Raffinose is not a reducing sugar because it does not have an aldehyde group available in an open-chain formation, and it can't form one in solution. A free aldehyde group is what allows a sugar to act as a reducing agent by donating an electron in a redox chemical reaction.
Raffinose is a closed-chain trisaccharide made of glucose, galactose and fructose. It is found widely throughout the plant kingdom, and it ranks second to sucrose in abundance as a soluble carbohydrate. Humans do not have the enzyme to break down raffinose, and therefore, it passes through the stomach and upper intestine undigested.