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ABSTRACT
The use of spray drying for dehydration of acid whey is generally limited by the appearance of uncontrolled thickening and solidifying of the whey mass during the lactose crystallization step. The origin of this physical change is still unknown and probably linked to complex interactions between physical properties and chemical composition of these products. To understand this phenomenon, we simulated the thickening of concentrated acid whey on a laboratory scale by measuring the flow resistance changes as a function of time and whey composition. The thickening process was characterized by an amplitude of torque and a lag time (induction time). Thickening of lactic acid whey concentrate occurred regardless of the presence of whey proteins or lactose crystals. Moreover, this work clearly demonstrated that the thickening process was due to the formation of filamentous structures corresponding to calcium lactate crystals and showed a large dependence on calcium and lactate contents, pH, and phosphate concentration.
Key words: acid whey, rheology, thickening, calcium lactate
INTRODUCTION
Spray drying is one of the most convenient techniques for producing milk, whey, and derivative powders. However, to maximize energy efficiency, these products must be concentrated at the highest possible TS content during a previous evaporating step, which is usually conducted in falling film vacuum evaporators. Additionally, between the evaporation and drying steps, whey powder manufacturing includes a lactose crystallization step that is often performed in a stirred tank over a few hours (Jensen and Oxlund, 1988). In the case of lactic acid whey powder manufacturing, unpredictable and drastic increases in the viscosity of the concentrate occur during the crystallization step, which prevent the concentrate from being pumped to the dryer and often lead to its solidification.
A number of researchers investigated the so-called phenomenon of age-thickening or age-gelation in either concentrated (skim) milk or condensed sweetened milk (Noda et al., 1976; Snoeren et al., 1984; Aoki et al., 2002; Bienvenue et al., 2003). Their results indicated that preheating of milk, storage temperature, and TS content of the concentrate or whey proteins appeared to play an important role in the thickening process. But they mainly indicated that the marked increases in viscosity observed during storage of concentrated (skim) milk and condensed sweetened milk had to be attributed to casein micelle...