Brown rot, blue mould and why rotten fruit is a safety matter

Brown rot is caused by Monilinia fructigena and related species. It produces an expanding brown lesion on the fruit with concentric rings of buff-coloured spore pustules, and it spreads readily by contact, so a single infected fruit in a heap will infect its neighbours. Infected fruit left on the tree dries into a hard mummy, which overwinters and provides the inoculum for the following season — which is why removing mummified fruit at pruning is a genuine sanitary measure rather than tidiness.

Blue mould is caused principally by Penicillium expansum. It produces a soft, watery rot with blue-green sporulation, and it is a wound pathogen: it needs an opening, which bruising, wasp damage, scab cracks and handling all supply. It is the classic rot of stored and ground-lying fruit, and it is the organism that makes rot a regulatory matter rather than only a sensory one.

Botrytis cinerea and a number of other organisms also cause fruit rots, and in practice a heap of neglected fruit will carry a mixed population. The distinction that matters operationally is not which species is present but whether the fruit is sound.

Patulin is a mycotoxin produced by Penicillium expansum and some related moulds in rotting fruit. It is not destroyed by pressing, and maximum levels in apple juice and apple products are set in food law in the UK, the European Union and many other jurisdictions, with stricter limits for products intended for infants and young children. That makes control of rotten fruit a legal obligation for anyone selling juice, not a matter of preference.

The controls that work are all in the orchard and the yard. Patulin is concentrated in and around the rotted tissue, so removing rotten fruit removes most of the toxin; trimming does not, because the toxin diffuses beyond the visible lesion. Prompt gathering, prompt milling, avoiding deep heaps that heat, and sorting on the way to the mill are the recognised measures.

Fermentation does reduce patulin substantially, which is why the sharpest regulatory attention falls on unfermented juice. This is not a licence to press rotten fruit for cider. Reduction is not elimination, the starting concentration determines the finishing one, and everything else that comes with rotten fruit — acetic acid bacteria, mould taint, degraded pectin — is unaffected by fermentation and often made worse by it.

The sensory consequences arrive before the safety ones become apparent. Mouldy fruit contributes musty, earthy, cellar-like aromas that are difficult to remove and that persist through fermentation and maturation. Heavily rotted fruit also carries acetic acid bacteria on its broken surfaces, and those bacteria convert ethanol to acetic acid at the first opportunity, raising volatile acidity towards the vinegar fault.

There are process consequences too. Rotting breaks down pectin and cell structure, so heavily rotted fruit presses badly and yields juice that is harder to clarify. Fungal enzymes can also interfere with pectin behaviour in ways that matter for keeving, where the entire process depends on the pectin fraction behaving predictably.

The folklore position — that a proportion of rotten fruit contributes complexity to traditional farmhouse cider — is worth addressing directly because it is repeated so often. What has genuinely been true historically is that farmhouse cider was made from fruit that would not meet modern standards, and that some of the resulting character was accepted as normal. That is a description of past practice, not an endorsement, and it cannot be reconciled with either modern food law or a clean fermentation.