Louis Pasteur, a French scientist, found a lasting solution to the wine spoilage problem. By heating wine to temperatures below its boiling point for a specified period, the product did not spoil for as long as there was no re-contamination.
His famous heat treatment method became known as pasteurization method after his own name.
What is milk pasteurization?
We can define milk pasteurization as the process of heating milk (or milk product) to a predetermined temperature for a specified period without re-contamination during the entire process.
The predetermined temperature usually depends on the heat resistance of spoilage microorganism that the pasteurization program is targeting to destroy.
Proper pasteurization is necessary for the following reasons:
- There is a public health concern since the public is going to consume the product. Pasteurization kills all the pathogenic bacteria that may cause infections to consumers.
- There is need to ensure that the product can keep for longer periods without expensive storage equipment. Pasteurization will eliminate spoilage bacteria and enzymes and extend the shelf life of the product.
The amount of heat and the length of time used in pasteurization depends on the Thermal Death Time of the target microorganism. The minimum combination should target the most resistant pathogen such as Coxiellae burnettii.
Different microorganisms have different D-values. However, these D-values follow a negative slope when plotted on a graph.
The video below illustrates how to derive different D-values for different microorganisms.
Milk pasteurization programs
Broadly, pasteurization can be categorized as either batch or continuous. The choice of the pasteurization method depends on several factors, which may not be limited to:
- Intended purpose of the pasteurized milk
- Access to sophisticated equipment
- Volume of milk to be pasteurized
Whatever the case, one can choose to carry out normal pasteurization or ultra pasteurization. Normal pasteurization will preserve milk for about two to three weeks while ultra pasteurization will preserve milk for even up to one year.
Different types of thermal processing methods
- Thermization: Heat the milk to between 57°C to 68°C and hold for 15 minutes. Thermization targets pathogenic bacteria while leaving the good bacteria in the product. The low temperatures do not alter the structure and taste of the milk.
- Batch pasteurization: Also known as low temperature long time (LTLT) pasteurization. Heat the milk to 63°C for 30 minutes. The extendend holding time causes alteration in the milk protein structure and taste.
- Flash pasteurization: also known as high temperature short time (HTST) pasteurization. Heat the milk to between 72°C to 74°C for 15 to 20 seconds. Targets resistant pathogenic bacterial spores (Clostridium botulinum spores).
- Ultra-high temperature (UHT) pasteurization: Heat the milk to between 135°C to 140°C for 2 to 4 seconds. The extreme heat targets Coxiella burnetii, which causes Q-fever. The heat kills all the vegetative forms of bacteria and the milk can survive for 9 months.
- Canned sterilization: This is a wet treatment of canned milk products in an autoclave/specialized treatment chambers. Heat to between 115°C to 121°C for 10 to 20 minutes.
1. High Temperature Short Time (HTST) pasteurization
This type of pasteurization is also known as flash pasteurization. Flash pasteurization involves heating milk to 71.7°C for 15 seconds to kill Coxiella burnetii, which is the most heat resistant pathogen in raw milk.
Since it is technically impossible to bring the milk to that exact temperature, it is always safe to work with a range of temperatures. To be safe, you can heat the milk to between 72°C to 74°C for 15 to 20 seconds. This will ensure that the milk is heated uniformly to the required temperature.
This method is most suitable in continuous pasteurization systems.
Flash pasteurized milk will keep for between 16 and 21 days. For commercial reasons, some manufacturers intentionally reduce the number of days to push the products out of the shelves.
How HTST pasteurization works
A standard milk pasteurization system consists of the following parts:
- Balance tank: maintains a constant head for the incoming milk
- Milk feed pump: creates suitable pressure that is necessary for efficient flow
- Flow control system: ensures that sufficient amount of fluid is in the conduits at any given time
- Filters and clarifiers: removes dirt from the products
- Homogenizer: divides fat globules into micro globules to avoid fat separation in standing milk
- PHE with regeneration, heating, holding and cooling sections: facilitates efficient pasteurization
- Flow diversion valves: Ensures that all the conditions for pasteurization have been met before the milk asses through.
- Instrumentation and control equipment: Increases system efficiency and reliability
- Peripheral sources of utilities such as steam, air, and water: Provides the necessary utilities for heating, cooling, and pressurized environments
- Conduits/piping system: Facilitate flow of milk and utilities from one point to the next
Each of the pasteurizer sections has been designed to increase the efficiency of the PHE.
Here’s how milk moves through the PHE for an effective pasteurization
- Chilled milk from the bulk milk tank at 4°C moves into the balance tank from where it is pumped into the regeneration section of the PHE.
- At the regenerative heating section of the PHE, chilled milk receives heat from the already pasteurized milk leaving the system. Heat exchange occurs across the PHE plates in a counter current motion of the two fluids of different temperatures (the video below illustrates this motion). The regenerative heating raises the temperature of milk to about 40°C to facilitate easy standardization. Heating then continues to 60°C to facilitate easy homogenization of the fat globules.
- After clarification, standardization, and homogenization, milk passes into the heating section where milk exchanges heat with steam across the PHE plates. The steam heats the milk to over 72°C, which is the perfect HTST pasteurization condition.
- Once the milk has attained the pasteurization temperatures, it moves into the holding tubes. The length of these tubes have been calibrated with the flow rate to ensure that the milk stays at that temperature for at least 16 seconds. This time is sufficient to destroy the target pathogen according to the D-values (outlined in the first video above.)
- If the milk fails to attain the required temperatures, the flow diversion valve diverts its flow back to the heating section to ensure that the temperatures are sufficient to kill all the target pathogens and their spores.
- Once the milk is fully pasteurized, it moves back to the regenerative heating section to raise the temperatures of the incoming chilled milk. In the process, the temperatures of the outbound pasteurized milk drops to about 32°C.
- The pasteurized milk then moves to the cooling section of the pasteurizer where chilled water (or PHE coolant) further lowers its temperatures to 4°C.
Advantages of regenerative heating
- Utilization of the incoming chilled milk to cool the outgoing hot pasteurized milk increases the efficiency of the PHE.
- A smaller amount of heat energy is required to heat the milk to pasteurization temperatures since the heating does not start from 4°C of the chilled milk.
- Reduces the amount of time required to pasteurize milk.
Note: When starting the process of pasteurization in the PHE, milk is circulated in the heating section until it attains the required temperatures before regenerative heating begins.
Video illustration of how the PHE works
2. Low Temperature Long Time (LTLT) pasteurization
Here, the temperatures used for pasteurization are reduced to 63°C and held for 30 minutes. The prolonged holding period alters the structure of the milk proteins making it better suited for making yogurt.
This method is best for batch pasteurization where the milk is held in a jacketed vat for effective pasteurization.
3. Ultra High Temperature (UHT) Pasteurization
This is a completely closed pasteurization method. The product is never exposed even for a fraction of a second during the entire process.
It involves heating milk or cream to between 135°C to 150°C for one to two seconds, then chilling it immediately and aseptically packaging it in a hermetic (air-tight) container for storage.UHT milk can keep for nine months without refrigeration. Click To Tweet
Despite the risk of Millard browning, UHT pasteurization remains the most popular milk preservation method for safe and stable milk.
Disadvantages of high temperature pasteurization
- There is a possibility of alteration of milk proteins. This can affect the properties of such milk when used to make other food products.
- High temperatures inactivate the enzymes that protect the product increasing the risk of spoilage.
- Elevated temperatures cause Maillard reaction, which discolors the product making it undesirable to consumers.
- High temperatures alter the protein structure and imparts a cooked flavor to the milk.
Application of high temperature pasteurization
Pasteurizing fluid milk
You can heat the milk to 63° C for not less than 30 minutes (low temperature long time pasteurization). Alternatively, heat the milk to 72° C for not less than 16 sec (high temperature short time pasteurization) or equivalent.
These temperature-time combinations have been proven to be sufficient for the destruction of pathogens and the enzyme phosphatase. A negative test result for the alkaline phosphatase test confirms the efficacy of pasteurization.
Frozen dairy dessert mix pasteurization
Very many frozen dairy products exist in the market. When pasteurizing ice cream or ice milk, heat the product to at least 69° C for not less than 30 min or 80° C for not less than 25 sec. Any other time-temperature combinations must be approved (e.g. 83° C/16 sec).
Pasteurization of enriched milk products
Milk based products with 10% butterfat or higher, or added sugar (e.g. cream, chocolate milk, etc) should be heated to 66° C/30 min or 75° C/16 sec for effective pasteurization.
Objectives of milk pasteurization
- The chief objective of milk pasteurization is to destroy pathogenic bacteria that could have a public health concern. By destroying these microorganisms, the product becomes safe for public consumption.
- Secondly, pasteurization eliminates destructive bacteria and enzymes that could cause spoilage of the product. This leads to a prolonged shelf life of the milk.
- Pasteurized milk is commercially sterile, which means that they are not entirely rid of bacteria. One should compound their preservation with another method (usually refrigeration).
In modern milk processing plants, the PHE is connected to a separator and a homogenizer in the same line. The milk separators help in butter fat standardization while the homogenizer breaks down fat globules into tiny microglobules that remain suspended throughout the milk. It prevents formation of cream layer when the milk is left standing for long.
Steps of pasteurization
Before you begin pasteurization, chances are high that you will be bulking milk to attain an economically viable volume. Milk being a highly perishable product, it requires extreme care to avoid incurring losses. For this reason, it is necessary to chill the milk to avoid spoilage.
a). Milk chilling
Chilling is not a pasteurization process but it is a necessary step when dealing with large volumes of milk. Milk leaves the cow’s udder at temperatures above the ambient, which encourages rapid bacterial multiplication that speeds up spoilage. However, reducing the temperatures to between 2° C to 5° C arrests bacterial growth and metabolism. This provides a head start at keeping the quality before proper pasteurization commences.
Chilling may affect the quality of the product negatively if it is kept for long. Psychrotrophic bacteria will cause proteolysis of protein, which leads to bitter flavor attributed to the released polypeptides.
Prolonged chilling introduces alterations to the structure of the casein micelles and increases the coagulation time. This leads to formation of less firmer curd and consequently low quality cheese.
Effects of chilling on milk
Impeding rennet/acid coagulation:
Lowering the temperatures to 2°C causes the milk not to coagulate even after rennet/acid treatment. This phenomenon has been utilized in continuous cheese making process in which the temperatures are raised after addition of acid/rennet. Coagulation begins when the temperatures reach 15°C to 30°C.
No coagulation of milk at isoelectric point:
Even after adjusting the pH of casein to isoelectric point, the milk will not coagulate if its temperature ranges between 2°C and 5°C. Low temperatures encourage the formation of many diffusible inorganic salts that distorts the micellar structure of casein leading to formation of more non-micellar (soluble) casein.
Consequently, one you have to lower the pH of the medium further to achieve complete coagulation. However, acid coagulation leads to formation of a less rubbery coagulum.
Chilling increases viscosity of milk:
Viscosity of milk largely depends on its colloidal components, of which proteins forms the bulky part. Chilling changes the structure of milk proteins leading to an increase in their bulk hence the increase in milk viscosity at chilled temperatures.
Chilling decreases in cheese curd firmness:
Milk chilling affects the ratio of calcium:phosphate hence their interaction in the colloid solution. A drop in this ratio leads to an increase in the duration it takes for the milk to coagulate. To counter this problem, add calcium chloride to cheese milk before cold aging starts.
Increases hydrolytic rancidity in milk:
Chilling exposes the casein micelle and release the lipases into the medium. As the temperatures rise gently or when the medium is gently agitated, the lipases get active and attack the fat globules and release the fatty acids leading to rancidity.
Increases foaming in milk:
Chilled milk foams easily due to the increased activity of ß-lactoglobulin, which is a surfactant. Milk proteins coalesce at the surfaces/lamellae of the protein, which also traps air leading to formation of air bubbles.
Chilling leads to an increased clustering of fat
Chilling milk encourages change formation in the surface of fat globules, which encourages the globules to stick together. The clustering of fat globules leads to an increased creaming rate in cold milk.
b). Pre-heating (regeneration) and standardization stage
After bulking, the chilled milk is heated to about 40°C to facilitate easy separation of butter fat during standardization. The system uses regenerative heating, i.e., it uses the heat of the already pasteurized milk to heat up the incoming chilled milk. The chilled milk, in a counter current flow, cools down the pasteurized milk.
The purpose of standardization is to obtain a product with uniform content of butter fat. Different products can be obtained from this process e.g. skimmed milk, standardized milk, low fat milk, high fat milk, etc.
After determining the type of product you are making, you can use a computer program or any standardization method to determine the amount of cream to separate. This will leave you with the desired amount of butterfat to standardize the milk.
c). Clarification stage
Clarification is essential for removing all foreign matter from the product. Large solid particles are removed by straining the milk through tubular metallic filters. A centrifugal clarifier (not the one used for standardization) is used to remove all soil and sediments from milk.
The filters, usually fitted in parallel twins permits continuous processing as one can be cleaned while the other is running. Clean the filters regularly (between 2 to 10 operational hours depending on the level dirt) to avoid growth of bacteria.
d). Standardization stage
It is important to standardize milk fat to ensure that you end up with a product of consistent quality in the market. Different consumers prefer different products. There are customers who will consume skim milk only while there are those who will take low fat milk. There are those who will take standardized milk while there are those who prefer high fat milk.
Standardization is necessary to ensure that all the customers are catered for. Again, it is during the process of standardization that you get to separate the butterfat that is used for making cream and other fat based products such as butter and ghee.
Here is an in-depth review of milk standardization.
e). Homogenization stage
Homogenization is a physical process of breaking down the the milk fat globules into tiny droplets to discourage cream separation. Tiny droplets of fat do not rise in a milk column since reducing their sizes also increases their density in the milk.
A milk homogenizer working at between 100 to 170 bars splits all the fat globules into very tiny droplets that increases the level of integration of the fat in the milk. As a result, the milk fat remains uniformly distributed in the milk.
f). Heating section
Utilizes heat from steam to raise the temperatures of the milk from about 60°C to the required 72°C that is effective to kill the Clostridium botulinum spores. The steam exchanges heat with the milk across the PHE plates in a counter current motion.
At the end if this section, there is a temperature sensor, which controls the flow diversion valve. Any milk that does not attain the required temperature is diverted back to the heating section until it attains the required temperatures.
g). Holding section
After heating, milk flows into the holding tubes whose lengths have been calibrated with the milk flow rate to ensure that milk takes at least 16 seconds in the tubes. All the milk must maintain the required pasteurization temperatures at the end of the tubes.
In case of a breach, a sensor will trigger the flow diversion valve to take the milk back to the heating section to bring the milk to the required temperature.
Once the milk has attained the required temperatures at the end of the holding tubes, milk flows back to the regeneration section to heat the incoming chilled milk while in itself being cooled down to about 30°C.
h). Cooling/chilling section
After regenerative cooling of pasteurized milk, it moves to the cooling section of the PHE where chilled water/PHE coolant lowers the temperature of pasteurized milk to 4°C. The chilled milk is then pumped to the packaging machines for aseptic packaging and subsequent storage in the cold room.
If the milk is to be used for making yogurt, there is no need to chill it. It will only require regenerative cooling to about 45°C, which is the suitable temperature for yogurt bacteria.