04 05 Transpiration - Copy - PLS 172 1 of 18 Water and...

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PLS 172 1 of 18 Water and Water loss in Postharvest Biology Michael S. Reid and Mikal E. Saltveit, Dept. Plant Sciences, UC Davis Table of Contents I. Introduction II. Effects of water loss A. Physical effects B. Physiological effects C. Economic effects III Physics of water loss from perishables A. Properties of water B. Psychrometrics: behavior of water in air C. Transpiration: water diffusion from tissue D. Diffusive resistance IV Measurement of transpiration rate A. Weight loss B. Direct measurement C. Diffusion porometer V Factors affecting water loss A. Commodity factors 1. Surface to volume ratio 2. Routes of water loss 3. Anatomy of the evaporating surface. 4. Physiological state of the commodity 5. Cultivar 6. Cultural conditions B. Environmental factors 1. Humidity 2. Air velocity 3. Temperature 4. Atmospheric pressure VI. Methods to reduce water loss A. Handling techniques B. Proper refrigeration design C. Packing D. Waxing E. Film wraps F. Curing VI. References I. INTRODUCTION Perishable products are mostly water. Typically, fresh fruits, vegetables, and ornamentals are 90 - 95% water. One could describe perishable crops as nutritious, attractive or interestingly packaged water. A comparison of the water content of milk and watermelon illustrates this point. On a weight basis, milk has more solids in it than watermelon. Because water is by far the most important com- ponent of the products with which we are con- cerned, a study of the ways in which water loss occurs from perishable crops, factors affecting its loss, and the technology available to prevent, or at least alleviate it, is an important part of our dis- cussion of the postharvest physiology of fresh horticultural crops. II. EFFECTS OF WATER LOSS Loss of water can have a wide range of effects on perishable crops (Table l). These range from physical (e.g., wilting) and economic (e.g., weight loss) effects, to subtle visual and physiological (e.g., softening, senescence) effects that may oc- cur at quite low levels of water loss. A. Physical Effects One of the most important functions of water in plants is the maintenance of form. Plant organs can be likened to assemblages of tiny balloons (cells) inflated with water under pressure. In addi- tion to ‘filling out’ the organ, turgor pressure is responsible, among other features, for texture and gloss of the organ. Reduction in turgor pressure as a result of as little as 2% water loss causes wilting of leafy vegetables, flaccidity of other vegetables, and shriveling and wrinkling of fruits. Water loss can also be indirectly responsible for deterioration during the postharvest period. In many perish- ables, packages are ‘bulge’ packed so that the product is tightly constrained even after the inevi- table loss of some water. Insufficient over-filling of the cartons or boxes can allow the product to
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