Air is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide and assorted other gases. Oxygen is consumed and carbon dioxide is produced during respiration in air. The diffusion of gases into and out of plant tissue is controlled by the difference in their concentration across a barrier, the thickness of the barrier and the permeability of the barrier; this relationship is summarized in Fick's law of diffusion. As fruit become larger, the surface area through which gases diffuse into and out of the fruit increases slower than the volume of tissue that is dependent upon this exchange of gases. For example, doubling the size of a spherical commodity (e.g. a tomato) will increase its surface area four-fold, while its volume will increase eight-fold. Tissue in the center of large fruit is therefore in danger of being unable to obtain sufficient oxygen or eliminate sufficient carbon dioxide to maintain normal aerobic respiration. This problem can be compounded by enhanced respiration induced by physical injury, ripening or exposure to ethylene, by surface waxing, by packaging, or by the intentional or unintentional modification of the composition of the external atmosphere.
Controlled atmospheres (CA) and modified atmospheres (MA) are used to extend the storage life in a number of crops. The concentration of oxygen is lowered and the concentration of carbon dioxide is elevated in both techniques; they just differ in the level of control exerted over the concentration of the gases. In CA, the concentration of the gases is periodically measured and intentionally adjusted, while in MA the concentration of gases is allowed to maintain itself through judicious selection of temperature, rates of gas diffusion and product respiration. While CA is usually employed in large, commercial-size storage rooms, MA can be scaled down to individual consumer packages in the form of a modified atmosphere package (MAP). The commercial success of the MAP remains limited because the control of temperature (a major factor governing respiration and therefore the atmospheric composition in the package) is often unreliable during marketing. The added expense of the package and the variability of the respiratory behavior of the product also contribute to its limited use. However, the MAP can be an excellent technology when the variables are properly controlled, as in bags of fresh-cut salad mixes.
160 Handbook of waste management and co-product recovery 7.4 Changes in composition
Harvested plant commodities are composed of water, carbohydrates, proteins, lipids, vitamins, minerals and molecules that contribute to flavor and aroma. Changes inevitably take place in the proportion of these constituents after harvest and thereby affect the quality and utility of the stored product. Water not only composes over 80% of most commodities, which contributes to the bulk of the tissue, but its relative distribution within the tissue can cause changes in turgor that alter both texture and enzyme activity. Loss of water therefore can alter not only the size and appearance of the commodity (i.e. weight loss, skin shrivel), but also its texture (i.e. crisp versus flaccid) and its physiological activity.
Water has two avenues of escape from the tissue. One is as water vapor, much like the diffusion of other gases, while the other is as liquid water that migrates through the tissue to the surface of the commodity where it evaporates. Water loss is a combination of both processes and is proportional to differences in the vapor pressure of water between the commodity and the storage environment. Air in void spaces within tissue and near its surface has a relative humidity close to 100% and its vapor pressure is fundamentally determined by its temperature. The vapor pressure of water in air is dependent on the air's temperature, but also on its relative humidity.
The fundamental manner by which storage rooms are cooled necessitates a lowering of the air's relative humidity. Heat is removed by passing warm storage-room air that has a high relative humidity over cold evaporator coils in a mechanical refrigeration system. Condensation on the coils removes water vapor from the air and makes it very difficult to maintain a relative humidity of greater than 80-90% in storage rooms. The evaporator coils are maintained at a low temperature to facilitate the required rate of heat transfer which is proportional to the difference in temperature. Water condenses onto the cold evaporator coils so that the cold air exiting the evaporator has a lower water content than when it entered as warmer air. The relative humidity of the cold air drops as it warms during its absorption of heat from the stored produce. This warmer, dryer air can now absorb water from the produce which it conveys to the cold evaporator coils. The need to have a large temperature differential to remove enough heat to maintain the cold storage temperature often results in the coils being maintained at below freezing temperatures. Water condenses and freezes on the coils and the layer of ice greatly reduces the effectiveness of the coils, so that they must be periodically defrosted.
Water loss could be reduced by using larger evaporators with smaller temperature differentials, but evaporators are expensive. The use of packaging that acts as a barrier to water loss, or the injection of water vapor into the storage environment are two ways to overcome this inherent limitation of mechanical refrigeration. However, packaging that restricts water loss may also reduce the penetration of cold air into the package and thereby interfere with maintaining the proper storage temperature.
Carbohydrates are constantly being metabolized by the commodity to supply energy (i.e. respiration) and smaller molecules for the synthesis of other macromolecules (e.g. proteins, lipids, hormones, etc.). Most commodities have sufficient carbohydrate reserves that their loss from metabolism during short-term storage does not significantly alter the composition. However, their conversion from complex molecules (e.g. starch, hemicel-lulose) to simpler molecules (i.e. mono- and disaccharides) can significantly alter the composition of the commodity. Arresting this enzymatic conversion with low temperatures, enzyme deactivation (e.g. blanching) or genetic engineering maintains quality. For example, all three approaches have been used to maintain the quality of sweet corn (preventing the conversion of sugars to starch) and potatoes (preventing the conversion of starch to sugars).
Commodities low in storage carbohydrates (e.g. melons, leafy greens, cut-flowers) can exhibit a significant reduction in quality and shelf-life as their limited reserves are depleted by respiration. The carbohydrate reserves of cut-flowers can be replenished by immersing their stems in sugar solutions. Such remedial action is unavailable for harvested fruits and vegetables, so their quality at harvest must be the highest possible, and their storage environment must be optimal to preserve their limited storage reserves.
While vegetative plant tissues (e.g. leafy greens) are usually low in both proteins and lipids, many reproductive (e.g. fruit and seeds) or storage (e.g. roots and tubers) organs can be excellent sources of these two nutrients. The nutritional quality of plant protein is not only determined by the amount of protein in the tissue, but also by the amino acids that compose the proteins. Proteins are made of 20 amino acids linked together to form long linear arrangements of hundreds of individual amino acids. Humans cannot synthesize all the amino acids we need to make functional proteins, so we must eat proteins that contain those essential amino acids. A 'balanced' diet therefore contains protein in both sufficient quantity and quality. Detrimental changes in protein content are minor, and usually entail the loss of activity of proteins functioning as enzymes. In contrast, there are many detrimental changes than can occur with lipids.
Lipids serve as food reserves in seeds (e.g. walnuts) and fruit (e.g. avocados), and are the major component of membranes (i.e. phospholipids) in all tissues. Their susceptibility to oxidation is dependent on their level of saturation. A fully saturated lipid has only single bonds between its linear backbone of carbon atoms, and is resistant to oxidation. In contrast, the more easily oxidized unsaturated lipids can have one, two or three double bonds between the carbon atoms. These unsaturated lipids are considered to be more healthy for humans than saturated lipids, but are more easily degraded by oxidation during storage and marketing. Oxidation products of lipids give the tissue a rancid taste and aroma.
Plant commodities are significant sources of vitamins A and C in the human diet. These vitamins are susceptible to respiratory loss by the plant after harvest. Their rate of synthesis is low in harvested tissue, and there are few reports that they can actually increase during storage. Postharvest techniques used to control other quality changes are equally capable of reducing the loss of these vitamins.
The mineral composition of tissues does not significantly change after harvest. However, the availability may change dramatically as other components (e.g. phenols) bind the minerals into inaccessible complexes.
The quality of many fruits and vegetables is closely linked to their aroma and flavor. Many of these compounds are volatile and therefore require continued synthesis to maintain the characteristic aroma and flavor of the commodities. Improper handling and storage (e.g. immaturity at harvest, excessive water loss, chilling temperatures) can reduce the capacity of the tissue to synthesize these compounds. Quality can also be lost by the synthesis of unwanted volatile compounds. For example, the oxidation of lipids produces rancid odors, while anaerobic respiration produces alcoholic aromas.
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