PROCESSING APPROACHES FOR LIMITED FREEZE DRYING

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Abstract
Methods are presented whereby “limited” freeze drying can be carried out to leave behind a predetermined and uniform moisture content (e.g., in the range 8–14%), so as to make the product suitable for compression. Experiments with mist‐rewetting of fully freeze‐dried products show a relatively uneven moisture content and, for beef, a tendency for browning. Desorption equilibrium data are presented for beef and turkey, including moisture contents and temperatures of interest for limited freeze drying. Two approaches for implementing limited freeze drying commercially were tested and evaluated. One involves modification and control of operating conditions of ordinary freeze dryers, while the other involves use of a hydrating salt as a humidity‐regulating water‐uptake medium in a circulating‐gas apparatus. Both methods give a quite satisfactory product in small‐scale tests. The hydrating‐salt process should provide a simpler and more self‐regulating situation for quality control of final moisture content, as well as giving more rapid drying rates.