Wine is a highly complex mixture of compounds which largely define its appearance, aroma, flavour and mouth-feel properties. The compounds responsible for those attributes have been derived in turn from three major sources, viz. grapes, microbes and, when used, wood (most commonly, oak). The grape-derived compounds provide varietal distinction in addition to giving wine its basic structure. Thus, the floral monoterpenes largely define Muscat-related wines and the fruity volatile thiols define Sauvignon-related wines; the grape acids and tannins, together with alcohol, contribute the palate and mouth-feel properties. Yeast fermentation of sugars not only produces ethanol and carbon dioxide but a range of minor but sensorially important volatile metabolites which gives wine its vinous character. These volatile metabolites, which comprise esters, higher alcohols, carbonyls, volatile fatty acids and sulfur compounds, are derived from sugar and amino acid metabolism. The malolactic fermentation, when needed, not only provides deacidification, but can enhance the flavour profile.

The aroma and flavour profile of wine is the result of an almost infinite number of variations in production, whether in the vineyard or the winery. In addition to the obvious, such as the grapes selected, the winemaker employs a variety of techniques and tools to produce wines with specific flavour profiles. One of these tools is the choice of microorganism to conduct fermentation. During alcoholic fermentation, the wine yeast Saccharomyces cerevisiae brings forth the major changes between grape must and wine: modifying aroma, flavour, mouth-feel, colour and chemical complexity. The wine bacterium Oenococcus oeni adds its contribution to wines that undergo malolactic fermentation. Thus flavour-active yeasts and bacterial strains can produce desirable sensory results by helping to extract compounds from the solids in grape must, by modifying grape-derived molecules and by producing flavour-active metabolites. This article reviews some of the most important flavour compounds found in wine, and their microbiological origin.

Yeast and bacterial modulation of wine aroma and flavour

Two key groups of organisms are involved in the production of red, white, and sparkling wine. The yeasts, typically strains of Saccharomyces cerevisiae , carry out the primary or alcoholic fermentation, in which sugars are converted to ethanol and CO2. Lactic acid bacteria (LAB), especially

Lactic acid bacteria as a potential source of enzymes for use in vinification

Malolactic fermentation (MLF) is the bacterial-driven decarboxylation of L-malic acid to L-lactic acid and carbon dioxide, and brings about deacidification, flavour modifications and microbial stability of wine. Pasteur first described the presence of ‘bacteria’ in wine nearly one-hundred-and-fifty years ago and the subsequent elucidation of the bacterial-driven malolactic reaction was described about fifty years later. Over the following years the occurrence of MLF became apparent in wines worldwide, and eventually Oenococcus oeni was identified as the principal organism involved in the process. O. oeni is remarkable in its ability to tolerate the nutritionally poor and challenging, harsh wine environment; however, it can be a difficult and sometimes unreliable organism to work with in the winery. A greater knowledge of its biology would undoubtedly facilitate the development of strains and practices, with improved performance outcomes. We already know a considerable amount about the biochemistry and physiology of O. oeni, and ironically, although we know little about its genetics, its genome has been sequenced. With this groundwork in place and molecular biology techniques at our disposal we are poised to increase our knowledge and understanding of this organism enormously.

Oenococcus oeni and malolactic fermentation – moving into the molecular arena

Malolactic fermentation (MLF) is conducted by lactic acid bacteria (LAB) and refers to the decarboxylation of Lmalate to L-lactate. This secondary fermentation is difficult to control and is mainly driven by Oenococcus oeni.  Uncontrolled MLF, especially in wines with a high pH, which are typical of warmer viticultural regions, may render the wine unpalatable or even cause spoilage. In this review we focus on wine compounds and emphasise factors that affect the growth of 0. oeni and MLF, and discuss practical applications.  We also explore alternative technologies that may enable better control over MLF.

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Control of malolactic fermentation in wine. a review

http://digitalknowledge.cput.ac.za/bitstream/11189/6447/1/Metabolic%20engineering%20of%20malolactic%20wine%20yeast.pdf

Metabolic engineering of malolactic wine yeast.

The malolactic enzyme of Leuconostoc oenos ATCC 23278 was purified 136fold. The molecular weight was estimated at 132,000 when determined by gel filtration. The enzyme contained two identical subunits (Mw = 66,000 using sodium dodecyl sulfate gel electrophoresis). The malolactic enzyme catalyzes the NAD(+)- and Mn(+)-dependent reaction L-malate----L-lactate + CO2. The apparent Km values for malic acid, NAD+, and Mn2+ were 17 mM, 0.044 mM, and 0.017 mM, respectively. The optimal pH and the optimal temperature for activity were 5.0, and 37 degrees C, respectively and the isoelectric point was pH 4.30. L-lactate and ethanol were non-competitive inhibitors, whereas succinate, citrate, and D-tartrate showed competitive type inhibitions.

Purification and properties of a malolactic enzyme from Leuconostoc oenos ATCC 23278

Bioconversion of acrylonitrile into acrylamide using a highly compact multiphasic reactor

Several steps to obtain a good malolactic bioconversion in wine and a new process are presented. These include the optimisation of the biocatalyst production, the purification and properties of malolactic enzymes from Lactobacillus sp. 89 and Leuconostoc oenos ATCC 23278, and the development of a bioconversion process together wih some applications to wine. The use of selected strains and processes could allow the control of malolactic transformation in all wines in spie of the presence of inhibiors such as sulphur dioxide.

Pascal Naouri

Papers

Purification and properties of a malolactic enzyme from Leuconostoc oenos ATCC 23278

Bioconversion of acrylonitrile into acrylamide using a highly compact multiphasic reactor

A new technology for malolactic bioconversion in wine