Shelf life of plant products is limited because plant products are still alive. After harvest, the quality of products changes, in most cases this means a quality reduction. Exceptions are climacteric fruits such as banana and tomato that can be harvested before they are ripe, or flowers that have not opened yet. During postharvest, continued respiration, transpiration, development (ripening) and other physiological processes. These processes, in combination with a change of the metabolic content, determine postharvest quality (Figure 5.13).
Climacteric fruit have an increased ethylene production and a rise in cellular respiration during ripening e.g.: Apples, bananas, melons, apricots, and tomatoes.
Figure 5.13: Quality of living plant products is determined by physiological processes and metabolic content.
Respiration is the major force of deterioration of plant products. It is the burning of carbohydrates using oxygen and producing carbon dioxide and water. It also produces energy in the form of ATP, and heat. This increase in temperature subsequently increases respiration, potentially increasing the deterioration of the product. Respiration also increases during ripening, wounding, harvest, and senescence, and it causes a reduction of carbohydrates potentially changing the nutritional composition of plant products. It can be slowed down by lowering the temperature and reducing the oxygen concentration and increasing carbon dioxide. Complete removal of oxygen however, and levels of carbon dioxide above 20% should be prevented because that can lead to fermentation and a rapid decline of quality. The production of water from respiration is sometimes visible as condensation if the plant product is packed in a plastic bag, for example in the case of lettuce.
Transpiration is a physical process, driven by the vapour pressure difference of the plant product and its surroundings. The boundary layer of the product also plays an important role. A thick wax layer, or hairs on the fruit such as in peach can prevent transpiration. Transpiration leads to water loss, which will reduce the weight and thus the value of the product. Control of physiological processes and prevention of breakdown of metabolites and phytonutrients in the postharvest chain is essential for the preservation of quality.