Preservation by Heat: Canning, Bottling and Sterilization
What this domain tests
This domain tests thermal death kinetics, the canning sequence, the acidity rule that decides whether a pressure retort is needed, and the identity of spoilage types.
Heat kinetics
- D value: time at a given temperature to reduce the population by 90%.
- z value: temperature change needed to change the D value tenfold; often about 10°C for bacterial spores.
- F0: equivalent minutes at 121.1°C.
- 12D concept: for Clostridium botulinum, a 12-decimal reduction is the minimum safety target.
Commercial sterility means the food is free of viable microorganisms able to grow under normal storage; it is not the same as absolute sterility.
Canning sequence
Raw material selection, washing, blanching, filling, exhausting (steam or thermal), seaming, retorting, cooling and storage. Acid foods (pH below 4.6), such as most fruits and tomatoes, can be processed in boiling water or with a light heat process. Low-acid foods (pH above 4.6), such as vegetables, pulses and meat, need a pressure retort at 115-121°C because C. botulinum spores must be destroyed.
Spoilage types
Flat sour (thermophilic Bacillus species, acid without gas), TA spoilage (gas without acid), putrefactive anaerobic spoilage (C. botulinum) and hydrogen swell from can corrosion.
Worked example
If D at 121°C is 0.25 minute, a 12D process needs 0.25 × 12 = 3.0 minutes at 121°C. If the z value is 10°C and the retort runs at 111°C, D becomes 2.5 minutes and 12D becomes 30 minutes. That is why lowering the retort temperature demands a much longer process.
Traps that cost marks
Boiling water cannot sterilize low-acid food. "Sterile" and "commercially sterile" are not interchangeable. Inadequate cooling encourages thermophilic spoilage, and insufficient headspace strains the seam.