CHAPTER 7
DRYING (cont'd)

MOISTURE LOSS IN FREEZERS AND CHILLERS

When a moist surface is cooled by an air flow, and if the air is unsaturated, water will evaporate from the surface to the air. This contributes to the heat transfer, but a more important effect is to decrease the weight of the foodstuff by the amount of the water removed.
The loss in weight may have serious economic consequences, since food is most often sold by weight, and also in many foodstuffs the moisture loss may result in a less attractive surface appearance.

To give some idea of the quantities involved, meat on cooling from animal body temperature to air temperature loses about 2% of its weight, on freezing it may lose a further 1% and thereafter if held in a freezer store it loses weight at a rate of about 0.25% per month. After a time, this steady rate of loss in store falls off; but over the course of a year the total store loss may easily be of the order of 2-2.5%.
A further consequence is deposition of frost and ice reducing heat transfer on the cooling evaporator surfaces.

                            q = UA DT                                                                                                            

For fixed q (determined by the cooling demand) and for fixed U (determined by the design of the freezer) a larger A will mean a smaller DT, and vice versa. Since the air leaving the coils will be nearly saturated with water vapour as it leaves, the larger the DT the colder the air at this point, and the dryer it becomes. The dryer it becomes (the lower the RH) the greater its capacity for absorbing water from the product. So a low DT decreases the drying effect. The water then condenses from the air, freezes to ice on the coils and must be removed, from time to time, by defrosting. Similarly for fixed U and A, a larger q means a larger DT, and therefore better insulation leading to a lower q will decrease weight losses.

Drying > SUMMARY, PROBLEMS

Back to the top