Milk does four things in a bake and a substitute rarely does all of them

Milk is four ingredients delivered in one liquid

Milk is mostly water — around seven-eighths of it — with a few per cent each of fat, sugar and protein suspended or dissolved in the rest. Each of those fractions is doing something specific in a batter, and because they always arrive together, recipes almost never separate them out. That is fine until the liquid is replaced by something with a quite different composition, at which point the recipe silently loses whichever contributions the substitute doesn’t make.

The four are easy to name once you look. Water hydrates flour and turns to steam. Fat tenderises and carries flavour. Lactose browns and adds a background sweetness. Casein and whey proteins set with heat and add structure, and whey in particular contributes to the skin that forms on a heated milk mixture. A plant milk may match one of these, occasionally two, and almost never all four.

Water is the easy part and it is genuinely easy

For the hydration function, almost anything works. Rice, almond, oat, soy and coconut drinks are all overwhelmingly water, and a bread dough or a plain cake that needs liquid for the flour is largely indifferent to which one it gets. This is why a dairy-free loaf is usually unremarkable and a dairy-free enriched brioche is a project.

The exception is consistency. Commercial plant drinks vary considerably in how much solid material they contain, and some are thickened with gums that behave in a batter rather differently from plain liquid. A drink formulated for coffee has been adjusted for foaming and acid stability, which isn’t the same specification as a drink meant for baking, though in practice it often works well.

Lactose is why a milk-enriched crust goes brown

Lactose is a reducing sugar, so it takes part in the Maillard reaction with the amino acids around it, and it also caramelises. That is a substantial part of why a milk-based bread crust, a brioche or a pastry brushed with milk goes a deeper colour than water alone would produce. Take the lactose out and a crust stays pale even when the bake is finished, which reads as underdone whether or not it is.

The plant drinks are unequal here. Oat drink is usually made by treating oats with enzymes that break their starch into maltose, so it contains a reducing sugar in quantity and browns well — which is a large part of why it has become the default in dairy-free baking. Almond and rice drinks contain much less and colour poorly. Adding a small amount of a sugar that browns, or a little maple syrup, does the job where colour matters.

Casein and whey do structural work that is hard to see

Milk proteins contribute more than people expect. They absorb water, they add to the network that sets in the crumb, and they compete with the starch for moisture in ways that soften the finished texture. Whey proteins denature and coagulate somewhere in the seventies Celsius, which is within the range a bake passes through, so they set alongside the egg and the starch rather than after them.

Soy drink is the closest match in protein terms and behaves most like dairy in a batter, which is why it holds up in custards and enriched doughs where the thinner drinks fall apart. Almond, rice and most coconut drinks carry very little protein, so anything relying on protein to set — a baked custard, a pastry cream, an enriched dough — needs either a different substitute or additional structure from egg, starch or a thickener.

Plant butters are mostly a water content problem

Replacing butter is harder than replacing milk, because butter is an emulsion of roughly four parts fat to one part water and its usefulness comes from being plastic across a range of temperatures rather than melting at a point. Coconut oil is solid below the mid-twenties Celsius and then melts quite sharply, which gives none of that range, so a laminated dough made with it moves straight from brittle to greasy with little useful territory in between.

Formulated plant butters are blends designed to imitate the melting curve, and the good ones do it reasonably well. Water content is the thing to check: a block sold for spreading holds far more water than one sold for baking, and the extra water shows up as spread in a cookie and as toughness in a pastry, since water that is present will find the gluten. Buttermilk is the simplest substitution of all — a plant drink with a spoonful of vinegar or lemon juice supplies the acid a soda-raised recipe needs, even though it supplies nothing else that dairy would.