To make your passage through the early chapters a bit smoother and to familiarise you with a few terms right up front, let’s summarise what’s in your glass of milk. Water makes up around 87% of milk. Aside from the water, milk is mainly composed of sugar (lactose), fat, and protein. The pie chart above gives you a rough idea of the proportions of dissolved and undissolved solids in milk.
In this image you can see the relative proportions of the non-water component of milk.
The protein comes in two main forms: whey and casein. Whey protein is key to giving milk foam its elasticity and stability. (See lesson 2.3) Casein protein in milk is an emulsifier, and it is an important source of nutrition. Casein protein clumps together in a way that not only makes the milk appear white and opaque but also helps it to form small clots in the stomach. This ensures better nutrient absorption for infants.
The milk sugar lactose imparts sweetness, though this sugar is significantly less sweet than sucrose. Like sucrose, lactose is a double sugar, or disaccharide. It is made of galactose and glucose bonded together.
Milk’s third principal constituent is fat. It takes the form of globules, similar to the casein clumps. The interplay between the casein and the fat is key to the successful homogenisation. Homogenisation refers to the process of making things uniform or similar (see lesson 1.6).
Milk is a minor source of vitamins, including C and B12, and a significant source of calcium, magnesium, phosphorus, potassium, selenium, and zinc. Research suggests that ‘intake of milk and dairy products contribute to meet nutrient recommendations, and may protect against the most prevalent chronic diseases, whereas very few adverse effects have been reported’ (Thorning et al 2016)
A barista’s preference for particular ratios of sugar, fat, and protein brings great variation in flavour intensity, texture, and glossiness to the coffee world.