Preservation of milk and milk products from microbial spoilage

Milk and its products consist of numerous nutrient content, it serves as an excellent growth medium for all of the microorganisms. Thus, various preservation methods are used to eliminate the growth of spoilage-causing microorganisms and maintaining the nutritional properties of milk. Several techniques have been used to limit the growth of organisms in milk and milk products.

A. Heat treatment methods

1. Thermisation

It is the most commonly used method used for milk preservation by heating the milk at a mild temperature at 57 – 68 °C for 15 – 20 seconds and rapidly cooling at <6 °C. This method is effective against spoilage-causing bacteria however it doesn’t eliminate the pathogens such as L. monocytogenes. The main objective of thermisation is to reduce the growth of psychrotrophic bacteria and extend the shelf-life of milk.

2. Pasteurization

Pasteurization is a method of food preservation that involves the application of heat, usually below 100° at a certain time. It aims to reduce the number of viable pathogens and spoilage-causing microorganisms (e.g., Coxiella burnettii, Brucella abortis, Mycobacterium tuberculosis) to extend the shelf-life of milk without harming the milk quality.  In milk, thermoduric species such as Micrococcus spp., Enterococcus faecium and Enterococcus faecalis, Bacillus subtilis, Bacillus cereus, and certain lactobacilli. are killed by the process of pasteurization.

Four common types of milk pasteurization vary with temperature and time the milk is held at that temperature.

1. Vat Pasteurization/Low temperature, long-time pasteurization in which the milk is heated at 63°C for 30 min.
2. High Temperature/Short Time (HTST) in which, the milk is heated at 72°C for 15 s.
3. Ultra-pasteurization (UP) in which the milk is heated at 138°to 150° C for one or two seconds
4. Ultra-High-Temperature (UHT) in which the milk is heated at 280° F for only two seconds.

The objective of pasteurization are:

• To destroy pathogenic organisms present in milk.
• To ensure the quality of milk and milk products.
• To destroy the unwanted organisms present in milk and milk products.

3. Sterilization or UHT

Sterilization is a method of food preservation that involves the application of heat, usually more than 100° at a certain time to kill almost all bacteria followed by packaging in air-tight containers either before or after heat treatment.  The sterilized milk can be stored at room temperature for a longer period.

There are two methods of sterilization. 

1. Conventional method / In-bottle sterilization method in which the product is packed before heat treatment and the packed product is heated at 105-110°C for 30-45 min.
2. UHT method / Aseptic method in which the product is heated at 135-150°C for 1-20 seconds followed by instant aseptic filling into sterile containers.

The objective of sterilization are:

• To ensure the quality of milk and milk products at room temperature without refrigerated storage making it safe for human consumption.
• To ensure the destruction of microorganisms (pathogenic and spoilage-causing microorganisms, vegetative and spore forms, viruses).

4. Dehydration

It is defined as the process of removal of water normally present in milk by the application of heat under controlled conditions by evaporation. In this method, the water activity of milk is reduced to prevent the growth of spoilage-causing microorganisms.

The objective of this methods are:

• To reduce the growth of spoilage-causing and pathogenic organisms and extend the shelf-life of milk.
• To reduce the volume and weight of milk without compromising the nutritive value.

The methods of dehydration used in milk preservation are:

1. Spray-drying: in this process, the pre concentrated liquid food is dispersed into a stem of hot gas that results in evaporation of water content resulting in instantaneous drying.
2. Drum drying: In this process, the pre-concentrated product is applied as a thin film on the outer surface of an internally heated rotating metal drum.
3. Fluid bed drying processing involves drying, cooling, agglomeration, granulation, and coating of particulate materials. The gas (usually air) is passed through a product layer under controlled velocity conditions to create a fluidized state.

5. Use of preservative agents

Preservatives are substances that are capable of inhibiting or retarding the growth of microorganisms. Such preservatives used in food can be divided into three types:

1. Natural preservatives
2. Bio preservatives
3. Chemical preservatives

Preservatives that are used in milk and its products are:

B. Low-temperature treatment

• The foods are stored at temperatures  0–5 °C.
• It causes minimal changes to the nutritional properties of food.
• The main objective of chilling is to reduce the rate of microbial growth and its enzymatic activities which extends the shelf life of milk and milk products.
• It has been used in combination with other methods of food preservations such as irradiation, pasteurization to extend the shelf life of milk and milk products.
• Milk, cheese, yogurt, butter, etc. are stored in chilled storage.

C. Other Methods of Treating Milk

1. Microfiltration

• It is used to remove significant numbers of bacteria from milk and extend its shelf life.
• It can be used in a combination with HTST pasteurization.

2. Bactofugation

• It is a centrifugation process that removes the bacteria present in milk.
• It has been used in the cheese industry to minimize the milk spoilage by clostridia that cause ‘late blowing’.

3. Ohmic heating

• Ohmic heating is the process of heating the product by passing it through an alternating electric current.
• It is a direct heating method in which the food is in direct contact with the electrode and food itself is a part of an electric circuit.

4. Microwave heating

• Microwave heating is a method in which electromagnetic waves are used to generate heat in food.
• Microwaves used in the food industry for heating are of frequencies 2450 or 900 MHz. 
• It has been in commercial practice for milk pasteurization for quite a long time.

5. Pulse Electric Field

• The pulsed electric field is one of the non-thermal food preservation technologies in which food is subjected to short pulses (1-100 µs) of high electric fields with a duration of nano to milliseconds and intensity of 10 – 80 kV/cm to foods placed between two electrodes.
• It has a lethal effect on vegetative bacteria, mold, and yeasts.

6. High-pressure process (HPP)

• It is a non – thermal pasteurization process in which food is subjected to high pressure in the region of 3300 -600 Mpa for about 10 minutes.
• The components of microorganisms are destroyed by high pressure that can inactivate pathogenic and saprophytic microorganisms.

7. Ultrasound

• Ultrasonic is a high-power sound wave at frequencies between 16 kHz and 100 MHz.
• In this method, the sonic wave is passed through the milk and the changes occur in the pressure which leads to cavitation, which causes gas bubbles in the liquid causing a bactericidal effect.
• An ultrasound application has been conducted on the B. subtilis spores in milk at a temperature range of 70-95 °C.

8. UV Radiation and Irradiation

• Ultraviolet (UV) radiation is electromagnetic radiation having a wavelength of about 10-400 nm.
• Gamma rays, X-rays, and accelerated electron beams are the sources of ionizing radiation used for the preservation of foods.
• It is applied in combination with the pasteurization of brine during cheese production.
• Irradiation can be used in dairy products to destroy pathogens or all microorganisms.

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