Oat milk is terrible with coffee. Not least because in acidic coffees (like the light roasts I prefer) it curdles and separates leaving a delicious mug of coffee sullied with floating goop.
Searching online didn’t net a solution, the whole coffee space has been SEO’d to shit and none of the first page results contained any useable information whatsoever.
Milks curdle entirely because they are introduced to too much acid. It’s why old milk gets chunky: the natural sugars in it starts to ferment and form lactic acid, lowering the pH of the milk and causing it to separate.
Plant milks seem to be much more sensitive to acidic environments than animal milks, and the combination of sugar or sweeteners and flavourings to a coffee further reduce the pH, turning the drink more acidic and making a smooth creamy coffee that much harder to achieve with oat milk.
I experimented with different water temperatures, and while it does help temporarily, it just slows the reaction and curdling, not stopping it.
Using darker roast coffees is an option, but not what I prefer, and doesn’t seem to stop oat milks curdling when a vanilla shot is added to the coffee like my partner prefers.
It’s disgustingly simple. The acidity can be reduced with a sprinkle of a pantry staple: Baking Soda.
It only needs to be a tiny amount too, from a few experiments I’ve found 0.1g baking soda to be the sweet spot for reducing the acidity enough for no plant milk to curdle (when added to a large 300ml coffee with 100ml milk) without imparting any flavour.
For us, that translates to 0.1% baking soda by weight compared to the milk, which means we can add 1g (or 1/4 teaspoon) to the 1L carton of milk, shake, and not have to worry for a week.
And voila! Your (kind of nasty) milk substitute no longer curdles!