Chemistry of Food Additives and Preservatives

By Stefano Tommasone Ph.D.Reviewed by Emily Henderson, B.Sc.

The practice of preserving foods and improving their properties has been known for millennia. Sugar and salt are typical examples: the first is a very common sweetener, while the second not only improves the flavor but also preserves food for long periods. Over the years, many more chemicals have been used as food additives.

Food Additives. Image Credit: giedre vaitekune/Shutterstock.com

It is evident how chemistry plays a crucial role in the food industry, enabling the exploitation of several useful functions. Additives used in foods serve various purposes, such as enhancing quality and taste, improving nutritional value, and increasing storage ability.

Common additives are:

• Preservatives – maintain freshness, prevent food spoilage and changes in flavor or texture;
• Sweeteners – for added sweetness with or without extra calories;
• Colors – prevent color loss due to exposure to the environment, enhance natural colors, provide color to colorless foods;
• Flavors – add a specific flavor;
• Nutrients – replace vitamins and minerals lost in processing, add nutrients lacking in the diet.

Moreover, other categories of food additives include enzymes, texturizers, and leavening agents.

The usage of acids and bases – both organic and inorganic – or buffering agents, are required to maintain a balance in foods with an extreme acid flavor (i.e. yogurts, fruit juices) or an extremely alkaline one, like egg whites. Some of the most widely used acidity regulators are citric acid, acetic acid, and lactic acid.

The need for preservatives and their function

The term “spoilage” is used to indicate any change that makes food unfit for human consumption. It is a naturally occurring process that, besides involving changes in appearance and texture, can also lead to biological contamination of foods. Several factors can trigger spoilage mechanisms, such as light, air oxidation, microorganisms, endogenous enzymes, and temperature.

Chemical preservatives are fundamental in tackling food spoilage. Depending on their target and mechanism of action, preservatives can be antibacterial, antioxidants, and some chemicals target enzymes in the food itself.

Examples of this last category are citric acid and ascorbic acid, which inhibit the activity of the enzyme phenolase by lowering the pH, preventing the browning of apples and potatoes. Some metal chelators instead (i.e. EDTA, ethylenediaminetetraacetic acid) can remove the metal cofactors that many enzymes need.

Nitrites are well known for their antibacterial properties, particularly sodium nitrite (NaNO₂) and potassium nitrite (KNO₂), used in cured meat products. They also contribute to the texture and flavor and are responsible for the characteristic cured meat color since they react (as nitric oxide – NO) with the muscle protein myoglobin to form nitrosomyoglobin.

Commonly used in wine preparation, sulfites preserve the spoilage of fruits and vegetables and exhibit antioxidant properties. Some naturally occurring organic acids are good antimicrobials, such as propionic acid in strawberries and apples, and benzoates found in cranberries.

When unsaturated fatty acids in oils and lipids are in presence of oxygen, free radicals can initiate peroxide formation at the fatty acid double bonds. The chain reaction propagates to other double bonds, and side products eventually build-up to create the rancid flavor of spoiled food. Chemicals, like butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and propyl gallate, stop the chemical breakdown of food and therefore are used as an antioxidant.

The search for “natural” alternatives

Food additives are strongly regulated and expert scientific groups conduct risk assessments to identify whether they pose appreciable risks to consumers.

Despite the many benefits and the effectiveness in their function, there are concerns for some preservatives on both human health and environmental impact, with some studies reporting risks for humans and animals. There is therefore a growing demand for natural alternatives while keeping the same standard in terms of food safety and efficacy.

Terpenoids and polyphenols express high antimicrobial activities and are responsible for the antimicrobial properties in essential oils, which make them interesting candidates as natural preservatives in the food industry. Terpenoids derive from terpene (C₅H₈)_n, where oxygen is added to the basic structure or the methyl group is removed. Polyphenols are natural secondary metabolites, characterized by multiple phenolic units.

Unfortunately, their supply is subject to limitations being mainly dependant on plant extraction. However, microbial biosynthesis could provide a more sustainable and cost-effective alternative for the mass production of these two chemicals. There have been many studies on the production of terpenoids and polyphenols by metabolic engineering of S. cerevisiae and their antimicrobial activities in foods is being explored.

The tendency towards the search for natural versions concerns food additives in general, particularly colors and flavors. Recently the Food & Drug Administration (FDA) approved a spirulina extract for blue and green confectionaries. The extract contains phycocyanobilin, a blue pigment that along with chlorophyll is responsible for the blue-green color of algae.

Although some additives are naturally occurring, like the vanilla flavor (vanillin), in the vast majority of the cases they are still produced by synthetic methods, since the same products obtained by natural sources have a massive impact on the production costs and ultimately lead to increased prices of the products made.

Conclusions

Humans have always resorted to chemistry to help in preserving and improving the properties of foods. Over the years, an increasing number of alternatives have become available, particularly considering large-scale food production, long-term preservation, and aesthetics.

Synthetic products, even when the same compound is found in nature, are still the elite choice. Nevertheless, recent progress on high-yield production methods, such as microbial biosynthesis, or extraction processes, is also fast-forwarding the search for natural products for commercial use in the future.

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