Physical and microbiological properties of yoghurt powder produced by refractance window drying

The present study aimed to determine the effect of refractance window (RW) drying comparatively with freeze drying on some physical properties and microbiological properties of yoghurt powder. Physical properties of yoghurt powders obtained by RW drying were generally better than those of yoghurt powders obtained by freeze drying except for colour properties. RW drying caused reduction in the counts of yoghurt bacteria during drying, and this reduction increased as the drying temperature increased. Spreading thickness did not play a critical role on the viability of yoghurt bacteria in dried samples. The viability of yoghurt bacteria in yoghurt powders was observed during 90 days of storage at 4 °C. At the end of storage period, the survival of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus in the yoghurt powders produced by RW drying at 50 °C was higher than those produced by RW drying at 60 °C and 70 °C.