Milk Frothing between Myths and Reality: Proteins

In my career as a barista and trainer, I realized that I had learned and often transmitted incorrect notions derived from information dictated by professionals, books, and coffee service materials. For this reason, over the past four years, I have been conducting extensive work to review and scientifically verify the information circulating in training environments at all levels.

In the past year, I have periodically encountered topics regarding milk frothing and have written a guide about it. For those who missed it, I provide the link below: “How to froth milk in a coffee shop: the complete guide“.

Milk frothing is perhaps one of the coffee shop topics where you can find the most urban legends and false myths, as well as improperly used terminology.

With experimental chemist Salvatore Impemba, we have begun co-writing a brief series of readings about the biggest myths transmitted during coffee service courses, starting with “The Chemical Composition of Milk“, which must be read before continuing to fully understand the concepts expressed. In this page, we will discuss the main changes regarding milk proteins during frothing.

We will finally answer important questions from the coffee industry.
If temperatures exceed 70°C while frothing milk, do proteins denature?
Above 70°C, can protein denaturation lead to a cooked taste?
Do denatured proteins destabilize frothed milk?
When milk is frothed, why isn’t the result the same when frothing it a second time?

Let’s begin…

What Happens to Proteins when Milk is Heated?

When a protein-containing food is heated, protein denaturation certainly occurs. The same happens in milk when it is frothed (we will see how and in what percentages later) but even before that when it is heat-treated for food use.

A protein can be viewed as a long chain formed by many amino acids linked to each other.(1) The chain can fold upon itself thanks to chemical interactions between the amino acids. The shape that the protein can take, called conformation, depends on the amino acid sequence and environmental conditions.

The lowest energy form that the protein assumes, that is, the initial one, is called the native form.(2) When discussing protein denaturation, we are not talking about a change in its amino acid sequence but simply a change in its native conformation, namely the structure it naturally takes in space.

This change can be induced by denaturing agents or simply by a temperature change. Whey proteins are completely denatured in 60 minutes at 77.5°C, in 30 minutes at 80°C, and in 5 minutes at 90°C. Caseins are less easily denatured by heat and don’t denature even at 100°C unless at least 12 hours pass, while at 155°C they denature in 3 minutes.(4)

Caseins, which represent 80% of proteins in milk, exist as spherical structures called micelles containing minerals inside. Due to their high concentration compared to other proteins, caseins strongly influence milk characteristics. The rest of the protein portion is represented by whey proteins that contain a high percentage of sulfur.(5)

Studies show that when milk undergoes mild heat treatments, up to 60°C, effects only occur on proteins that possess hydrophobic characteristics, such as β-casein and β-lactoglobulin with their gradual denaturation.(6)

After protein unfolding (denaturation), aggregation can occur. Protein unfolding is a reversible process, meaning it can go back, if heat is stopped before aggregation occurs.

In many texts, protein denaturation refers to the sum of both phenomena, unfolding and aggregation (insolubilizing denaturation).(8)

β-lactoglobulin is found on the surface of micellar caseins and when heated, it detaches from this surface causing a loss of heat stability in the micelles that can lose phosphorus and bind less calcium.(9) It remains the least denaturizable whey protein that shows a breaking of its sulfur-sulfur bonds around 140°C. (10)

β-casein is very sensitive to temperature changes and can easily detach from micelles. Separated from micelles, β-casein can form water-soluble aggregates with about 40 amino acid units. (11)

Glycopeptides, that is, proteins with carbohydrate portions, have been found in milk when it is heated to temperatures above 50°C.(12)

As we see in the graph below, insolubilizing denaturation (aggregation and subsequent insolubilization) of proteins remains a process that must take into account time and temperature – the longer the times and temperatures milk is subjected to, the greater the number of denatured proteins. For example, at 70°C in 5 minutes we have denaturation of just 6% of whey proteins and after 30 minutes about 25%.(15)

The graph shows the percentages of insolubilizing denaturation of whey proteins only, as casein denaturation at the temperatures involved is negligible. We also remind that the reported denaturation was measured after heating periods of 30 minutes, well above cappuccino consumption times.

Do Denatured Proteins Destabilize Frothed Milk?

Contrary to what is thought, denatured proteins favor the stabilization of frothed milk foam. This is true until excessive denaturation and consequent aggregation occurs; in this case, the opposite effect will occur, namely a lower foaming capacity and stability.

The probable misunderstanding could derive from the confusion created in the use of the terms “denaturation” and “aggregation and insolubilization”.

Not many studies have been found regarding the ideal degree of protein denaturation to maximize foamability and stability; however, it is plausible to think that the times and temperatures involved do not lead to protein denaturation sufficient to affect frothed milk stability.

If Temperatures Exceed 70°C while Frothing Milk, Do Proteins Denature?

We have learned to distinguish between protein denaturation and protein insolubilization, and that denaturation favors foam stability until it becomes excessive or leads to aggregation with consequent insolubilization.

As we have seen, protein denaturation derives from temperature but also and especially from the time they are exposed to such temperature. Protein denaturation exists at 50°C as at 70°C, however, strong denaturation can hardly exist at these temperatures for the time between cappuccino creation and customer consumption, and certainly heat-caused insolubilization is minimal.

It should be emphasized that the method used in coffee shops to insufflate air into milk is steam injection, which induces a thermal change in it and could have a significant effect not only on the physical structure of milk (formation of bubbles and increase in water) but also on its chemical composition and protein conformation. Consequently, steam action could be the biggest responsible for protein denaturation in milk frothing.

So should we be careful not to exceed 70°C for fear of denaturing milk proteins? We unanimously reject this false myth, unless significantly higher temperatures are reached or for longer times.

Above 70°C, Can Protein Denaturation Lead to a Cooked Taste?

In native proteins, -SH groups are hidden and are not very reactive. With protein denaturation, these groups become accessible and therefore reactive, and end up degrading due to strong heat. It is suspected that the degradation of these groups may confer the famous “cooked” taste, with the formation of hydrogen sulfide found in appreciable quantity after brief heating at 80°C or long heating at 70°C.(15) Unless reaching temperatures that would really burn the customer, we reject this false myth.

When Milk is Frothed, why Isn’t the Result the Same when Frothing it a Second Time?

Setting aside complex issues such as food hygiene, as we have seen, denatured proteins (if not aggregated) tend to return to their original form. One might think that by bringing the milk back to refrigerator temperature and frothing it again, the result would not change. In reality, we forget that through steam injection we add a good amount of water to the milk; the resulting milk will be watered down and practice shows us that, likely largely for this reason, it is impossible to obtain a result identical to the first formation of frothed milk.

With today’s reading, we have brought some order to the information regarding chemical modifications that proteins can undergo during milk frothing in coffee shops. We just have to wait for the next readings to talk about fats and carbohydrates.

BIBLIOGRAPHY:

1 – F. Franks, D. Eagland, The role of solvent interactions in protein conformation, CRC Crit. Rev. Biochem, 1975, 3, 165-219.
2 – V. A. Bloomfield, Association of proteins, J. Dairy Res., 1979, 46, 241-252.
3 – P. J. Flory, Statistical Mechanics of Chain Molecules, John Wiley & Sons, New York, 1969.
4 – C. Tanford, Protein denaturation, Advances in Protein Chemistry, C. B. Anfinsen, M. L. Anson, J. T. Edsall, F. M. Richards, Academic Press, New York, 1968, 23,122-275.
5 – D. G. Schmidt, Colloidal aspects of casein, Neth. Milk Dairy J., 1980, 34, 42-64.
6 – T. A. J. Payens, H. J. Vreeman, Casein micelles and micelles of α- and β-casein, Solution Behavior of Surfactants, K. L. Mital, E. J. Fendler, Plenum Press, New York, 1982, 1, 543-571.
7- J. N. de Wit, Structure and functional behavior of whey proteins, Neth Milk Dairy J., 1981, 35, 47-64; J. N. de Wit, G. Klarenbeek, A differential scanning calorimetric study of the thermal behavior of bovine β-lactoglobulin at temperature up to 160°C, J. Dairy Res., 1981, 48, 293-302.
8 – J. N. de Wit, G. Klarenbeek, Effects of various heat treatments on structure and solubility of whey proteins, J. Dairy Sci., 1984, 67, 2701-2710.
9 – L. K. Creamer, G. P. Berry, A. R. Matheson, The effect of pH on protein aggregation in heated skim milk, J. dairy Sci. Technol., 1978, 13, 9-15; S. Kudo, The influence of casein on the heat stability of artificial milks, J. Dairy Sci. Technol., 1980, 15, 245-254; G. R. Howat, N. C. Wright, The heat coagulation of caseinogen: the role of phosphorus cleavage, Biochem J., 1934, 28, 1336-1345; J. E. Kinsella, Milk proteins: Physicochemical and functional properties, CRC Crit. Rev. Food. Sci. Nutr., 1984, 21, 197-262.
10 – K. Watanabe, H. Klorstermeyer, Heat-induced changes in sulphydryl and disulphide levels of β-lactoglobulin and the formation of polymers, J. Dairy Res., 1976, 43, 411-418.
11 – R. N. Carpenter, R. J. Brown, Separation of casein micelles from milk for rapid determination of casein content, J. Dairy Sci., 1985, 68, 307-311; D. G. Dalgleish, The enzymatic coagulation of milk, Applied Science Publishers, London, 1982, 1, 157-183.
12 – E. J. Hindle, J. V. Wheelock, The release of peptides and glycopeptides by action oh heat on cows’milk, J. Dairy Res., 1970, 37, 397-405.
13 – J. E. Kinsella, Functional Properties of Proteins: Possible relationships between structure and function in foams, Food Chemistry, 1981, 7, 273-288.
14 – M. Levy, The Effects of Composition and Processing of Milk on Foam Characteristics as measured by Steam Frothing., Interfepartmental. Programme. Animal and Dairy Science. Louisiana State University, Baton Rouge, USA.
15- Charles Alais, Science of Milk (1988)