Rational primary drying temperature for freeze-drying of fermented dairy products

The influence of primary drying temperature on the quality of freeze-dried fermented dairy products was determined experimentally. The objectives of this research were to investigate the freeze-drying characteristics of yogurt, bio-yogurt and curdled milk dried at primary drying temperatures of -40 +/- 2 degrees C; -25 +/- 2 degrees C; -15 +/- 2 degrees C; -10 +/- 2 degrees C and -5 +/- 2 degrees C. The primary drying time and the total drying time of the tested products were determined for each primary drying temperature. The survival rate of lactic acid bacteria, active and titratable acidity, color and sensory quality evolution were also determined. Increasing the temperature of primary drying (p < 0.05) from -40 +/- 2 degrees C to -25 +/- 2 degrees C was observed to lead to the preservation of 75-80% of lactic acid bacteria. A further increase in the primary drying temperature to -15 +/- 2 degrees C and to -10 +/- 2 degrees C leads to a decrease (p < 0.05) of this parameter to 65-70% and 5-35%, respectively. It is noted that an increase in the temperature of primary drying leads to a decrease (p < 0.05) in pH to 3.7-3.8 and an increase (p < 0.05) in the titratable acidity to 93-95 degrees T for yogurt and bio-yogurt and it is up to 85 degrees T for curdled milk. An increase in the primary drying temperature affected the sensory characteristics of the freeze-dried dairy products. The optimal primary drying temperature for yogurt and curdled milk is found to be -25 +/- 2 degrees C, and for bio-yogurt this temperature is -15 +/- 2 degrees C. The application of the optimal primary drying temperature proposed in the industrial practice is expected to lead to significant savings in specific energy costs along with a reduction in the time of drying.