Finnish study touts xylans as starch replacer in future bioplastics

A new Finnish study suggests that xylans and mannans, plant cell wall polysaccharides derived from agricultural and forestry sidestreams, could replace starch as a base material for biodegradable packaging.

“Starch, another water-soluble polysaccharide, is widely studied and already used in commercially packaging materials,” Kirsi S. Mikkonen, one of the authors of the study, said.

“Xylans and mannans are not energy nutrients from humans, so it would be beneficial to utilise them as packaging materials rather than starch,” the researcher, from the Department of Food and Environmental Sciences at the University of Helsinki, said.

Xylans and mannans are the most common type of polysaccharides after cellulose, and are biosynthesized by trees and plants.

For example, xylans are the main hemicelluloses in angiosperms (flowering plants) and make up 25-35% of the lignified tissues in grasses and cereals.

Other sources are agricultural crops such as sugar cane and cereal husks, as well as pulp side-streamed from hardwoods.

Great packaging potential

Mikkonen said xylan- and mannan-based films have great potential in packaging applications. She said: “They are plant derived and biodegradable. They could potentially be obtained in large quantities from agricultural and forestry sidestreams, meaning they are both sustainable and economical.

"They are safe for food applications and they have potential as oxygen and grease barriers,” she added. According to the study, several patents have already been filed relating to the use of xylans and mannans in packaging.

One new patent application, for example, proposed a method of producing films by combining hemicelluloses with a crosslinking or hydrophobizing agent.

The resulting film would then serve as an oxygen, aroma and/or grease barrier and would be suitable for the coatings on fruit, cheese or paper. If xylan- and mannan-based films were to be used as biodegradable wrappers, plastic bags or trays, manufacturers would have to increase strength, stiffness and flexibility with plasticisers, crosslinking agents or blended polymers.

Nano-sized components

One interesting trend is the reinforcement of polysaccharide-based films with nano-sized components, such as cellulose or clay, according to the study. However, Mikkonen warned that the development of xylan- and mannan-based films is a relatively novel concept and research does not, as yet, indicate how these materials could be processed using existing manufacturing technology. For example, there are no studies on extruding these types of films.

“During the last five to ten years, the number of scientific papers published on this topic has greatly increased, but the lack of industrial processes for hemicellulose isolation and recovery has restricted their availability for large scale trials,” Mikkonen said.

She added: “There is still a lot of work to be done… but the development of the biorefinery concept and the necessity of more efficient utilization of renewable resources motivate the researchers and industry.”

Source: Trends in Food Science & Technology