coffee science : everything you need to know about milk -1

being a barista is a specialised craft revolving around three main products: coffee, water, and milk. the science behind espresso has already been covered at length, and there’s been a recent surge of information on water chemistry… yet milk hasn’t yet received the analysis it deserves.

so today, we’re going to break down exactly what makes up your milk. this will help you to make decisions about things like the best temperature for different drinks, whether to buy full-fat or skimmed, and more.

what is milk ?

let’s begin with the basics. this is a bit technical; you may even start to feel like you’re back in chemistry lessons at school. but don’t worry, because we’re going to really break it down for you.

milk’s chemical structure falls under a couple of classifications. scientifically, it’s referred to as an “emulsified colloid of liquid butterfat globules, dispersed within a water-based solution”. in english, that’s called “a bunch of teeny-tiny insoluble protein particles, fat, and other fun stuff, evenly distributed in water”. the major components are water; protein; fat and sugar (carbohydrates); and other vitamins, minerals, and salts.

and in the context of coffee-making, it’s essential for a barista to understand how these components interact. only this will tell you how they impact on your beverage.

lactose, aka how much should you heat that milk ?

lactose is a diassacharide (two sugars) made up of galactose and glucose. it kind of looks like this:

credit: uoguelph.ca

yet while lactose is classified as a sugar, it’s only 16/100 as sweet as regular sugar (sucrose). it also reacts rather interestingly to the application of heat and hydrolysis (using water to unbind the chemicals). you see, steaming milk adds both heat and water to the mix. this is why a steamed milk tastes sweeter than, say, milk heated on a stovetop or in a microwave.

the optimal temperature for steaming milk is a hot-topic amongst baristas, but the core of this debate is one question: “at what temperature does milk taste the sweetest?”

but the answer is really much less dependant on the temperature than it is on the lactose content! as much of a no-brainer this is, a milk with a higher lactose content will always taste sweeter, regardless of what temperature you’re steaming it at. conversely, a milk with a low lactose content (under 3%) won’t get that mellow, much-desired sweetness – no matter what you do to it. as a gauge, most commercial brands of milk you find in cafés have a lactose content of anything between 4-5%.

so why does hot milk taste sweeter? because the human tongue is naturally more sensitive to sweetness when things are hotter. this explains why a cold soda tastes refreshing and balanced, but a warm one is cloyingly sweet.

something interesting to note, though, is that it does affect the proteins – which is why “burnt” milk is thinner. “burning” milk is a no-no because of how it affects the consistency of the milk, not because it has any effect whatsoever on the sweetness of the milk.

another fun fact: in the culinary world, there are several classic french recipes, bechamel sauce being the best example, that require you to burn the milk. this makes sense because, with the thinner consistency of burnt milk, incorporating it into sauces is easier and it also reduces the chances of lumps forming.

there are several brands of lactose-free milks on the market that extract lactose with an enzyme called lactase, leaving a residue of the (much, much sweeter) glucose and galactose. as a result, lactose-free milk often tastes confusingly sugar-sweet.

protein, aka how good will my foam be ?

if fat and lactose were your friends, protein would be that third-wheeler that you never really invited to birthday parties. i think it’s partly because talking about protein isn’t particularly exciting, but also because it’s flavourless and has less of an obvious impact on us.

that said, protein, in the context of steaming and frothing, is probably the most crucial component of milk. yes, i mean it.

there are two main proteins in milk: caseins (80%) and amino acids, also called whey proteins (20%).

but what’s interesting to know is that milk proteins are partially hydrophobic. okay, i get it, you’re staring at me pretty blankly. but this basically means that one end of the chain wants to stick to water but the other end of it wants to get the hell away. this is what actually gives milk that opaque, white colour. and, most importantly, this property is responsible for building the foam structure.

let me explain how this works. imagine a giant ball-pit, the kind with the multicoloured plastic balls. now imagine, for a second, that all these balls were joined up with fishing line thread or something, forming a rope of balls, so to speak. these ropes are all tangled up and mixed together, so you can still jump in the pit and swim around in it without any obstruction.

suddenly, steam (aka heat and water and air) is added into the pit! argh ! chaos !

the heat is breaking down the chains into smaller chains; some of them are  even unravelling. and to make things even weirder, the balls are all repelling the water! they wrap around the air bubbles, since that’s the only thing they can kind of cling on to. suddenly, some kind of grid-like structure is formed… it kind of looks like foam !

a much looser foam for a better visual understanding. 

credit: modernistcuisine.com

what all of this means is that, if your milk has more protein, it will have a more stable foam structure.

naturally produced as-is milk usually has a protein content of 3.3%, but in most commercial milks, more protein is added to make the heat-related process of homogenisation easier. so your milk could probably contain anything between 3.6-4.1%.

article written by christine s.

edited by t. newton.

feature photo credit: chris pelliccione