Avoiding microbiological instabilities

The avoidance of microbiological spoilage problems during winemaking involves attention to a number of factors, many of them basic winemaking parameters. It is also important to remember that many of these factors are inter-related, and a small change in one may have magnified effects in others. This means that addressing all of these factors in combination, rather than just one or two, will have the greatest likelihood of success in preventing microbiological spoilage problems.

  • SO2 usage: The addition of some SO2 at the crusher is highly recommended as a very simple way of dramatically reducing the likelihood of microbiological spoilage in wine. Approximately 40 mg/L SO2 should be added for healthy red fruit, and approximately 60 mg/L SO2 should be added for healthy white fruit. If the fruit is damaged, more SO2 is required. SO2 usage throughout the winemaking process is critical in controlling microbial growth and preventing spoilage. A detailed article on this subject was published in the August 2003 edition of Technical Review, and is reproduced here.
  • Wine pH: There has been a trend with some winemakers to produce higher pH wines to achieve perceived benefits in mouthfeel. However, high pH conditions favour the growth of spoilage microorganisms, including Dekkera/Brettanomyces yeast, and could result in negative sensory effects that greatly outweigh any small mouthfeel benefits. The ideal pH range for dry red table wines is 3.3 to 3.6 and for dry white table wines is 3.0 to 3.4 (Rankine, 1989). Insufficient acid addition to fruit and large pH increases during malolactic fermentation (MLF) can contribute to a higher than ideal pH that will favour microbial growth. Additionally, at a higher pH, more SO2 is required to provide effective protection against microbial spoilage and oxidation (see the SO2 article for more detail).
  • Presence of residual sugar and/or malic acid: Residual sugar or malic acid in wine can provide potential substrates for microbial growth. It is recommended that enzymatic analysis be carried out to determine the concentration of sugar and malic acid in wine, as other simpler methods do not give accurate results at low levels. Industry services staff members are often asked what concentration of sugar or malic acid constitutes the wine being ‘dry’ or the MLF being ‘complete’. It is our experience that any measurable level of these nutrients can have the potential to act as a substrate for microbial growth. Winemakers should therefore be aiming to achieve concentrations below the detection limits for these nutrients, by enzymatic assay. These levels are 0.1 g/L for Glucose + Fructose, and 0.05 g/L for malic acid. One technique for reducing residual sugar that has been tried successfully in some wineries, is to keep fermentations warm during pressing, and for between 12 and 24 hours afterwards. This allows the fermentation to continue, rather than being stopped dead by a sudden drop in temperature.
  • Inadequate clarification or filtration: A trend is evident in Australian winemaking at present towards bottling wines with minimal filtration. The trend seems to be driven by concerns about the sensory impact on wine of filtration, but is also contributing to the occurrence of microbiological spoilage problems in bottle. It is the AWRI’s position that a well-performed filtration of the appropriate grade will not have a negative effect on wine quality, but that bottling without filtration poses a considerable risk of spoilage, with an associated significant drop in wine quality. At the very least, before bottling a hazy wine, it is imperative to determine what is causing the haze, rather than just assuming that it is something benign. Additionally, high turbidity will reduce the effectiveness of SO2 in wine, a factor that is discussed further in the SO2 article.
  • Timing of racking post-malolactic fermentation: On a related subject, studies have shown the importance of the timing of racking post-MLF. Chatonnet et al. (1995) found that when racking was delayed by approximately six weeks, the concentration of 4-ethylphenol (a spoilage compound formed by Dekkera/Brettanomyces yeast) in each of several barrels of a particular wine approximately doubled, and the percentage of barrels considered to be tainted with 4-ethylphenol rose from 57% to 100%. Chatonnet et al. (1995) considered that the concentration of free SO2 remaining in the wine after the delay in racking was too low to sufficiently protect the wine. Thus, if racking is delayed free SO2 may be bound up, Dekkera/Brettanomyces (and other spoilage microorganisms) may proliferate and the concentration of 4-ethylphenol and other spoilage compounds may increase markedly. It is advised that winemakers clarify red wines and add SO2 as soon as possible after MLF. It is also considered advantageous to identify at the earliest time point possible the completion of MLF, to allow unwanted microoganisms as little time as possible to proliferate prior to clarification and the addition of SO2 taking place. The SO2 added after MLF should be the single largest addition that the wine receives.

 

References

Chatonnet, P. Dubourdieu, D. Boidron, J.N. The influence of Brettanomyces/Dekkera sp. yeasts and lactic acid bacteria on the ethylphenol content of red wines. American Journal of Enology and Viticulture 46(4): 463-468; 1995.

Rankine, B. Making good wine : a manual of winemaking practice for Australia and New Zealand. Sun Books, South Melbourne (Macmillan Australia) xvii, 374 p. 1989.

Robinson, E. Godden, P. Revisiting sulfur dioxide use. Technical Review (145) : 7-11; 2003.