This section is a much more in depth look at milk and the chemistry of milk. Part of the interest in looking at things with this level of detail was to answer some of the following questions that I was curious about.
- Why does the milk seem to get better (ie, sweeter), when you steam it? Is it a change in the chemistry? The incorporation of air? Both?
- Why do some steamers make a "sweeter" milk than others do, even when using the same milk?
- Great foam, good foam, and no foam - what is it about the milk that plays a role, or is all in the skill of the Barista?
Note, I don't necessarily provide the answers in a specific way to these questions, but once you read this part of the guide, you'll see I managed to answer them in a roundabout way. Oh, and there's one more question:
- What is wrong (or right) with me that I care this much about coffee and creating the perfect milk to marry with it?
Hrmm. That is a question!
Before we go racing off willy nilly to create beautifully textured milk lets have a quick look at milk in a way that you (or I) never thought we would ever need to. We’ll introduce ourselves so to speak and get to know the milk a little bit better
Sugars, Fats and Proteins oh my!
Bovine milk is fascinating stuff. Complex and very nearly a perfectly complete food. In addition to all the vitamins, minerals etc. we have three things that require our attention, as they will play a starring role in the final outcome of perfectly prepared milk. They are, in no particular order, fat, protein and milk sugar (or lactose).
Milk is an ever evolving product, with changes in its composition fluctuating slightly but consistently, due to the feed of the cow, the type of cow producing the milk, the stage of lactation etc. These will result in subtle yet potentially noticeable changes in the quality of the foam you can produce, and the taste and texture of the cappuccino/drinks you prepare. This is especially true with high grade, "microfarm" type milk - the kind you get from a farm or coop of farms, as opposed to that big name brand you see on the Safeway shelves.
With all of this said, I do not want to hear excuses that the drink you made today wasn’t as good or the foam not as tight because all of a sudden this milk is obviously from a Jersey cow and you’re used to milk from a Guernsey cow or that the cows are into the clover and alfalfa these days and we all know that dry feed produces the best flavoured milk…and so on. We’ll assume that the milk is generally stable and that if the foam isn’t there and the flavour isn’t there…well, it’s you.
| Banana Milk! |
Use this milk and you won't have to worry about sweetness from lactose - it has added sugar! Milk and espresso, together at last!
The slightly sweet and pleasant taste we find in milk is primarily due to the relationship of lactose and chloride contents. Lactose is the milk sugar, a disaccharide of glucose and galactose to be precise. It is a solution (homogeneously mixed in a liquid) in milk. It is also less soluble (the ease with which it will dissolve in a liquid) than sucrose and therefore perceived as less sweet. Hmmm. On the scale of relative sweetness sucrose is 100, lactose rings in at a low, low 16.
However increasing the temperature of the milk (by steaming for example) has the effect of increasing the solubility of the lactose and in turn increasing its perceived sweetness, a good thing for us. That lovely increase in sweetness of steamed milk from the espresso machine or the hot milk your Mom made for you as a child is due to the increased solubility of the lactose at higher temperatures…in case you were wondering.
Wrap some tape around the middle of your glasses and push’em back up your nose. You’ve joined the milk science club and it’s only going to get worse.
I’m all about a little fat in the milk.
Milk is sold based on the quantity of fat it contains and can range from 0% in non-fat to approximately 4% in whole milk-Yum! Although not primarily concerned with the taste of the milk, milk fat gives body to the flavour…a fuller flavour so to speak. Fat can be a big player in the sumptuous mouthfeel and texture of our steamed milk.
Sure the foam is a big player too and we’re getting there but the fat lends and inherent richness to a milk based beverage (apart from the foam) that cannot be denied. Remember our mini-manifesto: richness is good.
There is some concern about dietary fat and it going straight to the hips. Milk fat is especially pernicious. This is where the size of the cup come into play and the logic behind the small is better ethos comes into focus. Do not drink 20oz lattes! If you do, you will need to use non-fat milk so you don’t get a fat ass and at the same time deny yourself the pleasure of sinfully rich whole milk. Think small. You are not a calf.
I've saved the best for last.
Proteins are responsible for our milk being able to be foamed. Technically very complex little structures, milk foam bubbles and how they are created can be tough to get a handle on. Case in point:
“Foam formation is mainly based on the effect that in the boundary layers of the phases, liquid and air molecules are enriched due to a boundary layer activity and therefore stabilize the boundary layers.”
(Milk and Diary Product Technology, Spreer & Dekker, 1998)
Hmmm, do I ask the audience, call a friend or choose the 50-50?
When you are steaming milk you are incorporating air into the milk. Proteins are important because they are adsorbed (defined as the adhesion in an extremely thin layer of molecules to the surfaces of solid bodies or liquids with which they are in contact, so don't email me saying I had a typo, and should have spelled it absorbed) by the thin film surrounding an air bubble giving stability to the entrapped air.
We want proteins. But it doesn’t end there. There are actually two types of foam in milk, which may appear separately or simultaneously. One foam appears to be a protein type and the other a phospholipid-protein type. So who cares?
Well the relationship of fat and protein can impact how easily the milk will foam and at what temperatures milk is most receptive to taking on air.
Proteins and Fats
| Treat Yourself! |
Not only does half and half (10% fat) milk froth well, but it adds an entirely new layer of richness and vitality to milk-based espresso drinks.
Foam stability decreases reaching a minimum at about 5% (whole milk is 4%) fat and then increases rapidly as fat is increased to 10%, with highly stable cream-type foams forming when the fat content is increased to above the 10% level (table cream at 18% or whipping creams at 35% etc.)
Increases in fat content also cause a decrease in foam volume as well, up to a level of approximately 5% fat. Therefore skim milk offers the potential for the greatest volume of foam and most stable foam. This potential decreases gradually through 2% milk down to whole milk - it has the lowest potential to create heaps of foam but to create heaps of stable foam. Get it? Whole milk, while "foamable", is more difficult than skim milk. Here's where it gets weird again - go higher in fat than that 4% whole milk, (eg, beyond a fat content of 5%), and you once again see a steady increase in both foam volume and stability. There's a reason why whipping cream is 35% fat content :)
Now if our goal is to create volumes of foam, non-fat milk gets the nod. It will create the most foam for us. Despite that fact I like a tasty drink and therefore recommend whole milk or fuller fat milk. This may confuse some of you. Yes whole milk will be more difficult to foam and work with but in the end massive volumes of foam is not the end all and be all, a fabulously satisfying drink is. The fat in whole milk will make for a tastier drink and in the hands of a skilled barista whole milk will create as much foam as you need.
Protein in Detail
There are two different types of proteins in milk; whey proteins and caseins. The later make up 80% of the total protein of milk, and both play an equally important role in the formation of foam.
Casein imparts good surface-active properties and thus plays a role in the functional properties of whipping/foaming. Whey proteins although offering less surface activity than casein, they offer far superior foam stabilizing properties creating a more rigid film at the air/water interface of the foam.
Once again who cares? If you're serious about your milk foaming, you should, if only to shed some light on how slight differences in the components of the milk will affect the properties of the foam we create. In fact, your typical PBTC (person behind the counter) or manager of a typical chain café should take note of this… after all, those are the places and people who most often resteam milk. Here's the scoop.
Of great interest is that both proteins are stable up to approximately 140F after which they become susceptible to denaturation. The proteins no longer maintain their native shape or charge and will not behave in the same way to facilitate the creation of foam. New proteins are needed, more milk must be added. By adding fresh milk to already steamed milk you introduce new, unchanged proteins and can foam again.
The above is a complicated way of explaining where the old adage that you can “foam milk once and steam it twice” comes from. To foam milk a second time won’t work because the proteins that facilitated the formation of the foam initially have become denatured. You can of course just reheat the milk a second time but even that is not recommended. Steam only as much milk as you need for a given drink. Start with cold fresh milk every time. Never resteam milk or add fresh milk to already steamed milk. It is considered poor form.
If you're in a shop, and paying $3 or $4 for that cappuccino or latte, ask, nay, demand they only use fresh, cold milk to make your beverage. You will notice a huge difference. If they challenge you, quote the above chapter and verse, or take your biz elsewhere. It's your hard earned money - why settle for denatured, reduced-charge milk!
Back on subject, it doesn’t end here. Foaming potential relates not only to the relationship of the proteins and the fats but is also influenced by the temperature at which the milk is foamed.
Effect of Temperature on Foaming Ability
| Other Milks |
There's a range of milks to try - some steam better than others. Soy can work well, but Buttermilk can be a disaster.
Low fat milk is most receptive to taking on air at low temperatures. This applies as well to both whole milk and cream, although to a lesser extent. So from approximately 40F (fridge temperature) up to about 100F, things are looking good for your milk and all the chemical changes you're bringing into play.
However, at approximately 100F, on up through to 160F the trend is reversed with the higher fat dairy products consistently exhibiting a greater volume (as seen as a percentage increase in volume due to foam) of foam being produced at any given point. In general temperature trumps the influence of the fat on foaming. All milk, regardless of fat content, creates the greatest volume of foam at cooler temperatures.
The Take Home Message
All this science. My brain hurts. But we're almost there.
Here's the bottom line: To assist yourself in creating as much foam as you are going to need for your drinks, you should start with cold, fresh milk and a clean, cold pitcher. Start with milk as cold as you can and create foam right from the gun to create the greatest volume of foam possible.
Ok pencils down; stop spinning your Erlenmeyer flasks. We’re about to skip on past the chemistry of things and get down to the process and the practical aspects of steaming milk. It’s about time!