Malic Acid in Wine: Origin, Function and Metabolism During Vinification (2025)

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Malic Acid in Wine: Origin, Function and Metabolism during Vinification H. Volschenkla, H.J.J. van Vuuren2 and M. Viljoen-Bloom1* (1) Department of Microbiology, Stellenbosch University, Private Bag XI, 7602 Matieland, South Africa (2) Wine Research Centre, University of British Columbia, Vancouver, B.C. V6T 1Z4, Canada (a) Current address: Department of Food and Agricultural Sciences, Cape Peninsula University of Technology, PO Box 652, 8000 Cape Town, South Africa Submitted for publication: May 2006 Accepted for publication: September 2006 Key words: malic acid; wine; deacidification The production of quality wines requires a judicious balance between the sugar, acid and flavour components of wine, i -Malic and tartaric acids are the most prominent organic acids in wine and play a crucial role in the winemaking process, including the organoleptic quality and the physical, biochemical and microbial stability of wine. Deacidification of grape must and wine is often required for the production of well-balanced wines. Malolactic fermentation induced by the addition of malolactic starter cultures, regarded as the preferred method for naturally reducing wine acidity, efficiently decreases the acidic taste of wine, improves the microbial stability and modifies to some extent the organoleptic character of wine. However, the recurrent phenomenon of delayed or sluggish malolactic fermentation often causes interruption of cellar operations, while the malolactic fermentation is not always compatible with certain styles of wine. Commercial wine yeast strains of Saccharomyces are generally unable to degrade i -malic acid effectively in grape must during alcoholic fermentation, with relatively minor modifications in total acidity during vinification. Functional expression of the malolactic pathway genes, i.e. the malate transporter (mael) of Schizosaccharomyces pombe and the malolactic enzyme (mleA) from Oenococcus oeni in wine yeasts, has paved the way for the construction of malate-degrading strains of Saccharomyces for commercial winemaking. INTRODUCTION production of quality wines. Acidity in wine directly or indirect- Acidity in wine originates mainly from two sources. The first is the ly affects several different levels of the winemaking process and organic acids extracted from grapes into the must during harvesting ultimately determines wine quality in terms of the perceived and crushing; L-tartaric, L-malic and citric acids are the predomi- organoleptic and aesthetic character. Wine acidity also influences nant acids (Boulton et al., 1996). The chemical composition of har- the ageing potential or the shelf-life of wine, as it determines the vested grapes therefore strongly influences the composition of physical, biochemical and microbial stability of wine. In addition, must at the onset of vinification and ultimately the final quality of wine acidity and pH affect the timely succession of cellar events the bottled wine. Secondly, the combined metabolism of yeasts and and the effectiveness of several techniques applied by winemak- bacteria during subsequent fermentation steps contributes to the ers (Margalit, 1997). pool of wine acids. The net contribution of these microorganisms Winemakers often experience problems when some wine acids to wine acidity is the sum of both the degradation of some grape exceed acceptable concentration levels. Since the artificial acids and the biosynthesis of some unique organic acids by yeasts manipulation of sugars and flavourants in wine is detrimental to and bacteria during and after alcoholic fermentation. Succinic acid wine quality and prohibited in most wine producing countries, is the major acid produced by yeast during fermentation. Lower winemakers can only modify the acidity component of wine by levels of other tricarboxylic acid (TCA) cycle intermediates are adding or removing certain acids. The adjustment of acidity in also present (Ribereau-Gayon et al., 2000). Lactic acid is mainly must or wine is complex and a number of factors must be taken produced by lactic acid bacteria (LAB) during malolactic fermen- into account to ensure the correct method and timing for rectify- tation, but small amounts can also be synthesised by yeast. Several ing wine acidity. Winemakers routinely employ bacterial malo- cellar procedures such as maceration and cold stabilisation also lactic fermentation to deacidify wine (Henick-Kling, 1993). influence the final acid composition of wine due to precipitation Although this step is considered the most natural method for wine phenomena (Jackson and Schuster, 1997). acidity adjustment, which also contributes to microbial stability The conversion of grape sugars to ethanol and carbon dioxide and organoleptic complexity, there are a number of pitfalls asso- is often described as the fundamental biochemical reaction ciated with this biological process. involved in winemaking, but an intricate ensemble of biological This review discusses, with special reference to L-malic acid, and spontaneous chemical reactions all contribute to the final the origin and evolution of organic acids in grapes and wine, the product. Besides the importance of flavour compounds, the pres- role of acidity in wine and the fate of these organic acids during ence or absence of organic acids in wine plays a pivotal role in the fermentation and downstream processing. Corresponding author: email: [emailprotected] [Tel: +27-21-808 5859; Fax: +27-21-808 5846] Acknowledgements: Mr. G Coetzeefor assistance in preparing the manuscript. S. Afr. J. Enol. Vitic, Vol. 27, No. 2, 2006 123 124 Malic acid in wine ORGANIC ACIDS IN GRAPES tion of L-malic and L-tartaric acid. Following the lag phase, there The principal organic acids in grapes are L-tartaric and L-malic is a second period of 'berry growth' (Stage III). The entry into acid (see Table 1), accounting for more than 90% of the grape Stage III begins with the sudden onset of ripening or "veraison", berry's acid content (Boulton et al, 1996). Although L-malic and which generally starts between 6 to 8 weeks after flowering and L-tartaric acids have similar chemical structures, they are synthe- lasts for 35 to 55 days depending on the grape cultivar (Coombe, sised from glucose via different metabolic pathways in grape 1992; Pratelli et al, 2002; Ribereau-Gayon et al, 2000). berries. L-Malic acid is formed via glycolysis and the TCA cycle, Veraison, which seems to be a stress-associated process, is while ascorbic acid is the principle intermediary product of L-tar- characterised by several drastic physical and biochemical taric acid biosynthesis. Slight differences in grape acidity among changes in the grape berry (Coombe, 1992; Davies and Robinson, different grape varieties are usually found, affecting especially 2000). The second most significant biochemical change during the ratio between L-tartaric acid and L-malic acid in different veraison is the rapid reduction of grape berry acidity, which coin- grape cultivars (Kliewer et al, 1967). L-Tartaric acid is usually cides with the change in sugar composition of the grape berry. present in grapes at average concentrations of 5 to 10 g/L Grape berries respire actively during the early stages of growth, (Ruffner, 1982), while mature grapes contain between 2 and 6.5 but the intensity of respiration slows down as they advance in age. g/L L-malic acid (Boulton et al, 1996; Ribereau-Gayon et al., During veraison, the availability of the respiratory substrate, 2000). Excessive amounts of malic acid (15 to 16 g/L) may be sucrose (via photosynthesis), becomes limited due to the degra- present in grapes harvested during exceptionally cold summers in dation of chlorophyll. The berry is therefore forced to shift its the cool-climate viticultural regions of the world (Gallander, metabolism from sugar to L-malic acid respiration. Prior to the 1977). Although tartaric acid is often found at higher concentra- onset of veraison, L-malic acid is the most abundant organic acid tions than L-malic acid and is the stronger acid of the two, its con- (up to 25 g/L) in the grape berry vacuole, resulting in the low centration is relatively constant. It is the fluctuating concentration internal pH of 2.5 of grapes (Ruffner, 1982; Ribereau-Gayon et of L-malic acid that usually poses problems to winemakers al, 2000). With the onset of veraison, the L-malic acid concen- (Margalit, 1997). tration rapidly decreases to between 4 and 6.5 g/L, or even as low Evolution of L-malic acid during grape berry development as 1 to 2 g/L, with a concomitant increase in internal berry pH (pHof ca. 3.5). The development of the grape berry displays a double-sigmoidal growth pattern (Kanellis and Roubelakis-Angelakis, 1996), char- The biochemistry related to the accumulation and rapid respi- acterised by three successive phases: Phase I is the green or ration of L-malic acid in grapes has been studied in detail (see herbaceous stage immediately after flowering (see Fig. 1). The Fig. 2). L-Malic acid accumulates in the berry vacuole before berries are hard and green, and undergo a short period of cell divi- veraison (Stages I and II, see Fig. 2) via the collective activities sion and cell enlargement resulting in rapid expansion of the of the phosphoenolpyruvate carboxylase (PEPC) and malate berry (Kanellis and Roubelakis-Angelakis, 1996; Terrier et al., dehydrogenase (MDH) enzymes (Blanke andLenz, 1989; Diakou 2001). Characteristic of Stage I is the increase in vacuolar size of et al, 2000; Or et al, 2000). The cytosolic PEPC enzyme, well the grape berry cells due to the rapid storage of L-malic and L-tar- known for its photosynthetic role in C4- and CAM-plants, cataly- taric acid (Fillion et al, 1999; Pratelli et al, 2002). Stage II com- ses the (3-carboxylation of phosphoenolpyruvic acid to yield prises a short lag phase during which berry growth ceases and oxaloacetic acid and inorganic phosphate. The resulting berry acidity reaches a maximum due to the continued accumula- oxaloacetic acid is further reduced by the NAD-dependent malate TABLE 1 Organic acids present in grapes and wine (Boulton et al, 1996).

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