Using stirring and homogenization to improve the fermentation profile and physicochemical characteristics of set yogurt from fresh, refrigerated and frozen/thawed sheep milk

摘要

The impact of stirring and homogenization on set yogurt production was assessed for fresh, refrigerated (4 days/7 degrees C) and frozen/thawed (1 month/-18 degrees C) sheep milk. Results showed that cold storage increased the fermentation time ( > 90min) and that yogurt from frozen/thawed milk was more sensitive to post acidification (50% higher pH drop) and had less firmness and adhesiveness (similar to 25% lower). Stirring and homogenization changed the milk structure, reducing the natural creaming occurrence in the first hours of fermentation (10-100%) and increasing the yogurt textural parameters. The overall evaluation of the physical processes made before the milk was fermented showed modifications that were greater for refrigerated and frozen/thawed milk than for fresh milk. After 28 days of storage from production, all yogurt samples showed no additional syneresis, no reduction in the lactic acid bacteria counts and an increase in the adhesiveness and firmness. Therefore, results suggest that both milk refrigeration and freezing alters yogurt fermentation time, buffering capacity and texture, and that stirring and homogenization improved the yogurt texture of all samples due to higher interactions of fat and proteins. This information is useful for sheep milk producers to improve the quality of the yogurt produced, especially when it is obtained from previously cold-stored milk.