An Overview of Postharvest Biology and Technology of Fruits and Vegetables

2010 AARDO Workshop on Technology on Reducing Post-harvest Losses and Maintaining Quality of Fruits and Vegetables 2-11 An Overview of Postharvest Biology and Technology of Fruits and Vegetables Chun-Ta Wu Department of Horticulture, National Taiwan University, Taiwan, ROC Abstract Harvested fresh fruits and vegetables are living products. They are characterized by high moisture content, active metabolism, and tender texture; as a consequence, significant losses resulting in senescence, desiccation, physiological disorders, mechanical injuries, and microbial spoilages occur at any point from harvest through utilization. The main objective of postharvest technology is to restrict deterioration of produce along the postharvest chain, and to ensure that maximum quality value for the produce is achieved. Temperature management and dehydration control are the essential and the two most important strategies to extend shelf life and retain quality of horticultural perishables. The other supplements such as controlled atmospheres and modified atmospheres, 1-methylcyclopropene fumigation, and heat treatments can further enhance their storability. Over the past few years, development and application of effective, safe, and environmental-friendly postharvest technology for edible horticultural commodities have become and will continue to be the number one concern by fresh produce handlers and consumers. Introduction Fruits and vegetables are considered as a commercially important and nutritionally essential food commodity due to providing not only the major dietary source of vitamins, sugars, organic acids, and minerals, but also other phytochemicals including dietary fiber and antioxidants with health-beneficial effects. In addition, fruits and vegetables provide variety in color, shape, taste, aroma, and texture to refine sensory pleasure in human’s diet. There is an increasing demand for fresh produce at the consumer level, because of the raising awareness of people about the superior of fresh, natural foods than processed products resulting in the active encouragement by health agencies and public media as well as several medical researches demonstrating various health benefits of fresh produce consumption13. Fruits and vegetables, unfortunately, are highly perishable in nature and may be unacceptable for consumption if not handling properly following harvesting3,4,7,13. Furthermore, fresh horticultural products are important items of international commence after the globalization of trade and free trade agreements. Longer shipments and distribution periods may eventually increase the potential of heavy losses; therefore, the importance of proper cares and techniques for handling fresh produce after harvest has been recognized and emphasized. Postharvest, the connecting link between the grower and the 3 consumer, is concerned with the biology of harvested plant materials and uses this knowledge to develop effective and feasible handling technologies that delay the rate of senescence3,13. The main purposes of applying postharvest technology to harvested fruits and vegetables are to diminish losses between harvest and utilization, to maintain best possible quality (appearance, texture, flavor, and nutritive value), and to ensure food safety 3,4,13. All fresh fruits and vegetables are living tissues. Due to high moisture content, active metabolisms, tender nature, and rich in nutrients, they are vulnerable to dehydration, environmental stresses, mechanical injury, and pathological breakdown and are usually considered to be highly perishable3,4,7,13. These characteristics strongly limit the storage life of fruits and vegetables and cause significant deterioration following harvest. Postharvest losses can occur at any point in the production and marketing chain. It is estimated that the magnitude of these losses due to inadequate postharvest handling, transportation and storage in fresh fruits and vegetables is relatively higher, 20-50%, in developing countries when compared to 5-25% in developed countries4. Kader (2005) commented that complete elimination of postharvest losses may be impossible and uneconomical, but to diminish them by 50% is possible and desirable5. Minimizing postharvest losses of produce that has invested substantial labor, materials and capital to grow is a very effective way to increase food availability without further boosting crop production4,13. To reduce these losses, understanding the causes of deterioration in fruits and vegetables is the fundamental step, and followed by utilizing appropriate and affordable technological procedures to delay senescence and conserve quality of produce. In this paper, the factors involved in deterioration of harvested fruits and vegetables are discussed first, and then some postharvest technologies generally used in the commodities are briefly summarized. Factors Involved in Postharvest Loss of Fruits and Vegetables The actual causes of postharvest loss in fresh fruits and vegetables are many and commodity specific, since horticultural products are diverse in morphological structure, composition, developmental stages, and general physiology13. However, the main causes of produce deterioration are continuous metabolism and growth, water loss, physiological disorders, mechanical damage and pathological breakdown3,13. 1. Continuous Metabolism and Growth Since fruits and vegetables are living biological systems, they are subject to metabolic and developmental changes even after harvesting. It is well established that the quality of the harvested commodities cannot be improved further but it can be retained till their consumption if the rate of metabolic activities are reduced by adopting the appropriate postharvest handling operations. One of the important parameters determining the metabolic activity of a horticultural product is its respiration rate, which is usually associated with the commodity deterioration. Respiration, involving enzymatic oxidation of organic substrates with energy production resulting in O2 4 consumption and CO2 and water production, represents sum of total of all the metabolic activities of the tissue3,5,7,13. Respiratory rate of produce after harvest is reversely proportional to its storage life, i.e., the higher the rate of respiration, the shorter is the storability, because the produce is detached from its source of photosynthates and is entirely dependent on its own food reserves5,7. Respiration rate of a produce is dependent on a wide range of variables, including commodity and environmental factors3,5,7,13. Among the external factors affecting respiratory rate of fresh fruits and vegetables after harvest, temperature is considered the most important in modulating this 3,5,7,13 physiological parameter . Temperature quotient (Q10), which is the ration of the rate of a reaction at one temperature (T1) versus the rate at that temperature plus 10℃ [(rate at T1+10℃)/rate at T1], is the measure usually quoted for respiration to give a general estimation of the effect of temperature on the overall metabolic rate of produce5,7,13. Within the 5℃ to 25℃ range, the velocity of respiration increase 2 to 2.5 folds for every 10℃ rise in temperature for most harvested produces, i.e., Q10 = 2.0-2.5. The gas composition, such as O2, CO2, or ethylene, surrounding the horticultural produce, also exerts a great impact on both its respiratory and general metabolic rate. The postahrvest technologists are mainly concerned with slowing down the rate of respiration for maintaining quality and maximizing storage life. Ethylene, the simplest olefin, is a gaseous phytohormone that can significantly elicit the respiration and senescence processes of a number of harvested fruits and vegetables in trace amounts (less than 0.1 μL L-1) 3,7,8,11,13. In higher plants, ethylene is produced from methionine via three enzymatic reactions: (1) methionine is converted to S-adenosyl-L-methionine (S-AdoMet) by S-AdoMet synthetase; (2) the conversion of S-AdoMet to 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, is the rate-limiting step catalyzed by ACC synthase (ACS) in this pathway; (3) ACC oxidase (ACO) degrades ACC to release ethylene8,11,13. The major physiological responses regulated by ethylene in harvested horticultural products include chlorophyll degradation, fruit ripening, senescence, and abscission of plant organ3,8,11,13. Since exposure to ethylene can be detrimental to quality of most fresh horticultural commodities, ethylene is of major concern to all produce handlers during postharvest period. Ripening is a developmental phase spanning from the last stage of maturation through the earliest stage of senescence in fleshy fruit and is commonly observed in many fruit products after harvest1,,3,7,11,13. It is widely accepted that fruit ripening is a senescence process, due to a breaking down of the cellular integrity of the tissue1,7,13. Several biochemical and physiological events involving in change of color, firmness, flavor, and aroma take place in this transitional period, which results in the transformation of unripe fruit into an edible ripe product1,3,8,11,13. Based on whether or not they produce a peak in respiration, the ripening behavior of fruits has been categorized as being either climacteric or nonclimacteric1,3,7,13. Climacteric fruit, such as apple, banana, and tomato, is defined by the dramatic increase of respiration rate and ethylene release during ripening. Nonclimacteric fruit, such as citrus, grape, and strawberry,

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