Pasteurisation

Milk Pasteurisation basics


Introduction

The process of pasteurization was named after Louis Pasteur who discovered that spoilage organisms could be inactivated in wine by applying heat at temperatures below its boiling point.

Pasteurization - destruction of all pathogenic (harmful to health) Bacteria

Keeping Quality - to improve the keeping quality of milk and milk products.

MICROBIOCIDAL CONTROL MEASURES

Pasteurization and other heat treatments of milk such (sufficient time/temperature combinations) that they practically eliminate specific pathogens.

Examples of typical microbiocidal control measures include the following:
Centrifugation / bactofugation: The removal of microbial cells of high density from milk using high centrifugal forces. Most efficient against microbial cells of high density, especially bacterial spores and somatic cells
Sterilization: Heating at high temperatures for a time sufficient to render milk or milk products commercially sterile. Products manufactured using this method are microbiological stable at room temperature.
Competitive bacterea: Reduction in the number of undesirable microorganisms by lowering the pH. Competition for nutrients and production of antimicrobial substances (such as nisin, other bacteriocins and hydrogen peroxide). Usually in cheesemaking the microbiological control measure applied by choice of starter cultures.
Electromagnetic energy : Electromagnetic energy results from high voltage electrical fields, which alternate their frequency millions of times per second (< 108 MHz). Examples are microwave energy (thermal effect), rdio-frequency energy (non-thermal effects) or high electric field pulses (10 - 50 kV/cm, nonthermal effects). The treatment destroys cells by establishing pores in the cell walls due to the build up of electrical charges at the cell membrane.
High-pressure treatment: Application of high hydrostatic pressures to irreversibly damage the membranes of vegetative cells. Microfiltration: Removal of microbial cells, clumps and somatic cells by recirculation over a microfilter. Normally, a pore size of ~0.6-1.4 µm is sufficient to separate most bacteria. Synergy in combination with heat treatment.
Pasteurization: Heating milk and the number of any pathogenic microorganisms to a level at which they do not constitute a significant health hazard. (based on thermal death of Salmonella)
UV : The application of (on e.g. packaging material, equipment and water) high intensity broadband light pulses of wavelengths in the ultraviolet, visible and infrared spectrum (~20,000 times sunlight) to destroy microorganisms. Due to the inability to penetrate in-transparent substances, the technology is only effective against surfaces, for instance, in the removal of biofilm and can therefore prevent cross contamination
Ripening (ageing): Ccheese ripening and subsequent PH reduction kills harmful bacterea
Thermization: The application to milk of a heat treatment of a lower intensity than pasteurization that aims at reducing the number of microorganisms. A general reduction of log 3-4 can be expected. Microorganisms surviving will be heat-stressed and become more vulnerable to subsequent microbiological control measures.
Ultrasonic: The application of high intensity ultrasound (18-500 MHz) that cause cycles of compression and expansion as well as cavitation in microbial cells. Implosion of microscopic bubbles generates spots with very high pressures and temperatures able to destroy cells. More effective when applied in combination with other microbiological control measures. When applied at higher temperatures, the treatment is often referred to as "thermosonication". Warm sealed packaging: The application of heat (80 to 95 °C) to a solid end product in connection with the packaging process, for instance to maintain the product at a viscosity suitable for packaging. Such process can be done in a continuous flow system or in batch processes. The product is sealed at the packaging temperature and chilled for storage/distribution purposes afterwards. When combined with low pH in the product, e.g. below 4.6, the warm sealed product may be commercially sterile as any surviving microorganisms may not be able to grow. A supplementary microbiostatic control measures is to ensure adequate cooling rates of packaged products to minimize potential for Bacillus cereus growth.

PASTEURIZATION OF MILK

Process

Pasteurization can either be carried out as a batch operation ("batch pasteurization" or "LTLTpasteurization" (Low Temperature, Long Time)), with the product heated and held in an enclosed tank, or as a continuous operation ("HTST-pasteurization" (High Temperature, Short Time)) with the product heated in a heat exchanger and then held in a holding tube for the required time.

Currently, the most common method of pasteurization is by means of heat exchangers designed for the HTST process (High Temperature Short Time). This process involves heating of the milk to a certain temperature, holding at that temperature under continuous turbulent flow conditions for a sufficiently long time, to ensure the destruction and/or inhibition of any hazardous microorganisms that may be present. An additional outcome is the delay of the onset of microbiological deterioration, extending the shelf life of milk.

To save energy, heat is regenerated, i.e. the chilled milk feeding the exchangers is heated by the pasteurized milk leaving the pasteurization unit.

Pasteurization carried out in a batch-process involves the heating of milk placed in a container to a certain temperature for sufficiently long time to achieve equivalent effects as in the case of the HTST process. The heat can be supplied externally or internally in heat exchangers or within a pasteurizer. Due to the noncontinuous flow conditions, heating and cooling takes longer and will add to the effect (cumulative).

The minimum pasteurization conditions are those having bactericidal effects equivalent to heating every particle of the milk to 72 °C for 15 seconds (continuous flow pasteurization) or 63 °C for 30 minutes (batch pasteurization).

When changes in the composition, processing and use of the product are proposed, the necessary changes to the scheduled heat treatment should be established and a qualified person should evaluate the efficiency of the heat treatment.

For instance, the fat content of cream makes it necessary to apply minimum conditions greater than for milk, minimum 75 °C for 15 seconds. Formulated liquid milk products with high sugar content or high viscosity also require pasteurization conditions in excess of the minimum conditions defined for milk.

The products subjected to pasteurization should show a negative alkaline phosphatase reaction immediately after the heat treatment as determined by an acceptable method. Other methods could also be used to demonstrate that the appropriate heat treatment has been applied.

Alkaline phosphatase can be reactivated in many milk products (cream, cheese, etc.). Also, microorganisms used in the manufacture may produce microbial phosphatase and other substances that may interfere with tests for residual phosphatase. Therefore, this particular verification method must be performed immediately after the heat treatment in order to produce valid results. Note: Low residual alkaline phosphatase levels in heat-treated milk (below 10 µg p-nitro-phenol equivalent/ml) are taken as assurance that the milk has been correctly pasteurized and that it has not been contaminated by raw milk. However, although this measure is still considered as being the most appropriate method of verification, the factors listed below influence the residual levels and should be taken into account when interpreting the results: Initial concentration in milk: the "pool" of alkaline phosphatase present in milk varies widely between different species and within species. Typically, raw cow's milk shows an activity much higher than goats milk. As pasteurization results in a log reduction of the initial level, the post-pasteurization residual level will vary with the initial level in the raw milk. Consequently, different interpretation according to origin of the milk is necessary and in some cases, the use of alkaline phosphatase testing to verify pasteurization may not be appropriate. Fat content of the milk: Phosphatase is readily absorbed on fat globules, thus the fat content in the product subjected to pasteurization influence the result (typical concentrations in cows milk: skim 400 µg/ml; whole 800 µg/ml, and 40% cream 3500 µg/ml). Application of pre-heating: The level of alkaline phosphatase is decreased with heat, such as at temperatures typically applied in separation and in thermization.

STERILIZATION OF MILK

Sterilization is a microbiocidal control measure that can be obtained by various heat treatments, the most common and [validated] methods being UHT (Ultra High Temperature) processing in combination with aseptic packaging or In-container Sterilization. UHT treatment is a continuous operation that can either be carried out by direct mixing of steam with the product to be sterilized, or by indirect heating by means of a heat exchanging surface, followed by further aseptic processing (eventual) and aseptic packaging/filling. Thus the UHT plant are constituted by heating equipment in conjunction with appropriate packaging equipment and, eventually, additional treatment equipment (e.g. homogenization). In-container sterilization may be a batch or continuous process.

Thermal processes necessary to obtain commercially sterile products are designed to result in the absence of viable microorganisms and their spores capable of growing in the treated product when kept in a closed container at normal non-refrigerated conditions at which the food is likely to be held during manufacture, distribution and storage.

For products at risk of contamination with Clostridium botulinum such as certain composite milk products (as identified as likely to occur by a hazard analysis), the minimum thermal process should be established in consultation with an official or officially recognized authority. Where the risk of contamination with Clostridium botulinum is lower, alternative thermal processes may be established by an official or officially recognized authority, provided that the end products are microbiologically shelf stable and verified. The combined effects of two or more treatments may be considered additive provided they comprise a single continuous process. UHT treatment UHT treatment is normally in the range of 135 to 150 °C in combination with appropriate holding times necessary to achieve commercial sterility. Other equivalent conditions can be established through consultation with an official or officially recognized authority.
Validation of milk flow and holding time is critical prior to operation.
The products subjected to commercial sterilization must be microbiologically stable at room temperature, either measured after storage until end of shelf life or incubated at 55° C for 7 days (or at 30 °C for 15 days) in accordance with appropriate standards. Other methods could also be used to demonstrate that the appropriate heat treatment has been applied.

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