3.2. Antioxidant activity
The antioxidant activity of black and green tea is undisputable.
The idea was to investigate the potentially higher antioxidant
activity of fermented samples compared to the pure teas as samples
blank. The samples obtained by fermentation with SC1 and
SC2 were especially interesting, because it was proved that native
kombucha culture produces the beverage with antioxidant properties
(Jayabalan et al., 2008), even if the kombucha originates are
from very different geographic regions. Hydroxyl and DPPH free
radicals were chosen as the undesired species because they are
two different types in terms of reactivity and origin. Hydroxyl radicals
are very reactive, but DPPH radicals are relatively stable. Also,
it is possible to generate hydroxyl radicals in the human body,
while DPPH radicals are synthetic products, which are usually used
for the investigation of antioxidant activity to relatively stable
reactive species.
Antioxidant activity, AAOH, of fermentative liquids and kombucha
beverages on the hydroxyl radicals is shown in Fig. 2. A significant
increase of the AAOH was observed in all cultivation mixtures
after three days of fermentation (average value 47.7%) (Fig. 2).
After that period, a very slight increase of the AAOH was observed,
until the end of fermentation. It is obvious that the compounds
produced by kombucha caused a significant effect. After comparison
among the AAOH values for samples obtained on different substrates,
but with the same starter cultures, it was possible to notice
the differences, i.e. the average value of the AAOH is higher for the
samples obtained from green tea inoculated with different cultures.
Also, it was noticed that the highest average AAOH was obtained
with the SC1 on the substrate with black tea (Fig. 2a),
while the highest activity was achieved with the Control starter
on the substrate with green tea (Fig. 2b). There could be several
reasons for such antioxidant behaviour, but the main is again the
fact that polyphenols composition and content in black and green
tea are not the same (Graham, 1992; Mukhtar & Ahmad, 2000).
The dynamics of changes of AADPPH were almost completely different
to the rate of changes of AAOH. The only similarity between
them was a significant increase after three days of fermentation
(average value 48.7%), but continual decrease of AADPPH was found
until the end of fermentation (Fig. 3).
Some similarities between antioxidant activities, AAOH and
AADPPH, were noticed however. Both types of antioxidant activities
achieved the highest values using SC1 on the substrate with black
tea (Figs. 2a and 3a). The highest values were achieved with the
Control on the substrate with green tea (Figs. 2b and 3b). The
lowest antioxidant activities were obtained with SC2 on both substrates.
On the substrate with green tea, using SC2 as an inoculum,
AADPPH decreases significantly until the end of fermentation
(Fig. 3b). This fact certainly does not recommend SC2 for kombucha
beverage production.
The highest number of mixed cultures of acetic acid bacteria
and yeasts was achieved with the Control at the end of the fermentation
on both substrates. There were approx. 5.2 106 of bacterial
cells/ml and 3.1 106 of yeast cells/ml of black tea substrate, and
4.9 106 of bacterial cells/ml and 2.8 106 of yeast cells/ml of
green tea substrate. The microbial count at the end of fermentation
of both SC1 and SC2 was slightly lower in comparison to the Control.
The facts mentioned above indicate that the microbial count
does not correlate to AAOH and AADPPH.