TREES ARE NATURE’S MIRACLE CHEMICAL FACTORY

Throughout history, forests have provided mankind with beneficial chemical products such as tar, tall oil, turpentine and acetic acid. In the last few decades, the world has witnessed the rise of two tree-based chemicals: xylitol, a sweetener derived from birch trees that fights tooth decay, and sitosterol, a compound that reduces cholesterol levels. One of the newest discoveries is HMR lignan, a highly effective antioxidant extracted from the inner branches of spruce trees. The compound was recently approved as a food additive in the United States.

“There is something fantastic about trees. There they stand in one place and constantly produce new molecules,” marvels Professor Bjarne Holmbom from Åbo Akademi University. Holmbom’s field is wood chemistry, which he has been studying for over 40 years. In recent years, his research team has focused on understanding the chemistry of trees on a molecular level.

The secret behind trees is their chemistry. They grow in a fixed location for decades, centuries, even millennia. They cannot flee or brush off diseases or predators. They must fight back with chemicals.

After millions of years of evolution, trees have developed a vast array of chemical compounds to fight off external threats. These compounds interest not only paper makers but also researchers of human health.

“A certain natural wisdom grows in trees. As wood chemists, we must understand how this wisdom is able to produce unique chemicals. Instead of manufacturing new substances, we study what Mother Nature has created and try to understand why and how this is so,” says Holmbom as he describes the work of wood chemists.

The average tree is half cellulose, a quarter lignin and hemicellulose, and a small rest is the extractives. Cellulose is a fairly well understood compound, as is its cooking process, which tries to eliminate both the lignin and most of the hemicellulose.

In the current processes, the black liquor produced from cooking pulp contains mostly lignin and hemicellulose and is burned as an energy source. The extractives are a source of tall oil and turpentine, though some of these chemicals are burned along with the waste sludge.

According to Holmbom, when lignin and hemicellulose become degraded, they are no longer very useful from a chemical standpoint. “Pure lignin can be useful in many ways, and the same applies to hemicellulose. The extractives are also interesting. It would be good if the different hemicelluloses and extractives could be extracted before the cooking process. There has been considerable research into the pre-extraction of hemicellulose, a process that can now be carried out reasonably well.”

Hemicelluloses are polysaccharides, meaning that they are complex chains of sugars. In pulp processing, hemicelluloses are broken down.

“If we could extract them first and then add them to the pulp, it would improve paper strength. It could also minimize resin problems, as the hemicelluloses’ molecules accumulate on the surface of sticky resin particles and make them less sticky. This could improve the runnability and cleanness of paper machines. This has been studied for many years now,” explains Holmbom.

It is more accurate to refer to hemicelluloses in the plural, because they are really a group of different hemicelluloses that vary between tree species. The cosmetics industry, for example, uses hemicelluloses as emulsifiers to prepare water and oil emulsions.

Holmbom says that research has also been carried out into hemicelluloses as immunomodulators or those properties that fight infections. The building blocks of hemicelluloses also include sugars with interesting physiological effects. One example of such a sugar is mannose, which has been shown to help combat certain stomach infections

“We are currently studying these monosaccharides and what we can do with them, for example converting xylose into xylitol and mannose into mannitol. These sugars are packed with potential and no one knows what’s in store next,” Holmbom says.

“If hemicelluloses are broken down into smaller pieces or so-called oligomers, there is evidence that these pieces are highly bioactive. There is also data that they promote tree growth or function as growth hormones.”

In the future, we may even enjoy a plate of “tree pasta”, as Holmbom believes that hemicellulose can also be used as a dietary fibre. Their sugars are so-called slow carbohydrates, which help balance blood sugar levels and promote weight loss. “Without a doubt, this is worth studying.”

Familiar compounds:

Turpentine, Tall oil, Tar
Sitosterol: a compound derived mainly from pine but also from birch and spruce. It is used in margarine that benefits health by lowering cholesterol levels.
Xylitol: a birch derivative used for example in chewing gum 
to fight tooth decay.