5.1.1.a Benefits of ageing in vats

Vats correspond to reductive ageing since there is little or no oxygen present in the environment unless it is intentionally added by micro-oxygenation or operations such as racking.
On an organoleptic level, this allows the preservation of all the aromatic components that are highly sensitive to oxygen, such as thiols. The materials are neutral in terms of flavour (except for wooden vats) making it possible to preserve a very fruity profile.
Logistically, maturing in vats is easy, especially with large volumes. There is less work and monitoring is less time consuming.
Different materials and shapes exist.
The available materials are:

• Steel
• Raw concrete
• Concrete with epoxy resin
• Wood
• Stainless steel
• Sandstone

Each material has its advantages and disadvantages but do not seem to have the same characteristics, mainly in terms of hygiene, but also with regard to the neutrality of the material (deterioration, microbiological risks, release of compounds into the wine) and respect for the environment (water consumption and savings).

Wood

Wood has several advantages such as thermal inertia, resistance to decay and aromatic properties when it is new or almost new. It is a "noble", traditional and aesthetic material. It also provides beneficial micro-oxygenation during ageing in wooden vats.
However, certain major disadvantages remain. The cost is relatively high since the vats are often made to measure and require more maintenance than stainless steel.

Risks of microbiological contamination are higher when the wood changes shape depending on the ambient humidity (growth of bacteria, risk of mould, organoleptic deviation when the wood is poorly dried or when the humidity is too high in the winery). It is also the only container that requires soaking with water before filling.
It is the only container that is not neutral (causing organoleptic modification of the must and wine). In general, wooden vats are only used for fermentation and are less versatile than stainless steel or concrete.

Stainless steel

Stainless steel is the material that seems to meet the criteria of resistance, versatility (fermentation, vinification, conservation/storage) and neutrality. Stainless steel is easy to clean and light. It is the material that best resists deterioration and is the most airtight. Stainless steel does not represent a danger in case of physical or chemical deterioration, it is a neutral material which, when intact, does not present any microbiological risk for grapes, must or wine. Lastly, it seems that stainless steel is the most environmentally friendly material, as its smooth surface reduces the amount of water needed for cleaning and descaling.
Some disadvantages may remain: it is the material with the lowest thermal inertia compared to concrete or wooden tanks. An absence of microbiological risks in the wine is only possible if there are no scratches in the vat (they are a means of adhesion for micro-organisms and tartar) and with good maintenance (gentle removal of limescale).

Stainless steel vats come in many sizes, which makes them convenient for small-volume vinification (by plot or partial plot, and use for wine storage). Stainless steel vats are more often vertical (higher than they are wide) but there are also models referred to as "horizontal" (wider than they are high) which means they can be adapted more readily to the characteristics of a small vat room. The most recent stainless steel vats have an integrated double-wall thermoregulation system.
There are several types of finish: cold-rolled or 2B stainless steel, "bright annealed" or "electropolished". These last two finishes are smoother and require less water to clean than 2B stainless steel because microorganisms have fewer attachment points. They are also the materials that produce the least amount of pollution in the rinse water and therefore require less product when descaling. Bright annealing is the most expensive finish.

Stainless steel tanks have the advantage that they may be fitted with floating caps with an air-filled tube surrounding the stainless steel rim. These tubes can be easily inflated by the operator and the mechanism maintained at a certain height by accessories on the vat that facilitate movement of the floating cap. The fact that the cap can be completely removed also enables better maintenance/cleaning. This system permits wine storage for varying tank levels without the need for inerting.

Other tanks are equipped with an integrated cap that can sometimes be more difficult to clean but ensures a good hermetic seal and preservation of the wine. These caps are perforated to enable inerting.

Concrete

Concrete has very good thermal inertia and overall resistance to deterioration. It is not very porous to air when epoxy coated, which makes it a versatile container (wine preservation/storage) but is more permeable when left raw. However, concrete has some disadvantages: it is the heaviest material, and deteriorates the most physically, although this depends on the coating and tank maintenance.
Water consumption is often high for cleaning concrete, as its rough surface is a prime attachment point for scale. Despite its strength, concrete can still crack over time, especially when cleaned with pressurized hot water. Cracks can make cleaning difficult, and provide sites for micro-organism contamination which increases the risks of organoleptic deviance ("musty taste").
Often cubic, they can also be egg-shaped. Concrete can be raw or epoxy coated. The latter has a lower water consumption than raw concrete, which has a greater resistance to the removal of scale.