By Wineland Media

Lynn Engelbrecht, Senzo Mtshali, Bronwen Miller & Maret du Toit

The results obtained showed that pH, ethanol and malic acid play an important role in the expression of the malolactic enzyme gene.Malolactic fermentation is a very important process in winemaking, resulting in deacidification, microbial stability and aroma modifications. The direct conversion of l-malic acid to l-lactic acid during malolactic fermentation is catalysed by the malolactic enzyme. Here we report on the results of two studies investigating the effects of pH, ethanol and malic acid on the expression of the malolactic enzyme gene (mle) from Oenococcus oeni and Lactobacillus plantarum. The expression of the mle gene was enhanced at lower pH levels (pH 3.2 vs. 3.8), as well as in the presence of malic acid, while expression decreased in the presence of ethanol. A higher expression level of the mle gene encoding the malolactic enzyme may be linked to a faster and/or successful malolactic fermentation and a better understanding of which parameters and how they affect mlegene expression, could aid in managing a successful malolactic fermentation. Our results also support the use of co-inoculation as a malolactic fermentation inoculation strategy.

Introduction

Genes are part of a living organism’s genome and are responsible for specific traits and characteristics. Each gene contains a set of instructions on how to produce a functional product, for example an enzyme. The process by which the information contained within a gene is used to produce this functional product or enzyme is called gene expression. Not every gene product is needed all the time. The organism assess the environment and then reacts on internal and external signals which triggers the expression of certain genes necessary for the development and survival of the organism at that specific moment.

Malolactic fermentation is a very important step in the winemaking process. By the conversion of l-malic acid to l-lactic acid, it contributes to deacidification of the wine, microbial stability, as well as softening, while the aromatic profile is also being influenced. Oenococcus oeni is the lactic acid bacteria mainly associated with malolactic fermentation and is the most favourable species used in malolactic starter cultures. However, the species Lactobacillus plantarum, which is also frequently found in grape must and wine, and effective in completing malolactic fermentation successfully,1 has also now been used in commercial malolactic starter cultures either as a single strain or mixed with O. oeni.

Ideally, O. oeni prefers to grow at a pH of 4.8, in a medium with £10% (v/v) ethanol and at a temperature of 22°C,2 whereas in wine O. oeni is exposed to harsh environmental conditions, including high ethanol concentrations (>12%), low pH (<3.8), sulphur dioxide, low temperatures (<18°C) and limited nutrients. However, it is able to survive this multi-stress environment and therefore the best adapted wine lactic acid bacteria. In order to survive these conditions, O. oeni employs different stress response mechanisms to preserve energy and to defend and protect the cell envelope. The main mechanism of survival is the metabolism of l-malic acid which generates a proton motive force, resulting in the production of energy through ATP synthesis and deacidification of the intracellular pH3 and in the presence of ethanol for example, O. oeni has showed to respond by increasing the fatty acid content in its membrane to regulate membrane fluidity.4,5

The direct transformation of l-malic acid into l-lactic acid by wine lactic acid bacteria is the result of the malolactic enzyme. A better understanding of the when, the where and what conditions promotes or prevents the expression of the gene coding for the malolactic enzyme, provides valuable information on predicting the effectiveness of malolactic fermentation.

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