GB922493A - Improvements in or relating to freezedrying of foodstuffs - Google Patents

Abstract

922,493. Freeze-drying food. EDWARDS HIGH VACUUM Ltd. April 6, 1961 [April 12, 1960], No. 13090/60. Class 49. [Also in Group XIII] Food material is freeze-dried in a Vacuum chamber by being sandwiched between shelves in the form of a stack of heated plates to which controllable pressure is applied by pneumatically or hydraulically operated flexible bags, e.g. of nylon, rubber, canvas or by metal bellows containing fluid. The plates are preferably thin, electrically heated, metal or resin (e.g. epoxy), sheets of low thermal mass, siliconetreated to prevent adhesion of food material, and, before insertion between the plates, the foodstuff may be pre-frozen, e.g. by vacuum evaporation, or, alternatively, the entire stack of food and plates may be refrigerated e.g. in a blast tunnel cooler. As shown, Fig. 1, food 2 is sandwiched between plates 3, which have heating elements 10 and which are clamped by rods 4, and the entire stack is enclosed in a vacuum chamber 1, pressure being applied to the plates by pneumatic bags 6 into which air is pumped or merely allowed to flow in. Expanded metal sheets 7, Fig. 2, are preferably interposed between food and heating plate on one or both sides of the food to provide points 8 of contact therewith and allow escape of vapour therefrom. Undue pressure at the bottom of the stack is relieved either by interleaved support plates 5 or springs (not shown) of graded strength, or by standing the shelf stack on edge in the vacuum chamber.