Why won’t my milk foam?

December 16th, 2006

The problem we have with milk is that drinkable doesn’t necessarily mean foamable. Many people tell me that they find skimmed milk easier to foam than full fat, and yet we know that it is all about the protein when it comes to milk foam. Are these people right? Yes and No.


A great and simple test the quality of your milk is to foam it up and then start to listen to it. That’s right – get your ear right into the pitcher. What do you hear? If your milk is in poor condition then it will sound like a freshly opened soft drink, almost fizzing away in pitcher – this is not good. Every tiny pop is another bubble bursting and your foam is falling to pieces. As you let it sit you notice that out of nowhere there are larger bubbles appearing at the edges of the pitcher. With great milk you’ll only hear a few little pops (though this will increase as you swirl the pitcher) but it should really be very quiet indeed.

Pouring

What has this got to do with full fat and skimmed milk? The answer is down to what is causing your bubbles to burst. It isn’t down to drainage as we see in crema, but the presence of glycerol. Glycerol provides the backbone to our triglycerides in our fat molecules – making up around 4% of our whole milk but only traces in our skimmed milk. When the milk is fresh the glycerol is busy being part of a bigger fat but various things can break it up letting it run free. Free glycerol interrupts the foaming process very effectively causing our bubbles to burst. So full fat milk is more susceptible to this than skimmed. However when the milk is fresh this shouldn’t be a problem for either product.

Often we can trace the problem back to storage. The big enemies of foamable milk is light and heat, so if you get your milk in a clear glass bottle that has been exposed to the morning daylight for some hours before delivery then it is likely to be in pretty poor condition. Again – light is a problem in some cafes where they store it in a glass pitcher in a glass fronted fridge. Store it somewhere dark and nice and cool. Interestingly the longer life fresh milks now come in more opaque packaging than many standard milks on the market. In the UK milk in a supermarket never really has more than 10 days shelf life but I tend to see a sharp dip in foam quality when you get with three of four days of the use by – even though it tastes great with my cereal. Anywhere along the way from cow to cup someone can ruin the milk, so it is hard ever to definitive in saying one dairy is better or worse than another.

Full Fat Contents

We are starting to see more and more “Cappuccino Milks” on the market. Often they have added protein (which we already have plenty of to start with) or in one bizarre case they have removed half the fat and then added thickeners and stabilisers to get the nice thick viscosity of full fat milk. This is also a UHT product which I am not wild about. If we look at the quantity of protein in the milk it is around 3% by weight. However only about 25% of these are whey proteins which are the ones that we are after. If you are choosing a milk for cappuccino then taste, sweetness and a creamy texture are what you are looking for in the raw product but often we just don’t demand the same of our milk supplier as we do our coffee supplier – yet your average cappuccino is way over 80% milk. We don’t think of it as a point of difference – milk is milk right? Try it – go out and buy 5 different brands of milk. Steam them all. How do they perform? How do they taste?

Any questions in the comments….

[tags]milk, cappuccino, milk chemistry, food chemistry, lipids, latte, latte art[/tags]

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