Impact of cream washing on fat globules and milk fat globule membrane proteins

Milk proteins, contained within the aqueous phase surrounding fat globules, should be removed before analysis of the composition of the native milk fat globule membrane (MFGM). The effect of the conditions applied during washing of cream on MFGM integrity has not been fully studied, and factors potentially effecting a modification of MFGM structure have not been systematically assessed so far. In this study, a cream separator was used to investigate the impact of cream washing on milk fat globule stability and the corresponding loss of MFGM proteins. Flow velocity, fat content, and type of washing solution were varied. Particle size measurements and protein analyses were carried out after each washing step to determine fat globule coalescence, removal of skim milk proteins, and efficiency of MFGM isolation. Significant differences in fat globule stability and protein amount in the MFGM isolates were measured using different washing conditions.     

Size-based fractionation of native milk fat globules by two-stage centrifugal separation

A two-stage centrifugal separation method, at various separation temperatures and feed rates, was employed to fractionate milk and cream on the basis of fat globule size. It involved a modified and a conventional centrifugal separation in first and second stages, respectively. In the first stage, two streams of milk: one rich in larger fat globules and another rich in smaller fat globules, were obtained by fractionation in a modified cream separator. In the second stage, the two streams from the first stage were each further fractionated in a conventional cream separator. Depending on the temperature and feed rate of the first stage, this double separation method was able to create streams with mean fat globule size (D [4, 3]) as small as 1.35 μm and as large as 4.28 μm without affecting the droplet integrity. The developed method has potential for size-based fractionation of native fat globules in industrial scale.Industrial relevanceThe developed method has potential for size based fractionation of native fat globules in industrial scale.     

Native milk fat globule size and its influence on whipping properties

The objective of this study was to investigate the influence of native milk fat globule size on the aeration of high fat dairy products with regard to maximum firmness time, gas inclusion and foam stability. The results showed that whipping time to maximum firmness was inversely proportional to mean fat globule size for both unhomogenised and slightly homogenised (2 MPa) creams. Additionally, increasing native mean fat globule size of the creams resulted in increased overrun. No significant differences in serum drainage were found between creams with different native milk fat globule size. Furthermore, when creams with native large mean fat globules were homogenised, the results showed that the maximum firmness time was in accordance with the mean fat globule size of non-aggregated creams. In the present study, cream fractionation was achieved by creaming or in a cost effective and fast manner using a modified centrifugal separator.     

Proteolysis of milk fat globule membrane proteins during in vitro gastric digestion of milk

The influence of gastric proteolysis on the physicochemical characteristics of milk fat globules and the proteins of the milk fat globule membrane (MFGM) in raw milk and cream was examined in vitro in simulated gastric fluid (SGF) containing various pepsin concentrations at pH 1.6 for up to 2 h. Apparent flocculation of the milk fat globules occurred in raw milk samples incubated in SGF containing pepsin, but no coalescence was observed in either raw milk samples or cream samples. The changes in the particle size of the fat globules as a result of the flocculation were dependent on the pepsin concentration. Correspondingly, the physical characteristics of the fat globules and the composition of the MFGM proteins in raw milk changed during incubation in SGF containing pepsin. The major MFGM proteins were hydrolyzed at different rates by the pepsin in the SGF; butyrophilin was more resistant than xanthine oxidase, PAS 6, or PAS 7. Peptides with various molecular weights, which altered with the time of incubation and the pepsin concentration, were present at the surfaces of the fat globules.     

Influence of enzymatic cross-linking on milk fat globules and emulsifying properties of milk proteins

The enzyme transglutaminase (TGase) can modify dairy protein functionality through cross-linking of proteins. This study examined the effects of TGase treatment on milk fat globules and the emulsifying properties of milk proteins. The extent of TGase-induced cross-linking of caseins increased with incubation time, with no differences between whole and skim milk. Extensive clustering of fat globules in extensively cross-linked raw whole milk occurred on homogenisation at 400 or 800 bar. Considerably less clustering of fat globules was observed when recombined milk (90 g fat L^–1) was prepared from TGase-treated skim milk and homogenised at 400 or 800 bar. TGase treatment did not affect fat globule size in cream, but prevented coalescence of fat globules therein, possibly through cross-linking of milk fat globule membrane components. TGase-induced cross-linking of milk proteins affected their emulsifying properties and may increase the stability of natural milk fat globules against coalescence.     

Reexamination of Fat Globule Clustering and Creaming in Cow Milk

Fat globule clustering, as characterized by cream volume and cluster time, was studied in raw milk, heated milk, homogenized milk, and in model systems. Immunoglobulin M was confirmed as the heat-labile component in fat globule clustering and was shown to function as a cryoagglutinin rather than as a cryoglobulin as previously indicated. Hapten inhibition studies demonstrated that the antigen is carbohydrate. Skim milk membrane was identified as the homogenization-labile component. Although immunoglobulin M can agglutinate milk fat globules to a limited extent, normal creaming requires both immunoglobulin M and skim milk membrane. Approximately 7% of the immunoglobulin M in normal milk participates in a single creaming. The lower portion of creamed milk (gravity separated skim milk) failed to support creaming on addition of washed fat globules but did so on addition of skim milk membrane. A theory of fat globule clustering consistent with observed experimental results depicts immuno-globulin M interacting in an antigen-antibody mode simultaneously with skim milk membrane and milk fat globules through specific carbohydrate moieties.     

Physical,textural, and rheological properties of whipped cream affected by milk fat globule membrane protein

This work aims at improving the textural and whipping properties of whipped cream by the addition of milk fat globule membrane protein. The determination of particle size distribution and average diameter of whipped cream showed that the small particle size was shifted to a larger range after milk fat globule membrane protein was added. The average particle size (d_3,2) of whipped cream reached a maximum value of 5.05 µm at 1% milk fat globule membrane protein, while slowly decreased with increasing milk fat globule membrane protein levels from 2% to 5%. In addition, the partial coalescence of fat increased with the increase of milk fat globule membrane protein levels, and the correlation between the whipping time and the overrun of whipped cream was positive. The addition of milk fat globule membrane protein also altered the rheological behaviour of whipped cream, resulting in the increase of modulus G′ and the loss modulus G″. The results also indicated that higher milk fat globule membrane protein level decreased the serum loss of whipped cream while improved its stability. While milk fat globule membrane protein levels had no significant effect on viscosity, its increasing levels effectively improved the hardness, consistency, and viscosity of whipped cream.     

Disruption of fat globules during concentration of whole milk in a pilot scale multiple-effect evaporator

The effects of heat treatment by direct steam injection (DSI) and concentration of whole milk using a pilot scale multiple-effect evaporator on fat globule size and the total milk fat globule membrane (MFGM) proteins were examined. In both nonpreheated and preheated whole milk, the size of milk fat globules decreased while the surface protein concentration of the fat globules increased as the milk passed through each effect of the evaporator. These results indicate that the fat globules were disrupted during DSI heating and evaporation and the proteins from the skim milk were adsorbed onto the fat globule surface.     

Interactions of fat globule surface proteins during concentration of whole milk in a pilot-scale multiple-effect evaporator

The changes in milk fat globules and fat globule surface proteins during concentration of whole milk using a pilot-scale multiple-effect evaporator were examined. The effects of heat treatment of milk at 95 degrees C for 20 s, prior to evaporation, on fat globule size and the milk fat globule membrane (MFGM) proteins were also determined. In both non-preheated and preheated whole milk, the size of milk fat globules decreased while the amount of total surface proteins at the fat globules increased as the milk passed through each effect of the evaporator. In non-preheated samples, the amount of caseins at the surface of fat globules increased markedly during evaporation with a relatively small increase in whey proteins. In preheated samples, both caseins and whey proteins were observed at the surface of fat globules and the amounts of these proteins increased during subsequent steps of evaporation. The major original MFGM proteins, xanthine oxidase, butyrophilin, PAS 6 and PAS 7, did not change during evaporation, however, PAS 6 and PAS 7 decreased during preheating. These results indicate that the proteins from the skim milk were adsorbed onto the fat globule surface when the milk fat globules were disrupted during evaporation.     

The effect of sonication and high pressure homogenisation on the properties of pure cream

The homogenisation of milk and cream has been widely studied but the effect of sonication on the structural and functional properties of cream is not well known. In this study, raw milk, ultrafiltration retentate and cream samples were sonicated at 20 kHz and the rennet and acid gelation properties of these sonicated samples investigated. High pressure homogenisation at 80 bar was also performed for comparison. Sonication of raw milk and retentate samples led to a decrease in the fat globule size. Conversely, the fat globules in cream samples sonicated at < 10 °C flocculated to form grapelike structures whereas the cream samples sonicated at 50 °C did not form such aggregates. High pressure homogenisation at 50 °C led to similar flocculated structures, but these were not observed at low temperatures. This suggests a potential benefit of sonication technology in allowing low temperatures to be utilised for cream homogenisation, reducing energy demand. However, a gel made using cheese-milk with sonicated cream resulted in separation of a fat layer rather than the incorporation of the fat globules into the gel matrix. Rennet gelation properties of both the sonicated or homogenised samples were significantly superior to a native control sample where the resultant gels had shorter coagulation times and decreased syneresis.Industrial RelevanceHomogenisation of dairy cream is normally carried out at temperatures of around 50 °C, to ensure that the fat is in the liquid state. In this work, we show that we can achieve comparable changes to the fat globules within the cream using ultrasound at much lower temperatures (< 10 °C). The ability to form flocculated fat particles at lower temperatures could lead to reduced costs through reduced energy demand.     

The Effect of Milk Processing on the Microstructure of the Milk Fat Globule and Rennet Induced Gel Observed Using Confocal Laser Scanning Microscopy

ABSTRACT: Confocal laser scanning microscopy (CLSM) was successfully used to observe the effect of milk processing on the size and the morphology of the milk fat globule in raw milk, raw ultrafiltered milk, and standardized and pasteurized milk prepared for cheese manufacture (cheese-milk) and commercial pasteurized and homogenized milk. Fat globule size distributions for the milk preparations were analyzed using both image analysis and light scattering and both measurements produced similar data trends. Changes to the native milk fat globule membrane (MFGM) were tracked using a MFGM specific fluorescent stain that allowed MFGM proteins and adsorbed proteins to be differentiated on the fat globule surface. Sodium dodecyl sulfate polyacrylamide gel electrophoresis confirmed the identity of native MFGM proteins isolated from the surface of fat globules within raw, UF retentate, and cheese-milk preparations, whereas only casein was detected on the surface of fat globules in homogenized milk. The microstructure, porosity, and gel strength of the rennet induced gel made from raw milk and cheese-milk was also found to be comparable and significantly different to that made from homogenized milk. Our results highlight the potential use of CLSM as a tool to observe the structural details of the fat globule and associated membrane close to its native environment.     

Effect of the fat globule membrane on in vitro digestion of milk fat globules with pancreatic lipase

The rate and extent of in vitro lipid digestion in raw and recombined milk were investigated by determining the release of fatty acids in simulated intestinal fluid containing pancreatic lipase. Changes in the globule size, surface charge and microstructure of fat globules during digestion were examined. In the absence of bile extract, the rate of lipid digestion was slower in raw milk than in recombined milk, suggesting that the composition of the milk fat globule membrane influences the rate of lipid hydrolysis in milk. Flocculation of fat globules occurred in the early stages of digestion; the globules then coalesced to form large particles, from within which triacylglycerols were removed. However, in the presence of bile extract, the changes in the size and microstructure of the fat globules during digestion were different. Bile extract may therefore affect physicochemical interactions of fat globules and hence alter the lipolysis during the digestion.     

Native vs. damaged milk fat globules: membrane properties affect the viscoelasticity of milk gels

The storage modulus G' of rennet and acid milk gels filled with milk fat globules was measured as a function of the fat globule surface composition (native milk fat globule membrane, caseins and whey proteins, or a mixture of the three due to mechanical treatments) and surface area (i.e., the fat globule size). By different technological procedures, it was possible to obtain fat globules of constant surface composition but various sizes, and vice-versa, which had never been done. For both rennet and acid gels, a critical fraction of the fat globule surface covered by caseins and whey proteins was identified (approximately 40%), beyond which G' increased. Below this threshold, the gel viscoelasticity was unaffected by mechanical treatments. When the diameter of native milk fat globules decreased, the G' of rennet gels increased slightly, whereas that of acid gels decreased sharply. For both types of gels, G' increased when the diameter of partially disrupted fat globules decreased. For recombined globules completely covered with caseins and few whey proteins, G' increased with fat globule surface area for rennet gels whereas it decreased for acid gels. With the help of confocal microscopy and in the light of general structural differences between rennet and acid gels, a mechanism is proposed for the effect of fat globule damage and diameter on G', depending on the interactions the globules can undergo with the casein network.     

Ultrasonication retains more milk fat globule membrane proteins compared to equivalent shear-homogenization

Ultrasonication, like common shear homogenization, can reduce the milk fat globule size and may change the milk fat globule membrane (MFGM). This work compared the effect of ultrasonication to equivalent shear homogenization on MFGM proteins and lipid-derived volatile components. Results showed that treating milk with ultrasound at 35 kJ/L would realize a similar size distribution of the milk fat globules as shear-homogenization at 20 MPa. Proteomics analysis revealed that in total 192 MFGM proteins were identified and quantified and a number of these proteins were lost after both treatments; however, more MFGM proteins remained after ultrasonication than after shear-homogenization. SDS-PAGE results showed that milk plasma proteins, and especially caseins, were absorbed on the milk fat globules after both treatments. In addition, the amount of the volatile free fatty acids increased after both treatments.Industrial relevance: Ultrasonication, as an innovative food processing technology, in comparison to traditional homogenization, was shown to equally efficiently decrease the MFG size, but lead to less damage to native MFGM proteins, which may be due to its longer homogenization time window. These results increased knowledge on the biochemical changes of milk fat globules after their size reduction and showed that ultrasonication could be used as a novel approach to improve dairy product quality.     

Effect of the fat globule sizes on the meltdown of ice cream

The meltdown of ice cream is influenced by its composition and additives and by fat globule size. The objective of this study was to examine the effect of fat globule size and fat agglomerate size on the meltdown stability of ice cream. Therefore, an ice cream mix (10% milk fat) was homogenized at pressures ranging from 0 to 30 MPa in single-stage, double-stage, and selective homogenization processes. The ice cream, produced on a continuous ice cream freezer, was characterized by an optimized meltdown test while, in addition, the fat globule sizes and the free fat content were determined in the mix and the molten ice cream. The meltdown was dependent on the fat agglomerate sizes in the unfrozen serum phase. Agglomerates smaller than a critical diameter led to significantly higher meltdown rates. Homogenization pressures of at least 10 MPa were sufficient to produce a stable ice cream. Furthermore, proof was provided that double-stage homogenization is not necessary for fat contents up to 10% and that selective homogenization is possible to produce stable ice creams. Based on these results a model was deduced describing the stabilizing mechanisms during the meltdown process.     

瓜尔豆胶对搅打稀奶油的搅打性能的影响

研究了不同浓度的瓜尔豆胶对搅打稀奶油乳状液的表观黏度、脂肪球粒度、脂肪球界面蛋白浓度、脂肪球部分聚结率、泡沫硬度和搅打起泡率的影响。结果表明，瓜尔豆胶对搅打稀奶油乳状液的表观黏度影响非常显著；瓜尔豆胶浓度过高或过低，都会使得解冻后的乳状液粒径变大；瓜尔豆胶的质量分数越高，脂肪球部分聚结速度越快，泡沫硬度也越大；搅打起泡率随着瓜尔豆胶质量分数增大而降低。     

The fat globule membrane in ice cream

A method for determining the average thickness of the membrane around the fat globules in homogenised ice cream mix from the mix viscosity is examined. The difficulties with this approach, such as the non-Newtonian behaviour of the mix, the polydisperse nature of the fat globule size distribution and the contribution of the micellar casein to the volume fraction of the aqueous phase, are discussed. Values of 60 to 90 nm for the effective “hydrodynamic” thickness have been obtained for ice cream mixes of 9% w/w vegetable fat and 10.5% w/w skim milk solids. It is suggested from these values, and from the interpretation of electron micrographs of similar mixes, that part of the micellar casein is attached to fat globules, thus increasing the effective volume fraction of the fat and accounting for the abnormally high viscosities of these mixes.     

Fat Content of Milk and Cream and Effects on Fat Globule Stability

Milk fat globules with crystalline fat content were sheared in defined flow conditions in order to ascertain the critical strain rate, which led to destabilization of the fat globules. Based on a fluid-mechanical model which considered the influence of the dispersed phase, a critical shear rate was defined. The model predicted a linear decrease in destabilization shear rate coupled with an increase in fat volume concentration. This interrelationship was confirmed in experiments. Fat globule stability, as related to shear strain rate, increased at decreasing temperatures. Critical shear rates for milk and cream should be calculated at a fat content not exceeding 45% for temperatures between 5 and 20°C. Any motion at higher fat content had a destabilizing effect on fat globules.     

Effects of Size and Stability of Native Fat Globules on the Formation of Milk Gel Induced by Rennet

Rennet‐induced gelation crucially impacts cheese structure. In this study, effects of the size and stability of native fat globules on the kinetics of rennet‐induced coagulation were revealed by determining the caseinomacropeptide release rate and rheological properties of milk. Moreover, the mobility and stability of fat globules during renneting was revealed using diffusing wave spectroscopy and confocal laser scanning microscopy. By use of a 2‐stage gravity separation combined centrifugation scheme, native fat globules were selectively separated into small (SFG, D_4,3 = 1.87 ± 0.02 μm) and large fat globules (LFG, D_4,3 = 5.65 ± 0.03 μm). The protein and fat content of SFG and LFG milk were then standardized to 3.2 g/100 mL and 1.2 g/100 mL, respectively. The milk containing different sized globules were then subjected to renneting experiments in the laboratory. Reduction of globule size accelerated the aggregation of casein micelles during renneting, giving a shorter gelation time and earlier 1/l^* change. The gel produced from LFG milk was broken due to coalescent fat globules and generated coarser gel strands compared to the finer strands formed with SFG milk. Structural differences were also confirmed with a higher final storage modulus of the curd made from SFG milk than that from the LFG. In conclusion, the size of fat globules affects the aggregation of casein micelles. Moreover, fat globule coalescence and creaming during renneting, also affects the structure of the rennet gel. A better understanding of the size of globules effect on milk gelation could lead to the development of cheese with specific properties.     

Development of the milk fat microstructure during the manufacture and ripening of Emmental cheese observed by confocal laser scanning microscopy

Changes in the physico-chemical properties and microstructure of milk fat globules were investigated during the manufacture and ripening of Emmental cheese. The measurement of fat globule size and apparent zeta-potential showed that they were slightly affected during cheese milk preparation, i.e. storage of cheese milk overnight at 4 °C and pasteurisation. After rennet-induced coagulation and heating of curd grains, coalescence caused the formation of large fat globules (i.e.>10 μm). The structure of fat in Emmental cheese was characterised in situ using confocal laser scanning microscopy (CLSM). The rennet-induced coagulation lead to the formation of a continuous network of casein strands in which fat globules of various sizes were entrapped. Heating of curd grains induced the formation of fat globule aggregates. Pressing of the curd grains resulted in the greatest disruption of milk fat globules, their coalescence, the formation of non-globular fat (free fat) and the release of the milk fat globule membrane (MFGM) material. This study showed that milk fat exists in three main forms in ripened Emmental cheese: (i) small fat globules enveloped by the MFGM; (ii) aggregates of partially disrupted fat globules and (iii) free fat, resulting from the disruption of the MFGM and allowing free triacylglycerols to fill voids in the protein matrix. The curd grain junctions formed in Emmental cheese were also characterised using CLSM: they are compact structures, rich in protein and devoid of fat globules.