The definition of pasteurization is as follows:
"The heating of every particle of milk or milk product to a specific temperature for a specified period of time without allowing recontamination of that milk or milk product during the heat treatment process."
The process of pasteurization is performed for two main reasons. Firstly, the prime purpose of pasteurization is to destroy potentially pathogenic microbes present in milk thus rendering the milk or milk product safe for human consumption. The secondary purpose of pasteurization is to extend the shelf life of the milk or milk product. It is not only organisms but also enzymes, which are involved with spoilage, and these are also inactivated by pasteurization.
The Pasteurization Marketing Ordinance (PMO) is a regulatory body founded by the US Public Health Service, in conjunction with the US Food and Drug Agency (FDA) and other agencies and institutions. It is the governing body that states limits and compliance standards for pasteurization in the United States of America. It should be pointed out that this is the accepted world standard in regard to pasteurization. The PMO has given the following statement regarding health reasons for pasteurization.
The public health value of pasteurization is unanimously agreed upon by health officials. Long experience conclusively shows it value in the prevention of disease which may be transmitted through milk.
Pasteurization is the only practical, commercial measure which, if properly applied to all milk, will destroy all milk-borne disease organisms.
It has been demonstrated that the time-temperature combinations specified by this ordinance, if applied to every particle of milk, will devitalize all milk-borne pathogens. Compilations of outbreaks of milk-borne disease by the US Public Health Service over many years indicate that the risk of contracting disease from raw milk is approximately 50 times as great as from milk labeled "pasteurized"
The extent to which micro-organisms and enzymes are inactivated is directly proportional to the degree (temperature) and duration (time) of pasteurization. Thus the higher the temperature and the longer the duration the greater the effect of killing micro-organisms and inactivating enzymes.
The two most heat stable pathogens found in fresh milk are Mycobacterium tuberculosis and Coxiella burnetti. The first being the bacterium causing tuberculosis and the second being the causative bacterium responsible for Q fever. The minimal degree (temperature) and duration (time) requirements for pasteurization of milk are based on thermal death time studies on these two organisms. To ensure the safety of all dairy products these temperature and time combinations are highly regulated. For milk and colostrum the temperature and duration of pasteurization has been established by the PMO and is as follows:
Milk Pasteurization - PMO Guidelines
As must be apparent the higher the temperature utilized the shorter the time required to destroy all potentially pathogenic microbes. It should be pointed out that for diary products containing 10% or more fat, or if they contain added sweeteners the PMO guidelines stipulate that the specified temperature shall be increased by 3C (5F).
The process of pasteurization has since it's inception undergone many improvements. In the beginning milk and other fluids were simply boiled and unfortunately many of the nutrients and bio-active substances were adversely affected. For a growing number of milk and colostrum manufacturers the preferred method of sanitizing milk and colostrum is by flash pasteurization - also know as High Temperature Short Time (HTST) pasteurization. By incorporating this method to sanitize or kill microbial contamination the biological integrity of the milk and colostrum is maintained and any loss of bioactivity is minimized.
Flash pasteurization is performed using a continuous plate and tube or heat exchange system. This counter current method ensures that the temperature of the product is elevated to the exact required temperature: held there for the required time and then swiftly lowered in the shortest possible time. Thus the milk and colostrum is heated to 72C in the shortest time possible - held at that temperature for a minimum of 15 seconds and immediately cooled. In this way complete destruction of microbial growth and inactivation of enzymatic activity is achieved. Microbiological testing is performed to assure sanitation and product safety. Of significant importance is the fact that this method ensures that maximal biological activity and nutritional benefits are retained.
The thermal stability of immunoglobulin preparations has long been the focus of many research studies. It has been observed that flash pasteurization incorporating high temperature short time (HTST) - 72C for 15 seconds has a minimal effect on protein denaturization and reduction in bioactivity. The following exemplifies the thermal stability of IgG in flash pasteurization.
Thermal Destruction of Immunoglobulins During Processing of Colostrum
Pasteurisation is a critical quality parameter used during the manufacture of dairy products. Data collected during processing has shown that minimal loss due to denaturization of immunoglobulins occurs when colostrum is pasteurized (Table 1.).
Table 2. D-Value of IgG in Bovine Colostrum
Temperature (ºC) D-Value (sec)
D-value = time required to inactivate 90% of the IgG
If a second pasteurization step is a requirement of further processing, research has shown that 65ºC for 30 minutes has no effect on the activity of IgG. Previous research conducted at 72ºC for 15 seconds found a reduction in IgG activity of approximately 0.5-10%. However, this reduction is likely to be dependent on the precise system into which the Colostrum is incorporated and the interactions of the components within this system. For example, salt causes IgG to become less susceptible to denaturization and aggregation during heat treatment. Further in a recent investigation it has been reported that IgG values will be only very slightly affected by HTST pasteurization at 72C for 15 seconds. In fact what was observed was an approximate drop of 2% in quantified IgG value which is well within the standard error of the assay used in the measurement.
Further antigen binding studies have shown that the bioactivity of antibodies is little affected by the process of flash pasteurization. In determining D-values , the time required to reduce the antigen binding activity of IgG (antibody) by 90% was determined (Table 2.)
Table 2. D-Values IgG antibody/antigen binding in bovine colostrums
Temperature (ºC) D-Value (seconds)
Thus modern dairying technology has developed to the point where the process of pasteurization, though effective in neutralizing any potential microbial health hazard, has very little negative effect on the bioactivity, biofunctionality, and nutritional composition of the various components found in milk and colostrums.
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