Once you have a batch on the shelf, most of the useful information about it is available to your nose and your eyes. This is a rough guide to what the changes over the three months mean.
Week one to two
The batch is loud. The lid hisses when you open it, sometimes dramatically. The smell is sweet and yeasty, a bit like bread dough. The liquid is cloudy and pale. Scraps are floating in a solid mat on the surface.
This is the yeast phase. Sugars are being converted into carbon dioxide and small amounts of alcohol. Nothing is wrong; the batch is just starting up.
If the lid is uncomfortably tight when you try to open it, tilt the container away from you before cracking the seal. The gas will find you either way, but you will not wear the liquid.
Week three to four
The pressure drops. Daily gas release is no longer strictly needed; once a week is enough. The smell shifts from yeasty to slightly sour, like young wine or fresh sourdough starter. The colour is starting to turn amber underneath the floating mat.
A thin white film on the surface is common at this point. This is kahm yeast. It is harmless. Stir it in when you next open the lid or skim it off; the batch continues normally either way.
Day thirty stir
Push the floating scraps back under the liquid with a wooden or plastic spoon. This is the only intervention the batch needs after the initial fortnight of gas release. Close the lid and leave it alone.
Month two
Quiet. Very little gas. The liquid is amber and mostly clear. The smell is sour, sweet, and starts to develop a vinegary note. The scraps are visibly breaking down; you can see them getting softer and paler.
If nothing at all is happening at this point — no smell change, no colour change — the batch has probably stalled. Check that it is not too cold and that the sugar was enough.
Month three
The liquid is a translucent amber, similar in colour to cold-brewed tea. The smell is fully developed: sour, sweet, and unambiguously vinegary. When you swirl the jar, small bubbles rise slowly rather than fizzing.
This is a finished batch. Strain, bottle, use. Harvest and filter covers the details.
What healthy looks like, at a glance
| Sign | Healthy | Worrying |
|---|---|---|
| Smell | Sour, sweet, vinegary | Putrid, rotten, faecal |
| Colour | Amber, translucent | Very dark brown or grey |
| Surface | Thin white film or bubbles | Fuzzy black or green spots |
| Sound on opening | Hiss (early), silence (late) | No hiss ever, or explosive release |
If the smell is genuinely putrid — the way spoiled meat smells rather than the way vinegar smells — discard the batch. Do not use it, and do not put it on plants. A healthy batch is sour and sweet, never rotten.
When in doubt
Your nose is right more often than your worry. If the smell has moved from sweet to sour over three months, the batch is fine. If the smell is one you would refuse in food, trust that instinct.