Natural yoghurt
Cultured milk bringing acid, body and liquid at once — the acid being the part that changes the chemistry rather than only the taste.
Also called plain yogurt (US); plain yoghurt (UK); greek yogurt (US); greek yoghurt (UK); natural yogurt (US)
Yoghurt is milk fermented by bacteria that convert lactose to lactic acid. The acid drops the pH far enough to set the milk proteins into a gel, which is why yoghurt is thick without anything being added to it.
In baking that acid does the work buttermilk’s does: it reacts with bicarbonate of soda to generate carbon dioxide, and it tenderises by interfering with gluten development. Its thickness means it contributes rather less free liquid than the same weight of milk, and strained styles — Greek and Greek-style — contribute less again, because straining is precisely the removal of watery whey.
That is the trap in swapping one yoghurt for another. Substituting strained yoghurt for ordinary yoghurt by weight leaves a formulation drier and stiffer, and the packaging gives no indication of it.
What changes if it is left out
Where bicarbonate of soda is the leavener, removing yoghurt removes the acid it needs and the bake rises poorly and can taste soapy. Buttermilk is the closest swap because it brings comparable acid; milk is not, unless an acid is added with it.
Follows from the acid-base leavening model, in the same way as the buttermilk record.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Related
What it does
Hydration
Yoghurt supplies liquid, and less of it per gram than milk does because the protein gel holds some of the water. Strained styles supply less again — straining is the removal of watery whey — which is why swapping one yoghurt for another by weight leaves a batter stiffer.
pH modification
Lactic acid from the fermentation reacts with bicarbonate of soda to generate carbon dioxide, which is why yoghurt appears in the same recipes buttermilk does.
Gluten inhibition
Acid interferes with gluten development, which is the tenderising half of what a cultured dairy liquid does to a scone or a soda bread.
Can stand in for
Buttermilk— for the acid that activates bicarbonate of soda, thinning it if the batter needs it
The closest available swap, because the acid is the part that does the chemical work and both carry it. Yoghurt is thicker, so a batter built for buttermilk may need loosening — and a strained yoghurt needs it more than an unstrained one.
Sour cream— for the acid, and not for the fat
Keeps the chemistry and loses the richness. Yoghurt is a cultured milk and sour cream a cultured cream, so the acid transfers and the shortening does not — a cake made each way is tenderer with the sour cream.
Used in
Water, acidity and fat together. The fat is what keeps it soft after cooking.