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All organisms produce globular protein molecules called enzymes. Enzymes serve as organic catalysts to help carry out or speed up chemical reactions that are necessary for the life functions of organisms. Specific enzymes are produced to bring about specific chemical reactions for specific purposes in specific locations. It is a constant, ongoing process. Without enzymes, the body's ability to function and repair itself would be severely affected.
Rennin enzymes are produced by the stomach cells of young mammals. Rennin is secreted in large amounts right after the birth and then its production gradually drops off. It is then eclipsed in importance by the Pepsin enzyme. Pepsin and rennin are the only two enzymes produced in the stomach.
Rennin is produced as the inactive prorennin. When milk enters the stomach, prorennin is activated by the hydrochloric acid in the gastric juice and converted into the active rennin enzyme.
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Function of Rennin Enzymes
The function of rennin enzymes is to curdle milk and separate it into semi-solid curds and liquid whey. Curdling of the milk is necessary if the milk is to be retained in the stomach long enough for the milk proteins to be digested properly. Young mammals would derive no benefit from the milk they drink if it passed through the stomach too quickly, which is what would happen if it was allowed to remain in its non-curdled state.
Let us understand how the rennin causes milk to curdle. Milk contains caseinogen protein. Caseinogen has four major molecule types. Three of these - alpha-s1 casein, alpha-s2 casein and beta casein - can be precipitated by the calcium in the milk; calcium causes no reaction on the fourth kappa casein.
The kappa casein prevents the alpha and beta caseins from being precipitated by calcium and thereby prevents milk coagulation in normal circumstances. Since this is not desirable in this instance, the rennin enzyme targets the kappa casein and makes it inactive. Milk can then be curdled, digested and all its benefits derived.
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Effect of Temperature
Temperature affects the rate of reaction of rennin and milk. At body temperature, which is 37º C, rate of reaction is most favorable.
Above 37º C, according to the principles of Kinetic Theory, the increase in temperature imparts the milk and rennin molecules with increased energy and thereby increased velocity. As the molecular movement speeds up, the frequency of molecular collisions goes up and so the rate of chemical reactions increase. But then, the rising temperature affects and breaks down the enzyme's hydrogen and ionic bonds. As the bonds disintegrate, so does the shape of the enzyme's active site – this is the part into which the substrate fits; the substrate being the molecule on which the enzyme acts, which, here, is the milk molecule. The altered active site no longer accepts the caseinogen molecules. The rennin stops affecting them.
Below 37º C, according to the principles of Kinetic Theory, the fall in temperature reduces the energy levels of the milk and rennin molecules. They slow down and there are fewer collisions. So the rate of reaction decreases. If the temperature falls to 0º C, reactions stop altogether as there is no molecular activity at this level.
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Due to its important role in curdling milk, rennin enzyme is widely used in the food industry, notably in the production of cheese.
Rennin for cheese-making was once derived mainly from the dried stomachs of calves and from some non-animal sources. This however makes for a limited supply of rennin, and industrial cheese production requires rennin in far greater amounts. So most of the rennin enzyme used in industrial cheese production is now produced by genetic engineering methods.