Citric acid: how this neat little molecule can help the environment

Citric acid is so ubiquitous and innocuous that it’s easy to gloss over it when you see it listed in the ingredient list for foods and cosmetics. But it’s a surprisingly versatile little molecule that lends itself to plenty of green innovations.
In this article, we’ll talk about why citric acid is so useful – and how it might become even more useful in the near future.

No one’s afraid of citric acid

What really sets the stage for citric acid is that it’s such a benign compound. No one’s about to tell you that you should be worried about citric acid being added to your food.

Perhaps part of the reason citric acid is so unintimidating is that it was named for lemon juice, from which it was originally extracted in the 1700s. Its other, more intimidating alias is 2-hydroxy-1,2,3-propanetricarboxylic acid.

It is a safe chemical that poses no dangers to humans nor to the environment – neither through its applications, nor in its production.

Besides its presence in lemon and other citrus fruits, citric acid is ubiquitous in nature. Almost all aerobic organisms have citric acid, since it plays an essential role in cellular metabolism via the citric acid cycle – also known as the Krebs cycle.

Another key to citric acid is that the production process doesn’t pose environmental harm. Crystallization from lemon juice was the main method of production up until about the 1920s, when it was discovered that microbial fermentation could obtain unprecedented yields. Today, natural extraction remains much rarer, with microbial fermentation accounting for the bulk of citric acid production. Despite that, citric acid still maintains the advantage of being regarded as a “natural” compound.

From the very beginning, extraction of citric acid was already useful, because it has excellent antibacterial and antimicrobial properties, and also works as an acidifying agent. With these properties, it is a useful additive in foods, cosmetics, pharmaceuticals, and cleaning agents. There are no safety concerns for its use as an additive, and it can often be used in place of other, more harmful ingredients, particularly in the case of cleaning products.

Citric acid innovations in food packaging

Citric acid goes beyond “do no harm.” We’ve already mentioned the traditional role of citric acid as a preservative. Soon, it could also have a huge positive impact on the environment and human health by reducing the harm caused by single-use plastics.

Single-use plastics from food packaging are a devastating threat to the environment. Not only do plastics pollute the environment and cause harm to wildlife, but microplastics find their way into food, and therefore into our bodies. Microplastics have been found in human tissue and human blood, and are thought to be a contributing cause of some diseases.

So what does all that have to do with citric acid?

Citric acid has a unique chemistry that lends itself towards functioning as a cross-linking agent. That means that it can be used to cross-link bio-polymers in order to create green, biodegradable food packaging materials.

Let’s take a look at the molecular structure of citric acid. Its functional groups include three carboxyl groups and one hydroxyl group, which lend themselves easily towards cross-linking with green packaging materials like starch, cellulose, and gelatin. This reaction can occur at ambient temperature with some materials, or in other cases requires high temperatures to create the energy needed to drive the formation of new bonds.

Citric acid acts not only as a cross-linking agent, but also as a plasticizer, providing increased flexibility to the packaging material.

A final added bonus? The biodegradable food packaging materials will now also have the properties of keeping food fresher and safer.

Citric acid was already widely used as a safe and effective antioxidant, antimicrobial, and antibacterial agent in foods. Part of its preservative activity is attributed to its role as a chelating agent, meaning it can bind metal ions and prevent them from catalyzing bacterial reactions taking place in our food. Now all of those properties could potentially be built into food packaging materials.

Just how near is the future of food packaging?

For now, more research is needed in order to establish efficient methods for the industrial production of citric acid cross-linked biofilms.

Citric acid provides an exciting prospect for the future of food packaging, serving a triple role as a cross-linking agent, a preservative, and a plasticizer. These innovations could help reduce the use of single-use plastics and their harmful effect on environmental and human health.

References

Ciriminna, R., Meneguzzo, F., Delisi, R. et al. Citric acid: emerging applications of key biotechnology industrial product. Chemistry Central Journal 11, 22 (2017). https://doi.org/10.1186/s13065-017-0251-y

Silva, M. M., and F. C. Lidon. “AN OVERVIEW ON APPLICATIONS AND SIDE EFFECTS OF ANTIOXIDANT FOOD ADDITIVES”. Emirates Journal of Food and Agriculture, vol. 28, no. 12, Jan. 2016, pp. 823-32, doi:10.9755/ejfa.2016-07-806.

Zhang, Wanli et al. “Cross-linked biopolymeric films by citric acid for food packaging and preservation.” Advances in colloid and interface science vol. 314 (2023): 102886. doi:10.1016/j.cis.2023.102886