The adjective “sterile”, depending on which dictionary is cited, will have several entries defining everything from geographic regions to intellectual quality. For the intent of this discussion we are focusing on the definition pertaining to an environment being devoid of living microorganisms, namely spoilage and pathogenic microorganisms. Sterility can be achieved by either chemical or physical means or a combination of the two. The two main types of shelf stable foods of interest in the realm of retorting are acidified foods and low acid canned foods, often designated LACF’s. Another factor placing those categories into our realm of interest is water activity.
Of the two types of shelf stable foods, the acidified food category uses chemical means or a combination of chemical (acid) and physical (heat) to kill microorganisms. This category must have an equilibrium pH of 4.6 or lower. LACF’s equilibrium pH range is anything above 4.6, alcoholic beverages being the exception. pH is a measure of acidity ranging from 0 to 14, 0 being the strongest acid and 14 being the opposite thereof, namely, basic or alkaline. Another attribute of shelf stable canned foods that determines processing requirements is water activity. The water in a food product that has been sealed into a container exists in two forms, bound water and available water. The bound water is either ionically bonded as water of hydration or bonded to cell structures and molecules. Bonded water has no bearing on biologically affected food stability. The amount of free water or water that is not bound to ions or otherwise is water activity or aw. It is this available water that will support microbial activity. Any foods with aw > .85 will support growth of pathogenic and spoilage microorganisms in foods meeting the pH criterium.
While enough heat will completely sterilize food, too much heat will render food unpalatable or inedible. It is for this reason that commercial sterilization was developed. Commercial sterilization is defined as the destruction of all pathogenic and spoilage organisms that can grow in food under normal storage and handling conditions. This includes both vegetative (metabolically active growing and reproducing) cells and their non-vegetative (metabolically inactive) spores. Some bacteria are genetically programmed to develop spores when conditions become too adverse for normal metabolism. The spore by natural design will remain dormant until conditions favorable to metabolism return. It is because spores are quite resistant to heat that commercial sterilization occurs at the elevated temperatures of around 250°F/121.1°C, or higher. Vegetative cells are killed at relatively milder temperatures of 190°F/87.8°C to 212°F/100°C – this is called pasteurization and is the method of rendering acidified foods shelf stable.
Because commercial sterilization is governed by science and required by governments, it must therefore be measurable. Lethality is the desired outcome of commercial sterilization. The measurement of lethality is through calculations with inputs such as heat resistance of the microbial spore, spore counts, temperature and time at temperature. The formulas and calculations of lethality are beyond the scope of a short introduction such as this, but, suffice it to say the unit of measure is F₀, pronounced as “f sub zero” or “f sub oh”. It refers to the equivalent of 1 minute exposure to moist heat at 250°F/121.1°C.
While not the only one, a primary target of commercial sterilization is of the Clostridia class of bacteria known as Clostridium botulinum or “C bot” for short. An interesting characteristic of this bacterium is that 1 kilogram of the biotoxin it produces – botulinum toxin – could kill the entire human race. The bacterial spores not destroyed by commercial sterilization, namely thermophiles, were introduced at the end of last month’s entry.