*Dr. Vikas Sangwan
The process of homogenization refers to the dispersion of the structural elements of food including globules, molecules, particles, and droplets etc. within a surrounding continuous medium. In the homogenization process, a fluid is forced to pass through a minute orifice causing various energy transformations directly impacting the dissolved, dispersed, or emulsified components. Significant mechanical stress on fluid in the form of shear, turbulence, and cavitation results in dispersion of structural elements.
In earlier days, homogenization was introduced in dairy industry as a manufacturing step to reduce the size of fat globules, improving the stability of the emulsion resulting in physical and chemical stability of milk. Over the years, food homogenization has grown from a normal manufacturing step to an established and essential operation, with role in wide array of fields. Homogenization performs multiple functions including particle size reduction, dissolution, mixing, dispersion, encapsulation, emulsification, and structuring.
Homogenization of Milk
Milk is a very complex food containing water, protein, fat, lactose, and salts. The main components of milk with industrial significance are protein and fat. As milk is an oil in water emulsion, fat globules are dispersed in a continuous phase of skim milk. When left undisturbed, fat globules in milk rise to the surface and form a layer of cream. Homogenization in the dairy industry is used principally to prevent or delay the formation of a cream layer in milk by reducing the diameter of the fat globules. Major percentage of the fat globules in unhomogenized raw milk are between 2 -12 µm while in homogenized milk the size range is 0.2-2 µm. Process of homogenization has a significant impact on the quality of dairy products such as condensed milk, curd, or ice cream.
Objectives of Homogenization of Milk
In dairy industry, the process of homogenization is employed in wide array of milk and milk products, including liquid milks, yoghurt, recombined milk products, some cheese varieties, cream, ice cream, infant formula, and other nutritional products. Following are the main purposes for which homogenization is being used:
Prevention of Creaming: Cream layer formation in packed milk is deemed undesirable. Homogenization is used to reduce the size of the fat globules, leading to significant reduction in rate of creaming. This process also improves the stability of milk towards partial coalescence.
Enhancement in Rheological Properties: Rheological properties of products can be greatly improved using homogenization. For instance, viscosity of the cream is improved by creating homogenization clusters and viscosity of yoghurt is enhanced by creating protein-stabilized fat globules.
Emulsification: Addition of fat from milk or any other source need emulsification. Homogenization prior to emulsification helps in creation of stable emulsion.
Benefits of Milk Homogenization
- Improves digestibility of milk.
- Improves the appearance and mouthfeel of tea or coffee due to increased milk viscosity.
- Prevents churning of fat during rough handling or excessive agitation.
- Reduces curd tension – forms a curd with brighter appearance, heavier body, rich flavor, and more palatability.
- Reduces fat separation during manufacturing of milk products like evaporated milk and ice-cream providing a smooth texture to the product.
- Reduces susceptibility of milk to oxidized flavor development.
Process requirements for Homogenization
Concentration and physical state of the fat contribute to the size and dispersion of the fat globules during homogenization. Further, low concentration of serum proteins also leads to clumping of fat globules in products with high fat content. Homogenization of cream having more than 12 % fat by normal high pressure is not efficient due to lack of casein resulting in cluster formation. Minimum requirement for a proper homogenization is around 0.2 g casein per gram of fat. There is a direct co-relation between dispersion of the lipid phase and temperatures of homogenization. Homogenization of solidified fat as in cold milk is almost ineffective. Processing at low temperature results in incomplete dispersion of the fat phase. For homogenization to be effective, the fat phase should be in a liquid state and in concentrations normal to milk. Normally homogenization is performed at 60 – 70°C, with a pressure range from 10 to 25 MPa, depending on the product.
In homogenizers used for homogenization of milk, a high-pressure pump forces the liquid through one or more narrow openings (homogenization valves). The pressurized fluid flows axially through homogenization valve with subsequent flow at an extremely high velocity through the valve gap. Finally, the fluid leaves the valve seat at atmospheric pressure. The pressure and velocity of the liquid in the homogenizing valve are determined by the size of the valve gap.
During homogenization, the liquid upstream of the valve has a high potential energy, which on entering the valve, is converted into kinetic energy. The high velocity in the narrow valve opening leads to intense turbulence; converting the kinetic energy to heat energy. Only a fraction of kinetic energy, less than 0.1%, is utilized for globule disruption.
Several processes occur concomitantly during homogenization. Total surface area increases due to deformation and disruption of droplets. Frequent collision of newly formed small droplets also occurs if surface is insufficiently stabilized. Multiple disruption and collision events occur during the passage through a homogenization valve. The rates of the various processes depend on a number of variables, such as power density, surfactant concentration, droplet size, and volume fraction, and each has a characteristic time.
Homogenizers can be single-stage, double-stage, or even multi-stage type. Single-stage homogenizers are equipped with only one homogenizing valve, on the other hand, double-stage homogenizers are equipped with an additional homogenizing valve. In dairy industry, generally two stage homogenizers are used with a pressure of 140 bar (2000 psi) and 35 bar (500 psi) in the first and second stage respectively.
Generally, homogenizer is placed before the final heating section of the heat exchanger. Normally in pasteurized milk production plants, homogenizer is installed after the first regenerative section. In plants where UHT milk products are manufactured, homogenizer is always placed downstream in direct system and upstream in indirect system except indirect system where products with high fat or protein contents are processed.
Types of Homogenizers
There are a number of different types of homogenizer. The three most common are rotatory type, high pressure models, and sonic disruptors.
Rotatory homogenizers are the most common type. These homogenizers have a long shaft and angled knives at the bottom and are capable of shearing large quantities of material. These types of homogenizers are generally quick and efficient. Usual operating pressure of these homogenizers is generally low. The construction is so designed that milk is subjected to grinding and shearing action. Foaming and aerosols can be a problem with certain types of materials.
High pressure homogenizers are usually used with liquid foods. High pressure homogenization is the most commonly used method for homogenization of milk. They work by forcing the material through small tubes or valves with a piston pump under very high pressures.
Sonic disruptors generate high intensity ultrasonic waves. These waves are inaudible to humans but exert a pressure of more than 500 atmospheres along with high temperature. The waves are amplified by a probe into a beam that produces the cutting or shearing effect on particles through a process called cavitation.
Types of Homogenization
Full stream homogenization
In conventional dairy process, a separator is used to separate raw milk into a low-fat phase (skim milk) and a fat-enriched phase (cream) prior to homogenization. Milk is then standardized to the final product fat content by mixing these two phases and then homogenized. Full stream or total homogenization is the most commonly used form of homogenization of market milk and milk intended for cultured milk products.
In partial stream homogenization processes, all of the skim milk is not homogenized, cream with a small portion of skim milk is homogenized. Using this method of homogenization, reduces the total power consumption by about 60 – 65 %, thus reducing the operating cost significantly.
Recent Evolution and Major Application
From being a simple manufacturing step to a process of multiple uses and benefits, significant technological developments have taken place in the field of homogenization. These developments have an impact on the design and geometry of homogenization valves, making it possible to work at higher pressures and with very short processing times.
High homogenization pressures were introduced as an alternative nonthermal treatment in the food industry. The application of high-pressure homogenization technology has been extended to industries other than dairy. There occurs a paradigm shift from standard homogenization of 0 – 50 MPa to high pressure homogenization of 50 – 300 MPa and ultrahigh pressure homogenization of pressure equal to or greater than 400 MPa due to changes in design of valves. Processing efficiency is modulated by applying pressure in different ranges or by applying the same pressure for different number of passes. Apart from normal application in disruption of fat globules, different modification in design and technology of homogenizers made it possible to extend its use to the following new areas:
- Reduction in food spoilage and improvement in food safety by decreasing the microbial load and inactivating enzymes – possibly due to effect of temperature and changes in structure of protein derived from mechanical stress.
- Recovery of different bioactive compounds (like polyphenols), various proteins, and fibrous materials.
- Increase in the functionality of food with respect to health effects by improving the bio accessibility and bioavailability.
The increasing interest of consumers and the food industry in improving the organoleptic and nutritional qualities of foods has boosted the use of homogenization process. Currently, the major factors driving the research interest in improving the bioavailability and bio accessibility of active components are concerns about food functionality and sustainability. For this purpose, alternative, soft and nonthermal technologies such as high pressures homogenization has huge potential.
*Research & Development, Mother Dairy Fruit & Vegetable (P) Limited