Usually because malolactic fermentation (MLF) has started. Bacteria, not yeast, convert malic acid into lactic acid and carbon dioxide, and in cider this often begins months after the yeast has finished, commonly in spring. A sudden return of gas in bottles or vessels at that point isn't spoilage in itself: it's MLF starting on a delay, and it makes the cider noticeably softer.
What does malolactic fermentation do to cider?
It swaps a sharp acid for a softer one. In one study cited in Home Cidermaking Science, malic acid in apple juice measured 2.65 g/L and dropped by more than 90% after fermentation, to 0.14–0.18 g/L. Other estimates put the total acidity reduction at roughly 50%. The carbon dioxide released is the fizz you see.
| Before MLF | After MLF | |
|---|---|---|
| Malic acid (one study) | 2.65 g/L | 0.14–0.18 g/L |
| Total acidity (other estimates) | 100% | about 50% |
Which bacteria cause it?
Two main groups: Oenococcus oeni (formerly Leuconostoc oenos) and species of Lactobacillus, principally L. collinoides. Both occur wild in cider without any intervention. Oenococcus cultures are also sold if you'd rather control the process than wait for it.
MLF in cider often doesn't start right away. It typically begins months later, commonly in spring, well after primary fermentation has finished and the cider seemed done.
Why didn't sulfite stop it?
Nobody has fully settled that. Research going back to the 1970s noted the paradox: in the laboratory, these bacteria are inhibited at sulfite concentrations below 10 ppm at pH 4.0, while typical cider must gets roughly 30 ppm of free SO₂ before the yeast is pitched. Yet cider regularly goes through MLF the following spring. Possible explanations: sulfite in real cider may suppress the bacteria rather than kill them, or strains that survive cidery conditions may be hardier than laboratory cultures.
Do you want MLF in your cider?
It's a style choice, not a rule. English and French cidermaking treat MLF as normal and actively encourage it. American craft cider often skips it on purpose, aiming for a younger, sharper, brighter profile. If you bottle a cider that hasn't been through MLF and it starts in the bottle, the extra gas adds pressure, so keep that in mind before bottling a cider that is still sharp and young in the cold months.
Measuring your juice's acidity and apple type: the Acidity Advisor in the Cider calculator (Pro). Sulfite at bottling: How Much Sulfite Should You Add to Cider at Bottling?
The malolactic change, and what comes after
Home Cidermaking Science, Chapter 7, covers MLF in cider and perry; Chapter 10 covers bottling and packaging, and Chapter 11 diagnosing faults.
Educational content. Rules on home fermentation vary by country; check the law where you live. Full disclaimer