Most of the time I drink my coffee black. However, there are times I want a dark roasted coffee with milk to accompany a sickly sweet dessert after a big Sunday dinner, and so do my family. Just adding cold milk doesn't always have the textural or flavour effects we want and so it is necessary to heat and foam the milk up. This article only relates to cow’s milk.
What can go wrong?
Heating milk up is not as simple as it seems. Too little heat and we don't get the desired foam. Too hot, and the milk loses body, sweetness and develops unpleasant sulphurous qualities. I’ve many a time seen milk over-heated or burnt at major, high street coffee chains and it’s no good. This is another reason to visit specialty coffee shops, for their trained baristas to make milk right. But I digress. We want to get milk right at home and here are a few things that’ll help.
Heating milk to optimum temperature
When there are lots of guests needing good milk in their coffee, I heat it in a sauce pan on a gas stove. I use a thermometer to measure the temperature and foam it with an electric milk whisk (below). I do not have a steam wand at home.
For best results milk should be heated to 60C (140F). Some say milk is sweetest at this temperature but the lactose sugar content will have a greater influence here. Much beyond this temperature, milk starts to burn and take on an unpleasant sulphurous/vomit-like aroma and flavour. It will also get harder to foam milk as the body thins out. A skin appears on the milk when it’s over-heated, meaning the proteins are denatured (chemical bonds are breaking down).
At this temperature we also perceive milk as sweeter. It's generally safe to drink and so there's no threat of scalding the tongue.
What is micro foam?
Milk is made up of proteins, fatty acids, sugar and water. This article on Dairy Processing Handbook provides more information on its chemistry and a description of what it is; an emulsion of oil in water. When we add air to milk heated to the point proteins are unravelling (given above), the protein starts to encase air bubbles, creating a foam.
Steam wands do this most effectively, adding steam and heat to the trained barista’s desire. Less injected air will create a thin, creamy micro foam which sits atop a flat white. Slightly more air for a latte. Yet more air will create a foam with a thicker structure, which made well should keep together and wobble on top of the coffee. This is cappuccino foam. Steam wands do this best as they add water too, which the proteins want to escape from, the hydrophobes that they are. They therefore cling to air bubbles and float to the top.
When using the milk whisk we also add air but no steam. It’s more difficult to produce a micro foam but the Nespresso milk frothers do a very good job. Sadly, if you don't have one, it’ll be hard to replicate the foam using the hand frother. Using whole fat milk will make this easier than skimmed, the latter creating a less silky, supple foam and larger bubbles. The extra fat globules in whole milk help create a more consistent and thicker foam.
Spicing things up
For a different style of milk with an added sweet spice, try heating your milk with cardamom pods and cloves. The milk will still easily be frothed but now you have a milk with the flavour of the orient. The sweet, warming spice makes a nice change from the monotonousness of having unflavoured milk. Some people add turmeric also, but I don't think it has the sweet qualities of cloves, cinnamon and nutmeg nor the citrusy pleasure of cardamom.
Extra reading - Characteristics of milk
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