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Cider science

Oxidation, from the mill to the bottle

What does oxygen do to cider?

In short

In fresh juice, oxygen and the enzyme polyphenol oxidase convert phenolic compounds to quinones within minutes, and those quinones polymerise into the brown pigments that make pressed juice go the colour of weak tea. This is enzymatic browning, and much traditional practice welcomes it.

In finished cider the enzyme is gone, and a slower chemical process takes over. Phenolics oxidise, generating hydrogen peroxide, which oxidises ethanol to acetaldehyde — the compound behind bruised-apple and sherry notes.

Neither process is inherently a fault. Controlled oxidation gives several traditional styles their depth; uncontrolled oxidation gives the flat, papery character that is a fault wherever freshness was intended.

How three conditions become faultsOxygen, wild organisms and residual sugar each open a route to a specific group of faults, and each has its own prevention.OxygenAcetic bacteriaVolatile acidityand oxidationKeep vesselsfull and coolWild organismsBrettanomycesPhenolic taintand mousinessSulphite andsanitationResidual sugarYeast in bottleOver-carbonationand gushingFilter, or leaveit truly dryWhat prevents it
Oxygen, wild organisms and residual sugar each open a route to a specific group of faults, and each has its own prevention.
Described in full
Layout
Three parallel columns, each running downwards: the condition, the organism or reaction it permits, the faults that follow, and the practice that prevents it.
Oxygen
Air in a part-empty vessel or at every transfer. It permits acetic acid bacteria to work, and it drives chemical oxidation of phenolics independently of any organism.
What oxygen produces
Volatile acidity and, at higher concentration, frank acetification — cider turning to vinegar. Alongside it, oxidation gives sherry, walnut, bruised-apple and cardboard notes as acetaldehyde and browning products accumulate.
Wild organisms
Present on the fruit and in old wood. Brettanomyces and certain lactic bacteria decarboxylate hydroxycinnamic acids to volatile phenols; some lactic bacteria also form the tetrahydropyridines behind mousiness.
What wild organisms produce
Phenolic character — leather, smoke, sticking plaster — which is a signature at low levels and a fault above them. Mousiness is not detectable on the nose and appears only as a lingering aftertaste, which is why it is so often missed.
Residual sugar
Any fermentable sugar left in a sealed container is a fuel supply for whatever yeast survives filtration or arrives afterwards.
What residual sugar produces
Refermentation in the bottle, and with it over-carbonation, gushing on opening and, in the worst case, a bottle that fails under pressure. This is a safety matter, not only a quality one.
Prevention
Keep vessels full and cool, and minimise transfers, for oxygen. Sulphite and sanitation for wild organisms. For residual sugar, either stabilise and filter, or leave the cider genuinely dry — the only condition that cannot referment is one with nothing left to ferment.

Enzymatic browning, in the mill and the press

Intact apple tissue keeps polyphenol oxidase and its phenolic substrates in separate compartments. Milling destroys that separation, and in the presence of oxygen the enzyme oxidises hydroxycinnamic acids — chiefly chlorogenic acid — and catechins to quinones. Quinones are reactive: they polymerise with each other and with other phenolics to form brown pigments, and they also bind proteins.

The visible result is the browning of pomace and juice within minutes of milling. The less visible result is a reduction in the phenolic material available to the finished cider, because the polymerised products are larger, less soluble, and partly removed with the lees. Controlled oxidation of juice is therefore used deliberately as a way of reducing bitterness and stabilising colour.

Where a pale, fresh cider is wanted, the enzyme is suppressed instead — by sulphiting the juice promptly, by minimising the time between milling and pressing, by working cool, or by excluding air from the press. Ascorbic acid can also be used, though it reduces the quinones back rather than inhibiting the enzyme and is consumed in the process.

Chemical oxidation, in the vessel and the bottle

Once the juice has fermented, polyphenol oxidase has been denatured and removed, and any further oxidation proceeds by a slower chemical route. Phenolics — particularly the catechin-type compounds — are oxidised by dissolved oxygen in a reaction catalysed by trace iron and copper, producing quinones and hydrogen peroxide.

The hydrogen peroxide is the important intermediate. Through Fenton-type chemistry it generates hydroxyl radicals, which oxidise ethanol to acetaldehyde. Acetaldehyde is the principal marker of an oxidised cider: it smells of bruised apple at low concentration and of sherry at higher ones, and it is what most drinkers are detecting when they call a cider oxidised.

Further oxidation of acetaldehyde gives acetic acid, and longer exposure produces the flat, papery, cardboard-like character associated with aged and heat-abused product. The progression from interesting to dull to faulty is continuous, and where the line falls is a matter of style rather than of chemistry.

Where oxidation is wanted

Controlling it

The three practical levers are exclusion, sulphite and yeast. Exclusion means keeping vessels full, sealing them, purging headspace with inert gas, and minimising transfers, since every racking and every bottling operation introduces oxygen. Ullage — the air space above the cider in a partly emptied vessel — is the standing enemy, because it supplies both oxygen and a habitat for film yeast and acetic bacteria.

Sulphur dioxide acts on several fronts: it scavenges hydrogen peroxide, binds acetaldehyde into an odourless complex, and inhibits the microorganisms that would otherwise take advantage. Its effectiveness is strongly pH-dependent, which is treated on its own page.

Live yeast is the third lever, and is why bottle-conditioned cider ages so well with little sulphite. The yeast consumes dissolved oxygen as it arrives, and continues to protect the cider for as long as the population remains viable.

Pinking is a separate phenomenonSome ciders develop a pink or salmon tint, most often after pressing or after filtration. It is associated with oxidation of particular phenolic fractions and is largely cosmetic, but it is not the same process as ordinary browning and is treated separately.

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