Chapter 21

The Role of Oak in Winemaking

Wine Barrel Cleaning, Sanitation, and Storage

With the cost of American or European oak barrels, it's in the winemaker's best interests to maximize a barrel's life span by adopting a maintenance program involving cleaning, sanitation, storage, and replacement. Barrels must be rigorously cleaned prior to sanitation, to remove any gross solids and color or tartrate coatings on the surface of the barrel that may impede any sanitation treatments.

Barrel Cleaning

Once emptied of wine, barrels should be inverted to near the 6 o'clock position, drained and then rinsed with high pressure (100-3,000psi) cold water for three minutes using a barrel washer with a rotating spray head inserted into the bung hole. Rinsing helps to loosen and remove the gross sediment, which generally consists of yeast lees, pigments, proteins, polysaccharides and tartrates.

Barrel Washing Systems

Barrel washing systems come in a variety of designs. The simplest and least expensive system is often referred to as the peg leg, a three- or four-leg assembly that stands inside the barrel and supports the wash head (Figure 21.7). The cellar worker is required to manually turn the barrel, insert the wash head and physically activate the system. Hand-truck washers have been around for some time and come in a wide array of designs, with different levels of sophistication and automation.

Barrel Sanitation

Barrel's harbor large amounts of microorganisms the day it arrives at the winery, and it will continue to do so until they are replaced. Brettanomyces yeast species are the most feared barrel spoilage organism. Oak provides the perfect environment for Brettanomyces because of the porous surface and the inability to use powerful detergents or chemical sanitizers.

Hot Water

Hot water is a useful disinfection treatment if the cooperage has become contaminated with spoilage microorganisms. A hot water treatment (85°C, 185°F) for 20 minutes is effective against in eliminating viable acetic acid bacteria from barrel wood (Jackson, 2008).

Steam

An alternative to using hot water for barrel sanitation is dry-saturated steam between 70 and 82 degrees C (160-180°F) (Figure 21.8). The vapor property of dry steam not only re-hydrates barrels but releases tartrates and old wine that have been absorbed by the wood as well as controls wild yeasts such as Brettanomyces.

Ozone

Many wineries have implemented ozone as a part of their barrel-sanitation program. Natural ozone gas is produced safely and economically through sophisticated, yet easyto- use integrated ozone generators. The way that a typical ozone generator works is by pulling oxygen from the local atmosphere and sending it through a small field that electrically excites the oxygen gas and changes the gas from oxygen to ozone.

High Power Ultrasound

High power ultrasound (HPU) cleaning works by converting electrical energy to ultrasonic sound waves above the range of human hearing of 12 to 20 Hertz. Within a liquid such as water, these sound waves generate high-energy micron-sized bubbles, which subsequently collapse, creating shock waves (that transfer kinetic energy), acoustic streaming, and vibration - this process is known as cavitation.

Storage of Empty Barrels

If used or emptied barrels are not to be immediately refilled with wine, then they can be stored dry or wet. Ideally, the winemaking schedule should be cyclic so that reusable barrels get emptied, cleaned, and refilled with new wine within a day or two.

Short-Term Dry Storage

Dry storage involves directly filling the barrel with sulfur dioxide gas or burning elemental sulfur producing gaseous sulfur dioxide within the barrel. Before sulfiting it is important that all the water is drained out of the barrel since any residual water standing in the barrel will combine with sulfur dioxide gas to form sulfurous acid which will soak into the wood. It has been clearly demonstrated that burning sulfur in damp wood provides much less effective sterilization than in dry barrels. It takes several days to obtain complete sterilization in damp barrels, but only a few hours in dry wood.

Long-Term Wet Storage

Wet storage involves filling barrels with acidified water/sulfite solution using potassium metabisulfite. Sodium metabisulfite can be used in-place of the potassium metabisulfite. This will keep the barrel staves tight against one another and free from leaks as well as free from spoilage.

Post Rinse Following Storage

Rinsing the barrels after wet or dry storage is very important as any residual sulfur can lead to high levels of bound sulfur dioxide in must wine, which can inhibit yeast or malolactic fermentations and generate strong reductive odors, i.e., hydrogen sulfide. Also, it is a good time to check for leaks.

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