Decontaminating nuts with compressed carbon dioxide

Food manufacturers are always trying to minimise the risk of recalls due to contamination. Almonds, a popular treat around Christmas time, are one of the more susceptible foods. Now, researchers at the Fraunhofer Institute for Environment, Safety, and Energy Technology UMSICHT have developed a new process to kill off germs that can befall almonds and nuts — using compressed carbon dioxide to decontaminate food.

Processing can affect the quality of food, particularly that of plant-based products that are consumed raw. These foods may be contaminated with Salmonella that can cause food poisoning. These bacteria can even spread to dry foods, which experts call products with low-water activity. This group of low-moisture foods includes almonds, nuts, dried fruits, spices, milk powder and even tea.

Karen Fuchs, a researcher at Fraunhofer UMSICHT in Oberhausen, said Salmonella can go dormant to survive on almonds.

“In the process, they produce additional biomass that protects them from desiccation. If water enters the picture, Salmonella then proliferate explosively.

“But it takes just ten to one hundred of these bacteria to cause food poisoning. Contaminated almonds that make their way into production facilities after harvesting can also contaminate other batches.”

In a joint project with the University of Alberta in Canada, Fuchs and her team investigated technologies that could serve to decontaminate almonds. The German Federal Ministry of Education and Research (BMBF) funded this research venture called MiDeCO2.

“It is common knowledge that pressurised carbon dioxide can kill pathogenic bacteria in liquids such as orange juice. Our research has shown that under certain conditions this also works with dry food,” Fuchs said.

Carbon dioxide is not harmful to the environment or health and can be separated from almonds without a trace of residuals. This does not involve any energy-intensive steps for purification.

Retaining the flavour of almonds

In one process step almonds are decontaminated and impregnated with antimicrobial oils using compressed carbon dioxide in a high-pressure autoclave.

© Karen Fuchs/Fraunhofer UMSICHT

The oil extract coats the almond, making it difficult for germs to re-contaminate the fruit. The reported advantage of this process is that almonds retain their characteristic flavour and quality. Fuchs and her team carried out tests with Staphylococcus carnosus, a surrogate organism known for an even more resistant reaction than Salmonella, proving that the process in the autoclave does not adversely affect the shelf life, rancidity or lipid composition of almonds.

“The oils are not just antibacterial; they also have antioxidant properties. They increase the oxidation potential and extend the shelf life of fats, meaning that almonds are not as quick to go rancid,” Fuchs said.

Fuchs said that the antibacterial and anti-oxidative properties are not the only potential benefit. Increasing the amount of oils that harmonise well with almonds’ flavour could also add a tasty touch of seasoning.

This process also lends itself to other foods. The increased lipid oxidation potential could benefit any food that is prone to oxidation.

Image credit: ©stock.adobe.com/au/pavelkant