One of the many amazing things about milk is that it can be transformed into silky foam. This happens because of its protein content, without which milk would be like water, i.e., with zero chance of frothing up. When the steam wand of an espresso maker or any sort of frothing device is used to turn milk into foam, millions of tiny air bubbles are injected into the milk. At this is happening, the milk proteins are destabilized by the heat (their structure, which resembles a ball of wool, begins to unravel) and the protein molecules start to wrap around the air bubbles. This creates a protective coating that keeps the bubbles from bursting.
On average, cow’s milk has a protein content of 3.325%, which is very low. This is why, from one type of milk to another, even a slight difference in protein content can have a dramatic influence on the amount of foam produced.
For example, a quick look at the protein content of different types of cow’s milk (see the chart) shows that skim milk and partially skimmed milk (1% m.f.) have a somewhat higher protein content than 2% and whole milk.
Sometimes, protein content can vary slightly depending on the time of year and how the milk is processed. In summer, for instance, milk contains less protein than milk produced in winter due to changes in the feeding of the dairy cows. At the processing plant, a filtering process (such as micro-filtration or ultra-filtration) can also slightly reduce protein content compared to unfiltered milk. This might explain differences observed among various brands of milk.
Unfortunately, these differences in protein content don't appear in the Nutrition Facts tables, since they are minimal and the numbers are rounded off. However, they do help to explain why we don't always get the same quality or the same quantity of foam for our coffee from one milk carton to the next.
Why does milk sometimes refuse to froth?
Even if you always use the same kind of milk or the same technique, sometimes the milk simply does not cooperate, forming foam that disappears in seconds. This is not only frustrating but mystifying. Part of the explanation is the presence of free fatty acids, monoglycerides and diglycerides, which are compounds that form when enzymes or bacteria attack the milk fat in a reaction called "lipolysis." If these new compounds are too numerous, they will cause the foam bubbles to burst. There are several reasons for this phenomenon, including the way in which the dairy cows are fed, the phase in their lactation cycle, poor weather conditions and even the technique used to chill the raw milk at the farm. At home, lipolysis can also occur when milk has been improperly stored. It is critical to always store milk in the refrigerator at 40°C (4°C). Also keep in mind that fresh milk usually froths better than milk that's been refrigerated for several days. In other words, when milk refuses to froth, there's not much you can do about it—other than hope that your next litre of milk will yield better results.
Consistency: a matter of fat
Unlike the fat in whipping cream, the fat contained in milk doesn't contribute to the formation of foam as such. All forms of milk can produce foam, regardless of their fat content. It is not the quantity so much as the quality of the foam that will vary greatly from one type of milk to another.
- Whole milk produces a soft and supple foam that flows easily on the surface of your coffee. It's the preferred choice of baristas who practice latte art, creating clever pictures using the flowing foam as their canvas. It's the fat globules present in whole milk and 2% that give this foam its suppleness.
- Skim milk,for its part, produces an abundance of foam, but the consistency is less supple than whole milk foam
Foam Test: Which milk should you choose?
We decided to put whole milk and skim milk to the test to see which one made the better foam. Rather than using the steam wand of an espresso machine (subject to variations in temperature and technique) we used an electric frother (Nespresso Aeroccino) under identical conditions for both types of milk, and took photos of the foam a few minutes after it was made.
Our findings?Both types of milk gave great results, producing an abundance of foam, with skim milk yielding slightly more foam than whole milk. Both foams were also quite stable—meaning they didn't disappear too quickly. This is significant because ideally the foam should be present right up until you finish drinking your coffee. The real difference between the two foams was their smoothness and taste, which basically comes down to personal preference. If you can't pick a favourite, you might want to consider 2% milk, a nice compromise between volume and smoothness.
Produces foam that is softer, creamier and thicker than skim milk foam. The rich, creamy taste is excellent and the foam's consistency tends to become firmer as you are drinking the coffee.
Produces more foam than whole milk does, with somewhat larger bubbles, a firmer consistency and a neutral taste. After a while, the foam loses its suppleness and becomes somewhat dry and stiff.
Milk with added calcium, lactose-free milk, UHT milk, almond or soy milk—there are so many varieties of milk and milk substitutes to choose from. How do they perform in a milk frother? Here's what we discovered.
Calcium-enriched milk features added modified milk substances (in this case, concentrated milk minerals and lactoserum proteins) that increase its calcium content as well as, to a lesser degree, its protein content. These types of milk froth very well and are an excellent option for anyone looking to boost the amount of calcium in their diet.
UHT milk (such as the Grand Pré brand, sold, in Tetra Packs) is processed at a very high temperature in order to sterilize it before packaging. This heat treatment improves the frothing properties of its lactoserum proteins. The result? This milk produces an abundant amount of fairly firm foam, which lasts longer than foam produced using regular pasteurized milk.
Natrel-brand lactose-free milkhas a higher protein content than other types of milk—11 g per cup (250 ml) versus 8-9 g for regular milk or other kinds of lactose-free milk—thanks to the company's proprietary method of removing the lactose from the milk. Those few extra grams of protein help to produce a greater volume of foam, with very fine bubbles.
Goat's milk and sheep's milk contain protein that is similar to that of cow's milk, and therefore they froth very nicely.
Soy milkwhich has 6-7 g of protein per cup (250 ml), produces a firm foam, but its stability varies from one brand to another. Some makers offer a soy drink specially designed for coffee shops that produces thicker, more stable foam. One such beverage, the So Nice "Barista Blend," is now sold in stores. In our tests, we found that this blend gives very good results, but its foam is pretty much the same as the ones obtained with other brands of regular soy milk.
Almond milk, which has a mere 2 g of protein per cup (250 ml), gave us a very pleasant surprise: It forms lovely, delicate and long-lasting foam. Milk allergy sufferers, take note!