With unpasteurized cream (we spell it with a z in this part of the world, sorry), you can't not make sour cream. The bacteria naturally present in cream causes a natural souring.

There is more than a little confusion over history and nomenclature, though. The French crème fraîche, the Russian smyetana, the Polish smietana are all sour creams, the result of letting cream sour naturally. A combination of bacteria naturally present in the cream and in the air produced a certain amount of acid and the buttery or nutty taste associated with these lightly soured creams.

Theoretically the term sour cream encompasses all these products and more. In practice, though, the term sour cream (at least in this part of the world) now refers to a manufactured cream product that is around 20% fat, that is pasteurized, inoculated and given time to sour and thicken, and pasteurized again to stop the fermentation. It has around .8% acidity.

Crème fraîche is 30% to 40% fat, and may be between .2% and .8% acid, so is likely to have a milder flavor than sour cream. Nowadays most crème fraîche in France is produced artificially with the pasteurization process. Rennet may be added to the production of crème fraîche in the United States to create a thicker product.

Smyetana and smietana are combinations of fresh cream and soured cream and have a lower acid content – and therefore milder taste – than sour cream.

Now, not all cream is the same, or at least not all bacteria is the same. We have left very thick unpasteurized cream out at room temperature for between 12 and 24 hours, and gotten a thickened cream with a mildly sour taste. Once when we did so, however, the cream tasted something like dirt (well, what we imagine dirt to taste like).

Commercially produced sour cream is manufactured to set standards, with the species of bacterial used tightly controlled. We have found homemade sour cream to be both thinner and with a lighter flavor than the commercial product, and also occasionally having off flavors. So in a narrow sense, we were wrong to suggest that you can make modern-day sour cream as long as you have access to unpasteurized cream. You can make a sour cream, but it will not be as thick or potent.

So our advice – or recipe – is to use a high-butterfat cream (30% or higher) that tastes good already. You can add a little strained lemon juice to boost the acidity. Room temperature in Maine may not be the same as room temperature in India, so what may have taken 24 hours here may take four or five hours where you are. Check it periodically until you get a sense for how long the souring process takes. Then use the cream quickly. Putting it in the refrigerator is going to retard the rate of souring/spoiling, but without pasteurization, it will shortly reach the point of no return.

A benefit of using a high-fat cream is that it will not curdle if you cook with it, while a commercial product, with a lower fat content, will.

(See how clever we were to bury the phrase "we were wrong" in the middle of a paragraph toward the bottom of the article? There's a lesson to be learned here, children….)