Condensed milk is a highly shelf-stable milk product made by evaporating much of the water from milk to create a thick, syrupy liquid, and adding sugar to it before canning the milk in a sterilized can. When well made, a can can last unopened on the shelf for up to two years. A related dairy offering, evaporated milk, is made in a similar fashion, but no sugar is added. Many grocery stores carry both types of milk for use as beverages and in baking projects.
The inspiration for condensed milk came in the mid 1800s, when it was difficult to obtain milk in regions isolated from dairies. Milk also tended to be collected in unclean conditions, and it could sometimes be unsafe to drink. Gail Borden, an inventor, experimented with the idea of removing some of the water content from milk and adding a stabilizer before canning the result, in the hopes that he could develop a shelf-stable milk product which could be easily transported and sold. In 1864, Eagle Brand Consolidated Milk began to be produced.
Initially, condensed milk was intended to be used as a beverage, either consumed plain or thinned with water. People who did not have access to fresh milk would have used condensed or evaporated milk instead, and many companies began to supplement their products to ensure that they would be nutritionally valuable to consumers. As pasteurization and refrigeration became more widespread, fresh milk replaced condensed as a beverage, but some recipes still call for the product, and it can also be useful for camping trips.
To make condensed milk, high quality milk is first pasteurized to remove any potential contamination. The pasteurized milk is transferred to a sealed evaporator in a closed pipe system and subjected to low pressure, which lowers the boiling point of the milk. As a result, a lower heat can be used to remove as much as 60% of the water content of the milk, which is homogenized, stabilized, sweetened, and canned in sterile containers. Sugar helps to fight bacteria, making the resulting milk particularly shelf-stable.
Some consumers confuse condensed milk and evaporated milk, since the two products are very similar. As a general rule, evaporated milk is unsweetened, while the condensed variety has been made with sugar. Some old recipes may specify the use of “sweetened condensed milk” to ensure that cooks do not use evaporated milk. This milk is often used in beverages as well, adding a thick texture and sweet milky flavor to things like Thai iced tea.