Chewing food, or masticating, aids the digestive process. It reduces food particle size so digestive juices have a
greater surface area on which to act. Chewing mixes the food with saliva to make swallowing easier. In ruminants,
remastication of large amounts of ingested food is important. After the ruminant has consumed large amounts of
food, it can regurgitate the food as a bolus (cud) by the process of rumination. It then chews the regurgitated food
for a second time. Because poultry have no teeth, the gizzard accomplishes the grinding and crushing process.
Enzymes are responsible for most chemical changes occurring in the digestion process. Various body cells make
these enzymes. The enzymes speed up the biochemical reactions of digestion. This occurs at regular body temperature
and does not use up the enzymes in the process. In hogs and horses, the enzyme ptyalin begins the digestion of
carbohydrates for conversion into maltose and dextrin. The salivary gland secretes ptyalin. Mucin in the saliva
lubricates the food to make it easier to swallow.
The pancreas secretes the enzyme amylopsin into the beginning of the small intestine. Amylopsin aids in the digestion
of starches and dextrins to form simpler dextrins and maltose. Another enzyme that helps with the digestion of
carbohydrates is sucrase. It breaks down sucrose into glucose and fructose. Maltase breaks down maltose into
glucose. Lactase breaks down lactose into glucose and galactose. Enzymes have nothing to do with the actions of
microorganisms on cellulose in ruminants.
Protein digestion begins in the true stomach of several species of animals. The abomasum is the “true” stomach of
ruminants. It is comparable to the true stomach of other mammals and the proventriculus in poultry. Cells in the true
stomach produce hydrochloric acid that activates the enzymes pepsin and rennin. Pepsin and rennin are important
to protein digestion. Pepsin, which is located in the gastric juices, begins protein digestion. It breaks protein down
into simpler compounds called proteoses and peptones. In young nursing animals, the enzyme rennin aids in the
digestion of protein in milk. The pancreas secretes the enzymes trypsin, chymotrypsin, and carboxypeptidase into
the duodenum of the small intestine. These enzymes continue protein digestion. They break down the more complex
substances into amino acids. Amino acids are the final product of protein digestion.
Lipase, which is also in the gastric juices, begins the enzymatic digestion of fats. The process by which lipase
converts fats into higher fatty acids and glycerol is limited in many species. Lipase converts fats into higher fatty
acids and glycerol. This process is limited in many species. The liver secretes bile into the duodenum of the small
intestine. Bile emulsifies fats and breaks them into smaller globules. This increases the total surface area of the fats.
The pancreas secretes the enzyme steapsin to complete the conversion of fats into higher fatty acids and glycerol.