Dairy products, like milk, butter, cheese, various canned foods, juices, syrups, low-alcoholic beverages (like beer and wine), almonds, eggs, vinegar, and even water are pasteurized to inhibit the growth of bacteria and other harmful pathogens present in these products.

✦ The process of heating wine to increase its shelf life was used by Chinese since 1117. This was first registered in the diary Tamonin-nikki in Japan in 1568. ✦ After nearly two centuries, in 1768, an Italian scientist Lazzaro Spallanzani experimentally proved that heat efficiently killed bacteria, and hermetic sealing of foods further prevented their growth. ✦ Later, in 1795, Nicolas Appert, a French chef and confectioner, began experimenting with methods to preserve foods. He invented the canning process in which food is processed, sealed, and boiled in airtight containers to increase its shelf life. However, Appert boiled the food for a long time and at very high temperature, which altered its flavor and nutrition value. Also, alcohol (boiling point 78°C or 172°F) present in alcoholic beverages used to evaporate during this process. ✦ Finally, in 1864, renowned French chemist and microbiologist Louis Pasteur was called upon to study the frequent souring of wines. At that time, fermentation was thought to be a chemical process. Pasteur's research revealed that fermentation was carried out by yeast, a living organism, which appeared round and plump under the microscope. After studying spoiled wine, he found that it contained a different rod-shaped microbe which caused the wine to turn sour. He also discovered that only some microbes, especially bacteria, are involved in the souring of wines. He then determined the temperature and duration of heating wine to kill bacteria present in it. He found out that heating wine below its boiling point, at a temperature which kills the bacteria, and then rapidly cooling it inhibits their growth. He, thus, came to the rescue of the French wine industry. This pasteurization process was later applied to beer, vinegar, and milk preservation.

We must first understand the structure of bacteria to know how pasteurization affects it. ✦ Structure of Bacteria » Bacteria are unicellular organisms made up of a cell envelope, cytoplasm, and flagella. » The cell envelope is made up of a cell wall and plasma membrane, its main function being protection. » The cytoplasm is where all the life processes take place, and flagella are structures that a bacterium uses for locomotion, attachment, or as a means of protection. To regulate these life processes, certain enzymes are secreted by bacteria. » The bacteria are, thus, made up of proteins, certain fatty acids, ribosomes, water, chromosomes, nutrients, and enzymes. Spoiling of food is a result of the metabolic activities carried out by bacteria present in it. ✦ Pasteurization Process » Pasteurization involves heating of liquids at certain temperatures, which are generally below its boiling point, and then rapidly cooling these foods. Rapid cooling in this process is important because any bacteria that are not destroyed during heat treatment, are done so, during the cooling phase. » Heating of foods causes the enzymes secreted by bacteria to denature, i.e., break down and change their shape. This modification stops the functions of bacteria and helps preserve food. » Heating also damages the cell envelope of the bacterium. The shape of proteins and fatty acids that make up the cell wall is modified, and as a result, the cell wall weakens. The internal cytoplasm of the bacteria tends to expand due to heating. The weakened cell wall is unable to sustain the buildup of internal pressure, which causes the cell to burst. » There are certain thermoduric bacteria which are more resistant to heat. They are harder to kill. Hence, to keep them in check and prevent them from multiplying, these heat-withstanding bacteria are refrigerated to keep them under control.