Page 10 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

MALOLACTIC FERMENTATION IN WINE
stimulation of the MLF. Costello et al. (1985) reported that growth of Pediococcus sp. was supported by the rapid cell death of O. oeni, and under high pH conditions the early growth of L. brevis will completely inhibit the growth of O. oeni. Recently, Gerbaux (in a personal communication) showed that wine conditions that stimulate MLF will inhibit the develop- ment of Brettanomyces spoilage yeast (Gerbaux et al. 2009).
THE INFLUENCE OF MALOLACTIC FERMENTATION ON WINE COMPO- SITION
MLF is not only the simple decarboxylation of L-malic acid to L-lactic acid and CO2. Wine components are consumed by wine LAB, which means they will produce metabolic end products from those components. This has an influence on grape-derived aroma compounds and on those arising from the AF, and confers biological stability on the final product (Davis et al. 1985). The growth of MLB is generally encouraged where MLF is required to reduce the acidity of the wine. The reduction of acidity is beneficial to the quality of wines made in cool winegrowing regions, because the grapes naturally contain high levels of organic acids. Worldwide consumer preference currently favours fruit-driven wines with moderate acidity, resulting in acid reduction becoming a critical issue with wines produced in cool climates. This, coupled with positive flavour changes associated with growth of MLB in wine, has made MLF a desirable process for almost all red wines and for certain styles of white wines. Growth of LAB in wine must be controlled to ensure desirable MLB that produce no off-flavours. In most cases, MLF should complete rapidly to save processing time and achieve early stability of the product. In no instance should indigenous strains of LAB be relied on to conduct the MLF.
Apart from producing lactic acid as a major end product of sugar catabolism (Henick-Kling 1993), LAB are known to produce other flavour-active compounds, such as acetaldehyde, acetic acid, diacetyl, acetoin and 2,3-butanediol. Diacetyl, acetoin and 2,3-butanediol mainly originate from the bacterial consumption of citric acid, and are of considerable importance to the flavour profile of wine. In lower concentrations, these compounds are thought to add complexity to flavour. At concentrations in excess of 5 mg/L, diacetyl can be overpowering, resulting in distinct buttery/nutty flavours. Depending on the pH and the oxidation-reduction potential, acetic acid can be another product of citric acid metabolism by wine LAB. Increased levels of alcohol, volatile esters and ethyl lactate were reported in wines undergoing MLF (Meunier and Bott 1979). Henick-Kling et al. (1992) described flavour contributions attributed to individual strains of MLB. Recently, numerous studies have been conducted on the impact of MLF on the wine sensory profile. Bartowsky et al. (2011) described effects on red berry fruity characters of red wine attributed to strain-specific influences by MLB. Sumby et al.
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