The co-cultured organism may compete for nutrients or may produce
metabolic products that may stimulate or inhibit each other's
growth. It has been reported that yeast may produce vitamins that
enhance the growth of LAB (Marshall, 1987). Growth of lactic acid bacteria
is believed to be promoted when co-cultured with yeasts, mainly
due to the excretion of specific amino acids and small peptides by the
latter, either during growth, and regardless of the antagonism for the
main carbon source (Gobbetti et al., 1994) or as a consequence of an accelerated
autolysis (Zambonelli et al., 2000).
The yeast when co-cultured with the L. acidophilus probiotic was
more competitive than in mono culture, regarding the final population
reached that showed an increase of 1 log CFU/mL. The yeast probably
caused a significant (p b 0.05) reduction in the population of bacteria
in 4 h of fermentation. After a reduction in the L. acidophilus (LACA 4)
population, these LAB increased their population and stabilized at
6.6 log CFU/mL. In the co-culture of S. cerevisiae and P. acidilactici, both
presented one log higher than when cultivated in pure culture.
Despite the slow growth start, the LAB reached viable counts of about
8 log CFU/mL after 16 h, and remained constant (p N 0.05) until the
end of the fermentative process.
The cultures showed a trend to decrease their populations when
they were kept at 4 °C. Fermentations conducted with L. acidophilus
(LACA 4) and P. acidilactici (UFLA BFFCX 27.1), and the fermentation
involving all strains were the ones that showed smaller effect of low
temperature on viable populations (p b 0.05). The fermentation involving
all cultures led to an increase of L. acidophilus (LACA 4) viable count
during 24 h under refrigeration.
In general, the results of the pH values followed an opposite trend to
that observed for titratable acidity measurements, i.e., as the acidity
increased, the pH decreased. Fig. 3a shows clearly that L. acidophilus
(LACA 4), in pure culture,was the only strain able to acidify significantly
the substrate (p b 0.05). Efficient acidification profiles were generally
achieved in co-culture fermentation (Fig. 3b).