Evaluation of the growth and survival of probiotic microorganisms in commercial bio-yogurt

[摘要] A review of the literature highlighting the importance of the 'therapeuticminimum' and the survival of the probiotic bacteria in fermented milk bioproductsis given in Chapter 2. Special reference is made to the historicalbackground of probiotics, its therapeutic value and the survival throughpassage in the gastrointestinal tract. In addition, technology of bio-yogurt,factors affecting the survival of probiotic bacteria in yogurt, and the media forthe differential enumeration of these microorganisms in dairy products arediscussed.In Chapter 3 existing media proposed for the selective enumeration of startercultures employed in the manufacture of bio-yogurt are compared andevaluated. It is essential for comparison reasons to standardize enumerationmethods for microbial analyses in order to study the incidence of the probioticbacteria in the presence of the conventional starter cultures. The mediaproposed by Chr. Hansen's laboratory proved to be the most suitable for theenumeration of the different cultures. It is essential that bio-yogurts meet the criteria of a minimum of 106 cfu/rnl ofprobiotic bacteria until the expiry date to induce any potential therapeuticadvantages for the consumer. Consequently, in Chapter 4 we evaluatedsamples of AB-yogurt obtained from supermarket outlets statistically based onthe enumeration of viable probiotic cultures, Lactobacillus acidophilus andBifidobacterium bifidum; as well as conventional yogurt starter cultures,Streptococcus thermophilus and Lactobacillus bulgaricus and the maintenancewith respect to the 'therapeutic minimum'. Based on the data obtained, the AB-yogurts examined comply with the criteria regarding the number of viablecells of L. acidophilus, but the consumer would not have received sufficientnumbers of B. bifidum cells at the time of consumption.In Chapter 5, we monitored the survival of viable cells of the probiotic culturesand starter cultures present in bio-yogurt at frequent intervals from day 1until the expiry date at day 31 stored at 4eC and loec. B.bifidum neverexceeded counts of 106 cfuZgin any of the samples and a constant decline inits numbers was observed. L. acidophilus, despite maintaining counts higherthan 106 cfu/ g in the yogurt samples, also exhibited a substantial decrease inits numbers during storage.Due to the poor survival of probiotic cultures in yogurt, we incorporated aprobiotic yeast species, S. boulardii as part of the starter culture in Chapter 6and monitored its progression and survival in yogurt and milk products.Despite good growth and the survival of the yeast species until the expiry date,excessive gas and alcohol production proved, however, to be major constraints. In order to further study the effect of yeast growth on the survival of probioticbacteria in bio-yogurt, pure cultures of Kluyveromyces marxianus, Issatchenkiaorienialis, Debaryomyces harisenii and Yarrotoia lipolytica were inoculated intocommercial AB-yogurt, sterile milk and pasteurised sweetened yogurt inChapter 7. The yeast species were able to progress in the bio-yogurt reachingmaximum counts exceeding 107 cfu/ g. Despite the inability of some species toutilise lactose, the yeast species utilised available organic acids, galactose andglucose derived from bacterial metabolism of the milk lactose, as well aspossible free fatty acids or free amino acids present in the dairy products andthereby sufficiently contributed to the retention or enhancing of the pH values.The production of excessive gas and alcohol was major constraints inimplementing Kluyveromyces marxianus, Issatchenkia orienialis. The inclusionof Y. lipolytica and D.hansenii in AB-yogurts, therefore, seemed the mostpromising in controlling the pH to assure the viability of the pro-bioticmicroorganisms. In Chapter 8, Y. lipolytica and D.hansenii cultures were inoculated intocommercial plain and fruit AB-yogurt at moderate (105 -106 cfujml) and lowlevel (10² - 10³ cfujml), directly after manufacture and with the ABT-starterbefore fermentation, to compare the effects that the yeast will have on viabilityof probiotic bacteria. Viable bifidobacteria counts remained virtually the samein the bio-yogurt inoculated with the yeast cultures during the refrigeratedstorage period. A rapid decrease in L. acidophilus occurred after 2 weeksstorage (2-4 log cycles) in the yeast-inoculated bio-yogurt suggesting possibleantagonistic action of the yeast against L. acidophilus. Addition of the yeastprimarily encouraged the growth of streptococci, which had an influence onthe pH of the yogurt environment. A gradual decrease in the pH of all the bioyogurtproducts was observed. pH was affected by enhanced growth ofstreptococci, utilization of organic acids by the yeasts and the fact that L.bulgaricus was excluded form the yogurt starter culture. The yogurt inoculatedwith D. hanserui was still acceptable and had a pleasant taste compared to thecontrol yogurt after 30 days storage. Inclusion of yeast as part of the starterculture for bio-yogurts seems promising. In Chapter 9, possible enhancement of the growth and survival ofBifidobacteria in bio-yogurt by the addition of a prebiotic was investigated.Commercial AB-yogurt was fortified with 1, 2 and 3% of the fructooligosaccharide,neokestose, and growth and survival of bifidobacteria as wellas L.acidophilus and S. thermophilus were monitored during storage at SoC over30 days. With the addition of neokestose the viable bifidobacteria countremained significantly higher in all the yogurts when compared to traditionalyogurts without the addition of neokestose. L. acidophilus reduction in viablecounts did not exceed a 30% reduction; therefore neokestose also had a bettersurvival effect on L. acidophilus. The addition of neokestose had no affect onthe survival of S. thermophilus.

[发布机构] University of the Free State