The accelerated freeze-drying (AFD) method of food preservation / a report on some work of the Research Establishment and Experimental Factory of the Ministry of Agriculture, Fisheries and Food, Aberdeen, Scotland, between 1955 and 1960.

  • Great Britain. Ministry of Agriculture, Fisheries and Food.
Date:
1961
Catalogue details

Licence: Public Domain Mark

Credit: The accelerated freeze-drying (AFD) method of food preservation / a report on some work of the Research Establishment and Experimental Factory of the Ministry of Agriculture, Fisheries and Food, Aberdeen, Scotland, between 1955 and 1960. Source: Wellcome Collection.

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    may not be suitable. Vacuum packing of rough irregular material places too | great a strain on the package and contents to be of much practical value. Vacuum packing has been used to some extent with powdered or small grain materials in which the particles are pressed together by the atmosphere to give a rigid mass of relatively high density. Such a package lacks mechanical strength and so is liable to damage. Where vacuum packing is required it is important to have the pack fit the contents closely before evacuation, otherwise the excess packaging material suffers sharp and haphazard folding and is liable to damage. When rectangular blocks are to be packed in this way the problem has been overcome by neatly folding and shaping the pouch over a wooden form before inserting the contents as described below. Considerable experience of this form of packaging was gained at the Experimental Factory, with 24 0z meat bars, which are typical compressed products, ideally suited to vacuum packing in flexible containers. The blocks, measuring 5-6 x 4°6 x 2-5 cm, were vacuum packed in pouches of a laminate of -oo15 in. Pliofilm + -018 mm aluminium foil + -oor in. Melinex polyester. The laminate was converted to sachet-type pouches, + in. fin-sealed on three sides and open on the fourth. The internal dimensions of the pouches were I10 x 90 mm. Since it was important to have a closely fitting pack the pouch was formed on a mandrel of cross-section 5-6 2-5 cm. It was then easy to insert the meat bar into the pouch and fold the open end so that it was again smooth and flat, and ready for sealing. It was essential that the sealed area should be flat and smooth since the presence of wrinkles resulted in subsequent failure of the seal. The vacuum sealer was specially constructed but was of a type which is readily available commercially. It consisted of a tilted rectangular metal cabinet hinged at the rear base edge so that the whole top and sides could be raised to expose the flat base, on which the packs were placed. A vacuum line was fitted to this cabinet and provision made for packing in either nitrogen or air (see Fig. 5). The jaws were located at the rear of the cabinet and were of the permanently heated type. The top jaw was fixed, and the lower one was raised for sealing by a piston operated by the difference in pressure be- tween cabinet and atmosphere. An electronic timer and solenoid valve con- trolled the sealing operation so that the evacuation cycle could be varied in length as required. The packed meat bars were laid four at a time on the platform with the flattened mouths of the pouches protruding between the sealing jaws. The lid with its rubber gasket was closed, and the cabinet exhausted to a pressure of approximately 1-o mm Hg. At this level the jaws were closed for a period of approximately 3 seconds and the pouches sealed. The cabinet was now opened and the pouches removed from between the open jaws. The top of the pouch was folded to produce a neat rectangular block-shaped pack. Testing. The pressure within a vacuum-packed container was measured by a simple non-destructive test. The package was placed in a small decom- pression chamber fitted with a heavy glass window and a vacuum gauge. The pressure was reduced and the point noted when the walls of the package just began to lift away from the contents. At this point the pressure in the chamber equalled that within the pack and could be read from the gauge.