In the past weeks, as the heat has swelled into the high 30s and I have remained confined to ever-lower levels of my house, I’ve found myself making all kinds of different iced drinks to keep myself cool and entertained. Perhaps my favorite of these to make is Brown Sugar Boba Milk, which goes by the names Tiger Sugar Milk, Milk Strike Wow, Mudflip Tea, and surely many others; in Singapore, Arif tells me that these drinks fall under the category of what are more often referred to as bubble teas or pearl milk teas.
What is Boba Milk?
Tiger Sugar Milk. Image from James Park/ny.eater.com
Despite the multitude of names, the core of the drink is incredibly simple: boba, small balls of tapioca starch mixed with boiling water, are poached in a brown sugar-laden syrup until soft and chewy, then topped with whole milk to complete the drink. Often, for visual appeal, the brown sugar syrup is spread along the sides of the glass before the drink is prepared. In this case, the drink can be shaken or stirred to incorporate the syrup before drinking.
The composition of the syrup, as well as the methods used to coat the cup with the syrup, are crucial to the visual presentation of the dish, and their secrets are closely guarded. Tiger Sugar, the most famous company in the US making the drink, reportedly goes as far as to build a separate room in each of their storefronts, inside of which one or two high-level employees coat the cups with the syrup before sending them to the main room. Curious if I would be able to replicate the drink at home, I decided to dig more into the rheology of this sticky substance.
The First Attempt
My first attempt at making the syrup produced a poor surface coating that was easily washed away by the milk
For my first attempt, I used a basic drink recipe. I took 1 cup of boba pearls (made from tapioca starch and water) and cooked them in 3 cups of boiling water for about 30 minutes. By the end of the boiling process, the water had cooked down significantly, leaving about ½ cup water in the saucepan. To this, I added a cup of dark brown sugar, stirring until it formed a syrup. I then cooked the boba in the syrup, stirring constantly, for about 5 minutes to give the boba time to absorb the brown sugar flavor. Removing the boba from the saucepan, I drizzled the remaining syrup on the walls of my glass to try and mimic the Tiger Sugar drink.
Though the flavor of the drink was pretty good (it’s hard to go wrong with brown sugar, boba, and milk), the visual appeal left much to be desired. The sugar syrup only formed a very thin layer on the outside of the glass, and began to flow down the sides when left unperturbed, making an uneven pattern. On the plus side, reincorporating the syrup back into the milk before drinking was easy: just a few stirs quickly made the drink homogeneous. Though this was a fun attempt, I was curious how I could better replicate the iconic stripe pattern of the famous commercial drink.
Rheological Design
From a rheological perspective, the most interesting aspect of the syrup was how it exhibited both elastic and viscous behavior. After the coating formed, it would flow down the side of the glass like a viscous fluid, but by extending it between my fingers it felt sticky and I could get it to briefly form stretchy elastic filaments. My hypothesis was that by increasing the elastic behavior of the syrup, I could make it stick more effectively to the side of the glass.
I believe that the most likely explanation for the elasticity of the syrup is that some of the tapioca starch from the boba pearls gets incorporated in the syrup during the cooking process. When starch is added to water and heated, it goes through a process called gelatinization. During this process, the heat causes starch granules to grow, and more water is irreversibly absorbed into the starch molecules. These enlarged starch networks cause a dramatic increase in the viscosity of the liquid, which is why corn and potato starches are used as thickening agents in cuisines around the world. At higher concentrations, gelatinized starch can show other rheologically interesting properties like the stretchiness that I observed.
Assuming that starch was the primary ingredient leading to the observed viscous and elastic behavior, I decided to make the boba drink once more, but add an extra 20g of tapioca starch to the boba just before adding the brown sugar and reducing. The effects were immediately noticeable: the sugar syrup was thicker than before, and also showed significantly more elasticity, which I observed by taking a small amount of syrup between two fingers and stretching them in opposite directions and seeing that the filament that formed was significantly more extensible and long-lived. An alternative approach to improving the coating behavior could be to simply increase the concentration of the sugar, increasing the viscosity without as much of a change to the elasticity. Based on my first experiment, I suspect that this still wouldn’t have formed a nice, even pattern, but I’ll leave testing this alternative for another time.
The Final Drink
To put my new starchy syrup to the test, I needed to try to coat a glass. After a couple of test runs, I found the best way to do this was to take a large spoonful of boba and syrup and put it in the bottom of the glass. Then, I tilted the glass and rotated it, spreading the syrup evenly around. As I did this with my new syrup, it became clear that my modification was successful at improving the coating behavior. The syrup formed a much thicker layer on the class, and a much more interesting stripe pattern was visible. Once I added the milk, my Instagram-worthy boba drink was complete.
However, there were a few downsides to my new starched-up drink. First of all, it was nearly impossible to incorporate the syrup on the sides of the glass into the milk mixture. This is not an issue in a boba shop, where the plastic lid makes vigorous shaking possible, but caused some problems at home. Additionally, the boba that had been cooked in the starch mixture had a much softer texture, which I found a little unpleasant. However, I do think this issue could be fixed by adding the starch later in the cooking process, which should have minimal effect on the final texture. Overall, I enjoyed the second drink and appreciated being able to see my rheological fiddling make a difference in the way I make one of my favorite foods.
Adding starch to the syrup resulted in a much more robust surface coating