Why Fruit Bubble Tea Can Curdle Milk, and How to Avoid It

Fruit bubble tea can curdle milk because of its acidity. Casein, the main protein in dairy milk, stays evenly spread through milk at its normal pH of about 6.6. Add enough acid to bring milk down to pH 4.6 and the casein clumps, and the milk separates into curds and whey. That is the split you see in the glass.

It is also the reason to follow the liquid in each TeaJoy kit's method: our fruit tea syrups go with water, and our milk teas and matcha with milk or a milk alternative. Below is the science in plain words, then what it means for each sachet in our kits.

What acid does to milk

About 80% of the protein in dairy milk is casein. It sits in the milk as tiny clusters, and one type of casein, kappa-casein, keeps those clusters pushing each other apart so they stay suspended in the milk. That is how a 2023 review of milk and plant proteins in the journal Foods describes it.

Acid weakens that push. The same review explains that as the pH drops, the proteins lose the force holding them apart, drift closer, bond to each other and clump. A 2003 study in the Journal of Dairy Science sums it up for milk: add acid, and the casein clusters become unstable, clump together and set into a gel.

The numbers make it clearer. Milk normally sits at about pH 6.6, according to a 2024 review of milk's heat stability, also published in Foods. Adding acid brings that number down. A teaching guide on milk and cheese from Cornell University gives the point where casein curdles as pH 4.6, a process it calls acid coagulation. Its cheese-making examples are vinegar, lemon juice and citric acid, added to milk with enough heat.

Heat pushes it further. With high heat and enough acid, the Cornell guide adds, the whey proteins curdle as well as the casein, and that is how ricotta is made. Nothing in our kit methods is heated: every one says no boiling or cooking is needed, and every drink finishes with ice.

Once acid has pulled the casein out, it does not simply go back. The 2023 review notes that casein separated from milk by acid will not dissolve in water, and usually has to be neutralised before it will.

Where the acid is in our sachets

Now to your glass. Every one of our fruit tea syrups lists fruit juice and citric acid in its ingredients. Our tea powders and our matcha powder list neither. The liquid in each method follows that same line.

Sachet	Kit	Fruit juice or citric acid in the ingredients?	Liquid in our method
Passion Fruit Tea Syrup	Fruit Tea Kit	Yes: passion fruit juice (13.7%) and citric acid	300ml water
Mango Tea Syrup	Fruit Tea Kit	Yes: mango juice (24.6%) and citric acid	300ml water
Strawberry Tea Syrup	Matcha Bubble Tea Kit	Yes: strawberry juice (10.6%) and citric acid	300ml water
Matcha Tea Powder	Matcha Bubble Tea Kit	No	300ml milk or milk alternative
Classic Tea Powder	Milk Tea Kit	No	300ml milk or milk alternative
Brown Sugar Tea Powder	Milk Tea Kit	No	300ml milk or milk alternative

The boxes say it plainly. The Fruit Tea Kit box reads "just add water and ice", and the Milk Tea Kit box "just add milk and ice". Our commercial kits for cafés keep the same split: the Classic Milk Tea box says "just add milk / plant milk and ice", and the three fruit boxes say "just add water and ice".

The one to watch is the Matcha Bubble Tea Kit, because one box holds both routes: it says "just add milk/water and ice". Its matcha powder goes into milk; its strawberry tea syrup goes into water. The full four steps for every kit are in how to make bubble tea at home without special equipment.

How to avoid a split drink

1. Match the sachet to its liquid from the table above: fruit tea syrups with 300ml of water, tea and matcha powders with 300ml of milk or a milk alternative.
2. With the Matcha Bubble Tea Kit, make the matcha drink and the fruit drink as two separate glasses. The strawberry syrup belongs in water even though the matcha next to it goes in milk.
3. Stir the sachet into its liquid until it dissolves before anything else goes in, as our steps do. The boba comes next and the ice goes on last.
4. Leave other brands' recipes with their own ingredients. Ours are written for the liquid on our pack.

If your drink splits

Start again the pack way. The clumps are casein that the acid has already pulled out of the milk, so stirring will not dissolve them again. Open fresh sachets, pour the liquid the pack gives, stir the syrup or powder in until it dissolves, add the boba and top with ice.

Each sachet is a single portion, and the pack says to consume it immediately once opened, so a remake starts from new sachets. A kit drink is ready in about 20 seconds, so it is a short job. To see our method beside the from-scratch way, how to make bubble tea at home: from scratch or in 20 seconds sets the two side by side.

What about plant milks?

There is no single rule. Everything above is about casein, the protein in dairy milk, and the 2023 Foods review, which looked at milk and plant proteins together, says each plant protein seems to react to acid in its own way. So we would not set one rule for every plant milk on the shelf. The method for the Milk Tea Kit and for the matcha drinks reads "milk or milk alternative", and our commercial milk tea box names plant milk outright.

Whichever milk you pour, it goes with the tea or matcha powder, and the fruit syrups stay with water. For a milk tea made either way, our brown sugar milk tea recipe takes you through the brown sugar sachet step by step.

Questions people ask

Can you add milk to fruit bubble tea?

With ours, use water. Every TeaJoy fruit tea syrup lists fruit juice and citric acid, and acid can curdle the casein in dairy milk. Our method for each fruit syrup is 300ml of water, then the syrup, the boba and ice.

At what pH does milk curdle?

Casein, the main protein in dairy milk, curdles once acid brings milk down to pH 4.6. Cornell University's teaching guide calls this acid coagulation. Milk normally sits at about pH 6.6.

Can you fix a curdled bubble tea?

Not by stirring. Casein that acid has pulled out of milk will not dissolve in water, so the clumps will not go back in. Make a fresh drink with new sachets and the liquid the pack gives.

Which TeaJoy kits are made with milk?

The Milk Tea Kit uses milk or a milk alternative for all four drinks. The Matcha Bubble Tea Kit uses it for its two matcha drinks and water for its two strawberry fruit teas. The Fruit Tea Kit uses water for all four.

Make yours the pack way

All three kits are in bubble tea kits for home: the Fruit Tea Kit, made with water, and the Milk Tea Kit, made with milk or a milk alternative, at £13.49 each, and the Matcha Bubble Tea Kit, which makes two drinks with milk and two with water, at £13.99. Sign up to our emails for 15% off your entire order.