Science of milk in coffee
Baristas handle milk to make flat whites, cappuccinos, lattes, and cortados nearly as much as they do coffee so it's important to have a good understanding of the science of this quite complex ingredient to make sure the coffee and milk marriage works at its best to produce a great tasting drink.
Early days of milk and coffee
The combination of coffee and milk goes back a fair way in the history of coffee. William H Ukers in All About Coffee, notes that the Dutch Ambassador Johan Nieuhof, was experimenting with milk in coffee in imitation of tea with milk in 1660, and Mark Pendergrast in The History of Coffee and How It Transformed Our World writes that 'by 1710, rather than boiling coffee, the French made it by the infusion method, with powdered coffee suspended in a cloth bag over which boiling water was poured. Soon they also discovered the joys of sweetened "coffied milk" or "milky coffee." The Marquise de Sevigne declared this form of coffee "the nicest thing in the world," and many French citizens took to café au lait, particularly for breakfast.'
And though we might not approve of the way it was prepared, we do approve of Tristram Shandy's 1759 breakfast in The Life and Opinions of Tristram Shandy, Gentleman of "two dishes of milk coffee (which by the bye is excellently good for a consumption, but you must boil the milk and coffee together-otherwise 'tis only coffee and milk)".
The combination of coffee and milk however was only perfected into the type of drink we now enjoy with the introduction of the espresso machine in 1903 when the Italian Desiderio Pavoni had the genius idea of adding a steam wand to Luigi Bezzera's new steam powered device invented for the quick production of coffee in busy cafes.
Why does milk work with coffee
Dairy products are classic enhancers of some of the flavour notes found in coffee, particularly its grain, sweet, chocolate and nutty leads, and milk also softens bitterness. The three components of milk that have an impact on its flavour and texture are sugar (lactose), protein and fat.
Even if you don't want any actual foam on top of your final drink you should still steam milk as the incorporation of air brings its flavours to life - as we will see steaming doesn't just heat, it also improves sweetness and texture. Milk that hasn't been foamed tends to taste flat and dull by comparison.
The quantity and quality of the foam you create in your milk is dependent on the techniques you employ to incorporate the air into the milk as well as the type of milk you use. The process of adding milk to air is called stretching because it increases the volume of the milk. Air is added to the milk via the steam wand and it is sucked in from the outside as a result of the whirlpool movement the wand creates in the milk.
Bubbles in milk can range in size enormously but if you foam well the bubbles will be so incorporated into the milk that they won't even be apparent as bubbles. Microfoam is the term used to describe frothed milk where the bubbles are so small and numerous that they can't even be seen, though they can be sensed on the palate as a lovely velvety texture. The milk will appear thicker and creamier.
Creating stable milk foam involves an intricate interplay between the desirable foaming properties of milk proteins with the destabilising properties of milk fat.
Heat for sweet: lactose
One of the things many baristas get wrong (often as a result of customer demand) is the temperature to which they heat the milk - too hot and its flavours destruct.
Milk contains a natural sugar compound in the form of lactose, also known as milk sugar. Lactose doesn't taste particularly sweet (about five time less than sugar) but when heated it breaks down into smaller sweeter tasting compounds. However, beyond the optimal sweet point of about 60°C the sugars break down beyond sweetness and the milk simply tastes scorched.
Protein for bubbles
Adding air to milk to make foam is not the same science as adding air to water. Steaming water doesn't produce a thick stable foam because water and air don't mix well. This is because air needs something to hold it in the liquid. In the case of milk it's the protein that traps the air. There are two different types of proteins in milk: whey proteins and caseins with caseins making up 80% of the total protein of milk.
Heating protein molecules causes them to unravel (denature) out of their normally balled up structure. The newly unravelled protein molecules now look like chains. One end of the protein chain (particularly in caseins) is attracted to water and the other is repelled by water. The ends that are repelled by water (the hydrophobic ends) all move to point inwards towards the water free interior of the air bubble (just introduced by the barista with his steam wand) while the ends that are attracted to water (the hydrophilic end) remain happily out in the water. Coated with these surface active proteins, the air bubble is now fairly stable. This action is known as adhesion. Proteins also bond to one another sideways, forming crosslinks which add to the stability of the foam as each air bubble is bound to the one beside it.
Just as the ideal temperature for milk sweetness was 60°C it is also the temperature at which proteins make the best foam. Above this temperature they denature too much and no longer work effectively in coating the air bubbles.
Fat for feel
To our mind, if your barista doesn't ask what type of milk you want in your coffee it's for one of two reasons. The first is that you are in front of an average barista who is dishing out low fat milk to everyone. The second reason is that you're lucky enough to have found yourself in the hands of a good barista who wouldn't consider using anything other than full fat milk.
After water, fat is the main component of milk and it's what gives it a rounded mouthfeel. The rounded mouthfeel is enhanced by steaming the milk because the fats melt - the more fat there is in the milk the silkier it becomes when steamed and the richer the flavour of the final drink. Full fat milk contains about 3.25 - 3.5% fat while low fat milks contain about 1%.
Milk fat destabilises foam because the fat molecules also have hydrophobic and hydrophilic ends that compete with proteins in the hydrophobic/hydrophilic environment. They get in the way of the protein molecules trying to gather on the surface of the air bubble and worse still the fats don't bond to each other side-to-side to form a reinforcing network. Instead they will compete with protein molecules in forming bonds.
Milk fat globules start denaturing, leading to the leakage of liquid fat, at between 10-40°C and it's at this point that the destabilizing influence of fat on the proteins that are trying to coat the air bubbles is strongest, thus destabilizing the foam. The situation improves once the temperature has reached over 40°C.
By definition then, milk with less fat produces bigger more stable air bubbles (macrofoam) but in a very dry foam, the sort of foam you see spooned out and floating in a disconnected way on top of the coffee. This very dry foam is mostly air with very little flavour and the natural sweetness of the milk is pretty much overwhelmed. To our mind it simply doesn't produce a cohesive drink.
Homogenization is a process that takes place during milk production whereby the fat that is suspended in the milk liquid is broken down into smaller particles so that it disperses more evenly in the watery base of the liquid. One way of homogenizing milk is to force it at high pressure through small holes. Homogenization can improve foaming by decreasing the size and increasing the stability of the milk fat globules thus overcoming the negative effect of natural milk fat globules on foaming of milk.
Keep it fresh and refrigerated and never reheat
When foaming it's important to use fresh milk that has been kept refrigerated. Refrigeration slows down the natural degeneration of the milk fats (lipolysis) which would eventually make the milk taste rancid. You will get a better foam if you start with cold milk because you've got more time to aerate it as you bring it up to desired temperature. Never reheat milk or add fresh milk to milk that has already been heated as the proteins will not do their job as well when heated the second time around.