Plants are obviously alive since we can see that they grow and develop over time. What may not be as obvious is that harvested plant tissues remain alive and continue to respire, develop and respond to their environment long after they have been detached from the parent plant. Many physiological changes alter the quality of trimmings and other food wastes; often at an accelerated rate due to injuries imposed during and after harvesting. Postharvest treatments are applied to control the rate of desirable changes while reducing the rate of deleterious changes, and to thereby maintain quality and market life of the commodity. These postharvest treatments range from the control of temperature and humidity, to chemical and gaseous treatments, to control of atmospheric composition, to cleaning and sanitation, and to packaging for transport and marketing.
As with any complex chain of sequential events, each step from harvest to consumption can have a significant impact on the product's final quality. Products diverted from the usual marketing chain because of defects or as waste from processing are typically of low economic value and preservation of their quality must be with cost-effective treatments. The management of waste and co-product recovery is becoming increasingly important as they are developed into sources of raw materials for new products.
All living tissues respire, and the respired substrates (e.g. sugars, organic acids) are often major components that determine product quality. Respiration involves the controlled transformation and ultimate oxidation of complex organic molecules. Sugars derived from complex carbohydrates (e.g. starch) and oxygen from air are the usually respired substrates, and the products are carbon dioxide, water and energy. While most of the energy is released as heat, a portion is captured in the form of high-energy bonds in molecules such as adenosine triphosphate (ATP). A constant supply of this biochemical energy is needed to counter the second law of thermodynamics and to maintain the complex organization and structure characteristic of living things.
The rate of respiration (e.g. consumption of oxygen and production of carbon dioxide) is of major concern to scientists studying the storage of harvested horticultural commodities (e.g. fruits, ornamentals and vegetables) because the rate of respiration is tightly coupled to the rate of many other metabolic reactions. Most endothermic enzymatic reactions that convert substrates into products utilize energy transferred from ATP. Therefore, the availability of ATP will govern the rate of these reactions. Many of these reactions are involved in some aspect of quality retention or loss after harvest. They include the hydrolysis of starch to sugars, the polymerization of sugars to polysaccharides and the synthesis of organic acids, precursors of lignin, pigments, and flavor and aroma compounds.
Other biochemical reactions that are not part of normal metabolism can be induced by the traumas of harvest or by the imposed storage technology. Plants have evolved a limited set of responses to biotic and abiotic stresses. These stresses can include diseases, feeding of insects, drought, high temperatures and physical injuries encountered during harvest and processing. The physical forces used during harvest to remove the plant or plant part from its growing environment, and the forces used during processing to segment or abrade the product, are by necessity sufficient to cause physical injury. The many steps between harvest and consumption offer abundant opportunities for additional injuries that often induce unwanted physical and physiological changes. Foremost among these changes are elevated rates of respiration and ethylene production, and the production and accumulation of phenolic compounds that participate in tissue browning and toughening.
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