Flat sour is a result of improper cooling procedures.
Flat sour is a type of food spoilage that results in food having an unappealing tasteless or off-flavor and a sour-like vinegar aroma. It is caused by thermophilic bacteria surviving the canning process because the jars of food were kept hot after canning for too long a period. These particular bacteria produce no gas (unlike some others), so even if the jar originally sealed, there will be no bulges which later cause the lid to become unsealed. Food with flat sour show no signs of spoilage until the jar is opened. While not harmful, this food should be discarded.
Flat sour can be avoided by following proper cooling procedures:
- Promptly after the processing time is completed and any requisite waiting time has elapsed (5 minutes after the lid is removed for boiling water or steam canners; 10 minutes after the weight is removed for pressure canners), remove jars to a cooling rack or towel – do not let them sit in the canner overnight.
- Leave sufficient space between jars – at least 1” to 3” – on the cooling rack.
- Do not cover the cooling jars with a towel or anything that will maintain heat – allow the jars to cool naturally.
For further information on preserving, visit the National Center for Home Food Preservation (NCHFP) or contact your local Cooperative Extension Office.
Brought to you by the UC Master Food Preservers of El Dorado County
Last updated July 2025