What happens if we keep heating milk?

You can heat fresh milk at 140 °C around 10 min. If you keep heating coagulation will occurs. It happens due to the aggregation of the milk proteins. Milk contains 85-88 % of water depending on the source animal and rest of them are solids. Usually cow’s milk solid contains lactose (4.9%), fat (3.4%), protein (3.3%) and 0.7% minerals (0.7%).

This articles has two parts.

1. Physical changes
2. Chemical changes

Physical changes that happen if we keep heating the milk

As you already know the majority portion of milk is water. If you continue heating at a moderate fashion the water will evaporate and milk will become more concentrated. Eventually, you will get milk skin. It’s the protein that forms sticky milk skin. If you continue heating the milk skin will become thicker as more and more protein will come together (a process called coagulation). If you even heat the milk further the color will change from white to yellowish or brown. The sweetness of milk also increases with heating. I hope you should stop at this point unless you really want to do an experiment. At this point, the milk is still edible. In fact, many people in the sub-continent (Bangladesh, Pakistan, India) like it. By the way, if you really want to keep heating the milk; eventually all the water will evaporate and milk solids will be burned to black solids. Please read the second part if you are interested in what happens at the molecular level if you keep heating your milk.

Chemical reactions that happen if we keep heating milk

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.

Heat induced changes of milk can be classified into five groups:

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

 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.

Usually the reactions of lactose and protein falls under the umbrella of Maillard reactions.  Casein modifications include the dephosphorylation and proteolysis reactions. The most important proteolysis reaction is the cleavage of the peptide bonds in κ-casein. Please check this article to learn more about the chemical changes.

Enjoy a glass of warm milk!!!

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.