Chemical Methods of Food Preservation

Lucy Bell-Young

by Lucy Bell-Young

6th January 2021

Blog

Using chemical food preservatives is one of the many ways of preserving food. Food preservation has been part of human activity since long before civilization was developed. In fact, evidence suggests that as many as 14,000 years ago, people in the Middle East and in the Orient actively used sun drying techniques to preserve their food. Chemically preserving food is an ancient practice, particularly the use of salt in drying fish and meats. 

What Are Chemical Preservatives In Food?

Chemical food preservatives are additives that help inhibit fungal and bacterial growth in food. These preservatives can preserve food without the need for drying it. Unlike the traditional and ancient way of drying food, which involved adding salt and/or exposing the food item to the sun, many types of chemical preservatives don’t require desiccation.

Sugar

Some foods, like fruits and vegetables, can be preserved by taking advantage of the chemistry of sugar. Sugar is a good chemical preservative because it helps prevent microbial growth and can easily be prepared from various organic sources like sugarcane, honey, and refined crystal sugar. For example, fresh fruits, such as berries and apples, can either be preserved in sugar syrup or made into jams.

Vinegar

Vinegar is another common chemical that has been used for millennia to preserve food. Using vinegar to preserve food items like vegetables is called pickling. Vinegar is actually a dilute solution of ethanoic acid, more commonly known as acetic acid. This is what makes vinegar such an effective antimicrobial agent: the acidity of the acetic acid in this household condiment prevents bacteria from multiplying, thereby stopping the decomposition process.

It’s possible that pickling was accidentally discovered when ancient people tried to preserve food in wine. Wine becomes vinegar when the fermenting organisms like yeast reach a saturation point of breaking down the sugar content in the mixture. Aside from vegetables, fish is also commonly pickled. 

Salts

Table salt, or sodium chloride, is one of the most common chemicals used in food preservation. Virtually all cultures from ancient times to the present day have used mineral or sea salt to preserve food.

The main principle behind food preservation using salt is dessication through osmotic pressure, where the water inside the cells of food like fruit or meat is absorbed by the salt. Most bacteria cannot grow in an environment with high salt concentration because the salt draws all the water out, and, like every other living organism, bacteria need water to survive. 

There are other types of salts, like nitrite compounds, that help preserve food by killing specific bacteria. For instance, nitrites kill Clostridium botulinum in cured meat products, preventing botulism or food contamination and poisoning due to the accumulation of toxic metabolic products from the bacteria.

Pickled vegetables in glass jars
Using vinegar to preserve food items like vegetables is called pickling

What Chemicals Are Used In Food Preservation?

Modern and industrial levels of preserving food, like canning and meat curing, typically involve three types of chemical preservatives:

  • Benzoates (such as sodium benzoate)
  • Nitrites (such as sodium nitrite)
  • Sulphites (such as sulphur dioxide)

Before adding chemical preservatives to food, the food product undergoes other steps in the preservation process, including cooking, pasteurisation, and irradiation, in which heat, high energy or ionising radiations are applied. By bombarding food with heat or high energy radiation, bacteria and fungi are killed, thus extending the shelf life of the food product. 

The process of food irradiation involves applying gamma rays from Co-60 or Cesium-137 radioisotopes. Electron beams are also used, with energy of up to 10 MeV, as well as X-rays, with energy output of up to 5 MeV. Irradiation is also applied to food packaging to ensure sterility. Exposing food to high doses of ionising energy or radiation may sound dangerous, but it’s actually an approved practice used in many countries as a way of effectively killing pathogens, bacteria, insects, and even moulds. 

Benzoates As Food Preservatives

Chemically, the formula for sodium benzoate is C6H5COONa, and it is a salt of the conjugate base. However, when it dissolves in water, sodium benzoate dissociates into its ions. The benzoate ion acts as base, forming the conjugate acid and a hydroxide ion. Sodium benzoate can be produced by the chemical reaction between sodium hydroxide and benzoic acid.

Sodium benzoate also occurs naturally. It can be found in many food sources, especially in berries like cranberries. Seafood, such as prawns, are another good source of this chemical. It’s also found in many dairy products, like cheese and yoghurt.

Sodium benzoate is commonly used as food preservative because of its ability to inhibit bacterial and fungal growth. A solution of sodium benzoate is acidic, which makes it effective at killing microorganisms, which are unable to survive in acidic environments. 

Regulations limit the use of sodium benzoate as a food preservative and drinks additive to 0.1%. In the UK, it has been largely replaced by potassium sorbate as the additive in the majority of soft drink products. Sodium benzoate is generally not harmful at low concentrations, but the presence of vitamin C can produce benzene when exposed to heat or sunlight. This makes it potentially carcinogenic when mixed with beverages that contain ascorbic acid.

Glass bowl of sodium benzoate with 'E211' card poking out
Sodium benzoate is a commonly used food preservative with the E number E211

Nitrates And Nitrites In Food Preservation

Nitrates and nitrites are commonly used as chemical food preservatives for meat products. Like sodium benzoate, these chemicals inhibit bacterial and fungal growth, particularly the growth of Clostridium botulinum, which causes botulism. These chemicals also preserve the reddish, fresh appearance of processed meat, as well as the flavour. 

Nitrates are used in manufacturing cheese products to prevent them from bloating while being fermented. Vegetables naturally contain nitrates from soil where nitrogen-fixing bacteria are abundant, mostly in green leafy vegetables like spinach and lettuce, and in agricultural lands, nitrates are even artificially added.

In Europe, the safety and health standards for food additives, including nitrate and nitrites, are implemented by the European Food Safety Authority (EFSA) under the European Commission. Currently, the acceptable daily intake for nitrite set by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 2002, is 0.07 milligrams per kilogram of body weight per day (mg/kg bw/day). Meanwhile, the standard maximum daily intake for nitrate is 3.7 mg/kg bw/day.

Sulphites In Food Preservation

Sulphites are commonly used as chemical food preservatives for dried fruits, radishes, and dried potato products. Sulphites are primarily used for aesthetic purposes: they make food appear fresh by stopping the oxidation process that makes fruits and vegetables turn brown.

Sulphites can be applied to fruits such as apricots, fresh oranges, and grapes to maintain their appearance. They also extend the shelf life of fresh food by killing bacteria and fungi, and help in slowing down the natural breakdown of vitamins C and A.

Disclaimer

All content published on the ReAgent.co.uk blog is for information only. The blog, its authors, and affiliates cannot be held responsible for any accident, injury or damage caused in part or directly from using the information provided. Additionally, we do not recommend using any chemical without reading the Material Safety Data Sheet (MSDS), which can be obtained from the manufacturer. You should also follow any safety advice and precautions listed on the product label. If you have health and safety related questions, visit HSE.gov.uk.

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