The effect of different dairy matrices and different probiotic strains on the viability of probiotic microorganisms after in vitro digestion

The aim of this thesis is to investigate the viability levels of probiotic yogurt, fermented beverages, and cheese matrices produced using Lacticaseibacillus casei and Lacticaseibacillus rhamnosus probiotic cultures in the oral, gastric, and small intestinal stages of the in vitro gastrointestinal digestive system. The physicochemical properties of the produced probiotic yogurt and fermented beverage samples were analyzed at 1, 7, 14, 21st, and 28th days, and the probiotic cheese samples at the 1st, 15th, 30th, 45th, and 60th days of storage. The results were statistically evaluated. In vitro digestion analysis was performed on the 1st day of storage for probiotic yogurt and fermented beverage samples and on the 60th day of storage for probiotic cheese samples, and the results regarding the viability of probiotic bacteria obtained in the mouth, stomach and small intestine stages were evaluated. The pH values and %lactic acid content in probiotic yogurt, fermented beverage and cheese samples progressed in a consistent manner throughout the storage period. When the physicochemical properties of the probiotic samples were examined, it was concluded that the results obtained from the study were similar to those in the literature. The viability of Lacticaseibacillus casei was detected at levels of 9 log cfu/g in probiotic yogurt and fermented beverage samples and 8-9 log cfu/g in probiotic cheese samples during storage. The viability of Lacticaseibacillus rhamnosus was detected at levels of 9 log cfu/g in probiotic yogurt and fermented beverage samples and 8 log cfu/g in probiotic cheese samples throughout storage. It was determined that the probiotic microorganisms in all produced yogurt, fermented beverage and cheese samples maintained their probiotic levels throughout storage. It was determined that Streptococcus thermophilus in probiotic yogurt and fermented beverage samples exhibited a better symbiotic effect in samples containing L. rhamnosus compared to those containing L. casei after 28 days of storage. When the viability of probiotic bacteria was evaluated in vitro after gastrointestinal digestion of probiotic yogurt, fermented beverage, and cheese samples produced in the thesis study, the number of viable bacteria after digestion in probiotic samples containing L. casei ranged from 61.40% to 77.00%, while in samples containing L. rhamnosus, post-digestion bacterial viability ranged from 64.75% to 78.58%. Differences in probiotic bacterial viability between the oral, gastric and small intestinal stages of digestion in different dairy product matrices were found to be significant (p<0.05). For both probiotic bacteria, the probiotic level was maintained in post-digestion probiotic yogurt and fermented beverage samples, but a decrease in the number of live probiotic microorganisms was observed in probiotic cheese samples, resulting in viability below the 10⁶ cfu/g level, thus failing to maintain the probiotic level. When the data obtained on the viability of probiotic yogurt, fermented beverages, and cheese matrix during in vitro digestion stages were examined, it was determined that the highest viability was in the yogurt matrix. It is believed that the probiotic bacteria used in the study had a good symbiotic effect with yogurt starter bacteria and that yogurt was a good carrier matrix. The fermented beverage matrix was the carrier matrix that provided the second-highest viability levels for the probiotics used in the study, following yogurt. It is thought that the good interaction between S. thermophilus and the probiotic bacteria positively influenced this viability. Although the probiotic cheeses produced in the study had a better protein and fat complex compared to yogurt and fermented beverages, they showed lower viability levels for L. casei and L. rhamnosus compared to other matrices. It is believed that in vitro digestion studies conducted on different dairy product matrices produced using different probiotic cultures in the future will contribute to the literature. In particular, based on the data obtained from our study, it has been concluded that L. rhamnosus, which is known to have positive effects on health, may be included among probiotic culture alternatives for fermentation purposes in R&D studies and industrial applications due to its high viability in fermented dairy product matrices during its passage through the gastrointestinal system. It is also considered that in vitro digestion analysis applied to a harder cheese matrix, such as Cheddar, Kashar, and Ezine cheese, could be a potential alternative for future studies. In addition, since the positive effect of microencapsulation on the viability of probiotic microorganisms during GIS passage has been supported in soft cheeses, further studies in this area are recommended.