Make Boba for Bubble Tea
Ever wondered how the boba in bubble tea are made? Bubble tea or boba tea is a sweetened drink made of flavored tea, milk and bubbles. The translucent, squishy bubbles called boba are remarkably easy to make. You only need three ingredients: tapioca flour, water, and brown sugar. The skill lays in one little detail: the temperature of the water used. Curious? Try it out and make bubble tea from scratch!
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The bubbles in bubble tea -also called boba or tapioca pearls- are made from tapioca flour, a starch extracted from the Cassave plant. Unlike wheat flour which contains starches, protein, and fiber, tapioca flour only contains starch, and the secret to making bubbles lies in the way starch particles interact with water.
Starch particles are a large number of glucose units – the sugar that rushes through your blood to give your cells energy – joined together. When these particles are mixed with cold water, they disperse and float around in the water. This type of mixture is called a suspension, and the suspension of starch in cold water is often referred to as goo or Oobleck. Note that the starch particles do not change when mixed with cold water. When you leave the goo out, the water will evaporate and you will have your starch particles again.
The story changes when you add heat. Starch particles swell and break apart when mixed with hot water. The smaller pieces then create new connections and form a network that can hold water. This process is called starch gelatinization. When this solution cools, it becomes more gel-like. With time, it will lose water and become stiffer, but no matter how long you wait, it will never turn into starch particles again. The addition of heat changed the particles.
Extra: Make a cup of tea, let it cool, and sweeten it with some of the brown sugar syrup. Add boba and enjoy!
Extra: Leave the cup of your first dough (the one made with cold water) out for a day or two, stirring occasionally. How does it change? Why would this happen? Would the same happen with the dough made with boiling water if that dough was left out for several days?
Extra: The bubbles (tapioca pearls) get harder when they cool and when they are left out. Explore what happens when you reheat them.
Extra: Substitute the tapioca flour by another flour like corn starch, potato starch, or wheat flour. Can you predict which flours will give similar bubbles, and which ones will not?
Observations and Results
Was cold water not enough to make tapioca balls? That is expected! Tapioca flour is a starch and starch particles spread out and float around when mixed with cold water. They do not create new connections that can keep dough together.
On the contrary, starch particles mixed with boiling hot water break into smaller pieces that partially dissolve in water. The pieces make new connections and as the starch-water mixture cools, even more connections are formed. As long as there is not too much water, this mixture can hold its structure.
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. The pearls get their typical chewy, gel-like, and translucent appearance.
If you tried letting this mixture of starch particles and cold water sit for a couple of days, stirring it occasionally, you might have noticed you end up with pure starch flour again. The water evaporates and you are left with unaltered starch particles. And, if you tried letting the tapioca dough created with hot water sit for a couple of days, you might have noticed it dries out but does not become starch flour again. The starch particles were altered when they came into contact with hot water.
More to Explore
The Scientific Secret of Stretchy Dough, by Scientific American
Make Your Own Gelatin Pearls, by Scientific American
Science Activities for All Ages!, from Science Buddies
Sabine De Brabandere, PhD, Science Buddies
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