There are two types of buttermilk. Traditional buttermilk is a thin, cloudy, slightly tart but buttery-tasting liquid that is left after cream is churned to make butter. These days, however, it is more commonly sold as a thick liquid produced commercially by adding an acidifying bacteria – and sometimes flavouring and thickening agents – to milk. This commercial product can be thought of as a gentler, thinner yogurt, with any buttery flavour likely added.
Buttermilk is traditionally a drink, but is more often used in baking now. When used with baking soda, it reacts to form carbon dioxide, thus helping mixtures such as soda bread, rolls, scones and waffles to rise.
It also used as a marinade as the acidity can help to make meat more tender and flavourful. You'll find buttermilk used in this way in some chicken dishes.
Traditional buttermilk is rarely available. Commercially-produced buttermilk is often stocked in larger branches of most supermarkets.
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All buttermilk will continue to ferment to some degree and thus become more acidic, so pay attention to use-by dates.
Buttermilk should be kept refrigerated and used quickly once opened.
When using buttermilk as leavening with baking soda, it’s best to let it come to room temperature before use. This reaction happens immediately so is much faster than baking powder, which works only when heat is present. If the buttermilk is still refrigerated, the dough might set before the reaction has time to work fully and the result will be heavier.
If you cannot get buttermilk for baking, or do not have enough, plain yoghurt works just as well but will give a slightly different flavour.