Spore-forming bacteria present one of the greatest challenges to the production of shelf-stable, heat-processed foods. The genera Bacillus (aerobic or facultatively anaerobic) and Clostridium (obligately anaerobic) produce endospores with extreme heat resistance, enabling them to survive thermal processes that kill vegetative cells. The concept of “commercial sterility” recognizes that a heat process sufficient to destroy C. botulinum spores may not eliminate all thermophilic spore formers, but the product is safe if spore germination and growth are prevented by storage conditions.
Bacillus stearothermophilus (now Geobacillus stearothermophilus) is the classic indicator organism for flat sour spoilage in low-acid canned foods. It is a thermophilic, obligately aerobic spore former with a D₁₂₁ of 1.5-4.0 minutes, making it more heat-resistant than C. botulinum. Flat sour spoilage is characterized by a pH drop (due to acid production without gas) and an off-flavor described as “medicinal” or “metallic,” with no visible can swelling. This spoilage typically occurs in canned vegetables stored at elevated temperatures (above 43°C), such as in tropical climates or warehouse heat abuse.
Bacillus subtilis and Bacillus coagulans (the latter is a major spoilage agent in acid and acidified foods, pH 4.0-4.6) cause a variety of spoilage types. B. subtilis produces proteolytic enzymes that cause textural degradation and off-odors. Bacillus cereus can cause both foodborne illness (emetic and diarrheal syndromes) and spoilage, particularly in pasteurized dairy products and cooked rice. Its psychrotrophic strains are capable of growth at refrigeration temperatures, making them relevant to chilled food safety and spoilage.
Among clostridia, Clostridium thermosaccharolyticum is a thermophilic, obligately anaerobic spore former that causes “TA spoilage” (thermophilic anaerobic spoilage) in canned foods. It produces hydrogen, CO₂, and organic acids, resulting in swollen cans and a sour, cheesy odor. Unlike flat sour spoilage, TA spoilage is accompanied by gas production and can swelling. Clostridium putrefaciens and C. sporogenes cause putrefactive spoilage characterized by production of H₂S, mercaptans, and other sulfur compounds. Control measures include adequate thermal processing, rapid cooling after retorting, and storage below 40°C to prevent germination of thermophilic spores. Spore-forming spoilage organisms are closely related to pathogenic Clostridium species. Thermal processes designed for commercial sterilization target C. botulinum but may not eliminate thermophilic spoilage spores. Pasteurization is insufficient for spore control.
