Lesson 25. Chemical And Biological Methods Of Food Preservation

25. Preservation Using Chemicals

Salt and sugar

Salt and sugar have long been used as effective means of extending shelf life of various products as these solutes bind water, leaving less water available for the growth of microorganisms. Essentially the water activity (aw) of the product is reduced, and since most microorganisms require a high water activity, they areunable to survive.

Salt and sugar in concentrated solutions have high osmotic pressure. When these are sufficient to draw water from microbial cells or prevent normal diffusion of water into these cells, a preservative condition exists. (Morris et al.,2004)

The critical concentration of sugar in water to prevent microbial growth will vary depending upon the type of micro-organisms and the presence of other food constituents, but usually 70% sucrose in solution will stop growth of all micro-organisms in foods. Less than this concentration may be effective but for short period of time unless the foods contain acid or they are refrigerated.

Salt becomes a preservative when its concentration is increased and levels of about 18% to 25% in solution generally will prevent all growth of micro-organisms in foods. Except in the case of certain briny condiments, however, this level is rarely tolerated in foods.


Acids such as citric acid, acetic acid (vinegar) and ascorbic acid are also known to confer protection against product deterioration. In these cases, the pH of the product is shifted to being low, that is, more acidic, where very few molds, yeast and bacteria are able to grow and multiply.

Acetic acid is a general preservative inhibiting many species of bacteria, yeasts and to a lesser extent moulds. It is also a product of the lactic-acid fermentation, and its preservative action even at identical pH levels is greater than that of lactic acid. The main application of vinegar (acetic acid) includes products such as pickles, sauces and ketchup.

Other acidulants

  • Malic and tartaric (tartric) acids is used in some countries mainly to acidify and preserve fruit sugar preserves, jams, jellies, etc.

  • Citric acid is the main acid found naturally in citrus fruits; it is widely used (in carbonated beverages) and as an acidifying agent of foods because of its unique flavour properties. It has an unlimited acceptable daily intake and is highly soluble in water. It is a less effective antimicrobial agent than other acids.

  • Ascorbic acid or vitamin C, its isomer isoascorbic or erythorbic acid and their salts are highly soluble in water and safe to use in foods.

25.1 Preservatives

Chemical food preservatives are those substances which are added in very low quantities (up to 0.2%) and which do not alter the organoleptic and physico-chemical properties of the foods at or only very little.They are used to improve the colour and keeping qualities of the final product for some fruits and vegetables.

Food additives such as benzoate and sorbate are quite commonly used in the fruit drink industry to protect against microbial spoilage, while nitrites are used in meat processing. These chemicals work best at acidic pH ranges and when the products are pasteurized. A combination of heat and chemicals where applicable is usually more effective than either one on its own.

Treatment with preservatives takes place after blanching or, when blanching is not needed, after slicing. The composition and strength of the preservative solution vary for different fruit and vegetables. As a general rule, preservatives are not used for treating onions, garlic, leeks, chilies and herbs.

Preservation of food products containing chemical food preservatives is usually based on the combined or synergistic activity of several additives, intrinsic product parameters (e.g. composition, acidity, water activity) and extrinsic factors (e.g. processing temperature, storage atmosphere and temperature).

This approach minimises undesirable changes in product properties and reduces concentration of additives and extent of processing treatments.

25.2 Lypophilic acid food preservatives

Benzoic acid

Benzoic acid in the form of its sodium salt, constitutes one of the most common chemical food preservative. Sodium benzoate is a common preservative in acid or acidified foods such as fruit juices, syrups, jams and jellies, sauerkraut, pickles, preserves, fruit cocktails, etc. Yeasts are inhibited by benzoate to a greater extent than are moulds and bacteria.

Sorbic acid

Sorbic acid is generally considered non toxic and is metabolized; among other common food preservatives the WHO has set the highest acceptable daily intake (25 mg/kg body weight) for sorbic acid. Sorbic acid and its salts are practically tasteless and odourless in foods, when used at reasonable levels (< 0.3 %) and their antimicrobial activity is generally adequate.


Sorbates are used for mould and yeast inhibition in a variety of foods including fruits and vegetables, fruit juices, pickles, sauerkraut, syrups, jellies, jams, preserves, high moisture dehydrated fruits, etc.

Potassium sorbate

Potassium sorbate, a white, fluffy powder, is very soluble in water (over 50%) and when added to acid foods it is hydrolysed to the acid form. Sodium and calcium sorbates also have preservative activities but their application is limited compared to that for the potassium salt, which is employed because of its stability, general ease of preparation and water solubility.

25.3 Gaseous chemical food preservatives

Sulphur dioxide and sulphites

Sulphur dioxide (SO2) has been used for many centuries as a fumigant and especially as a wine preservative. It is a colourless, suffocating, pungent-smelling, non-flammable gas and is very soluble in cold water (85 g in 100 ml at 25°C).

The various sulphite salts contain 50-68% active sulphur dioxide. A pH dependent equilibrium is formed in water and the proportion of SO2 ions increases with decreasing pH values. At pH values less than 4.0 the antimicrobial activity reaches its maximum. The antimicrobial action of sulphur dioxide against yeasts, molds and bacteria is selective, with some species being more resistant than others.

Sulphur dioxide and sulphites are used in the preservation of a variety of food products. In addition to wines these include dehydrated/dried fruits and vegetables, fruit juices, acid pickles, syrups, semi-processed fruit products, etc. In addition to its antimicrobial effects, sulphur dioxide is added to foods for its antioxidant and reducing properties, and to prevent enzymatic and non-enzymatic browning reactions.

Carbon dioxide

Carbon dioxide (CO2) is a colourless, odourless, non-combustible gas, acidic in odour and flavour. In commercial practice it is sold as a liquid under pressure (58 kg per cm³) or solidified as dry ice.

Carbon dioxide is used as a solid (dry ice) in many countries as a means of low-temperature storage and transportation of food products. Besides keeping the temperature low, as it sublimes, the gaseous CO2 inhibits growth of psychrotrophic micro-organisms and prevents spoilage of the food (fruits and vegetables, etc.).

Carbon dioxide is used as a direct additive in the storage of fruits and vegetables. In the controlled/ modified environment storage of fruit and vegetables, the correct combination of O2 and CO2 delays respiration and ripening as well as retarding mould and yeast growth. The amount of CO2 (5-10%) is determined by factors such as nature of product, variety, climate and extent of storage.


 The various forms of chlorine constitute the most widely used chemical sanitizer in the food industry. These chlorine forms include chlorine (Cl2), sodium hypochlorite (NaOCl), calcium hypochlorite (Ca(ClO)2) and chlorine dioxide gas (ClO2).

These compounds are used as water adjuncts in processes such as product washing, transport, and cooling of heat-sterilised cans; in sanitising solutions for equipment surfaces, etc.

 25.4 General rules for chemical preservation

  • Chemical food preservatives have to be used only at a dosage level which is needed for a normal preservation and not more.

  • "Reconditioning" of chemical preserved food, e.g. a new addition of preservative in order to stop a microbiological deterioration already occurred is not recommended.

  • The use of chemical preservatives MUST be strictly limited to those substances which are recognized as being without harmful effects on human beings' health and are accepted by national and international standards and legislation.

25.5 Preservation by Fermentation

  • Food fermentation is usually carried out with the use of micro- organisms, producing alcoholic, acetic or lactic acid as end products as a result of their action on simple sugars present. These reactions take place in the absence of oxygen, that is anaerobic, and are responsible for the production of wine, vinegar, yoghurt and pickled vegetables among others. Having pure cultures of the desirable microorganism in the food products being fermented is critical to obtaining  clean”, characteristic flavours of end products.

  • It is a process in which food spoils, but results in the formation of an edible product. Perhaps the best example of such a food is cheese. Fresh milk does not remain in edible condition for a very long period of time. Its pH is such that harmful pathogens begin to grow in it very rapidly. Early humans discovered, however, that the spoilage of milk can be controlled in such a way as to produce a new product, cheese.

  • Bread is another food product made by the process of fermentation. The addition of yeasts brings about the fermentation of sugars present in the mixture, resulting in the formation of a product that will remain edible much longer than will the original raw materials used in the bread-making process.


  • Audrey Morris, Audia Barnett and Olive-Jean Burrowsb (2004) Food Preservation “Food and Nutrition Resource Manual for theSmall-scale Food Processor in the Caribbean”. Vol. 37, No.3, pg-199

  • GEA,Membrane Processing: State of art technology Vol.2, Issue 2, Nov,2005 Technology Watch.

  • J.vanKooij (1981) Ch:Food preservation by irradiation in IAEA BULLETIN.Radiation and agriculture VOL. 23, no.3

  • LMMappe_UK_30722 Modified Atmosphere Packaging (MAP) in the food industry, WITT

  • Matthew G. Green (2001) Solar Drying Technology for Food Preservation, infogate

  • Richard Coles, Mark Kirwan3, Gary S. Tucker(2001) Ch. Food Biodeterioration and Methods of Preservation in “Food and Beverage Packaging Technology” Ed.:2

  • Sally Tobler, (2006). “The History of Food Preservation, How Science in 19th Century changed the Food Industry”

  • Volker Heinz & Roman Buckow (2009)Food preservation by high pressure J. of Consumer Protection and Food Safety

Suggested Reading:

  • M. ShafiurRahman (ed.), (1999)Handbook of Food Preservation, Marcel Dekker, New York,

  • P. Zeuthen and L. Bogh-Sorensen, (2003)Food Preservation Techniques, CRC Press, Boca Raton

Last modified: Thursday, 22 August 2013, 9:17 AM