Biopolymers are macromolecules that are synthesized by living organisms. They have many functions. Some, like DNA, can have very specialized roles in information storage and transfer. Others are produced on a much larger scale and provide structural integrity or protection in the form of hard shells. These ‘structural biopolymers’ represent a diverse range of compositions and chemical functionality and can be broadly classified as polypeptides, polysaccharides or polyphenols. Many of these biopolymers can be extracted from ‘biomass’ and indeed this is often done on a multi-metric ton scale.
One of the most interesting things about biopolymers is that many of them form gels or viscous solutions in water. This is used widely in the food industry to control rheological properties and stability.
Gelation occurs when individual biopolymer chains coil together into double or triple helices. These helices act as junction zones within an extended three-dimensional network that traps water into a solid-like gel. In some biopolymers the gelation is driven simply by cooling whereas other liquid-gel transitions can be triggered by a change in pH (e.g. chitosan, alginate) or addition of multivalent metal cations (e.g. alginate, carrageenan). An excellent description of gelation in biopolymers can be found on the London South Bank University website.
Another factor that affects the properties of biopolymers is the source. Obviously the structure and properties of a polysaccharide are very different from a polypeptide. But there can even be dramatic variation in properties of a single biopolymer, depending on the species that it was sourced from. A good example of this natural varibility is alginate, which is extracted from seaweed. Alginate is a linear copolymer of β-D-mannuronate and α-L-guluronate. The segments are not random, but contain blocks of identical or alternating residues (MMMMMM, GGGGGG or MGMGMG). The strength of the polymer depends on this composition, which varies greatly between different species and growth conditions as well as within different parts of the plant. Polyguluronate segments bind metal cations particularly strongly, forming crosslinked ‘egg-box’ regions.
The natural variation in biopolymers is used widely to fine-tune the properties of foods. However, the possibility of harnessing this resource is only just starting to be explored in materials chemistry.