Question 1

A thin white film on the surface

Accepted Answer

Almost always kahm yeast — a harmless surface film that forms when a fermenting liquid meets air. It looks alarming and it is nothing. Stir it back into the batch when you next open the lid, or skim it off with a spoon. The batch continues as normal. Do not discard.

Question 2

Fuzzy black or green mould

Accepted Answer

Contamination, usually because a scrap floated above the liquid line and dried out. Small spots are fixable; widespread growth is not. Small isolated spot: scoop it and any surrounding pulp out, dissolve a tablespoon of brown sugar in a little warm water and add it, close the lid. Widespread mould covering most of the surface: discard the batch and start again.

Question 3

Container is bulging or hard

Accepted Answer

Normal CO2 pressure from an active fermentation. The reason we use plastic and not glass — glass can crack under this pressure. Release the gas daily during the first month, then weekly. Tilt the container away from you when you crack the lid so any liquid that follows the gas does not hit you. Never leave a bulging batch sealed for weeks.

Question 4

No bubbles at all after a week

Accepted Answer

Three usual causes: the batch is too cold, the lid is so tight you cannot see the activity, or there is not enough sugar to feed the fermentation. Move the batch to a warmer spot between roughly 20 and 30 degrees, out of direct sun. Open the lid and stir; you should see fine bubbles rise. If nothing changes after another week, add half again as much sugar dissolved in a little water, and give it a fortnight.

Question 5

Fruit flies, or larvae in the batch

Accepted Answer

The lid was not sealed properly between openings. Flies find fermenting sugar within minutes. Skim off anything visible on the surface. Close the lid firmly and keep it closed except during gas release. If larvae are through the whole batch, discard it. The batch does not need to breathe; the lid should be shut tight the rest of the time.

Question 6

A putrid or rotten smell, not sour and sweet

Accepted Answer

Something spoiled instead of fermenting. Usually oil, cooked food, meat or dairy got in, or the sugar was too low and putrefaction won the race against acidity. A healthy batch smells sour, sweet and vinegary — never rotten. Discard the batch. Do not use it, and do not put it on plants or down a drain you care about. Start again with fresh scraps and pay attention to the ratio.

Question 7

The liquid is separating into layers

Accepted Answer

Perfectly normal. Denser sediment settles to the bottom, lighter pulp floats to the top, clearer liquid sits in the middle. It happens more visibly late in the batch. Do nothing until harvest. At day ninety, strain through cloth to catch the sediment, and the bottled liquid will be uniform.

What is your batch doing?