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As a novice cheesemaker, I have read that Ultra High Temperature prosessed milk is unsuitable for cheesemaking, as casein proteins are denatured and cannot be curdled with rennet. I have verified that claim experimentally. Although curd has formed, its quality was poor (I managed to make whey-like cheese after further heating though).

As casein is the majority of milk protein and curdling with rennet is important component of milk digestion, I started to worry if feeding UHT milk to toddlers is a good idea. (I also wonder, whether boiling milk may lead to a similar casein denaturation.)

Unfortunately, Wikipedia addresses only the cheesemaking aspect of UHT processing. I do not know whether it is possible to digest temperature-denatured casein (without proper curdling). Moreover, after some more research I am not sure what is mechanism of milk digestion in infants: Which enzyme curdles milk in human infants?

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  • $\begingroup$ Why are you asking specifically about infants and toddlers? They have the same enzymes as adults and they typically digest milk better than adults. $\endgroup$ – Jan Aug 9 at 8:32
  • $\begingroup$ @Jan It is exactly because they typically digest milk better than adults. I want to avoid answers based on age-related enzyme production loss. I have been taught in school that our infants produce rennet while adults don't (which I am not so sure any more). For sure a lot of adults do not produce enough lactase - thus I expect there may be similar case with rennet. Also, because a toddler is the main milk consumer in my family. ;) $\endgroup$ – abukaj Aug 9 at 9:46
  • $\begingroup$ Goat’s milk is easier to digest but did you mean cows milk? $\endgroup$ – Solar Mike Aug 10 at 11:27
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UHT treatment increases the digestibility of casein.

Ultra High Temperature Treatment, but Not Pasteurization, Affects the Postprandial Kinetics of Milk Proteins in Humans (Journal of Nutrition)

Moreover, the UHT treatment of milk causes structural changes in its protein system. The main change is the denaturation of whey proteins and their interaction with casein micelles. Consequently, it was expected that the behavior of the UHT milk would be similar to that of casein micelles, as observed with native casein. However, it has been shown that the interaction between casein micelles and whey proteins is also accompanied by a large increase in number of very small, soluble particles resulting from the desegregation of casein micelles. Consequently, by weakening the interaction forces between caseins, heat treatment provokes both a loss of the micelle framework and the formation of small aggregates. Correlatively, an increase in the amount of nonsedimentable casein has been observed in UHT milk following high-speed centrifugation (33). Two effects of these changes on milk properties may be softer coagulation and a higher susceptibility to enzymatic hydrolysis due to loosening of the micellar structure and the presence of small aggregates that may be more accessible to enzymes.

In the stomach of adult or infant, gastric acid and the enzyme pepsin denaturate whey and casein proteins from either raw or heated milk Critical Reviews in Food Science and Nutrition). The denatured proteins are then nearly completely digested by the enzymes pepsin in the stomach and by trypsin, chymotrypsin and peptidases in the small intestine (Medical Libretexts.

Humans, including infants do not have the enzyme rennin (chymosin).

Rennin, also called chymosin, protein-digesting enzyme that curdles milk by transforming caseinogen into insoluble casein; it is found only in the fourth stomach of cud-chewing animals, such as cows. Its action extends the period in which milk is retained in the stomach of the young animal. In animals that lack rennin, milk is coagulated by the action of pepsin as is the case in humans. A commercial form of rennin, rennet, is used in manufacturing cheese and preparing junket (Britannica).

Chymosin is secreted in the neonatal stomach of ruminants (cattle, goats, camels), pigs, cats, and rats. Animals including humans, chimps, and horses have inactivating mutations in their chymosin gene and do not secrete the enzyme (vivo.colostate.edu).

There's one study that mentions the presence of chymosin in humans, but the evidence seems to be very weak as described in another Biology SE answer. Some articles, like this one mention chymosin in humans without proper references.

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  • $\begingroup$ Does it mean that UHT processing does not affect latter cuddling in the stomach? $\endgroup$ – abukaj Aug 12 at 10:50
  • $\begingroup$ UHT does not affect casein, but it does affect whey protein. I'm not sure what exactly this means in regard to milk curdling in the stomach. But stomach acid and pepsin denaturate both casein and whey; this does not prevent their digestion or absorption, but it can slow it down, which can be actually a good thing, because it results in more even nutrients supply. In general, various types of heating do not slow down the digestion or absorption of nutrients. In other words, stomach acid affects milk proteins more than heating. $\endgroup$ – Jan Aug 12 at 11:01
  • $\begingroup$ From what I have read so far curdling is important to prevent proteins from leaving the stomach too early. Also, from slide #22 of this presentation on UHT treatment (no further references though) I infer that denatured whey protein may disturb casein curdling. $\endgroup$ – abukaj Aug 12 at 11:02
  • $\begingroup$ @abukaj, at the top of my answer, I added a quote from the ScienceDirect article, which says that heating makes both casein and whey proteins more digestible, which means they are both more accessible for pepsin, which breaks them down. So, this first phase of digestion of proteins in the stomach is increased; the second phase, which occurs in the small intestine, does not seem to be increased, but in summary, the digestion time is shorter. $\endgroup$ – Jan Aug 12 at 13:40
  • $\begingroup$ Hesting milk to 90°C does not count as UHT processing. The milk is not even boiled. $\endgroup$ – abukaj Aug 13 at 16:06

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