Changes in physical properties of bovine milk from the colostrum period to early lactation

The aim of this study was to analyze individual cows’ samples from the colostrum, postcolostrum, and early lactation periods to investigate how milk composition, physical properties, stability, and suitability for processing change throughout this period. Attention was paid to the first week postpartum in which the composition of bovine mammary secretion can change markedly. Properties including pH, titratable acidity, ethanol stability (ES), rennet clotting time, and casein micelle size were analyzed, together with some compositional factors such as fat, total protein, lactose, total and ionic calcium, magnesium, citrate, phosphorus, sodium, and potassium. Total Ca (36.2 mM) and free ionic Ca (2.58 mM), Mg (5.9 mM), P (32.2 mM), and Na (24.1 mM) appeared to be high on d 5 postpartum, having decreased substantially over the first 5 d; they gradually decreased thereafter. The average pH on d 5 was only 6.49, compared with 6.64 at 1 mo postpartum. Stability measurements showed that the average ES on d 5 was 70% and the rennet clotting time was 12.2 min, which were significantly lower than values at later stages. A number of milk properties including ES, pH, protein content, and Ca²⁺ concentration could be useful for identifying the point of transition from colostrum to the early lactation period. Knowing the composition and physical properties of colostrum and postcolostrum secretions will help establish when such milk is suitable for processing and determine the best use for that milk.     

Factors affecting the clotting properties of sheep milk

A Formagraph was used to test the effects of some of the exogenous factors that can affect the processing properties of milk (pH, soluble calcium, rennet concentration, coagulation temperature), and two of the endogenous factors (protein and fat concentration), on the comparative clotting properties of sheep and cows' milk, namely renneting time (r), rate of firming (k20) and curd consistency (A30). A lower pH decreased r and k20 and increased A30 in both sheep and cows' milk. The addition of calcium chloride did not affect the clotting properties of sheep milk, but in cows' milk it decreased r and k20 and increased A30. Increasing the concentration of rennet decreased r and k20 and increased A30 for both sheep and cows' milk. Increasing the coagulation temperature from 30 to 38 °C decreased r for both sheep and cows' milk, but it decreased k20 and increased A30 only in cows' milk. Increasing the protein concentration decreased r in both sheep and cows' milk; it did not affect k20 of sheep milk, but it decreased that of cows' milk and increased A30 in both milks. Increasing the fat concentration had little effect on r and k20 in either sheep cows' milk, but it decreased A30 in both milks. In general, sheep milk had faster renneting times and rates of firming and greater curd consistency than cows' milk, and its clotting properties tended to be less affected by changes in the clotting conditions. © 2002 Society of Chemical Industry     

From goat colostrum to milk: Physical,chemical, and immune evolution from partum to 90 days postpartum

This study focused on the study of the changes originated in the milk from partum until d 90 of lactation. Ten multiparous Majorera goats, bred carefully under animal health standards, with a litter size of 2 kids (the average in this breed is 1.83 prolificacy) and similar gestation length (149 ± 1 d) were used. Goat kids were removed from their dams to avoid interferences with the study. Compositional content (fat, protein, and lactose) were measured, as well as some other properties, including pH, density, titratable acidity, ethanol stability, rennet clotting time, and somatic cell count. Moreover, immunity molecules (IgG, IgA, and IgM concentrations and chitotriosidase activity) received great attention. Fat and protein content were higher in the first days postpartum, whereas lactose content was lower. Density, titratable acidity, rennet clotting time, and somatic cell count decreased throughout the lactation period, whereas pH and ethanol stability increased. Relative to the immunological parameters, each measured parameter obtained its maximum level at d 0, showing the first milking as the choice to provide immunity to the newborn kids. On the other hand, this study might be used to establish what the best use is: processing or kid feeding.     

Influence of some polyvalent organic acids and salts on the colloidal stability of milk

Fifteen polyvalent organic salts and acids increased, some very markedly, the heat stability of milk progressively with increasing concentration; acetate and mesaconic acid had no effect, and maleate caused destabilization. Citrate, oxalate and EDTA markedly reduced the [Ca⁺] in the milk and increased its ethanol stability by shifting the ethanol/pH profile to lower pH values. These additives ≥ 20 mmol/l prevented the coagulation of milk by rennet, but the other additives caused only slight increases in RCT .
Citrate, EDTA and oxalate disrupted casein micelles and would therefore not be suitable additives: however, the other acids and salts did not alter the appearance of the milk and might be recommended as suitable additives to improve the heat stability. In terms of the response to the level of Ca²⁺ and colloidal calcium phosphate in milk, there appeared to be a correlation between ethanol stability and rennet coagulation time.     

Effect of sodium chloride and pH on the rennet coagulation and gel firmness

The effect of sodium chloride concentration and pH of milk at renneting on the rennet clotting time (RCT) and gel firmness was studied. The results showed that rennet clotting activity and gel firmness decreased with increasing NaCl concentration in milk. The RCT increased as the pH of the salted milk decreased below 6.4. Milk containing 5 and 10 g NaCl/100 g did not coagulate at pH 5.0. The gel firmness increased with decreasing milk pH to 6.2 for unsalted milk. However, the firmness decreased as the pH of milk decreased below 6.2. In case of using salted milk, the firmness decreased as the pH at renneting dropped below 6.4.     

Plasminogen activation system in goat milk and its relation with composition and coagulation properties

The activity of plasmin (PL), plasminogen (PG), and plasminogen activator (PA) and their correlation with goat milk components and milk clotting parameters were investigated. Seven late-lactating Saanen goats were used to provide milk samples that were analyzed for PL, PG, and PA activity (colorimetric assay) fat, protein, noncasein nitrogen, nonprotein nitrogen, casein content, and somatic cell count (SCC). Milk clotting parameters (rennet coagulating time = coagulation time; K20 = firming rate of curd; A30 = curd firmness) were measured with a formagraph. Average milk yield and composition were similar to those previously observed in other studies. Plasmin, PG, and PA activity, expressed as units/ml, were, respectively, 20.04 +/- 0.94, 3.21 +/- 0.04, and 1154 +/- 57.61. Plasminogen activity was surprisingly low compared with other species (bovine, ovine), but it was consistent with the high activity of PA. A negative significant correlation was observed between PL and milk casein content. The correlation coefficients between PL and casein/protein ratio and PA and casein/protein ratio were negative and significant. A positive significant correlation was observed between PL and rennet clotting time and PA and rennet clotting time. Also positive was the correlation between PL and K20 and PA and K20. The plasmin activity was negatively correlated with A30. High plasmin and plasminogen activator activity in goat milk appeared to be negatively related with coagulating properties in late lactation, most probably via degradation of casein due to plasmin activity.     

Rennet-induced gelation of calcium and phosphate supplemented skim milk subjected to CO2 treatment

A Doehlert design was performed to study the effect of calcium and phosphate supplementation at 0 to 25 mmol/kg and 0 to 16 mmol/kg, respectively, on the rennet gelation of reconstituted skim milk subjected to pH-reversible CO(2) acidification. Supplemented reconstituted skim milk samples were acidified to pH 5.80 by the addition of CO(2) under pressure and depressurized under vacuum to restore the initial pH value. The second-order polynomial models satisfactorily predicted the effect of salt addition on the micellar molar Ca:P ratio and the average diameter of the casein micelles, whereas only trends were used in the analysis of the rennet-clotting behavior of salt-supplemented, CO(2)-treated milk. Whether added Ca was the most determinant factor on the micellar molar Ca:P ratio, added Pi (a mixture of Na(2)HPO(4) and NaH(2)PO(4)) was the most determinant factor on the other responses studied, and its effect was most pronounced when Ca was simultaneously added. By comparison with control samples, changes observed in this study were essentially due to salt supplementation and not to the CO(2) treatment. Therefore, this CO(2) treatment could be considered as an entirely reversible treatment rather than only pH-reversible, and predictions might be applied to untreated milk. In the case of Ca-supplemented milk, the micellar molar Ca:P ratio increased, the average micellar diameter decreased, and the rennet-clotting properties were improved, whereas opposite effects were observed upon Pi supplementation. Since modification of the micellar molar ratio is the result of change in the chemical composition of micellar calcium phosphate, the effect of calcium and phosphate supplementation on the rennet clotting of milk was found to be also dependent on the nature of the interaction between caseins and colloidal calcium phosphate.     

Reformation of casein particles from alkaline-disrupted casein micelles

In this study, the properties of casein particles reformed from alkaline disrupted casein micelles were studied. For this purpose, micelles were disrupted completely by increasing milk pH to 10.0, and subsequently reformed by decreasing milk pH to 6.6. Reformed casein particles were smaller than native micelles and had a slightly lower zeta-potential. Levels of ionic and serum calcium, as well as rennet coagulation time did not differ between milk containing native micelles or reformed casein particles. Ethanol stability and heat stability, >pH 7.0, were lower for reformed casein particles than native micelles. Differences in heat stability, ethanol stability and zeta-potential can be explained in terms of the influence of increased concentrations of sodium and chloride ions in milk containing reformed casein particles. Hence, these results indicate that, if performed in a controlled manner, casein particles with properties closely similar to those of native micelles can be reformed from alkaline disrupted casein micelles.     

Factors associated with milk processing characteristics predicted by mid-infrared spectroscopy in a large database of dairy cows

Despite milk processing characteristics being important quality traits, little is known about the factors underlying their variability, due primarily to the resources required to measure these characteristics in a sufficiently large population. Cow milk coagulation properties (rennet coagulation time, curd-firming time, curd firmness 30 and 60 min after rennet addition), heat coagulation time, casein micelle size, and pH were generated from available mid-infrared spectroscopy prediction models. The prediction models were applied to 136,807 spectra collected from 9,824 Irish dairy cows from research and commercial herds. Sources of variation were investigated using linear mixed models that included the fixed effects of calendar month of test; milking time in the day; linear regressions on the proportion of Friesian, Jersey, Montbéliarde, Norwegian Red, and “other” breeds in the cow; coefficients of heterosis and of recombination loss; parity; stage of lactation; and the 2-way interaction parity × stage of lactation. Within- and across-parity cow effects, contemporary group, and a residual term were also included as random effects in the model. Supplementary analyses considered the inclusion of either test-day milk yield or milk protein concentration as fixed-effects covariates in the multiple regression models. Milk coagulation properties were most favorable (i.e., short rennet coagulation time and strong curd firmness) for cheese manufacturing in early lactation, concurrent with the lowest values of both pH and casein micelle size. Milk coagulation properties and pH deteriorated in mid lactation but improved toward the end of lactation. In direct contrast, heat coagulation time was more favorable in mid lactation and less suitable (i.e., shorter) for high temperature treatments in both early and late lactation. Relative to multiparous cows, primiparous cows, on average, yielded milk with shorter rennet coagulation time and longer heat coagulation time. Milk from the evening milking session had shorter rennet coagulation time and greater curd firmness, as well as lower heat coagulation time and lower pH compared with milk from the morning session. Jersey cows, on average, yielded milk more suitable for cheese production rather than for milk powder production. When protein concentration was included in the model, the improvement of milk coagulation properties toward the end of lactation was no longer apparent. Results from the present study may aid in decision-making for milk manufacturing, especially in countries characterized by a seasonal supply of fresh milk.     

Clotting and proteolytic properties of plant coagulants in regular and ultrafiltered bovine skim milk

Regular and ultrafiltered (UF; 1×, 2× and 4× concentrated) skim milk samples were treated with a range of enzymes including calf rennet, ficin and papain. The clotting properties, curd casein profiles and free amino acid (FAA) contents were determined. In general, UF milk samples coagulated faster and formed firmer curds irrespective of protein concentration. Furthermore, both ficin and papain had a more significant effect on proteolysis in curd formed from regular and 1× UF milk than on 2× or 4× UF milk. Cardoon extract and calf rennet had very similar clotting properties, although the former caused both the capillary electrophoresis profile of caseins and FAA measurements to show slightly more extensive hydrolysis in the curd. The results suggest that the UF process may cause structural changes to proteins or other milk constituents with a resultant change in clotting properties and proteolysis of the casein molecules.     

Evaluation of Harp Seal Gastric Protease as a Rennet Substitute for Cheddar Cheese

A crude preparation of gastric proteases from Harp Seal (Pagophilus groenlandicus) was found to coagulate milk over a wider pH range than porcine pepsin and had a higher ratio of milk clotting to proteolytic activity with hemoglobin at pH 1.8. Cheddar cheese prepared with seal gastric protease (SGP) gave significantly higher sensory scores than cheese made with calf rennet. Chemical analysis of the cheeses revealed a lower concentration of citrate-HCl soluble nitrogen and less free and peptide-bound amino acids in SGP cheese than in the cheeses made with calf rennet and Mucor miehei protease.     

Protein interactions in heat-treated milk and effect on rennet coagulation

Skim milk was heated at different pH values to cause differential association of whey proteins (WP) with the casein micelles. All of these milk treatments coagulated poorly with rennet. To understand this in more detail, the casein micelles from heated milk were redispersed in unheated serum or unheated micelles were suspended in the sera from heat-treated milk. Systems containing micelles from milk heated at pH 6.7 and 7.1 were marginally better than the heated milk, but that from milk heated at pH 6.3 was not. The sera from milk heated at pH 6.7 and 7.1 impaired the clotting of native micelles but that from the pH 6.3 milk did not. Native casein micelles were suspended in permeates or dialyzed (against unheated milk) sera from heat-treated milk. Permeate systems free of WP/κ-casein complexes produced significantly stronger rennet gels; as did dialyzed systems. The impaired rennet clotting of heat-treated milk was attributed to a synergistic effect of the casein micelles with their heat-modified surfaces, the soluble serum WP/κ-casein complexes, and other dialyzable serum components.     

Use of Oscillatory Deformation Technique to Determine Clotting Times and Rigidities of Milk Clotted with Different Concentrations of Rennet

A curd firmness tester that measures rigidity of gels subjected to oscillatory deformation was used to determine clotting times and gel rigidities (G) of milk treated with different concentrations of rennet. Clotting times were directly related to reciprocal of enzyme concentrations. Increases in gel rigidity quite closely obeyed the relationship G = Ge-λ/t where λ is the time to attain a value of G/e and t is the time subsequent to clotting. Rates of increases in rigidity during the first half of gel assembly were increased by increases in rennet concentration but maximum gel rigidity (G) was not affected.     

Effect of milk protein genetic polymorphisms on rennet and acid coagulation properties after standardisation of protein content

The effects of milk protein genetic polymorphisms on the rennet and acid coagulation properties of milk after protein standardisation were investigated. Skim milk samples were adjusted to a protein concentration of 6.07 ± 0.06% by ultrafiltration (UF) before evaluating rennet coagulation and acid coagulation properties. Only the β-lactoglobulin (β-LG) genotypes influenced the rennet-clotting time before standardisation for the total protein concentration by UF; however, this effect was confounded with the β-LG concentration. After UF-concentration, a similar protein concentration between the samples was achieved in the retentate, then the rennet clotting time and rennet curd firmness at 30 min were significantly influenced by both the κ-casein (κ-CN) and β-LG genotypes. κ-CN genotypes significantly influenced the acid coagulation properties of both skim milk and retentate. Variations in the concentration of milk proteins (mostly α_S2-CN-12P) explained most of the differences in the rennet and acid coagulation properties of milk after protein standardisation by UF.     

Modeling Milk Clotting Activity in the Continuous Production of Microbial Rennet from Mucor miehei

Production of microbial rennet, a milk clotting enzyme, from a commercial strain of Mucor miehei (NRRL 3420) was investigated in a continuously fed fermenter system for prolonged periods. The spherical film-wise growth of the culture has been accomplished and the effects of medium pH, mixing and dilution rates, and feed of D-glucose concentration on milk clotting activity was investigated. In model simulation studies, maximum milk clotting activity was generated from a multiple linear function. This was expressed in terms of fermentation medium pH, D-glucose, dissolved oxygen concentrations and dilution rate at the time of maximum milk clotting activity.     

Effect of Temperature of Milk Acidification on Rennet Gel Properties

Changes in mineral solubilization and rennet reaction rate were investigated after decreasing milk pH to 6.3 by lactic acid addition at a temperature of acidification (TAC) of 25 or 35 °C with a short period of equilibration. With increasing TAC, casein micelles retained higher amounts of Ca and P, and at a given temperature of coagulation, rennet clotting time was increased, and dG'/dt decreased. This effect was confirmed by the microstructure of casein micelles during the first stage of the enzymic coagulation indicating that the aggregation of para-k-casein was observed later at higher TAC. The effect of TAC on rennet milk gel formation could be attributed to the nature of the micellar mineral content and the conformational state of casein micelles before rennet action.     

Characterization of curd formation during the rennet coagulation of milk by an optical microscopic method

Optical microscopic observation of the coagulation of rennet-treated milk was performed using a conventional light microscope under standardized conditions. The microscopic images, i.e., number and average area of milk aggregates, were analyzed using Optiscan software. The evolution of the latter parameters was followed as a function of the rennet coagulation time of milk. The curves presented two inflection points which correlated with the rennet flocculation time and the rennet clotting time values, as determined by reference methods. These relationships were validated on milk supplemented with different concentrations of calcium (5.0–7.5 m ) and subjected to varying temperatures for rennet coagulation (22.5–40°C). This optical microscopic method with associated image analysis software allowed (i) the observation of structural changes occurring during gel formation, even at low magnification (200X) and (ii) the determination of the rennet flocculation and clotting times.     

Ethanol Stability of Casein Solutions as Related to Storage Stability of Dairy-based Alcoholic Beverages

Ethanol stability characteristics of casein in a simulated cream liqeur environment were examined. Stability decreased with increasing concentration of Ca ions and decreasing pH in line with expectations from previous studies with milk. Monovalent cations also had a destabilizing effect while increase in casein concentration improved stability through enhanced Ca chelation. Homogenization with milk fat reduced ethanol stability. Rapid acid production occurred during storage of the simulated system at 55°C when the system contained reducing sugar, leading to a drop in pH and a destabilization of the protein to added ethanol. These characteristics have implications for storage stability of dairy-based alcoholic beverages.     

RHEOLOGICAL ASPECTS OF THE RENNETING OF MILK CONCENTRATED BY ULTRAFILTRATION

The kinetics of clotting by rennet of ultrafiltrated milk were studied by determining induction and clotting times, and development of the rigidity modulus as a function of rennet concentration and protein content in the sample. Creep cmpliance-time studies were conducted on the resultant gels at three different stresses. Clotting time increased with protein content and decreased with rennet concentration. The protein effect on clotting time decreased with increasing rennet concentration and was practically nil at 200 mg enzyme/kg sample. The ultimate rigidity and the viscoelastic parameters of gels depended on the protein concentration in a double logarithmic relationship. The calculated values of the constants indicated that the viscosities were influenced to a greater degree by the protein concentration than were the elastic moduli. The renneted milk gels exhibited creep compliance-time response at low shear stresses indicative of the viscoelastic behaviour of an interlinked three-dimensional network. Gelation followed a second order reaction in its initial stages and an approximately third order reaction in the final stage. The results are discussed in terms of potential mechanism and type of bonds involved.     

微小毛霉与米黑毛霉凝乳酶的制备及其凝乳质构特性研究

将经乙醇分步沉淀所制得的微小毛霉和米黑毛霉凝乳酶以小牛皱胃酶为对照在不同温度、不同时间和不同pH值下凝乳,以下压过程的平均力为衡量标准,对其物性进行了比较,得出在相同条件下,米黑毛霉凝乳酶的凝乳要好于微小毛霉凝乳酶,并在高温和酸性环境要比微小毛霉凝乳酶稳定。