Agriculture Reference
In-Depth Information
Malolactic Fermentation
The conversion of malic acid to lactic acid by
Oenococcus oeni
is often referred
to as a secondary fermentation or malolactic fermentation (MLF), although in the
strictest sense this is simply a microbial transformation as there are no direct energy
production. Generation of a proton motive force across the bacterial cell wall does
lead to energy formation and thus improved growth rates of lactic acid bacteria
capable of conducting MLF in wine. The evolution of carbon dioxide during this
process is likely to have been the catalyst for the misuse of the term fermentation as
this mimics the evolution of gas during the alcoholic fermentation process. MLF is
important in wines for several reasons; firstly it minimise the opportunity for micro-
bial growth in wines post packaging as a substrate capable of use as a carbon source
is removed; secondly the conversion of the diprotic malic acid to monoprotic lactic
acid softens the acidity and enhances the textural mouthfeel aspects of wine; finally
specific sensory notes such as buttery aromas may be imparted into the wine by the
formation of diacetyl. This reaction is shown below:
Wine is an extremely harsh environment for bacterial growth and it is the highly
adapted species of lactic acid bacteria with tolerance to low pH, low nutrient con-
centrations and high ethanol concentrations that manage to grow successfully in
wine that complete the MLF. Whilst several species of
Lactobacillus, Pediococcus
and
Leuconostoc
have been reported to occur in wine, the preferred species for
MLF is
Oenococcus oeni
due principally to the ability of many strains of this organ-
ism to grow without the production of off flavours in the wine. Numerous reviews
on starter culture selection, conducting and the impact of malolactic fermentation
and in wine are published and the reader is directed to these more comprehensive
publications (Powell et al.
2005
; Bauer and Dicks
2004
; Zhang and Lovitt
2006
; du
Toit et al.
2011
) to supplement information present here.
Traditionally MLF is encouraged in red and some white wines such as char-
donnay and like alcoholic fermentation, winemakers may choose to use specific
starter cultures to induce MLF in their wines. Starter cultures have advantages in
that the organisms have well characterised attributes that confer tolerance to condi-
tions prevalent in wine (low pH, nutrients, etc), nutrient requirements and impact
