In some instances weight losses can be excessive; for example, when blanching mushrooms, weight losses in excess of 19% and volume losses between 11 and 15% have been recorded (23). It has been shown that weight loss from vegetable tissue during water blanching occurs by two main mechanisms. In the typical temperature range of 50 to 55°C the cytoplasmic membranes that enclose the cell contents become disorganized, and on loss of turgor the cells contract and express some cell solution. Simultaneously, the damaged cell membranes allow free diffusion of solutes out of the cells. Because of continued diffusion of solutes out of the tissue during the blanch time, the net tissue weight loss also increases. The example of whole peas blanched in the laboratory at 85°C is shown in Figure 3. The kinetics of mushroom shrinkage have been described by three apparently first-order reactions (24). Artificially altering the water content of the vegetable prior to blanching will also influence subsequent weight loss, although the effect on the diffusional loss of solutes is small (25).
The blanching of dried vegetables has been studied since it was found that the standard rehydration step of around 18 h in cold water could be avoided by an extended blanch of 10-70 min at 71-85°C, depending on the vegetable. During this time sufficient water was taken up to compensate for lack of soaking, with resulting improved overall quality (26).
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