Date of Award


Degree Name

MS in Agriculture - Dairy Products Technology


College of Agriculture, Food, and Environmental Sciences


Phillip S. Tong

Advisor College

College of Agriculture, Food, and Environmental Sciences


Probiotics, especially Lactobacillus acidophilus, strains have increasingly been incorporated into food products as dietary adjuncts due to their purported health benefits. Dairy products, especially yogurt, are frequently used as probiotic delivery systems. To realize therapeutic effects, it is important to consume a high amount of probiotics. However, maintaining a sufficient amount of viable probiotics in yogurt throughout the shelf life has been a challenge as starter cultures have shown to possess some deleterious effects on probiotic survival. The main objective of this thesis project was to determine the extent of antagonistic effects imposed by yogurt starter cultures (Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus) and their metabolic products on the survival of five different L. acidophilus strains designated as ATCC700396, LA-5, NCFM, PIM703 and SBT2062. These probiotic strains were incorporated as adjuncts in yogurts, which were made with and without starter cultures. To simulate pH reduction in regular yogurt, yogurt without starter cultures was manufactured by adding an acidulant (glucono-delta-lactone). The pH of both yogurts was approximately 4.6 throughout the 28-day storage period at 4°C. Viable cell counts of the probiotic strains were enumerated weekly on a selective medium (MRS-maltose agar). While cell count of L. acidophilus PIM703 and SBT2062 remained steady (~6 x 107 CFU/g) in yogurt with starter cultures during storage period, the survival of ATCC700396 and NCFM was reduced significantly (p < 0.05) by 3 log and 4.6 log, respectively. In contrast, in yogurt without starter cultures, all L. acidophilus strains survived well with a maximum of 1-log reduction. Our results indicated that while the presence of starter cultures (especially L. delbrueckii ssp. bulgaricus) had a negative effect on the survival of some probiotic strains, low pH was not an important factor impairing their viable cell counts. In addition to survival studies, gene expression of selected stress genes (dnaK and groEL) was determined using semi-quantitative reverse-transcription polymerase chain reaction (RT-PCR) method. This preliminary study demonstrated the important of dnaK in L. acidophilus NCFM to maintain its viability under various stress in yogurt during refrigerated storage period. These studies underscore the selection criteria of probiotic strains in fermented food should include their abilities to coexist with the starter cultures. Our future work is to provide a better understanding of strain differences in survival of probiotics in yogurt.

ENG Abstract_THESIS.doc (95 kB)
ENG Abstract_THESIS.doc