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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    APPLIED PLATE PRESSURE It was soon obvious that leafy vegetables, fruit and meat minces could not withstand a high plate pressure, either because the resulting compaction of the bed caused excessive stifling of the vapour flow, or because the structure of the tissues was too weak mechanically. Furthermore, since it was advisable to increase the pressure gradually from zero, it followed that it was necessary to establish only the maximum useful pressures for the range of foods considered. It seemed possible that this feature would require to be studied in con- junction with the temperature of the heating plates, and a series of experiments was carried out using pre-frozen cod steaks. The technique evolved was to raise the heating water temperature to its maximum as soon as possible, maintaining the plate pressure at a low value, since if this were increased too rapidly some thawing always occurred on the surface of the product. The maximum plate pressure was thus not applied until after about 2 hr, and the effect was to drive the points of the expanded metal sheet into the surface of the dry layer, so reducing the heat path from the metal mesh to the internal ice front, and further increasing the heat flux by increasing the contact area. Expanded metal was obtainable in a fairly large range of mesh formations and dimensions, but in order to achieve significant penetration of the dry tissue with a practical applied plate pressure, it was necessary to select those forms which had points on one side of the diamond configuration. The other side should preferably have been flat to give good contact with the metal tray and lid but this form was not commercially available and the cost of producing small test pieces for experimental purposes was prohibitive. The dimension of the aperture in the mesh was important, since it had to be small enough to ensure a high proportion of contact area with the foodstuff and yet large enough to permit easy escape of the water vapour. In addition the overall thickness of the mesh, as measured between two flat sheets placed on either side, had to be adequate to allow easy escape of the evolved water vapour to the edge of the heating plate. The metal used to form the mesh did not appear to affect the rate of drying. Both stainless steel and aluminium were tried, and although these materials have very different thermal conductivities, the main barrier to heat transmission was the dry layer of tissues on the product surface, and both metals were equally effective. A number of different mesh forms were tested, and eventually it was decided to standardize on an aperture size $ in. x i in. witha strand 7g in. wide formed from 20 G. aluminium sheet, and having an overall thickness of about 2-5 mm. Aluminium was chosen because it was only one-third the price of stainless steel, it was readily available and, although it was more easily damaged by careless handling, it could be obtained in perfectly flat sheets whereas the samples of stainless steel which were tried were in most cases badly buckled. Using this mesh it was then shown to be advantageous to employ a final maximum applied plate pressure on pre-frozen blocks of some materials, e.g., meat and fish, in the region of 8 lb/in.@) This value gave a penetration into the dry layer of about 1 mm on both top and bottom surfaces, giving a significant reduction in heat path to the ice front. As the expanded metal was driven into the product the available cross-sectional area, through which water vapour could escape to the edge of the tray, was progressively reduced, and it is probable that the use of higher plate pressures with this particular mesh would have resulted in stifling of the vapour flow again.