HEAT AND MASS TRANSFER ANALYSIS IN COLD PRESERVATION OF FOOD

Abstract

The designers of freezers and cold storages in India borrow food property data from literature, which gives these data for the products harvested in foreign countries, particularly Europe and America. Since the growing conditions are quite different in India, it is important that the thermal properties and storage characteristics of fruits and vegetables of Indian ori-gin be determined experimentally and thermal behaviour during cooling process be known. In the present investigations experimental measure-ments have been made for the specific heat, density, water content and thermal diffusivity of apples, oranges, peas and potatoes during precooling and those for peas during freezing as well. The specific heats of the four products, in unfrozen state, were measured by calorimetric method, the density was determined by measuring mass and volume of the product sample and the water content was measured gravimetrically through infrared drying of the food commo-dity. Each property was measured for six to eight samples of every commodity and the arithmetic mean value was calculated. Statistical analysis was performed in each case to determine the standard deviation and coefficient of variation, which were found to be very satisfactory. A method was developed to calculate the thermal diffusivity, thermal conductivity and surface film conduc-tance, through the transient heat transfer analysis by the temperature measurements at three locations in the food flesh and at the surface. For these measurements, apples, oranges, unpeeled and peeled off potatoes and slab, cylin-drical and spherical shaped pea packages in copper con-tainers. were employed In precooling experiments, involving time-temperature measurements, food samples were cooled in a 4 m long gal-vanised iron air-duct of 0:3 m x 0.25 m section, insulated with 15 mm thick glass wool. Air was cooled by passing it over the cooling coils of a R-12 refrigeration system and its velocity in the test duct was maintained constant at 6.6 m/s. The temperature inside the test duct could be maintained constant between 0.5°C and 6°C and the relative humidity of the circulating air was maintained constant at about 80 X. When desired state of circulating air was reached„,the food samples were suspended in the air stream. Transient cooling temperatures were measured by 30 SWG copper constantan thermocouples, imbedded at five points in the food flesh, through a data logger, at equal time intervals and at a scan rate of 40 per second. Measurements were made on 12 apples with radius 2.7 to 3.5 cm, 13 organges with radius 2.6 to 3.2 cm, 13 potatoes with radius 2 to 2.8 cm, 14 peeled off spherical potatoes with radius 1.75 cm each, 10 slab shaped pea packages with half thick-ness 1.6, 1.8 and 2.35 cm, 13 cylindrical pea packages with 1.8 and 2.52 cm radius and 12 spherical pea packa-ges with 3.0 and 5.7 cm radius.