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Viability of probiotic cultures in commercial Australian yogurts

Authors: N. Micanel, I.N. Haynes and M.J. Playne


The investigation of various media formulations, using pure cultures and commercial yogurts, provided the basis for selecting suitable methods to enumerate probiotic cultures. Full and reduced-fat yogurts from four manufacturers of commercial probiotic yogurts were sampled one day post manufacture and analysed for constitutive microflora. Samples were stored at 4°C and 10°C, with subsequent sampling and testing at approximately two weekly intervals, until product 'use-by' date was reached. Enumeration methods utilised M17 agar for streptococci, MRS pH 5.3 agar for Lactobacillus delbrueckii subsp. bulgaricus, MRS + 0.2% ox bile for L. acidophilus, and Bifidus-blood agar for bifidobacteria. For product held at 4°C, Streptococcus thermophilus survival was universally highest in all four products, with lowest counts being 4.1 x 108 cfu/g after six weeks. A single yogurt contained L. delbrueckii subsp. bulgaricus, which also survived well. Minimum counts were 7.4 x 106 cfu/g at this temperature. L. acidophilus viability varied widely. One product retained levels of >107, another >106 cfu/g. A third reduced slowly, but maintained levels of >105 cfu/g. No viable organisms (<103 cfu/g), were detected in the fourth product. Of the three products incorporating bifidobacteria, one maintained high levels (>106 cfu/g), another showed a steep decline from 1.5 x 105 to <103 cfu/g within two weeks post manufacture, and no viable cultures (<103 cfu/g) were detected in the third. The effect of storage at 10°C had little effect on the viability of all organisms, however, lower pH levels resulted in most cases. Similarly, fat levels had no noticeable effect on the survival of cultures.

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