Final answer:
The increased acidity during the digestion of milk proteins and lipids is due to the presence of gastric acids like HCl for protein digestion and the release of fatty acids from lipid digestion, which are inherently acidic and may slightly increase the stomach's acidity.
Step-by-step explanation:
Digestion of Proteins and Lipids in Milk
The process of digestion involves the enzymatic breakdown of food, which includes the chemical digestion of proteins and lipids present in milk. Intestinal digestion of proteins begins in the stomach with the help of HCl and the enzyme pepsin, continuing into the small intestine where digestive enzymes from the pancreas and intestinal lining further break down proteins into amino acids. The acidity in the stomach rises due to HCl, which is necessary for the activation of pepsin and rennin, enzymes that initiate protein digestion.
The digestion of lipids mainly occurs in the small intestine. Lipids enter this part of the gastrointestinal tract largely undigested. Pancreatic lipase and bile, secreted by the liver, are necessary to emulsify and break down lipid molecules into smaller units, such as micelles, which have increased surface area for the action of pancreatic lipase. As lipids are broken down into their constituent fatty acids, the acidity increases, since fatty acids are acidic in nature.
In cases such as lactose intolerance, the absence of sufficient lactase to digest the milk sugar lactose may cause undigested lactose to ferment in the intestine, leading to a reduction in pH and thus contributing to increased acidity. Additionally, fatty acids released during lipid digestion can diffuse into the stomach wall, entering the portal vein and slightly increasing the stomach's acidity. This digestion and absorption of milk nutrients, therefore, impact the acidity levels within the digestive system.