Milk Science
This month for our Level-up, we’re going to be travelling to Swallow Hill Jersey Dairy to meet our milk partners – both human and bovine! In preparation for that, here’s some milk science to look over. If you’ve been with us for a while, you might remember some of this information from a Level-up last year. If you’re new to Deeper Roots, this will hopefully help you understand more about why we use the milk we use, and the science behind milk steaming.
Milk is around 87% water – the remaining 13% is what gives milk its flavor and unique qualities. Of that 13%: 27% is lactose (milk sugar), 31% is fat, 26% is protein, and 6% is made up of various minerals. As coffee professionals, we’re mainly concerned with lactose, fat, and protein – especially protein. To achieve this makeup, a balance of nutrients is needed in the cow’s diet. Farmers usually provide three sources of food, often year-round: grass from the pasture, silage (a fermented, high-moisture feed – think sauerkraut for cows), and cake (a supplemental protein source made from cereals and soy). Cattle are usually kept out of the pasture during winter months, when their hooves can churn up wet ground. This is important for us because it means that there IS a difference between summer and winter milk! You’ll likely notice a difference as the weather begins to turn.
When it comes to steaming milk, or creating microfoam, a couple of things are happening. The act of steaming milk itself is a two-step process. First, we “entrain” air into the milk. It introduces air into the liquid of the milk, thereby texturizing it and creating a silky mouthful. This also increases the volume of the milk by about 33%. Second, we are heating the milk to about 155* for ideal drink making. This also helps keep microfoam stable.
microfoam at 60x magnification (the white line is 1mm) - courtesy of Barista Hustle
Several things also happen chemically to the milk as we introduce air and heat it – specifically two that are important to us as baristas. Milk sugar (lactose) is a “disaccharide,” meaning it is made up of two sugar molecules: glucose and galactose. When we heat milk past 110*, lactose splits into its constituent monosaccharide molecules. That’s important – because it’s easier for your palate to break down simpler sugars. Steamed milk will taste sweeter than non-steamed, even though no additional sugars are created. The second involves the most important component of milk for microfoam: protein. Protein molecules naturally exist as tightly coiled strands. One side of the molecule is hydrophobic, meaning it repels water; one side is hydrophilic, meaning it attracts water. As milk is heated, these molecules begin to unspool and form a grid-like structure. The hydrophobic part of the strand wraps around the microfoam bubbles, while the hydrophilic part draws in the water close around the bubbles. This is what creates luxurious, voluptuous microfoam for the best drinks.
courtesy of Barista Hustle
(It’s also why it’s so hard to steam oat and macadamia milks; they have almost no protein compared to whole milk.)
Heating milk too hot, above 165*, will cause these protein molecules to “denature,” or fall apart. Practically speaking, this is the upper limit for how hot we can steam cow’s milk because in addition to tasting burnt or scalded, any microfoam you create will not hold up.
This is best summarized by Professor Steven Abbott’s Five Foam Factors:
1. Wet – having a high liquid content with a relatively low aeration percentage: 33% is ide
2. Fine bubbles with a small diameter – this creates lower surface tension
3. Stability – milk foam above 98* will remain relatively stable
4. Moderate viscosity – unhomogenized whole milk is best for a luxurious mouthfeel (Swallow Hill whole milk is unhomogenized!)
5. Low yield stress – it takes little effort to disrupt the structure – pour within 10s of steaming for best results
Good cows produce good milk, and good milk leads to good latte art and good tasting drinks. And that’s Good for the People.
courtesy of Barista Hustle