Fat globules in milk

Figure 3.15 Average diameter of the fat globules in milk of Guernsey or Friesian cows throughout lactation (from Walstra and Jenness, 1984).

A thin membrane surrounds the fat globules in milk (King 1955). It contains a complex mixture of proteins and lipids. Some of its proteins are enzymes and are classified, along with the other enzymes present in milk, according to the nomenclature proposed by the International Commission of Enzymes set up by the Union of Biochemistry and adopted by the ADSA Committee on Enzyme Nomenclature (Shahani et al. 1973). 

Phipps, L. W. 1974. Cavitation and separated flow in a simple homogenizing valve and their influence on the break-up of fat globules in milk. J Dairy Res. 41, 1-8. 

In addition, to release lipids from source material, such as those in starch, fish meal, or milk, it might be necessary to treat the sample with an acid prior to lipid extraction (see Basic Protocol 4). In the case of milk, addition of ammonium hydroxide is necessary to dissolve casein prior to lipid extraction, which will release the lipids from its surrounding matrix (e.g., from the film surrounding the fat globules in milk). Furthermore, in certain cases, it is necessary to predry the sample in order to allow efficient and complete extraction of lipids. Particle size reduction is another factor that may improve lipid extraction efficacy. 

Factors that Affect the Surface Layers of Fat Globules in Milk and Cream... 

The surface layers of the fat globules in milk are affected by various treatments. Effects of homogenization are described in Sections 5.13 and 5.14 however, other treatments such as cooling and heating, as well as environmental conditions, also influence the surface layers of milk fat globules and are described in this section. 

Figure 5.3. Schematic illustration of the relative effects of heating and homogenization on fat globules in milk. MFG = milk fat globule, CM = casein micelle, WP = whey protein, dWP = denatured whey protein.

With an increase in dispersed-phase volume fraction, the viscosity of an emulsion increases. This increase in viscosity is linear at a low droplet concentration (McClements, 1999) the viscosity of an emulsion of milk fat globules in milk plasma increases linearly with fat content up to 30% (Bakshi and Smith, 1984 Kyazze and Starov, 2004), whereas the viscosity of low-fat milk (<2.0% fat) increases in a near linear fashion with fat content (Phillips et al., 1995). However, above a certain volume fraction of the dispersed phase, the droplets in emulsions are packed so closely that flow is impaired, giving the emulsion a gel-like character (McClements, 1999). For instance, the viscosity of cream increases rapidly with increasing fat content when the fat content is >50% (Prentice, 1968 Mulder Walstra, 1974). 

Wade, T., Beattie, J.K. 1997. Electroacoustic determination of size and zeta potential of fat globules in milk and cream emulsions. Coll. Surf. B. 10, 73-85. 

Ultrasound may be used for disruption of fat globules in milk and is an alternative to homogenization for this purpose. It also has the potential to alter the functionality of milk, as demonstrated by its effects on the properties of yoghurt (Vercet et al. 2002). These authors showed that the simultaneous application of heat and ultrasound (12 s at 20 kHz) under moderate pressure (2-kg pressure) improved the textural properties of yoghurts. 

Figure 3.24 Effect of homogenization on the size (volume distribution) of fat globules in milk (modified from Mulder and Walstra, 1974).

This is not to say that milk has no colored substances. It does. Milk contains riboflavin, or vitamin B2, an important enzyme cofactor that has a greenish-yellow hue. Riboflavin is water soluble, so its color is easiest to see in the whey that forms when the liquid portion of milk is separated from the solids, as it is in cheese making. The fat globules in milk have a yellow tinge due to carotenoids, such as beta-carotene, which come from the diet of the milk source. Beta-carotene has nutritional importance because it is a precursor to vitamin A. Summer cow s milk is yellower that winter cow s milk because of what the cows have been eating — fresh green grass has more beta-carotene than hay does. 

Formation of emulsions has been reviewed by Walstra (1983). In contrast to solid colloids, there appears to be much less control of the sizes of emulsion drops. Mechanical devices called homogenizCTs can reduce fat globules in milk to sizes where they take much longer to flocculate and cream to the top. Homog-enizers capable of making nearly uniform size drops are beginning to appear. 

Electron microscopy studies have revealed that the fat globules in milk have small particles at-... 

From the surface tension isotherms reported it can be generalized that interactions between protein and amphiphile in the bulk solution are closely related to adsorption behaviour at the air/water interface. Association of lipid-like substances to proteins results in plateau regions of the isotherms, within which neither changes in the amphiphile concentration, nor the supposed consecutive unfolding of the protein structure is reflected in the zly-value. Replacement of protein in the surface film by a lipid type of amphiphile, when no interaction occurs, was seen in the mono-caproin-ovalbumin isotherm, and also in the SDS-protein isotherms at sufficiently high amphiphile concentrations. Similar effects have also been observed in the membrane of the fat globule in milk [24], by addition of a nonionic amphiphile at emulsification. 

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