## 3 Results and Discussion  
### Kinetic Measurements  
The commercial fermentation time and the strength of the formed casein network are very important quality factors in yogurt preparation. In this research, the kinetics of acidification of yogurt samples by lactic acid bacteria were studied as enriched with extract of LSB of varied concentrations (0% \(w/w\), 0.5% \(w/w\), 1% \(w/w\), 2% \(w/w\), and 3% \(w/w\)). The addition of aqua extracts of LSB differentiates milk's initial pH, as shown in Table 1. As the added concentration % \(w/w\) of extracts of LSB increased, the pH value of the initial milk mixture at fermentation time 0 min decreased probably due to the organic acids which are contained in the extract. In addition, the extract of LSB affected the initial acidity of the samples, but, further changes in the pH values of yogurt samples took place during fermentation and these changes represent the effects of lactic acid bacteria [24,25] and the studied fermentation kinetic parameters, as shown Table 1. In yogurt with 0.5% \(w/w\) extract of LSB, the maximum acidification rate was observed as 11.22 \(\times\) 10\({}^{-3}\)\(\pm\) 0.39 \(\times\) 10\({}^{-3}\) upH/min which was significantly different compared to the other concentrations (\(p<\) 0.05). Sea buckthorn contains organic acids, phenolic acids, etc., which even in the small concentration of 0.5% extract of LBG could be responsible for accelerating the growth of LAB and causing a rapid drop in pH. Samples with 0% and 2% \(w/w\) extract of LSB showed similar V\({}_{\text{max}}\) values, 8.9 \(\times\) 10\({}^{-3}\)\(\pm\) 0.00 \(\times\) 10\({}^{-3}\) upH/min and 8.0 \(\times\) 10\({}^{-3}\)\(\pm\) 0.33 \(\times\) 10\({}^{-3}\) upH/min, respectively. The results are in contrast to previous research on guarana aqua extracts added to yogurt [16] where the yogurt with 0.5% \(w/w\) was similar to the control sample and the peak of V\({}_{\text{max}}\) was noticed at 1% \(w/w\) guarana extract followed by a drop at concentration increase to 2% \(w/w\) guarana extract. Also, in another research study, moringa extract added at varied concentrations to yogurt increased V\({}_{\text{max}}\) according to the concentration increase [26]. Probably aqua extracts of LSB contain fibers, organic acids, and phenolics which could be responsible for these different effects on V\({}_{\text{max}}\). The required time for yogurt samples to reach a pH of 5 was also determined in Table 1. The highest time required for yogurt with 0% \(w/w\) extract of LSB to reach a pH of 5 was T\({}_{\text{pH5}}\) 308 \(\pm\) 50.48 min and the smallest for the yogurts with the highest concentrations of extract of LSB 2% \(w/w\) (175 \(\pm\) 13.88 min) and 3% \(w/w\) (165 \(\pm\) 3.22 min). The presence of bioactive compounds and indigestible polysaccharides in LSB extracts could influence acid lactic bacteria growth. Similar results were found in other studies, in yogurts enriched with extracts such as _Pleutrotus ostreatus_[27], guarana aqua [16], and green tea [28]. Acidification of milk influences yogurt's physicochemical stability as the way that the pH lowers casein micelles makes them lose their negative charge and enhances the attraction of proteins. When the pH reaches the value of 4.6, the agglomeration of caseins takes place [16]. In this study, the fermentation completion time was the largest for the sample with 0% \(w/w\) LSB extract 421.00 \(\pm\) 1.22 min (\(p<\) 0.05) and the yogurt samples