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Coffee Science: Everything You Need to Know About Milk

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 theres been a recent surge of information on water chemistry yet milk hasnt
yet received the analysis it deserves.
So today, were 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.

SEE ALSO: Hold the Dairy: A Barista Guide to Alternative Milk

What is Milk?
Lets begin with the basics. This is a bit technical; you may even start to feel like
youre back in Chemistry lessons at school. But dont worry, because were going to
really break it down for you.
Milks chemical structure falls under a couple of classifications. Scientifically, its
referred to as an emulsified colloid of liquid butterfat globules, dispersed within a
water-based solution. In English, thats 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, its essential for a barista to understand how
these components interact. Only this will tell you how they impact on your

Lactose, AKA How Much Should You Heat that Milk?

Lactose is a diassacharide (two sugars) made up of galactose and glucose, and
believe it or not its far more exciting for coffee-brewers than that sounds. It kind
of looks like this:

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Yet while lactose is classified as a sugar, its 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

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 youre steaming it
at. Conversely, a milk with a low lactose content (under 3%) wont get that mellow,
much-desired sweetness no matter what you do to it. As a gauge, most
commercial brands of milk you find in cafs 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

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tastes refreshing and balanced, but a warm one is cloyingly sweet. Heating milk has
zero impact on the lactose content.
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 It might not make for much of a balanced cup, but it is something
definitely interesting to try if you get the chance!

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Removing lactose from milk. Credit:

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 its partly because talking about
protein isnt particularly exciting, but also because its 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%).

Amino acids: sticks and stones may break my bones but protein chains excite
me! Credit:
But whats interesting to know is that milk proteins are partially hydrophobic. Okay,
I get it, youre 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.

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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 thats the only thing they can kind of cling
on to. Suddenly, some kind of grid-like structure is formed it kind of looks like

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A much looser foam for a better visual understanding. Credit:
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 3.64.1%.

Fat, AKA Do You Want Flavour & Foam or Mouthfeel?

Fat is both your best and worst friend. On one hand, it tastes frikkin fantastic, but
on the other hand, fat is detrimental for foam stability and too much of it will mask
the flavour of the coffee.
As I mentioned earlier, milk is a type of emulsion, i.e. its a combination of fat and
liquid. Vinagrettes and hollandaise sauce are common examples of culinary
emulsions. There are also temporary emulsions (salad dressing) that split over time
and permanent emulsions (mayonnaise) that dont. Unprocessed milk falls into the
former category and processed milk into the latter.

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Butterfat, the primary type of fat found in milk, is a pretty hefty globule. More than
95% of the total milk lipid is in the form of a globule ranging in size from 0.1 to 15
m in diameter. This triglyceride (an ester made up of three fatty acids) is so large
and heavy that it weighs down air bubbles, making foam collapse. Butterfat is
usually broken up in the homogenising process, to prevent a layer of fat forming on
the top of your milk and eventually forming a solid layer when chilled a common
phenomenon in unpasteurized farm-fresh milk.

Fat separation in raw milk. Credit: GrassFood

Another reason why a fattier milk shouldnt be your one-stop solution to a better
tasting latte is its flavour-masking properties.

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Since fat globules are so huge, it physically blocks the other solubles in the water
from reaching your tongue. In other words, it coats your tongue and forms a film
that repels certain flavours.
To best understand this effect for yourself, try making three cappuccinos: one with
non-fat milk, one with whole milk, and one with half-and-half. Try to identify which
one tastes more of coffee.
All that being said, you shouldnt reach for the skim just yet because what fat
delivers more than anything else is the magical factor of mouthfeel.
Recent scientific studies have shown that the reason why fatty foods taste so
satisfying has very little to do with the actual flavour of fat, but instead with the
mouthfeel of it. Imagine drinking unsalted melted butter now imagine the taste of
cold butter spread over hot toast. That creamy mouthfeel is what seals the deal,
Human tasters working for food companies report on the sensation of fat by
rubbing their tongue against the roof of their mouths. Alternatively, a machine
called a tribometer, sometimes made with a pigs tongue, makes quantitative
measurements of mouthfeel. And based on these quantitative studies, high-fat food
items like cheesecake, chocolate ganache and cream sauces all register as
immensely satisfying just because of the way it feels when in contact with the

The effect of fatty emulsions on the touch receptors in the tongues papillae. Credit:
Something else you should know about fat is that, while is does block off certain

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flavours, it accentuates others. In a study of how flavor is released in low-fat versus

high-fat ice cream, food scientists found that fat could have different effects
depending on the flavor. Cherry, for example, becomes less intense in the
presence of fat, but the opposite is true for vanilla.
Flavour is made up of these things called volatile components, and fat affects how
these volatile components reach your tongue. Some flavours like to stick to fat and
so make for a more prolonged release. This explains why adding milk to coffee
brings out certain nutty, or caramel-esque, flavours.

Understanding Milk Labels

So now that you know about each individual component of milk, its time to bring
the whole family together!
Make it a habit to read milk labels. I urge you to go to a supermarket and literally
read every single different milk carton. You might look eccentric doing it, but youll
learn so much.
As an example, lets take two of the most common brands of milk in Singapore and
compare the nutritional contents using precisely what weve looked at so far in
this article.

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Exhibit A: Meiji Whole milk. Credit:

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Exhibit B: Magnolia Barista milk. Credit:

Comparing the two, its easy to see the immediate differences: Meiji has a much
higher lactose content but a lower protein content. It also has a higher fat content
and added sodium.
Meiji was designed to be drunk, and its the most common brand in Singaporean
cafes. Its easy to see why, because its deliciously buttery, and the slight saline
content adds an extra dimension to the milk.
Barista milk, on the other hand, was a type of milk specifically designed to be used
with coffee. Hence the higher protein content (more stable foam) and less fat (to
draw out more of the flavours of the coffee). The modest lactose content might
have something to do with the desire to not throw the flavours of the coffee offbalance, too.
That said, the taste of milk as with coffee is highly subjective. Deciding what
brand of milk to use in a cafe is almost as big a decision as the type of bean to use!

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However, with an in-depth understanding of the different components of milk, that

decision doesnt need to be guesswork anymore.
Article written by Christine S. and edited by T. Newton.
Feature Photo Credit: Chris Pelliccione
Perfect Daily Grind.

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