Cause of Flat Sour in Canned Food

Flat sour spoilage in canned foods is identified by a sour taste and odor. Unlike spoilage caused by gas-forming bacteria, it typically does not cause the can to swell or bulge because the gas produced is either minimal or reabsorbed by the food.

Identifying the Spoilage Bacterium

The primary cause of flat sour spoilage in canned foods is the bacterium Bacillus stearothermophillus.

Reasoning for Flat Sour Spoilage

• Thermophilic Nature: Bacillus stearothermophillus is a bacterium that thrives at high temperatures (thermophilic).
• Spore Survival: Its spores are resistant to heat and can survive the temperatures used during the canning process.
• Growth Conditions: When canned food containing these surviving spores is stored at elevated temperatures (generally above 30°C or 86°F), the spores germinate and the bacteria grow.
• Acid Production: During its growth, Bacillus stearothermophillus ferments sugars in the food, producing acids, mainly lactic acid. This acid production is responsible for the characteristic sour flavor and odor.
• Limited Gas Production: This bacterium produces very little gas, or the gas produced is reabsorbed, preventing the can from swelling and leading to the 'flat' condition associated with this type of spoilage.

Therefore, the heat resistance and specific acid-producing, low-gas-producing metabolism of Bacillus stearothermophillus make it the cause of flat sour spoilage in canned goods.