Juice reception and settling tanks

Sizing follows from the press cycle rather than from the day’s tonnage. A tank that holds one press load plus a margin lets the press start its next cycle without waiting; a tank that holds the whole day means nothing downstream has to keep pace at all. Which is right depends on how much tank the operation can afford to have standing idle for eleven months of the year.

What the tank does for quality is mixing. Juice from the beginning and the end of a press run differs — the first-run juice is freer and lower in solids than the pressings that follow — and juice from different bins of fruit differs more. A reception tank makes one lot of them, which is exactly what a producer wants if the intention is a blend and exactly what they do not want if the intention was to keep lots separate. Deciding that before the juice runs together is one of the quiet decisions of a pressing day.

It is also where additions are made, for a simple reason: an addition is only as good as its mixing, and mixing a sulphite or enzyme dose evenly into a full tank with a pump loop or an agitator is straightforward, whereas dosing into a fermenter that is already stratified is not. The pH reading that governs a sulphite decision is taken here too, on a representative sample.

Particles fall through a liquid at a rate governed by their size, by how much denser they are than the liquid, and by how viscous the liquid is. In fresh press juice the coarse fraction — skin fragments, pulp, pip and stalk debris — is large enough to fall out over hours. The fine and colloidal fraction is not, and no amount of standing still will drop it.

Pectin is what holds the middle fraction up. It raises the viscosity of the juice and it stabilises fine particles as a colloid, which is why commercial practice adds a pectinase before settling rather than after: the enzyme breaks the pectin down, the viscosity falls, the colloid destabilises, and material that would have stayed in suspension indefinitely settles in a day. This is the single largest lever on how well a settling tank works.

Cold matters, but not for the reason usually given. Chilling actually raises viscosity slightly and so slows the fall of a particle; what it buys is time, by suppressing the wild fermentation that would otherwise start within a day and stir the tank thoroughly from the bottom up. Cold settling is settling that is allowed to finish.

What comes off the top is a juice with most of its gross solids removed, and what is left behind is the gross lees. The trade is between clarity and volume: draw off high and the juice is clean but a good deal is left with the deposit; draw low and the last of it carries solids back into the fermenter. Producers commonly recover the lees separately, by a second settling, by filtration or by centrifuge.

A cone-bottomed tank concentrates the deposit into a small volume that can be dropped out of a bottom valve, which makes racking off almost trivial and loses very little juice. It is the arrangement most worth having if a tank is being bought for settling. A dished or flat-bottomed tank spreads the deposit over the whole floor, so the clear juice has to be drawn from a side valve or with a floating suction, and the last few centimetres are a compromise.

Multiple side valves at different heights are the cheap version of the same idea: draw from the highest valve that is running clear, then the next, and stop when the flow clouds. Sight glasses help and lie a little, because the deposit at the wall is not always at the same level as the deposit in the middle.

Height is the other variable. A tall narrow tank gives a long settling distance, so the same particle takes proportionally longer to reach the bottom; a shallow wide one settles faster but presents more surface to the air and takes more floor. This is one of the few cases where the vessel geometry changes the outcome of a process directly and predictably.

Keeving depends on a settling vessel too, but what forms is a floating gel — the chapeau brun — rather than a bottom deposit, produced when pectin is demethylated and cross-linked by calcium and lifted by fermentation gas. The clear juice is drawn from between the floating cap and the bottom deposit, which is why keeving vessels are typically wide and shallow and why the drawing-off point is set by observation rather than by a fixed valve.

That also explains why a keeved juice cannot be treated the way a settled juice is. Pectinase, the tool that makes ordinary settling work, destroys the mechanism keeving depends on, and heavy shearing at the mill and the press disturbs it as well. The vessel and the enzyme decision are made together, not separately.