From river stain to your cup of tea: the secret world of tannins
- Written by The Conversation
While stopped in heavy Melbourne traffic recently, I noticed that what looked like a shadow under a row of spotted gums (Corymbia maculata) along a major road was actually a stain on the concrete kerb.
As a botanist, it caught my attention; biological stains always have an interesting story attached.
Stains like these – under many tree species, on your car after certain leaves have fallen on them, and on your timber deck after rain has washed leaves onto it – are from tannins leaching out of foliage.
Tannins are astringent and bitter-tasting chemicals found in many leaves; they’re what add flavour to red wines, chocolate and tea. Oak timber is high in tannins, and it’s the tannins in oak barrels that enrich the flavour of some wines.
So tannin stains on concrete, cars and decks may be unsightly, but that doesn’t mean tannins are unimportant.
Important to plants
When it rains, materials such as amino acids (the building blocks of proteins) and sugars can be washed or leached from leaves of trees. These contribute to the complex chemistry of soils. Many of the microflora and fauna in the ground, which contribute to healthy soils, depend on these chemicals for their growth and proliferation.
Among the many chemicals that are leached from leaves are tannins.
Tannins are important to plants as their bitter taste makes the leaves unpalatable; it’s the plant’s way of trying to dissuade animals from eating their leaves.
Some caterpillars and grasshoppers are turned off by tannins; koalas and possums cope with tannins by having specialised gut microbes that allow them to consume high-tannin diets.
If you spot a water-filled cavity or hollow in a tree trunk, or in between the trunks and a branch, it is often dark brown or even black due to the tannins that have leached into it.
These tannins can be efficient in preventing insects and other pests from growing in the water, although mosquito larvae can be quite resilient if the concentration of tannin is low.
Sometimes forests and felled timber leach so much tannin into streams and rivers they create a blackwater river, where the water may look and taste bad, but is often safe to drink.
The brown stains seen in Tidal River at Wilson’s Promontory, Victoria, and the Franklin River, Tasmania, are caused by tannins.
The dark colour of tannin streams does not mean they are unhealthy, and may indicate the tree canopy cover upstream is in good nick.
Tannins leached into soil can play an important role in the rate of litter decomposition, which is important to ecosystem function.
When tannin levels are high, they slow down litter decomposition. That means the leaf litter can be a food source for bugs for a long time. It also reduces soil drying and protects soil microbes.
Useful to humans, too
A number of tree species contain tannins that contribute to the durability and the distinctive colours of their timbers.
The name tannin comes from their use, particularly in days gone by, in the tanning of leather. However, they are also used in the dyeing of fabrics and as wood preservatives.
Tannins range in colour from pale yellows through orange to dark browns that are almost black. Their chemical structure means they bind well with fibres such as cotton and linen for long-lasting and environmentally-friendly colours.
We are just learning of their many environmental roles, and their impact on human health has yet to be fully explored (we do know they can be anti-oxidants and anti-carcinogenic).
As for that tannin stain I spotted while stuck in traffic, it’s likely it’ll still be there next time I drive past. Concrete is very porous and the tannins from the leaves above will be topped up each time it rains. So stains like this may be more or less permanent.
Tannin stains can generally be washed from vehicles and other non-porous surfaces quite easily, but a high pressure spray may be required to clean up tannin-embedded concrete, slate or stone paving surfaces. Warm or hot water may help.
For such a common stain on concrete, there is much we don’t know about tannins and so much to learn.
Read more https://theconversation.com/from-river-stain-to-your-cup-of-tea-the-secret-world-of-tannins-271730
