This study was an attempt to optimize the formulation of a shelf-stable fermented milk beverage using response surface methodology. The effect of whole milk powder-to-whey ratio, agar, starch, pectin, fructose and cream on the viscosity and water-holding capacity of the beverage was investigated. Whole milk powder-to-whey ratio, agar and starch were identified as the most significant factors using the Plackett-Burman design (P < 0.05). The levels of the four factors were optimized using response surface methodology with Box-Behnken design. A response surface model was developed indicating the relationship between each of the response variables and the four factors. The optimal combination obtained from the model was whole milk powder-to-whey ratio 5:1, pectin 1 g/L, starch 10 g/L and pectin 0.5 g/L.

In the manufacturing process of long shelf-life yogurt, there are many factors that may affect its quality, including pasteurization temperature and time, the homogenization degree of stirred mixtures, and filling temperature. The shelf life of yogurt can be extended by choosing a proper pasteurization temperature and time taking the product quality into account, and its texture can be improved by selecting a suitable homogenization approach, which is also beneficial to more uniformly heating yoghurt during sterilization, avoiding the collapse of the system. The recovery of yogurt viscosity can be affected by filling temperature. A proper filling temperature should be selected to reach the requirements of quality design during the manufacturing of yogurt.

The influence of whey protein concentrate (WPC80) added to fermented skim milk drink on its stability was studied. The stability of fermented skim milk drink with different concentrations of whey protein concentrate was evaluated by particle size distribution, centrifugal analysis and Turbiscan analysis. The results showed that fermented milk drink containing 30% WPC80 had the smallest particle size and smallest centrifugal sediment. The turbiscan analysis showed distinctly that the fortified drink produced less sediment than the fermented milk drink with no added WPC80.