A previous review article dealt with the microbes that occur in wine. Bacteria constitute one group of micro-organisms that play a role in vinification. Their widespread occurrence impacts on wine quality. This impact may be positive or negative, therefore it is important for winemakers to heed their presence and be able to identify and rectify potential problems, if required.
Bacteria are common in nature. One gram of soil contains 40 million bacterial cells, 1 million cells occur in a millilitre of fresh water and they also play an important role in various industry processes or the production of products such as yoghurt, cheese, chutney, sauerkraut, cacao, coffee, vinegar and wine. Considering that a 1 000 times magnification is required to see them, bacteria may be described as an invisible friend or foe of the wine industry. Pasteur confirmed the presence of bacteria in fermenting grape juice 150 years ago, but it was only in the 1960s that the most important bacteria responsible for malolactic fermentation (MLF) were initially classified as Leuconostoc oenos and later as Oenococcus oeni. The positive role of MLF is not only the conversion of malic acid to lactic acid, which removes a carbon source of possible bacterial spoilage, thereby creating microbiological stability in the wine, but moreover it may impart flavours described as buttery, fruity, berry-like, floral, fusel, rancid or cheesy. The kind of flavour and its concentration in the wine are determined by the particular bacterial strain and the stage of MLF in the wine. Unfortunately winemakers also have to contend with undesirable bacteria that occur in wine. Undesirable bacteria may occur at different stages of the vinification process (Bartowsky et al, 2009).
Various species of bacteria occur on grapes and in grape juice. Of the acetic acid bacteria the most prevalent on healthy grapes is Gluconobacter oxydans. Damaged, unhealthy grapes or grapes infected with Botrytis cinerea contain high populations of Acetobacter aceti and Acetobacter pasteurianus. Various lactic acid bacteria may also occur on damaged or unhealthy grapes. To control the spoilage microbe numbers at that stage, it is recommended to keep grapes cool, maintain the pH-levels of the juice below 3.7, add approximately 50 mg/l SO2 to the crush of the grapes and 100 to 200 mg/l lysozyme, if the cellar has a history of lactic bacteria spoilage. As soon as alcoholic fermentation starts, the population of acetic acid bacteria decreases considerably. Some Acetobacter species may still occur, but Gluconobacter species cannot survive. If alcoholic ferments are prolonged or get stuck and the formation of CO2 starts to decrease, making oxygen readily available again, acetic acid bacteria may grow again and stimulate the formation of acetic acid. During a typical alcoholic fermentation the population of lactic acid bacteria will decrease. The lactic acid bacteria Oenococcus oeni are able to survive alcoholic fermentation, however, because they are able to resist higher alcohol contents and pH-values below 3.5. Although lactic acid bacteria are usually unable to survive the alcoholic fermentation, the following factors or practices may promote their survival and cause prolonged or stuck alcoholic ferments:
- SO2 is added to the grape juice at low dosages or not at all.
- The pH of the juice is so high (above 3.8) that free SO2 does not exist and consequently negates the impact of SO2.
- A high initial bacteria count.
When prolonged or stuck alcohol ferments occur, lactic acid bacteria are able to cause a considerable increase in the volatile acid content of young wines. Following the completion of normal alcoholic fermentations, the desirable Oenococcus oeni is usually the dominant bacterial species, although the undesirable Lactobacillus and Pediococcus species, which may form undesirable compounds such as acetic acid, diacethyl, acrolein and biogene amines, are also able to conduct the MLF if pH-values above 3.7 occur in the wine. After completion of MLF various bacterial species may grow and cause spoilage. Acetobacter species are able to multiply rapidly in the young wine if oxygen uptakes in the wine are promoted by oxidative racking, low humidity in barrel cellars and the occurrence of oxygen spaces in containers. Acetobacter pasteurianus may even survive for long periods under relatively anaerobic conditions. After completion of the alcoholic fermentation and MLF the undesirable Lactobacillus and Pediococcus lactic acid species may form various spoilage components in wines. It is therefore essential to apply the following standard practices after alcoholic fermentation and MLF.
- The routine monitoring of wines for spoilage microbes.
- The racking of wines from the thick lees.
- The filtration of wines if undesirable bacteria occur or if the cellar has a history of bacterial spoilage.
- Maintaining humidity levels above 80% in the barrel cellar(s).
- Keeping containers full.
- The methodical handling of empty barrels by filling them with SO2 gas, a solution of SO2 and citric acid or wine.
- Maintaining oxygen levels below 1.0 mg/l during wine storage.
- Maintaining wine storage temperatures below 16C.
- Maintaining pH-levels below 3.7 in wines.
- Maintaining molecular SO2 levels above 0.5 mg/l.
Bottling is the final stage in the vinification chain and it is essential to ensure microbiological stability in the bottle. The microbiological contamination of bottled wines is especially promoted by:
- Bottling unfiltered microbiologically unstable wine.
- Failure of the membrane filtration of a microbiologically unstable wine during the bottling process.
- Insufficient application dosage of a preservative, if applicable, before bottling.
- Contamination caused by unsanitary bottling.
The anaerobic environment of bottled wines usually prevents the growth of Acetobacter species, but they may survive and cause spoilage when the bottle is opened and exposed to oxygen. Lactic acid bacteria require very little oxygen for growth and may therefore survive and grow in bottled wines if nutrients such as sugar, malic acid and citric acid occur in the bottled wine. The spoilage that may occur in such instance(s), is a secondary fermentation that accompanies the development of CO2 (Tracy, 2009).
In addition to the above-mentioned traditional standard practices that may be applied to prevent bacterial spoilage in wines, new techniques such as ultra high pressure, high energy ultrasound, ultraviolet radiation and pulsating electrical fields are currently receiving attention (Bartowsky et al, 2009).
Bartowsky, E.J., Costello, P.J., Abrahamse, C.E., McCarthy, J.M., Chambers, P.J., Herderich, M.J. & Pretorius, I.S. 2009. Wine bacteria friends and foes. Wine Industry Journal 24(2): 14 – 16. This article is also available on the website www.winebiz.com.au.
Tracy, R. 2009. Spoilage microbe population fluctuations during winemaking causes, effects, solutions. Practical Winery & Vineyard 30(1): 94 – 97.