Let’s talk about milk…
We all know the micro-foam in milk is really important to a great cup of coffee. But how and what makes this? Why can you only steam milk to a particular temperature? Why does it get sweeter when heated and then drop off if you over heat? For me these were really interesting questions that I had to look into.
I found there are a range of things that have a subtle but noticeable change to the foam texture, the overall taste, quality of latte art and texture of the cappuccino/latte:
1. Foam ability / stability
2. Milk Source
3. Fat Content
4. Milk Sugar
So ‘foam’ formation is all about the creation and stabilisation of gas bubbles and is measured by ‘foam ability’ and ‘foam stability’.
•Foam stability – how long the foam lasts
•Foam ability /volume – the amount of foam formed or the amount of space
Steam injection and agitation (as in cappuccino machines) is what helps forms these air bubbles in milk.
2. Milk Source:
• breed of cow
• diet of the cow (grain, grass, corn)
• health of the cow
• stage of lactation
Most we have no real control over but it pays to know your milkman. Keeping it local is the key to a good supply.
3. Fat Content
The fat content also has a direct relationship with the amount of ‘foam’ produced, but also the ‘mouth feel’ and ‘texture’ of the foam.
Milk directly from the cow has over 4% fat content. Store milk has around 2% fat and skim (depending on brand) has most of the fat removed leaving just water, proteins and sugars.
Call me naive, but I was surprised to read that milk manufacturers alter the fat content by first removing all of the fat and then adding it back depending on the particular product i.e. skim, low fat, extra fat…milk products. The amount of fat varies considerably between different brands.
In terms of impact of fat on foam, fattier milk is generally ‘thicker’, has better ‘mouth feel’ or ‘body’ a fuller taste and heavier feeling on the tongue. Where skim milk can taste watery, lacking in substance and the taste will not stick around in the mouth as long (some may want this!!)
Fat content affects the surface tension of milk. Generally speaking the fattier the milk the greater the surface tension. Basically the fat coats the air bubbles. One would think the greater the surface tension then the greater the foam…right??? Not quite…there are two things to keep in mind:
• ‘Foam stability’ – this increases as fat content increases (compare ‘whole milk’,’table cream’&‘whipping cream’)
• ‘Foam Ability’ – this decreases as fat content increases.
So fattier milk has more ‘foam stability’, it gives bubbles better resistance to bursting and stabilises the foam better than skinnier milk. Skinnier milk however has the ability to produce greater ‘foam volume’ but less ‘foam stability’.
So, what’s the goal??? One needs to ask, if the goal is to create volumes of foam, non-fat milk gets the thumbs-up. However, if your goal is to produce foam that is ‘thicker’, has better ‘mouth feel’, ‘body’, great ‘stability’ and better ‘foam texture’ then this comes with fattier milk.
So, in summary:
- Whole Milk – Mouth Feel, Foam Stability & Foam Texture
- Skim Milk – Foam Volume
4. Milk Sugar
Milk contains natural sugars (lactose) and the sweetness we get when we sip on milk is directly related to how the milk is heated to unlock this sweetness. The average lactose content of milk varies between 4.7 and 4.9%, though milk from individual cows may vary more.
Lactose is a carbohydrate made up of a combination of two sugar molecules, ‘galactose’ and ‘glucose’. In a cooled down form, these two molecules are not known for their solubility and don’t taste very sweet. However, the solubility of the sugar and also sweetness increases with increasing temperature to a point, after which the molecules break down and the sweetness is dissolved going the other way.
So, as you heat or steam the sugar during frothing it starts break down the suspension-ability of the lactose or milk sugars into smaller, simpler and more soluble sugars which by themselves have a sweeter taste into our cups. The breaking down of lactose continues until around 75-90 degrees where temperatures above this the milk begins to then lose its sweetness. At this point, every second of overheating, milk sweetness is lost.
This is why baristas refuse to overheat your latte…Now you know.
Protein has an important role. Milk foams are stabilised by milk proteins. The proteins in milk, especially the whey-proteins are largely responsible for the foaming capacity in milk. Normal milk contains 30–35 grams of protein per litre.
At 60°C when you add steam to milk, hot air bubbles are pushed into the milk. At this stage the proteins in the milk become denatured, that is unfolds, and are at a stage where the proteins have unravelled enough to start to stick, coat the surface and surround the air bubbles and essentially together with the fats they form a cage around each air bubble. The protein protects the bubble and gives it stability on the top of milk drinks causing them to ‘float’. Continuously applying steam in the right manner until 95°C will lock in the bubbles at a tiny size and become the much revered micro foam.
If you keep steaming the milk beyond this point, the protein will become completely denatured – the foam goes past the point of being resilient and fluid, and they become rigid and stiff. What you get is more of a sea foam texture and a big solid funky blob. When you taste it, the milk generally will not slide across your tongue the same way that micro-foam milk does. This is not good.
6. Homogenisation and Pasturisation
I’ve always wondered what the difference is between pasteurized and homogenized milk. I thought I’d try to break it down. Basically they are two different processes.
Homogenization compresses milk by breaking up the fat globules to such a small size that they remain suspended evenly in the milk rather than separating out and floating to the surface. Non-homogenized milk is visually more natural and something that is more like fresh farm milk and is separated into layers.
Homogenization improves steam frothing. These smaller/tiny fat globules increase the viscosity of milk which provides greater richness and smoothness to the milk from the consumer’s perspective. When you compare the fat globules in the milk you find that in unhomogenised milk the fat globules have an average diameter of 2-4 micrometres and with homogenization they average around 0.4 micrometres. They are also more thickly covered in milk proteins which help stabilise the milk. These tiny protein covered fat globules also help become the much revered micro foam.
Pasteurization on the other hand is the process in which milk is heated to kill the bacteria. Most milk sold at retail is pasteurized prolonging the shelf life of milk. There are two types of pasteurisation, heating milk to a minimum of 72 degrees Celsius for at least 15 seconds and the other is heating milk to 62 degrees Celsius for 30 seconds. Pasteurisation is also results in fat globules being reduced in size to allow a smooth consistency. If milk isn’t pasteurized, the cream tends to separate and rises to the top of the container.
With UHT milk, which is milk that has been sterilised by heating for an extremely short period, around 1–2 seconds, at a temperature exceeding 135°C (275°F) to kill spores in milk, its whey proteins are around 70% denatured and therefore froths better than pasteurized milk which is around 20% denaturation of the whey proteins.
Milk expands when heated and so does its density however, liquid fat collapse as the milk is heated.
Therefore, the foaming properties of milk as a function of temperature vary depending on fat content and the processing conditions used in manufacture.
All milk, regardless of fat content becomes more receptive to the taking in of air and creates the greatest volume of foam at cooler temperatures. Therefore you want to start off with coldest milk and start foaming as soon as possible.
Skim milk foams were most stable when formed at 45 °C. Milk fat had a detrimental effect on foam formation and stability of whole milk especially in the temperature range 15–45 °C. The detrimental effects of milk fat on foaming properties were reduced by homogenization and ultra-high-temperature (UHT) treatment.
However, from approximately 45°C temperature up to about 75°C, things look better for your milk. After 75°C the trend begins to reverse with the higher fat dairy products consistently exhibiting a greater volume of foam being produced at any given point. In general, temperature trumps the influence of the fat on foaming.
Also, after 70°C, a higher temperature tends to be harder for the tongue to analyse. Milk with a temperature well over 70°C can stat to burn the tongue and is not desirable. Milk starts to boil around 80 C.
The best milk temperature for Café Latte or Cappuccino is between 60°C and 70°C.
So, why not start with icy milk? Well, I read that the ice crystals in the milk are jagged and pierce the protein coating around the bubbles.
Well…there you have it!!!
Hopefully this gives an understanding of why not all milks are created equal and why different cafes use different milks.
Finding the right milk that works is where it is all at.
The ability to have milk that produces the best ‘mouth feel’, foam stability’, ‘foam volume’ and ‘foam texture’ will give a café its ultimate edge.
So in summary you want:
- Good stable milk source – fresh and healthy cows. Know your milk man.
- Fattier milk
- Milk with the highest protein
- Homogenised milk and pasteurised milk
- The Coldest Milk possible to start with (Including a cold pitcher)
- Temperatures around 60°C that don’t exceed 70°C
So this post tells us what good milk is made up of ….but to achieve good micro foam you also need to understand how to use the steam wand to produce the best milk consistently. Perhaps a discussion for another day.