The majority of our milk supply comes from extraction from cows during or right after pregnancy. Milk is a colloid (a liquid that contains a microscopic substance(s) distributed throughout another substance) of emulsified liquid butterfat, casein proteins, and calcium phosphate dispersed throughout a water solution. The reflection of light off of the casein proteins and calcium phosphate give milk its white appearance. Milk is classified based on its composition of butterfat. Whole milk contains 3.25% milk fat, reduced milk contains 2% milk fat, and skim milk contains less than 0.5% milk fat.
Milk has many culinary applications: Milk can moisten many dry doughs or batters, it can be used to replace part of the cream in many desserts to reduce fat content, and it can be drunk either cold or hot with flavorings such as chocolate, caramel and vanilla.
Evaporated milk is milk that has been evaporated to concentrate the milk flavors. This evaporation removed 60% of the water in the milk. Evaporated milk contains at least 25% milk solids (proteins, minerals, etc) and 7.5% milk fat. It is possible to add ½ cup of water to ½ cup of evaporated milk to get whole milk.
Condensed milk is milk that has been concentrated. It also has 60% of its water content removed, but it differs from evaporated milk in that it has sugar levels of 40-45% of its total weight. It also contains at least 28% milk solids and 8% milk fat.
Spoiled milk is caused by bacteria within milk. Bacteria in milk digest lactose as their source of energy. This digestion produces lactic acid, which lowers the pH of the milk. The increased acidity causes the proteins in milk to denature and curdle producing the clumpy appearance of milk. The sour taste of spoiled milk is caused by the lactic acid.