Preservation of milk and milk products from microbial spoilage

Milk is a nutritious and versatile food that contains many essential nutrients such as protein, calcium, vitamin D, potassium, and phosphorus. However, milk is also highly perishable and can spoil quickly if not stored properly. Spoiled milk can cause food poisoning and waste valuable resources. Therefore, it is important to preserve milk and extend its shelf life as much as possible.

There are many methods of preserving milk, each with its own advantages and disadvantages. Some methods aim to reduce the number of microorganisms that cause spoilage and disease, while others aim to remove or reduce the water content that supports microbial growth. Some methods also alter the physical or chemical properties of milk, such as its flavor, texture, color, or nutrient content.

The following are some of the common methods of preserving milk:

1. Refrigeration: storing milk in proper containers at low temperatures (1-4°C) to slow down microbial growth and enzymatic activity. This method can preserve milk for 1-4 weeks depending on the type and quality of milk.
2. Freezing: storing milk in proper containers at very low temperatures (-18°C or below) to stop microbial growth and enzymatic activity. This method can preserve milk for up to 6 months, but may affect its texture and flavor.
3. Dehydration: removing water from milk by applying heat under controlled conditions by evaporation. This method can reduce the water activity of milk and prevent microbial growth. Dehydrated milk can be stored as a powder for 2-10 years depending on the storage conditions.
4. Canning: heating milk in sealed containers at high temperatures (105-150°C) for a certain time to kill almost all microorganisms. This method can preserve milk for up to 2 years, but may affect its flavor and nutrient content.
5. Use of preservative agents: adding substances that can inhibit or retard the growth of microorganisms in milk. These substances can be natural (such as salt or sugar), bio-preservatives (such as lactic acid bacteria or bacteriocins), or chemical preservatives (such as benzoic acid or sorbic acid). These agents can extend the shelf life of milk by varying degrees depending on the type and concentration of the agent.
6. Other methods of treating milk: applying physical or electrical treatments to milk that can reduce or eliminate microorganisms without affecting its quality. These methods include microfiltration, bactofugation, ohmic heating, microwave heating, pulse electric field, high-pressure process (HPP), ultrasound, UV radiation, and irradiation.

These methods can be used alone or in combination to preserve milk and its products. The choice of method depends on several factors such as the availability of resources, the cost-effectiveness, the safety, and the consumer preference. The main objective of these methods is to ensure the quality and safety of milk and its products for human consumption.

Thermisation is a method of sanitizing raw milk with low heat. It involves heating milk at temperatures of around 63–65 °C for 15–20 seconds, followed by rapid cooling to below 6 °C. Thermisation is a subpasteurization heat treatment that reduces the number of spoilage bacteria in milk with minimal heat damage.

The main objective of thermisation is to extend the shelf-life of milk when it cannot be immediately used in other products, such as cheese. Thermisation can also be used as a pre-pasteurization or post-pasteurization treatment of dairy products. Thermisation inactivates psychrotrophic bacteria in milk, which may release heat-resistant enzymes that degrade the quality of milk if allowed to grow. Thermised milk can be stored below 8 °C for three days, or at 0–1 °C for seven days.

Thermisation does not eliminate pathogens such as Listeria monocytogenes, which can survive and grow at refrigeration temperatures. Therefore, thermised milk is not suitable for direct consumption and requires further processing or treatment before being consumed. Thermised milk can be pasteurized, sterilized, or used to make cheese or other dairy products. Cooling thermised milk before reheating is necessary to prevent the outgrowth of bacterial spores that may have germinated during the first heating.

Sterilization or UHT is a method of food preservation that involves the application of heat, usually more than 100°C 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:

• Conventional method / In-bottle sterilization method: in which the product is packed before heat treatment and then heated at 105-110°C for 30-45 min.
• 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 objectives 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).

UHT treatment requires both a sterilizer and an aseptic unit (for packaging the product). It is used for low acid (above pH 4.6) products such as UHT milk, UHT flavored milk, UHT creams, soya milk, and other dairy alternatives. Aseptic packaging is crucial in preserving the long shelf life of UHT milk without the need for refrigeration. The containers used for aseptic packaging are typically made of materials such as cartons, bottles, or pouches. These packaging materials undergo a sterilization process to eliminate any microorganisms present on their surfaces.

The advantages of UHT milk are:

• It has a longer shelf life than pasteurized milk and can be stored for up to six to nine months without refrigeration.
• It retains most of the nutritional value of fresh milk and does not require any preservatives.
• It is convenient and economical for consumers who do not have access to refrigeration or frequent milk supply.

The disadvantages of UHT milk are:

• It has a slightly different taste and smell than fresh milk due to Maillard browning and other chemical changes that occur during heating.
• It may lose some heat-sensitive vitamins such as vitamin B12 and folate during processing.
• It may not be suitable for some dairy products such as cheese and yogurt that require live cultures.

Dehydration 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 objectives of this method 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:

• 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.
• 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.
• 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.

Dehydrated milk products include milk powder, skimmed milk powder, whey powder, casein powder, etc. These products have a longer shelf-life than liquid milk and can be reconstituted by adding water when needed. Dehydrated milk products are also used as ingredients in various food products such as bakery, confectionery, ice cream, etc.

Dehydration has some advantages and disadvantages as a method of milk preservation. Some of the advantages are:

• It reduces the transportation and storage costs of milk.
• It preserves the nutritional quality of milk to a large extent.
• It offers convenience and flexibility to consumers and manufacturers.

Some of the disadvantages are:

• It requires high energy input and sophisticated equipment.
• It may cause some loss of flavor, color, and solubility of milk.
• It may alter some functional properties of milk proteins.

Summary:

Dehydration is a method of milk preservation that involves the removal of water by evaporation. It reduces the water activity and inhibits the microbial growth in milk. It also reduces the volume and weight of milk and extends its shelf-life. Dehydration can be done by spray-drying, drum drying, or fluid bed drying. Dehydrated milk products include various types of milk powders that can be reconstituted or used as ingredients. Dehydration has some advantages and disadvantages as a method of milk preservation.

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:

• Natural preservatives
• Bio preservatives
• Chemical preservatives

Preservatives that are used in milk and its products are:

• Natural preservatives: These are substances that are naturally present in milk or derived from natural sources, such as lactoperoxidase, lactoferrin, lysozyme, organic acids, and antimicrobial peptides. These compounds can inhibit the growth of various bacteria, yeasts, and molds by affecting their cell membrane, enzyme activity, or metabolism. Natural preservatives have the advantage of being safe, effective, and compatible with the sensory properties of milk and its products.
• Bio preservatives: These are living microorganisms or their metabolites that can exert a beneficial effect on the shelf-life and safety of milk and its products. The most common bio preservatives are lactic acid bacteria (LAB) and their bacteriocins, such as nisin. LAB can produce organic acids, hydrogen peroxide, diacetyl, and other compounds that lower the pH and inhibit the growth of spoilage and pathogenic microorganisms. Nisin is a peptide produced by some strains of Lactococcus lactis that has a broad spectrum of activity against Gram-positive bacteria, including spore-formers. Nisin is approved by the Food and Drug Administration (FDA) for use in dairy products such as cheese and yogurt. Other bio preservatives include enterococci, bacteriophages, and endolysins. Enterococci are Gram-positive bacteria that can produce bacteriocins such as enterocin. Bacteriophages are viruses that infect and kill specific bacteria. Endolysins are enzymes that degrade the cell wall of bacteria. These bio preservatives have the potential to control spoilage and pathogenic microorganisms in milk and its products without affecting their quality or safety.
• Chemical preservatives: These are synthetic compounds that are added to milk and its products to prevent microbial growth and extend their shelf-life. The most commonly used chemical preservatives in dairy products such as cheese and yogurt are benzoate, sorbate, and natamycin. Benzoate and sorbate are weak organic acids that lower the pH and inhibit the growth of yeasts and molds. Natamycin is an antifungal agent that inhibits the growth of molds by interfering with their membrane function. Chemical preservatives have the advantage of being effective at low concentrations and having a wide range of application in the food industry. However, they also have some disadvantages such as possible adverse effects on human health, sensory changes in food products, and development of microbial resistance.

The use of preservative agents in milk and its products depends on several factors such as the type of product, the processing method, the storage conditions, the regulatory requirements, and the consumer preferences. The choice of the best preservative agent should consider the balance between the efficacy, safety, quality, and acceptability of the product.

Besides the methods mentioned above, there are some other methods of treating milk that can improve its quality, safety, and shelf life. Some of these methods are:

• Microfiltration: This is a process that uses a membrane filter to remove bacteria and spores from milk. It can be combined with pasteurization or UHT to achieve a longer shelf life and better flavor.
• Bactofugation: This is a process that uses a centrifuge to separate the bacteria from the milk fat. It can reduce the bacterial load and prevent spoilage problems such as late blowing in cheese making.
• Ohmic heating: This is a process that uses an electric current to heat the milk uniformly and rapidly. It can inactivate microorganisms and enzymes without affecting the nutritional and sensory properties of milk.
• Microwave heating: This is a process that uses electromagnetic waves to heat the milk. It can be used for pasteurization or sterilization of milk and has advantages such as energy efficiency and minimal heat damage.
• Pulsed electric field: This is a process that uses short pulses of high voltage electric fields to disrupt the cell membranes of microorganisms. It can extend the shelf life of milk and preserve its freshness and nutrients.
• High-pressure processing: This is a process that uses high pressure (up to 600 MPa) to inactivate microorganisms and enzymes in milk. It can improve the safety and quality of milk and its products without affecting their flavor and texture.
• Ultrasound: This is a process that uses high-frequency sound waves to create cavitation bubbles in the milk. These bubbles can damage the cell walls of microorganisms and spores and enhance the heat transfer during heating.
• UV radiation and irradiation: These are processes that use electromagnetic radiation to kill or reduce microorganisms in milk. UV radiation can be used for surface disinfection of packaging materials or for treating milk directly. Irradiation can be used for sterilizing milk or reducing pathogens in dairy products.

These methods are not widely used in the dairy industry yet, but they have potential applications for improving the safety, quality, and shelf life of milk and its products.

References:

• Preservation of Milk and Dairy Products by Using Biopreservatives - SSRN
• Milk Preservatives and Adulterants: Processing, Regulatory and Safety Issues: Food Reviews International: Vol 31, No 3
• Diversity and Control of Spoilage Fungi in Dairy Products: An Update
• Use of Preservatives in Dairy Industry - DSS Imagetech
• Preservation of Milk and Dairy Products by Using Biopreservatives

We are Compiling this Section. Thanks for your understanding.