Technological, microbiological and sensory quality attributes of goat's milk kefir: comparison between plain and flavored types

Introduction: Kefir is a popular probiotic drink that strengthens the immune system, reduces lactose intolerance and lowers blood cholesterol levels. It can be made from various milk sources, including goat's milk, which has superior nutritional qualities such as higher protein content and better digestibility compared to cow's milk. The objectives of this study were: firstly, to optimize the production parameters for goat's milk kefir, focusing on the incubation time, temperature and grain/milk ratio to achieve standard technological and microbiological qualities of kefir; and secondly, to monitor the changes in the physicochemical, textural and microbiological properties of kefir during shelf life. Two types of kefir (plain and flavored) were assessed and also compared in terms of consumer preference. Methods: The optimization of the incubation time, temperature and grain/milk ratio was carried out by using a Box-Behnken design and response surface to analyse the pH, acidity, syneresis, mesophilic and lactic acid bacteria concentrations, firmness, consistency, cohesiveness and viscosity index of plain kefir (control). After optimizing the production process, the physicochemical, textural and microbiological quality properties of plain and lemon-flavored kefir were assessed over time during a two-week storage at 4 ºC. The treatments were compared by linear modelling. The sensory attributes of appearance, odor, taste, sourness, smoothness and acceptability were assessed by a consumer panel and statistically analyzed on variance to determine differences between the two kefir types. Results: The optimization experiment revealed significant effects (p < 0.05) of three factors on quality properties of kefir. The acidity was maximized at a 0.66% grain/milk ratio, 25.3 °C and 22.8 h, with incubation temperature having the greatest impact. Specific conditions optimized technological properties, particularly firmness (> 5 g) and viscosity index (< 1 g.s). Syneresis varied greatly with temperature; the highest values were found at lower temperatures (15-20 °C) and shorter incubation times (16 h). However, lower temperatures or longer incubation times could produce the desired acidity and texture, but only at higher grain/milk ratios (>1.0%). In the second phase of the experiment, the flavored kefir maintained a lower pH (4.00), which was beneficial for pathogen inhibition, and exhibited less pronounced proteolysis, contributing to better textural stability. While both kefir types reached similar firmness and cohesiveness, the flavored kefir showed a higher viscosity index and consistency, which had a positive effect on mouthfeel and consumer preference. Microbiologically, the control kefir presented slightly higher lactic acid bacteria and mesophilic populations (7.7 log CFU/mL and 7.6 log CFU/mL, respectively) than the flavored kefir (7.4 log CFU/mL and 7.2 log CFU/mL, respectively). The sensory evaluation revealed a clear preference among panelists for lemon-flavored kefir in almost all sensory attributes. Conclusion: This comprehensive study offers crucial insights into the production of goat's milk kefir by elucidating the effects of key processing variables on product quality. It provides valuable information on the quality characteristics and storage stability of both the natural and flavored variants of kefir and enhances our understanding of quality changes over time. These findings not only contribute to optimizing production parameters for high-quality goat's milk kefir, but also lay a solid starting point for future research in this field.