The technology behind disruption of fat globules
Homogenization has become a standard industrial process, universally practised as a means of stabilizing the fat emulsion against gravity separation. Gaulin, who invented the process in 1899, described it in French as “fixer la composition des liquides”.
Homogenization primarily causes disruption of fat globules into much smaller ones (Figure 6.3.1). Consequently, it diminishes creaming and may also diminish the tendency of globules to clump or coalesce. Essentially, all homogenized milk is produced by mechanical means. Milk is forced through a small passage at high velocity.
The disintegration of the original fat globules is achieved by a combination of contributing factors such as turbulence and cavitation. The homogenization reduces fat globule size from an average of 3.5 µm in diameter to below 1 µm. This is accompanied by a four- to six-fold increase in the fat/plasma interfacial surface area. The newly created fat globules are no longer completely covered with the original membrane material. Instead, they are surfaced with a mixture of proteins adsorbed from the plasma phase.
1) studied a fat-protein complex produced by the homogenization of milk. They showed that casein was the protein half of the complex and that it was probably associated with the fat fraction through polar bonding forces. They postulated further that the casein micelle was activated at the moment it passed through the valve of the homogenizer, predisposing it to interaction with the lipid phase.
The physical state and concentration of the fat phase at the time of homogenization contribute materially to the size and dispersion of the ensuing fat globules.
Homogenization of cold milk, in which the fat is essentially solidified, is virtually ineffective. Processing at temperatures conducive to the partial solidification of milk fat (i.e. below 40 °C) results in incomplete dispersion of the fat phase.
Products of high fat content are more difficult to homogenize and also more likely to show evidence of fat clumping, because the concentration of serum proteins is low in relation to the fat content. Usually, cream with higher fat content than 20 % cannot be homogenized at high pressure, because clusters are formed as a result of lack of membrane material (casein). Increasing the homogenization temperature decreases the viscosity of milk and improves the transport of membrane material to the fat globules.
Homogenization temperatures normally applied are 55 – 80 °C, and homogenization pressure is between 10 and 25 MPa (100 – 250 bar), depending on the product.
When the liquid passes the narrow gap, the flow velocity increases (Figure 6.3.2). The speed will increase until the static pressure is so low that the liquid starts to boil. The maximum speed depends mainly on the inlet (homogenization) pressure. When the liquid leaves the gap, the speed decreases and the pressure increases again. The liquid stops boiling and the steam bubbles implode.
Many theories of the mechanism of high pressure homogenization have been presented over the years. For a low-viscous oil-in-water dispersion like milk, where most of the droplets are in the order of one µm (10–6 m) in diameter, two theories have survived. Together, they give a good explanation of the influence of different parameters on the homogenizing effect.
The theory of globule disruption by turbulent eddies (“micro whirls”) is based on the fact that a liquid jet is formed at the outlet of the gap. As the jet is broken up many small eddies are created. Higher pressure equals higher jet velocity that gives smaller eddies and more energy rich eddies. If an eddy hits an oil droplet of about the same size, the droplet will be deformed and finally break up. This theory predicts how the homogenizing effect varies with the homogenizing pressure. This relation has been shown in many investigations.
The cavitation theory, on the other hand, claims that the shock waves created when the steam bubbles implode disrupt the fat droplets. According to this theory, homogenization takes place when the liquid is leaving the gap, so the back pressure which is important to control the cavitation is important to homogenization. This has also been shown in practice. However, it is possible to homogenize without cavitation, but it is less efficient.
Single-stage and two-stage homogenization
Homogenizers may be equipped with one homogenizing device or two connected in series, hence the names single-stage homogenization and two-stage homogenization. The two-stage system is illustrated in Figure 6.3.5.
In both single-stage homogenization and two-stage homogenization, the whole homogenization pressure (P1) is used over the first device. In single-stage homogenization, the back pressure (P2) is created by the process. In two-stage homogenization the back pressure (P2) is created by the second stage. In this case the back pressure can be chosen to achieve optimal homogenization efficiency. Using modern devices, the best results are obtained when the relation P2/P1 is about 0.2. The second stage also reduces noise and vibrations in the outlet pipe.
Single-stage homogenization may be used for homogenization of products with high fat content demanding a high viscosity (certain cluster formation).
Two-stage homogenization is used primarily to reach optimal homogenization results and to break up fat clusters in products with a high fat content. The formation and break-up of clusters in the second stage is illustrated in Figure 6.3.3.
Effect of homogenization
The effect of homogenization on the physical structure of milk has many advantages:
- Smaller fat globules leading to less cream-line formation
- Whiter and more appetizing colour
- Reduced sensitivity to fat oxidation
- More full-bodied flavour, and better mouthfeel
- Better stability of cultured milk products
However, homogenization also has certain disadvantages:
- Somewhat increased sensitivity to light – sunlight and fluorescent tubes – can result in “sunlight flavour” (see also Chapter 8, Pasteurized milk products).
- The milk might be less suitable for production of semi-hard or hard cheeses because the coagulum will be too soft and difficult to dewater.
A high-pressure homogenizer is a pump with a homogenization device. A homogenizer is generally needed when high-efficiency homogenization is required.
The product enters the pump block and is pressurized by the piston pump. The pressure that is achieved is determined by the back-pressure given by the distance between the forcer and seat in the homogenization device. This pressure P1 (Figure 6.3.8) is always designated the homogenization pressure. P2 is the back-pressure to the first stage.
The high-pressure pump
In Figure 6.3.4, the piston pump is driven by a powerful electric motor (1), via belts (2) and pulleys through a gearbox (3) to the crankshaft (10) and connecting-rod transmission, which converts the rotary motion of the motor to the reciprocating motion of the pump pistons (9).
A piston pump is a positive pump and its capacity can only be adjusted by changing the speed of the motor or changing the size of the pulleys. To handle higher pressures, pistons with smaller diameter are installed. This will reduce the maximum capacity, as each machine size has a maximum crankshaft speed. A larger machine has a longer stroke length and/or more pistons. In many cases these pistons also have a larger diameter.
A high-pressure pump has normally three to five pistons (9), running in cylinders in a high-pressure block (8). They are made of highly resistant materials. The machine is fitted with double piston-seals. Water is supplied to the space between the seals to lubricate the pistons. A mixture of hot condensate and steam can also be supplied to prevent reinfection when the homogenizer is placed downstream in aseptic processes.
A piston pump will always generate a pulsating flow. The acceleration and deceleration of the liquid will create a pulsating pressure in the suction pipe. To avoid cavitation in the pump, there is always a damper on the suction pipe to reduce the pulsation. On the outlet side, the pulsation might create vibrations and noise, why the outlet pipe is also equipped with a damper.
As it is a positive pump, a piston pump should not operate in a series of other positive pumps, unless there is a bypass – otherwise the result can be extreme pressure variations and damaged equipment. If the flow can be stopped downstream of a high-pressure pump, a safety device must be installed that opens before the pipe bursts.
The homogenization device
Figure 6.3.5 shows the homogenization and hydraulic system. The piston pump boosts the pressure of the milk from about 300 kPa (3 bar) at the inlet to a homogenization pressure of 10 – 25 MPa (100 – 250 bar), depending on the product. The pressure to the first stage before the device (the homogenization pressure) is automatically kept constant. The oil pressure on the hydraulic piston and the homogenization pressure on the forcer balance each other. The hydraulic unit can supply both first and second stage with an individually set pressure. The homogenization pressure is set by adjusting the oil pressure. Actual homogenization pressure can be read on a pressure gauge.
Homogenization always takes place in the first stage. The second stage basically serves two purposes:
- Supplying a constant and controlled back-pressure to the first stage, giving best possible conditions for homogenization
- Breaking up clusters formed directly after homogenization as shown in Figure 6.3.3.
The parts in the homogenization device are precision-ground. Its seat is at an angle that makes the product accelerate in a controlled way, thereby reducing the rapid wear and tear that would otherwise occur.
Milk is supplied at high pressure to the space between the seat and forcer. The distance between the seat and the forcer is approximately 0.1 mm or 100 times the size of the fat globules in homogenized milk. The velocity of the liquid is normally 100 – 400 m/s in the narrow annular gap. The higher the homogenization pressure, the higher the speed.
Homogenization takes 10 – 15 microseconds. During this time, all the pressure energy delivered by the piston pump is converted into kinetic energy. Part of this energy is converted back to pressure again after the device. The other part is released as heat; every 40 bar in pressure drop over the device gives a temperature rise of 1 °C. Less than 1 % of the energy is utilized for homogenization, but nevertheless, high-pressure homogenization is the most efficient method available.
The purpose of homogenization varies with the application. Consequently the methods of measuring efficiency also vary.
According to Stokes’ Law, the rising velocity of a particle is given by:
vg = velocity
g = force of gravity
p = particle size
ρhp = density of the liquid
ρlp = density of the particle
t = viscosity
in the formula:
It can be seen that reducing the particle size is an efficient way of reducing the rising velocity. Therefore, reducing the size of fat globules in milk reduces the creaming rate.
Analytical methods for determining homogenization efficiency can be divided into two groups:
Studies of creaming rate
The straight forward way of determining the creaming rate is to take a package, store it at the recommended storage temperature until the last day of consumption, open it and check if the cream layer is acceptable or not.
The USPH method is based on this. A sample of, say, 1 000 ml is stored for 48 hours, after which the fat content of the top 100 ml is determined, as well as the fat content of the rest. Homogenization is reckoned to be sufficient if 0.9 times the top fat content is less than the bottom fat content.
The NIZO method is based on the same principle, but with this method, a sample of 25 ml is centrifuged for 30 minutes at 1 000 rpm, 40 °C and a radius of 250 mm. The fat content of the 20 ml at the bottom is divided by the fat content of the whole sample, and the ratio is multiplied by 100. The resulting index is called the NIZO value. The NIZO value of pasteurized milk is normally 60 – 70 %.
Size distribution analysis
The size distribution of the particles or droplets in a sample can be determined in a well defined way by using a laser diffraction unit (Figure 6.3.6), which sends a laser beam through a sample in a cuvette. The light will be scattered and absorbed, depending on the size, refractive index and numbers of particles in the sample.
The result is presented as size distribution curves. The percentage of the volume (fat) is given as a function of the particle size (fat globule size). Three typical size distribution curves for milk are shown in Figure 6.3.7. It can be seen that the curve shifts to the left as a higher homogenization pressure is used.
Note that fat globules can aggregate during storage and that this can increase the creaming rate.
Energy consumption and influence on temperature
The electrical power input needed for homogenization is expressed by the formula:
E = Electrical effect, kW
Qin = Feed capacity, l/h 10 000
P1 = Homogenization pressure, bar 200 (20 MPa)
Pin = Pressure to the pump, bar 2 (200 kPa)
ηpump = Efficiency coefficient of the pump 0.85
ηel. motor = Efficiency coefficient of the electrical motor 0.95
The efficiency coefficients are typical values. From the figures for feed capacity and pressures given on the right above, the electric power demand will be 68 kW. Of this, 55 kW is used for pumping and converted to heat in the homogenization device, and 13 kW is released as heat to the cooling water and to the air.
As was mentioned above, part of the pressure energy supplied is released as heat. Given the temperature of the feed, Tin, the homogenization pressure, P1, the pressure after homogenization, Pout, and that every 4 MPa (40 bar) in pressure drop raises the temperature by 1 °C, the following formula is applicable:
The energy consumption, temperature increase and pressure decrease are illustrated in Figure 6.3.8.
Tin = 65 °C
P1 = 200 bar (20 MPa)
Pout = 4 bar (400 kPa)
Tout = 70 °C
The homogenizer in a processing line
In general, the homogenizer is placed upstream, i.e. before the final heating section in a heat exchanger. In most pasteurization plants for market milk production, the homogenizer is usually placed after the first regenerative section.
In production of UHT milk, the homogenizer is generally placed upstream in indirect systems but always downstream in direct systems, i.e. on the aseptic side after UHT treatment. In the latter case, the homogenizer is of aseptic design with special piston seals, sterile steam condenser and special aseptic dampers.
However, downstream location of the homogenizer is recommended for indirect UHT systems when milk products with a fat content higher than
6 – 10 % and/or with increased protein content are going to be processed. The reason is that with increased fat and protein contents, fat clusters and/or agglomerates (protein) form at the very high heat treatment temperatures. These clusters/agglomerates are broken up by the aseptic homogenizer located downstream.
An aseptic homogenizer is more expensive to operate. In some cases it is sufficient if just the second stage is placed downstream. This arrangement is called split homogenization.
Note that the whole section, including the heat exchanger, between the first and the second stage in the homogenizer, has to withstand a fairly high pressure.
Full stream homogenization
Full stream or total homogenization is the most commonly used form of homogenization of UHT milk and milk intended for cultured milk products.
The fat content of the milk is standardized prior to homogenization, as is the solids-non-fat content in certain circumstances, e.g. in yoghurt production.
Partial stream homogenization means that the main body of skim milk is not homogenized, but only the cream together with a small proportion of skim milk. This form of homogenization is mainly applied to pasteurized market milk. The basic reason is to reduce operating costs. Total power
consumption is cut by some 80 % because of the smaller volume passing through the homogenizer.
As sufficiently good homogenization can be reached when the product contains at least 0.2 g casein per g fat, a maximum cream fat content of 18% is recommended. The hourly capacity of a homogenizer used for partial homogenization can be dimensioned according to the following example.
Qp = Plant input capacity, l/h 10 000
Qsm = Output of standardized milk, l/h
Qh = Homogenizer capacity, l/h
frm = Fat content of raw milk, % 4.0
fsm = Fat content of standardized milk, % 3.5
fcs = Fat content of cream from separator, % 35
fch = Fat content of cream to be homogenized, % 18
The hourly output of pasteurized standardized milk, Qsm, will be approx. 9 840 l. Inserted into Formula 2, this gives an hourly homogenizer capacity of approx. 1 915 l, i.e. about one-fifth of the output capacity.
The flow pattern in a plant for partially homogenized milk is illustrated in Figure 6.3.9.
- 1. 1) Fox, K.K., Holsinger, Virginia, Caha, Jeanne and Pallasch, M.J., J. Dairy Sci, 43, 1396 (1960).