Proper sanitation practices in wine cellars prevent the development of undesirable micro-organisms. Although the alcohol concentration of wines limits the microbial spoilage of it, in comparison with fruit juices and food, certain harmful micro-organisms can cause considerable sensory problems in wines. During the winemaking process favourable conditions must also be created for certain micro-organisms, without also favouring the undesirable micro-organisms.

Micro-organisms occur everywhere in a cellar and can mainly be divided in three groups, namely moulds, bacteria and yeasts. Sources of the different micro-organisms include the grapes, harvesting equipment, cellar equipment and tools, hoses, barrels, sampling equipment, corks and bottling lines. The mould Botrytis cinerea which occurs in rotten grapes, can cause premature oxidation in grape juice, but can also as noble rot contribute to the making of noble late harvest wines.

The most known bacteria are acetic acid bacteria and lactic acid bacteria. Acetic acid bacteria like Acetobacter can cause high volatile acid concentrations. Lactic acid bacteria (LAB) include Oenococcus, Pediococcus and Lactobacillus and can convert malic acid to lactic acid during malolactic fermentation (MLF). Lactobacillus can however also cause a mousy flavour in wine, which cannot be removed easily. Pediococcus can cause ropiness in wine which will spoil it visually. Diacetyl, which has a buttery flavour, can in excess lead to a rancid character and unhealthy biogenic amines with off-flavours, can also be formed during MLF.

The most-used yeast is Saccharomyces species, although some winemakers prefer spontaneous fermentation to create unique characteristics in their wines. The results of the spontaneous yeasts are however unknown and can also deplete the nutrients in the juice, which can limit the fermentation of Saccharomyces yeasts. The wild yeasts can also contribute to the undesired formation of volatile acid, ethyl acetate and aldehydes and Zygosaccharomyces, Picchia and Candida yeasts have a resistance against SO2. Brettanomyces bruxellensis is however the most important spoilage yeast and can form off-flavours like wet dog and band-aid. It requires few nutrients and can for example utilise the sugars occurring in toasted barrels. It is also difficult to prevent, because it can have resistance against low pH, high alcohol and SO2.

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Unacceptable sanitation.

Acceptable sanitation.

 

The growth of micro-organisms is promoted by factors like available sugar, malic acid, nitrogen nutrients, pH’s higher than 3.6, high temperatures and available oxygen. Their growth is however limited by SO2, ethanol, pH’s lower than 3.6 and low temperatures. In order to maintain sound cellar hygiene, it is advisable to keep the cellar temperature below 15°C and maintain the pH of wines below 3.6.

In order to manage the sanitation status of a cellar properly, it is essential to test for the presence of micro-organisms and monitor their status. It is recommended that the stage between alcoholic fermentation and malolactic fermentation (MLF) during maturation until the blending of wines and the stage prior to bottling, are seen as the three most important critical control points during winemaking. Tests must obviously also be executed if fermentation problems occur or off-flavours are identified. The monitoring and detection of micro-organisms are of cardinal importance to prevent spoilage. General techniques that are used to do this, are microscopic investigation, microbiological culture plating or the use of laboratory equipment, which uses PCR (polymerase chain reaction) for analyses. Luminometers are bioluminescence equipment used to detect the presence of ATP (adenosine triphosphate). ATP is energy molecules, which occur in all living organisms. The analysis of swab samples taken from cellar surfaces, is consequently a good indication of the micro flora population, although specific organisms cannot be identified. It is consequently a rapid method to monitor the efficiency of cleaning and sanitation practices.

Effective sanitation also includes the prevention of microbiological distribution. Seeing that micro-organisms are not locality bound and can occur anywhere, it can be distributed by cellar workers and wine flies. The maintaining of workers’ hygiene and limitation of wine flies are consequently essential supporting actions of cellar sanitation. It is important to distinguish between cleaning and sanitation. Cleaning is the removal of soil and solid residue, while sanitation is the decreasing of micro-organisms.

The cleaning and sanitation procedures can consequently be divided in five steps:

  • A pre-rinse action to loosen solid residue.
  • The application of cleaning agents and a scrubbing exercise.
  • The rinsing of the cleaning agent.
  • The application of the sanitiser.
  • The rinsing of the sanitiser (in some cases unnecessary).

The execution of effective cleaning and sanitation procedures will prevent the formation of biofilms. Biofilms in the cellar, which are layers of organic matter of slime or moulds on cellar or equipment surfaces, or mineral or tartrate precipitation on tanks and barrels, can be an important source of microbiological contamination.

Cleaning agents can be surface active and have either an acid base (for the cleaning of mineral precipitates) or an alkaline base (for the cleaning of tartrates). It can be applied together with foaming, soaking, or spraying at high pressure. Heat is the base of most sanitation procedures, either as hot water at 77 to 85°C or steam. The surface that needs to be sanitised and the focus organism will determine which one is used. The chemical sanitisers used in cellars, changed considerably over recent years. Chlorine products were very popular, but due to the problems with trichloro-anisole (TCA) it is not used anymore. Iodine products, quaternary ammonium products, acid-ionic combinations (like peracetic acid), hydrogen peroxide, ozone and ultraviolet are more frequently used nowadays. No sanitising agent is perfect and different aspects must be considered before a specific product is chosen (see the Winetech Tegnies article on the sanitation of barrels: September 2014). It is also important that cleaning and sanitising products are rotated at least weekly, because micro-organisms can develop product resistance. It must also be used at the right concentration to ensure efficiency, but unnecessary high concentrations are a waste of money and safety and regulatory prescriptions can be exceeded.

The sanitation of barrels and bottling lines require specific attention and both are critical points to prevent contamination and later losses. Brettanomyces can penetrate barrels up to 8 mm and heavy toasted barrels are more susceptible to contamination by it. Even new barrels must also be treated before use to prevent contamination.

The following procedures can be used as guidelines for the sanitation of barrels:

  • Barrels must be cleaned before sanitation practices are applied.
  • The burning of sulphur wicks, containing 5 g sulphur, is effective and has a long term influence. It must however be burnt completely and barrels must be sealed effectively during storage afterwards.
  • The steaming of barrels for 10 minutes or water at 82°C is effective to a depth of 8 mm.
  • Treatment with 1 ppm ozonised water for 10 minutes decreases the contamination of barrels.
  • Peracetic acid, also known as peroxi-acetic acid (PAA), at a concentration of 120 ppm can decrease the contamination of Brettanomyces by 90% within one week.
  • Chlorine dioxide is effective for stainless steel tanks, but not barrels.

The cleaning and sanitation of sampling equipment are also important to prevent cross contamination. PAA, citric acid and alcohol can be used for it.

The sanitation and monitoring of the filling bowl and filling nozzles are the most important actions prior and during bottling. If mobile bottlers are used, it must be ensured that effective sanitation and control are applied by them before bottling commences to prevent contamination from other cellars.

Different anti-microbial products and processes can be used during the different stages of winemaking to prevent the development of microbes, decrease the micro population or eliminate it. Sulphur dioxide is probably the most used. It is however important that the molecular sulphur dioxide concentration is more than 0.8 ppm. Different factors determine it, but the pH of the wine is the most important factor. Lower pH increases the percentage. Chitosan is a polysaccharide extracted from Aspergillus niger and kills micro-organisms. Lysozyme, extracted from egg white, is effective against malolactic bacteria and is mainly used to prevent MLF or stabilise wine microbiologically after MLF. Dimethyldicarbonate (DMDC), also known as Velcorin, is very effective against yeasts and is mainly used to prevent secondary fermentation in bottled sweetish wines. Sterile filtration with membrane filters using a pore size smaller than 0.45 micron prior to bottling, is the final process to sterilise wine prior to bottling (Rieger, 2015).

Reference

Rieger, Ted, 2015. Microbial monitoring and winery sanitation practices for quality control. Wine Business Monthly, October 2015: 42 – 46.

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