GERM THEORY THE PASTEURIZATION PROCESS OF MILK
A germ is a microorganism that causes food items to spoil. It was originally not identified where the germs came from until Pasteur discovered that a germ originates from other germs. He therefore suggested that if all germs were killed in a certain location, there would be no possibility for food to be spoiled. To kill such germs, Pasteur heated food in a process called pasteurization, it is a process that is still being used to preserve perishable foods especially milk. This discovery gave way to other discoveries by Pasteur, in the nineteen sixties and seventies. He discovered a parasite that attacked silk worms and later on a bacterium responsible for anthrax in domestic animals, Heldman (1997 pp 25-30). He discovered that the germs causing these diseases were capable of surviving in dead tissues and through air. When a carcass infected by the bacteria is not disposed off well, re-infection will occur when plants grow on it. The bacterium would grow in plants and then diffused to the air through spores. However, he suggested that such infection could be stopped through sterilization.
Milk is an essential animal product that has various nutritional values to the human body. This product is however very delicate and can be spoiled easily. Through the process of pasteurization, a process named after the founder Louis Pasteur, milk is preserved for a much longer time by the heating process that kills all the possible germs contributing to spoilage. Milk is heated and maintained at certain high temperature for a considerable time, this allows the heat resistant germs in the milk to die. For the destruction of pathogenic microorganisms, time and temperature has to be regulated. According to Ontario regulations of pasteurization, milk has to be maintained at 63° C for a minimum of thirty minutes or at 72° C for not less than sixteen seconds. After the completion of the process the milk would be tested to find out if all pathogens were destroyed, this is facilitated when milk tests negative for alkaline phosphates, Herbst (2006 pp 45-46). For freezing milk products like ice cream, it is required that the products be maintained at 69° C for a minimum of thirty minutes or at 80°C for not less than twenty five seconds.
There are two methods used to pasteurize milk, batch and continuous method. Batch method is the simplest process of preserving milk and milk products. This process consists of a vat pasteurizer that is made of a jacketed vat surrounded by steam or heating coils, Weeks (2007 p 28). Milk is heated in the vat and thereafter held at a certain temperature for a given period of time. This method is commonly preferred for ice crème due to its quality reasons other than for preservation. The process can be modified by allowing the milk to be separately heated before passing it into the vat.
Continuous method of pasteurization is the most appropriate and commonly used due its advantages of conserving time and energy. In this process, a pasteurizer known as high temperature short time (HTST) is used. A plate heat exchanger is used to complete the heating treatment; it is an equipment of grooved stainless steel plates that are fastened together in a frame. Gaskets are used as boundaries that prevent leakage; the heating medium for this method is either hot water or vacuum steam.
The high temperature short time processor operates when raw milk at 4° C is passed through a regenerator where it is warmed between 57 and 68° C. The heat from the regenerator comes from the hot pasteurized milk that flows in a counter current direction in the opposite side of the thin stainless steel plates. The raw milk that is still under suction goes through a timing pump that delivers it to the HTST system. The raw milk is heated to a temperature of 72° C by the hot water that continuously flows on the opposite sides of the plates, Gradle (2008, pp 38-43). After the milk is pasteurized, it flows into a holding tube where it is held for a minimum of sixteen seconds. The surface friction, timing pump and the length and diameter of the holding tube determine the maximum speed of the system.
To ensure that all particles of the milk are heated at the right temperature, the milk will be passed through a low diversion device from an indicating thermometer and a recorder controller. If the FDD assumes a forward flow, then the milk is properly heated but if it remains normal then the right temperatures have not been achieved. The unheated milk will pass through a diverted flow line of the FDD back to the constant level tank where the process is repeated. The well heated milk passes through the forward flow section of the FDD to the regenerator section where its heat is re-utilized in the raw product as it is cooled from 32 -9° C. The milk which is warmed is cooled to a maximum of 4°C on the sides of the thin stainless steel plates and then to a vacuum breaker in the HTST to a storage tank for packing.
The continuous method is the most economical process of pasteurizing milk. Less heat is used during the process and time used effectively, it is designed to ensure that the heat and cool air produced by both the raw and pasteurized milk are re-used in the process Herbs (2006 pp 47-48). The HTST system is self-contained with different equipments of measuring and regulating heat and temperature. It is designed in a way that no amount of heat or cool air escapes before it is utilized maximally. Through the flow diversion device, the milk that is well heated and that not heated well are separated into other stages for appropriate action.
Pasteurization is the safest method of preserving milk and milk products. The milk products preserved through this process can stay for a minimum of seven to sixteen days as long as correct storage measures are taken, Weeks (2007 pp 24-27). The product can be consumed safely without having to observe other safety measures. The maximum safety of the milk will however be determined by the packing procedures. When milk is being packed, right temperatures should be maintained and packages screened for safety. Once the packed milk is sealed and stored well, it will remain fresh for a stipulated period. Milk is pasteurized to not only kill the bacteria but also to improve and maintain the quality of the milk. This method has been medically approved to offer maximum protection to milk and other milk products.
Gradle H. Bacteria and the Germ Theory of Disease: BiblioBazaar, LLC, 2008 pp 38-43
Heldman D. Principles of Food Processing: Springer1997, pp 25-30
Herbst J. Germ Theory: Twenty-First Century Books 2006, pp 45-48
S. Microbes and Society: Jones & Bartlett Publishers, 2007, pp 24-28