HEAT TRANSFER STUDIES DURING COLD PRESERVATION OF FOOD PRODUCTS

Abstract

Food products of perishable nature are needed to be preserved from spoiling using the precooling technique: Precooling is the process of cooling fruits and vegetables as soon as possible after the harvest and prior to transportation over long distance to a cold storage warehouse and marketing. The present work is an attempt to investigate experimentally the heat transfer behavior during precooling of fruits and vegetables in rectangular duct under forced convection. In the experimental study done in RAC lab at (MIED, IITR), on rectangular duct the food commodities investigated are apple and sapodilla. Forced air cooling is achieved by suspending the food inside a 4 m long rectangular air duct of 0.3x0.3 m section of Galvanized Iron sheet which is insulated with 1 cm thick puff sheet. The humidity inside the duct is to be maintained constant. The temperatures of the cold air outside package and of the food product at different locations inside the package are to be measured at regular time intervals. The air is circulated through the duct by means of a blower powered by 1.5 hp electric motor. The air is then made to pass through a coil. This coil is the evaporator coil of 5 ton capacity vapor compression plant. The test programs were performed on two types of fruits, apple and sapodilla. The air velocity, V=2.5m/s was selected to examine the effedts of cool air velocity on the cooling process. The temperature was monitor at three locations (surface, middle and centre) for apple and sapodilla. In the present study, a heat transfer coefficients have been obtained based on the transient temperature measurement techniques. Two cases adopted to determine the surface heat transfer coefficient namely, low Biot number and large Biot number. By product the slop of the dimensionless temperature vs time curve with aid of one dimensional transient heat equation, the Biot number of the cooling process can be predicted which, in turn, the heat transfer coefficient obtained. It has been shown that the dimensionless temperature of apple and sapodilla decreases exponentially with the time. High dropping in the temperature was occurred during the first 30 minutes of cooling which follows a lower rate of reduction in temperature.