Succinic acid production by wine yeasts

[摘要] ENGLISH ABSTRACT: One of the most striking qualities of wine is its tart, sour taste. The sensory perceptionof sourness is mainly attributed to the presence of hydrogen ions (protons) at highconcentrations. Large amounts of weak carboxylic acids (organic acids) are the mainsources of these ions within wine. Once wine enters a person's mouth, the dissociableprotons of the weak organic acids within wine are partially neutralized or, in other words,titrated by the saliva secreted inside one's mouth. This explains why the duration andintensity of a wine's sourness is related to its titratable acidity content. The sour taste ofwine is usually considered refreshing and it helps balance wine flavour. In fact, winesbecome watery when its titratable acidity content is too low.After alcoholic fermentation, the titratable acidity of wine will usually be less thanthat of the grape juice from which was made due to ethanol-induced precipitation ofpotassium bitartrate crystals and partial consumption of malic acid by fermentingwine yeasts. Occasionally however, increases in titratable acidity are observed duringalcoholic fermentation. If wine is produced from grape juice with optimum levels oftitratable acidity, unforeseen increases in titratable acidity during alcoholic fermentationcan be detrimental to the quality of the final product.Although the net production of malic acid by wine yeasts contributes to increases intitratable acidity seen during grape juice fermentations, the production of succinic acid isregarded as the primary contributor. In fact, succinic acid accounts for approximately90% of the non-volatile acids produced during fermentation of grape juice. Between 0.5and 1.5 g/L succinic acid is normally found in wine, but higher concentrations thereof(up to 3.0 g/L) have been detected within certain red wines.Acidity adjustments should preferably be carried out before the onset of alcoholicfermentation to allow better integration of the added compound(s) and to ensure thatconditions during fermentation favour the quality and microbial stability of the finalproduct. In doing so unfortunately, winemakers run the risk of ending up with wines thatmay taste too sour if they are unable to accurately predict and take into considerationthe amount of succinic acid produced during alcoholic fermentation. Knowledge withregard to the factors involved in succinic acid's production by fermenting wine yeasts istherefore required in order to manage the titratable acidity of wines more accurately.Ever since Louis Pasteur first noticed succinic acid amongst the by-products ofalcoholic fermentation, attempts have been made to determine the metabolic pathwaysand factors involved in its production by fermenting wine yeasts. Up until now however,it remains unclear why wines sometimes end up with exceptionally high levels ofsuccinic acid.For these reasons it was decided to investigate the possible causes of very highsuccinic acid concentrations within wine. Due to complexity of grape juice's chemicalcomposition and the problems associated with sterilizing grape juice, fermentationexperiments were conducted within a chemically defined grape juice-like medium. Succinic acid production by nine different industrial wine yeast strains was studiedunder various conditions with regard to the nutrient status of the synthetic grape juice,temperature and availability of molecular oxygen during alcoholic fermentation.The amount of succinic acid produced during alcoholic fermentation was found todepend on the yeast strain, fermentation temperature and chemical composition of thesynthetic grape juice. Out of the nine commercial yeast strains selected for this study,strain WE372 produced the largest amount of succinic acid in synthetic grape juice at28°C. Strain WE372 produced significantly smaller amounts of acetic acid than theother yeast strains of this study and very little acetic acid at 28°C, which indicated thatstrain WE372 may have less acetaldehyde dehydroganase activity than the other yeaststrains of this study under the conditions tested. The effect this has on NAD: NADHbalance is the probable cause for its ability to form more glycerol, succinic and malicacid than the other strains.Results from our study show that succinic acid production is influenced primarily by themetabolizable fraction of YAN, which we termed metabolically available nitrogen (MAN).Succinic acid production by fermenting yeasts will be favoured by moderate to highfermentation temperatures (20°C to 28°C) in grape juice with a nicotinic acid and/ ornicotinamide deficiency, high sugar content (200 g/L to 240 g/L), moderate amounts ofmetabolically available nitrogen (300 ± 50 mg/L MAN), the presence of flavonoids andlarge supplies of unsaturated long-chain fatty acids. Even higher concentrations ofsuccinic acid were produced when oxygen was made available to fermenting yeasts byaerating the fermenting grape juice. Fermentation temperatures below 18°C, too muchmetabolizable nitrogen (> 450 mg/L MAN), very high concentrations of fermentablesugar (> 240 g/L), lipid deficiencies and a lack of pantothenic acid, thiamine, biotin orpyridoxine will decrease the amount of succinic acid produced fermenting yeasts.

[发布机构] Stellenbosch University