Erin Silva, University of Wisconsin
Adapted from: Silva, E. 2008. Influence of preharvest factors on postharvest quality. In Wholesale success: a farmer's guide to selling, postharvest handling, and packing produce (Midwest edition). Available online at: http://www.familyfarmed.org/wholesale-success/ (verified 19 May 2011).
Obtaining the optimum postharvest quality of vegetables actually begins very early in the farm planning process. The effects of preharvest factors on postharvest quality are often overlooked and underestimated. However, many of the decisions that we make during crop production can greatly influence the postharvest quality of crops. It is critical to remember that vegetable quality is only maintained postharvest – it is not improved during the harvest and storage processes. Thus, it is of utmost importance to consider the preharvest factors that allow us to maximize the quality of the vegetables going into storage. These factors encompass production and management decisions concerning soil fertility, variety selection, irrigation, and pest management.
Maintaining good, long-term soil health and quality remains a primary goal of organic production systems. Achieving this goal will ultimately benefit the postharvest quality of vegetables grown on the farm, as the availability of the optimal levels of plant nutrients throughout the growing season will allow for optimal quality of the vegetables throughout the packing and distribution processes. Deficiencies or overabundances of certain plant nutrients can affect positively or negatively a crop’s susceptibility to physiological disorders, disease, and negative composition and textural changes. When optimizing soil fertility to improve postharvest quality, it is important to remember that these may not be the same soil nutrient levels that produce the highest yields.
Nitrogen is an important mineral element that is used by almost all crops. Nitrogen, as a key component of plant proteins, plays an important role in plant growth and development (Ritenour). Because of nitrogen’s involvement in protein synthesis, soil nitrogen deficiencies may lead to lower protein concentrations in vegetables, thereby affecting the nutritional composition of the crop. Adequate soil nitrogen supplies allow for the optimal development of vegetable color, flavor, texture, and nutritional quality.
Excess soil nitrogen can be problematic as well. Research has shown that too much soil nitrogen can reduce the vitamin C content of Green leafy vegetables such as swiss chard (Comis, 1989). Excess nitrogen may lower fruit sugar content and acidity. In certain situations, leafy green plants may accumulate excess soil nitrogen, leading to high concentrations of nitrates in the harvested greens.
Specific examples of excess nitrogen negatively affecting crop quality include (Ritenour):
- Altered celery flavor
- “Brown-checking” of celery
- Weight loss of sweet potato during storage
- Hollow stem in broccoli
- Soft rot in stored tomatoes
Phosphorus and Potassium
Phosphorus and potassium also play very important roles in plant growth and development. Phosphorus is a key component of DNA and plant cell membranes. This element also plays a key role in plant metabolic processes. Potassium is important in plant water balance and enzyme activation. High levels of soil phosphorus have been shown to increase sugar concentrations of fruits and vegetables while decreasing acidity. High levels of soil potassium often have a positive effect on the quality of vegetables. Increased soil potassium concentrations have been shown to increase the vitamin C and titratable acidity concentrations of vegetables and improve vegetable color. Potassium also decreases blotchy ripening of tomato.
Elemental calcium is an important to plant cell walls and membranes (Ritenour). Deficiencies in soil calcium have been associated with a number of postharvest disorders, including blossom end rot of tomato, pepper, and watermelon; brownheart of escarole; blackheart in celery; and tipburn of lettuce, cauliflower and cabbage. High soil calcium concentrations reduce these disorders and are associated with other postharvest benefits, including increased vitamin C content, extended storage life, delayed ripening, increased firmness, and reduced respiration and ethylene production
The texture of the soil on which certain vegetable crops are grown may also affect the postharvest quality. For example, carrots grown on muck soils have been shown to have a greater concentration of terpenoids, a chemical that imparts a bitter flavor, than carrots grown on sandy soil.
Adequate soil moisture during the preharvest period is essential for the maintenance of postharvest quality. Water stress during the growing season can affect the size of the harvested plant organ, and lead to soft or dehydrated fruit that is more prone to damage and decay during storage. On the other hand, vegetables experiencing an excess of water during the growing season can show a dilution of soluble solids and acids, affecting flavor and nutritional quality (Shewfelt and Prussia 1993).
Excess moisture on the harvested vegetable can also increase the incidence of postharvest diseases. To minimize the amount of water on the harvested vegetable brought into storage, it may be beneficial to choose surface or subsurface irrigation rather than overhead irrigation. Vegetables harvested in the early morning, during rainy periods, and from poorly ventilated areas can also experience increased postharvest decay.
Insect pest problems during the growing season can also affect postharvest quality, both obvious and no-so obvious ways. Visible blemishes on the vegetable surface caused by insect feeding can have a negative effect on the appearance of vegetables, thus decreasing their appeal to consumers. Feeding injury on vegetables by insects can lead to surface injury and punctures, creating entry points for decay organisms and increasing the probability of postharvest diseases. In addition, the presence of insect pests on vegetables entering storage leads to the possibility of these insects proliferating in storage and becoming an issue.
Selection of Vegetable Varieties
The selection of the right vegetable variety for your farm and market channel can greatly influence the subsequent postharvest quality (Shewfelt and Prussia 1993). Certain varieties are more suited for the longer-term storage that is essential for marketing to larger wholesale outlets. Other varieties may optimize taste, essential for the post harvest quality of vegetables going to farmers markets or CSA’s.
When planning which vegetable varieties to grow on your farm, it is important to consider which harvest windows are needed. Vegetables harvested at the incorrect stage of maturity will have a significant decrease in postharvest quality. Quality characteristics such as texture, fiber and consistency are greatly affected by stage of maturity at harvest. Fruits and vegetables that are harvested while immature are highly susceptible to shriveling and mechanical damage. Fruits and vegetables harvested at an overripe stage often have poor texture and flavor. Suboptimal harvest dates lead to a greater susceptibility to post-harvest physiological disorders than harvest dates closer to the proper stage of maturity.
Other Production Considerations
Certain production techniques can also help to attain the optimal postharvest quality. These techniques include:
- Staking of tomato crops allows the fruit to remain off the ground during the growing season. By keeping the fruit off the ground, fruit blemishes and decay are minimized. This, in turn, leads to less postharvest decay in storage. Certain staking techniques also may allow more light penetration and air circulation through the canopy, increasing fruit yield and size (Hanson et al. 2001).
- Pruning certain crops (such as tomatoes) can alter the microclimate around the plants in ways that benefit postharvest quality. For instance, removing some of the plant foliage can allow for better air circulation and thus minimize excess moisture around the fruits, leading to less decay and postharvest disease issues.
- Row covers over leafy greens can minimize physical damage to certain vegetables, especially leafy greens. By minimizing physical injury to the plant tissue, fewer entry points for microorganisms are present on the vegetable surface, thus minimizing the potential for postharvest diseases to manifest.
References and Citations
- Comis, D. 1989. Nitrogen overload may shrivel vitamin content. Agricultural Research. July. p. 10-11.
- Hanson, J.T., C.G. Chen, R. Kuo, R. Morris, and R.T. Opeña. 2001. Suggested cultural practices for tomato [Online]. AVRDC – the World Vegetable Center. Available online at: http://www.agrifoodgateway.com/articles/suggested-cultural-practices-tomato (verified 3 March 2010).
- Ritenour, M. Undated. Plant nutrition impacts on vegetable quality [Online]. University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center. Ft. Pierce, FL. Available at: http://postharvest.ifas.ufl.edu/post%20harvest%20info/Pre%20Harvest%20fa... (verified 3 March 2010).
- Shewfelt, R.L., and S.E. Prussia (eds.) 1993. Postharvest handling: a systems approach. Academic Press Inc. San Diego, CA.