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Ale Efecto de la condensación de vapor sobre la velocidad de calentamiento de los productos refrigerados expuestos a la atmósfera húmeda: aplicación a la predicción de vida útil de leche fluida
ABSTRACT
The paper deals with the contribution of vapor condensation and a temperature gradient to warming up of a refrigerated food product exposed to a warm and humid calm atmosphere. Heat and masstransfer coefficients were found to follow the common natural convection pattern. At night relative humidity, heat dissipation through the condensing vapor may equal heat transfer by the temperature gradient. The experimental transfer coefficients were used in the computation of the time temperature simulation and milk deterioration (acidity type) kinetics, with a view to predicting the shelf life ofmilk exposed to variable conditions of temperature and humidity.
INTRODUCTION
THE SHELF LIFE of refrigerated perishable food products is highly dependent on the time-temperature relationship during their handing and storange. For example, a poor cold chain at the manufacturing, retailing or consumer level results in exposure of the product to a warm and humid atmosphere. In turn, warming up ofrefrigerated food products exposed to humid and warm atmospherenis accelerated by the latent heat of the water vapors condensing upon the product’s cold surface. It was found that the shelf life of a perishable product may be reliably predicted by incorporation of time temperature data in the analysis of its deterioration kinetics, In the case under consideration, the heat and mass transfercoefficients should be evaluated. Of special importance are the natural convection coefficients which are aperative an environment at rest.
In simultaneous heat and mass transfer by natural convection, the rate coefficients are commonly expressed in the following demensionless formm
The {r and Sc values represent the properties af the surrounfing medium and the two Gr values, buoyancy effects dueto the density defferences caused by the temperature and vapor concentration gradients, respectively
There are no experimental data regarding the heating accompanied with watwer vapors condensing on a cold body (typical of refrigerated food products exposed to humid and warm atmosphere), thus the present study was underlaken with a view to:
a) Determining the heat and mass transfercoefficients for cold vertical bodies under the conditionsnin question: and
b) Use os the above rate coefficients, in conjunction with the deterioration kinetics, for predicting the shelf life under variable condition of temperature and humidity (model refrigerated fluid milk).
EXPERIMENTAL
Experimental set-up
Experiments were carried out in a walk-in chamber (244x122x244 cm) constructed of 20 mmthick plywood panels and, provided with an acces (15). The walls were insulated on the outside with 50 mm foamed polystyrene plates and on the inside with 10 mil polythylene sheets at water vapor barrier.
Heating was provided by 250w element (9)controlled by a contact thermometer (6) via a relay(8). Humidity was controlled by generating know small quantities of water. The relative humidity wasdetermined with a set of dry and wet bilb thermometers (7) (10) ventilated by a fan (12). After 12 hr equilibration at the prescribed temperature and humidity, the heating and evaporation units were turned off.
Transfer coefficients were determined on waterb filled glass cylinders (7cm diam and 10, 15 and 25 cmheight, 2 mm walls), with top and bottom bases insulated by 5 cm thick polystyrenediscs. The cylinders (16) were introduced through the hatch, one at a time {by an operator wearning rubber gloves (2)}, and mounted on a frame (13) provided with a polysturene-foam tray (4) for collecting condensation drops and suspended under a semianalyticla balance (5) (Sartorius model 2257, accuracy 0.005 g).
The increase in weight due to condensation, and the change of water temperature,...
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