Lesson 21 : Pentose Phosphate Pathway or HMP Shunt


Digestion of starch actually starts in the mouth. The enzyme amylase, which is responsible for starch degradation is present in saliva. Starch can be considered as a long chain of many concentrated glucose molecules. Amylase splits this chain in many small two-glucose units (called maltose).

However, most of the starch taken up with food is degraded in the small intestine (amylases are also produced by the pancreas and are excreted to the duodenum from which they reach the small intestine). In the stomach, amylases are not active, i think due to low pH. Infants cannot digest starch as they lack amylase production.

Digestive enzymes have problems digesting crystalline structures. Raw starch will digest poorly in the duodenum and small intestine, bacterial degradation will take place mainly in the colon. Resistant starch is starch that escapes digestion in the small intestine of healthy individuals. In order to increase the digestibility, starch is cooked.
The enzymes that break down or hydrolyze starch into the constituent sugars are known as amylases.

Alpha-amylases are found in plants and in animals. Human saliva is rich in amylase, and the pancreas also secretes the enzyme. Individuals from populations with a high-starch diet tend to have more amylase genes than those with low-starch diets; Beta-amylase cuts starch in maltose units.

Starch can be also be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two

Food products made in this way include

  1. Maltodextrin, a lightly hydrolyzed starch product used as a bland-tasting filler and thickener.
  2. Various glucose syrup / corn syrups viscous solutions used as sweeteners and thickeners in many kinds of processed foods.
  3. Dextrose commercial glucose, prepared by the complete hydrolysis of starch.
  4. High fructose syrup, made by treating dextrose solutions with the enzyme glucose isomerase, until a substantial fraction of the glucose has been converted to fructose. In the United States, high fructose corn syrup is the principal sweetener used in sweetened beverages because fructose has better handling characteristics, such as microbiological stability, and more consistent sweetness/flavor. High fructose corn syrup has the same sweetness as sugar.
  5. Sugar alcohols, such as maltitol, erythritol, sorbitol, mannitol, hydrogenated starch hydrolysate are sweeteners made by reducing sugars
Last modified: Saturday, 21 January 2012, 11:19 AM