Pasteurization is a process where many substances such as milk, are treated with controlled heating at temperatures well below boiling (in honour of its developer Louis Pasteur). Milk, beer and many other beverages are now pasteurized. Pasteur examined the spoiled wine and detected the presence of microorganisms like bacteria which were responsible for the production of lactic acid and acetic acid fermentations which resulted in the spoilage of wine. He then discovered that brief heating at 55 to 60°C would destroy these microbes and preserve wine for long periods. Hence, pasteurization does not sterilize a beverage or milk but kills any pathogens present and slows spoilage by reducing the level of non-pathogenic spoilage microbes. Milk in older methods of pasteurization was held at 63°C for 30 min. Now, mostly two methods are used, flash pasteurization or high temperature short-term (HTST) pasteurization, which consists of quick heating t about 72°C for 15 sec and then rapid cooling. The other method used in dairy industry is ultrahigh-temperature (UHT) sterilization, where milk and milk products are heated at 140 to 150°C for 1 to 3 sec. The products pasteurized by this method needs no refrigeration and can be stored at room temperature for about 2 months.

Dry heat sterilization can also be used on many objects in the absence of water. The items to be sterilized are placed in an oven at 160 to 170°C for 2 to 3 hours. Oxidation of cell constituents and denaturation of proteins results in the death of microbes. Most laboratories sterilize glass Petri dishes and pipettes with dry heat. This method though is not suitable for heat sensitive materials like many plastic and rubber items.

Low temperatures

Another convenient method to inhibit the growth and reproduction of microorganisms is to use lower temperatures like freezing or refrigeration. Mostly this method of control is used in food microbiology. Freezing items at -20°C or lower stops microbial growth because of the absence of liquid water, and the ice crystal disruption of cell membranes at this temperature. This method is also used for long term storage of microbial samples in the laboratories at -30 to -70°C in the form of glycerol stocks. This method of control at low temperatures slows microbial growth and reproduction, but does not halt it completely. Fortunately, most pathogens, are mesophilic and do not grow well at low temperatures around 4°C. Thus refrigeration is a good technique only for short-term storage of food and other items.