A Comparative Study of Skimmed Milk and Cassava Flour on the Viability of Freeze-Dried Lactic Acid Bacteria as Starter Cultures for Yogurt Fermentation

Concentration and preservation of lactic acid bacteria (LAB) starter cultures for food production guarantee the long-term delivery of stable cultures in terms of viability and functional activity. One method that has commonly been used to prepare dried starter cultures for food applications is freeze-drying. During freeze-drying, the bacterial cells are exposed to stresses such as freezing, drying, and long-term exposure to low-water activity. Additionally, another added stress is introduced during the rehydration process. The purpose of this study was to evaluate the survival rates and fermentation performance of three (3) freeze-dried lactic acid bacterial cultures previously isolated from Ghanaian traditional fermented milk. LAB cultures, i.e., Lactobacillus delbrueckii, Lactococcus lactis and Leuconostoc mesenteroides were frozen in the chamber of a Telstar (Lyoquest) laboratory freeze dryer for 10 hrs at -55 °C (as single and combined cultures) using skimmed milk and cassava flour as cryoprotectants held in plastic or glass cryovial. For viability during storage, freeze-dried LAB cultures were stored in a refrigerator (4 °C) and at room temperature (25 °C) for 4-weeks. The survival of freeze-dried cultures was determined by growth kinetics at 600nm (OD600). The performance of freeze-dried LAB cultures after 4-weeks storage was determined by their growth and acidification of milk during yogurt fermentation and consumer sensory evaluation of fermented milk using a 9-point hedonic scale. The survival rates for LAB ranged between 60.11% and 70.91% following freeze-drying. For single cultures, the highest survival was recorded for Lactobacillus delbrueckii (L12), whereas for combined cultures, the highest survival was observed for Lactococcus lactis (L3) combined with Lactobacillus delbrueckii (L12). During the fermentation process all the freeze-dried LAB cultures were able to acidify yogurt to a pH below 4, while yogurt produce from the spontaneous fermentation was characterized by low acidification. Yogurt fermented with freeze-dried lactic acid bacteria cultures, either single or combined strains, showed improved acceptability as compared to the spontaneously fermented yogurt. The consumer acceptability results showed that yogurts produced with combined starter culture of Lactoccus lactis and Lactobacillus delbrueckii or single culture of Lactococcus lactis were the most preferred products with Lactococcus lactis and Lactobacillus delbrueckii possessing high survival rates and high consumer acceptability in yogurt production. These findings are crucial and can be adopted for large-scale production and commercialization of yogurt production, however, in-depth investigation on the effects of freeze-drying and long-term storage on survival and performance of selected LAB cultures are needed.