Milk protein coagulation: effects on digestion

Summary of a review by Huppertz & Chia 2020

Proteins play an important role in many processes in the body. For dietary proteins to be utilized efficiently, the protein needs to be digested to free amino acids and di- and tripeptides in the intestine. A protein source that allows this is considered a well-digestible protein source. In general, most animal protein sources, including milk, are characterized by high digestibility. Several factors and processes affect protein digestibility, with one of them being gastric coagulation. In this summary of a review by Huppertz & Chia, the effects of gastric coagulation of milk proteins on digestion will be discussed.

Three classes of proteins are distinguished in milk: caseins, whey proteins and milk fat globule membrane proteins. The latter class only represents a small portion of the total milk protein, whereas caseins and whey proteins are the most abundant classes. In raw milk, the proteins involved in gastric coagulation are caseins and milk fat globules. In processed milk, whey proteins and milk fat globules are involved.

Physiological relevance of gastric coagulation

Gastric coagulation of milk proteins ensures a controlled transit of dietary protein through the stomach. This facilitates a more sustained release of amino acids in the bloodstream and a maximum utilization of protein. In addition, a controlled release of protein ensures that the digestive capacity of the intestine does not become overloaded. This is particularly relevant for people with a reduced digestive or absorption capacity, such as infants and elderly adults.

During gastric coagulation, the so-called curds that are formed by caseins can differ in terms of firmness. This has been shown to affect digestion. Studies conducted in mostly infants show that consumption of soft-curd milk leads to reduced digestive difficulties compared with consumption of hard-curd milk. Different factors determine whether milk protein will form hard or soft curds in the stomach, for instance milk composition, cow breed and processing of milk.

Impact of processing on gastric coagulation

Two processing steps appear to have a large effect on the gastric coagulation of milk: heat treatment and homogenization. For heat treatment, the effects on gastric coagulation are largely related to the denaturation of whey proteins. For homogenization, the effects are primarily related to changes in milk fat globules. Studies have shown that both heat treatment and homogenization lead to the formation of softer curds, and can possibly improve digestibility.

It should be noted that while heat treatment leads to so-called soft-curd milk, heat treatment may also affect protein digestion negatively: during heat treatment, protein glycation can occur which impacts digestibility and limits the uptake of the essential amino acid lysine.

Conclusion

Gastric coagulation of milk protein regulates gastric emptying, making the process essential in efficient supply and utilization of protein in the human body. Processing of milk leads to the formation of softer curds, which may lead to easier digestion.

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Reference

Huppertz, T., & Chia, L. W. (2020). Milk protein coagulation under gastric conditions: A review. International Dairy Journal, 104882.