Live and Dead Bacteria Counts of Different Yogurts Before and After Expiration

Abstract

Abstract Objectives The objective of this study was to investigate differences in live and dead bacteria counts a month before the expiration date and right after the expiration date and observe any differences in bacteria counts between types of yogurt, brands, and stores tested. Methods Two yogurt containers with the same expiration date were collected for each brand at three stores. The yogurts collected were three national brands of Greek yogurts and five regular yogurts. The first container was tested one month prior to expiry. The second container was tested at the expiration date. After being thoroughly mixed, triplicate 50–75 mg of yogurt were sampled from each carton. Yogurt samples were diluted 9-fold with freshly prepared peptone water. A second dilution was prepared by mixing 25 µL of the yogurt peptone mix with 975 µL of freshly prepared sterile PBS. The samples were incubated for 15 min. at 37°C with 1 µL SYBR green and 5 µL propidium iodine and iced for 1 hour in the dark. 200 µL of dyed yogurt were diluted with 800 µL of PBS and analyzed by flow cytometer (BD Accuri™ C6 Cytometer). Each yogurt sample was run in triplicate on slow speed setting. Flow cytometer parameters were set to differentiate live and dead bacteria based on fluorescence as well as yogurt matrix artifacts. The data was analyzed by a least-squares fit model to test for significance among variables. Results Greek yogurts had significantly higher live and dead bacteria counts compared to regular yogurts. The Greek yogurts differed amongst themselves. Brand 3 had significantly lower live counts than brand 2 and 1. Brand 1 had significantly higher counts of dead bacteria compared to the other two. While differences between Greek yogurt are statistically significant, they are most likely not clinically significant. For the regular yogurt live and dead counts there were no significant differences. No significant differences were observed based on the timing of yogurt sampling or the store the yogurt came from. Conclusions Preparing Greek yogurt involves more straining and whey removal and yields a higher yogurt concentration, so it is no surprise that Greek yogurts have higher bacteria counts. This study demonstrates that when considering yogurt for its probiotic qualities, the most relevant factor is the type of yogurt. Yogurt brand, store, or when it is sampled have little effect on the probiotics count. Funding Sources USU.