Patent GB-1032857-A (GB)
Title: Improved method and apparatus for producing readily reconstitutable food products byfreeze drying
IPC: F26B5/06
Filed: 1963-04-09
Abstract: 1,032,857. Freeze drying. CRYO-MAID Inc. April 9, 1963 [April 9, 1962; Aug. 29, 1962; March 6, 1963], No. 14038/63. Heading F4G. [Also in Division A2] In a method for freeze drying water-laden, heat-sensitive material, the material is quick frozen to a temperature below that at which thawing occurs in any part thereof and the frozen material is then agitated in an evacuated air-free system in proximity to and in unrestricted relationship to a cryoplate condenser, while radiant energy is supplied to the material to cause sublimation of the ice crystals contained therein to water vapour which condenses on the cryoplate condenser, the conditions of operation being such that the conductance of the system is greater than the rate at which water vapour is formed by the sublimation of the ice crystals, the pressure is maintained sufficiently low to prevent thawing in any part of the material and the material is agitated in a manner such that it presents changing surfaces for the absorption of the radiant energy. Materials such as coffee extract, tea extract, blueberries, tomato paste, tomato soup, pea soup, orange or grapefruit juice, chicken broth, shrimps and chicken pieces, after pasteurization (if necessary), are frozen and ground whilst frozen to a particle size of 4-16 mesh (preferred) and the ground particles are then passed through an air lock into a freeze drying chamber 25, Fig. 2, through which they are passed down a series of horizontal or inclined vibrating trays 21, the freeze dried product being discharged to atmosphere through a further air lock 32. The trays are heated by radiant energy of which 50%, preferably at least 80%, has a wavelength longer than 2.5 microns; radiation of the required wavelength may be produced by flat stainless steel jackets 24 heated by a number of quartz-sheathed infra-red heaters such that the amount of radiant energy supplied to the material diminishes progressively as the water content of the material decreases. Water vapour evolved from the particles condenses on the cryoplate condensers 20 maintained at below - 50‹ F., preferably below - 90‹ F., by the use of dry ice, mechanical refrigeration or liquid cryogens. Ice formed on the cryoplates is periodically removed from each cryoplate in turn, as by passing hot material through the cryoplate, by heating or flexing the surface of the cryoplate itself, or by actual mechanical scraping; the ice is broken in a breaker 27a and removed through air locks 27b. Cooling conduits or coils 29 are embedded within or attached to the underside of the trays 21 so that the frozen particles will not thaw and agglomerate or stick to the tray surface. A pressure below about 150 microns of mercury, usually 7-45 microns, is maintained in the chamber. Conductive heating may supplement the radiant heating. The vibration of the trays may be stopped occasionally for periods of not longer than about 30 to 60 seconds. One example is directed to the production of a freeze dried readily reconstitutable coffee powder having a moisture content of less than about 3% by weight and a surface area of at least about 7m<SP>2</SP>/g. In certain examples the amounts of radiant heat supplied, expressed in terms of temperature in degrees Fahrenheit, in 12 consecutive zones were as follows: 600‹ (zones 1 to 3); 300‹ (zone 4); 500‹ (zones 5 and 6); and 450‹ (zones 7 to 12).