The thin, smooth white film that sometimes forms on top of homemade pickle brine is usually a harmless kahm yeast layer that lands from the open air; it can be skimmed off with a spoon, and the pickles are still safe to eat.
Real spoilage looks different: coloured mould spots (green, black, pink), brine that has turned cloudy and smells foul, a jar lid that has bulged, or vegetables that have gone soft and slimy. If any of these show up, the whole jar should be discarded.
How do you tell kahm yeast from mould?
Kahm yeast looks flat, thin and white, ripples slightly when the jar is nudged, and has no sharp smell. Mould, on the other hand, has a fuzzy, raised texture, usually shows up green, black or pinkish, and gives off a damp, musty odour. Pickles with kahm yeast are fine once the layer is skimmed off cleanly; mouldy pickles cannot be safely picked around, and the whole jar should go.
When it is hard to tell, touching the layer briefly helps: kahm yeast lifts away cleanly without sticking to the spoon, while mould leaves a sticky, stringy trace behind.
Does cloudy brine always mean trouble?
In fermented pickles, brine turning slightly cloudy with white sediment at the bottom in the first days is a normal sign of fermentation, caused by lactic acid bacteria souring the pickles. The problem is when that cloudiness turns into a foul-smelling, slimy mess or the brine's colour changes noticeably — at that point it is spoilage, not fermentation.
If opening the jar gives off a sour but clean smell, the process is on track; a sharp, rotten smell means the cloudiness has gone well past ordinary fermentation.
What does a bulging lid or gas smell mean?
A slight release of gas and a small pop when opening the lid is normal during fermentation. But if the lid is clearly bulging outward, or opening it releases a sharp smell like rotten eggs, unwanted bacteria may have taken over — that jar should be poured out without even a taste test. Loosening the lid slightly once a day during the first week of fermentation to let built-up gas escape keeps it from bulging too much.
How should pickles be stored?
The vegetables should always be weighed down so they stay fully submerged under the brine, since the parts exposed to air spoil first. Once fermentation is done, keeping the jar in the fridge both slows further fermenting and noticeably extends how long the pickles keep.
Do vinegar pickles and naturally fermented pickles spoil differently?
Vinegar-based pickles spoil far more rarely thanks to their high acidity, but once opened they should still go in the fridge and be discarded if the brine turns cloudy and smells off. Naturally fermented pickles have a more delicate microbial balance: a jar left too long at room temperature can let unwanted bacteria outcompete the lactic acid bacteria, turning the brine foamy and overly soft.
For both types, keeping any utensils that touch the jar clean, and avoiding grabbing vegetables with bare hands, noticeably lowers the risk of spoilage. Using a clean spoon or ladle every single time keeps outside bacteria from being introduced into the jar and helps the brine stay clear for much longer.