Sugarless Dalgona Coffee Design
May 31st, 2020
Last month, I shared some experiments I had done with Dalgona Coffee, trying to figure out what was stabilizing the gas bubbles in this sweet coffee drink. This month I tried to design a “better” drink, motivated by some comments by my co-worker who said that her husband couldn’t stand how sweet Dalgona Coffee is.
Last time I briefly discussed how sugar is likely crucial in stabilizing the foam by significantly viscosifying the liquid between air bubbles, preventing them from contacting and merging with each other. To replace sugar’s role, I decided to create a secondary gel microstructure in the interstitial fluid. This is a similar approach I took in my paper on designing a highly extensible yield-stress fluid. In that work, I took an oil-in-water emulsion—which has a jammed microstructure very similar to a foam—and introduced the concept of a transiently crosslinked network in the interstitial water phase. Unlike then, I’m not trying to make the Dalgona Coffee highly extensible or elastic since it would likely be extremely unpleasant to drink. I’d like to have minimal impact on the texture or mechanical properties, just remove the sugar. By the way, this type of goal is often the objective when food scientists design fat-free or sugar-free food.
What is a gel?
Glasses and gels are the two types of microstructures that can produce yield-stress fluids
In my very first entry, I mentioned that the word “gel” can be a bit confusing and warrants its own discussion. A somewhat well-known quote credited to Klaas te Nijenhuis is that “a gel is a gel, as long as one cannot prove that it is not a gel.” In part, this confusion stems from competing definitions of the word. Colloquially, a gel is most often defined according to visual and somewhat vague tactile properties. I think that when many ordinary people hear the word “gel”, it conjures images of hair gel or hand sanitizer, materials that are transparent, “thick”, semisolids. Unfortunately, neither of these example materials would be considered a gel from a microstructural perspective.
As a rheologist who wants to design materials, I want to use “gel” to describe the microscopic structure of a material that gives rise to particular material properties. From this perspective, most generally, gels are materials that have a networked microstructure that spans the entire volume of that material. One may then apply all sorts of adjectives that I won’t discuss to a gel to get more precise in terms of structure and mechanical properties like, “chemical”, “physical”, “critical”, “transiently crosslinked”, etc.
Since I’m trying to create a drink that suspends air bubbles inside it, here I’m only interested in gels that form a yield-stress fluid. Thus, our gel must be a material that can bear a static load for long time scales, and the networked microstructure can break apart (i.e. yield), flow, and reform at a critical stress. In terms of how many ways there are to get a yield-stress fluid, these networked gel microstructures contrast with jammed repulsive microstructures like the foams we’ve been discussing before this. Hair gel and hand sanitizer are often composed of soft, spongy particles jammed together in water, and thus are not gels from this microstructural perspective.
Making the coffee drink
The high stiffness and yield strength of this drink made it difficult to pour and mix with the milk
For our Dalgona coffee, we could in theory take either the gelled or jammed approach for modifying the interstitial fluid of our foam, however I’m limited by needing to be able to drink this fluid (food-safe ingredients only!), and what I’m able to get delivered to my house during a lockdown here in Singapore. Thus, I chose to use Xanthan gum, a food grade polymer that self-associates (i.e. sticks to itself) and is often used as a flavorless way of thickening sauces and drinks, particularly for people with swallowing disorders. I didn’t want to overpower the texture of the coffee foam with too high of a yield-stress or viscosity and based on my prior experience with Xanthan gum as an archetypal example of polymer gel yield-stress fluids, I chose a concentration of 1 wt% as the water phase. I dry mixed approximately 0.3 grams of Xanthan gum powder with the 2 Tbsp of instant coffee powder before adding the 2 Tbsp of hot water. Using an electric hand mixer on medium speed after a few minutes I obtained a foam that was noticeably stiffer than the ordinary Dalgona Coffee. This material unlike the ordinary case was definitely a yield-stress fluid; the stiff peaks that formed on my eggbeater held their shape for a very long time as you can see at the top of the page.
If I were to make this drink again, I would slightly reduce the amount of Xanthan gum. In terms of replacing the stabilizing functionality of the sugar, I was able to successfully produce an extremely fine foam that while long-lived and pleasantly creamy, was a bit too stiff to nicely pour on top of or mix with the milk easily. As a person who does enjoy sweet things, I would also add some sort of sweetener back into this drink, as the foam I made was extremely bitter and nearly undrinkable. Perhaps differences in taste or texture induced by the Xanthan gum were masked by the assault on my taste buds by the bitter coffee, but I believe I this additive strategy was relatively successful in not negatively affecting those qualities.