From gelatinization to caramelization, learn the science behind the sweet and squishy bubbles in this wildly popular Asian drink along with a recipe that captures this "QQ" texture and caramel sweetness perfectly!
Background
Bubble tea, or boba tea, is a sweet and creamy refreshment known for its chewy black pearls. Its most basic drink form, like the images pictured to the left, contains black tea, milk, and bubbles that are made from tapioca starch - an extract of the South American cassava plant. However, there are now a variety of other toppings and drinks available in the many stores that have recently been popping up around the world (my favorite go-to order is winter melon green tea with honey boba and lychee jelly - delicious!).
But what exactly is a "QQ" texture? "QQ" (very Q) is the bouncy and chewy consistency that is a signature characteristic of boba and is what makes bubble tea so addicting. This perfect chewiness is often hard to attain, similar to the al dente texture people crave in pasta. It is found in many other Asian foods as well, such as mochi or noodles, so "QQ" is clearly a treasured aspect of the cuisine.
Bubble tea is one of my favorite drinks (I have been known to go a little crazy over it), even if it's not the most healthy, so I decided to try making my own boba from scratch. I have tried using premade boba before and the results haven't been too bad, but the taste and texture were sometimes varied. Instead of braising them in a sugar syrup (a crucial step as I have found), I merely drizzled honey on top in an effort to recreate the honey boba of various bubble tea shops. However, the honey would just mix with the tea once I had poured it over the boba, leaving the boba themselves flavorless.
Above: bubble tea I made with premade boba - peach mango green tea, milk black tea, and matcha tea from left to right.
With this homemade boba, I wanted to infuse the boba with a more complex taste and sweetness in every bite. More importantly, I strove to attain the ideal "QQ" texture, as I know from personal experience that boba that is too soft or firm just doesn't taste as good. I decided to experiment and find the best way to make the boba extra chewy. I would also try the boba with and without braising them in the sugar syrup to see the differences in taste. If I knew the factors that affected the final bubbles, perhaps I could control them in a way to end with the product I wanted.
Procedure
Making boba only requires three simple ingredients: tapioca flour, water, and brown sugar (muscavado sugar is preferred). The process takes a couple hours, but the end results are definitely worth it. For my specific recipe with servings and measurements click here.
I started by boiling some water on the stove and dissolved some of my brown sugar before adding the tapioca flour to make the boba dough. At first, the flour wouldn't stick together and I was left with bits of dough all over the pot. However, as I began to furiously mix with a spatula and knead the dough, it began to form into a sticky, smooth blob. I took the dough out of the pot, cut it into portions, and began to roll out one of the portions into a long log. I cut the log into small squares before rolling them into spheres. It was a bit like playing with playdough which was quite fun. As time went on, I quickly noticed that it was getting harder to roll out the dough, which was hardening and cooling. I wondered why this was happening and if cooling the dough would affect the boba's texture after cooking. But we'll get to that later.
Once the boba were rolled out, I placed them in a bowl with a little tapioca flour to prevent them from sticking. I sifted out the extra flour and dumped them into a pot of boiling water. I continued to stir them to prevent sticking (they are very, very sticky throughout this whole process) and waited for them to float. They seemed to expand in the hot water, but as soon as I poured them into a bowl of cold water, they shrunk and became firmer.
They looked a bit different from the boba usually found in stores, more dark brown than black. Upon trying one, I found that it tasted bland with only a hint of sweetness. The chewiness was there though! The next step was to braise the boba in a brown sugar syrup to really coax out those complex and sweet flavors. I added water and even more brown sugar (this is why boba isn't healthy but a little sugar never hurt anyone :)) to a pot. I dropped the chilled boba in once all the sugar had dissolved and let the pearls braise in the syrup. I started with a pot full of water, but eventually most of it evaporated until I was left with a thick and viscous syrup. The boba were now a deep brownish-black and one bite caused an explosion of flavor in my mouth. The rich, nutty taste of the caramelized brown sugar was amazing! Why was this flavor so much better than the previous honey boba I had tried in bubble tea stores and with the premade boba? Keep reading to find out!
Above: Braising the boba in brown sugar syrup - the images show the water evaporating into a thick syrup and the boba turning a deeper brown.
Science!
First, the secret behind the chewiness of boba is in one factor: the temperature of the water. This all lies in the fact that tapioca flour only consists of starch, in comparison to something like wheat flour which has starches, protein and fiber. The "QQ" of the bubbles is determined by how the starch particles interact with water. Starch particles are created when a large number of glucose (sugar) molecules join together. In cold water, these starch particles disperse and float around, creating a suspension or goo, similar to the Oobleck you may have made before with cornstarch and water. This Oobleck is a non-Newtonian liquid, which acts similar to a solid but flows like a liquid. Here, a reaction has not taken place - the two compounds have merely mixed. If the goo is left outside, the water will eventually evaporate the starch particles will be left behind.
However, the starch begins to change when you add heat and a process called gelatinization begins to take place. When mixed with hot water, the starch granules absorb water and swell as they become less crystalline (water increases the randomness in the starch granule structure). Subsequent heating causes the granules to burst after they can no longer hold any more water. Molecules in the granule leach out and disperse in the surrounding liquid, forming a network that can hold water and gelatinizing. When this solution cools it becomes like a gel or paste. With time it will lose water and become stiffer. This is an irreversible reaction, as no matter how long you wait, it will not turn into starch particles again. On cooling, gelatinized starch tries to re-organize itself by forming new hydrogen bonds to hold on to the water which it absorbed during swelling. This process is called retrogradation and leads to textural changes in the starch.
So how does all of this relate to a "QQ" texture? Well, if the starch is overheated or overcooked, the boba can’t hold its round shape and chewy texture after being cooked. Thus, it's important to work quickly when forming the dough so that it doesn't become too overcooked. It's also important to make the bubbles rapidly so that the dough doesn't harden too much during retrogradation. When you cook the dough, more changes occur in the starch particles. The tiny gas bubbles you see appearing in the pearls indicate this change, as they get their typical chewy, gel-like and translucent appearance.
Next, onto the flavor! Caramelization is the process of removing water from a sugar, such as sucrose or glucose. The sugar starts by melting at high temperatures, followed by boiling. At this stage, saccharose (sugar) decomposes into glucose and fructose. This is followed by a condensation step, in which the individual sugars lose water and react with each other. The last series of reactions include both fragmentation reactions for the rich, nutty flavor production and polymerization reactions for the dark brown color production. One example is diacetyl, an important flavor compound produced during the first stages of caramelization, which causes a buttery or butterscotch flavor.
For boba, we prefer to use dark brown sugar because it contains nearly twice as much molasses as light brown sugar does (6.5% as compared to 3.5%), which gives it a richer caramel flavor. Braising the boba in a syrup made by caramelizing the dark brown sugar causes all those rich flavors to come out and be absorbed by the bubbles. Depending on the type of sugar used, the initial caramelization temperature will be different, so you can control the sweetness and flavor with how hot you make the syrup. This method creates sweet and delicious boba in every bite, in contrast to the flavorless honey boba or boba in a white sugar syrup.
Conclusion
There are so many chemistry concepts behind cooking boba, including gelatinization and caramelization. Understanding them allows you to control the necessary variables to create the perfect texture and flavor. Further experimentation could include trying to make boba from different kinds of starch, like potato starch, corn starch, or taro. The boba can also be frozen to be cooked later if you want to whip up a batch for a tasty snack. I hope you enjoyed learning about the science of boba. Now, go try making it yourself!
Link to my brown sugar boba recipe.
Link to my post on the history & culture of boba.
Want to Learn More? (always cite your sources, kids)
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