Vegetable oil
A category, not a substance: any neutral liquid oil sold for cooking. Liquid at every temperature a kitchen reaches, which decides almost everything it can and cannot do.
Also called cooking oil (INTERNATIONAL); canola oil (US); rapeseed oil (UK); sunflower oil (UK); corn oil (US); groundnut oil (UK); peanut oil (US); neutral oil (INTERNATIONAL)
“Vegetable oil” is a shelf label rather than an ingredient. In the UK a bottle so labelled is usually rapeseed, sometimes a blend; in the US it is commonly soybean or a blend; sunflower, corn and groundnut are sold under their own names and behave near-identically in baking. For structural purposes they are interchangeable, and the differences between them — smoke point, fatty acid profile, keeping — matter for frying and for nutrition rather than for how a cake sets.
WHAT MATTERS IS THAT IT IS LIQUID. It has no crystal structure, so it cannot be creamed, cannot be rubbed in and cannot be laminated. There is nothing for beaten air to be held in. Any recipe whose aeration depends on the fat is not a recipe an oil can be substituted into, and this is a hard boundary rather than a matter of degree.
What it does well is lubricate. It coats flour particles readily and it stays liquid at eating temperature, which is why oil-based cakes feel softer and less dry days later than butter cakes of the same age. That is a mouthfeel property, not a water content one — a point worth holding on to, because oil brings no water whatsoever.
Its other advantage is that it is forgiving. There is no correct temperature for it, no emulsion to break, and no window to miss.
Composition and specification
Water content
Effectively none. Substituting oil for butter by weight therefore removes water from a formulation at the same time as it adds fat, because butter is only about four-fifths fat and oil is essentially all of it.
An oil is by definition the separated fat phase. BakeHQ publishes no substitution percentage for the adjustment, because the right figure depends on what the water was doing in that particular recipe.
RecordedTrue by definition rather than by measurement
What changes if it is left out
Removing the oil from a batter that had no other fat removes tenderising and lubrication, and the result is tougher and drier-eating rather than merely less rich. It removes no structure, no aeration and no liquid, so the bake will still rise.
Follows from the shortening-effect mechanism recorded under fat plasticity, which carries its own support.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Does oil really make a cake moister?
It makes a cake feel moister, and “moist” is doing two different jobs in that sentence.\n\nIf moist means holding more water, oil cannot be the reason: butter carries water into a batter and oil carries none, so swapping butter for oil by weight makes a batter DRIER in water terms while making it richer in fat.\n\nWhat oil does is stay liquid. Butter resolidifies as a cake cools, and a crumb shot through with set fat crystals reads as firmer and drier in the mouth than the same crumb lubricated with something still liquid at room temperature. Two days later the gap widens, because the perception of staling is partly a perception of dryness.\n\nSo the observation is real and the usual explanation is wrong. BakeHQ records the property as lubrication and mouthfeel rather than as moisture, and does not publish a claim that oil cakes contain more water, because they do not.
Related
What it does
Gluten inhibition
It lubricates and interrupts the continuity of the protein and starch structure, which is the shortening effect. Recorded under the fat mechanism even though there are no crystals in a liquid oil — the vocabulary names the family of effect, and the tenderising route is the same one.
The science behind it
Fat plasticity and crystal structure
It sits permanently outside the plastic range, which is a complete account of what it cannot do.
Can stand in for
Butter— for tenderising only, and for nothing else butter was doing
SCOPED DELIBERATELY AND NARROWLY. Oil replaces butter’s lubrication and replaces none of its structure, none of its aeration, none of its water and none of its flavour. In a muffin or a brownie that is usually the whole of butter’s job and the swap works; in anything creamed, rubbed in or laminated the recipe depended on a solid, and the result is not a variation but a failure. BakeHQ publishes no conversion ratio, because the correct adjustment depends on how much of butter’s water the formulation was counting on.
Covered in
Choosing a fat, and substituting one honestly
The fat that can do exactly one of the five jobs, which makes it the clearest test of whether a substitution question has been asked properly.
Used in
Liquid fat, which is why there is no creaming stage.
Compared in
Olive oil vs vegetable oil in baking
One side of the comparison.