HTST, LTLT, and UHT Pasteurization & Effects on Milk Curdling

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 the difference between pasteurization and homogenization?

Many people tend to confuse pasteurized milk with homogenized milk. Let’s first look at these two processes before going deeper into the purpose and process of pasteurization.

What is milk homogenization?

Homogenization is a mechanical process that is applied to milk to break down fat globules in milk so that the butter fat does not from the aqueous phase.

Non-homogenized milk has a tendency of forming a layer of cream on top of the milk, which many consumers do not like.

Milk homogenization usually take place during pasteurization. We shall see how this happens when we shall be discussing continuous pasteurization process in this article.

What is milk pasteurization?

We can define milk pasteurization as the process of heating milk to a predetermined temperature for a specific duration to kill specific pathogens and reduce the load of spoilage microorganisms without re-contamination during the entire process.

The predetermined temperature usually depends on the heat resistance of pathogenic 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 product is meant for public consumption. 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 Mycobacterium tuberculosis or Coxiella burnetii.

When deriving the thermal death time of any microorganism, the temperature must remain constant and holding time varied to kill the specified number of cells.

Thermal death time (D-value) is the duration it takes to kill a certain bacteria at a given temperature.

Difference between D-value, Z-value, and F-value?

The decimal reduction time, D-value, is the amount of time under specified conditions to reduce microbial population by one decimal. This time varies and is dependent on the temperature and the target microorganism. One decimal reduction (1D) is equivalent to a population drop by one log cycle or 90% reduction.

For instance, let’s say we can reduce the population of microbe X to 10% by exposing it to 121°C for 4 seconds. We Denote the D-value of microbe X as D_121°C = 4 seconds. For spore formers like Clostridium botulinum, the treatment should achieve 12 log cycle reduction in original bacterial population.

Z-value is the measure of change in the rate of death due to change in temperature. It is the change in temperature required to change the decimal death time by one log cycle or one decimal (1D).

In other words, Z-value is the measure of bacterial sensitivity to heat treatment. It is the change in temperature that will reduce the D-value by a factor of 10. You can obtain a Z-value by plotting two D-vales against temperature.

F-value is the duration it will take to kill a known bacterial population. It is usually set at 12 log cycles (12D) to kill the most resistant mesophilic spores in a food sample.

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.

Video by R. Paul Singh

Milk pasteurization programs

Broadly, pasteurization can be categorized as either low or high temperature pasteurization methods. Both of these can either be batch or continuous processes.

Low temperature pasteurization is majorly concerned with food safety and aims at killing all pathogenic microorganisms and reducing spoilage types in a food sample. Milk that has undergone low temperature pasteurization is suitable for making cheese because it encourages syneresis.

Low temperature pasteurization can assume various temperature/time combinations such as 63°C/30 minutes or 72°C/15 seconds. Mild heating kills all pathogenic bacteria and reduces the load of spoilage bacteria but preserves most physico-chemical properties of the milk.

On the other hand, high temperature pasteurization aims at killing the vegetative pathogenic and spoilage bacteria as well as denaturing as much serum protein as possible. High temperature pasteurized milk is more suitable for making yogurt because Syneresis will not occur. The serum proteins are denatured hence they will not separate.

High temperature pasteurization involves intense heating and may involve temperature/time combination regimes such as 140°/2 seconds, 85°C/30 minutes, or 90°C/20 minutes. Intense heating aims at destroying serum proteins to avoid syneresis.

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,
• Target microorganism, etc

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 chamber. Heat to between 115°C to 121°C for 10 to 20 minutes.

How to test for the effectiveness of milk pasteurization

High temperature pasteurization denatures peroxidase enzyme. The enzyme is more resistant to heat treatment than all pathogenic microorganisms found in milk. If the heat treatment is sufficient to denature this enzyme, it is a confirmation that all the pathogenic microorganisms have been killed been killed in the process.

The principle of the test is premised on the effect of heat on alkaline phosphatase, a natural enzyme present in raw milk. When heated at pasteurization temperatures, the enzyme is deactivated.

When milk containing the phosphatase enzyme is incubated with p-nitro phenyl di-sodium ortho phosphate, the reagent hydrolyses the substrate and liberates para-nitro phenol, which gives a yellow colour in and alkaline environment.

The stronger the colour, the higher the quantity of the enzyme present in milk. Properly pasteurized milk should, therefore, produce a negative result for peroxidase test. Practice caution when preparing samples since recontamination can also lead to a positive test result.

Alkaline phosphatase may not be very reliable because research has shown that it can be reactivated, especially if the pasteurized milk has high fat content.

Different Pasteurization Regimes

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 has been identified to be the most heat resistant pathogen of public health concern 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.

High quality flash pasteurized milk will keep for between 16 and 21 days. For selfish commercial interests, some manufacturers will 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: remove 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 passes through.
• Instrumentation and control equipment: Increase system efficiency and reliability
• Peripheral sources of utilities such as steam, compressed 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 without possibility of contamination

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 has 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 drop 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

Video by Sondex A/S

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 cheese.

This method is best for batch pasteurization where the milk is held in a jacketed vat for effective pasteurization. There are many designs of batch pasteurizers in the market that are suitable for both domestic and commercial use.

Let us look at some of the best batch pasteurizers for home use. We’ll also review an ice cream mix / cream pasteurizer that is capable for commercial use.

The best batch pasteurizer to buy for home/small farm use

a) Milky 3.7-Gallon Pasteurizer: Review

The Eco Mini 3.7-Gallon Stainless Steel Pasteurizer from Milky is a mini processor of no mean repute. As small as it is, it will pasteurize about 14 ltrs /3.7 gal of milk in an hour.

The water bath provides gentle heating to control the pasteurization process. It also has an additional insulation to avoid unnecessary loss of heat making it an efficient energy conserving machine.

The water bath has an inlet and outlet to allow controlled pasteurization. When the heating and holding is completed, just connect cold water and the temperature will come down in under 20 minutes.

The temperature gauge is calibrated to allow controlled heating during processing.

Quirks:

• The inner pail can be very heavy when full. The machine weighs 13.5lbs when unloaded. Coupled with the heat, you may not be able to lift it out easily without having cooled it properly first.
• It also comes with a 120V AC connection. You may need a convertor if your outlet is different.

Advantages

• Versatile machine with adjustable temperatures. Can pasteurize milk, ice cream base, and even other foods such as juices and fruits
• Made of stainless steel for easy cleaning and hygiene
• Easy to operate. Comes with a programmable timer and electronic control buttons
• Excellent customer support
• Comes with a warranty

  Buy from Amazon  

b) Commercial Refrigerated Pasteurization Machine

This is the perfect starter pack pasteurizer for a small farm or a small business owner who want to assure the quality of their dairy products. It is made of food grade stainless steel and is designed with usability in mind. You can easily reach its corners and curves to clean it.

It has high heating power (rated at 6000W) making sure the pasteurization conditions for the contents are attained efficiently. The inbuild cooling system ensures that the pasteurized milk is cooled to the required temperature for storage or further processing into different products.

The onboard microcomputer helps the operator to control the quality of the products under processing. The jacketed wall ensures thermal efficiency during heating and cooling respectively.

Issue of concern

• It comes with a 220V/380V power rating.

Advantages.

• Food grade stainless steel for hygiene and quality preservation.
• You can also use it to pasteurize juices/beer/sauce
• Comes with a warranty
• Comes with adjustable thermostat to a different temperature range.
• Comes with a control panel
• Has onboard radiator for cooling

  Buy from Amazon  

c) H&M HMIX30CP-236W Commercial Dairy / Custard / Milk Pasteurizer

Pasteurizing ice cream mix can be quite a daunting task due to the viscosity of the product. Insufficient heating will result in a poorly pasteurized product that will lead to formation of defects.

Defective butter is not safe for consumption. This can lead to serious losses, especially given the expenses associated with procurement of cream.

H&M’s commercial ice cream / cream pasteurizer eliminates the risk of poor pasteurization by the use of their computerized HMIX30CP-236W, which is specially designed to handle thicker mixes.

All the surfaces that come into contact with food is made of stainless steel for hygiene and food safety. All the parts are easy to assemble and disassemble by following the simple user manual that comes with the machine.

The machine has soft-cooking technology that is facilitated by the use of cycled glycol that evenly distributes heat during pasteurization. It can heat the mix to 100°C without burning it.

The machine uses environmentally friendly coolant (R404A).

It is easy to control by feeding data through the digital interface that then allows the machine to run automatically without additional intervention. It has an optional to print the product info and working cycles.

Cleaning is simple as the machine comes with an installation for the cleaning water supply and the spray tap.

Quirks:

• The machine is heavy and will need a dedicated space in your kitchen / production area to avoid moving it often. The place should be easy to access and proximal to utilities such as power and water.
• It also needs a heavy capital expenditure to install.

Advantages

• Flexible usability and easy to control. Has a touch screen for easy manipulation of the process. You can program the settings for custom made mixes
• Parts are made of stainless steel for maximum hygiene and food safety.
• Machine is easy to disassemble for maintenance purposes
• Machine uses glycol to evenly distribute heat during pasteurization. This eliminates the risk of cooking the product and ensures efficient pasteurization
• Uses an environmentally friendly R404A gas as a coolant

  Buy from Amazon  

d) H&M HMIX30-236W Commercial Ice Cream Pasteurizer (30Liter/3phase/Water Cool)

Comes with 8 presets for processing 8 different bases / ice cream mixes. It is computer controlled to make it easy to control during pasteurization and ageing of the ice cream mix.

It has a memory chip to memorize the procedure for the next batch to avoid monotonous work of setting the machine each time a new batch is loaded.

The machine also has an automatic power recovery to pick up processing from where it left before the power failure.

The machine also comes with an option to print the working cycles.

  Buy from Amazon  

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.

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

1. 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.
2. Secondly, pasteurization eliminates destructive bacteria and enzymes that could cause spoilage of the product. This leads to a prolonged shelf life of the milk.
3. 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 in 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 – 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 firm 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 (IP), 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 overview of milk standardization.

e) Homogenization stage

Homogenization is a physical process of breaking down 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.

Note\\

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.

Non-conventional Methods of Pasteurization

Conventionally, normal heating is used to pasteurize milk and other food products. However, there are other non-conventional methods of pasteurization. Some of these methods involve heating while others are completely devoid of heat.

They include:

1. Microwave Heating.

The method is currently still under development and has only been accepted for commercial sterilization of canned foods.

Microwave heating is highly effective on low acid foods and can be used in a continuous and batched process.

2. Ohmic Heating.

Ohmic heating involves passing of electrical pulse through food via charged electrodes. The resistance to electrical flow produces heat which pasteurizes the food sample.

Heating is uniform and more efficient producing a greater quality product than other heating methods for pasteurization.

The process can be applied to liquid foods and foods with high moisture content like fruits.

3. High-Pressure Processing.

High hydrostatic pressure pasteurization is completely non-thermal process. It relies on high water pressure that is applied isostatically to the packaged food product.

The pressure disrupts the cell membranes of the target microorganisms leading to leakage of the protoplasm hence death of the microorganism. This method promises the advantage of leaving the food structure intact and maintaining the nutritional and sensory quality of the food product.

It is also cheaper than heating since it uses tap water to apply the pressure to the food. Since pasteurization is done after packaging, there is no chance of food re-contamination after pasteurization.

4. Pulsed Electric Fields.

This method uses high voltages of electricity (>20 kV) that is applied to food in very short pulses lasting microseconds.

The electrical pulses interfere with the metabolic processes of the microorganisms and kills them rendering the food safe for human consumption.

Additionally, this process leaves the food material unaffected structurally and in terms of the nutrient content.

Comparing Curdling Between Low and High Temperature Pasteurized Milk

The milk samples are subjected to two different pasteurization regimes. One sample is heated to 85°C and then held for 15 minutes, after which it is cooled to 45°C. The other sample is heated to 63°C where it is held for 30 minutes, after which it is cooled to 45°C.

The aim of this experiment is to find out the curdling rates of these two samples of milk when rennet and/or yoghurt culture is added.

The experiment

1. Sample A was subjected to low temperature long time (LTLT) pasteurization while sample B was subjected to high temperature short time (HTST) pasteurization.
2. Each of the batches were sampled into separate beakers, their pH measured and recorded.
3. Three (3) mls of yoghurt culture was added into each beaker followed by 3 mls of activated rennet.
4. The pH was then determined after every five-minute interval for the next 10 minutes.

Meanwhile, the remaining samples were inoculated with yoghurt culture and activated rennet and left to curdle.

The Results

Sample	Initial pH	pH on addition of rennet	pH after 5 minutes	pH after 10 minutes
A	6.6	6.56	6.44	6.32
B	6.49

Upon cutting the curd, sample A formed a thick curd while sample B remained practically curd-less.

Conclusion

From the results obtained in this experiment, it is safe to conclude that milk to be used for cheese-making should be subjected to low temperature long time (LTLT) pasteurization.

This will encourage faster curd formation because low heat does not denature the whey proteins, which are instrumental in the bond formation of the curd. Also, the undenatured whey proteins promote whey separation.

On the contrary, milk intended for making yoghurt should be subjected to high temperature pasteurization (90°C for up to 30 minutes) to completely denature the whey proteins. This will ensure that the yoghurt does not undergo syneresis.