The digestion of lipids starts in the stomach, catalysed by an acid-stable lipase, which originates in the glands at the back of the tongue, known as lingual lipase. However, the rate of hydrolysis is very slow because the lipid is not emulsified for the digestion process. Lipase can hydrolyse or degrade triacylglycerol only in the lipid–water interface. However, the rate of hydrolysis is slow because the lipid does not get emulsified, and the enzyme can only degrade the triacylglycerol in the lipid water interface.

The triacylglycerol molecule consists of fatty acids of short or medium chain length (milk), which can be catalysed by a diff erent enzyme called gastric lipase. The optimum activity of gastric lipase is found to be in neutral pH and therefore this enzyme is not of much use. In adults, the pH in gastric juice is low. However, in infants, whose gastric juice pH is nearer neutral and whose diet is only milk lipids, gastric lipase may digest the milk lipids in such neutral pH. Overall, in adults, dietary lipids are not digested in mouth and stomach and the digestion starts only in the small intestine. The overview of the lipid digestion is shown in Figure 10.1.