The functional group formed is an ester group. When a fatty acid reacts with glycerol, the carboxyl group (−COOH)(-COOH)(−COOH) of the fatty acid reacts with a hydroxyl group (−OH)(-OH)(−OH) of glycerol, releasing water and forming an ester linkage (−COO−)(-COO-)(−COO−), and three such ester bonds together make a triglyceride.