If you who enjoy your coffee with a splash of milk, to go without is frustrating to say the least. To avoid running into this problem, you may keep a carton or can of shelf-stable milk in your kitchen cabinets. Without any refrigeration, the milk can be poured right into coffee or cereal. But that begs the question: What's in shelf-stable milk? After all, Americans tend to think of milk as a fresh, perishable product that can spoil in a matter of hours. So how is it possible that some milk can sit out unrefrigerated for months on end without declining in quality?

Some cartons of milk become shelf-stable due to a processing and packaging treatment known as ultra-high temperature processing (UHT). According to the US Food and Drug Administration, UHT milk is created by by heating milk at or above 280°F for at least 2 seconds. The milk can undergo UHT processing either before or after it’s packaged to create an extended shelf life for the product. 

Although UHT milk is popular throughout Europe, it didn’t reach the US until 1993, when the company Parmalat started distributing its shelf stable milk. A carton of Parmalat whole milk contains just UHT milk and “Vitamin Ds,” (added to fortify the milk after some naturally occurring vitamins are killed off in processing). Parmalat’s UHT milk comes in a sealed carton, like chicken or vegetable broth, and can sit on the shelf of your pantry for up to six months (unlike milk powder, which can stay shelf stable for years). Of course, after it’s opened, the milk should be refrigerated and used within one week.

Another type of shelf-stable milk you may be familiar with is known as evaporated milk. Unlike Parmalat, or other brands of milk advertised simply as “shelf-stable,” evaporated milk isn’t plain milk in a can. Carnation, one of the most popular brands of evaporated milk explains on their website that the product is “milk with about half the water removed.” While it can be used straight from the can in recipes as a milk replacer to yield creamier porridge or baked goods, evaporated milk should be mixed with water if you want it to taste more like regular milk. In addition to concentrated milk, a can of evaporated milk contains disodium phosphate, an emulsifier; sodium ascorbate for vitamin C; cholecalciferol for vitamin D3; and carrageenan, a gelling and thickening agent extracted from seaweed). If you’re looking for a milk that stays fresh at room temperature for longer than UHT milk, evaporated is the answer—unopened, a can will stay fresh for about a year.

Both UHT milk and evaporated milk can be used in any recipe or method that calls for regular milk, but don’t expect the flavors to be indistinguishable from the real deal. According to a study in the Journal Diary of Science, the extreme heat needed to kill spores while making UHT milk can cause the Maillard reaction, which you might know as that unique toasty flavor that happens when baked good become golden brown. This can alter the smell and flavor of the milk. Due to its concentrated nature, evaporated milk is also a great deal creamier and richer than regular milk in flavor and bit thicker in texture.