Agriculture Reference
In-Depth Information
on the sensory properties of the wine and these are traditionally added following the
completion of alcoholic fermentation. However, even with the use of commercial
starter cultures, delayed or stuck MLF can arise from poor or inappropriate nutrient
management, the production of inhibitory compounds from yeast, or the extremely
harsh physicochemical properties of the wine matrix. Timely completion of MLF
enables more efficient winemaking practices and blending decisions to take place.
Problems may arise from late inoculation of starter cultures into wine for MLF.
An unexpected temperature drop combined with nutrient depletion and high ethanol
concentration can make the management of MLF challenging. It is not uncommon
for wineries to have some wines that have not completed MLF prior to winter, and
these must be left with low concentrations of sulfur dioxide until seasonal tem-
peratures rise, which allow bacterial growth and metabolism to resume, or have ex-
pensive temperature control to ensure appropriate conditions conducive for growth
of the MLF organisms. The cellaring of wines without adequate sulfur dioxide to
prevent oxidation and limit growth of micro-organisms can be deleterious to wine
composition, as spoilage organisms can commence growth along with
O. oeni
when
warmer temperatures permit (Alexandre et al.
2004
).
On the other hand, the growth of certain LAB, including
O. oeni
, early in the
winemaking process is considered by some to be undesirable for wine composition.
Increased acetic acid and D-lactic acid levels may arise from heterofermentative
degradation of fermentable carbohydrates (Lonvaud-Funel
1999
), hence the reluc-
tance by some winemakers to use MLF starter cultures early in the winemaking pro-
cess. Inhibition of bacterial growth by nutrient depletion arising from yeast metabo-
lism may be augmented by the production of inhibitory excretory products of yeast
growth, the most obvious being ethanol, sulfur dioxide, low molecular proteins and
medium chain fatty acids (Comitini et al.
2005
; Capucho and San Romão
1994
).
Several reports have recently described the microbial interactions of concurrent
growth of
S. cerevisiae
and
O. oeni
in either model wines—wine made from white
grape varieties or bench top vinifications (Jussier et al.
2006
; Alexandre et al.
2004
).
Loss of bacterial viability when MLF cultures are added during alcoholic fer-
mentations has been reported, although successful MLF is still possible (Rosi et al.
2003
). Yeast metabolic activity will be highest during fermentation, and it can be
reasonably expected that production of inhibitory compounds such as ethanol and
sulfur dioxide will be greatest at this time (Larsen et al.
2003
). Ethanol has an in-
hibitory impact upon the growth of
O. oeni
at concentrations greater than 6 % v/v,
although the effect is strain dependant (Henick-Kling
1995
), hence an advantage
of early inoculation with MLF starter cultures is the gradual acclimatisation of the
bacteria to rising alcohol levels (Silveira et al.
2004
), along with greater levels of
organic acids, which improve ethanol tolerance (Zhang and Lovitt
2006
). Medium
chain fatty acids derived from yeast autolysis are generally higher in concentration
if yeast lees are allowed to accumulate (Powell et al.
2005
). Therefore, expeditious
inoculation for MLF following completion of alcoholic fermentation is also war-
ranted if winemaking practices prevent early inoculation.
The simultaneous inoculation of yeast and bacterial starter cultures for winemak-
ing may not be a universally acceptable or a practical approach in the production
