Research – Of Milk and Foam

I am obsessed with latte art. Sadly, it is probably because I am so awful at it that I have such an unhealthy attachment to the barista sport of pouring steamed milk into espresso. While there is no mystery behind the creation of a beautiful latte rosetta (it takes lots of practice), there is a certainly an enormous amount of confusion associated with the creation of the sweet, silky, steamed foam integral to even the most basic of latte pours. Most times the internet can point the way, but I have found time and again that when it comes to the science of food, there is a lot of pointing but not many worthwhile destinations.

If you have any desire at all to flex even the puniest of barista muscles, you need to understand what microfoam is. The barista behind the counter of your local coffee shop didn’t pour you a latte heart because they are hitting on you (unless you are really, really, ridiculously good looking), it is because they know how to make some good microfoam. In fact, they probably took at least a basic in-house course in milk chemistry and physics in order to understand how to succeed at sculpting majestic lattes that customers are willing to happily throw ludicrous amounts of money at. Now, the good thing for you is that you don’t need a course in milk chemistry and physics to make proper microfoam, because we can distill the entire course into one basic principle.

You need fresh good milk.

If you are a fan of ultra-high temperature (UHT) pasteurised milk that can sit in a box at room temperature for a year then I suppose you don’t need fresh milk. For us normal people, fresher is better, though it by no means guarantees great milk as you will read below. I suppose I could also add that you need to practice steaming said fresh milk quite a bit in order to learn how to reproducibly produce this paint-like hipster ambrosia, but the fundamental principle for getting any type of milk to foam is that it should be fresh. There is a lot of misinformation out there when it comes to latte science. I mean, there is really a LOT of misinformation, so in the name of higher knowledge I am going to try and lift the milky veil and provide some established arguments that explain for example, why people often complain that organic milk won’t foam. And while I am it why not test some milk and see what happens? After all, what good is all this food science if it doesn’t make our cappuccinos taste better?

Let me address some of the myths right now in order to get things going on the right track. The ability of a milk to foam is not affected by the presence of glycerol. The reason why steamed milk is sweeter and tastier than cold milk is not because lactose has a better solubility in milk at higher temperatures. It is also not because lactose is breaking down into smaller, sweeter-tasting sugars.

We will tackle the sweetness question first, because it is probably the most misunderstood. Lactose is indeed a combination of two smaller sugars – galactose and glucose – no debate. Although it is definitely possible to break lactose into its constituent parts, doing this with a wand immersed in a pitcher of milk is not going to work. Not unless it is Harry Potter’s magic wand, because studies have already examined the stability of lactose in milk and shown that under the conditions experienced in an espresso machine the breakdown is too small to be noticeable. So the increased sweetness isn’t because of galactose or glucose being formed. It also isn’t because of increased solubility of lactose; while this is true, it has no bearing on the question since the lactose is already in solution in your milk, as this handy solubility curve illustrates. The same amount of lactose is in solution in the milk in the fridge as in the steamy microfoam.

Darn. So there is no extra glucose, no extra galactose, and no extra lactose – no extra sugars at all. But it really tastes sweeter, right? What is that all about? Taste perception is to this day very much a poorly understood phenomenon, but it is known that temperature plays a very important role in how we gauge flavours, especially sweetness. It has been shown by several teams of researchers that temperature alone can provoke a sensation of sweetness in a high percentage of individuals, even in the absence of any normally sweet tasting substances. In a properly foamed latte, additional factors such as the organization of the fats in the steamed milk, or the dispersion of the air all contribute to an altered mouthfeel that makes the beverage taste sweeter and more sumptuous.

The issue of foaming has also been studied in detail – enough to have its own defined term, the Steam Frothing Value (SFV). Yes, there is actually a scientific apparatus out there that investigates the foaming of milk, and researchers putting it to use have concluded that the single biggest factor when it comes to foaming derives from breakdown of the fats in milk. What this group found was that glycerol alone has no influence on frothing – rather, it is the levels of mono- and diglycerides that result when fats are destroyed via a number of processes that are umbrella termed lipolysis. These mono- and diglycerides, along with the fatty acids that are formed at the same time, have a negative effect on the microfoam. Microfoam is possible because the high temperature of the steam transforms the milk protein casein, changing its shape so that it now behaves as a stabilising blanket for the tiny air bubbles. Mono and diglycerides and fatty acids displace the casein, weakening the bubbles and causing them to collapse. Calcium has also been found to play a role, with added calcium serving to increase the amount of casein surrounding the bubbles. You should then expect that milks with calcium added should develop better microfoam than those without.

If the best way to get good foam is from milk that has low levels of free fatty acids and mono- and diglycerides, then it makes to sense to try and understand what causes it. Unfortunately, the list is incredibly long and ranges from how the milk was treated during processing to what the cows were feeding upon and even their stage of lactation. Good luck finding that information on the side of your milk carton, hmmm? Depending on the amount of treatment, such as pasteurisation, that the milk has seen it will contain different amounts of the enzyme lipase that is the catalyst for all of that lipolysis running amok and turning fats into the by-products that ruin our foam. With high temperature sterilisation the lipase is inactivated and is unable to perform this operation, so this is one factor keeping your milk fresher longer. Bad news for raw milk or certain “organic” milks, but good news for skim milk which contains low levels of fat and thus a much lower potential for breakdown products to build up to the point where the foam collapses.

A lot of scientific terminology, but where does it leave us? Six long-winded paragraphs what I wrote at the very beginning, all to say:

You need fresh milk.

OK, OK – even fresh milk is not guaranteed to be great milk, but it is certainly going to give you the best chance. Can’t stress it enough, really – your milk should be cold and high-quality, and if you want the increased mouthfeel that pushes your latte to the next level then go beyond the boring, bubble-friendly skim milk and opt for a bit of fat. Your tastebuds will thank you!

I imagine I could leave things here and let all of this Einstein babble sink in, but as I wrote at the top of this post, if we aren’t making the world a better place with this science stuff then what are we doing? Shouldn’t we try a few types of milk with a rudimetary paleo-test that provides far more fun than reputable data? The answer is “Yes!” Let’s begin by taking a look at the different types of milk I have selected:

Lactantia Calcium 2% Milk just before steaming

Lactantia Calcium 2% Milk

This milk is advertised as having 35% more calcium compared to regular milk. If we can believe the conclusions outlined previously, then more casein should surround the bubbles and the microfoam should be more stable. Tastier would be nice, but a beautiful, foamy white counterpoint to the deep brown of the espresso whets my aesthetic appetite as well.

Lactantia Pur Filtre 2% Milk ready to be steamed

Lactantia Pur Filtre 2% Milk

The control group for this test. When I was a child, only the health freaks were drinking 2% milk. Nowadays it is positively decadent, and when you open a friend’s fridge and see this on a shelf it is like finding a pack of cigarettes or a Kenny G album. Still, it makes a nice baseline for our test. No judgments here.

Lactantia Pur Filtre Skim Milk ready to undergo the foam test

Lactantia Pur Filtre Skim Milk

After all of that skim milk bashing it seemed pretty obvious that a chance for redemption would be at hand. Well, let’s get started with the bashing again, because this skim milk has ZERO grams of fat. None. The labelling states there are 5 milligrams of cholesterol, but there are ZERO grams of fat. It may be past life as a scientist, but I find a claim of none at all a bit hard to believe. My skepticism aside, bureaucracy is bureaucracy and in Canada you can have anything up to 0.5% fat and still call it Skim Milk. Maybe it is alright to simply say there is fat in there, but it is at least one quarter as much as the 2% variety.

The test

For our test we need to fix a few parameters. Given the fact that my skill at foaming the milk is not barista level there is going to be some slight variation here in the amount of foam I will produce, so I will do two trials with each milk in order to keep things on the scientific up-and-up. The milks all have a date of expiration between the 26th of December and the 8th of January, so they are more or less in the same ballpark in terms of freshness. Temperature is one variable that we can actually control pretty well, so I will be aiming for the latte sweet spot of 150ºF to 155ºF (65ºC to 68ºC) each time, using milk that has been stored in my fridge at 45ºF (7ºC). Fahrenheit, I know – ugh. What can I say – most cooking supply stores stock thermometers that read in Fahrenheit and I am just not willing to fork out uberdollars in order to get a dual reading digital one. Plus, this thermometer is so darn pretty.

Reading the milk temperature with a good old fashioned thermometer

The second variable that must be fixed is the quantity of milk to use. I settled for a tidy 200 mL, not the least because my measuring cup has 100 mL graduations, and it is more or less how much milk I usually add to a latte. I am going to use the same machine for all the tests, namely a Breville Barista Express, and I will do everything possible to use the same technique to work the steam wand each time. I will be measuring two different things – the amount of foam produced and the strength of the foam. Measuring is the easiest, and involves a simple glance at the side of my measuring cup. For the strength of the foam I will use the time-tested scientific method of placing more or less a half teaspoon of sugar in the middle of the foam and timing how long it takes to sink through using the stopwatch function on an iPhone. More or less as rigorous as Scientology.

Winner – Pur Filtre Skim

I decide to test the skim milk first, probably because as someone who is not a fan of this bland white water I am secretly (ummm…openly) hoping for it to fail. I fire up the Breville and within the first 10 seconds start wishing I had grabbed the handle of the glass measuring cup instead of the wall. The foam is building, but I seriously wonder whether I will reach 150ºF before the pain receptors in my hand demand I let go. Not willing to waste money on more skim milk I burn my fingers finishing the test, wait 1 minute for the foam to settle and proceed to measurement. I then place an eye-balled half teaspoon of sugar on top of the foam and start the timer, waiting for the sugar to completely sink into the foam before I stop measuring. This is definitely the least accurate of the two measurements, so I am not quite sure what I am expecting.

Interesting – there is a lot of foam here! I run through all my other first trials in order to get practice foaming the milk in the same manner, and then I take the second trial as the firm result.

Microfoamed skim milk

IMG_3085
What is that – about 180 mL of liquid? The timer says 4 minutes and 57 seconds, but to be truthful I just had better things to do at this point. The sugar was only maybe half-way through the foam and I was getting antsy. I am quite surprised at just how solid this foam is.

Runner Up – Calcium 2%

Microfoamed Calcium 2% Milk

IMG_3084

Fairly impressive. Something like 195 mL of liquid and the rest is foam. The foam is much less stable than the skim milk, though it is better than the 2% milk without the calcium added, as we are going to see below.

Last Place – Pur Filtre 2%

Microfoamed steamy 2% milk in a beaker

IMG_3082

The least amount of foam as well as the weakest. Time to put together some type of summary here.

What can we conclude?

Without a doubt the skim milk produced the strongest foam, and the largest amount. The calcium enriched milk produced slightly more foam than the product without added calcium, and the foam seemed to be stronger as well. This may only be due to the greater amount of foam present since our test doesn’t account for that. To be honest, I am surprised any useable results at all came from this. I think the single biggest conclusion was that the steamed skim milk did not taste awful. It was surprisingly OK. Yes, I did a taste test at each stage so that I could give a little commentary on how temperature affected sweetness. Cold skim milk tastes like water gone wrong, but this is coming from someone who drinks luxurious 3.25% milk. It is possible that I have become overly sensitised to the taste of good ol’ Homo milk, but Sara and I both agreed that the Calcium 2% milk tasted terrible when steamed. Yes, even worse than the skim milk. That is truly saying something.

What should you do?

Keep drinking lattes. If you are a foam junkie your best bet is skim milk. If you want a bit of extra fat with your foam go for the added calcium when you can find it. And keep practicing that latte art!

UPDATE (2015-01-22)

A very recent publication in the Journal of Dairy Science has touched on differences between milk labeled organic, and that labeled conventional (non-organic). The authors conclude that names such as “organic” or “non-organic” can be deceptive, and do not give meaningful information on the true origin of differences between the products. Cow breed, diet, health, and environment all contribute to the final composition of the milk, once again illustrating how complex the dairy equation really is.

4 Comments

  1. Marie-H
    December 28
    Reply

    I loooove my latte in the morning and I start every day with it… I always have a selection of good coffees from around the world and I choose my hotels with good capuccinos and refuse to go where they don’t serve it so this topic interested me… Mmmm … You say milk freshness for foaming??? BIG DOUBT here! I used different froating types : one attached to my my Brevilleespresso machine, the Trudeau electric one , the hand air push froater and now I use the Nespresso automatic that makes NO NOISE . This one, you don’t have to moove your arm to get foam -so scientifically, It could have serve better use for your trst, as the Tester can’t be the reason why there is more or less foam!!! He doen’t have to touch it after choosing temperature and pour milk in it.

    The problem with FRESHNESs is here: I use ALMOND milk , in carton boxes on the shelf, the fresh in the fridge I used don’t get foamy!!! The foam I put in my espresso is of course lighter than the one I pour in my husbands coffee , made from cow milk. But there isa good foam, and I tried a few kinds of milk : hemp does not get foam at all, sunflower milk as a light foam but the taste is horrible for me, soy milk: some brands get foamy some not, rice milk: no foam at all , mixed nuts milk didn’t do it , and Ihave tried others but the milk in my espresso is a carton box shelf unsweetened almond Breeze… But, strangely, and that’s the main reason I read your article, sometimes, I buy a carton that just don’t do its foamy job… I buy them by caisse 6/box and if one carton doesn’t do it, I now know the complete 6 pack will not foam, so I get them all back to the store… But the same brand, same sort, with a different production date is usually THE ONE for me and I brong it in all my vacations hotels unless they have options (I wright upfront to know)
    So : no casein, on the shelf, no lactose, unsweetened but fatty from almonds 3.5 g. Carbohydrates 2g 1%

    As I am allergic to it, not lactose like usually is the trouble, for me it is the milk protein ( casein) that causes reactions… I don’t know more about why some carton will foam and some won’t 🙁

    • Immortal Pestle
      January 1
      Reply

      Very interesting, indeed! Let me start by saying that you are completely correct – using a machine like the Aeroccino would be a much better method for this test. Although I have used and even owned one in the past I don’t have one right now, so I am kind of stuck with my poor old hands and not very metronomic wrist. C’est la vie!

      I should clarify that I was really talking about DAIRY milk, so while I think that what I wrote should provide at least a tiny bit of insight into the problems arising when trying to get cow’s milk (or goat’s milk for that matter) to foam, it is a different matter when it comes to other types of “milk” products that are non-dairy.

      Still, the basic problems all involve stabilisation of bubbles. You need some combination of fats, protein, water, and/or other additives that will form a strengthening layer around the gases. Almond milk is naturally lower in protein than dairy milk, so that is one factor going against it in terms of achieving a brilliant foam. However, carrageenan is added to introduce an emulsifier that assists in thickening and foam stabilisation. While I really can’t tell you exactly what has gone wrong with the boxes that you pick up from time to time it definitely seems that you are on the right track and that something has gone quite wrong with the batch that you unfortunately purchased. Either some type of process depleted or damaged the proteins in the almonds, or a similar process did likewise to the carrageenan. I don’t really have the intimate knowledge of the almond milk manufacturing process to help you with a firm answer, but you could always write the company and put your question to them. I am sure they would be happy to answer!

      Thanks for helping us out with your experiences. And as always – keep drinking those lattes 🙂

  2. JP
    January 5
    Reply

    Dude, my Aeroccino is all yours on weekdays (and nights) if you want to do some testing. As you know, I am one of those complaining about the froathability of organic milks. So far, I’ve only had sucess with Lactacia’s PūrFiltre Biologique 1% which foamed perfectly in the Aeroccino but our local supermarket was out of it last week so it could be a fluke. For sure, all of the Liberté and Nutrinor organic stuff did not froth. I am more of a soy milk guy now and the So Nice Barista blend is amazing.

  3. […] Interested in getting up close and personal with your favourite beverage? Don’t forget to check out the companion article on milk and foam when you finish! Now, read […]

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