By Phil
Beakerhead’s goal is to unite the arts and science communities, which sometimes stand divided. I think it’s an awesome initiative, and the result is creative and inspiring. We’ve participated in Beakerhead for the last two years, and as this year’s approached, I couldn’t wait to see what crazy and innovative ideas they would conceive!
In this edition, we participated in two Beakerhead events as a part of the Chemistry of Food category. At the event Torched, we whipped-out a number of flame-powered siphon brewers and demonstrated the method and offered coffee samples to visitors.
We hosted the second event at the Simmons Building and it was called Café au Lait Scientifique. This event was part of the Beakerhead sub-theme of “milk”, which is right in our wheelhouse, of course! For Café au Lait Scientifique, I hosted a talk along with preeminent UBC chemistry professor Dr. Stephen McNeil and we undertook two tasting exercises with the participants. For this session, Stephen and I decided to focus on how and why applying heat affects the taste of milk. As anyone who enjoys Lattes or Cappuccinos can attest, steamed milk tastes much sweeter than cold milk. Stephen and I setout to understand the chemistry behind this transformation. To clarify, when I say we both setout to explore this, what I really mean is that he researched it and explained it to me! Here is a quick summary of what Stephen explained to me regarding the increased perceived sweetness of milk as it’s heated. Please bear with me, as this gets pretty science-y. If you’re not so science inclined but still curious to understand the reason, skip to the bottom and I’ll explain in my words what it all means!
Milk gets sweeter with initial heating (about 65°C/150°F) because some of the lactose (a disaccharide, sweetness 16% of sucrose) breaks apart into two monosaccharides, galactose and glucose (60% and 74% respectively). It doesn’t happen a lot, but it doesn’t have to, because each lactose molecule that does that reaction boosts the effective sweetness of those atoms by over 800%. So that part’s easy.
At 82°C/180°F, the whey proteins denature. That process is pretty well understood: the proteins unfold, free thiol groups and hydrophobic residues that were formerly inside the protein structure become exposed, the free thiol groups can react with thiols on other proteins (both whey and κ-casein) to form disulfide bridges, and the hydrophobic surfaces on different proteins are drawn together. This results in aggregation of the whey proteins. It’s not obvious how that in and of itself would lead to a dramatic loss of sweetness at that temperature, but some possibilities suggest themselves from the research.
1) The sugar concentration drops.
Possible culprit: Maillard reaction of the sugars with exposed amino groups from the denatured whey proteins. If you keep heating, that will eventually lead to small decomposition molecules that look brown and taste burnt, but even the initial step will remove sugar molecules from solution and decrease sweetness. Alternative culprit: sugar molecules decompose to organic acids (mostly formic). These are potentially related effects, i.e. the first step in the cascade of chemical changes that leads to formic acid could begin with a sugar molecule reacting with a denatured protein.
2) Other flavours develop that mask the taste of the sugar.
If the sugars turn into formic acid, the acid gives a sour, vinegar taste.
If fatty acids are oxidized, they produce a range of ketones which produce a “stale” or “oxidized” flavour. If the whey protein thiols and resulting disulfide bridges are further oxidized, sulfur compounds like H2S and CH3SH are generated, and even a tiny tiny amount of those will mess up the flavour profile, generating a cooked, burnt, cabbage-like flavour.
The flavour profile change at 180° F could be due to some or all or none of these, but I think they’re all worth mentioning as possibilities, and I like the fact that a couple of them trigger from denaturization of the whey protein, which is the one thing it’s well established happens right at that temperature.
To summarize the above, the milk tastes sweeter because with the application of heat, the lactose sugars break apart into much sweeter galactose and glucose sugars. As subsequently more heat is applied, more sugars break apart so the milk gets perceptively sweeter and sweeter. This only continues up to a point though. As the temperature reaches 180°F/82°C the maillard reaction begins to play an important role and galactose and glucose sugars disappear as they react with the now available amino acids (proteins). The result is a reduction of perceived sweetness! The bottom line is that there exists a “sweet spot” between the maximum production of galactose and glucose prior to the point that the sugar browning strips them away. Also, an important consequence is that it’s not just about sweetness, the taste of the milk will also change.
Getting back to the actual event, the first tasting exercise explored two coffee-to-milk ratios, and I felt that due to some setup challenges (different temperatures of each ratio), the experiment was unsuccessful.
The second tasting exercise in the session was designed to explore milk taste alone, and it was very successful! We steamed three samples of milk to 130°F/54°C, 160°F/71°C, and 180°F/82°C. We then put all of these samples into a 130°F/54°C water bath to bring them all to the same temperature and hold them there until tasting. The purpose of the last step was to eliminate the effect of final tasting temperature on the perceived sweetness. I honestly thought that no one would like the 180°F/82°C samples. I was quite wrong! The group of participates were really divided on which milk they preferred. Some said that the 160°F/71°C was the sweetest and preferred that and some preferred the less sweet and more complex tasting 180°F/82°C sample. It’s debatable that it was more complex in a good way, but this is where personal taste preference came into play!
All-in-all the session was a smashing success, and I found working with Stephen a true pleasure and wonderful learning experience. I can’t wait until next year!
-Phil
