Many micro-organisms prefer pHs higher than 4 and lower pH wines consequently have more resistance against spoilage. Wines with pH lower than 3.5 are preferred, because the percentage active molecular form of sulphur dioxide is also higher at a lower pH (see Table 1).
Irrespective of the form of sugar, whether it is glucose, fructose or sucrose, the presence thereof in wine can be problematic. The concentration of sugar in wine will determine which micro-organisms can metabolise it. Concentrations higher than 0.2 g/ℓ can promote the growth of Brettanomyces bruxellensis. This species can proliferate under the aerobic conditions of bottled wines and off-flavours like 4-ethylphenol and 4-ethylguajacol can be formed. Zygocaccharomyces species can cause problems at high alcohol concentrations. It is also resistant against preservatives like sulphur dioxide. Zygosaccharomyces rouxii is particularly osmotolerant and can cause fermentation in grape juice concentrate. Zygosaccharomyces bailli is resistant against preservatives and often a contaminant of food and beverages.
The process stage in wine cellars also plays an important role in spoilage risks, because conditions differ during the different processes. During the initial stage of alcoholic fermentation, most micro-organisms can proliferate as result of the high sugar and low alcohol concentrations. Spontaneous fermentation with Saccharomyces or non-Saccharomyces species can impart desirable sensory characteristics to the wine and also save costs, but if it is not managed properly high volatile acid and aldehyde concentrations can be formed. The formation of the undesired Brettanomyces flavours usually takes place during the latter stages of winemaking, because it can resist high alcohol and sulphur dioxide concentrations and is also a slow growing yeast. Although Pichia species can also form these undesirable flavours, it can fortunately not grow at alcohol concentrations above 6% A/V. The use of sulphur dioxide in the vineyard and prior to alcoholic fermentation can eliminate many of the non-Saccharomyces species. Stuck fermentations can be problematic, because residual sugar can, in the absence of sulphur dioxide, be metabolised by Acetobacter species and lactic acid bacteria to form significant concentrations of volatile acid.
After the completion of alcoholic fermentation and MLF, Brettanomyces bruxellensis, Acetobacter species and to a lesser extent lactic acid bacteria are the sources of potential spoilage. It is unknown at which stage the first-mentioned forms the specific off-flavours and oak barrels are frequently a source of contamination. Sterile filtration followed by the addition of sufficient sulphur dioxide should prevent potential Brettanomyces spoilage. A molecular sulphur dioxide concentration of 0.825 mg/ℓ is recommended by the AWRI (Australian Wine Research Institute) to prevent the growth of undesirable micro-organisms (see Table 1). Seeing that Acetobacter is aerobic, its spoilage can be limited by keeping containers on ullage and maintaining a sufficient sulphur dioxide concentration. Lactic acid bacteria can be problematic if MLF is not completed and it happens spontaneously in containers or after bottling. It is consequently important that the MLF status of wines is monitored regularly, in order to implement necessary actions to prevent or facilitate it.
Bottling is the last stage in the production process and it is important to differentiate between different wine types. Low alcohol wines with a high sugar concentration and a low concentration of anti-microbial agent or preservative have a high spoilage risk and except for inline sterile filtration sufficient sulphur dioxide levels must be maintained and oxygen uptake be limited. It is also essential that the micro status of wines are monitored regularly during storage (Seabrook, 2015).
Seabrook, Alana, 2015. Minimising the risk of wine spoilage. Wine & Viticulture Journal, November/December 2015: 30 – 32.