Just like coffee, milk is an extremely complicated and diverse beast. No two milks are alike. If you make decisions about the milk used at a cafe or even in your kitchen at home, it really pays to know what’s going on.

Let’s dive straight in.

Milk is mostly water with four other components: sugar, fat, protein and ash (don’t worry it’s not actually ash). We’re going to run through each component, then talk about the whole picture.

Sugar

Lactose is the primary sugar in milk. There are other sugars in there like glucose and galactose, but there’s not enough of them to take notice.

Sugars are carbohydrates (aka saccharides), which kind of translates to “dense source of energy”. When a calf drinks milk the easily digestible and long lasting energy comes in the form of lactose.

Lactose isn’t super sweet. If sucrose (table/caster sugar) has a sweetness intensity of 1.0, lactose only has 0.16. It’s more than 5 times less intense! This means you need to add or remove a lot of lactose to notice any significant change in sweetness. The sweetness of lactose is much softer and what I would describe as ‘lower-register’ in comparison to sucrose. It’s round and mellow, not sharp and candy-like.

Fun fact: the sweetness of lactose -and other sugars- changes according to the temperature that you experience it. (edited: see comment below)

Fat

Butterfat is the name for the collection of fats present in milk. When you see that a milk is advertised as skim, trim, skinny, low fat or otherwise, it’s this portion of the milk that has been removed.

Butterfat is kind of complicated. It’s a combination of fatty acids bundled up in esters called triglycerides. If that sentence was a bit of blur, I hear you. Just think of triglycerides as a delightful sampler pack of tasty fats that are less dense than water. Roughly 65% of the fats in these triglycerides are saturated, including Palmitic (found in palm oil), Myristic (nutmeg) and Lauritic (cocoa butter). 35% of them are unsaturated like Oleic and Linoleic (olive oil). These triglycerides bunch together in large clusters called fat globules. The globules are about 14 microns (0.014mm) wide, which is absolutely massive on the scale of molecules and fats. Their sheer size means they’re unable to dissolve or hang suspended in the milk. Homogenised milk undergoes a process that breaks apart these globules, which then spread throughout the milk evenly. You can spot unhomogenised milk by the thick layer of cream at the top of the bottle (a rare and delightful sight these days).

Fat gives the milk a whole lot of heft and mouthfeel. The contributions of proteins and sugars to mouthfeel shouldn’t be underestimated, but in milk, fat is certainly doing most of the of heavy lifting in this department. Fatty milk is rich, dense and luxurious whereas skim milk is generally thinner, lighter and (in my opinion) unsatisfying. Food scientists around the world are scrambling to emulate the experience of fat in foods, but they’ve been coming up empty-handed for years. You just can’t replace the unctuous, mouth-filling, satisfying feeling of fat on your palate. Embrace it!

Fat (unfortunately?) also reduces the amount of coffee flavour that reaches your taste buds. If you think of a black coffee, it’s made of only water and coffee particles. The solution is free-flowing and there’s not much stopping the flavour in those particles from moving towards and hitting your taste buds. Introduce butterfat (with an average globule size of 14 microns) and the solution is very quickly gummed up. Your taste buds can only register flavours that are touching them; anything else that’s in your mouth will only contribute to general mouthfeel.

A similar effect can be observed when you eat or drink something fatty to subdue the effects of spicy foods. The capsaicin or spice that’s causing discomfort is dissolved in the fat, reducing how much of it gets to your tongue.

Protein

Milk proteins are special because they can only be found in the milk of a mammal. For coffee drinks, protein’s main role is to assist the creation and stabilisation of milk foam. It does this by acting as a barrier or film between the air and milk, which stops the foam’s tiny bubbles from popping or destabilising. It’s other lesser role is to provide some of the gentle caramel flavour you experience drinking heated milk.

76-86% of the protein in milk is collected in little bundles called casein micelles. Casein is the protein type and is pretty much only found in mammalian milk. A micelle is a chemical denomination for the structure that large molecules take on when in solution. This allows them to float around while not actually being dissolved in the liquid (this is what’s called a colloidal suspension and is why milk looks white!) Thanks casein!

The rest of the proteins in milk are known as the serum or whey proteins. Beta-lactoglobulin, the most common whey protein, helps us create milk foam. Normally, the exterior of these proteins attracts water. By whipping or heating them they ‘unfurl’, exposing the hydrophobic sections of the protein. Those water-repelling sections will then adhere to any air bubbles reducing their surface tension and making the bubbles stable. (edited: see comment below)

Ash

Milk was designed by nature to nurture a young calf into adolescence. This means there’s a whole lot of other beneficial stuff packed into our humble glass of milk. Ash is just the name the dairy industry uses for this group of vitamins, minerals and enzymes.

Milk contains a whole bunch of water soluble vitamins: thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), vitamin B6 (pyridoxine), vitamin B12 (cobalamin), vitamin C, and folate. It also has the fat-soluble vitamins A, D, E and K. Low fat milk has much less vitamin A, and is usually fortified with extra vitamin A to achieve a similar nutritional value to full-fat milk.

The minerals include calcium, magnesium, phosphorus, potassium, selenium, and zinc.

There are heaps of enzymes in milk. Some of them degrade the fat and protein (reducing shelf life) and others are antibacterial (increasing it). The enzymes have nothing to do with our digestion of the milk, and a lot of them have purposes completely unknown to us—though the cows probably know.

Reading the Label

Reading milk bottle labels is a hobby of mine, and I hope it will become one of yours too. The nutritional information on a bottle of milk can tell you a lot about what it will taste like. Knowing the breakdowns of various milks and tasting their differences is a great palate development exercise. It’s also delicious. That said, two milks with identical fat/sugar/protein levels can taste rather different depending on the cows they came from.

For me, (and the many milk enthusiasts I know) a full-fat milk will always trump its low-fat cousin. One trap (that I’ve fallen into) is to look for as much fat as possible to improve the mouthfeel. A noble cause, but this strategy has limiting returns. Remember, the more fat you have, the less you can taste the coffee. Balance, as always, is necessary for fatty, delightful success. Milk fat is typically anywhere from 2.5-6.0%. I prefer milks with a higher 4.5-5.5% fat content. Cappuccinos from a milk like that will be rich, satisfying and dessert-like. If your coffee is roasted and made well, this level of butterfat shouldn’t be a problem for coffee flavour. Fat is the most important number to keep track of. Sugar and protein have less impact on flavour, but shouldn’t be forgotten.

Protein is a little less obvious than the others. It’s role as emulsifier for microfoam is crucial, yet we have more than enough in almost all milks. Don’t be too concerned with protein levels, unless out of the 2.9-5.0% range. Every milk that you’re likely to pick up in the supermarket has more than enough protein to support milk foam, so don’t worry too much about it.

Sugar is easy to taste and discern. If you can’t find the sweetness in milk, there’s a problem. Remember though that humans perceive lactose as far less intense than sucrose and glucose. This means that large swings in lactose need to happen before you see a big change in flavour. 3.6-5.5% is the usual zone for sugar.

Keeping track of the Ash components is a little difficult and has little to no bearing on flavour.

The Cowculator – Part 2

Next week I’m going to release an interactive calculator to help you understand precisely what’s going on with your milk when you add it to coffee. It’s super interesting, and I hope it will help some of you wrap your head around what’s really going on when you mix coffee and milk together!

Interesting Milk-Related Links:

Milk Chemistry (Where I got most of the numbers. Also has great chemistry explanations.)
Different sugars and sweeteners and their relative sweetness.
A great wikipedia article on Milk. (Super detailed!)
An SCAA Symposium lecture by Thom Huppertz on the science of milk foam.

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