Studying the chemistry of milk can get, well – nerdy and technical, but it is necessary for the perfection of things like beautifully textured milk and coffee art. So take a pencil out of your pocket protector, fellow nerds, and let’s get started.
Don’t worry, you are not going to need a chemistry degree to get through this, but the chemistry of milk is slightly technical. It is susceptible to changes in the dietary conditions of cows (we’ll restrict our discussion to bovine milk – you should thank me), the health of the cow and the food and region a cow eats and lives in. All of these change the taste and makeup of milk. To keep our discussion as simple as possible we’re going to concentrate on skim up through whole or full cream milk and assume all cows are raised in the same environment and eat the same food (you should thank me again).
There are three things in milk that directly influence the taste of coffee beverages; milk sugar, fat and protein content. Beyond this you’ll need more pocket protectors.
The sweetness we get from milk has to do with the lactose, or milk sugar, that is in the milk. In reality lactose is a combination of two sugar molecules, galactose and glucose, held together is suspension in our milk. It is not very soluble, i.e., it doesn’t break down in water, so the lactose stays in suspension in milk liquids and while cold it does not taste extremely sweet. If you compare lactose to, let’s say table sugar, it is way down on the list of “sweetness”. The amazing thing is that as the lactose is heated the solubility breaks down and the sweetness of our beverages increases. Such is the case with milk being textured in an espresso machine. The steam breaks down the suspension-ability of the lactose, releasing the sugars into our cups. To put it another way, when you were a child and came in from the cold if your Mum served you hot chocolate made with real milk it tasted sweet. The reason is that the milk was heated to release the milk sugars.
Put two cups of milk side by side, one whole and the other skim and taste the difference. The skim milk will be lighter over the tongue, the taste will not remain in the mouth as long and it will taste somewhat watery. Now take a mouthful of the whole milk. It will feel like it has more body, thicker and heavier. The watery taste is gone and it feels more like a solid food substance. The difference between the two is the fat content. Milk directly from the cow has over 4% of its content from fat and in fact if you let milk directly from the cow sit for a few hours the cream – full of most of the fat – will float to the top. What is below is basically whole, or 4% fat, milk.
Through mechanical extraction, producers can alter the fat content to 2% (50% extracted), ½% (almost all fat extracted) and skim (where the fat has been removed, basically leaving water, proteins and sugars). Arguably whole or full cream milk will give the coffee a fuller body and taste but which type of milk you use as a barista will be dictated by what your customer orders.
Proteins are the third main concern for the barista. Simply put they give us our froth. But the simplicity ends there. When you add steam to milk it is the proteins that stick to the outside of the air molecules. They basically build a cage around each air molecule and cause it to ‘float’, protect it and give it stability on the top of our drinks. But beyond that the fat content of the milk greatly affects how proteins react during this process, as does the temperature, because remember fat is influenced by temperature which influences the proteins which influences………..! So here in a nutshell is what you need to know:
As the fat content increases from skim to whole, the ability to make large volumes of froth and the stability of that froth, decreases. Therefore as we know it is easier to get more and denser froth with skim milk than it is with whole milk. BUT, as the fat content continues to rise above 4%, say from 5% onwards, froth volume and stability go up once again. You’ll see high volumes and stability in your froth in a cream with about 10% fat. (Note that table cream is at 18% fat and there is a reason why whipping cream is 35% fat…volume and stability).
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