of birch sap includes several types of yeast, including those of genus Candida, among
which, of course, there are pathogens. In addition, there is an idea of undesirability of
a large number of yeast presences in foods spreading in recent years. Therefore, in
order to prevent yeast and micromycetes' growth in the fermented juice, it was decided
to pasteurize it and to conduct the fermentation process by pure cultures of lactic-acid
bacteria only. It is known that most yeast and spores of micromycetes die at
temperatures from 70°C to 80°C. At this temperature some of the biologically active
compounds (that make up the therapeutic value of birch sap) may be destroyed, but we
assume that this loss may not be very high and will be compensated by useful
metabolites of lactobacilli. Further experiments were carried out with pasteurized juice.
When choosing a pasteurization mode, the same portions of birch sap were set for
10 minutes at a temperature of 70°С, 80°С and 90°С, after which they were placed into
a thermostatic cabinet at a temperature of (25 ± 2)°С for two days. We were drawing
conclusions about the effectiveness of pasteurization by the transparency or haze of
juice. The results of visual control were confirmed by the aliquot inoculation of such a
sap on cabbage, malt extract and meat-peptone agars.
After pasteurization at 70°C, such findings as yeast – 20 CFU/cm3,
enterococcus – 20 CFU/cm3, bacteria – 40 CFU/cm3 were detected in the juice. The
pasteurization regime of 10 minutes at a temperature of 80°C was considered as a
satisfactory one as this juice showed 10 CFU/cm3 of bacteria on meat-peptone agar.
We assumed their presence permissible, because in the process of lactic fermentation,
they die under the influence of lactobacilli metabolites as we have proven earlier.
To initiate lactic-acid fermentation, we used a starter culture of one of L. casеi ssp.
аlactosus strains on sterile birch sap that was being grown during a full day at a
temperature of (25 ± 1)°С. This is the minimum steady temperature that can be
provided by the thermostatic cabinet, while the temperature constant is required to
obtain starter cultures with the same cell yield.
Fermentation was carried out at a room temperature of 16 to 20°C in containers
filled up to the lid to avoid a contamination with mold and film yeast. Seed material
was introduced in the amount of 3.5% and 7% of juice volume to determine its
preferred dose. Periodic monitoring showed that after introduction of 7% starter
culture, a steady state of lactic-acid bacteria's growth was achieved at the level of cell
yield (1,2 - 1,3)×108 CFU/cm3 in 12-13 days, which was only 3 to 5 days earlier than
when a smaller amount of seed material of strain No. 6 was applied. A similar
correlation was also established for the strain No. 12 (Table 1). Therefore, the
application of a high seed dose as well as juice fermentation temperature's increase are
economically counter-productive, when conducting the process in aseptic conditions
and taking into account the future long-term storage of fermented juice [2].
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