Microscopy Techniques Are Effective Tools for Implementing Studies in Dairy Science and Technology (original) (raw)
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Observations of casein micelles in skim milk concentrate by transmission electron microscopy
LWT - Food Science and Technology, 2007
The microstructure of casein micelles in ultrafiltrated (UF) skim milk concentrate at pH 6.5 and 5.8 was investigated by transmission electron microscopy using three different preparation methods. The volume fraction of the casein micelles in the UF concentrate was 62.8% (v/v) at pH 6.5 and fixation by glutaraldehyde revealed the close packing of micelles in the UF concentrate as well as a higher degree of micelle aggregation at pH 5.8. No details of the microstructure of the micellar surface or core could, however, be observed. Freeze-fracture of cryoprotected, i.e. glycerol, UF concentrate on the other hand, exposed the finer structures of the micellar core but no pH dependent differences were observed. As cryoprotection includes a dilution of the sample with glycerol, the packing of the micelles in the UF concentrate could not be observed. Undiluted UF concentrate exposed to rapid freezing using a propane jet followed by freezefracture exhibited development of ice crystals but rough areas on the micrographs were identified as fractured casein micelles. The micellar core appeared rougher and differences in the micellar core microstructure due to changing pH could be observed when this preparation method was used.
2010
Mastitis deteriorates the quality of raw milk and higher proteolytic activity in mastitic milk has been reported. There are no published studies on the effects of health status of the animal on the casein micelle microstructure, therefore the specific objective of this study was to determine the effects of somatic cell count (SCC) on the dimension and microstructure of casein micelles in raw milk. Raw milk samples were collected from the individual quarter of the dairy cattle in three different levels of SCCs (i.e., low: SCC<200,000, medium: 200,000<SCC<800,000 and high: SCC >800,000 cells/ml) and prepared for study by Scanning Electron Microscopy (SEM). The results suggested that the size of casein micelle changes in mastitic milk, and there were significant differences in casein micelle microstructure between high and low SCC milk samples. The mean diameter of micelles in medium and high SCC specimen decreases dramatically and SEM micrographs revealed that aggregation of casein micelles increases considerably in raw milk samples, especially in high SCC milk, except for that of low SCC. Increase the activity of proteolytic enzymes such as plasmin and lysosomal enzymes (Elastase, Cathepsin B, D and G, etc) was realized as the main reason of hydrolyzing casein in milk, especially in milk with high SCC, which can be the cause of changes in size and tendency to aggregation of casein micelles in mastitic milk. In addition, decreased milk synthesis ability and reduced steric and electrostatic repulsion among casein micelles might be other causes for this phenomenon.
Microstructural properties of milk fat globules
International Dairy Journal, 2013
The surface of milk fat globules consists of a biological membrane rich in polar lipids and glycoproteins. However, high shear stress applied upon homogenization disrupts the membrane and leads to the adsorption of casein micelles, as the major protein fraction of milk. These changes in the interface properties could affect the interactions between native or homogenized milk fat globules and the surrounding protein matrix, at neutral pH and upon acidification. In this study, macroscale rheometry, microscopic observations, nanoscale AFM-based force spectroscopy and physico-chemical analysis were combined to examine the interfacial composition and structure of milk fat globules and to evaluate their interactions with casein micelles. We showed that the surface properties of milk fat globules (biological membrane vs. caseins) and pH govern their interactions with casein micelles. The adhesion between individual fat globules and casein micelles was higher upon homogenization, especially at acid pH where the work of adhesion increased from 3.3 x 10-18 to 14 x 10-18 J for native and homogenized fat globules, respectively. Consequently, casein-coated homogenized fat globules yield stiffer milk acid gels. These findings cast light on the importance of colloidal particle's surface properties and pH on their connectivity with the surrounding matrix, which modulates the bulk microstructure and rheological properties with potential functional consequences, such as milk lipid digestion.
Comparison of casein micelles in raw and reconstituted skim milk
Journal of Dairy Science, 2007
During the manufacture of skim milk powder, many important alterations to the casein micelles occur. This study investigates the nature and cause of these alterations and their reversibility upon reconstitution of the powders in water. Samples of skim milk and powder were taken at different stages of commercial production of low-, medium-, and high-heat powders. The nature and composition of the casein micelles were analyzed using a variety of analytical techniques including photon correlation spectroscopy, transmission electron microscopy, turbidity, and protein electrophoresis. It was found that during heat treatment, whey proteins are denatured and become attached to the casein micelles, resulting in larger micelles and more turbid milk. The extent of whey protein attachment to the micelles is directly related to the severity of the heat treatment. It also appeared that whey proteins denatured during heat treatment may continue to attach to casein micelles during water removal (evaporation and spraydrying). The process of water removal causes casein and Ca in the serum to become increasingly associated with the micelles. This results in much larger, denser micelles, increasing the turbidity while decreasing the viscosity of the milk. During reconstitution, the native equilibrium between colloidal Ca and serum Ca is slowly reestablished. The reequilibration of the caseins and detachment of the whey proteins occur even more slowly. The rate of reequilibration does not appear to be influenced by shear or temperature in the range of 4 to 40°C.
Interactive Effects of Milk Fat Globule and Casein Micelle Size on the Renneting Properties of Milk
Food and Bioprocess Technology, 2014
The size of the casein micelles (CM) and the milk fat globules (MFG) vary depending on farming factors, such as seasonal variation and stage of lactation, and cow genetics. The MFG and CM size of milk can influence the renneting behavior and texture of manufactured dairy products. In this work, we investigated the combined effects of MFG and CM size on the onset of gelation, the maximum rate of gelation, the value for G′ 60 min (the final storage modulus) and G″ 60 min (the final loss modulus), and tan δ upon renneting. Fractionation of MFG on the basis of size was carried out using laboratorybased centrifugation, whereas milk of predominantly large (184-218 nm) or small (147-159 nm) CM was selected naturally on-farm. Casein micelle size had the dominant effect on curd firmness and gelation rates of milk, where small CM milk formed rennet gels earlier and resulted in a firmer gel than milk with large CM. However, MFG size also influenced the renneting properties. The strongest rennet gels were obtained when large MFG (3.88-5.78 μm) was combined with small CM (153-159 nm). Selecting milk on the basis of the microstructure of key milk components could be achieved by natural selection of dairy cows or via fractionation technologies. Selection may provide a useful tool for efficient manufacturing of different dairy products based on the desirable characteristics specific to each.
International Dairy Journal, 2008
Fluorescence microscopy studies of fat globules in milk and milk products using fat-soluble stains (Nile Blue or Nile Red) allow visualisation of the neutral fat (core) of the globules, but provide no information about the milk fat globule membrane (MFGM). We applied the lipophilic probes 1,1 0-dioctadecyl-3,3,3 0 ,3 0tetramethylindocarbocyanine-5,5 0-disulfonic acid (DiIC 18 (3)-DS) and N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide (FM4-64), as well as fluorescent conjugates of the lectin wheat germ agglutinin (WGA488, WGA594 and WGA647), to milk to stain specifically the MFGM in its native environment. Using fluorescence microscopy, we observed various distribution patterns of the probes. This demonstrates that the MFGM of fat globules in harvested milk is structurally and chemically heterogeneous both within and among globules from the same species and probably between species. We propose that the use of membrane-specific fluorescent probes has significant potential for providing real time structural and chemical information about the MFGM in matrices such as mammary gland tissue, harvested milk, and milk products.
LWT - Food Science and Technology, 2004
Reconstituted skim milk samples at pH between 6.5 and 7.1 (heating pH) were heated at 80 C, 90 C or 100 C for 30 min (heating temperature). The particle size of the casein micelles was measured at pH 4.75-7.1 (measurement pH) and at temperatures of 10 C, 20 C and 30 C (measurement temperature) using photon correlation spectroscopy. The particle size of the casein micelles, at a measurement pH of 6.7 and a measurement temperature of 20 C, was dependent on the heating pH and heating temperature to which the milk was subjected. The casein micelle size in unheated milk was about 215 nm. At a heating pH of 6.5, the casein micelle size increased by about 15, 30 and 40 nm when the milk was heated at 80 C, 90 C or 100 C, respectively. As the heating pH of the milk was increased, the size of the casein micelles decreased so that, at pH 7.1, the casein micelles were B20 nm smaller than those from unheated milk. Larger effects were observed as the heating temperature was increased from 80 C to 100 C. The size differences as a consequence of the heating pH were maintained at all measurement temperatures and at all measurement pH down to the pH at which aggregation of the micelles was observed. For all samples, size measurements at 10 C showed no aggregation at all measurement pH. Aggregation occurred at progressively higher pH as the measurement temperature was increased. Aggregation also occurred at a progressively higher measurement pH as the heating pH was increased. The particle size changes on heating and the aggregation on subsequent acidification may be related to the pH dependence of the association of whey proteins with, and the dissociation of k-casein from the casein micelles as milk is heated.
Properties of casein micelles reformed from heated mixtures of milk and ethanol
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003
Addition of an aqueous solution of ethanol ( /30% ethanol) to skim milk, followed by heating to /60 8C, resulted in the mixture becoming translucent; objectively, its Hunter L -value was reduced. The degree of translucence increased with increasing ethanol concentration and temperature; the effect was not fully reversible on cooling, as cooled mixtures had higher turbidity values than unheated mixtures. Following removal of ethanol by vacuum rotary evaporation and readjustment to the original milk solids concentration, the milk contained large protein aggregates (referred to as reformed micelles), the properties of which were studied. Milk containing reformed micelles had a type B heat coagulation time Á/pH profile and was less stable to ethanol than control milk. The mechanism responsible for these changes in the properties of milk appeared to be independent of calcium or other milk salts, but was probably linked to ethanol-induced denaturation of whey proteins. # address: a.kelly@ucc.ie (A.L. Kelly). Colloids and Surfaces A: Physicochem. Eng. Aspects 213 (2003) 265 Á/273 www.elsevier.com/locate/colsurfa 0927-7757/02/$ -see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 -7 7 5 7 ( 0 2 ) 0 0 5 1 9 -8
Molecular weight and size distribution of bovine milk casein micelles
Biochimica et biophysica acta, 1974
We have characterized the distribution of hydrodynamic radii, sedimentation coefficients, and molecular weights of native bovine casein micelles, using samples fractionated by rate-zone centrifugation in sucrose density gradients. Diffusion coefficients were measured by inelastic laser light scattering, and were combined with sedimentation coefficients to yield molecular weights. The distribution of molecular weights is quite broad, with a ratio of weight-average to number average of about 3.