The Science of Food

Today I want to get back to dairy. In particular I want to get back to milks sexy cousin: cream. Cream comes from milk and we already know that milk is an emulsion of fats dispersed in a watery continuous phase. We also know that the fats in milk are packaged into a globule called a milk fat globule that stabilises the emulsion and prevents the fats clumping together and splitting. To understand cream this is all you really need to know. Cream is just milk with a lot more fat globules and lot less water. Much like mayonnaise, where egg yolk proteins emulsify a large amount of fat in a relatively small amount of water, the milk fat globules emulsify and stabilise a potentially large amount of butterfats in a watery matrix. Almost all the chemistry behind the luxurious mouth feel of cream, it’s ability to be whipped into structured foams and its ability to enrich sauces stems from the fact that cream has a stack of fat packaged into little globules floating in water.

Any one who has encountered raw or unhomogenised milk knows that if left to sit cream naturally forms as a layer on top of the milk. This process, called creaming, occurs because, although the fat globules are well emulsified, they are less dense than water. As a consequence they are buoyant and they will, over the course of 12-48 hours, float to the top of the milk and form the layer of cream. This is different from splitting, the fat globules are not excluding water and clumping together because of hydrophobic forces, they are just floating to the top. Creaming is easily reversed by giving the milk a bit of a shake but a better idea is to skim off the layer of fat globules and then you will have cream. This process has given rise to a technical meaning of the word ‘creaming’: in the world of emulsions it is the name given to any dispersed phase that pools together, i.e. sinks or floats, under the influence of buoyancy.

For most of human history skimming cream manually from milk was the only real way to make cream. In those days the cream was gathered from pooled milkings that were a day or two old and so the cream also had a bit of fermentation going on which provide a different taste. These days cream is made using fresh pasteurised milk in centrifuges. This not only speeds up the creaming process but it is also much more efficient with most of the dairy fat globules being purified from the milk. It has also provided a measure of control, allowing manufacturers to produce creams with a well defined proportion of fat. So in the supermarket we can select a creams with different fat content: lite, whipping, double and heavy creams for example.

Annoyingly, in different countries these creams have different names so when using a recipe from another country it can be easy to get a cream with an unsuitable fat content. For example something called heavy cream in the USA would be called double cream in Australia. The USA, according to Wikipedia, has light, whipping, heavy and something called Half and Half that I thought was some type of low fat milk when I lived in America but which is actually half milk, half cream. The only thing that really distinguishes these creams is the fat percentage. So when following recipes it’s a good idea to have some idea how the cream is being used in the dish and not just try to decipher the name used to describe the cream. If you know what you are using it for and you know roughly the fat content you want for that application you can normally work out an equivalent cream available in your local supermarket¹.

Apart from drinking it straight from the bottle cream has a bunch of applications around the kitchen. One of the most common is adding some luxuriousness and ‘mouth-feel’ to to a sauce, soup, curry or stew. Because cream is full of fat it slightly thickens and improves the texture of liquids while providing fatty flavours that increase the richness of the dish. Cream can be used this way, adding it to hot or potentially acidic foods or even deglazing a hot pan, because it is a lot less prone to curdling than milk. We already know that milk curdles because caseins denature and clump together to form curds when exposed to acidity or high heat. But cream, apart from having less caseins, is stabilised in adverse conditions by the ability of the fat globules to interact with caseins molecules as they begin to denature.

If a cream has a high enough fat content, about 25% according to Harold McGee, there are enough fat globules to soak up denatured casein proteins preventing them from interacting with other denatured proteins and forming curds. Because of this cream will not curdle in conditions where milk would probably curdle. That is not to say that cream wont curdle. It can curdle but a simple rule of thumb would be the heavier the cream the less likely it is to curdle. So if you’re using cream at high temperature, or if there might be some acidity to your dish, you would want a heavier cream because it has more fat globules, less caseins and less chance of splitting.

Another common application of cream is to whip it. This gives cream, at the same time, airiness and structure. The differing amount of structure that can be obtained, by using creams of different fat content, gives whipped cream great versatility. Whipped light cream forms a less dense foam that can be dolloped on fruit, pies or coffees while whipped heavy cream forms a denser foam that can form layers in a cake or be piped to form long lasting decorations (sometimes with the help of stabilisers like gelatin that provide a protein framework that helps stabilise the cream and prevent weeping). Whether lightly or strongly whipped, whipped cream is a foam, a stabilised network of air bubbles dispersed throughout a liquid. And the magic behind cream’s ability to become a foam comes down to what happens to milk fat globules when cream is agitated.

When we first attack some cream with a whisk we are repeatedly forcing air through the mixture. Initially this air bubbles out of the cream as there isn’t enough surface tension to trap the air and it can just slide out between the fat globules and water molecules. But as you keep whisking the fat globules are bashed together and begin to lose parts of their phospholipid membrane exposing patches of fat. We know that fats are hydrophilic and don’t like interacting with water so they will either stick to another globule’s exposed fat patch or align to face an air bubble. Over time you will start producing a network of globules bound together by their fat patches, a network that will eventually provide enough surface tension to start trapping the air bubbles. The more you develop this network the more air bubbles you’ll trap and the more structure you’ll have in your whipped cream. But be warned. If you go too far you’ll damage the globules too much and start liberating free fats that will coagulate. When that happens you’re on your way to making butter. This is why over-whipped cream is grainy, you’ve got bits of butter in it.

This description is a bit simplistic, proteins play a big role as well (start here if you want to know more), but for cooking purposes it is a good enough model to impart some lessons. Firstly, to construct the stabilising network you need a lot of fat globules so you’ll need at least a 30% fat cream to easily make whipped cream. The more fat in the cream the stiffer you’ll be able to make your whipped cream. So depending on what you are using the whipped cream for you’ll need to consider the fat content of the cream you start with. If you want a lighter whipped cream and you only have heavy cream you can add some milk to make it lighter. If you have a light cream and you want a stiff whipped creamed then it’s a trip to the shop for some heavier cream. You can make a heavy cream from a light cream by adding melted butter but because the butter fats are free instead of packaged in globules whipping wont work. It would work if you were wanting to add it to a sauce where the globules in cream will melt anyway but butter has it’s own flavour profile though so you’d need to take that into account as well.

Another important consideration when whipping cream is temperature. When whipping cream you are building structure using fats as your building material and we all know that fats like to melt. Milk fat globules package a range of different fats (technically fatty acids and triacylglycerols) that have a correspondingly wide range of melting points. So you want to start cold, 5-10C, and you want to keep it cold because any melted, liquid fat will affect the structure of your whipped cream. A lot of sources recommend chilling the mixing bowl which probably isn’t a bad idea as whisking itself causes a temperature rise, though if you are using an electric mixer you might be quick enough. Depends how lucky you are feeling.

You also want cream that has been chilled for a prolonged period of time, what is called ‘ageing’ your cream in the whipping business. This promotes the formation of some butterfat crystals that will help remove the fat globules outer layer speeding up the whole process. You can also make it easier to whip cream by adding acid. Acid denatures the proteins on the surface of the milk fat globules again making it easier to disrupt the out layer and expose the fat. Be careful though, too much acidity will affect the taste and weaken the structure of your whipped cream. Harold McGee recommends 5 ml lemon juice per 250 ml of cream, which seems a good place to start experimenting.

Cream by itself, straight or whipped, has a fairly neutral flavour. The most noticeable aromas being caused by common chemical building blocks called lactones (in chemistry-speak cyclic carboxylic esters) that cause the “fatty” aromas that are also present in peaches, coconuts and barrel-aged beers (I can imagine cream with peaches or coconut but beer? I’m not so sure). This neutrality makes it perfect for something like strawberries and cream where it brings a richness and mouth feel while leaving the flavour to the fruits. Lactones are also important as they can form from fatty acids in cream when it is heated, or when added to a sauce, in a process called esterification (that we’ve seen before). These new-born lactones, especially the γ and δ-lactones, contribute to the richness imparted by adding the cream.

Because of creams neutral flavour it probably didn’t take long before humans were tarting up whipped creamed with sugar or other flavours. Today, the most well-known sweetened whipped cream is Chantilly cream, a whipped cream that includes vanilla flavouring and castor sugar. The French, of course, claim it was invented in the French town of Chantilly in the 17th century but it is likely to have been an Italian invention. A medieval dish called Milk Snow consisting of whipped cream was mentioned in Italy as early as 1500 and the first recorded recipe for Milk Snow can be found in the 1545 English cookbook called ‘A proper newe booke of cokerye’ (you can read this cookbook online here – the recipe is titled ‘To Make a Dyschfull of Snowe’). Milk Snow not only included sugar but also egg whites, that presumably helped with getting a firmer consistency, sprigs of rosemary and a whole apple, which I think would have made it quite something to behold (people have made ‘a Dyschfull of Snowe’ and described the result here and here).

Another medieval cream oddity was known as cabbage cream. The dish contained no cabbage but was constructed by the skins formed on shallow pans of cream that were arranged on a plate in the ‘fashion of a cabbage’. Sugar and rose water would be thickly applied between the layers of cream skin. Probably not something that would be enjoyed today but if you are interested you can see a version of the recipe here. This version is also interesting as they appear to have pasteurised the milk before making the cream².

Another 16th century English form of cream, but one that has survived to the present day is clotted cream. Though less known outside of England clotted cream is still popular in it’s home country and is served on scones or fruit. Essentially it is super-high fat content cream that is produced by heating milk almost to a boil for several hours and then letting it cool before skimming off the cream. The boiling gets rid of some of the water, hastens the rise of the milk fat globules and melts some of the fat to create butter fat. In England, by law, it needs to be at least 55% fat, though typically it’s around 65% fat, so not something you want to eat by the tub full. The cooking gives it cooked aromas and the pockets of butterfat also give it buttery flavour. Although typically associated with England (Devon in particular) it is also has a cousin in Turkey, where it is called kaymak, and there is a Persian version called sarshir.

Finally we come to our fermented creams. As I mentioned above, in medieval times the cream had probably already undergone some fermentation. In the absence of modern technology our ancestors had to wait for the cream to rise to the surface and during this time lactic acid fermentation would have occurred. Because we are able to separate our cream a lot more efficiently the cream we buy will not have had the time for any fermentation, and the milk is also typically pasteurised before creaming, so to achieve products like sour cream or creme fraiche manufacturers need to add bacteria to ferment the cream.

Creme fraiche and sour cream are quite similar and are both produced commercially by inoculating cream with lactic acid bacteria and fermenting. The main differences between the two are that creme fraiche has a higher fat content and is accordingly thicker than sour cream which has a lower fat content and so is thinner but more acidic and and ‘tangy’. Having said that the sour cream I buy generally seems to be just as thick as creme fraiche so I suspect someone is adding thickeners to make my sour cream thicker. In general though, because of the fat content creme fraiche has a greater resistance to splitting than the lower fat sour cream so if your going to simmer for a while use creme fraiche. If using sour cream, which you might want to do because of the acid it will bring to your dish, add it late in the process. If you want to make them at home there are some simple recipes here.

I didn’t really expect this post to be that long but turns out there is more to cream than I thought. As I alluded to above, whipped cream is a waypoint on our way to butter. But it is a delicious waypoint and it’s been well worth the time working out the chemistry of cream. A post on butter is imminent so I guess it’s not the end of dairy quite yet, and there is cheese as well, but for now I think I’ll go and have some clotted cream and jam scones. Jam on first of course, Cornish style, not cream first like those savages in Devon.

Footnotes

1. Some creams will have added thickeners like gelatin. In Australia we have ‘thickened cream’, that is a thicker cream for ‘dolloping’ and which can be easier to whip. There is nothing you can’t do with a pure cream that you can do with a thickened cream so I’ll stick to pure cream here. ↩︎
2. As an aside the cookbook this recipe can be found in is called ‘The Queen-Like Closet’ from 1670 and it was written by Hannah Wolley who can probably claim to be England’s first professional food writer. ↩︎