## 3 Results and Discussion
### Microbiological Properties of Kefir and Reconstituted Kefir Samples  
Since the functional potential of fermented dairy products in general, and kefir in particular, is related to the microorganisms that ferment milk, their increased population in the final product is an important factor affecting their quality and functionality. Evaporative drying processes such as spray drying cause significant cellular damage to living bacterial cells, DNA denaturation, and cell-wall membrane breakdown, resulting eventually in reduced survival rates [14]. Thus, improved microorganisms' viability, following spray drying processes, is a considerable factor regulating kefir powder quality. Figure 5 shows the microbial population of kefir and reconstituted kefir samples as affected by final fermentation pH and low-temperature storage. As can be seen, the population of lactococci (Figure 5a), lactobacilli (Figure 5b), and yeasts (Figure 5c) changed significantly as a result of the studied factors. The highest decrease in the microbial population of reconstituted kefir when compared to fresh product, which was most pronounced in the cases of lactococci and lactobacilli, was observed in the CONTROL. Lactococci population was reduced by 1.5 logarithmic cycles, from 9.2 log cfu/g to 7.8 log cfu/g, lactobacilli by approximately 2 logarithmic cycles, from 9.2 log cfu/g to 7.8 log cfu/g, with yeasts exhibiting the highest decrease, by losing almost 5 logarithmic cycles from 6.8 log cfu/g to 2.0 log cfu/g. For lactococci and lactobacilli, the reconstituted kefir samples prepared with kefir grains and a final fermentation pH of 4.4 exhibited lower a reduction in their populations when compared to the CONTROL. The reduction was even lower for lactobacilli (Figure 5b) when the sample was subjected to refrigeration temperatures for prolonged periods (4.4-7). In the case of samples with higher final fermentation pH and regardless of low-temperature treatment, no statistically significant reduction between reconstituted and fresh samples for the lactococci and lactobacilli populations was observed. Figure 4: Adsorption isotherms of kefir powder samples of period of 7 days as affected by fermentation pH and low-temperature treatment (CONTROL: sample prepared with commercial starter culture, without storage prior to drying; 4.4-0 and 5.2-0: samples prepared with kefir grains to final fermentation pH of 4.4 and 5.2, respectively, without storage prior to drying; and 4.4-7 and 5.2-7: samples prepared with kefir grains to final fermentation pH of 4.4 and 5.2, respectively, subjected to storage at 4 °C for 7 days). With regard to yeasts, refrigerated storage at a fermentation pH of 4.4, and increased final fermentation pH of 5.2, without storage prior to drying, reduced their populations from 7.6 log cfu/g to 5.0 log cfu/g and from 6.4 log cfu/g to 3.8 log cfu/g, respectively, showing a drop to the reconstituted samples by up to almost three logarithmic cycles when compared to the respective fresh kefir. For other samples in the study, this corresponding reduction was higher than 3 logarithmic cycles and was up to five for the CONTROL. In agreement to the results of the present study, [PERSON] and [PERSON] [12] reported low yeast survival in kefir powder samples and optimum survival for lactobacilli and lactococci at 1 \(\times\) 10\({}^{-2}\) and 2.75 \(\times\) 10\({}^{-2}\), respectively. Concerning yeast populations in powder kefir samples, milk powder was reported to be effective in promoting the survival of yeasts, since their numbers decreased from 2.57 \(\times\) 10\({}^{7}\) in kefir to 2.34 \(\times\) 10\({}^{5}\) in kefir powder without milk powder and to 2.64 \(\times\) 10\({}^{7}\) in kefir powder with 10% milk powder addition [16]. The low thermotolerance of yeasts might be attributed to their increased thermal sensitivity [12] and could be related to the increase in the cell surface-to-volume ratio that in turn causes membrane condensation and restructuration during the dehydration process [14].