Oak vats and wooden tuns

A vat is coopered from riven or sawn staves held by iron or wooden hoops, with a head fitted into a croze groove. Oak dominates in England and much of France; chestnut is common in Asturias and in parts of France and Italy, and other timbers appear regionally. The staves swell when wet and it is that swelling, not any sealant, that makes the joint tight — which is why an empty vat that dries out leaks when refilled and needs to be swelled with water before use.

Upright vats have a small surface-to-volume ratio relative to a barrel of the same capacity, so they lose heat slowly and, at large size, hold fermentation warmth well. That is an advantage in an unheated stone building in November and a liability in a warm September, and the traditional response was to control the timing of the fill rather than the temperature of the vessel.

Large vats are effectively fixtures. They are built or assembled in place, and the head is fitted with a manway a person can climb through. That manway is the reason for the single most serious hazard in a traditional cider house.

Oxygen ingress through wood is slow but continuous, and it is dominated by the head and the joints rather than by diffusion through the stave face. A ferment in wood therefore has a small, steady oxygen supply that a sealed stainless tank does not, which supports yeast health early on and supports acetic acid bacteria later if the vat is not kept full.

The wood surface, and particularly a vat that has been in use for decades, holds a resident population: Saccharomyces strains, lactic acid bacteria that will carry out malolactic conversion without inoculation, Brettanomyces and Dekkera, film yeasts, and acetic acid bacteria. A traditional producer relies on this. It is the mechanism behind the claim that a particular vat makes a particular cider, and there is a real basis to it, though the population is not stable and a vat can turn.

Large old vats give very little flavour extraction from the wood itself. The extractable compounds — oak lactones, vanillin, the tannins of new oak — are long gone, and the inner surface is coated with tartrate and organic deposit. A hundred-year-old vat is best understood as an inert porous container with a biofilm, not as a flavouring vessel.

Wood cannot be sanitised in the way stainless can. The surface is porous to a depth of several millimetres, it is scored and checked, and hot caustic — the tool that solves this in steel — attacks lignin and degrades the timber. What is available is hot water, physical scrubbing, mild alkali such as soda ash, steam, and historically the burning of a sulphur wick inside the closed vessel to fill it with sulphur dioxide gas.

That means a wooden vat is managed rather than cleaned. It is kept full, kept wet, kept cool, and monitored; a vat that has produced a spoiled batch is treated as suspect for the next several fills. Where a vat has gone thoroughly acetic, the honest position is that it may not be recoverable and that continuing to use it will spoil what goes into it.