Effect of whey powder rehydration and dry-denaturation state on acid milk gels characteristics

Protein enrichment of the milk base as well as heat treatment are two inevitable practices in yogurt formulation. In this study, the native and denatured whey protein isolates (WPI) powders by dry-heating were studied to elucidate their rehydration mechanism and the potential effect of dry-denaturation on acid gel formation. The added native or denatured WPI powders were allowed to rehydrate into the milk bases at various time ranges (5, 180, 300, 480, 900, and 1,440 min). Rehydration kinetic of these powders was monitored by tracking particles size variation via the Qicpic granulo-morphometer. WPI rehydration into the milk base turned to be a quick two-step process. While rehydration condition seems not to affect the gel characteristics while, dry denaturation from its parts appears to delay onset of gelation and weakness the acid-gel elasticity.

Practical applications

Commercial preparation of yogurt involves different processing steps. Among them, protein fortification of the milk base as well as heat treatment appears as crucial steps that attribute towards the change in textural properties. Rehydration of the added powder is realized by the industry before applying the corresponding heat treatment, permitting thus to limit grainy texture. Throughout heating in the milk base whey protein isolates are denatured resulting thus in increased firmness. These aspects are of high significance in the preparation of dairy products, that is, different type of set and stirred yogurts, and this study shows that whey protein fortification is beneficial in their native form rather than in a dry-denatured form. On the other hand, their rehydration mechanism appears to be so fast, requiring few seconds permitting thus to limit the industrial usual time of rehydration (of 3 hr) before further treatment.