June 2015 P R A C T I C A L W I N E R Y & V I N E YA R D 61
W I N E M A K I N G
in precursor plasma mem-
brane lipid components in
samples from difficult-to-fer-
ment juices compared to the
easy-to-ferment control juices
from the same vineyard.
6
Ma n n itol is ver y
infrequently found in
Saccharomyces cerevisiae and is
thought to specifically indi-
cate the presence of oxidizing
agents and oxidative stress in
the environment.
9,10,11
To assess the impact of
oxidation of juice compo-
nents on fermentation rates,
we conducted studies in
Chardonnay juice spiked
with the oxidant hydrogen
peroxide (H
2
O
2
). Levels of
hydrogen peroxide ranging
from 1 to 10 mM H
2
O
2
were
evaluated and fermentation
progress monitored along
with cellular accumulation of
mannitol.
In this experiment, man-
nitol accumulated in the pres-
ence of H
2
O
2
, but progression
of fermentation was not
noticeably impacted. We eval-
uated the impact of molecu-
lar oxygen and of hydrogen
peroxide on a larger fermen-
tation scale (200 L) using a
Torrontes juice with a higher
phenolic content.
By contrast with the
Chardonnay juice experi-
ments, in the Torrontes juice
both oxygen treatments and
treatment with 1.45 mM H
2
O
2
led to arrest of fermentation.
Oxygen had a more pro-
nounced effect than hydrogen
peroxide. This experiment
suggests that there is some
component of juice that, when
oxidized, can be inhibitory
to yeast fermentation and
induce mannitol accumula-
tion.
Certain conditions and
combinations of low nitro-
gen and/or low vitamin lev-
els can also be inhibitory to
yeast metabolism. Our work-
ing model is that the phenolic
profile of these grapes may
have changed due to envi-
ronmental conditions, and
the resulting accumulation of
