Fruit and vegetable purees as cryoprotectants for vacuum freeze-dried fermented milk products

Fresh fermented milk products have a limited shelf life that can be extended by vacuum freeze-drying. Cryoprotectants are used to increase the survival of lactic acid microorganisms during freeze-drying. The most effective cryoprotectants are those of natural origin. Literature offers little information on the cryoprotective effects of fruit and vegetable purees. Therefore, we aimed to evaluate the effectiveness of fruit and vegetable purees in increasing the survival of lactic acid microorganisms during the freeze-drying and storage of fermented milk products. We studied bioyogurt samples containing pumpkin, fig, and banana purees. Rational modes of freezing and freeze-drying were established on the basis of thermal analysis. The cryoscopic temperature was determined by differential scanning calorimetry. The proportion of frozen moisture was calculated using the Nagaoka formula. Standard methods were employed to evaluate the sensory characteristics of bioyogurts and determine their protein, fat, and non-fat milk solids contents, as well as titratable acidity and microbiological indicators. The addition of pumpkin puree increased the cryoscopic temperature and reduced the freeze-drying stage and the total drying time by 13 h, depending on the amount of puree. However, the addition of sweet fig and banana purees decreased the cryoscopic temperature and increased the freeze-drying stage and the total drying time by 0.5–1.5 and 1.5–3 h, respectively. Based on the sensory evaluation of the freeze-dried bioyogurts, we selected the formulations with 15% of pumpkin and fig purees and 10% of banana puree. We found that the freeze-dried bioyogurts with puree had higher counts of lactic acid bacteria compared to the control. In the freeze-dried samples, the counts were higher at a storage temperature of 4 ± 2°C than at 20 ± 2°C. Pumpkin puree provided the best survival of lactic acid microorganisms during freeze-drying and storage.