Certain products cannot be kept in the liquid form for an appreciable time and must be reduced to dry powders, which can then be kept at chill or ambient temperatures. The water must be removed to make them into powders, but any heating above ambient to boil off the water would lead to rapid deterioration. The water must therefore be removed at low temperature, requiring low pres­sures of the order of 125 Pa.

The process is carried out in a vacuum chamber fitted with refrigerated contact freezing plates, heaters and a vacuum pump. Between the chamber and the pump may be a refrigerated separator to prevent too much of the mois­ture entering the pump. The product is placed in containers on the plates and frozen down to about — 25°C, depending on the product, but sometimes as low as — 50°C. The vacuum and, at the same time, a carefully controlled amount of heat, is then applied, to provide the latent heat of sublimation (ice to vapour) without allowing the temperature to rise. As the water is driven off, the product collapses to a dry powder. This is extremely hygroscopic and must be packed in air-tight containers as quickly as possible on completion of the cycle.

This process was developed for the preservation of antibiotics, but is now in widespread use for other products such as ‘instant’ coffee, tea, soup, etc.

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