Catechins are a group of flavonoid polyphenols found mainly in the seeds and to a lesser extent in the skin. They occur naturally as a defense mechanism against berry infections, so their concentration is higher in varieties grown in humid weather. They are responsible for the bitter notes in the taste of the wine and, because they have antioxidant properties, they contribute to provide color stability during the ripening process. The content of catechins in the sample is directly related to the crushing process and the period of contact with the grape skin.