On Milk, Maillard Reactions and Protein Denaturation

Milk is an important part of cafes, and so the cafe industry has many opinions, practices and standards regarding the proper treatment of milk - some of them well-informed, and others moreso the relic of generations of passed-down pretentiousness. I do generally believe that all of these beliefs were put forward with the best of intents, but some of the false ones do produce negative consequences for the industry.  

World Championship Coffee
In 2023, I had the privilege of experiencing Boram Um's practice routine for the World Barista Championship. Amongst many other unique elements, he used a lactose-free, freeze-distilled milk that he heated to 58°C, then chilled over metal balls to 50° in front of the judges. The lactose-free milk is a simple choice - lactose-free milk is made using lactase to break down lactose into its component sugars, glucose and galactose. Although the mass of sugar in the milk is the same, glucose and galactose taste quite a bit sweeter than the larger lactose disaccharide. Freeze-distillation removes water from milk, in this case removing >90% of the water from the milk, concentrating the sugars, fats, salt, and proteins significantly. An increase in fat concentration also tends to release and spread out the flavors of the coffee, beyond contributing a textural and flavor component.   
Initially Boram did not explain why he steamed then chilled the milk, but when I asked, he stated the reason for steaming the milk to 58° is to break down the simple sugars and increase sweetness, and chilling the milk by 8° was meant to slow down oxidation that occurs at the higher temperature. He incorporated the explanation into his final routine at the World Barista Championship, which you can watch here.

The idea that milk needs to be heated to around 58° to 65° for optimal sugar breakdown is a fairly pervasive one that most baristas will have heard, while the most common argument for sub-65° milk that I've heard is that the temperature range at which liquids tastes the sweetest is at around 55°. This latter idea is true, the former is false, at least in the vast majority of cafe and at-home cases. Sugars, whether they are simple sugars such as glucose or galactose, or disaccharides such as lactose, do not break down/caramelize and undergo minimal Maillard reactions when brought to 65 degrees for a few seconds. High water concentration is a pretty strong antagonist against Maillard and caramelization reactions, and milk is typically 85-90% water. Lactose is a generally less reactive sugar than its component sugars; lactose-free milks with galactose and glucose are likely to undergo a bit more in terms of Maillard reactions at this temperature compared to regular milk, however I am still very skeptical that the tiny possibility of Maillard reactions that occur are noticeable in terms of taste. In Boram's case, he was using a milk where he had removed >90% of the water content, and so it is plausible that some Maillard reactions were occurring to produce sweeter tasting compounds. If you are not using a heavily distilled milk, just heat your milk to the temperature that you want. 

A simplified overview of caramelization reactions and Maillard reactions.

A general rule of thumb with biochemical reactions is that for every 10° increase, reactions double, and so for a decrease of 8°, it is likely that the rate of oxidation has roughly halved. From a practical standpoint, it seems unlikely that the oxidation of milk is particularly noticeable at 58°. This idea may be worth further investigation though, as Boram did go on to become the 2023 World Barista Champion.

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Thermal Degradation
What happens if you heat milk past 65°? The large issue is protein denaturation - proteins usually have a set 'folded' structure, but a denatured protein is 'unfolded' or at least not folded in the correct orientation. Whey proteins in milk are usually hiding free thiol (i.e. sulfur containing) groups when folded correctly, preventing the thiol group from reacting with the rest of the milk.  Once these proteins are denatured, the thiol group is exposed and easily reacts to form volatile sulfuric compounds that are easily detectable at low concentrations (such as hydrogen sulfide or methanethiol). These compounds are usually what our brains interpret as eggy (hydrogen sulfide) and cooked/cabbage/asparagus (methanethiol) in really hot milk. Proteins are also responsible for emulsifying fat in the milk, which they become unable to do when fully denatured, leading to poor texture.  

Quick detour to eggs
As eggs and milk both contain proteins, we can use eggs as an a comparative example to explore milk protein behavior. When cooking eggs, we often aim to denature the proteins in the egg white to make it solid - albumen has a high protein content, and when these proteins denature at around 60°, it becomes solid and opaque (although some eggs have proteins that are still translucent when denatured). An important attribute is that the egg must reach 60° for the majority of proteins to begin denaturing, regardless of time (pictures of eggs held at different temperatures here and here). Some protein denaturing does occur in the egg yolk at longer time frames (40 minutes versus 2 hours), however these lead to pleasant gelling reactions. This is a relatively easy experiment to conduct at home if you have a sous-vide or a device that can control temperature fairly accurately (such as a temperature control kettle); set your temperature to 57° (a good egg pasteurization temperature), and leave your eggs in there for 8 hours. Without that extra three degrees, the texture of the egg white will not significantly change. The proteins also do not immediately unfold once 60° is reached; you cannot poach an egg by tossing it into 60° water and then turning off the heat. Similarly, the vast majority (98%) of the proteins in milk begin denaturing only after 65°, and do not all immediately denature at this temperature.

Back to milk
A common piece of knowledge passed onto baristas is that you cannot re-steam your milk without getting off flavors, which is true if you are steaming past 65°. However, if you are steaming below or around 65°, there is no issue with off flavor development because proteins have not denatured. In fact, Rowan Garleff of Phil & Sebastian Coffee based his 2024 Canada Barista Championship routine on this and actually found that people preferred double steamed milk in a blind tasting. His additional reasoning for supporting re-steaming was for environmental and economic reasons: e.g. if a barista has an extra 10% when measuring milk for a 300mL beverage that they toss, and a cafe sells 130 of these beverages a day, then the cafe is essentially throwing away thirty 4L jugs of milks a month. Similar to an experiment with eggs, you can sous-vide milk at 60° for a couple hours without any negative effects. 

We held milk in a ziploc inside a Fellow Stagg for 3 hours, chilled, then tasted. 

If you are a cafe wanting to waste less milk, I would highly recommend using a milk thermometer. I am in agreement with Scott Rao in that neither of us understand baristas insistence and pride on using their hands as thermometers. I am sure a focused barista can get milk to the right temperature zone, however a barista can also do other things rather than focus on a task that a $6 tool can easily do. For a cafe, beyond the use of a thermometer, it is important that sanitation and hygiene are paramount. Milk should be kept outside of the 'danger zone' of microbial growth for extended periods of time for food safety - there are unsafe ways to reuse and re-steam milk, and this article is not advocating for that. In preparation for his routine, Rowan had talked with Alberta Health Services who had given their stamp of approval on his recommended process. 

Final Thoughts
An oversight on my part is that I am unsure of the effects on flavor on re-steaming UHT milk - the milk that I use is HTST pasteurized, meaning it has been heated to at least 72° for at least 16 seconds to kill harmful microorganisms. UHT milk on the other hand is heated to roughly double the temperature (around 138-160°) for two to five seconds, and is sometimes the more common milk found in other parts of the world. UHT milk does taste different, sometimes already tasting a bit 'cooked'. 

Steam wands and steam do get Very Hot™ , which is another reason why creating a whirlpool when steaming is important - not only does constant movement help emulsify the milk, it also means that you are not heating a specific region of milk to the same temperature as the steam (thereby denaturing proteins, even if the final temperature is below 65°). 

It's not possible to cover every alternative milk out there, but it should be noted that many alternative milks do not contain whey protein and therefore will not have issues with producing off-flavored sulfurous compounds when heated to high temperatures. As seen in Boram's presentation, milk also acts differently if the composition is changed. Due to the common wisdom that milk degrades significantly once over 65°, cafes do not often experiment with high temperature milk. We have found that milk should be heated to over 90° and the proteins denatured intentionally for masala chai lattes at their best flavor profile and customer preference - but that will be for part II.

If you have any questions or other perspectives, please feel free to contact us for discussion!

Further reading

Li, Chengkang, et al. “Cysteine Residues Are Responsible for the Sulfurous Off-Flavor Formed in Heated Whey Protein Solutions.” Food Chemistry Molecular Sciences, vol. 5, 1 Dec. 2022, pp. 100120–100120, https://doi.org/10.1016/j.fochms.2022.100120.