Impact of extended refrigerated storage and freezing/thawing storage combination on physicochemical and microstructural characteristics of raw whole and skimmed sheep milk

The impact of freezing (1 month + thawing at 7 or 25 °C) and extended refrigeration (4 days, 7 °C) on physicochemical and microstructural characteristics of raw whole and skimmed sheep milk were assessed. Refrigerated storage resulted in higher sedimentation and creaming (whole milk), possibly due to proteases and agglutinins. Freezing/thawing processes in whole milk increased the particle size and creaming when samples were thawed at 7 °C. Skimmed milk showed an increase in buffering capacity and a reduction in soluble calcium immediately after thawing at 25 °C, suggesting that although the changes in fat are the main alterations caused by slow freezing of sheep milk, minor changes in saline balance can occur. An evaluation of the results showed that frozen and thawed milk in domestic equipment (commonly found in smallholdings) alter the milk microstructure, and it is therefore preferable to use extended refrigeration to accumulate the milk before dairy production.     

Effects of high‐pressure homogenisation on physicochemical characteristics of partially skimmed milk

The changes in partially skimmed milk (0.5% fat) physicochemical properties and proteins after high‐pressure homogenisation (HPH) at 100, 200 and 300 MPa were investigated. Processing parameters and changes in pH, ethanol precipitation stability, lightness, whey protein denaturation, hydrophobicity and viscosity were evaluated. No significant differences were found between milk pH and nonprotein nitrogen content before and after HPH. Ethanol stability, lightness and hydrophobicity increased when pressure was increased from 100 MPa to 300 MPa. Whey protein denaturation, evaluated through noncasein nitrogen, occurred only at 200 to 300 MPa, and viscosity increased just at 300 MPa. Therefore, HPH changed some milk physicochemical characteristics, mainly those related to protein content. These results highlight that HPH processing is a promising technology to improve partially skimmed milk mouth feel being suitable for dairy products manufacturing.     

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.     

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.     

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.     

High pressure homogenisation of raw whole bovine milk (a) effects on fat globule size and other properties

Although widely adopted by the chemical and pharmaceutical industries in recent years, little published data is available regarding possible applications of high pressure homogenisation for dairy products. The objective of this work was to compare the effects of conventional (18 MPa, two-stage) and single or two-stage high pressure homogenisation (HPH) at 50-200 MPa on some properties of raw whole bovine milk (approximately 4% fat). Fat globule size decreased as HPH pressure increased and, under certain conditions of temperature and pressure, HPH yielded significantly smaller fat globules than conventional homogenisation. Fat globule size was also affected by milk inlet temperature. The pH of all homogenised milk samples decreased during 24 h refrigerated storage. Total bacterial counts of milk were decreased significantly (P < 0.05) for milk samples HPH-treated at 150 or 200 MPa. Whiteness and rennet coagulation properties of milk were unaffected or enhanced, respectively, as homogenisation pressure was increased. Average casein micelle size decreased slightly when skim milk was homogenised at 200 MPa. Thus, HPH treatment has several, potentially significant, effects on milk properties.     

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.     

κ-卡拉胶影响大豆分离蛋白乳浊液稳定性的研究

研究了卡拉胶对大豆分离蛋白乳浊液粒度分布、乳析率和离心沉淀率的影响,在此基础上分析了静置过程中乳浊液粒径与乳析率、离心沉淀率之间相关性,结果表明:乳浊液粒径随卡拉胶浓度由小到大依次为0.03%<空白样<0.06%<0.09%;而体系的表观粘度随卡拉胶浓度的增大而升高;静置过程中乳浊液顶部粒径d3,2与乳析率有较好的相关性;乳浊液底部d3,2与离心沉淀率有很好的相关性。进一步分析了其可能的作用机理:卡拉胶低浓度时,卡拉胶分子被吸附到液滴的蛋白质正电荷区域,增加了液滴间的静电排斥,从而增加了体系的絮凝稳定性;随着卡拉胶浓度增大,卡拉胶会引起体系排斥絮凝。     

Kinetic and thermodynamic study of thermal inactivation of the antimicrobial peptide P34 in milk

The knowledge on thermal inactivation of biopreservatives in a food matrix is essential to allow their proper utilisation in food industry, enabling the reduction of heating times and optimisation of heating temperatures. In this work, thermal inactivation of the antimicrobial peptide P34 in skimmed and fat milk was kinetically investigated within the temperature range of 90–120 °C. The inactivation kinetic follows a first-order reaction with k-values between 0.071 and 0.007 min⁻¹ in skimmed milk, and 0.1346 and 0.0119 min⁻¹ in fat milk. At high temperatures, peptide P34 was less resistant in fat milk, with a significant decrease in residual activity as compared with skimmed milk. At temperatures below 110 °C, the fat globules seem to have protective effect to the peptide P34. Results suggest that peptide P34 is heat stable in milk with activation energy of 90 kJ mol⁻¹ in skimmed milk and 136 kJ mol⁻¹ in fat milk.     

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.     

浓缩与冻结方式对牛乳品质的影响

通过膜分离将鲜牛乳浓缩成不同总固形物含量的乳样(11. 83%、14. 94%、17. 61%、23. 01%),采用4种冻结方式(冰箱冻结、冷库冻结、螺旋冻结、平板冻结)对乳样进行冻结处理。测定乳样的色度、p H值、酸度、粒度、乙醇稳定性和热稳定性,并结合共聚焦显微镜和十二烷基硫酸钠-聚丙烯酰胺凝胶电泳进行对比分析。结果表明,随着总固形物含量的升高,牛乳的p H值、酸度、乙醇稳定性和热稳定性降低,总色差值、平均粒径增大。经冻结处理后乳样中β-乳球蛋白含量均降低。在4种冻结方式中,平板冻结速率最快,乳样的乙醇稳定性和热稳定性均最高;冰箱冻结的乳样平均粒径和乳脂肪球粒径均最大。综上,推荐将鲜牛乳浓缩至总固形物含量为23. 01%,然后采用平板冻结的工艺,可以更好地维持冻结乳样的品质。     

乳化剂的复配比例和用量对花生乳稳定性影响的研究

研究了乳化剂中蔗糖酯与单甘酯的复配比例(1:2～2:1)和用量(0.03%～0.07%)对花生乳的粒度分布,离心乳析率、离心沉淀率和储存稳定性的影响,研究结果表明:随着乳化剂中蔗糖酯(以单甘酯计)的比例上升,花生乳的粒径、离心乳析率和离心沉淀率均呈先减小后增大趋势,当蔗糖酯和单甘酯的复配比例为2:3时储存稳定性最好;当乳化剂用量低于0.05%时,随其用量的增加,花生乳的粒径、离心乳析率和离心沉淀率均呈下降趋势,储存稳定性逐渐提高,当乳化剂用量高于0.05%时,花生乳的粒径、离心乳析率,离心沉淀率和储存稳定性变化不明显;综合考虑,当乳化剂中蔗糖酯和单甘酯的复配比例为2:3,用量为0.05%时,花生乳的稳定性最好.     

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.     

The stabilisation of acidified whole milk drinks by carboxymethylcellulose

Carboxymethylcellulose (CMC) was used as a stabiliser for acidified whole milk drinks. The stability of the acidified whole milk drinks was investigated by observation of the evolution of size and zeta potential of colloidal particles during acidification along with sedimentation measurement and was compared with the stability of acidified skim milk drinks. It was found that the presence of fat resulted in larger particle sizes, but it did not disturb the absorption of CMC onto casein micelles below pH 5.2. The absorption of CMC endows the casein micelles with electrostatic and steric repulsions, essential to the stability of the acidified milk drinks. Acidified whole milk drinks can be stabilised by CMC, which is capable of effectively preventing the unwanted creaming of fat embedded in the clusters of CMC and caseins as well as preventing the aggregation of casein micelles as it did in skim milk drinks.     

Effect of the size and interface composition of milk fat globules on their in vitro digestion by the human pancreatic lipase: Native versus homogenized milk fat globules

Although the bioavailability of dietary lipids is of primary importance in human nutrition and health, the mechanisms involved in lipid digestion are not fully understood and are of growing interest. The objective of this study was to determine the effect of the size of milk fat globules and of the composition of their interface on the activity of the human pancreatic lipase (PL). Native milk fat globules of various sizes covered by their biological membrane (MFGM) and homogenized fat globules of various sizes covered by milk proteins were prepared from whole milk and underwent lipolysis by the human PL with colipase and bile salts. A lag phase preceding the hydrolysis of milk TAG occurred with all native milk fat globules samples but not with homogenized milk samples. The kinetic parameters of human PL were determined by measuring the enzyme activity either after the lag phase for native milk fat globules samples or immediately after the addition of the enzyme for homogenized milk samples. The catalytic efficiency of human PL is 4.6-fold higher on small (1.8 μm) than large (6.7 μm) native milk fat globules, related to a 3.6-fold larger available surface. Despite the 25-fold larger available surface, milk TAG from homogenized milk are only 2-fold better hydrolyzed compared to native milk fat globules, as a possible result of a less favourable interface covered by milk proteins. The potential mechanisms involved in native vs. homogenized milk fat globules digestion by the human PL are discussed. Our study highlights the crucial role of the MFGM in the efficient digestion of milk fat globules and brings new insight for the design of dairy products and infant formulas.     

Milk quality and automatic milking: fat globule size, natural creaming, and lipolysis

Thirty-eight Italian Friesian first-lactation cows were allocated to 2 groups to evaluate the effect of 1) an automatic milking system (AMS) vs. milking in a milking parlor (MP) on milk fat characteristics; and 2) milking interval (≤480, 481 to 600, 601 to 720, and >720 min) on the same variables. Milk fat was analyzed for content (% vol/vol), natural creaming (% of fat), and free fatty acids (FFA, mEq/100 g of fat). Distribution of milk fat globule size was evaluated to calculate average fat globule diameter (d₁), volume-surface average diameter (d₃₂), specific globule surface area, and mean interglobular distance. Milk yield was recorded to calculate hourly milk and milk fat yield. Milking system had no effect on milk yield, milk fat content, and hourly milk fat yield. Milk from AMS had less natural creaming and more FFA content than milk from MP. Fat globule size, globular surface area, and interglobular distance were not affected by milking system per se. Afternoon MP milkings had more fat content and hourly milk fat yield than AMS milkings when milking interval was >480 min. Milk fat FFA content was greater in AMS milkings when milking interval was ≤480 min than in milkings from MP and from AMS when milking interval was >600 min. Milking interval did not affect fat globule size, expressed as d₃₂. Results from this experiment indicate a limited effect of AMS per se on milk fat quality; a more important factor seems to be the increase in milking frequency, generally associated with AMS.     

Significance of frictional heating for effects of high pressure homogenisation on milk

High pressure homogenisation (HPH) is a novel dairy processing tool, which has many effects on enzymes, microbes, fat globules and proteins in milk. The effects of HPH on milk are due to a combination of shear forces and frictional heating of the milk during processing; the relative importance of these different factors is unclear, and was the focus of this study. The effect of milk inlet temperature (in the range 10-50 degrees C) on residual plasmin, alkaline phosphatase, lactoperoxidase and lipase activities in raw whole bovine milk homogenised at 200 MPa was investigated. HPH caused significant heating of the milk; outlet temperature increased in a linear fashion (0.5887 degrees C/ degrees C, R2=0.9994) with increasing inlet temperature. As milk was held for 20 s at the final temperature before cooling, samples of the same milk were heated isothermally in glass capillary tubes for the same time/temperature combinations. Inactivation profiles of alkaline phosphatase in milk were similar for isothermal heating or HPH, indicating that loss of enzyme activity was due to heating alone. Loss of plasmin and lactoperoxidase activity in HPH milk, however, was greater than that in heated milk. Large differences in residual lipase activities in milks subjected to heating or HPH were observed due to the significant increase in lipase activity in homogenised milk. Denaturation of beta-lactoglobulin was more extensive following HPH than the equivalent heat treatment. Inactivation of plasmin was correlated with increasing fat/serum interfacial area but was not correlated with denaturation of beta-lactoglobulin. Thus, while some effects of HPH on milk are due to thermal effects alone, many are induced by the combination of forces and heating to which the milk is exposed during HPH.     

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.     

Stability and physical properties of recombined dairy cream: Effects of soybean lecithin

Recombined dairy cream, which primarily comprises anhydrous milk fat and milk protein, has significant advantages compared to natural cream; however, its most notable disadvantage is poor stability. The objective of this study was to investigate the effects of lecithin on the stability and physical properties of recombined dairy cream (20% fat, and 1.5% protein) in terms of the creaming rate, mean oil droplet size and distribution, surface protein concentration, ζ-potential, and apparent viscosity. The results clearly showed that lecithin can significantly improve the stability of recombined dairy cream by decreasing the creaming rate, especially at a concentration of 0.6% (w/w). Increasing the lecithin concentration decreased the mean oil droplet size and the surface protein concentration but slightly increased the ζ-potential. The apparent viscosity decreased and surprisingly increased at 0.6% (w/w). We can infer that lecithin initially displaces proteins from the oil surface and may interact with both proteins and polysaccharides, forming a much more stable structure.     

Relationship between activity of some fat globule membrane enzymes and the lipidic fraction in ewes' milk: Preliminary studies

The aim of this preliminary study was to improve the knowledge about milk fat globules by studying the relationships between the morphometric characteristics of fat globules, the fatty acid composition and the activity of membrane enzymes evaluated in 14 milk samples from Massese ewes: xanthine oxidase (XO), xanthine dehydrogenase (XDH), γ-glutamyl transpeptidase (γ-GT), alkaline phosphatase (AP) and 5′-nucleotidase (5′-N). A negative correlation was found between the activity of XO and XDH and fat globule size, and a positive correlation between AP activity, the number of globule per mL and medium fat globules (2–5 μm). A significant relationship was found between the enzymatic activity of γ-GT, XO, XDH, 5′-N and various fatty acids in milk.