Preserved fruit and vegetables in glass bottles

Small-scale commercial preservation of acid foods

The acidity of a food affects its ability to resist spoilage. Food preservation techniques must take account of acidity to ensure optimal quality.

  • 28 April 2010 | Updated 14 October 2011

Acid foods

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Acidity is an important factor affecting the growth and survival of bacteria and other micro-organisms in foods. Food with high acidity can be safely processed by a small manufacturer.

Low acid foods usually require more complex processing equipment and specially trained staff.

The production of shelf stable acid foods, that is heat treated foods which may be safely stored at room temperature, appears to be growing in popularity.

Frequently these are mixed foods well received at a domestic or catering level which a manufacturer wishes to launch commercially. Example are vegetables, relishes, mushrooms marinated in vinegar, and pickled seafoods including octopus and mussels.

Most foods naturally contain acids. Depending on the type and amount of acid present, they are called high acid or low acid foods (see the figure below).

The amount of acid present, and therefore the foods ability to support the growth of food poisoning bacteria, is measured by the pH of the food. The pH scale extends from 0 to 14. A pH of 7 is neutral; a pH above 7 is called alkaline.

Very few foods have a pH above 7 but many staple foods have a pH in the range 4.5 to 7.0. The pH value of 4.5 is critical in food processing because below this pH, Clostridium botulinum, the most dangerous and most heat resistant of the food poisoning bacteria, is unable to grow.

The pH scale
Acid Foods (pH less than 4.5)
Pickled vegetables
These foods have a pH of about 4.5 and should
only be bottled with the addition of citric acid
Low Acid Foods (pH between 4.5 and 7)

Sweet corn















The pH scale (a measure of acidity)

As the table shows, those foods with a pH value greater than 4.5 are termed low acid and require pressure cooking to make them shelf stable.

Those with a pH less than 4.5 are called acid foods and may be safely processed at temperatures around 100° C - the boiling point of water. Pressure equipment is not required.

Low acid products can be made acid by adding sufficient food acids such as vinegar (acetic acid) or citric acid to reduce the pH to below 4.5.

Shelf stable acid products have been successfully produced in very large quantities.

Canned and bottled fruit and tomato products dominate this group of foods while various pickled and fermented foods such as gherkins and olives are familiar items of trade.

It is not the purpose of this leaflet to discuss the processing of major items of commercial production such as fruit and tomato products.

Rather it is to point out to potential manufacturers the microbiological problems they face when trying to transfer successful kitchen procedures to a product which will be stable on supermarket shelves.

Further reading

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.