Polyphenols are also often called tannins. These compounds are most associated with the bitterness and astringency of hard cider but can also impact color and mouthfeel. There are several types of polyphenols including flavonoids and phenolic acids. Apples, like most fruit, concentrate polyphenols in the peel and the core where the seeds are located. Why do the peel and the core have more polyphenols? It is nature’s way of protecting the fruit.
Polyphenols are antioxidants. They have the ability to suppress free radicals by inhibiting oxidation. For hard cider, suppressing oxidation is generally positive because many spoilage reactions are oxidative. Many bacteria are suppressed by polyphenols. Beer with the addition of hops is a great example. The hops along with alcohol create an environment that prevents many bacteria and oxidative reactions.