Compound
Citric acid
A minor acid in apples and a much more significant one in pears, whose metabolism by lactic bacteria is the reason perry gains more butter and more vinegar from malolactic fermentation than cider does.
- Class
- Acids
- Formula
- C6H8O7
- How often it matters
- Regularly encountered
What it does in cider
- Contributes a small fraction of apple juice acidity and a substantially larger fraction of pear juice acidity.
- Is metabolised by lactic acid bacteria alongside malic acid, but by a different route that yields acetic acid and diacetyl rather than lactic acid.
- Therefore raises volatile acidity and buttery character when malolactic fermentation runs in a citrate-rich juice — a perry problem more than a cider one.
- Chelates metal ions, which is why it appears as a permitted processing acid in some jurisdictions and as a component of some cleaning regimes.
How it is perceived
What the compound registers as, and at roughly what concentration. Perception is not a property of the molecule alone: sugar, tannin and carbonation all change where a threshold falls.
- bright
- citrus
- immediate sourness
Reads as a quicker, more lifted sourness than malic acid, arriving and departing faster on the palate.
Descriptors it is responsible for
Sensory records that name Citric acid as a cause. Each states the perception and the mechanism behind it.
- Butter — The buttery note of diacetyl, produced by lactic acid bacteria during malolactic conversion.
- Fresh pear — Clean, watery, faintly green pear aroma, the base fruit note of perry and of pear-fruited cider.
The structure it moves
| Dimension | What it is |
|---|---|
| Acidity | The sharp, mouth-watering quality that makes a cider taste fresh rather than flat. |
| Freshness | Whether the drink smells and tastes of live fruit or of time and air. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Acid adjustment
Correcting the acidity of low-acid juice before fermentation, usually with malic acid, and the difference between the pH question and the titratable acidity question.
Acid balancing
Bringing a cider to the sharpness it needs, which requires separating perceived sharpness from titratable acidity from pH — three related things that do not move together.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Faults it is implicated in
Being implicated is not the same as being a fault. Several of the compounds on this site are ordinary constituents of a sound cider and define a named fault only above a concentration.
Volatile acidity
The measurable fraction of acidity that can be steam-distilled off, dominated by acetic acid and read as vinegar sharpness in the nose and a hard, hot finish.
Excess diacetyl
A butter, butterscotch or popcorn character from diacetyl, produced by lactic acid bacteria metabolising citric acid, and by yeast under stress.
Lactic off-flavours
Sauerkraut, sour milk, silage or cheesy notes from lactic acid bacteria working on sugars and other substrates rather than on malic acid alone.
Low acidity
A cider without enough acid to give it definition, tasting soft, heavy and dull — and sitting at a pH that leaves it exposed to spoilage organisms.
Excessive acidity
Acidity high enough to dominate everything else, leaving a cider that is thin, hard and sour rather than fresh.
Metal haze
A haze or dark cast caused by dissolved iron or copper picked up from equipment, forming insoluble complexes with phosphate or with the cider’s phenolics.
Metallic taint
A metallic, inky or blood-like taste from dissolved iron, copper or zinc picked up from equipment, usually accompanied by accelerated oxidation.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
Oenococcus oeni
The acid-tolerant lactic acid bacterium that carries out most deliberate malolactic fermentation, converting malic acid to lactic acid after the yeast has finished.
Lactiplantibacillus plantarum
A versatile lactic acid bacterium, renamed out of *Lactobacillus* in 2020, capable of malolactic conversion and of a range of faults depending on conditions.
Leuconostoc mesenteroides
A heterofermentative lactic acid bacterium common on fruit and early in fermentation, and a classic producer of the dextran that causes ropiness.
About Citric acid
Citric acid is a footnote in apple juice and a real presence in pear juice, and that difference has a consequence nobody expects until they meet it. Lactic acid bacteria do not treat citrate the way they treat malate. Malate is decarboxylated to lactate and the story ends there. Citrate is taken into a different pathway whose products include acetate and the four-carbon compounds — diacetyl and acetoin — that taste of butter.
So a malolactic fermentation running in a perry does something a malolactic fermentation in a cider largely does not: it raises volatile acidity and it generates diacetyl in quantity. A perry that has gone through malolactic can therefore arrive at a buttery, slightly vinegary character that the maker did not choose and would not have got from an apple juice with the same bacterial population.
This is worth stating because malolactic fermentation is usually described as a single process with a single set of consequences. It is not. Its outcome depends on what organic acids the juice brought with it, and the acid profile of a pear is not the acid profile of an apple.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Acids
Malic acid
The acid of apples, which supplies almost all the sharpness a cider has and, through pH, decides how vulnerable that cider is to everything that could spoil it.
Acids
Lactic acid
The softer acid that replaces malic when malolactic fermentation runs, halving the acid a cider carries and changing its texture as much as its sharpness.
Acids
Acetic acid
The vinegar acid, made by bacteria oxidising ethanol whenever air reaches a cider, and the one fault in cider that no later processing can undo.
Ketones
Diacetyl
The butter compound, made mainly by lactic acid bacteria and perceptible at very low concentration, which is a defining part of some traditional ciders and an obvious fault in others.
Sugar alcohols
Sorbitol
The unfermentable sugar alcohol that pears carry in quantity and apples carry only in trace, and the single reason a fully fermented perry keeps a sweetness a fully fermented cider cannot.
What people ask next
Questions readers ask about the things this page mentions. Each one goes to the section that answers it rather than to a page written to receive the question.
- What is malic acid and why does it matter in cider — Malic acid is the acid of apples and supplies almost all the sharpness a cider has. It also sets pH, which decides how well sulphite works and how exposed the cider is to spoilage, so it does far more work than its taste suggests.
- Can you fix a cider that has gone vinegary — Not really. Volatile acidity cannot be removed at small scale, and blending it away only dilutes it. The realistic options are to accept it, to blend a lightly affected batch into a much larger sound one, or to let it finish becoming vinegar.
- How much acetic acid is normal in cider — A small amount is present in every cider and contributes lift. Sound Asturian sidra natural has been measured at 0.2 to 0.3 g/L, and a clean modern cider much above about 0.7 g/L is generally showing a fault rather than a style.
- Why does cider contain lactic acid — Because malolactic bacteria converted the malic acid into it. Malic acid has two acid groups and lactic acid has one, so roughly half the titratable acidity disappears and the cider tastes softer.
- Why is sorbitol important in perry — Sorbitol is a sugar alcohol that pears carry in quantity and that Saccharomyces cannot ferment. It passes through the whole fermentation untouched, so a perry can taste sweet while being dry by measurement.
- Which bacterium carries out malolactic fermentation in cider — Oenococcus oeni, in cider as in wine. Genome studies find that the cider strains are genetically distinguishable from the wine ones and that the strain most basal to the whole species was isolated from cider.
Where to go next
- All compounds — Grouped by what the fruit brings and what the ferment makes of it.
- Sensory — Every descriptor, with the compound or process that causes it.
- Microbiology — The organisms whose metabolism most of this chemistry belongs to.
Sources
What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.
Research on pear juice composition and sorbitol in perry
Various journals and institute reports · peer-reviewed literature · registered as competent for this subject
The evidence base for the single most important chemical difference between cider and perry: pears carry substantial sorbitol, which yeast does not ferment, so a fully fermented perry retains sweetness a fully fermented cider cannot.
The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Perry Pears
L. C. Luckwill and A. Pollard (eds), published for the National Fruit and Cider Institute by the University of Bristol, 1963 · reference work · bibliographic record verified, not opened 2026-08-25 · covers 1963
Bibliographic record verified on 2026-08-25 against Open Library, which records the work as Luckwill, 1963, published for the National Fruit and Cider Institute by the University of Bristol. The catalogue entry names Luckwill alone where the standard citation names Luckwill and Pollard as joint editors; CiderHQ keeps both names and records the discrepancy rather than silently following one catalogue. This is the foundational descriptive work on English perry pears and the ancestor of most later variety lists. No copy was opened: it is sixty years old, out of print, and held in research libraries rather than in open digital collections.