Small-scale commercial preservation of acid foods
Spoilage of processed acid foods
Under some conditions some food poisoning bacteria have been shown to grow in foods with a pH below 4.5. However, many years' commercial experience indicates that provided the equilibrium pH of a food or food mixture is below 4.5, the risk of food poisoning is virtually eliminated.
There are, however, numerous harmless micro-organisms that may spoil these foods if processing is not sufficient or if the closure of the container leaks. These include non-sporing and spore forming acid tolerant bacteria, yeasts and moulds.
The most important group of bacteria which spoil acid foods are the lactic acid producing bacteria usually Lactobacilli. Many produce gas and are capable of vigorously fermenting a wide range of foods. They are very acid tolerant and are capable of growth at pH values below 3.5. While able to tolerate more heating than most yeasts and moulds, Lactobacilli can be controlled by moderate heat treatment at temperatures in the range of 85° C – 90° C.
Due to their low heat resistance, yeasts should not pose a problem in heat-processed acid foods although they can be a serious problem if they gain entry to the product after heating is completed. Some yeasts can grow at pH values around 3.0 but fortunately are killed by temperatures greater than 85° C.
Many years' commercial experience indicates that provided the equilibrium pH of a food or food mixture is below 4.5, the risk of food poisoning is virtually eliminated.
Moulds vary greatly in heat resistance but again the greatest risk with spoilage of acid foods is if they gain entry after heating is completed. Most vegetative and sporing forms of mould will be controlled in acid foods by temperatures greater than 85°C. A few moulds are extremely heat resistant and it is not practical to nominate processes which will kill these moulds in all acid foods without causing heat damage to the food itself. Commercially, one attempts to ensure that contamination with these moulds is avoided or raw material likely to be contaminated, e.g. some fruit pulps, is checked for their absence before use.
Some bacteria form heat resistant bodies as part of their normal life cycle. These heat resistant bodies are called spores and are important in determining the heating procedure necessary to produce a food which will not spoil.
The best way to minimise the risk of spoilage of acid preserves by this type of organism is to maintain the pH as low as possible and preferably not greater than 4.0. Some manufacturers are reluctant to alter their formulations to reduce the pH and this means a more severe cook is required if the pH is in the 4.0 – 4.5 range and particularly if the pH is near 4.5.
It is a matter of judgment if the heat process given to acid foods should be designed to take account of spore-forming bacteria. The decision has to be made on the basis of the pH of the product and the association of spore formers with raw materials used in the product.
Hersom, A.C. and Hulland, E.D., Canned Foods: Thermal Processing and Microbiology, 7th Ed., Churchill Livingstone, 1980.
National Canners Association Laboratory. Manual for Food Canners and Processors, vol. 1., Avi Publishing Co., 1968.