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This article summarises the “dark side” of microbial transformation that can only be prevented by the watchful eyes, and if needed, interventionist actions of winemakers.

 

Bacterial spoilage

Oenococcus utilises malic acid to produce lactic acid, making it the preferred bacteria for malolactic fermentation. However, it can also utilise residual glucose and fructose resulting in the production of alcohol, lactic acid and other metabolites. Oenococcus is, therefore, only considered a microbiological risk in bottled wines containing a residual malic acid of >0.2 g/L or a combined glucose fructose level of >1 g/L. Re-fermentation will result in the production of small amounts of CO2 and a granular sediment. Oenococcus is sensitive to free molecular SO2 levels of 0.8 mg/L and is inactivated by the addition of lysozyme.

 

Lactobacillus in wines with residual sugars can lead to the production of acetic acid (VA) and other off-odours and flavours. These off-odours and flavours include:

  • Acrolein: Wines containing acrolein are known to be excessively bitter. Degradation of glycerol results in the formation of acrolein, which further reacts with the phenolics in wine producing extremely bitter compounds.
  • Diacetyl: Diacetyl at the correct concentrations is not considered a fault, but contributes positively to the buttery, nutty character of the wine. However, at high concentrations wines have an excessive buttery and lactic aroma. Diacetyl is produced from citric acid by the Lactobacillus populations present.
  • Mannitol: Certain strains of Lactobacillus may produce mannitol from fructose resulting in viscous, slimy wines having aromas of acetate, vinegar, excess lactic, buttery, or diacetyl aromas.
  • Mousiness: In the presence of ethanol, Lactobacillus can produce the compounds 2- ethyltetrahydropyridine, 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine, the compounds associated with strong mouse-cage, popcorn and basmati/jasmine-rice characters. As these compounds are not very volatile, they will be more prominent on the palate than the nose.
  • “Geranium” taint: Certain strains of Lactobacillus can reduce sorbic acid, a preservative added to wine to inhibit the fermentation activity of Saccharomyces, to the compound 2- ethoxyhexa-3,5-diene. The production of this compound results in a wine with a strong aroma of crushed geranium leaves.Certain strains of Lactobacillus are known to utilise tartaric acid (generally regarded as microbiologically stable) contributing to the production of acetic acid, lactic acid and CO2. Furthermore, Lactobacillus growth in bottles may cause turbidity and a granular sediment. Residual malic acid can be utilised by Lactobacillus to produce lactic acid, CO2 and in a few cases have even led to crystal formation. Crystal formation may occur within bottles as the Lactobacillus ferment malic acid, resulting in an increase in pH impacting the tartrate stability of the wine. Growth is encouraged in wines with high pH levels (pH >3.60). Lactobacillus is sensitive to free molecular SO2 levels of 0.8 mg/L and above, and most strains are inactivated by the addition of lysozyme. Lactobacillus is removed by sterile filtration (0.45-micron).

 

Pediococcus can use any of the sugars (including the un-fermentable pentoses) and malic acid to produce CO2, a granular sediment and adverse flavours and aromas; such as excessive amounts of diacetyl and acrolein. Diacetyl is produced from citric acid and depending on the concentrations, can either positively alter the wines by giving them a buttery, nutty character or when present in high concentrations, results in an excessive buttery and lactic aroma. Acrolein, on the other hand, is formed when Pediococcus metabolises glycerol and further reacts with the phenolics in wine, forming an extremely bitter compound. Acrolein formation may occur after fermentation has completed. Pediococcus growth is encouraged in wines having a high pH (pH >3.60). Pediococcus is sensitive to free molecular SO2 levels of 0.8 mg/L and above, and most strains will be inactivated by the addition of lysozyme. Certain strains of Pediococcus, usually Pediococcus strains resistant to SO2, can cause ropiness, resulting in highly viscous, oily wines that become increasingly viscous when agitated or moved. Ropiness may be developed either during fermentation, maturation and may even be formed in the bottle. Pediococcus is removed by sterile filtration (0.45-micron).

Acetobacter requires oxygen for growth and may continue to grow in even very low oxygen environments, such as oak barrels. Acetobacter utilises sugar to produce acetic acid (VA) and ethanol to produce acetaldehyde and ethyl acetate. Acetaldehyde will give the wine a typical “sherry” aroma (bruised apple, nutty and straw-like), while ethyl acetate results in an acetone aroma associated with paint thinners or nail varnish remover. Acetobacter cannot grow in the oxygen-poor environments of full storage vessels and is only capable of growth in bottled wines until oxygen levels are depleted. In half-full tanks Acetobacter can grow on the wine surface resulting in the production of volatile aromas capable of contaminating the wine. Like in half-full tanks, in bottled wine Acetobacter can grow on the surface, or cling to the sides of the bottle near the surface. Once the microbes have died off and the film is disturbed, a granular sediment may form. Growth of Acetobacter can be prevented by keeping wine storage containers full and following strict barrel topping regimes.

 

Yeast spoilage

Saccharomyces utilises glucose and fructose to produce alcohol and CO2. Wines having residual glucose/fructose levels of more than 1 g/L are at risk of re-fermentation by Saccharomyces. Re-fermentation will result in turbidity, sediment in the form of a yeast lees deposit and the production of CO2, sometimes to the point of the cork being expelled from the bottle. Dry wines (wines with no residual glucose and fructose) are microbiologically stable with respect to Saccharomyces growth.

Zygosaccharomyces, a fermentation yeast, utilises residual glucose and fructose causing turbidity, a sediment in the form of a yeast lees deposit, and the production of CO2, sometimes to the point of the cork being expelled from the bottle. However, Zygosaccharomyces is also capable of growth without fermenting the wine, resulting in no alcohol or CO2 production. In these cases, Zygosaccharomyces utilises sorbic acid as a growth substrate resulting in the formation of a large granular sediment. Zygosaccharomyces is particularly a problem in wineries that sweeten wines with grape concentrate at bottling and once contamination of the bottling line has occurred, it is very difficult to eliminate. Zygosaccharomyces is resistant to free molecular SO2 levels of up to 3 mg/L. Dry wines (wines with no residual glucose and fructose) are microbiologically stable with respect to Zygosaccharomyces growth.

Viable Brettanomyces populations may produce 4-EP and 4-EG, the compounds associated with Brettanomyces spoilage. Brettanomyces growth may continue in wines with glucose + fructose levels of as little as 0.2 g/L. Various other wine components can be utilised by Brettanomyces as a carbon source, such as non-fermentable sugars, glycerol, ethanol, malic acid and cellobiose from barrels. When present in higher populations, Brettanomyces growth may cause an increased turbidity and a slight granular sediment. Brettanomyces growth is inhibited by free molecular SO2 levels of 0.4 – 0.8 mg/L and suppressed by higher alcohol levels (>15%). Brettanomyces populations can be reduced by egg white fining followed by racking, or completely removed by filtration (0.65-micron).

Common surface film yeasts in wine include yeasts of the genera Candida, Pichia and Hansenula. Saccharomyces and Brettanomyces are also capable of forming surface films. Alcohol in the wine is utilised by surface film yeasts to produce acetaldehyde and other off-flavours. The presence of acetaldehyde in wine will result in a “sherry-like” aroma, described as bruised apple, nutty and straw-like. Surface film yeasts are aerobic and can be observed as a film on the wine. Growth is, therefore, only possible in bottled wines until all oxygen is depleted and can either be observed as a film at the surface of the wine or cling to the sides of the bottle near the surface. When this film is disturbed, a granular sediment may result. Good winery sanitation is critical in preventing the growth of surface film yeasts.

 

Non-wine microbes and moulds

Non-wine microbes, both yeast and bacteria, are incapable of growing in wine due to the low pH, alcoholic environment present. Non-wine microbes may be found in the cellar, however, will not continue to grow in bottled wines and will, therefore, not cause the wine to deteriorate.

Moulds are alcohol-intolerant, aerobic microbes and will not grow in wine. However, mould growth in the vineyard or cellar may indirectly affect the wine through the production of off-flavours and aromas.

 

Abstract

It is said that wine is the microbial transformation of grape juice. However, as pleasant as the end product might be if things go right, the microbial transformation can occasionally go very wrong.

 

– For more information, contact Caitlyn McCartney at Vinlab – micro@vinlab.com.

 

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