Sources of acetic and other fatty acids and their role in sluggish and stuck red wine fermentations

[摘要] ENGLISH ABSTRACT:The quality of wine is influenced by numerous factors. These factors include the quality ofthe grapes, winemaking techniques and quality control throughout the winemakingprocess. It is thus very important that any process leading to the lowering of the quality ofthe wine be prevented. Evidence in the wine industry shows that bacterial spoilage is stillvery much a common problem in many wineries. The spoilage of wine by bacteria canlead to amongst other problems, elevated volatile acidity levels, of which only a certainconcentration limit in wine is permitted. Usually more than 90% of the volatile acidity ofwine consists of acetic acid. Different yeast strains, heterofermentative lactic acid bacteriaand acetic acid bacteria (which can all be spoilage microorganisms) can produce aceticacid in high concentrations. It is thus important to be able to prevent the formation of thisacid by controling the unwanted growth of these spoilage microorganisms. Acetic acid andother medium chain fatty acids, octanoic- and decanoic acid, can also lead to stuck orsluggish fermentations. A stuck or sluggish fermentation can also lead to wine spoilage,due to sugar remaining in the fermentation which can be utilized by spoilagemicroorganisms. Acetic- and other fatty acids enter the yeast cell by passive diffussionand releases its proton in the cytoplasm, thereby acidifying the cytoplasm and inhibitingsome enzymes. These acids can also work synergistically with ethanol and its inhibitoryeffect is also dependent on the temperature. Yeast strains can also differ in theirresistance to acetic and other medium chain fatty acids and these acids can also influencethe growth of lactic acid bacteria.How acetic acid bacteria influence the winemaking process and the used measures tokeep these bacteria from spoiling wine have been the subject of very little attention in thepast. This was due to the belief that the anaerobic conditions prevailing in wine and theuse of sulfur dioxide are enough to control these bacteria, since acetic acid bacteria werealways described as being strictly aerobic microorganisms. Recently, some evidenceshowed that acetic acid bacteria can survive and even overcome the limits that thewinemaking process places on its growth. These bacteria are also known to inhibit theyeasts growth and fermentation ability due to the production of acetic acid and otherfactors. A research programme on the origin of volatile acidity in South African wines hadbeen initiated at the Department of Viticulture and Enology and at the Institute for WineBiotechnology at the University of Stellenbosch after increases in volatile acidity indifferent South African wines had been reported. This spurred us to investigate theoccurrence of acetic acid bacteria in South African red wine fermentations, which formspart of this study, and to identify the dominant acetic acid bacterial strains. The sulfurdioxide resistance of five representative strains were also determined, as well as the effectof metabolites which were produced by these bacteria on yeast growth and fermentationability.Our results indicate that acetic acid bacteria can occur in high concentrations in thefresh must and during alcoholic fermentation. In the 1998 harvesting season acetic acidbacteria occurred at 106-107 cfu per ml in the fresh must. In 1999 these numbers were104-105 cfu/ml. Acetic acid bacteria numbers decreased in 1998 to 102-103 cfulml duringfermentation. The survival of these bacteria in 1999 correlated with the pH of the must, aswell as sulfur dioxide dosages in the must. In must with a low pH and higher sulfur dioxidethe number of acetic acid bacterial numbers decreased more drastically than in the highpH, low sulfur dioxide musts. This was also true for acetic acid bacterial counts duringcold soaking of musts, with the number of acetic acid bacteria increasing during the coldsoaking period in musts with a high pH. In musts with a low pH and higher S02 dosagesacetic acid bacterial counts did not, however, increase during cold soaking.Gluconobacter oxydans dominated in the fresh must with Acetobacter liquefaciens andespecially Acetobacter pasteurianus dominating during the fermentation. Differentbiochemical and physiological tests revealed that 52% of the 115 isolates tested belong toA. pasteurianus. The high occurrence of A. liquefaciens with A. pasteurianus duringfermentation showed that the dominant acetic acid bacterial species in South Africadiffered from reports from other wine producing countries. The sulfur dioxide resistance ofthe acetic acid bacteria tested also differed in white grape juice, with a molecular sulfurdioxide concentration of 0.64 mg/I being necessary to eliminate all the acetic acid bacterialstrains tested. The A. hansenii strain was found to be the most resistant to sulfur dioxideand G. oxydans the least resistant. The latter strain was eliminated by only 0.05 mg/Imolecular sulfur dioxide, while A. hansenii was only eliminated by 0.64 mg/I molecularsulfur dioxide. The A. pasteurianus, A. liquefaciens and A. aceti strains tested displayedvarying degrees of resistance to sulfur dioxide. The volatile acidity produced by thesebacteria profoundly influenced the growth and fermentation ability of yeast, which led toslow/stuck fermentation. The A. hansenii and A. pasteurianus strains produced the mostvolatile acidity in grape juice, with up to 4.02 g/I for A. hansenii within 4 days, which led toa stuck alcoholic fermentation. This was, however, prevented by inhibiting or eliminatingthe acetic acid bacteria with sufficient sulfur dioxide additions prior to yeast inoculation.Compounds produced by acetic acid bacteria can also influence wine quality. Certainorganic acids were produced and metabolized by acetic acid bacteria, as well as acetoin.We could not, however, detect any other fatty acids that are inhibitory to yeast (producedby these bacteria).This study clearly showed that acetic acid bacteria could occur during fermentationand that certain winemaking techniques, like the maintenance of a low pH in the must andsulfur dioxide additions can influence the growth and survival of acetic acid bacteria.Acetic acid bacteria also influence both the winemaking process by inhibiting yeast as wellas the quality of the wine by producing acetic acid and/or other compounds. This studyalso shed some light on the occurrence of acetic acid bacterial species in the SouthAfrican context and could be important in assisting the winemaker, as well as the scientificreseacher, in finding ways to inhibit acetic acid bacteria in the ongoing battle against thesespoilage microorganisms of wine.

[发布机构] Stellenbosch University