A typical Australian coffee order is only 10% coffee; the remaining 90% is milk. As a coffee roasting company, we naturally spend a lot of time talking about coffee origins, roasting and brewing, but now it’s time to give some love to coffee’s greatest friend – milk.
So, what is milk? Milk is a nutrient rich liquid produced by mammals which is used as the primary source of nutrition for their infants. However, thanks to the magic of evolution, many of us can still enjoy milk as adults. However, when we say ‘milk’ we usually mean cow’s milk.
In the 24-48 hours it takes to get milk from a cow to a café, it goes through many changes which affect its flavour, texture and longevity. The key factors are the breed of cow, diet, pasteurisation, homogenisation and skimming.
Cow breeds
Since the domestication of cattle began approximately 10,500 years ago, five species have been domesticated into over 800 recognised breeds. The breed of a cow influences many things from the size of the cow to the shape of the horn. It also has a significant effect on the production of both fats and proteins in their milk. Jersey cows have been shown to produce the highest levels of milk fat amongst dairy cows. The highest levels of protein production are shared between Jersey and Guernsey cows.
Diet
Cows primarily eat grass, but they are also fed silage and cake. Silage is fermented grass which provides extra vitamins and helps digestion. Cake is a protein rich supplement which cows get given on their birthday – or when they need to bump up the protein in their diet! The specific grass, silage and cake that a cow consumes will result in a very different milk production.
Pasteurisation
Raw milk is milk which hasn’t been pasteurised. Australian law forbids the sale of milk for consumption which hasn’t undergone some form of pasteurisation. Pasteurisation was developed by Louis Pasteur in the 1880s as a way of inactivating harmful bacteria in wine. However, it has since been applied many food products including beer, fruit juice and milk. The pasteurisation process involves heating milk to a specific temperature for a specific time, thus eliminating pathogens and extending the shelf life of dairy products. There are several ways to pasteurise milk, however the method that you are probably most familiar with is High Temperature Short Time or HTST. Another method you may have heard of is Ultra-High Temperature or UHT which is used for long life milks.
The higher the temperature, the longer the shelf life of milk is extended. Pasteurisation can even remove the need for refrigeration in the case of UHT. However, lower temperatures will typically result in a truer representation of the raw milk’s flavour. A typical HTST treatment would hold the milk at 72°C for 15 seconds, while a typical UHT treatment would hold the milk at 138°C for three seconds or less.
Homogenisation
After a milk has been pasteurised, it will typically undergo a homogenisation process. The fat in milk is comprised of different sizes of fat clusters and individual globules. The clusters are less dense and will rise to the top, forming a cream layer. Homogenisation is the process during which milk is pumped at high pressure through small tubes, breaking up fat globules into smaller sizes and distributing them evenly throughout the milk. Homogenised milk typically has a richer texture but tastes more bland. Homogenisation also has the added benefit of helping to create a more stable foam structure in steamed milk.
Skimming
The amount of fat that ends up in milk is dependent on a variety of factors from the genetics of a cow to the food it eats. This can result in inconsistent fat levels in the final product. To achieve a more consistent product, producers will often remove some fat – even in full cream milks. Skimming is the process of removing the fat from a milk to reach a desired fat level.
CLSM 3D image of milk fat globules from raw cream stained with Rd-DOPE. Scale bar=5µm. SOURCE: National Center for Biotechnology Information
Milk under the microscope
A typical sample of raw milk from a cow will contain 5% lactose, 4% fat, 3.5% protein and 1% ash (amongst water and a bundle of other things). Let’s take a second to examine what each of these components are and their importance in a glass of milk or a flat white.
Lactose
Lactose is the sugar in milk. It is a disaccharide comprised of galactose and glucose. It has a mild sweetness when cold. However, heating milk can break down some of the lactose back into galactose and glucose – resulting in higher sweetness. Humans have an enzyme called ‘lactase’ which breaks lactose down into glucose and galactose. Lactose intolerant people have less of this enzyme and therefore have difficulty breaking lactose down.
Fats
Fat globules are important to the texture and taste of milk. However, fat has a detrimental effect on both the creation and consistency of milk foam. Higher fat content has been shown to cause the liquid in foam to seep out, resulting in lower foam volumes. This destabilisation effect is reduced by homogenisation due to the smaller fat globule size and further reduced by increased milk temperatures. Many of the fats in milk are solid at colder temperatures and will only melt once the milk reaches a temperature of 40°C. In foamed milk, this helps create a more stable foam structure.
Proteins
There are two main proteins in milk – casein and whey. These proteins have a large impact on coffee flavour. Casein has been shown to reduce bitterness in coffee and contributes to the milky flavour. Only a small amount of protein is required to create milk foam. However, the proteins are important for foam stability, preventing the drainage of the liquid which is contained in the foam layer.
Alternative milks
Alternative milks have exploded onto the Australian café scene in recent years to the delight of vegans and to the annoyance of many baristas. Many cafes offer soy milk, along with everything from coconut to oat milk. While a coffee order comprising an alternative milk is great news for the environment, it can be a source of frustration for baristas using acidic coffees. Baristas and soy drinkers are quite accustomed to seeing a latte glass filled with a curdled, coffee coloured tofu. The curdling is caused by a chemical reaction which occurs when you add an acid to certain proteins at high temperatures. The reason it often happens to soy and other alternative milks and not to dairy milks is due to the fragile nature of their proteins. Proteins are usually negatively charged, causing them to repel each other. Once you add an acid, this negative charge is removed, allowing the proteins to clump together which draws them into a separate solid protein layer with a distinct liquid layer underneath. Many alternative milk companies attempt to overcome this by adding an acidity regulator. If you love acidic coffees and don’t want to give up your alternative milk, you can reduce the acidity of your espresso by adding a pinch (0.2g) of bicarb soda to your espresso and stirring it in before adding your milk. You should even be able to achieve some nice latte art.
Now that you have a little more context about the 90% of liquid that goes into your cup, take a second to appreciate the long and complicated journey it took from the cow (or coconut) to the cappuccino.
This article was initially written in part of a curated cupping which was put together in September 2018.
