Coffee Chemistry : Milk and Heat

(Above: A pasteurization process in Murgon Button Factory, Queensland. Credit: Wikipedia.)

Recently, I took a look at a whole bunch of different canned coffees and started noticing some, let’s say trends amongst the flavor profiles across various brands and treatments – across the board, there was lots of sweetness to be found in every drink. And while there’s definitely a lot of reasons to throw a ton of sugar into a bottled milk coffee (say, to mask low quality coffee), I had forgotten about one aspect of that flavor when considering these drinks’ flavor – that the milk itself would be gumming up the flavor works with the way it was handled from cow to factory to mouth.

A lot can happen to a milk’s flavor, from animal to factory to mouth.

Now, much of the changes in milk’s flavor both pre- and post-containment comes from the process of pasteurization,  which most countries require to ensure a certain degree of stability and safety in their edible products. Particularly pertinent to dairy products, where the presence of microorganisms can lead to disastrous spoilage or foodborne disease, this process heats up a given food product for a long enough period of time to kill off any nasty buggies that might accelerate its degradation, but short enough to not cause any curdling or mass breakdown of the product itself. For milk, the range of pasteurization times and temperatures can be as low as 145°F for a half hour (for standard products that still necessitate refrigeration) to over 275°F for a few seconds (so-called “ultra-pasteurization” or “ultra-high temperature (UHT) pasteurization.”)

The extension in stability and  shelf life for dairy-containing products from ultra-pasteurizing (and similar aseptic) methods is so effective that upon its introduction to the market at large, refrigeration no longer becomes necessary. While obviously useful from a production and retail standpoint, ultra-pasteurized milk actually had some difficulty gaining traction in the US due to people simply not trusting milk that did not come from refrigerators. Image and perception can be a powerful thing, especially for something as perishable as milk.

What, you have this stuff just hanging out next to the Kool-Aid or something? Witchcraft! Sorcery!
What, you have this stuff just hanging out next to the Kool-Aid or something? Witchcraft! Sorcery!

However, taste is an even more important thing, and the sudden shock to the many fats, proteins, and sugars hanging out in milk don’t always take it very well. Ever wonder why organic milk seems to taste sweeter than its regular counterpart? Like the shelf stable stuff, most organic milk is treated via UHT pasteurization processes (one of the requirements to receive an organic label) and as such is subject to those same high (though brief) temperatures.

Now usually, most milk will already have been pasteurized to at least some degree before entering the equation for something like a coffee milk drink – even milk powders are usually required to come from similar sources before being dried out. It’s the second stage that necessarily occurs after a bottled coffee milk has been assembled, where things start to diverge a bit from your standard milk-with-coffees, which are both yet different from the smooth, foamy goodness of a nicely steamed milk.

So what exactly does happen in there to cause those changes? Well, lots of things.

A Party of Sugar, Fats, and Proteins

Mammals have evolved their milk to contain a plethora of all sorts of good stuff.

At its core, milk (in general, not just from cows!) is designed to deliver as much nutrition to its recipient, usually a baby [insert animal here]. Because of this, mammals have evolved their milk to contain a plethora of good-stuff – calorie-dense fats and sugars, high-utility proteins, and a whole mess of minor vitamins and minerals that will foster early growth and development in the most efficient way possible. Much of this efficacy comes from the presence of lactose, a disaccharide compound that infant mammals are super good at metabolizing. (Incidentally, most adult mammals, and actually, pretty much everyone not of European descent, are not so awesome at said metabolization, and eventually starts producing less lactase, the enzyme needed to break it down, resulting in the gassy awful mess that is lactose intolerance. Such is life.)

Lactose, in all its carbohydrate-y goodness.
Lactose, in all its carbohydrate-y goodness. (Source: Wikipedia.)

Upon heating, lactose splits into its component monosaccharides, glucose and galactose, hang out and combine into other sugars, all bringing a round sweetness that is not normally present in lower-temperature processed milk. Some of these sugars and proteins also intermingle with each other, causing some minor Maillard reactions typical of baking/roasting processes to occur. The result is a distinct ‘cooked,’ nutty flavor to the milk, despite there being nary a roaster in sight. There’s a ton of research that goes into the exact reactions that go on in that process – like all Maillard reactions, there are hundreds of possible reaction products, depending on the temperature, acidity, and heating of the milk!

It takes quite a bit of high, sustained heat for these molecules to break down, and if one starts to see some of the indications that the proteins are starting to go (like, say, a thick film forming at the bottom of a saucepan) – suffice to say you’re getting outside of the temperature windows where you’re getting outside of milk drink territory and more into the realm of custards, yogurts, and friends. In the context of what that means for your steamed milk, though, you’d be looking at changes in the overall consistency and mouthfeel of the beverage that you’re drinking.

Proteins help everyone play nice with each other in the entire ecosystem of the milk.

See, lots of proteins are also pretty surface-active, which means they act kind of an intermediary force between the fats and sugars and water that are hanging out in the milk. By sticking their non-polar (fat-like) ends closer to the milk fats in the milk while the polar (charged, water-like) ends closer to dissolved sugars and moisture, many proteins help everyone play nice with each other in the entire ecosystem of the milk. Heating milk up gently makes things even better – some of the reacted proteins allow for finer bubble formation and air retention, which makes your lattes taste super silky and your cappuccinos super foamy.

Keep heating that milk (say, above 140°F/60°C) and you get denaturation, which gives you that previously mentioned, not-milk-drink-ish kind of stuff. When this happens, the same compounds that were helping interfaces between air and water and fats stay happy start to bend, break, or rearrange,  which leads to flat feeling drinks with a big cap of tacky, gummy stuff sitting on top of your sad overcooked latte. No bueno.

Unfortunately, lots of pasteurization processes happen to skirt this line, resulting in some less-pleasant drinking experiences than one would like. To avoid this, lower-end companies may try to finagle the properties of the drink to replicate the original functions of the formerly-intact proteins, with varying levels of success. You’ll see stuff like xanthan gums and carrageenan in drinks to up viscosity and thickness, propping up your drinks and getting back some of that lovely foamy airiness… While more desirable than having a watery-feeling drink, it’s a pale substitute for the genuine article. But then again, pasteurization also prevents botulism, so I’ll take the good with the bad.

All of this, food for thought. Something to consider the next time you take a sip into your lattes. Enjoy, and until next time, take care. 🙂

References

  • Tomoko Shimamura and Hiroyuki Ukeda (2012). Maillard Reaction in Milk – Effect of Heat Treatment, Milk Protein, Dr. Walter Hurley (Ed.), ISBN: 978-953-51-0743-9, InTech, DOI: 10.5772/50079. Available from: http://www.intechopen.com/books/milk-protein/maillard-reaction-in-milk-effect-of-heat-treatment
  • CoffeeGeek – “Hello Milk!” http://coffeegeek.com/guides/frothingguide/milk

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