Fat particles in milk

STORE S LAW. (1) The rate at which a spherical particle will rise or fall when suspended in a liquid medium varies as the square of its radius the density of the particle and the density and viscosity of the liquid are essential factors, Stoke s lav/ is used in determining sedimentation of solids, creaming rale of fat particles in milk, etc. (2) In atomic processes, the wavelength of fluorescent radiation is always longer than that of the exciting radiation,... 

Almost any material can be observed within the SEM. Scientists routinely look at materials as diverse as contact lenses and engine parts, through to the fat particles in milk and crystals in diesel oil. The common factor between all types of specimens is the need to make them ready for viewing in the microscope. 

Wellings, S. R., Grunbaum, B. W. and Deome, K. B. 1960B. Electron microscopy of milk secretion in the mammary gland of the C3H/Crgl mouse. II. Identification of fat and protein particles in milk and in tissue. J. Natl. Cancer Inst. 25, 423-437. 

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. 

Singh, H., Sharma, R., Taylor, M.W., Creamer. L.K. 1996. Heat-induced aggregation and dissociation of protein and fat particles in recombined milk. Neth. Milk Dairy J. 50, 149-166. 

Casein is the most important protein body in milk. Besides its chief function as nitrogenous food it plays the important role of protective colloid for the fat particles and also for calcium phosphate with which it forms a colloidal combination. It keeps the fat particles in a fine state of emulsion and prevents them from uniting into clots, probably due to the formation of a fine membrane around the particle. Casein may be precipitated from milk by acids. It is insoluble in water and also in the solutions of neutral salts, except sodium fluoride and potassium oxalate. Casein has acid properties, drives carbon dioxide out of carbonates, and is soluble in alkalis. The ammoniacal solution possesses a high protective effect toward colloidal gold. The gold number 0.01 indicates that it belongs therefore to the protective colloids of the first class. The behavior toward bases has been care-... 

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

Pasteurization may be carried out by batch- or continuous-flow processes. In the batch process, each particle of milk must be heated to at least 63°C and held continuously at this temperature for at least 30 min. In the continuous process, milk is heated to at least 72°C for at least 15 s ia what is known as high temperature—short time (HTST) pasteurization, the primary method used for fluid milk. For milk products having a fat content above that of milk or that contain added sweeteners, 66°C is requited for the batch process and 75°C for the HTST process. For either method, foUowiag pasteurization the product should be cooled quickly to <7.2° C. Time—temperature relationships have been estabHshed for other products including ice cream mix, which is heated to 78°C for 15 s, and eggnog, which must be pasteurized at 69°C for 30 min or 80°C for 25 s. 

Cocoa butter is the common name given to the fat obtained by subjecting chocolate Hquor to hydrauHc pressure. It is the main carrier and suspending medium for cocoa particles in chocolate Hquor and for sugar and other ingredients in sweet and milk chocolate. 

Control of the polymorphic forms in cocoa butter is further compHcated by the presence of other fats such as milk fat. The fat in a chocolate can be likened to the mortar between the bricks in a mason s wall. The soHd particles in a weU-conched chocolate bed down better than the soHds in a coarsely refined and poorly mixed one (30). 

But the microscopic fat particles suspended in milk have an average diameter in the range 10-7 to 10-5 m, i.e. much larger than k of visible light. A beam of incident light is scattered rather than transmitted by a suspension of particles - a phenomenon known as the Tyndall effect. 

In summary, milk appears cloudy because the suspended fat particles scatter any incident light. 

Chicken broth is cloudy because it is colloidal, containing microscopic particles of chicken fat suspended in the water-based soup. Like milk, cream or emulsion paint, the cloudy aspect of the soup is a manifestation of the Tyndall effect. Adding the eggshells to the colloidal solution removes these particles of fat, thereby removing the dispersed medium. And without the dispersed medium, the colloid is lost, and the soup no longer shows its cloudy appearance. We say we have broken the colloid. 

A phase can be defined as a domain bounded by a closed surface in which parameters such as composition, temperature, pressure and refractive index are constant but change abruptly at the interface. The principal phases in milk are its serum and fat and the most important interfaces are air/serum and fat/serum. If present, air bubbles, and ice, fat or lactose crystals will also constitute phases. Forces acting on molecules or particles in the bulk of a phase differ from those at an interface since the former are attracted equally in all directions while those at an interface experience a net attraction towards the bulk phase (Figure 11.6). 

In milk plasma, fat may be present as extremely small globules, water-soluble fatty acids and other lipids, water-dispersible lipids, or lipoprotein particles. The amount is small, 0.02-0.03%. Obviously, most of the lipid is TG in the core of the globules. 

Milk or skim milk is standardized to meet milk fat and total milk solids compositional requirements in the final dry milk product e.g., NFDM must contain <1.5% milk fat DWM must contain >26% and < 40% milk fat, and both must contain <5% moisture on an MSNF basis (CFR, 1982). Dry milk products must also meet U.S. Department of Agriculture and American Dry Milk Institute standards for titratable acidity, scorched particle content, solubility, and bacteria content. Product users may also have specialized requirements for color, flavor, and dispersibility. 

The structure of the interfacial layers in food colloids can be quite complex as these are usually comprised of mixtures of a variety of surfactants and all are probably at least partly adsorbed at interfaces which even individually, can form complex adsorption layers. The layers can be viscoelastic. Phospholipids form multi-lamellar structures at the interface and proteins, such as casein, can adsorb in a variety of conformations [78]. Lecithins not only adsorb also at interfaces, but can affect the conformations of adsorbed casein. The situation in food emulsions can be complicated further by the additional presence of solid particles. For example, the fat droplets in homogenized milk are surrounded by a membrane that contains phospholipid, protein and semi-solid casein micelles [78,816], Similarly, the oil droplets in mayonnaise are partly coated with granular particles formed from the phospho and lipo-protein components of egg yolk [78]. Finally, the phospholipids can also interact with proteins and lecithins to form independent vesicles [78], thus creating an additional dispersed phase. 

A food colloid comprising a suspension of cocoa, sugar and solid milk particles in a continuous fat phase. 

Daily articles of life Milk is an emulsion of fat dispersed in water stabilised by casein and as all know is practically a complete food. Ice cream, is an emulsion, in which ice particles are dispersed in cream, stabilised by gelatin. Artificial beverages, coffee, fruit jellies are all emulsions in nature. The cleansing action of ordinary soap is due to a large extent on the production of OAV emulsion. 

While free fat remains a controversial concept in liquid dairy products, it undoubtedly has an important role in whole milk powder (WMP) for certain applications (e.g., chocolate manufacture) high-free fat WMP is favored. Keogh et al, (2003) reported that the particle size in chocolate mixes after refining and the viscosity of the molten chocolate decreases as the free fat content of the WMP increased such changes have significant implications for the mouthfeel and smoothness of chocolate. For more information on the role of fat in milk powder, see Chapter 13... 

Hillbrick G.C., McMahon D.J., Deeth H.C. 1998. Electrical impedance particle size method (Coulter Counter) detects the large fat globules in poorly homogenized UHT processed milk. Aust. J. Dairy Technol. 53, 17-21. 

Michalski, M.C., Michel, F., Geneste, C. 2002. Appearance of submicronic particles in the milk fat globule size distribution upon mechanical treatments. Lait. 82, 193-208. 

---