2% milk is 2% milk, except…

Mr. Beaudry specifies that the two brands that froth poorly are milks that come from Ontario. But the origin does not usually make a big difference, immediately indicates Julien Chamberland, researcher in dairy processing at Laval University.

Basically, he explains, the milk will make little bubbles because the air and milk don’t mix well (at all). Small amounts of air can be forced to stay trapped in bubbles for a period of time, but air and milk simply don’t mix with each other.

Proteins, allies of foam

Now there are, in milk, molecules which have “dual citizenship”, so to speak: these are proteins which have one end having affinities for water (therefore for milk), and one end which is hydrophobic and which has more affinities with air. These proteins “form a thin layer at the water/air interface with their hydrophobic fragments oriented towards the air and the hydrophilic fragments towards the water. This structure protects air bubbles [et prolonge leur durée de vie]“, explains Mr. Charmberland.

This is what allows the milk to froth well: the protein networks that form on the wall of the bubbles. Nothing too extraordinary, the “normal” characteristics of 2% milk usually make it possible. But, casually, there are still quite a few things that can go wrong in this whole story.

First, Mr. Chamberland points out, fats are important in this equation because they give a creamy texture to the mousse. But there shouldn’t be too many, and they should have the right characteristics.

Thus, milk fat tends to form globules which, if they are too large, will disturb the protein networks on the surface of the air bubbles and, often, cause them to burst. Fortunately, homogenization greatly reduces the size of these globules, but this explains why raw milks generally froth much less well than “industrial” milks.

A recent Australian study found that “the foamability of our raw milk samples was three to four times lower than that of processed milk samples. [quand on les faisait mousser mécaniquement]».

“Enemies” of moss

Interestingly, this same study also found seasonal differences in milk from the same company. Its customers had reported that its milk frothed well in summer, but very poorly in spring, and researchers found discrepancies – undoubtedly due to the cows’ diet being not the same all year round. — who could explain it.

In particular, they measured higher concentrations of “free fatty acids”, which are among the worst “enemies” of foam, in summer milk.

In general, living beings keep their fats in the form of “triglycerides”, a large molecule composed of a piece called glycerolto which three fatty acids cling. But due to the action of bacteria or enzymes that are naturally present in milk, these triglycerides are gradually broken down and their fatty acids are “released.”

However, these are known to displace proteins on the surface of the bubbles, which obviously makes them more unstable. A few years ago, researchers measured the extent to which these free fatty acids harmed the “cappucinability” of milk, by frothing samples in a very standardized way. At around 500 milligrams of free fatty acid per liter (mg/l), the lowest concentrations their milks contained, their test produced nearly 100 ml of foam, and this had a half-life (the time for until half disappears) for about 40 minutes.

But by doubling this concentration, they only obtained 60 ml of foam, and its half-life shortened to 20 minutes. And at the highest levels of free fatty acids tested (around 1500 to 2000 mg/l), they obtained almost no more foam (around 30 ml on average, but sometimes only 5 ml) and it sometimes disappeared in less of one minute.

However, these free fatty acids can come from various sources, sometimes very subtle — even the frequency of milking of cows can make a difference, that says it all. Thus, in a study published last month in the Journal of Dairy ScienceMr. Chamberland and his team observed that cheese made with milk from cows milked three times a day contained almost 60% more free fatty acids than that made with milk from cows that were milked only twice a day .

Several other factors that the average consumer may hardly know also influence fatty acids, such as “mechanical processing of milk, number of pumpings of raw milk, cooling speed, somatic cell content [des cellules de la vache, qui contiennent beaucoup d’enzymes pouvant changer la composition du lait]», says M. Chamberland.

But since the concentration of fatty acids inevitably increases over time due to enzymes and bacteria, as we said above, “the best tip I can give is to look at the consumption date,” says the researcher. The fresher the milk, the more likely it is to froth well. And when you find a brand that foams well, you can keep it. These are pretty much the only things the consumer can do.”

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