The Science of Food

I think humans have an interesting relationship with milk. Most of us start our food journey with human breast milk yet many, long after we’ve out grown any biological need for milk, chose to consume the milk and milk products of other species. To support this we have set up a $800 million dollar a year market for milk served by a logistics network that produces milk by impregnating animals, killing the male children and, like a dietary version of The Matrix, harvesting the milk that was meant to nourish their young. We have done this despite the fact that the majority of the worlds adult population are unable to consume milk or many milk products. We go to a lot of effort to produce a food stuff that our bodies evolved to only consume in childhood, that many do not have the necessary biological capacity to digest, makes many people explosively sick and has some questionable ethics around our treatment of animals. But, despite all this, without milk we would have no butter, cheese or cream, so yes, for this author at least, it is a very interesting relationship indeed.

When we talk about milk as adults we are generally talking about milk from cows, sheep, goats or even camels but of course a lot of us started life drinking human breast milk. The ability of mothers to nurture their newly born babies on breast milk is one of the defining attributes of mammals, the evolutionary group to which we belong. It is right there in the name, the word ‘mammal’ stems from the Latin ‘mamma’ for breast. It is also the only thing, with the possible exception of honey, that we consume as food that is created specifically to be a food. Milk, regardless of its species of origin, is mostly water but it contains a mix of nutrients and living cells that are intended to provide a nutritious food specifically tailored, over many thousands of years of evolution, to the nutritional requirements of a new born baby¹.

In the kitchen milk usually means cows milk and, in the modern age, almost always milk that has been pasteurised and homogenised. Cows 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.

Because cows milk is what we’ll generally encounter in the kitchen, for the remainder of this post when I say ‘milk’ I mean cows milk. Though just about everything I say about cows milk also holds for milks from any mammalian species, including ours²; the main difference being the ratio of fats, proteins and sugars. Before any processing, cows milk is on average 3.7% fat, 3.4% protein and 4.8% sugars³. Human milk is much lower in protein and higher in sugars and pig milk has a massive proportion of fat, nearly 8.5%. This makes me think pigs milk has real potential as the bacon of the milk world but apparently it is quite difficult to milk a pig so sadly the economics don’t pan out.

Although milk is a complex mixture it is really only the fats, proteins and carbohydrates (aka sugars) we need to think about in the kitchen. 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 and it is this sugar that gives milk it’s sweetness and also it’s ability to make a caramel, as the sugar is able to react with the 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 when boiling 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 human babies produce an enzyme called lactase which breaks down lactose into glucose and galactose in the small intestine from where they are absorbed through the gut wall. But production of this protein begins to decline between two to five years of age, and, for the majority of humans, has ceased 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. As we now know, fermentation produces gases such as hydrogen, carbon dioxide and methane which in turn cause the intestinal pain, bloating and diarrhoea that is associated with lactose intolerance.

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 and so 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).

It is not all bad for lactose intolerant people. Most can consume small amounts of lactose, generally less than 12g, especially when spread out over the day and when consumed with other foods. The processes involved in the production of dairy products, like cheese and yoghurt, can also remove a greater or smaller amount of the lactose which may make them available to lactose intolerant people. But there is always a lot of variety in human biology so do not just take my word for this if you are lactose intolerant. Every lactose tolerant person is the same but all lactose intolerant people are intolerant in their own way.

Apart from lactose milk is also a rich source of dietary fat and in milk these fats exist as droplets in the water phase bound up in what are known as milk fat globules. I talked at reasonable length on micelles in the mayonnaise post and these globules are basically complicated micelles with an outer shell of phospholipids, cholesterol, sphingolipids and proteins. Unlike micelles the outer shell is composed of an outer lipid bilayer, like the membrane of a cell, and an inner single layer of phospholipids. The shell surrounds a core made up of hydrophobic triacylglycerols which are esters of three fatty acids and glycerol, or what we would call fat.

The fat globules are constructed when milk is produced in the mammary glands and they allow the otherwise insoluble fats to be dispersed in water in much the same way that low density lipoproteins allow cholesterol and lipids to be carried around the blood stream. Without them the fats in milk would clump together into one large fat globule just like when oil and water separate in a vinaigrette. In fact we can call milk an emulsion because of the emulsiying effect of the milk fat globules that allow the fats to be dispersed through the watery continuous phase.

Because it is an emulsion raw milk can and will split. Just like the oil in a vinaigrette, over 12-48 hours the milk fat globules in raw or unhomogenised milk will float to the surface of the milk and form a top layer of fat in a process called creaming. This layer of milk fat globules is of course cream and when skimmed off the surface of the milk the remaining milk is referred to as ‘skim’ milk and it is lower fat because a lot of it’s fat globules has been skimmed off in the cream. This process is at the heart of the production of two of my favourite ingredients, butter and cream, but I’ll go more into these foods, and creaming, in later posts. This process is prevented by homogenisation which is why you wont see it in your grocery bought milk.

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 type of 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, as I’ve discussed elsewhere, proteins tend to have a definite structure that is stabilised by various interactions between different parts of the protein. Hydrophobic interactions are one of the more important types of interactions and, in water, they tend to force the hydrophobic parts of the protein into the centre of the protein structure where they are protected from water by more hydrophillic (water loving) parts of the protein.

Caseins, however, have little secondary structure, in technical terms this is partly because they have a high proportion of proline, the only amino acid which has a rigid ring structure that limits the ways that the protein can fold, and they are also quite hydrophobic. The consequence of this is that in milk, which is mostly water, they will tend to clump together (just like hydrophobic interactions drive the development of gluten in pasta) and form larger structures called, confusingly because they are not actually the micelles we’re familiar with, casein micelles. These micelles are held together by calcium ions and the hydrophobic properties of the caseins but the exact structure is not precisely known although a few different models have been proposed. The most recent model is shown below and it proposes that sub-micellar structures are formed from 15-25 casein molecules that then aggregate into casein micelles that can contain 100’s of the sub-cellular components.

You might be wondering why any of this is interesting but caseins are named after the latin word for cheese and this is because caseins are the solid bits you get in curdled milk, they are the curds and it is curds that make up the majority of the cheeses. I’m not going to go into cheese making here, that’s a whole post in itself, but it is important to notice that one of the casein proteins, kappa-casein, is important in forming the casein globules and they tend to coat the exterior of the micelle. A part of kappa-casein protein extends out from the surface of the globule and this part of the protein has a negative charge. So essentially each casein globule is haloed by a negative charge that will repel other casein globules with the same negatively charged halo preventing the micelles from clumping together.

It is when this negatively charged halo is removed that milk curdles. Without the negative charged halo the casein micelles clump together to form curds. In cheese making this is encouraged by treating milk with an enzyme that snips off the negative part of kappa-casein thus removing the repulsive force. But we also know that acidity can alter the charges on a protein, at a pH of around 5.5 kappa-casein loses its negative charge and so the micelles will start clumping together and start becoming curds. At this pH the acidity also starts dissolving the micelles so the casein proteins will also begin to scatter. But as the milk becomes even more acidic, at around pH 4.5, the caseins lose other charges on their surface and they will start clumping together again. 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.

In fact, when using milk in cooking, acidity is the main thing you should be thinking about, even more so than 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 a genus called Lactobacillus can survive pasteurisation and it is these bacteria that sours 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.

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 freshness is the first thing to keep in mind when cooking with milk or cream, secondly be gentle, it takes time to get milk to stores and then it sits in your fridge building acidity all the time, so by avoiding boiling and bringing it carefully to heat you can help avoid curdling. Also think about the acidity in your dish, if it is a very acidic dish it will curdle any milk that you add.

Lactobacillus bacteria are also used 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 more accessible to lactose intolerant people; the bacterial fermentation removes some or all of the lactose. 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 about this if you put the following DOI into SciHub – 10.1021/pr300890m).

Apart from caseins there are a few dozen different proteins in milk and they are generally referred to as whey proteins as they remain dissolved in the water when the casein proteins coagulate out of solution. These proteins have various functions, mostly defence, with many of these proteins being antibodies (or immunoglobulins), they can also act as milk allergens, for example beta-lactoglobulin. Although some cheeses are made from the whey, the most famous being ricotta, the whey was usually thrown out during cheese making but it is now marketed as a high protein super food. I’m not convinced, I can see how it could be good for body builders who need some extra protein but not sure about the many other health benefits that are claimed, all the proteins get broken down before entering the body so I’m not sure where the extra health benefits are coming from if taken orally (there is a recent review here that could be a start if you are interested in this).

So that’s it for milk for the moment. There is a lot more to milk than I have talked about here, from milk we get cream, butter and cheese and each of these have their own complicated but interesting story. But for now we have the basic chemistry of milk sorted which sets us up for better understanding these next steps in our complicated relationship with milk.

Footnotes

1. 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 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. ↩︎
2. There is a thriving online market for human breast milk intended for adults but read here for an explanation why this may not be a good idea. ↩︎
3. I got these figures from Harold McGee but there is some variation in the various sources I looked at but these seem to be reasonable averages. ↩︎