Oxygen toxicity is considered significant in the poor survival of probiotic bacteria such as Lactobacillus acidophilus and Bifidobacterium spp. in yoghurts. This study investigated methods to protect these bacteria from oxygen exposure. To confirm the accuracy of the reported survival estimates of L. acidophilus or Bifidobacterium spp. in yoghurts, the reliability of several enumeration media was evaluated with different commercial yoghurts. None of the media however, was found reliable thereby casting doubts on the reported cell numbers of probiotic bacteria in yoghurts. After much research,it was found that although oxygen can be detrimental to L. acidophilus and Bifidobacterium spp.in culture broths, it may not be significant for their poor survival in yoghurts. Nevertheless, techniques such as oxidative stress stress adaption, alternative packaging materials and microencapsulation as investigated in this study, can serve as general protective techniques to help yoghurt manufacturers in maintaining the recommended numbers of probiotic bacteria in their products. This would eventually assist in the efficient delivery of probiotic health benefits to yoghurt consumers. / Doctor of Philosphy (PhD)

Probiotic bacteria are added to commercial yoghurts as adjunct cultures, to impart health benefits to consumers. To gain maximum therapeutic benefit, the bacteria must remain viable over the shelf life of the yoghurt. Studies have shown, however, that the viability of these bacteria decreases significantly over this period and in some products, is negligible prior to the expiry date. Some strains of probiotic bacteria are oxygen sensitive. Yoghurt has been found to contain a significant concentration of dissolved oxygen and it has been proposed that this has a negative effect on probiotic viability. In this research, several tests were conducted and observations made. Experiments were conducted with non-commercial probiotic cultures to observe the effect of low oxygen environment on probiotic viability. No significant difference existed in viability between probiotic bacteria stored in oxygen reduced yoghurt and regular yoghurt. All yoghurt stored in oxygen barrier packaging material displayed enhanced shelf-life properties, this was observed in replicated experiments. Oxygen barrier packaging combined with an oxygen scavenging material was found to be the most effective oxygen removal system, particularly when used with set type yoghurt. / Doctor of Philosophy (PhD)