Growing fruits and vegetables requires knowing how to plant, provide pest control, irrigating when needed and harvesting at the proper time. But along with the effort it takes to do all this, knowing how to handle the produce after harvest will assure that the grower will sell sound produce that will not be subjected to premature breakdown and spoilage. If good care after harvest is not followed, all the grower’s efforts before harvest are for naught.
Ideally, a grower should sell most or all of their produce immediately after it’s harvested. But if they can’t sell all of it at one time, they need some type of cold storage to hold it for a few days before a market is found.
All fruits and vegetables cannot be held at the same temperature and humidity. Some prefer a cold temperature with a high relative humidity; others prefer slightly higher temps and lower humidity levels. Knowing what temperature range is desirable will keep produce fresh before sale. Cooling of produce extends the storage life of fruit and vegetable crops and reduces the reproduction of spoilage microorganisms and many food-borne pathogens. The faster the field heat is removed from the produce, the longer the storage time can be under refrigeration. That is why produce should be harvested early in the morning, before air temperatures get very hot, or at night, using lights to illuminate the field.
The proper temperatures should be maintained during cold storage and during transportation if the produce is to be shipped long distances. This maintenance of the constant ideal temperature is known as the “cold chain.” It should never be allowed to fluctuate; otherwise, the post-harvest life of the produce will be drastically shortened.
A grower’s cold storage facility should hold most produce between 90-95% relative humidity to prevent premature drying out and shrinkage. The exception to this rule would be for hot peppers, pumpkins, winter squash and sweet potatoes, which do best at a relative humidity between 65-85%.
Ethylene (C2H4) is a natural hormone that plants produce and use to regulate growth and development. Generally, ethylene rates increase with maturity and when produce is injured. During storage, ethylene can damage sensitive crops. For example, exposure to ethylene causes russet spotting on lettuce and yellowing in broccoli. Ethylene damage generally does not occur in less than 24 hours of exposure. Exposures are cumulative. Rapid and efficient cooling help prevent damage. Growers often see ethylene damage when they store apples, which release high ethylene levels, with sensitive crops like broccoli.
Transpiration & Water Loss
Each hour of exposure to warm, dry air results in over twice as much water loss as holding produce in high-humidity cold storage for one week. Most fresh produce is 85-95% water when harvested. Within growing plants there is a constant flow of water. Liquid water is absorbed from the soil by the roots, then passed up through the stems, and finally lost from the aerial parts, especially leaves, as water vapor.
The passage of water through the plants is called the transpiration stream. It maintains the high water content of the plant. A lack of water will cause plants to wilt and perhaps die. Fresh produce continues to lose water after harvest, but unlike in the growing plant, it can no longer replace lost water with water from the soil. Instead, it uses up water in the harvested produce. This loss of water from fresh produce after harvest is a serious problem that causes wilting and shriveling as well as loss of weight.
When harvested produce loses 5% or 10% of its fresh weight, it begins to wilt and shrivel and becomes unusable. To extend the usable life of produce, its rate of water loss must be as low as possible. The rate of water loss varies with the type of produce. Leafy green vegetables, especially spinach, lose water quickly because they have a thin skin (epidermis) with many pores. Others, such as potatoes, which have a thick corky skin with few pores, have a much lower rate of water loss.