Sweetened, thickened milk in a can. Keeps forever.
In-game description

Condensed milk is a food item in The Long Dark.

This can requires a can opener, hunting knife, or hatchet to be opened while retaining its full content. It is also possible to smash the can open, but this will result in losing some of its contents to spillage. It also replenishes a moderate amount of thirst, enough to top off the bar but not nearly as much as drinks such as sodas. It is essentially a straight upgrade from the Pinnacle peaches, offering a similar hydration bonus but for a much better weight-to-calorie ratio.

Contrary to its in-game description, condensed milk actually spoils very quickly when compared to other processed food.

Condensed milk is packaged as a product called Little Cow.

Food and drink
Canned Condensed milkDog foodPinnacle peachesPork and beansTomato soup
Animal meat Black bearMooseRabbitVenisonWolf
Fish Coho salmonLake whitefishRainbow troutSmallmouth bass
Plant Cat tail stalk
Miscellaneous Beef jerkyCandy barEnergy barGranola barMilitary-Grade MREPeanut butterSalty crackersTin of sardines
Water UnsafePotable
Soda OrangeGrapeSummit
Miscellaneous Herbal teaCoffeeEnergy drink