Milk Pasteurization and Sterilization

by Devy Nandya Utami

Liquid milk for consumption is mostly either pasteurized or sterilized. Pasteurization is a mild process, designed to inactivate the major pathogenic and spoilage bacteria in raw milk. Further improvements in shelf-life can be obtained by careful control of post-pasteurization contamination (PPC), by use of good quality raw milk and manipulations of the processing conditions. Eventually, however, such milks will spoil due to survival and growth of thermoduric bacteria or any post-pasteurization contaminants.

To keep milk for longer than few days at ambient temperature, it needs to be sterilized. The traditional process involves heating milk in a sealed container in the temperature range 114-120 Celcius degree for 20-30 minutes. More recently UHT processes have been introduced. These are continuous sterilization processes and involve temperatures in excess of 135 Celcius degree for times of greater than 1s, followed by aseptic packaging.

One of the main purposes of heat treatment is to reduce the microbial population in raw milk. Also, when milk is heated enzymes are inactivated, chemical reactions take place and there are changes in physical properties. Some important ones are a decrease in pH, precipitation of calcium phosphate, denaturation of whey proteins and interaction with casein, Maillard browning and modifications to the casein micelle.

The two most important kinetic parameters are the rate of reaction or inactivation at a constant temperature and the effect of temperature change on reaction rate. The heat resistance of vegetative bacteria and microbial spores at a constant temperatures is characterized by their decimal reduction time (D value), this is the time required to reduce the population of 90% or one decimal reduction (one log cycle). The number of decimal rductions (log N0/N) can be evaluated from:

log (N0/N) = heating time/D

where N0 = the initial population, N = final population

The two important points follow from this. Firstly, it is not possible to achieve 100% reduction. Secondly, for a spesified heat treatment, the final population will increase as the initial population increases.

Pasteurization

The first stage in the history of pasteurization between 1857 and the end of the nineteenth century might well be called the medical stage, as the main history in heat-treating milk came chiefly from the medical profession interested in infant feeding. In 1927, North and Park established a wide range of temperature-time conditions to inactivate tubercle bacillus.

milk pasteurization

HTST (high temperature-short time) continuous processes were developed between 1920 and 1927 and for some time the ability of the HSTS process to produce safe milk was questioned. One method of pasteurization produces as good bottle of pasteurized milk as does the other when good methods are used and when conditions are comparable. These included test of the following:

1. Raw milk quality (platform test)
2. Pasteurizability (survival of thermodurics)
3. Efficiency of pasteurization (pathogens and phosphatase)
4. Recontamination (thermophilic and coliform bacteria and the methylene blue test)
5. General bacteria quality, including organisms surviving pasteurization plus contaminating organisms (plate court)

Enzymes in raw milk may give rise to problems in pasteurized milk. However, it is unlikely that bacterial lipases and proteases, which are very heat resistant, will cause problems in pasteurized milks because of their relatively short shelf-life and refrigerated storage conditions.

In general, the lower the storage temperature, the better is the keeping quality. Raw milk is typically stored at 4 Celcius degree, temperatures in the cold chain are slightly higher and they are likely to be higher still in domestic refrigerators.

There is a requirement to further increase the shelf-life of pasteurized products, both for the convenience of the consumers and to provide additional protection against temperature abuse. However it is important to avoid the onset of cooked flavor, which would result from more severe pasteurization temperatures.

Sterilization

Sterilization of milk become a commercial proposition in 1894. Milk can be sterilized either in bottles or other sealed containers or by using ultra-high temperature (UHT) processing, which involves continuous sterilization followed by aseptic packaging.

Foods have been sterilized in sealed containers, such as cans, for over 200 years. Milk was originally sterilized in glass bottles sealed with a crown cork but more recently plastic bottles are used. The main aim is to inactivate heat-resistant spores, thereby producing commercially sterile product with an extended shelf-life.

Ultra-high temperature (UHT) offers some distinct advantages over in-container sterilization. Chemical reactions are less temperature sensitive so the use of higher temperatures, combined with more rapid heating and cooling rates, helps to reduce the amount of chemical reaction. There is also a choice of indirect heat exchangers for milk, such as plate or tubular types, as well as direct steam injection or infusion plants, all of which heat products at different rates and shear conditions.

For extended shelf-life and UHT products, aseptic packaging should be used of which a number are available. They are involve putting a sterile product into a sterile container in an aseptic environment. superheated steam has been used for sterilization of cans. Irradiation may be used for plastic bags.

Package should be inspected regularly to ensure that they are air-tight, again focusing upon those more critical part of the process. Sterilization procedures should be verified. The seal integrity of the package should be monitored as well as the overall microbial quality of packaging material itself. Rinsing, cleaning, and disinfecting procedures are also very important.

Reference:

Smit, Gerrit (editor). 2000. Dairy Processing: Improving Quality. Cambridge: Woodhead Publishing Limited. Improvements in the pasteurisation and steriliation of milk by M.J. Lewis, The University of Reading, UK.