Evaluation of the growth and survival of probiotic microorganisms in commercial bio-yogurt
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Date
2000-11
Authors
Hattingh, Analie
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
A review of the literature highlighting the importance of the 'therapeutic
minimum' and the survival of the probiotic bacteria in fermented milk bioproducts
is given in Chapter 2. Special reference is made to the historical
background of probiotics, its therapeutic value and the survival through
passage in the gastrointestinal tract. In addition, technology of bio-yogurt,
factors affecting the survival of probiotic bacteria in yogurt, and the media for
the differential enumeration of these microorganisms in dairy products are
discussed.
In Chapter 3 existing media proposed for the selective enumeration of starter
cultures employed in the manufacture of bio-yogurt are compared and
evaluated. It is essential for comparison reasons to standardize enumeration
methods for microbial analyses in order to study the incidence of the probiotic
bacteria in the presence of the conventional starter cultures. The media
proposed by Chr. Hansen's laboratory proved to be the most suitable for the
enumeration of the different cultures. It is essential that bio-yogurts meet the criteria of a minimum of 106 cfu/rnl of
probiotic bacteria until the expiry date to induce any potential therapeutic
advantages for the consumer. Consequently, in Chapter 4 we evaluated
samples of AB-yogurt obtained from supermarket outlets statistically based on
the enumeration of viable probiotic cultures, Lactobacillus acidophilus and
Bifidobacterium bifidum; as well as conventional yogurt starter cultures,
Streptococcus thermophilus and Lactobacillus bulgaricus and the maintenance
with respect to the 'therapeutic minimum'. Based on the data obtained, the AB-yogurts examined comply with the criteria regarding the number of viable
cells of L. acidophilus, but the consumer would not have received sufficient
numbers of B. bifidum cells at the time of consumption.
In Chapter 5, we monitored the survival of viable cells of the probiotic cultures
and starter cultures present in bio-yogurt at frequent intervals from day 1
until the expiry date at day 31 stored at 4eC and loec. B.bifidum never
exceeded counts of 106 cfuZgin any of the samples and a constant decline in
its numbers was observed. L. acidophilus, despite maintaining counts higher
than 106 cfu/ g in the yogurt samples, also exhibited a substantial decrease in
its numbers during storage.
Due to the poor survival of probiotic cultures in yogurt, we incorporated a
probiotic yeast species, S. boulardii as part of the starter culture in Chapter 6
and 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 probiotic
bacteria in bio-yogurt, pure cultures of Kluyveromyces marxianus, Issatchenkia
orienialis, Debaryomyces harisenii and Yarrotoia lipolytica were inoculated into
commercial AB-yogurt, sterile milk and pasteurised sweetened yogurt in
Chapter 7. The yeast species were able to progress in the bio-yogurt reaching
maximum counts exceeding 107 cfu/ g. Despite the inability of some species to
utilise lactose, the yeast species utilised available organic acids, galactose and
glucose derived from bacterial metabolism of the milk lactose, as well as
possible free fatty acids or free amino acids present in the dairy products and
thereby sufficiently contributed to the retention or enhancing of the pH values.
The production of excessive gas and alcohol was major constraints in
implementing Kluyveromyces marxianus, Issatchenkia orienialis. The inclusion
of Y. lipolytica and D.hansenii in AB-yogurts, therefore, seemed the most promising in controlling the pH to assure the viability of the pro-biotic
microorganisms. In Chapter 8, Y. lipolytica and D.hansenii cultures were inoculated into
commercial plain and fruit AB-yogurt at moderate (105 -106 cfujml) and low
level (10² - 10³ cfujml), directly after manufacture and with the ABT-starter
before fermentation, to compare the effects that the yeast will have on viability
of probiotic bacteria. Viable bifidobacteria counts remained virtually the same
in the bio-yogurt inoculated with the yeast cultures during the refrigerated
storage period. A rapid decrease in L. acidophilus occurred after 2 weeks
storage (2-4 log cycles) in the yeast-inoculated bio-yogurt suggesting possible
antagonistic action of the yeast against L. acidophilus. Addition of the yeast
primarily encouraged the growth of streptococci, which had an influence on
the pH of the yogurt environment. A gradual decrease in the pH of all the bioyogurt
products was observed. pH was affected by enhanced growth of
streptococci, utilization of organic acids by the yeasts and the fact that L.
bulgaricus was excluded form the yogurt starter culture. The yogurt inoculated
with D. hanserui was still acceptable and had a pleasant taste compared to the
control yogurt after 30 days storage. Inclusion of yeast as part of the starter
culture for bio-yogurts seems promising. In Chapter 9, possible enhancement of the growth and survival of
Bifidobacteria 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 well
as L.acidophilus and S. thermophilus were monitored during storage at SoC over
30 days. With the addition of neokestose the viable bifidobacteria count
remained significantly higher in all the yogurts when compared to traditional
yogurts without the addition of neokestose. L. acidophilus reduction in viable
counts did not exceed a 30% reduction; therefore neokestose also had a better
survival effect on L. acidophilus. The addition of neokestose had no affect on
the survival of S. thermophilus.
Description
Keywords
Food -- Biotechnology, Bacterial starter cultures, Saccharomyces, Thesis (Ph.D. (Microbiology and Biochemistry))--University of the Free State, 2000