Pasteurization is a process whereby fluids such as wine and milk are heated for a predetermined time at a temperature that is below the boiling point of the liquid. The treatment kills any microorganisms that are in the fluid but does not alter the taste, appearance, or nutritive value of the fluid.
The process of pasteurization is named after the French chemist Louis Pasteur (1822–1895), who is regarded as the founder of the study of modern microbiology. Among Pasteur's many accomplishments was the observation that the heating of fluids destroys harmful bacteria .
The basis of pasteurization is the application of heat. Many bacteria cannot survive exposure to the range of temperatures used in pasteurization. The energy of the heating process is disruptive to the membrane(s) that enclose the bacteria. As well, the bacterial enzymes that are vital for the maintenance of the growth and survival of the bacteria are denatured, or lose their functional shape, when exposed to heat. The disruption of bacteria is usually so complete that recovery of the cells following the end of the heat treatment is impossible.
The pasteurization process is a combination of temperature, time, and the consistency of the product. Thus, the actual conditions of pasteurization can vary depending on the product being treated. For example heating at 145°F (63°C) for not less than 30 minutes or at 162°F (72°C) for not less than 16 seconds pasteurizes milk. A product with greater consistency, such ice cream or egg nog, is pasteurized by heating at a temperature of at least 156°F (69°C) for not less than 30 minutes or at a temperature of at least 176°F (80°C) for not less than 25 seconds.
Particularly in commercial settings, such as a milk processing plant, there are two long-standing methods of pasteurization. These are known as the batch method and the continuous method. In the batch method the fluid is held in one container throughout the process. This method of pasteurization tends to be used for products such as ice cream. Milk tends to be pasteurized using the continuous method.
In the continuous method the milk passes by a stack of steel plates that are heated to the desired temperature. The flow rate is such that the milk is maintained at the desired temperature for the specified period of time. The pasteurized milk then flows to another tank.
Several other more recent variations on the process of pasteurization have been developed. The first of these variations is known as flash pasteurization. This process uses a higher temperature than conventional pasteurization, but the temperature is maintained for a shorter time. The product is then rapidly cooled to below 50°F (10°C), a temperature at which it can then be stored. The intent of flash pasteurization is to eliminate harmful microorganisms while maintaining the product as close as possible to its natural state. Juices are candidates for this process. In milk, lactic acid bacteria can survive. While these bacteria are not a health threat, their subsequent metabolic activity can cause the milk to sour.
Another variation on pasteurization is known as ultra-pasteurization. This is similar to flash pasteurization, except that a higher than normal pressure is applied. The higher pressure greatly increases the temperature that can be achieved, and so decreases the length of time that a product, typically milk, needs to be exposed to the heat. The advantage of ultra-pasteurization is the extended shelf live of the milk that results. The milk, which is essentially sterile, can be stored unopened at room temperature for several weeks without compromising the quality.
In recent years the term cold pasteurization has been used to describe the sterilization of solids, such as food, using radiation. The applicability of using the term pasteurization to describe a process that does not employ heat remains a subject of debate among microbiologists.
Pasteurization is effective only until the product is exposed to the air. Then, microorganisms from the air can be carried into the product and growth of microorganisms will occur. The chance of this contamination is lessened by storage of milk and milk products at the appropriate storage temperatures after they have been opened. For example, even ultra-pasteurized milk needs to stored in the refrigerator once it is in use.
See also Bacteriocidal, bacteriostatic; Sterilization
Pasteurization is a process that uses heat to kill microorganisms. While the French chemist Louis Pasteur (1822-1895) first applied this process to wine, pasteurization is more usually recognized as being a treatment for milk.
Pasteur was called upon to tackle some of the problems plaguing the French wine industry. Of special concern was the spoiling of wine. This caused great economic loss and tarnished France's reputation for fine vintage wines. Vintners wanted to know the cause of l'amer, a condition that was destroying the best burgundy wines. Pasteur looked at wine under the microscope and noticed that when aged properly, the liquid contained little spherical yeast cells. When the wine turned sour, however, there were numbers of bacterial cells producing lactic acid. Pasteur suggested heating the wine gently at about 120 degrees. This killed the bacteria that produced lactic acid and let the wine age properly.
A Second Revolution
Pasteur's book Etudes sur le Vin ("Studies Regarding Wine") was published in 1866. It was a testament to two of the researcher's great passions—the scientific method and wine. Pasteur suggested that greater cleanliness was needed to eliminate bacteria; this could be done with heat. Some wine-makers were aghast at the thought, but doing so solved the industry's problem.
Milk Joins the Revolution
With the practice of heating wine to kill bacteria firmly established, researchers turned to other liquids like milk. The idea of pasteurization was born. Several decades later in the United States, the pasteurization of milk was championed by American bacteriologist Alice Catherine Evans (1881-1975), who linked bacteria in milk with the disease brucellosis (a type of fever found in different variations in many countries). Pasteurization is now applied to most liquid food products produced commercially.
Pasteurization , a process discovered by Louis Pasteur (while trying to inactivate spoilage organisms in beer and wine), occurs when a product is heated to a specific temperature for a specified length of time. This process is now applied to a wide array of food products, such as milk, fruit juice, cheese, and water. Milk is heated to 145°F (63°C) for thirty minutes (or to 160°F [71°C] for fifteen seconds) and then rapidly cooled to 50°F (10°C) for storage. In developing countries, heating water to 149°F (65°C) for six minutes will kill enough contaminates to make the water safe to drink. Pasteurization protects consumers from harmful pathogens such as Mycobacterium tuberculosis and Coxiella Burnetii in milk, and pasteurized products benefit from longer shelf life.
see also Food Safety.
Diane L. Golzynski
In flash pasteurization, the product is held at a higher temperature, but for a shorter time, so that there is less development of a cooked flavour.
Pasteurization of milk destroys all pathogens, and although it will sour within a day or two, this is not a source of disease. It is achieved either by heating to 63–66 °C for 30 minutes (holder method), followed by immediate cooling, or (the high‐temperature short‐time process) heating to 71 °C for 15 seconds.