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The driving force and the end point of the shrinkage process, during dehydration, were evaluated using poly(acryl amide) and poly(acrylic acid) gels, as a model of food systems. The maximal shrinkage of the gels is practically the extent of their full collapse when water is removed and replaced by air. The drastically diminishing polymer mobility due to transition into its glassy state is one of the two mechanisms that determine the end point of the contraction process. Addition of low molecular weight solutes, such as sugars, lowers the moisture content required for the onset of the glass transition and thus extends the rubbery region at which shrinkage may take place. However, their own volume of the sugars sets a bottom limit for contraction, thus resulting in less shrinkage the higher the sugar content. The second mechanism that stops contraction may come into effect when a rigid filler is present in the system. The percolation of its particulates, while the polymer is in its rubbery state, forms a rigid solid matrix that resists further bulk shrinkage. However, until it reaches its glassy state, the polymeric network may continue its contraction around the filler particulates, thus forming a porous structure. |
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