Is the heating of milk a chemical process?

Yes. There are several chemical reactions that take place during the heating of the milk.

Heat induced chemical reactions of milk can be classified into five groups:

• Acid development
• Precipitation of calcium phosphate
• Maillard reactions
• Casein modification and
• Interaction of sulphydryl groups

In fresh milk all the ingredients remain in a large number of equilibria that includes the solubility and ionization of salts, the interactions of salts with proteins and the association of proteins. Since heating affects these equilibria the properties and the taste of milk changes with temperature.

pH of fresh milk is around 6.8 and it decrease with heating. pH is the measurement of acidity. If pH decreases the acidity increases and vice versa. During heating lactose degrades to organic acids that increases acidity. Additionally, primary and secondary calcium phosphate precipitated as tertiary calcium phosphate that also releases hydrogen ions (H+). These combined effect increases the acidity of milk during heating. With increasing temperature, the solubility of calcium phosphate decreases.

Usually, the reactions of lactose and protein fall under the umbrella of Maillard reactions. Millard reactions happen in different stages and depend on the pH of the milk. Initially, an Amadori product is produced, and then progresses to the 3-deoxyosone or 1-deoxyosone route. In the case of disaccharides, the third option is 4-deoxyosone route. The final products of Maillard reactions is the formation of melanoidins which is responsible for the browning of milk during heating.

 Casein modifications include the dephosphorylation and proteolysis reactions. The most important proteolysis reaction is the cleavage of the peptide bonds in κ-casein.

In the dairy industry, heat treatments of milk are mostly applied to extend the shelf-life of the product. The mild heat treatment during thermization and pasteurization process does not alter the nutritional qualities of milk.

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References:

1. Rehydratation and Solubility Characteristics of High Protein Dairy Powders. Editions Springer Edition.
2. Tomoko Shimamura and Hiroyuki Ukeda. Maillard Reaction in Milk – Effect of Heat Treatment. DOI: 10.5772/50079.