5.6.2 Theoretical overview of factors influencing the appearance of microorganisms

The environment's physicochemical parameters

Certain environmental parameters directly or indirectly influence cell viability. Consequently, these are key factors to take into account.
Residual sugars favour the growth of lactic acid bacteria and B. bruxellensis. For the latter, 0.35 g/L of residual sugar (glucose, fructose, sucrose, trehalose) are sufficient to produce 1000 cells/mL. A quantity of 1 g/L of sugar can be dangerous. It is therefore essential that as much sugar as possible is broken down during fermentation. Ageing on lees can release trehalose-type sugars, so it is necessary to be vigilant and not prolong the presence of lees in the environment if there is a risk of development of B. bruxellensis.

As far as unfavourable factors are concerned, alcohol is clearly significant. The higher the ABV, the more difficult it is for microorganisms to develop in the environment.
The acidity, expressed by the pH, is also a crucial parameter. A higher pH will favour the development of microorganisms. At a lower pH, i.e. below pH 3.6, development is slowed but not necessarily stopped.
A high alcohol/low pH combination is particularly effective from a hygiene point of view, although they are not the only criteria that need to be taken into account.

Temperature and oxygen supply

Acetic acid bacteria require oxygen, so the first key to controlling their growth is controlling the wine's oxygen supply.
Regular topping up of the barrels with bungs on the top and controlling exposure to oxygen during wine operations are essential to control this bacteria. Inerting the wine or even adding CO2 during the ageing process reduces the potential for oxygen dissolution in the wine, and therefore the risk of growth.
For batches aged in vats or amphorae, the container must be filled to the maximum and if there is any space, it should be systematically inerted with a gas such as nitrogen, carbon dioxide or argon.
B. bruxellensis and lactic acid bacteria need little or no oxygen to grow, but oxygenation can encourage them. B. bruxellensis breaks down sugars faster in the presence of oxygen.

As far as temperature is concerned, the lower it is, the more limited growth is. For example, to prevent Brettanomyces, a temperature between 12 and 15°C is optimal for slowing their development. Generally speaking, the lower the temperature, the more restricted their growth. Be careful, however, not to reduce the temperature too much, as this could increase the dissolution of oxygen in the wine, and dry out the air excessively, thereby increasing wine evaporation through the barrels.

Hygiene and cleaning of equipment

Rigorous hygiene is obviously a good way to reduce contamination and avoid bacterial population growth. To achieve this, rigorous hygiene measures must be applied to tools and containers, with regular descaling and disinfection.
For wood, steam is the most common and effective option. Used barrels should be cleaned with care because the wood's structure is microporous, making it an ideal place for microorganisms to grow. For more details, see the section on Barrel ageing.
For stainless steel, peracetic acid is a powerful disinfectant. It is a much easier material to clean.

All these practices must of course be preceded by rigorous cleaning prior to disinfection and more specifically thorough descaling. Limescale crystals that are deposited on the walls are very favourable areas for microorganisms to develop. Descaling using soda and soaking or in a closed circuit is effective. For more details, see the section on the preparation and cleaning of vats.

Sulphiting

Sulphiting is always effective but acetic acid bacteria are less sensitive to sulphites than lactic acid bacteria, for example, the latter being generally inhibited by the standard dosages.
Particular attention must be paid to the active SO2. Its concentration is affected by temperature, pH, ethanol content and the amount of free SO2.

To begin with, the higher the free SO2, the higher the active SO2. Then, for a given free SO2 concentration:

• The higher the temperature, the higher the active SO2. For each 1°C, the active SO2 increases by 7% for a given free SO2.
• The lower the pH, the higher the active SO2. For a given pH, pH -0.2 increases the active SO2 by 50%.
• The higher the ethanol content, the higher the active SO2. For an additional 1% ABV, the active SO2 increases by 5%.

• Online tools exist to Calculate active SO2.

For bacteria, an active SO2 of 0.35 mg/l will inhibit their development. For Brettanomyces, at least 0.5 mg/l is required. For optimal protection, the active SO2 must be 0.6 mg/l. To be lethal, a value greater than 0.7 is required.
For example, for a concentration of free SO2 of 25 mg/l, with a pH of 3.6, an ABV of 13.5% and a temperature of 14°C, the active SO2 is 0.4 which is sufficient to inhibit the development of B. bruxellensis. Under the same conditions but with a pH of 3.8 the active SO2 is then 0.25 mg/l which is insufficient. A free SO2 of 35 mg/l would be needed to obtain a minimum active SO2 of 0.35 mg/l.
Given the current pHs of wines, it is difficult to obtain an effective active SO2. To prevent Brettanomyces, SO2 is not the sole factor to work on.