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In infants, rennin helps in digestion of milk. Pepsin is also present in their stomach.

Why do infants need rennin for milk digestion, at the first place? Why does pepsin not act on the milk proteins in infants? Does the production quantity of rennin play a role in inhibiting the action of pepsin?

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Simple Answer: Nothing.

Background: First of all, I must tell that your statement So why doesn't pepsin acts on the milk proteins? is based on wrong assumption that pepsin does not act on milk proteins.

Milk contains a protein called casein, which has many different types (usually, we talk about $\kappa$-casein). Infants, which are dependent solely on milk for their survival, have a special protein for digestion of casein, known as rennin (aka chymosin). What chymosin does is breaking casein into para-casein and a glycomacropeptide. The reaction is like this:

chymosin on kappa caseinsource

In general terms, it is said to be a coagulation of milk. It is important to coagulate milk so that milk doesn't pass on to intestine just like water.

Back to the reaction, converting casein to para-casein is all chymosin does. The rest of the digestion of para-casein is carried out by pepsin only, meaning that there is no requirement to prevent pepsin from acting on casein. Also, the quantity of chymosin produced has nothing to do with it. I hope it gets clear to you now :)

PS: the glycomacropeptide is a biologically active protein and has been shown to stimulate the release of cholecystokinin in the body2.

EDIT: Adults too can digest casein with the sole help of pepsin. But pepsin requires lower pH for digesting casein, as low as pH 2.03. At pH higher than this (about 4.0 to 7.0), pepsin gives unusual peptides on digestion4. Infants have a higher pH than this, which makes pepsin incapable of digesting casein in their body5. This also, partly, explains why infants need chymosin for this purpose.

References:

1. Harper’s Review of Physiological Chemistry, 5th edition (p. 177, 1955)

2. Keogh JB, Woonton BW, Taylor CM, Janakievski F, Desilva K, Clifton PM. Effect of glycomacropeptide fractions on cholecystokinin and food intake. Br J Nutr. 2010;104:286–90

3. Qi W, Su R, He Z, Zhang Y, Jin F. 2007. Pepsin-induced changes in the size and molecular weight distribution of bovine casein during enzymatic hydrolysis. J Dairy Sci 90:5004–5011. doi:.10.3168/jds.2007-0230

4. Ono T, Takagi Y, Kunishi I. Casein phosphopeptides release from casein micelles by successive digestion with pepsin and trypsin. Biosci Biotechnol Biochem. 1998;62:16–21. doi: 10.1271/bbb.62.16.

5. Dallas D. C.; Underwood M. A.; Zivkovic A. M.; German J. B. Digestion of protein in premature and term infants. J. Nutr. Disord. Ther. 2012, 21121–9.

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    $\begingroup$ @mesentery Pepsin can too digest casein, but at lower pH, and yes, chymosin is just another name of rennin...I'll add that part. Quite clever BTW :D $\endgroup$ – another 'Homo sapien' Mar 22 '17 at 12:02
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Milk contains a protein called caesin which gets digested in an adult body with the sole help of pepsin.But pepsin requires lower pH for digesting caesin. Since infants are basically fed on milk and they have a higher pH which makes pepsin incapable of digesting caesin in their body.Therefore infants need an enzyme named renin for this purpose.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    $\begingroup$ Hello Pooja. Can you please add some references to support your answer? Good references include peer-reviewed articles, books and websites of scientific organizations. I should also point out that you have not added anything more than what is already mentioned in the existing answer. Moreover, the picture you have put is irrelevant. You should remove it. $\endgroup$ – WYSIWYG Mar 8 at 14:38

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