Pasteurization has been used for many years as a way to keep milk safe. In ancient times, many people tried to have their own dairy animals. They would produce and consume the product themselves, there was no mass distribution. In more modern times, urbanization of our populace led to people being removed from food production and a need to distribute food much more widely. This led to quite different timelines between the point of production and consumption. Milk is a highly perishable food and will spoil quite rapidly; it also has potential to carry pathogens, disease causing organisms that are different from those that cause spoilage. When Louis Pasteur developed the principles of food microbiology, he also determined that heat could be used to inactivate pathogenic organisms. Based on his early work, heat treatments to inactivate pathogenic bacteria were introduced as a way to keep milk, and other related dairy products, safe. At the same time, refrigeration technology was being developed and these two technologies, pasteurization and refrigeration, started to be used in tandem to keep milk safe and extend the shelf life. These to factors, safety and shelf life, need to be distinguished from each other for the following discussion.