5.3.4 Barrel maintenance

What does cleaning and disinfection involve?

First and foremost, it is important to define the terminology employed. Cleaning consists of removing debris in order to ensure the barrel's cleanliness. The main contamination in the barrel is scale. It is important to remove the scale before disinfection. Disinfection is the removal of microorganisms from the environment. Disinfection is necessary to eliminate all spoilage microorganisms. At this stage, the most common ones are acetic bacteria and B. Bruxellensis. Various techniques exist and have varying degrees of effectiveness. Traditionally, barrels are rinsed with cold water then cleaned and finally disinfected.

What are the possible cleaning methods?

Following racking or when a used barrel has been empty for some time, it is important to clean the barrel to avoid any organoleptic deviations. Different approaches are possible.

Pressurized hot water

Water at approximately 70°C under pressure is applied to the barrel. This is an efficient system to eliminate scale, but the temperature is too low to properly disinfect the surface or in depth. Pressure is crucial for quality descaling, it is generally between 100 and 120 bars.
This method is widely used because it is very easy to set up and highly effective for cleaning barrels.

Chemical agents

The best known is caustic soda, but this technique is rarely used because it is difficult to rinse. It is used when the barrel has been contaminated by acetic bacteria, and then provides a fairly efficient method for removing acetic acid.

Ultrasound

The barrel is filled with hot water and then placed in an ultrasonic chamber. The cavitation in the water removes both the scale and microorganism membranes. This method is not yet widely implemented in Europe. A major drawback is the amount of water required. However, its ability to combine cleaning and disinfection makes it quite advantageous.

What are the possible disinfecting methods?

Steam

Steam is one of the most common and effective disinfection methods. However, it has little effect on limescale. The steam is sent into the barrel with a pipe connected to a steam generator. The steam is at 100°C. The duration of the steam treatment depends on the generator's output and the desired in-depth temperature to be reached. The treatment time typically ranges from 6 to 12 minutes.

Ozonated water

A generator produces ozonated water. The water is sprayed into the drum for about 5 minutes. The free radicals formed are intended to destroy microorganisms. This method does not clean and its effectiveness on microorganisms is limited according to research because the effectiveness of ozonated water is disrupted by organic molecules. Moreover, its action is limited to the surface.

UV treatment

A UV gun can be used to disinfect the barrel's surface. The barrel must first be cleaned using another method. This technique is very effective on the surface but not within the wood. It is a possible alternative to burning sulphur discs when barrels have been kept empty a long time and one does not want to saturate the barrel with SO2.

Burning sulphur disks

Burning sulphur disks is an ancient technique to disinfect barrels. After cleaning, a wick or a sulphur disk is burned inside the barrel, which will release gasous SO2. The advantage of burning sulphur disks is that it goes deep into the wood and microbiologically stabilizes the impregnation front for a relatively long period of time.
The sulphur dosage during the process can vary from 2.5 to 10 grams per barrel. The higher the dose, the higher the free SO2 in the wine.

In practice, what protocols can be applied?

Regardless of the cleaning/disinfecting method employed, a cold water rinse is always appropriate to remove coarse impurities. Several techniques can be combined to properly maintain barrels. A cleaning technique is often coupled with a disinfection technique.
For example:

• Rinsing with cold water
• Cleaning with pressurized hot water (85°C) for 4 minutes
• Steam cleaning for 6 minutes
• Burning a 5 g sulphur disk

This method is quite common and ensures thorough disinfection of the barrel. Hot water increases the temperature of the barrel before the steam, thus taking advantage of its heat contribution. Under the conditions mentioned above, after four minutes of steam, the first 5 mm of the barrel will be at a temperature above 55°C, for a depth of 15 mm, 7 minutes 30 seconds is required.