How cider fruit develops and ripens

The number of cells in an apple is fixed within a few weeks of fruit set. After that the fruit enlarges by cell expansion, which is driven by water uptake and by the accumulation of solutes inside the cells. This division of labour explains a number of otherwise puzzling observations: why conditions during and just after bloom affect ultimate fruit size, why drought in July costs size that a wet August cannot fully recover, and why fruit on an overcropped tree stays small no matter how good the season.

Crop load competes throughout. All the fruit on a tree draw on the same pool of carbohydrate produced by the same canopy, so a heavy crop gives many small fruit and a light crop gives fewer large ones. For cider this matters less through size than through concentration: fruit on a heavily cropping tree tends to have lower soluble solids, and juice from an overcropped orchard tends to come in at a lower gravity.

Light interception is the ultimate limit. A dense, shaded canopy fixes less carbon than an open one of the same leaf area, which is why pruning to keep light in the tree is a sugar decision as much as a disease decision. The interior of an unpruned veteran tree carries fruit that never receives full light and never accumulates the reserves that outer fruit does.

Photosynthate arrives in the fruit as sorbitol and sucrose and is stored, in large part, as starch. Starch is not fermentable and contributes nothing to gravity, so an apple at maximum starch is an apple whose potential is locked up. As maturity approaches, the fruit begins hydrolysing that starch to sugars, and soluble solids climb sharply over a period of days to weeks.

This conversion is what the starch–iodine test visualises, and it is the single most important physiological fact about picking date for a cider maker. Fruit picked before conversion is well advanced yields juice of lower gravity and less alcohol. Fruit picked after it is complete has nothing further to gain on the tree and begins to soften and become vulnerable to rot.

Apples are climacteric: at the end of development they produce a burst of ethylene which triggers the coordinated changes of ripening — starch conversion, softening, aroma development, and eventually senescence. Cultivars differ enormously in how fast this runs. Early cider varieties go over within days of maturity, which is why they must be milled immediately; late varieties can hang on the tree or be stored for weeks, which is what makes an extended pressing season possible.

Malic acid dominates apple acidity and is highest in young fruit, declining through development as it is respired and as the fruit enlarges around it. A sharp cultivar picked late is measurably less sharp than the same fruit picked early, which gives the maker a lever: picking date moves acid and sugar in opposite directions, and the choice is a trade rather than an optimum.

Phenolics behave differently again. The procyanidins and hydroxycinnamic acids responsible for bitterness and astringency are largely synthesised early in fruit development, and their concentration falls as the fruit expands around a fixed quantity. Tannin content therefore tends to be diluted by anything that makes fruit larger — a light crop, a wet season, generous nitrogen — which is one reason a bittersweet grown in one place and season does not necessarily give the tannin figure recorded for it elsewhere.

In pears there is a further component. Pears accumulate sorbitol, a sugar alcohol that most cider yeasts cannot ferment, and it remains in the finished perry as a source of sweetness and body. This is the chemical basis of the frequently repeated observation that perry retains a sweetness that a fully fermented cider does not, and it is a property of the fruit rather than of anything the maker did.