Freezing and freeze-drying of strawberries with an additional effect of micro-vibrations.

Our research focuses on the formation of ice crystals and evaluating the structure of preserved frozen and freeze-dried strawberries. Strawberries were frozen in two ways. One-half of strawberries were frozen at - 30 °C under conditions of convective heat exchange. The other half of strawberries were frozen under the same conditions with an additional effect on the strawberries of micro-vibrations created in the air of the freezer according to a specific program. A digital frequency synthesizer that generates 250 W/m3 electromagnetic field rectangular pulse packets in the frequency bands of 2500-5000 kHz creates micro-vibrations. The microstructure of strawberries, the number of cells that have retained their structure and firmness were determined in frozen strawberries. The strawberries retained 25-30% of the cell structure of their total number during traditional freezing, and 65-70% of the cell structure when frozen under micro-vibration. The data of the penetration and shear stress showed that the strawberries frozen under micro-vibration conditions were 10-15% stronger. Then researched strawberries were vacuum freeze-dried. The primary drying temperature was 30 + 1 °C below zero and at the secondary drying the temperature was 38-40 °C. The microstructure and firmness of strawberries were researched in dried samples also. Freeze-dried strawberries frozen under micro-vibration had small and evenly distributed capillaries and their firmness was 8-10% higher than freeze-dried strawberries frozen by the traditional method. Thus, freezing strawberries with the additional effect of micro-vibration have a positive effect on the firmness of both frozen strawberries and freeze-dried strawberries. © Association of Food Scientists & Technologists (India) 2020.