Key Concepts
Chemical change
Cooked, white boba

Introduction

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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Background

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.

Materials

  • Stovetop
  • Pot
  • Water
  • Measuring cup and spoons
  • Brown sugar
  • Slotted spoon
  • Large bowl
  • Tapioca flour, also called tapioca starch
  • Small spoon
  • Two small bowls or cups
  • Cutting board
  • Butter knife
  • Adult helper
  • Optional: glass of cold tea with milk

Preparation

  1. Ask an adult for help to prepare for the activity, and anytime you need to handle the stove or hot water.  
  2. Heat three cups of water until it boils, add one cup of brown sugar and stir until the sugar dissolves and forms a syrup. Pour the syrup into a large bowl and set aside to cool.
  3. Just before you start the activity, heat a small amount of water until it boils.

Procedure

  1. Boba, also called tea bubbles or tapioca pearls, are tapioca flour balls cooked in a sugary syrup. The recipe for the dough is easy: six teaspoons (tsp) tapioca flour mixed with two teaspoons water. You will make two batches, one using cold or room temperature water, and another made with boiling hot water. Do you think using cold, room-temperature or hot water will impact the result?  If so, how?
  2. Scoop six teaspoons tapioca flour in a small bowl. Repeat with the other small bowl.
  3. Take one bowl with tapioca flour. Have a small spoon ready so you are ready to start mixing. Pour two teaspoons of room temperature or cold water on the tapioca flour and instantly mix the flour with the water. Mix well and set aside.
  4. Take the second bowl with tapioca flour. Ask an adult to reheat the water until it boils. Have a small spoon ready and ask an adult to pour two teaspoons of boiling water on the tapioca flour. Start mixing the flour with the hot water as soon as it is poured in. Mix well.
  5. Look at the two batches of dough. How are they similar, and how are they different?
  6. One of your doughs should clump together while the other one does not. Why do you think this happens?
  7. Set aside the dough that is not clumping.
  8. Kneed the clumping dough with your hands to make a ball. The dough should feel like playdough. Add a little hot water if the dough is too dry, and a little flour if it is too sticky.  As you cannot overmix this dough, you can play with the dough as long as you want.
  9. Transfer the dough on a cutting board and roll it into cylinders with a thickness of a pencil (about five millimeters). Cut each long cylinder into short pieces that are about as long as they are wide. Roll each little piece into a ball. These are your homemade tapioca balls!
  10. While you are rolling balls, ask an adult to heat up two cups of the syrup (the solution of brown sugar in water). Leave one cup in a cool place, you will use it later.
  11. Add the balls to the boiling syrup and cook them. Do the balls initially sink or float? Does that change as the cooking process takes place? Why would that happen?
  12. Be patient and check around every five minutes to see if your bubbles are soft and gooey. It takes about 20 minutes, sometimes shorter and sometimes longer. Smaller balls cook faster than larger ones. The balls will become more translucent, and bubbles will appear in the balls as they cook. Check one or two balls for doneness.
  13. Once you feel the texture is just right, let them cook a little bit longer. Why would you overcook them?
  14. Scoop the boba out of the boiling syrup with a slotted spoon and place them into the leftover cold syrup to cool.
  15. Let the boba cool and have a bite!  Are they overcooked or is their tenderness, just right?  
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

Mac and Cheese Science: What Makes a Thick Sauce?

It's a Solid... It's a Liquid... It's Oobleck!

Science Activities for All Ages!, from Science Buddies

Credits

Sabine De Brabandere, PhD, Science Buddies

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Key Concepts
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