Evaluation of different preservation techniques on the storage potential of Kefir grains.

摘要

The impact of 4 different preservation techniques (frozen stored without milk at -18 degrees C, refrigerated at 4 degrees C, air dried at room temperature for 3 weeks in a dessicator or freeze-dried) on the substrate pH, titratable acidity (TA), lactose and lactic acid levels of kefir was investigated at 0, 1, 3 and 10 months of storage. Fermentation was conducted using Lactococcus lactis subsp. lactis, cremoris and diacetylactis. It was shown that the frozen and refrigerated kefir grains developed acidic and alcoholic odours after 10 months of storage. The lyophilized grain retained the colour and appearance of fresh grains but were extremely brittle, while the air-dried grains developed a brownish colour, foul odour and a tough structure, followed by mycelial fungal growth after 2 weeks of drying. Mass losses over the 10-month storage period were small and insignificant in the frozen, refrigerated and lyophilized grains. Storage period had very little effects on the pH of frozen grains. Refrigerated and air-dried grains showed good retention of the initial acidification activity, while the decrease in the pH of lyophilized grain was significantly slower compared to the other treatments after 3 and 10 months of storage. The TA profiles of frozen and refrigerated grains was similar to that of milk stored at 10-18 h (0.76-0.88%) after 1 and 3 months but then decreased after 10 months of storage. The TA of the air-dried and lyophilized grains also significantly decreased at 10 months of storage. There were no significant differences in the lactose contents of the grains in all treatments after 10 months of storage, while the highest production of lactic acid was observed in frozen and refrigerated grains after 10 h of fermentation. However, the lactic acid contents were similar in all treatments after 18 h of fermentation. In conclusion, all of the preservation techniques used in this study are suitable for the preservation of kefir grains. However, air drying is not recommended for commercial use due to the unacceptable colour and smell of the grains.