The first time I went to Menchies, a self-serve frozen yogurt chain, the topping called “popping boba” amazed me the most. It looked like caviar but when I bit it, a burst of juice exploded in my mouth. It was very light, refreshing and fun to eat. Turns out, we have food science to thank for these spherical balls of fruit juice.
Making “popping boba” relies on a technique called spherification that is the reaction between calcium chloride, water and sodium alginate. Calcium chloride is a white powder and sodium alginate is a yellow one. Sodium alginate is a salt, a product of neutralization. It is derived from the reaction between alginic acid and a base like sodium hydroxide. Alginic acid is found in the cell walls of brown algae and is extracted by drying and processing seaweed. To start spherification, sodium alginate is added to the desired juice. A good tip is to use a blender to incorporate alginate to ensure it is evenly distributed. The juice should also be cold. If it is hot, the alginate will solidify before adding it to the calcium and you get jelly instead of juice inside the boba. Not only that, the juice must also have a pH greater than 3.6 which is hard considering the pH of orange juice is about 3.5. Too much acidity converts sodium alginate back into alginic acid which thickens it. A solution to this problem is to add “alkaline” that neutralizes acids giving it the ability to raise the pH of a substance. An example of an alkaline is sodium citrate.
Another name for alginate is “hydrocolloid” which is defined as a substance that forms a gel when added to water. This explains the next step in spherification. A syringe is used to add the juice one drop at a time into a calcium chloride water mixture. In water, calcium chloride dissociates into ions and according to the reactivity series, calcium is more reactive than sodium resulting in the formation of calcium alginate. On top of that, the Na has a 1+ charge while calcium has a 2+ charge. This means twice the bonds are required in calcium alginate. An increase in the number of bonds form a more ridged molecule creating a thin gel layer on the outside of the boba while keeping the desired juices on the inside. One final tip is to to store the boba in the same juice as its interior. The gel layer is very thin and permeable to small molecules. If it is stored in water, diffusion will occur giving you a very dilute inside.
This is just the very basic type of spherification. The reason why it comes out as a sphere is because the syringe creates droplets in the calcium chloride. With other tools, the shape can be manipulated to create ravioli, gnocchi etc. The possibilities are just endless!