Changes in kefir made from different kinds of milk during the storage of refrigerated and frozen conditions

This study was carried out to determine the changes during storage of kefir samples made from cow (İK) and goat (KK) milks in refrigerator and deep-freezer. In first stage, the kefir samples were produced and storaged at refrigerator (+4 °C) for 35 days. The chemical, physical, biochemical, microbiological, sensorial properties and also organic acids and flavor components in the kefir samples were determined on 1st, 7th, 14th, 21th, 28th and 35th day of storage period. In the second stage, the production of the kefir samples were repeated by using cow and goat milks. After keeping the samples at refrigerator up to the best storage period revealed at first stage, they were frozen (at -35 ºC for 24 h) and kept at -18 ºC for 45 days. The same analyses were done on 0th, 15th, 30th and 45th day of frozen storage period. According to the obtained results, dry matter and pH were higher in the samples of KK kept at refrigerator. The values of viscosity, L* and b* were higher in İK samples. The values of lipolysis in KK samples were higher, but the values of proteolysis were lower. During the storage, the values of acidity, viscosity, lipolysis and proteolysis increased in both samples while the values of syneresis decreased. On contrast, the counts of lactococci, lactobacilli, leuconostoc, total mesophilic aerobic bacteria (TMAB), acetic acid bacteria and yeast decreased during the storage period. Amounts of lactic, acetic, citric, pyruvic and oxalic acid were found higher in the samples of KK. The amounts of acetaldehyde, diacetyl and acetoin were higher in İK samples while ethanol value was higher in KK samples. Sensorially, İK samples were mostly preferred by the panelists during the storage. Sensorial scores and diacetyl/acetaldehyde ratio were the highest on the 14th day of the storage for İK samples and on the 21st day for KK samples. During frozen storage, the values of dry matter, protein, viscosity, L*, a* and b* of İK samples were higher. Lipolysis values were higher in KK samples while proteolysis values were higher in İK samples. The counts of the groups of microorganisms analysed decreased throughout frozen storage period in both samples. Even so, the counts of TMAB was over 7 lof cfu/g. The amounts of lactic, acetic, citric, oxalic acids, and also acetaldehyde and ethanol were higher in KK samples while diacetyl and acetoin values were higher in İK samples. Sensorially, İK samples were mostly preferred by the panelists. However, frozen storage had negative effect on sensorial properties of KK samples.