Influence of microwave - vacuum drying and hydrocolloids on the survival of starter bacteria and color indices of instant yoghurt

Yoghurt is a fermented product (using lactic acid bacteria as the starter) with a short shelflife which its shelf-life can be prolonged by drying. Therefore, in this study, the effects of various parameters such as dry matter content (10, 20 and 28 %), microwave power (35, 135 and 260 W), type and combination of hydrocolloids added to initial yoghurt, as well as incubation of reconstituted yoghurt (37º C for 7 hours) on the color indices and the survival of starter bacteria in instant yoghurt were evaluated. The results showed that the higher microwave power and dry matter content of initial yoghurt the more contrary effects on the color of yoghurt powder and the number of starter bacteria. Based on our findings, the most appropriate dry matter content and microwave power were 10% and 35W, respectively. Moreover, the presence of hydrocolloids did not show any significant effect on the survival of starter bacteria. On the other hand, incubation of reconstituted yoghurt led to a significant reduction on the survival of yoghurt starter bacteria especially Streptococous thermophilous.