10.0 Homogenizers and Homogenization
(Consult: Potter, Food Science; Arbuckle: Ice Cream)
10.1 Purpose. The purpose of homogenization is to make more permanent and uniform suspension of the fat by reducing the fat globules to a very small diameter, preferably not more than 1µ (0.001 mm). Homogenization is usually accomplished by forcing the liquid through a very small orifice under suitable conditions of temperature and pressure (50°C and 200 ATM, 1st stage and 50 ATM, 2nd stage are usual).
| 10.2 Diagram of a single stage homogenizing valve. The pressures used are in the range of 100-200 ATM. For milk, single stage homogenization may not be sufficient, because the process may lead to some degree of clumping of the small fat globules. However, this is easily corrected by constructing a 2-stage homogenizer, where the second stage valve is adjusted to a pressure of about 50 ATM or less, which is sufficient to disrupt the clumps. The physical effects of homogenization, relate to the altered nature of the fat globule surface. In the native state, the globules are protected by a membrane, which originates from the milk cell wall. The membrane is rich in phospholipids, such as lecithin, which is a surface-active component. It concentrates in the interface between the fat globules and the water in the milk serum. Thus, milk is an emulsion of insoluble fat |
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droplets which are dispersed throughout the water phase. The globules or droplets are prevented from coalescing, because they are protected by the membrane.
10.3.0 Surface Area Increase. During homogenization, the fat droplets become finely subdivided and the surface increases enormously. Consider the following calculations for surface area and volume for different size globules:
10.3.1 Increase in Surface area with diminishing globule size.
Diameter |
Volume |
Surface area |
Area/Volume |
|
10 |
523.60 |
314.16 |
0.60 |
|
9 |
381.70 |
254.47 |
0.67 |
|
8 |
268.08 |
201.06 |
0.75 |
|
7 |
179.59 |
153.94 |
0.86 |
|
6 |
113.10 |
113.10 |
1.00 |
|
5 |
65.45 |
78.54 |
1.20 |
|
4 |
33.51 |
50.27 |
1.50 |
|
3 |
14.14 |
28.27 |
2.00 |
|
2 |
4.19 |
12.57 |
3.00 |
|
1 |
0.52 |
3.14 |
6.00 |
| 10.3.2 Volume and Surface Area. The volume and surface area of a sphere are calculated as follows, where R is the radius. |
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10.4 Displaced Native Membrane. The table shows, that as globules are subdivided to a finer and finer size, the surface area increases relative to the volume. In the case of milk fat globules, there is no longer sufficient membrane material to cover the new surface and the globules become instead coated with milk protein, which also has surface-active properties.
10.5 Artificial Membrane. The following diagram shows how casein micelles may participate in this coating process. Casein micelles are small in comparison with fat globules ( about 1/100th of a micron). The micelles are built of sub-micelles, so that they resemble a raspberry. The outer surface of a micelle has a hydrophilic character (loves water; hates fat), while the portion, which points to the inner core, is more hydrophobic (hates water; loves fat).
10.5 Proteins in the Artificial Membrane. When the fat globule is subdivided, the fresh fat surfaces represent an attraction for the hydrophobic side of the submicelles, and they now coat the fat globules forming a new membrane. Therefore, the difference between native fat globules and the homogenized particles is not only the size, but the different membrane material. Notice, that it is only a small portion of the micelles, which are needed for the coating; the majority of the micelles remain intact.
10.6 Uses of Homogenization. Homogenization is used for making milk more stable against creaming and for giving the product a richer mouthfeel due to a slight increase in viscosity. Homogenization is also used in ice cream making. The homogenized mix gives a creamier product, when the mix is frozen. However, experienced ice cream makers know, that it is beneficial to store homogenized ice cream mix a few hours at low temperature (4°C) before freezing. This aging process permits any added emulsifying agent (glycerol mono- and di-stearates) to partially displace some of the casein submicelles from the fat surfaces. The reason, this is beneficial, is not well understood, and is still a subject of research.
10.7 Study Questions.
1. What are the benefits of homogenization of retail milk?
2. What is the purpose of homogenizing ice cream?
3. Should homogenization precede milk separation?
4. Should homogenization precede pasteurization?
5. What are the risks associated with homogenizing raw milk?
6. What is understood by "two-stage" homogenization and why is it used?
7. Distinguish between the native fat globule membrane and the artificial membrane created by homogenization.
