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Model for transpiration rate at water vapour saturated conditions

Pramod Mahajan, Guido Rux, Oluwafemi Caleb, Manfred Linke, Werner Herppich, Martin GeyerDepartment of Horticultural Engineering, Leibniz Institute for Agricultural Engineering, Potsdam Bornim, Germany

Water vapour saturation is commonly observed in the packaged fresh produce. The reason is moisture evolved from the product and insufficient water vapour permeability of packaging material. Under such conditions, even minor temperature fluctuations result in condensation inside the package causing sliminess, decay, enhancement of microbial growth, and browning of produce surface. This study investigated the moisture loss behaviour of mushroom, strawberry and tomato under 100% relative humidity saturated storage environments. Mass and surface temperature of each product was continuously measured using electronic balance and infrared thermometer connected to the data logger. A perforated plastic sphere filled with water storing polyacrylamide granulates was used as dummy evaporation sphere. Despite water vapour saturation, mushrooms, strawberries and tomatoes transpired at rates of 712, 122 and 18 mg of H2O kg-1 h-1, respectively. Respiratory heat production led to produce surfaces temperatures higher than that of the surrounding air. In turn, this resulted in an outward directed water vapour gradient. This was confirmed with no mass loss from the evaporation sphere. Interestingly, when mushroom was exposed to lower humidity (65%), there was rapid increase in mass loss. Consequently, evaporative cooling significantly reduced surface temperature below the surrounding air temperature. A generalised mathematical model was developed to predict mass loss as a function of temperature and relative humidity of storage environment and respiration rate of fresh produce. Application of such model to design modified humidity packaging was demonstrated for the selected fresh produces.

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