There are two types of buttermilk. Traditional buttermilk is a thin, cloudy, slightly tart but buttery-tasting liquid that's 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's 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. You can make your own by adding a squeeze of lemon juice to whole milk, and letting it stand for a few minutes to thicken.
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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 can't get buttermilk for baking, or don't have enough, plain yogurt works just as well but will give a slightly different flavour.
Use buttermilk as a marinade for crunchy, tender fried chicken in a buttermilk fried chicken recipe. Add buttermilk to batter to make fluffy pancakes with maple apples. A classic pound cake uses buttemilk to get the texture spot on, try it in a lemon & buttermilk pound cake. It also makes a fantastic dressing for crunchy salads and slaws. Try a green salad with buttermilk dressing.
Try kefir or yogurt.