Plastic microbeads, tiny particles present in personal care products during the 1990s and 2000s, caused significant environmental damage. These tiny particles, smaller than a sesame seed, passed through wastewater treatment plants, accumulating in oceans and rivers and posing a threat to the marine ecosystem.

Fortunately, soaps and scrubs with plastic microbeads are no longer available on the market. In recent years, many countries have recognized microbeads as a source of marine pollution and banned their use in personal care products. These bans have paved the way for more environmentally friendly alternatives, allowing consumers to enjoy deep cleaning without harmful environmental impacts.

Instead of synthetic plastics, research shows that biological waste contains numerous possibilities. One such resource is brewers' spent grain (BSG), a byproduct of beer production. Cheap and abundant, BSG is used as animal feed, in the production of biogas, compost, and fertilizer.

Recently, BSG has also found application as a protein and fiber-rich ingredient in crackers, bread, and cookies. Our research has discovered that BSG is suitable for use in personal care products in the form of sustainable exfoliating microbeads.

Chemical properties of cellulose
Cellulose, the main molecule in plant cell walls, is a key component of brewers' spent grain. For more than a century, scientists have been preparing large quantities of cellulose-based materials by transforming trees through a relatively simple chemical process. Trees are cut, peeled, chopped, pulped, and bleached, and the remaining cellulose is shaped into the desired final form.

Cellulose fibers do not dissolve in most solvents, which is fortunate, as otherwise, cotton shirts would be washed away in the rain, and tissues soaked in acetone would dissolve instead of removing nail polish.

In the cellulose processing industry, there are few chemicals available to overcome cellulose resistance. Most options are known for their instability, high toxicity, high cost, or poor recyclability.

However, sodium hydroxide dissolved in water in various concentrations provides a more sustainable option. Additionally, with sodium hydroxide, cellulose can be converted back into a solid state through a simple neutralization reaction.

This alkaline process can produce pure cellulose microbeads, which were first prepared about ten years ago. Cellulose pulp is dissolved in aqueous sodium hydroxide, then neutralized, drop by drop, in an acidic bath. When the acidic bath is drained, spherical cellulose microbeads remain.

Refining the process
Our research considered the possibility of creating microbeads from the abundant amount of cellulose-rich biological waste from the agri-food industry. With BSG as our cellulose material and targeted production of exfoliating microbeads, we began experimenting in the laboratory.

BSG posed a challenge due to the complexity of its composition. In addition to cellulose, BSG contains hemicellulose, lignin, proteins, lipids, and small amounts of ash, all carefully intertwined to form various plant cell structures.

To overcome this obstacle, dilute acid hydrolysis softens the cellulose and other fibers (hemicellulose and lignin) from BSG. Rough filtration rinses out simple sugars and proteins, leaving behind cellulose and lignin-enriched pulp.

The next steps include refining the sodium hydroxide solution. Only at specific temperatures and concentrations are sodium hydroxide solutions stronger than the bonds holding cellulose fibers together; this is also true for the more complex BSG pulp.

Our experiments revealed a narrow process window in which BSG pulp completely dissolves, with the help of small amounts of zinc oxide. Then, by introducing these BSG solutions, drop by drop, into an acidic bath, we simultaneously achieve shaping and hardening.

After a few hours, the acidic bath is drained, and smooth, spherical BSG-based microbeads remain.

Finally, strength and stability tests showed that BSG beads have the necessary strength to withstand compared to conventional plastic microbeads. When incorporated into soaps, BSG-based microbeads showed better performance than other plastic microbead alternatives currently available, such as ground coconut shells and apricot pits.

Creative solutions
Transforming beer waste into exfoliating microbeads is another step toward a more sustainable future. By leveraging the properties of cellulose and lignin present in BSG, this innovation demonstrates the potential of waste materials to contribute to sustainable solutions.

This success highlights the importance of research and innovation in the transition to more environmentally friendly practices. Ultimately, it encourages the exploration of other similar possibilities to reduce our environmental footprint.

If it is possible to transform beer waste into a valuable ingredient for personal care products, just imagine what other possibilities can be found in waste.

Original:
Amy McMackin
Doctoral Researcher, Sustainable Food Processing, Swiss Federal Institute of Technology in Zurich
Sébastien Cardinal
Professor of Organic Chemistry, Université du Québec à Rimouski (UQAR)