White Milk

I believe there may be a slight confusion in the name, as there is no food item called "white milk" that is unique to restaurants in the USA. However, I believe you might be referring to "buttermilk," which is a common ingredient and occasionally a standalone offering in some restaurants.

Buttermilk is a fermented dairy beverage that is a byproduct of churning butter from cultured cream. Traditionally, buttermilk was the leftover liquid from this process. Today, most commercial buttermilk is made by adding lactic acid bacteria to pasteurized non-fat or low-fat milk. This gives it a tart, slightly thickened texture and a tangy flavor.

In the United States, buttermilk is often lower in fat compared to regular whole milk. It is commonly used in baking, particularly in recipes like pancakes, biscuits, and muffins, to provide a tender crumb and subtle tang.

As a standalone drink, buttermilk can be consumed chilled and plain or sometimes sweetened and flavored, especially in Indian and Middle Eastern cuisines. As a drink, it may provide nutritional benefits such as being a source of probiotics, calcium, and vitamins.

To summarize, the food item you referred to as "white milk" is likely buttermilk, which is a fermented dairy beverage often found in restaurants and grocery stores across the USA. It has a tangy flavor and is commonly used in baking, as well as being consumed as a standalone drink.