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|Nutritional value per 100 g (3.5 oz)|
|Energy||169 kJ (40 kcal)|
|†Percentages are roughly approximated using US recommendations for adults.|
Buttermilk is a dairy drink. Originally, buttermilk was the liquid left behind after churning butter out of cultured cream. This type of buttermilk is now specifically referred to as traditional buttermilk and the fermented dairy product is known as cultured buttermilk.
Cultured buttermilk is common in warm climates (e.g., Afghanistan, the Balkans, India, the Middle East, Nepal, Nicaragua, Pakistan, Sri Lanka, Turkey, and the Southern United States) where unrefrigerated fresh milk sours quickly, as well as in colder climates, such as Scandinavia, Ireland, the Netherlands, Germany, Poland, Slovakia, Slovenia, Croatia, and the Czech Republic. This fermented dairy product, known as cultured buttermilk, is produced from cow's milk and has a characteristically intense sour taste caused by lactic acid bacteria. This variant is made using one of two species of bacteria—either Lactococcus lactis or Lactobacillus bulgaricus, which creates more tartness.
A quick version of buttermilk, known as acidified buttermilk, is used to make paneer. This version is simply cow's milk with a food-grade acid like lemon juice or vinegar added and left to sit for at least 10 minutes. That is when the milk has curdled.
The tartness of buttermilk is due to acid in the milk. The increased acidity is primarily due to lactic acid produced by lactic acid bacteria while fermenting lactose, the primary sugar in milk. As the bacteria produce lactic acid, the pH of the milk decreases and casein, the primary milk protein, precipitates, causing the curdling or clabbering of milk. This process makes buttermilk thicker than plain milk. While both traditional and cultured buttermilk contain lactic acid, traditional buttermilk tends to be less viscous, whereas cultured buttermilk is more viscous.
Buttermilk can be drunk straight, and it can also be used in cooking. Soda bread is a bread in which the acid in buttermilk reacts with the rising agent, sodium bicarbonate, to produce carbon dioxide which acts as the leavening agent. Buttermilk is also used in marination, especially of chicken and pork, whereby the lactic acid helps to tenderize, retain moisture, and allows added flavors to permeate throughout the meat.
Originally, buttermilk referred to the liquid left over from churning butter from cultured or fermented cream. Traditionally, before the Advent of homogenization, the milk was left to sit for a period of time to allow the cream and milk to separate. During this time, naturally occurring lactic acid-producing bacteria in the milk fermented it. This facilitates the butter churning process, since fat from cream with a lower pH coalesces more readily than that of fresh cream. The acidic environment also helps prevent potentially harmful microorganisms from growing, increasing shelf-life. However, in establishments that used cream separators, the cream is hardly acidic at all.
On the Indian subcontinent, the term "buttermilk" refers to the liquid left over after extracting butter from churned cream. Today, this is called traditional buttermilk. Traditional buttermilk is still common in many Indian, Nepalese, and Pakistani households, but rarely found in Western countries. In Nepal, buttermilk is called mohi and is a common drink in many Nepalese homes. It is served to family members and guests, and can be taken with meals or snacks. In many families, it is most popularly served with roasted maize. 
Buttermilk can be made at home by whisking a cup of non-homogenized cream until it separates into butter and buttermilk.
Commercially available cultured buttermilk is milk that has been pasteurized and homogenized (with 1% or 2% fat), and then inoculated with a culture of Lactococcus lactis plus Leuconostoc citrovorum to simulate the naturally occurring bacteria in the old-fashioned product. Some dairies add colored flecks of butter to cultured buttermilk to simulate residual flecks of butter that can be left over from the churning process of traditional buttermilk.
Cultured buttermilk, often known simply as "buttermilk" now, was first commercially introduced in the United States in the 1920s. It was popular among immigrants, and viewed as a food that could slow aging. It reached peak annual sales of 517,000,000 kilograms (1.140×109 lb) in 1960. Buttermilk's popularity has declined since then, despite an increasing population, and annual sales in 2012 reached less than half that number.
Condensed buttermilk and dried buttermilk have increased in importance in the food industry. Buttermilk solids are used in ice cream manufacturing, as well as being added to pancake mixes to make buttermilk pancakes.
Acidified buttermilk is a related product made by adding a food-grade acid (such as lemon juice) to milk. It can be produced by mixing 1 tablespoon of vinegar or lemon juice per 1 cup of milk and letting it sit until it curdles, about 10 minutes. Any level of fat content for the milk ingredient may be used, but whole milk is usually used for baking. In the process which is used to produce paneer, such acidification is done in the presence of heat.
One cup (237 ml) of whole milk contains 157 calories and 8.9 grams of fat whereas one cup of buttermilk contains 99 calories and 2.2 grams of fat. Buttermilk contains vitamins, potassium, calcium, and traces of phosphorus.
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