after harvest: respiration continues. Water is lost to the air surrounding the produce, and decay and rot may develop.
Respiration is a continuing process for harvested vegetables. During respiration, the vegetable will transform stored sugars into energy, releasing water, carbon dioxide and heat. Respiration rates vary among commodities: vegetables such as cabbage and potatoes have low respiration rates; others such as asparagus and sweet corn have high respiration rates. For all vegetables, respiration is affected by temperature- as the temperature decreases so does the respiration rate.
Water loss also occurs, resulting in wilting and shriveling which damages appearance and consumer appeal. Water loss from produce occurs through the opening in the cuticle of the vegetable, often as a result of mechanical damage during harvest. Any cut vegetable (such as asparagus) can lose much of its water through the cut end of the stalk. Some vegetables are more susceptible to quality changes due to water loss, with leafy green vegetables being the most susceptible, and hard shell vegetables like winter squash and pumpkins being less susceptible. Besides the relative humidity of the air surrounding the vegetables, water loss is affected by temperature. As the temperature decreases so does the rate of water loss.
Decay and rot are caused by organisms whose maximum growth rate often coincides with temperatures encountered during harvest. Some decay organisms do not grow at temperatures below 40 degrees F, while others will continue to grow, but at a slower rate, at temperatures of 32 degrees F.
Holding perishable vegetables at low temperatures is an important tool in controlling quality losses due to respiration, water loss and postharvest decay. The more rapidly we can cool vegetables, the less changes will occur in sugars (sweetness), appearance (wilting and shriveling) and decay. Recent research in Australia with asparagus illustrates that change in postharvest quality for vegetables is a time-temperature phenomenon. Similar to the degree-day concept that we use to gauge plant growth and development, the quicker we get produce cool, the longer we have before noticeable quality changes occur.
Cooling methods vary based on availability and traditional use. Selection of the best type of cooler for a particular operation depends on several considerations include the mix of commodities handled, the length of the cooling season, and the need (or desire) for fast cooling. Current choices for cooling include room cooling, forced-air cooling, hydro-cooling, ice-cooling, and vacuum cooling.
Room cooling is very common for vegetables and fruits produced in Michigan. The vegetables, either packed in cartons or in bulk boxes, are placed in a cold room and will cool through air movement between the produce and the room air. This method is generally the slowest way to cool perishable vegetables.
Forced-air cooling includes a variety of methods for forcing more cold air through the packaged produce so that temperatures will be reduced rapidly. An advantage of forced-air cooling is that the vegetables do not get wet. A disadvantage is the extra handling between the forced-air cooling and holding areas in the cold room.
Hydrocooling uses cold water, either sprayed over the vegetables or with the vegetables immersed. The use of water increases the rate of cooling relative to air because of the higher amount of energy that can be absorbed by the water. A disadvantage of hydrocooling is that the vegetables are wet which can aggravate decay in some vegetables.
Ice cooling uses solid or liquid-slush placed in the carton with the vegetables to cool them and provide low temperatures during transport and storage. It is limited to a few commodities that have traditionally been cooled with this method including broccoli and cut flowers.
Vacuum cooling uses special equipment to put the produce in a low-pressure environment. The rate of water loss by the produce is increased by the low pressure, and the cooling is accomplished as the water evaporates. Vacuum cooling is expensive, both in capital cost and energy, and should be reserved for leafy green vegetables that have high respiration and water loss rates.
