Mould in bokashi: white, green or black, what should you do? The complete diagnosis

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You lift the lid of your bucket and find a fuzzy layer on the waste. Don't panic: in bokashi, mould is not necessarily a bad sign. It all depends on its colour, how far it has spread and, above all, the smell that comes with it.

A white film is actually rather reassuring. Green, blue or black patches, on the other hand, mean you need to act, but rarely that you have to throw everything away. Here is how to interpret what you see, respond step by step and stop it happening again.

White film: a good sign

A fine, fluffy white down, sometimes in small patches, covering the surface of the waste or the grains of bran: this is the most common sign of successful fermentation. All reliable sources agree on this, our instructions as well as those of the manufacturers and gardening guides. It usually comes with a sour smell, like pickles or vinegar, sometimes slightly alcoholic.

Don't remove it and don't mix it in: simply carry on filling your bucket as usual, layer by layer. You will often find this same white film on the contents of a bucket that is ready to bury, and even on our activator or on well-fermented homemade bran. To learn how to recognise a bucket that has finished, see the guide How long does fermentation take?

Did you know?

Bokashi barely changes the appearance of food: a carrot stays a carrot, just softened. It is the sour smell and the white film that tell you fermentation has taken hold, not the look of the waste.

Green, blue, black: what it means

Coloured mould, whether green, blue-green, grey or black, shows that in that spot lactic fermentation has not won out. Air, excess moisture or a lack of bran have allowed other micro-organisms to take hold. If the smell remains sour, the problem is localised. If it becomes putrid (rotten eggs, spoiled meat, sewage), decomposition has started to replace fermentation.

Don't confuse it with

• Food that was already mouldy before going into the bucket: a bread crust or a piece of fruit forgotten in the fridge arrives with its own mould. The patch then stays limited to that item. Remove it, or avoid adding leftovers that are visibly mouldy.
• The natural colour of the waste: coffee grounds, tea leaves, avocado skins or green vegetables darken as they ferment. This is not fuzz, it is the food itself.
• Coloured liquid: brown, orange or cloudy depending on what you have added, it is nothing to worry about as long as it smells sour.

The causes: air, too little bran, excess moisture

Coloured mould almost always has one or more of these causes:

• Air: lid not closed properly, dirty seal, tap left open, opening too often, waste not pressed down enough, or a bucket left almost empty for a long time at the start.
• Too little bran: each layer must be completely covered. Meat, fish and very wet leftovers need more.
• Too much moisture: liquid not drained often enough, soggy peelings, leftovers in sauce. A bucket bottom saturated with liquid encourages coloured mould.
• Too much heat: a bucket in full sun or against a radiator can start to rot.
• Bran that is too old or poorly stored: if it smells musty or no longer smells of anything, it will not start fermentation properly. See the guide Bokashi bran.

A few patches: remove, add bran, press down, drain

One or two coloured patches, the size of a coin or a hand, and a smell that is still sour? The bucket can easily be rescued:

1. Remove the affected area with a spoon, along with a little of the material around it, and dispose of it with your food waste or in your garden composter.
2. Sprinkle a good handful of bran over the whole surface, more generously than usual.
3. Press down firmly to push out the air.
4. Drain the liquid, then check that the tap is properly closed.
5. Clean the rim and the lid seal before closing.

Next time you add waste, have a sniff: if the smell is distinctly sour, fermentation has taken over again. Carry on as normal, being a little more generous with the bran for a few layers.

Emy's tip

Lay a sheet of newspaper, or a clean freezer bag, flat on the top layer before closing. It limits the air in contact with the waste and absorbs condensation. Remove it when you next add waste and put it back on top.

Bucket overrun and a putrid smell: should you throw everything away?

When coloured mould covers a large part of the surface, or keeps coming back despite your corrections, and the smell has become putrid, the fermentation has failed. There is no point in continuing to fill this bucket: the contents will not turn into a good pre-compost. But « throwing it away » doesn't necessarily mean « putting it in the bin ».

Our instructions advise emptying mouldy contents into the household rubbish. Today we qualify that advice: only a bucket that is genuinely overrun and foul-smelling needs to be disposed of, and there are better options than the ordinary bin.

How to dispose of it properly

• Bury it away from your crops: this is the most common solution. Dig a hole at least 30 cm deep in a corner of the garden that won't be planted in the coming months, away from roots. Empty the contents into it, sprinkle two or three handfuls of bran, mix with the soil and fill the hole back in well. Leave it for at least a month, and longer in cold weather.
• Add it to an active composter, only if it mainly contains plant waste: place it in the heart of the heap and cover it with a thick layer of brown material. Avoid this option if the bucket contains a lot of meat or fish.
• Your local council's food waste collection, if there is one where you live: it is a simple solution with no risk of smells in the garden.
• The ordinary bin, as a last resort, if you have no other option.

Open the bucket outdoors and avoid breathing over it while you empty it. Wash your hands afterwards. Rinse the bucket with very hot water, carefully scrub the seal, the grid and the tap, then leave it to air-dry with the lid open before starting again. No need for detergent: a thorough rinse and scrub are enough.

Key points

A step-by-step response: a white film, carry on; a few coloured patches with a sour smell, remove them, add bran, press down and drain; a bucket overrun and smelling rotten, bury it away from your crops and start again.

Preventing it next time

• Fill in layers of 3 to 5 cm, each one fully sprinkled with bran, then pressed down. When starting out, begin with at least one or two litres of waste so that the bucket isn't left half full of air for long.
• Be generous with the bran, especially at the beginning, with meat and fish, and for wet waste. A 20 ml measuring spoon helps keep your dosing consistent.
• Open it as little as possible: collect your waste in a small kitchen caddy and empty it into the bucket once a day, or every other day.
• Drain the liquid regularly, every two or three days, and let soggy peelings drip before adding them. All the techniques are in the guide Looking after your bokashi.
• Keep the bucket in the shade, ideally between 18 and 25 °C.
• Store your bran somewhere dry, in a tightly closed container, and replace it if it smells musty. Our Bokashi EM compost activator keeps easily this way.

If your bucket smells bad without showing any mould, the diagnosis is a little different: see our guide Bad smells, too much liquid, fruit flies.

In summary

In bokashi, a white film is a good sign: it goes hand in hand with successful fermentation and needs no action. Green, blue or black mould points to too much air or moisture, or too little bran. A few patches are easily put right by removing the area, adding bran, pressing down and draining. Only a bucket that is overrun and smells putrid needs to be disposed of, preferably by burying it away from your crops rather than putting it in the bin. After that, well-sprinkled and pressed layers, a bucket that is rarely opened and liquid drained regularly are enough to stop it happening again.

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Other topics in the guide

Vermicomposting30 sheets →Composting25 sheets →EM micro-organisms26 sheets →