Effect of production parameters on physicochemical, microbiological and sensorial properties of kefir, and determination of peptide profile in produced kefir

In this study, kefir was produced by using different heat treatment norms (at 90°C for 5 minutes and at 90°C for 10 minutes), starter culture types (kefir grain and commercial kefir starter culture), incubation temperatures (20°C and 25°C) and final incubation pH values (pH 4.4 and pH 4.8). Physicochemical, microbiological and sensory properties of kefir samples were determined during storage at 4±1°C for 30 days. Additionally, peptide and bioactive peptide profiles of the kefir samples produced by different production parameters have been tried to be presented and the effect of the production parameters on these peptide profiles in the kefir samples were studied. Heat treatment norms used in the production did not have significant effect on total solids, ash, pH, titratable acidity, firmness, stickness and syneresis values of the kefir samples; but they were found to be effective on proteolytic activity values of the kefir samples. Regarding total solids and ash values of the kefir samples, no significant differences were found between kefir samples produced by using kefir grain and commercial kefir starter culture; in contrast type of starter culture has significant effect with varying levels on pH, titratable acidity, proteolytic activity, firmness, stickness and syneresis values. Furthermore, it was determined that incubation temperature used in the production did not make significant differences on ash and pH values of the kefir samples; but its effect on total solid, titratable acidity, proteolytic activity, firmness, stickness and syneresis values was significant. Regarding total solids, ash and proteolytic activity values, there were not significant differences between the kefir samples produced by using different final incubation pH levels. However, at the final stage of the storage period, proteolytic activity and syneresis values of the kefir samples increased; pH, titratable acidity, firmness and stickness values of the kefir samples did not change. In the study it was detemined that the effect of tested process parameters on apparent viscosity values of the kefir samples was significant. In addition, apparent viscosity values of the kefir samples increased during the storage. As a result of examination of the rheological properties of the kefir samples according to the Power Law model, all kefir samples exhibited non-newtonian pseudoplastic behavior and have thixotropic flow properties with respect to time. During the storage, the counts of total mesophilic aerobic bacteria, lactococci, lactobacilli, acetic acid bacteria and Leuconostoc spp. in the kefir samples decreased; in contrast the counts of yeasts in the samples increased. Moreover, it was found that there was a significant effect of the starter culture type on the number of microorganisms counted in the kefir samples. Regarding lactococci and yeast numbers, no significant differences were found between kefir samples produced by using different incubation temperatures. Besides, the final incubation pH levels used in production were determined to have no significant effect on the counts of lactobacili detected in the kefir samples, but also count results of all microorganisms were not influenced by the heat treatment norms. Based on the results of sensory analysis, given total scores of the kefir samples produced using different production parameters decreased during the storage. The total scores of the kefir samples produced with commercial kefir starter cultures were higher than that of the kefir samples produced with kefir grain. In addition, it was determined that the total scores of kefir samples with final incubation pH 4.4 were higher than that of the kefir samples with final incubation pH 4.8. Determination of the peptide profile of kefir was performed in this study by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) and nanoelectrospray ionization (nano-ESI) LTQ Orbitrap mass spectrometry coupled to nano ultra performance liquid chromatography (nano-UPLC). A total of 257 peptides including multiphosphorylated peptides were identified in kefir samples, which were mainly derived from β-casein, αS1-, κ- and αS2-casein. Sixteen of 257 peptides were determined to have various bioactivities. However, 236 of these peptides were unique for kefir and not detected in milk which showed that these 236 peptides were formed during the fermentation process and peculiar to kefir. In addition, relative intensity values of some peptides determined in the kefir samples were influenced by the starter culture type, incubation temperature and final incubation pH used in the production in variable significant levels; in contrast heat treatment norms have not significant effect on relative intensity values of the peptides in the kefir samples.