Abstract
Bifidobacterium is a putative probiotic bacterium associated with health benefits such as prevention of growth of pathogens, diarrhoeal diseases and cancer in the intestine, when consumed in sufficient numbers (>106 CFU/g). The poor survival of this organism has been a major obstacle to the manufacture of bio-yoghurts. A study of bio-yoghurts in the UK showed an adequate population level of bifidobacteria at the time of sale and identified the organism used as Bifidobacterium animalis ssp. lactis. B. animalis ssp. lactis is a strain of animal origin. In comparison to a number of human associated Bifidobacterium spp., it showed significantly (P<0.05) better survival and resistance to acidity, bile and oxidative stress. It was concluded that B. animalis ssp. lactis was used primarily for technological reasons since the health benefits of animal bifidobacterial strains have not been researched to the same extent as human associated strains. Storage time, temperature, titratable acidity, pH, dissolved oxygen, and redox-potential (Eh) all determined the survival of bifidobacteria in fermented milk. Bifidobacteria survived longer in products with low initial Eh, as adjusted by ascorbic acid. Interactions between yoghurt (S. thermophilus and L. delbrueckii ssp. bulgaricus) and probiotic (Bifidobacterium and Lactobacillus) bacteria had an effect on the viability of probiotic organisms in the product. Bifidobacteria survived longer in bio-yoghurts produced with S. thermophilus and B. longum only. S. thermophilus consumed dissolved oxygen, lowered Eh of the medium and improved bifidobacterial survival while L. delbrueckii ssp. bulgaricus appeared to have an inhibitory effect on B. longum. The apparent mutualistic relationship between S. thermophilus and B. longum merits further investigation. Detailed modelling of the survival kinetics showed that B. longum was more susceptible to acidity and survival was a log-linear function at low pH (pH 4, 4.25) whereas at high pH (pH 4.5, 4.75) pronounced shouldering and tailing effects were observed. B. animalis ssp. lactis generally showed a simple log-linear reduction over time irrespective of pH. B. animalis ssp. lactis recorded the highest T4D (time to 4-log reduction) value over a range from 4-8°C. Lowering the Eh by adding reducing agents appears to be the best way to enhance the survival of bifidobacteria in fermented milk. B. longum NCTC11818, a Bifidobacterium strain of human origin, can be successfully incorporated into buffalo curd (a Sri Lankan fermented milk) to produce a probiotic product with improved sensory properties such as taste and mouth-feel resulting in a significantly (P<0.001) higher overall acceptability.