The main source of phenolic compounds in our diet are fruits, vegetables, seeds of leguminosae, and cereal grains. For many years, polyphenols have been treated as anti-nutrient substances, but, now, the importance of these components as active food ingredients has increased as a result of investigations conducted during the recent years and referring to the pro-health effect of those plant raw materials on human organism. Polyphenols are regarded food antioxidants the activity of which is by several times higher than of vitamins A, E, and C. The effect of processing operations on the antioxidant activity of plant raw material is not clear-cut, and changes in their antioxidant activity referring to a given technological process depend on the plant raw material applied. There are several processing operations after which the antioxidant activity does not change or gets increased; they include: short thermal treatment, blanching/freezing, and alcoholic fermentation.