粉末狀食品在低水分含量狀態(tài)下的結構轉變

ABSTRACT

Caking of free-flowing powders during storage is a deleterious phenomenon ubiquitous in the feed, fertilizer, detergent and pharmaceutical industries, and of economical importance to low moisture foods. Among other subjects related to caking phenomena, the following aspects are reviewed in this paper: i）morphological changes occurring during caking; ii） proposed mechanisms of caking; iii） quantitative procedures to assess caking; iv） factors affecting caking kinetics; and v） mode of action of anticaking agents. However, the relationship between storage-induced caking processes and the glass transition of amorphous powders is analyzed in more detail.

Water relations, glass transition temperatures, structural collapse and microstructural changes of a protein hydrolyzate sample were investigated as a function of sample degree of hydrolysis, storage conditions （temperature and relative humidity） and time. The Gordon-Taylor equation was a good predictor for the plasticizing effect of water on the glass transition temperature of the powder （Tg）. It appeared that the difference △T between the storage temperature （T） and Tg was the driving force for structural collapse. Volume shrinkage, induced by viscous flow of the matrix and leading to collapse was first noticeable for AT > 30°C, corresponding to a water activity> 0.4. The functional dependence between caking kinetics and AT was modeled using a Williams-Landel-Ferry （WLF） type relationship. The viscosity of the matrix at the onset of structural collapse was estimated using the WLF equation was 7 log cycles lower than the viscosity of the amorphous solid. Scanning electron microscopy studies demonstrated the viscous flow nature of caking by thickening of pore walls and appearance of drop-like and semicrystalline microstructures.