What is Tannin?
Tannin is a chemical that naturally exists in tea and coffee as well as wine and other alcoholic beverages. It can stain crockery and leaves a dirty brown ring around mugs and splotchy marks on saucers. A common misconception is that bleaching will make the stains go away, but it only makes them invisible, it doesn't remove the stain. The stain is still in the glass or cup and will affect beer head retention and will affect the drink taste and customer experience.
The build-up of the tannin can occur over a period, and will be even worse is the glasses aren't pre-washed. The glass washer machine will start to loosen the tannin at around 60°C, but when the hot rinse occurs at approximately 90-100°C, it bakes back onto the porcelain and makes the marks harder to remove and more stubborn. A lack of rinse aid can further exacerbate this if you think your machine isn't using enough rinse aid, you can check this by marking the bottle with a marker pen or some tape and then completing a few washes.
What can be done about Tannin stains?
Tannin Remover is hugely effective at getting rid of tannin stains, either by using it directly in the dish/glass washer or by soaking the stained item in the chemical before running it through the machine.