Occasionally an older recipe for bread, pancakes, or other baked goods will call for “sour milk” in the ingredients. The date sour milk was first used is unknown, but cookbooks and other records show that it was commonly used in the colonial United States in the 1700s. Back in those days without refrigerators or pasteurization fresh milk spoiled rather quickly. The bacteria in the milk would multiply and convert lactose sugar into lactic acid. This would give the milk a sour, acidic taste. The lactic acid also caused the milk proteins to coagulate, causing the milk to turn thick and lumpy. Sour milk was often mixed with a base such as potash or baking soda to help baked goods rise. The sour taste and thickness of the milk also added richness to the finished product.

There are two ways to make sour milk. The first is simply to let regular milk sit beyond its selling date. (This does not appear to be entirely accurate, see heppigirl's addition below). The milk will slowly become sour and thick as the bacteria produce more lactic acid. Interestingly, using spoiled, sour milk in baked goods does not appear to pose a health hazard. Many people apparently swear by this method but I haven’t worked up the nerve to try it. I prefer the second, simpler method where an acidic liquid is added to fresh milk. To make your own sour milk, put a tablespoon of lemon juice or plain vinegar into a liquid measuring cup. Add milk until you have a cup of liquid. Let the mixture sit at room temperature for about 15 minutes. After this time the milk should be somewhat thick and lumpy when stirred and taste mildly sour.

Buttermilk and sour milk are often used interchangeably in recipes and I regularly substitute sour milk when I didn’t remember to purchase buttermilk. Sour milk tends to be thinner and have a milder acidic taste than buttermilk, which slightly alters the taste and texture of the finished baked good.

heppigirl says : milk that goes off in the fridge is different to sour milk. My folks eat sour milk with a plate of boiled potatoes - it's a Polish thing I guess, but you are supposed to pour the fresh milk in a large bowl and leave it on the bench top (in summer for example) until it just turns, then stick it in the fridge. The first lot is never that good, so you then use a bit of the first lot as a starter culture for the next batch which tastes much better :)

jmpz says : in the Caribbean there is a dessert made entirely of sour milk, sugar, eggs and cinnamon. it is a granular sweet concoction that is extraordinarily tasty...

Let's start by defining some terms. There are two kinds of milk, pasteurized milk and 'raw milk'. Raw milk is the milk as it comes out of the cow (although the cream is usually removed). In many parts of America it is illegal to sell raw milk because it is too delicious and healthy it can carry disease. In most countries the milk that you buy at the supermarket will be pasteurized, or even ultrapasteurized milk. There are downsides to consuming pasteurized milks, but that's another node.

Most of us have experienced pasteurized milk that has gone sour, and wish that we hadn't. It is essentially rotten milk, and it is undeniably disgusting and inedible. It will make you sick. Needless to say, that is not what this node is about.

Some of us have had the fortune to be introduced to raw milk that has gone sour, preferably with a known and delicious culture of healthy bacteria. This milk is healthy and (yes really) delicious. The most common cultures are of the genus Lactobacillus, a probiotic that can improve the health of the human gastrointestinal tract, and which are also found in products like kefir and yogurt. Americans would generally recognize all forms of sour milk as 'buttermilk', although if you are lucky you may also find them under the labels fermented milk or cultured milk. Sour milk can have many different flavors, but all types are tangy and strong and sour; some are sweeter, some are more 'fermented' and tangy; in my experience, most are not as strong and thick as the buttermilk found in American supermarkets.

There are still many parts of the world in which sour milk is the norm, and you have to specify 'sweet milk' if that's what you want (and it may be mixed up from powder unless your hosts have recently done the milking). A good culture can start souring milk in a matter of hours, and it is not uncommon to add the remains of the last batch of sour milk into the sweet milk as soon as you're done milking. The benefits of sour milk are many, and do indeed include taste. In areas without refrigerators, sour milk can be stored at room temperature for a couple days with no ill effects, and its life may be extended even more if you have a cool place in which to store it. And perhaps most importantly, because the bacteria break down the lactose, it is easier to digest than is sweet milk.

The downside to non-pasteurized sour milk is that any bacteria present in the milk, which might include some big nasties like tuberculosis, diphtheria, and typhoid, are still present in the soured milk. If you have healthy and vaccinated cows and milk them in a sanitary fashion there is not much of a risk of catching anything, but you do have to trust your milk source.

Sour milk is good plain, on cornbread (dried or fresh), in grits or any sort of hot breakfast cereal, and, if all else fails, over dry biscuits. If you have a sweet tooth or are a bit queasy when faced with sour milk, you can add sugar (I highly recommend at least trying sugar and sour milk over dry cornbread). There are thousands of regional recipes that use sour milk in any way that you can imagine; I recommend trying as many as you can.

Sour milk may also refer to sweet milk that has been 'soured' with addition of an acid, usually citric acid (in the form of lemon juice, for example) or acetic acid (in the form of vinegar). If you need sour milk for baking, it does not matter what kind of sour milk you use, and you can 'sour' milk by adding one tablespoon of the aforementioned acidic substances to one cup of milk.

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