There’s a lot of science in a glass of milk.

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If you start thinking about milk too much it begins to seem a little bit like a dystopian science fiction story.

An $800 million a year industry that impregnates animals, often culls the male offspring and harvests the mother’s glandular secretions for our own use. The Matrix has nothing on this.

Even more baffling, we do this despite the fact that the majority of the worlds adult population are unable to consume milk or many milk products.

Yet, nothing, in some cultures at least, seems as wholesome as a glass of milk. Not to mention the fond memories of milkshake bars and ice-creams that many of us retain from our childhood.

Even more importantly, without milk we would have no butter, cheese or cream. French cooking could not exist without milk.

Yes, milk is complicated, even more so when you start cooking with it. Just like it’s position in our culture the science of milk is a lot deeper than you might think.

What is Milk?

Few of us need an introduction to milk. Many of us first encountered it as newborns, when we were breast fed by our mother.

The ability of mothers to nurture their newly born babies on breast milk is, of course, one of the defining attributes of mammals, the evolutionary group to which we belong.

Milk and microbiomes

Every mammalian species produces a milk that is specifically tailored to it’s own children and humans are no different.

The consensus at the moment is that breast milk is the gold standard for new-borns and I’m not going to get into the breast-feeding debate here.

But I will mention a fascinating concept that has emerged recently is that breast milk is not only nurturing for the new born but it may also be nurturing for our gut microbiota.

Many studies have shown differences in the composition of the naturally occurring bacteria in our gut between infants raised on breast milk and those on baby formula and it is quite possible that breast milk includes nutrients, such as some oligosaccharides (big sugar molecules), that promote the growth and health of specific classes of bacteria in our gut (see here if you are interested in this story).

That bacterial populations have a big impact on our health is already well established and it is an intriguing concept that breast milk has evolved to not only feed a new born but also to nurture a healthy gut microbiome.

Milk, regardless of its species of origin, is mostly water but it also contains a mix of nutrients and living cells that are intended to provide a nutritious food specifically tailored to a newborn baby.

Milk Composition Across Species

In the kitchen, though, we are very, very rarely talking about human breast milk.

When cooking, we are usually talking about cow’s milk, though milk from sheep, goats or even camels sometimes makes an appearance. In the modern age, we are also almost always talking about milk that has been pasteurised and homogenised.

Is drinking human breast milk as an adult a good idea?

There is a thriving online market for human breast milk intended for adults but read here for an explanation why this may not be such a good idea.

Cow’s milk is a watery mixture of fats, proteins and sugars along with other trace elements like calcium, vitamins A and B12, riboflavin and potassium, just to name a few.

In raw cows milk there are also living cells, including immune cells from the mother as well as a several different species of bacteria but these are mostly killed during pasteurisation.

The main difference between cow’s milk and milk from other species is just the ratio of fats, proteins and sugars.

Before any processing, cow’s milk is on average 3.7% fat, 3.4% protein and 4.8% sugars. Human milk is much lower in protein and higher in sugars and pig’s milk has a massive proportion of fat, nearly 8.5%.

If you are thinking that this gives pig’s milk real potential as the bacon of the milk world, you’re not alone. But apparently, it is quite difficult to milk a pig so sadly the economics don’t pan out.

The Chemistry of Milk

Although milk is a complex mixture it is really only the fats, proteins and sugar we need to think about in the kitchen.

The Sugar

In milk the primary sugar is lactose, a disaccharide composed of one glucose molecule and one galactose (see my sugars post if this doesn’t mean much to you). The only place that this carbohydrate is made is in the mammary gland of mammals and it is this sugar that gives milk it’s sweetness.

Dulce de leche

As a sugar, lactose can react with milk proteins in Maillard reactions (see my steak post for more on Maillard reactions).

It is these Maillard reactions that are important when, like my grandmother liked to do, you boil a can of condensed milk to make caramel or dulce de leche.

At high temperature the lactose and the proteins react producing the browning and caramel flavours that are both the hallmark of the Maillard reactions.

If this is something you think you might like to try be careful. If the contents of the can reach a high enough temperature it can result in an explosion so keep at least two inches of water above the can at all times so that the contents cannot reach a temperature higher than 100C (see here for a detailed recipe).

Lactose is also the reason that most of the human race can’t drink milk.

All humans produce an enzyme called lactase when they are babies. In the small intestine lactase breaks down lactose into glucose and galactose.

In many, if not most, people expression of this protein begins to decline between two to five years of age, ceasing altogether by adulthood.

Our bodies did not anticipate that we would be drinking milk as adults and as lactose does not occur anywhere but in mammalian milk it sensibly saves some resources by shutting down production of lactase after childhood.

If someone who does not have lactase consumes lactose, it will pass through the small intestine and be fermented by bacteria in the large intestine.

Fermentation produces gases like hydrogen, carbon dioxide and methane and it is these gases which cause the intestinal pain, bloating and diarrhoea that is associated with lactose intolerance.

It is not all bad for lactose intolerant people. The processes involved in the production of dairy products, like cheese and yoghurt, can remove a greater or smaller amount of lactose, which may make them available to lactose intolerant people.

The evolution of lactose tolerance

Approximately 65% of the world’s population is lactose intolerant but some historic populations, most famously in northern Europe, lost the ability to turn off lactase production in childhood.

When this happened individuals gained the ability to digest lactose in adulthood.

In northern Europe this mutation is thought to have arisen some 7500 years ago and, in the context of neolithic life, it was a very useful mutation indeed as it meant that a new, easily obtained and significant source of life sustaining calories was available to the adult population.

This is why in central and northern Europe only 10% of the population are lactose intolerant, while in the rest of the world anywhere from 30-100% of the population are lactose intolerant (it’s a little bit more complicated than this and if you want to read more put the following DOI identifier into SciHub – 10.1038/500020a).

The Fat

Milk is also a rich source of dietary fat. But, if you have read my emulsions post, you’ll know that fats and water don’t mix terribly well.

For this reason, the fat in milk is contained in structures known as milk fat globules. These structures emulsify the fat, making milk yet another example of an emulsion.

The globules are similar to the micelles that we encountered in mayonnaise, in that they have an outer shell of lipids that can interact with both water and fat.

These lipids are arranged into a bilayer, and another single layer of phospholipids, that surrounds a core of hydrophobic triacylglycerols, or what we would call fat.

The fat globules are constructed when milk is produced in the mammary glands. They allow the otherwise insoluble fats to be dispersed in water, preventing them from clumping together and splitting out of the milk.

Milk fat globules will become much more important when we get to cream. The milk fat globules play an essential role when we whip cream and when we turn cream into butter.

The Protein

The final important constituents of milk, from the cooks perspective, are the milk proteins.

Milk is rich in proteins and there are several dozen different types of protein that can be found in milk but an easy way to think about them is as caseins and everything else.

In cows milk caseins are the dominant protein, there are four types of casein (alpha S1, alpha S2, beta and kappa) and they outnumber the other proteins in cows milk by about 4 to 1.

Caseins are a slightly unusual type of protein, in that they are relatively hydrophobic, or, in simpler terms, they don’t like water very much.

The consequence of this is that in milk, which is mostly water, they associate into larger structures called casein micelles (a little bit like the way hydrophobic proteins associate in the development of gluten in pasta).

These micelles are held together by calcium ions and the hydrophobic properties of the caseins. The precise structure of these micelles is still not known although a few different models have been proposed.

You might be wondering why I’m going on so much about caseins, but their name gives us a hint. Casein is derived from the latin word for cheese, caseus.

Caseins are the solid bits you get in curdled milk, they are the curds and most cheeses are made from the curds of milk (ricotta is the odd cheese out, it is made from whey proteins).

Milk and Acidity: The Science of Curdling

Caseins are also the reason the acidity is the arch-nemesis of milk. We know that acidity can alter the charges on a protein and as the pH drops, as we are adding acid that is, the charges on casein will start changing.

At first, this causes the casein proteins to start repelling each other, breaking down the casein micelles. As the pH continues to drop, the caseins lose their charges, and they start clumping together as large solids.

In other words, acid makes milk curdle.

This is what happens when milk goes sour and spoils but it is also the process underlying the production of fermented milk products like yoghurt and sour cream.

Acid and Cooking with Milk

Acidity is so important that, when you are cooking with milk, the main thing you should be worrying about is acidity, not heat.

Both the milk fat globules and proteins in milk are remarkably resistant to heat, milk, and also cream, can be reduced to a fraction of their volume without curdling.

Any one who has curdled their milk over high heat would probably disagree with this but often it is acid that is the true culprit behind curdling milk.

Naturally occurring bacteria, from the Lactobacillus genus, can survive pasteurisation and it is these bacteria that sour milk as they, over time, break down lactose to lactic acid.

The older your milk the more lactic acid will be produced by Lactobacillus making it more acidic and thus more likely to curdle when heated.

In fact, this used to be a dairy industry test for the freshness of milk, called the ‘curdle on boil‘ test. Milk was boiled and if there were clots then the milk had spoiled.

So, the take home message is, use fresh milk when cooking. Even in the fridge Lactobacillus will be making your milk more acidic.

Can You Prevent Soured Milk By Putting a Frog in It

That Lactobacillus bacteria sours milk is also at the root of the Russian tradition of putting a frog into milk to keep it fresh.

The specific species of frog used, the Russian Brown frog Rana temporaria, secretes anti-microbial molecules that kill the bacteria and prevent them from souring the milk (you can read more about this here).

Lactobacillus bacteria aren’t always the bad guys though. They are used deliberately in the production of fermented milk products like yoghurt and sour cream.

The breakdown of lactose into lactic acid by these bacteria explains the high acidity of those products but also why these products may be better for those who are lactose intolerant; the bacterial fermentation removes some or all of the lactose.

Conclusion

I hope I’ve convinced you that milk is one of natures miracles. What, on the face of it, seems to be a pretty boring white liquid is actually a highly complex emulsion with a whole bunch of chemistry going on.

It’s this chemistry that really comes into play when we start looking at things like cheese, butter and cream. Things that many would say are much more interesting, culinary speaking, than milk.

But I like milk, I especially like a milkshake, it’s my go to hangover cure. And as a chemist you have to be impressed that evolution could come up with something so nutritious to baby mammals that, with a little bit of extra chemistry, is also so good on pizzas.