By Dominique Fink, MSc Biology, President OENOSCIENCE
In a previous article, we explored microbiological consequences under low sulphite conditions. We saw that Brettanomyces-type yeasts can thrive if sulpĥite levels are low and multiply greatly. However Brettanomyces yeasts are not the only microorganisms capable of causing alterations. This is what we will see in this short article.
Microorganisms involved
Acetic bacteria: These bacteria will develop upon contact with oxygen like during racking operations. They often form a film on the surface of cider or wine. This results in acetic spoilage. When in contact with oxygen, acetic bacteria will transform alcohol into acetic acid and acetaldehyde.
Lactic bacteria: Often present at the end of alcoholic fermentation, lactic bacteria can consume residual sugars resulting in acetic acid and lactic acid which causes lactic spoilage.
Saccharomyces-type yeasts: These yeasts are responsible for the alcoholic fermentation. If an issue occurs during filtration or bottling operations, in-bottle refermentation can occur if residual sugars are still present.
Brettanomyces-type yeasts: Even is absence of residual sugars, these yeasts can stay viable and in a latent state after bottling. They are responsible for production of volatile phenols which can appear long after bottling, especially under low sulphite conditions.
Other wild yeasts: Yeasts like Candida sp, Pichia sp, or Hansenulaspora sp can cause spoilage by oxidizing ethanol in acetaldehyde and acetic acid. The result will be a loss of ethanol and an increase in volatile acidity.
Molds: Molds can only develop in presence of oxygen. However, they can persist in a latent state well after botlling, even in a low oxygen environment. They will produce off-compounds that will affect aromas significantly.
Pre-Bottling Microbiology Testing
We often talk of prevention and microbiological testing is a very good example of “better safe than sorry”. A microbiological test before bottling cost a few tens of dollars whereas fixing a problem after bottling will cost a lot more. Microbiological tests are therefore a very good investment as they will prevent you from loosing money if problems arise.
Microbiology is a very powerful tool to ensure that a product is stable or to assess if a contamination occurred during operations. For example, wines and ciders with residual sugars and L-malic acid are especially vulnerable to aroma defects or refermentation if bacteria and yeasts are present. A pre-bottling microbiological test will:
Help verify the filtration efficiency or the need for a filtration. If no viable microorganisms are present, perhaps a filtration step is not needed.
Help detect the presence of Brettanomyces yeast before bottling hence prevent an increase in volatile phenols.
Confirm the absence of lactic bacteria. If detected, these bacteria could perform a undesirable malo-lactic fermentation if levels of L-malic acid are present.
Help avoid an increase in volatile acidity.
Help adjust conditions for sulphiting, filtration and bottling.
If microorganisms are detected prior bottling, it is the best moment to intervene and implement a stabilization strategy such as sulphiting, filtration or pasteurization. Your oenologist is the best professional to consult.
Post-Bottling Microbiological Testing
The bottling step is a crucial moment in production. Even with every precautions, a contamination may occur on the bottling line. Post-bottling microbiological tests are useful for:
- Identifying re-fermentation risks.
- Verifying if a problem occurred with filling nozzles or consumables (corks, bottles...)
- Reassuring your distributors and your clients.
- Documenting your lots and those of your suppliers.
- Reassuring your external bottling service provider that your wine is stable prior operations.
When microorganisms are detected post-bottling, a risk evaluation must be done depending on the type of microorganism detected.
Case Studies
If the wine contains residual sugars, there is a risk of refermentation in the bottle 25 / L of glucose-fructose will generate a pressure of 8 bars. Therefore if glucose-fructose is measured at 5 g/L, a pressure of 1.6 bars could be produced. The question to ask is what is the risk for bottle explosion?
Risk is minimal because yeasts don’t have any substrate to consume.
If the wine has a low level of sulphites or has not been sulphited and contains residual sugars, there is a high risk of lactic spoilage. Bacteria will consume sugars and L-malic acid (in a white wine). Volatile acidity will increase as well as L-lactic acid. In a red wine, the risk is low unless citric acid has been added prior to bottling. Citric acid may become a substrate for lactic bacteria and it will be converted to acetic acid.
An increase in volatile phenols constitute an important risk. Brettanomyces yeasts can persist in a dormant state in a dry red wine and give off-flavors. The problem will worsen over time.
What Are The Solutions?
The oenologist is the professional to contact when a microbial contamination occurs. Depending on results from the Laboratory, several treatments are available to the winemaker.
Fining Agent : Novel agents like chitosan or chitin-derived compounds as well as many bentonites are efficient in reducing the amount of microbes through physical interactions.
Sterile Filtration : Corse filtration followed by a final filtration using a 0.45 micro cartridge is generally recommended for efficient stabilization. The oenologist’s expertise is well suited for this operation.
Enzymes : Microorganisms have a membrane composed of proteins and glycans (peptido-glycans). Specific enzymes can be used to degrade these structures which will lead to the elimination of the microorganisms.
To Conclude
Un-corking thousands of bottles costs time and money. Microbiological tests have great value because of their low cost and high benefits and therefore, constitute a great investment. The current trend in the industry is using lower sulphites levels at every steps. SO2 is an excellent preservative and so using less sulphites comes with consequences. A well-designed strategy to monitor microbial activity is an excellent way to prevent spoilage and off-flavors. A professional oenologist combined with an accredited laboratory are your best partners for producing high quality wines. Contact us at 514-564-2050. We are happy to help you.
