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.     

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.     

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 use of sedimentation field-flow fractionation in the size characterization of bovine milk fat globules as affected by heat treatment

Size distribution of fat globules affects the appearance, taste and stability of milk and milk-based products. Full-fat, semi-fat and chocolate bovine milk were subjected to heat treatment within a temperature range of 50–125 °C for 1 h. Sedimentation field-flow fractionation was employed to determine the changes in mean particle diameter of milk fat globules as affected by heat treatment. The mean particle diameter of fat droplets increased with increasing heating temperature for most samples. The particle size of fat globules increased on average 40 nm (4.65%) for full-fat and 72 nm (8.52%) for semi-fat milk following the heat treatment (50–125 °C). Chocolate milk exhibited considerable increase in particle size (104 nm, 12.53%) within a certain temperature range (50–110 °C), followed by a decrease in particle size when heated at 125 °C for 1 h. Heat-induced flocculation due to attractive interactions between hydrophobic sites on denatured protein molecules on different droplets was assumed to be mainly responsible for the increases in particle size observed in this study. Extensive heat-induced denaturation of milk proteins was also indicated by Native PAGE. Sedimentation field-flow fractionation proved to be a useful technique for adequately monitoring heat-induced changes in particle size distributions in milk.     

Behaviour of homogenized fat globules during the spray drying of whole milk

The changes in milk fat globule size and fat globule surface proteins of both low-preheated and high-preheated concentrated milks, which were homogenized at low or high pressure prior to spray drying using a disc atomization drier, were examined. The average fat globule size (d₃₂) of the spray-dried milk powders was smaller than that of the corresponding concentrates, but a small proportion of very large globules (4–80 μm) was also formed during spray drying. As a consequence, total surface protein (mg protein g⁻¹ fat) increased due to the adsorption of casein micelles at the fat globule surface during spray drying. Confocal micrographs of the powders showed some apparent spreading of the fat on the surface of the powder particles, particularly when the concentrates were homogenized at low pressure. These results indicate disruption of the milk fat globules during spray drying, which consequently causes changes in the fat globule surface protein layer.     

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.     

Gravity separation of raw bovine milk: fat globule size distribution and fat content of milk fractions

This project determined effects of time and temperature on changes of fat globule size distribution and fat content in milk fractions during gravity separation. Fresh raw bovine milk was gravity separated at 4 or 15 degrees C. After 2, 6, 12, and 48 h, seven fractions, from bottom fraction (F1) to top fraction (F7), were successively drained from a separation column. Higher temperature resulted in a faster rate of fat separation. Within 2 h, large fat globules had already moved to the top, and the volume mean diameter of F7 increased from 3.13 microm (without separation) to 3.48 and 3.64 microm, respectively, at 4 and 15 degrees C. In F7, there was little change in globule size distribution after 2 h, but fat content continued to increase with separation time. The fat content of F7 reached 26.6% after 48 h at 4 degrees C, achieving a 58.8% creaming capacity. For F1 to F6, longer separation time resulted in smaller fat globule sizes and lower fat contents, especially for F1. After 48 h at 4 degrees C, the volume mean diameter of F1 decreased from 3.23 microm (without separation) to 1.16 microm, and fat content decreased from 3.75% (without separation) to 0.20%. Gravity separation may have unique applications in the dairy industry today. Its simplicity makes it an effective procedure for small-scale dairy product manufacturers to produce milks with a range of fat contents without using a centrifugal cream separator.     

Influence of formulation on the structural networks in ice cream

Six different formulations, corresponding to three types of fat (hydrogenated or refined coconut oils or refined palm oil) and two mixtures of mono- and di-glycerides, namely saturated and partially unsaturated were chosen to investigate the influence of the oil phase nature and the low molecular weight emulsifier type on the networks present in ice cream. Ice creams were characterized for particle size distribution of fat globules, melting resistance and amount of proteins in the aqueous phase. Variation of rheological parameters as a function of temperature allowed following the ice network melting, the fat globule aggregation state and destabilization, and the structural arrangement of proteins. Presence of unsaturated fatty acids in the emulsifier promoted an increased percentage of agglomerated fat globules, increased melting time and higher storage modulus values at 5 °C. The influence of the fat type on ice cream characteristics was mainly illustrated by different rheological parameters and, to a lesser extent, by melting time, whereas the amount of proteins in the aqueous phase did not allow discriminating among the formulations.     

Effect of physical properties of milk fat globules on Brucella ring test sensitivity.

Variations in Brucella milk ring test reactions have been attributed to size and disparity in size of fat globules, fat content of milk, inhibitory factors, and cream-rising capacity. Physical properties of milk fat globules were studied in individual milk samples that had diverse sensitivities for detection of Brucella agglutinins. Size or disparity of globule size could not be correlated with the milk ring test sensitivity. Inhibitory and enhancing creaming factors could be transferred from the cream to the skim milk and reacted with other cream to show changes in sensitivity. The proportion of clustered milk fat globules was related directly to the quantity of agglutinins detected by the test. Sensitivity of the test was attributed to a fat globule agglutinin that caused clustering and to an inhibitory factor.     

Modeling of particle size distribution of sonicated coconut milk emulsion: Effect of emulsifiers and sonication time

Coconut milk was extracted from grated coconut meat with addition of water by a hydraulic press. This milk was mixed with emulsifiers (Gum Acacia and maltodextrin) at different ratios of emulsifier to fat (4, 2.75, and 1.5) and different maltodextrin (MD) to Gum Acacia (GA) ratio (0.67, 1.085, and 1.5). The emulsion was sonicated for different time durations (0.5–3.0 min). Particle size analysis of the sonicated coconut milk emulsion revealed that the number fraction of fat globules below 2 × 10⁻⁶ m increased as the emulsifier to fat ratio increased. However, at the highest ratio of emulsifier to fat this trend reversed even at higher sonication times. Similar slump in the increasing trend of globules below 2 × 10⁻⁶ m were observed at the highest value of ratio of MD to GA (1.5) studied. Modeling of particle size distribution by Rosin–Rambler–Sperling–Bennet relation was found to be a good tool for prediction of uniformity of distribution n and statistical average globule diameter M. The n values varied between 0.981 and 1.318 where as M values lied between 2.11 × 10⁻⁶ and 4.12 × 10⁻⁶ m. Linear regression fitting of n and M values as function of ratio of emulsifier to fat, ratio of MD to GA and the sonication time resulted in a good fit with relative deviation of 3.96% and 10.77%, respectively. These linear regression equations provided a suitable model to predict the sonication time required to achieve certain degree of size reduction with a relative deviation of 2.33%.     

High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h⁻¹ homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100–300 MPa) and milk inlet temperature T_in (4°C, 14°C or 24°C) on milk temperature T₂ at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The T_in values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High T_in and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d_4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at T_in=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (T_in=4°C, 14°C or 24°C) further decreased d_4.3 diameters to about 0.2 μm and narrowed the size distribution. At all T_in tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than 1 log cycle, provided homogenisation pressure was ⩾200 MPa at T_in=24°C (T₂∼60°C), 250 MPa at T_in=14°C (T₂∼62°C), or 300 MPa at T_in=4°C (T₂∼65°C). At all T_in tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria.     

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.     

The manufacture of miniature Cheddar-type cheeses from milks with different fat globule size distributions

A novel 2-stage gravity separation scheme was developed for fractionation of raw, whole bovine milk into fractions enriched in small (SFG) or large (LFG) fat globules. The volume mean diameter of fat globules in SFG, LFG or control (CTRL) milk was 3.45, 4.68 and 3.58 microm, respectively. The maximum in storage modulus (index of firmness) decreased with increasing fat globule size for rennet-induced gels formed from SFG, LFG or CTRL milks. Miniature (20 g) Cheddar cheeses were manufactured using each of the 3 milks. There were no significant (P > 0.05) differences in the pH, moisture and fat in dry matter levels between cheeses made using any of the 3 milks, however, the fat content of the cheese made using SFG milk was approximately 1% lower than that of cheese made using LFG or CTRL milk in each of the 2 trials. Image analysis of confocal scanning laser micrographs of the cheeses illustrated that the star volume of fat globules in the cheeses decreased significantly (P < or = 0.05) as the size of fat globules in the milks used for cheesemaking was reduced. This indicates that it is possible to manipulate the size distribution of fat globules in Cheddar cheese by adjusting the fat globule size distribution of the milk used for cheese-making. The concentration of free fatty acids (FFA) increased in all cheeses during ripening. At 120 d of ripening, the concentration of FFA varied significantly (P < or = 0.05 and P < or = 0.001 for trials 1 and 2, respectively) with fat globule size, with cheeses made in trial 2 from LFG, SFG or CTRL milks having total FFA levels of 3391, 2820 and 2612 mg/kg cheese, respectively.     

Monoacylglycerols in dairy recombined cream: I. The effect on milk fat crystallization

The effect of monoacylglycerols (rich in oleic acid, stearic acid or lauric acid) on milk fat crystallization in recombined cream is examined using differential scanning calorimetry to study crystallization kinetics, nuclear magnetic resonance to measure solid fat content during storage and interfacial tension analyses to analyze interfacial properties. The long-chain saturated monoacylglycerols (stearic acid) form upon cooling a two-dimensional crystal at the interface, called chain crystallization, which induces interfacial heterogeneous nucleation. Also, the crystal growth and α–β′ polymorphic transition were accelerated. On the contrary, long-chain unsaturated monoacylglycerols (oleic acid) didn't affect the crystallization behavior while mid-chain saturated monoacylglycerols (lauric acid) showed intermediate behavior. None of the monoacylglycerols influenced the solid fat content after storage for 5 days at 5 °C. The observed differences in nucleation mechanism and crystallization kinetics may influence the microstructural arrangement of the milk fat inside the fat globules and consequently the partial coalescence rate and hence the whipping properties of recombined cream.     

Addition of olein from milk fat positively affects the firmness of butter

Butter is highly valued for its characteristic flavor and aroma; however, it has the disadvantage of unsatisfactory spreadability at low temperatures. The functional characteristics of butter can be modified by changing its composition or physical structure. The objective of this study was to evaluate the effect of olein on structure and composition of butter. Olein was obtained by two stage dry fractionation process of the anhydrous milk fat (AMF) and added to commercial cream prior to butter manufacture. The fractions were characterized for triacylglycerols composition, solid fat content, crystallization isotherm, and thermal behavior. Butter was manufactured using commercial cream or cream containing 50% olein. Butter samples were characterized for physicochemical composition, instrumental color, crystallization parameters, and firmness after 1 and 7 days of storage at 10 °C. The firmness of butter subjected to room temperature was also evaluated. Butter containing olein differed significantly from the control and had darker yellow color, higher crystallization time, and lower solid fat content after 120 min at 15 °C, and hence lower firmness after 1 and 7 days of refrigerated storage. Although lower firmness was observed over time for all samples at room temperature, butter containing olein exhibited lower firmness after both 1 and 7 days, thus suggesting changes in organization of solid fat crystal network in the liquid fat. The addition of olein to butter allowed obtaining a softer product, with more intense color and possible nutritional benefits due to the medium chain triglycerides and higher carotene levels.     

Effect of cream homogenization on textural characteristics of low-fat Iranian White cheese

The effects of cream homogenization of cheese making milk on textural and sensory characteristics of Iranian White cheese were studied. Cream was homogenized in a two-stage homogenizer at 6.0/2.5 or 9.0/2.5 MPa. Cheese samples were analyzed for rheological parameters (uniaxial compression and small amplitude oscillatory shear), meltability, microstructure, and sensory characteristics. Cream homogenization increased fat content leading to increased meltability. This effect increased as the homogenization pressure increased. The values of storage modulus, stress at fracture and Young's modulus of elasticity for cheeses from homogenized treatments were lower than those of unhomogenized cheese. Cream homogenization at 6.0/2.5 MPa effectively improved the textural, functional and sensory characteristics and enhanced the yield of low-fat Iranian White cheese. This cheese had the lowest values of storage modulus and stress at fracture, probably due to the high number of small, evenly dispersed fat globules in microstructure and especially its lower protein content. Cheeses with homogenized cream had improved texture, flavor and appearance.     

Short communication: Effects of prill size of a palmitic acid–enriched fat supplement on the yield of milk and milk components,and nutrient digestibility of dairy cows

The objective of our experiment was to evaluate the effects of prill size of a palmitic acid–enriched fatty acid supplement (PA; 85% C16:0) on feed intake, nutrient digestibility, and production responses of dairy cows. Twenty-four primiparous and multiparous Holstein cows were assigned based on parity and production level to replicated 4 × 4 Latin squares balanced for carryover effects with 21-d periods. Treatments were a control diet (no added PA), or 2.0% PA added as a small prill size (PA-SM; 284 ± 12.4 µm), a medium prill size (PA-MD; 325 ± 14.7 µm), or a large prill size (PA-LG; 600 ± 17.4 µm) supplement. Overall, PA treatments increased milk fat content (4.25 vs. 3.99%), milk fat yield (1.48 vs. 1.39 kg/d), 3.5% fat-corrected milk (39.2 vs. 37.7 kg/d), and improved feed efficiency (fat-corrected milk:dry matter intake; 1.51 vs. 1.42) compared with control. Compared with control, PA treatments did not affect dry matter intake, body weight, body condition score, or yields of milk, protein, and lactose. The PA treatments increased neutral detergent fiber digestibility (44.8 vs. 42.4%) and reduced the digestibility of 16-carbon fatty acids (72.3 vs. 79.1%) and total fatty acids (76.6 vs. 80.3%). Compared with control, PA treatments reduced the contents of de novo synthesized milk fatty acids (23.0 vs. 25.8 g/100 g of fatty acids) and preformed milk fatty acids (36.3 vs. 39.1 g/100 g of fatty acids), but did not affect their yields. In contrast, PA treatments increased the content (40.8 vs. 35.1 g/100 g of fatty acids) and yield (570 vs. 436 g/d) of 16-carbon milk fatty acids compared with control. The PA prill size had no effect on dry matter intake, yield of milk and milk components, or feed efficiency. However, PA-LG tended to increase milk fat content compared with PA-SM (4.28 vs. 4.22%), and it increased 16-carbon fatty acid digestibility compared with PA-MD (74.2 vs. 71.0%) and PA-SM (74.2 vs. 71.7%). Additionally, PA-LG increased total fatty acid digestibility compared with PA-MD (78.1 vs. 75.6%) and PA-SM (78.1 vs. 76.0%). Results demonstrate that PA increased milk fat content and yield, and feed efficiency. Reducing prill size decreased fatty acid digestibility, but it had no effect on animal performance under the dietary conditions and prill sizes evaluated.     

Direct measurement of phase transitions in milk fat during cooling of cream—a low-field NMR approach

Low-field ¹H nuclear magnetic resonance (NMR) transverse relaxation (T₂) was measured during cooling (31–4 °C) of cream with low or high content of long-chain fatty acids (FA). Distributed analysis of the T₂ relaxation data revealed marked differences in the T₂ relaxation characteristics of the liquid fat population (10–100 ms), which was ascribed to differences in the size of the fat globules in the two types of cream. Principal component analysis (PCA) of the obtained T₂ relaxation decay data showed clear shifts in NMR relaxation behavior at 17 and 22 °C for cream with low or high content of long-chain FA, respectively. Differential scanning calorimetric measurements on the creams revealed that these shifts took place at the onset of crystallization of fat. Consequently, the present study demonstrated that low-field NMR can be used to measure phase transitions in cream.     

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.     

Polar lipids,sphingomyelin and long-chain unsaturated fatty acids from the milk fat globule membrane are increased in milks produced by cows fed fresh pasture based diet during spring

Milk lipids are an interesting source of bioactive molecules with functional and nutritional properties. Although the composition of milk lipids is of utmost importance for food processing and human consumption, it is far from being fully known. The objective of this study was to perform a comparative analysis of the chemical composition of lipids from bovine milks produced in French Brittany during spring (fresh pasture based diet) and winter (corn silage based diet). The polar lipid content and relative proportions of the glycerophospholipids and sphingomyelin were determined using HPLC/ELSD. The fatty acid composition of total lipids and polar lipids was determined using GC. The milks collected in spring contained i) a lower amount of total lipids: 39.7 ± 0.8 g/kg vs 41.7 ± 0.5 g/kg in winter, ii) a higher amount of polar lipids: 138 ± 11 vs 112 ± 8 mg/kg milk; 3.5 ± 0.3 vs 2.7 ± 0.4 mg/g fat, which was related to a smaller size of fat globules, and iii) a higher amount of sphingomyelin, 32 mg/kg milk vs 25 mg/kg milk in winter. Interestingly, the polar lipids from the milk fat globule membrane contained a higher concentration of unsaturated fatty acids in spring (C18:1 n − 9, C18:2 n − 6, C18:3 n − 3 and long-chain n − 3 fatty acids). Milk from cows fed a fresh pasture-based diet during spring is an interesting source of dietary long chain polyunsaturated fatty acids for human consumption.