Interactions of Calcium,pH, Temperature,and Chymosin During Milk Coagulation,

Holstein milk samples with good and poor chymosin-coagulation characteristics were coagulated in the Formagraph using different combinations of five levels of chymosin, three pH, and three temperatures in the presence and absence of .02% added calcium chloride.All the main factors significantly altered both coagulation time and curd firmness. Multiple comparisons of mean coagulation times showed that lower concentrations of chymosin (.01, .02, and .03 rennin units/ml milk) were significantly different in altering coagulation time and were different from higher concentrations (.04 and .05 rennin units/ml milk). The three pH produced significantly different mean coagulation times. Addition of more than .02 rennin units/ml milk was not necessary for adequate curd firmness in 30 min after chymosin addition where the pH of the milk was 6.4 or lower. Addition of .02% calcium chloride to milk was not necessary for adequate curd firmness 30 min after chymosin addition if other milk coagulation factors were adequately adjusted (pH ≤ 6.4; chymosin concentration = .02 rennin unit/ml milk; temperature = 37°C).     

Effects of Milk Composition and Genetic Polymorphism on Coagulation Properties of Milk

Milk samples from 31 Holstein cows of different phenotypes for β-casein, κ-casein, and β-lactoglobulin were collected monthly over the entire lactation. These samples were analyzed for total solids, fat, protein, casein fractions, lactose, urea, citric acid, somatic cell count, and pH. Rennet clotting time, rate of firming, and curd firmness as measured by a Formagraph were not significantly influenced by phenotypes for β-casein and κ-casein. Phenotype AA for β-lactoglobulin gave the best clotting time (3.91 min) and firmness of curd (36.30 mm) when compared with AB and BB phenotypes. Relative percentages of the different caseins and α-lactalbumin affected significantly rate of firming and curd firmness at cutting. Amount of κ-casein in milk was the most significant factor that affected curd firmness with a coefficient of regression of 15.96.     

Effects of Somatic Cell Counts and Milk Composition on the Coagulating Properties of Milk

Forty-two Holstein cows were selected to provide monthly milk samples with varying SCC for 1 yr. Coagulating properties of samples measured as rennet clotting time, rate of curd firming, and curd firmness at cutting were determined by a formagraph. Milk samples were analyzed for fat, protein, lactose, total solids, casein, individual caseins, urea, SCC, and pH. Least squares analyses of data, after adjustments were made for the effect of milk composition, indicated that elevated SCC were associated with a significant increase in rennet clotting time and slower rate of curd firming. An increase of SCC from 100,000 to 500,000 SCC/ml resulted in an increase of approximately 2.1 and 2.2% in RCT and K20, respectively. A further increase of SCC to above 1,000,000/ml resulted in an overall increase of 20.7 and 13.84% in RCT and K20, respectively. Regression analyses indicated that K20 was decreased by 5.42 min and curd firmness at cutting was increased by 12.92 mm for every percentage in milk casein. Rennet clotting time, rate of curd firming, and curd firmness at cutting were increased by 3.52, 3.41, min and decreased by 9.45 mm, respectively, for every unit increase in milk pH.     

Effects of pH, calcium chloride, and chymosin concentration on coagulation properties of abnormal and normal milk

Individual Holstein cow milk samples were selected for good and poor chymosin-coagulation characteristics. The effect of pH adjustment, addition of .02% calcium chloride, and variation in chymosin concentration on coagulation properties of good and poor coagulating samples was evaluated. Pooling 50% good and 50% poor coagulating samples did not improve the average coagulation properties of the poor samples. Reducing milk pH to 6.3 caused a significant decrease in coagulation time but a less marked increase in curd firmness. The greatest increase in curd firmness was obtained by a combination of reducing milk pH, addition of .02% calcium chloride, and reducing chymosin concentration. High-chymosin concentration at reduced pH decreased coagulation time without substantially increasing curd firmness. Curd disintegration was more apparent at high-chymosin concentration in the poor coagulating samples.     

Effect of genetic polymorphism of milk proteins on rheology of chymosin-induced milk gels

Individual milk samples from 121 cows in mid lactation of the Swedish Red and Swedish Holstein breeds with known protein genotypes of β- and κ-casein and β-lactoglobulin were analysed. Chromatographically pure chymosin was added to skim milk and rheological properties of the gels were measured using a Bohlin VOR Rheometer. Coagulation time (CT) and curd firmness after 25 min (G′₂₅) were registered for each sample. The B allele of κ-casein was associated with improved coagulating properties of milk, whereas the E allele showed a negative effect on these traits. The β-casein A²A² genotype was associated with inferior milk coagulation characteristics. Total protein concentration of milk was positively associated with curd firmness, but showed no association with milk coagulation time.     

Genetic analysis of coagulation properties,curd firming modeling,milk yield,composition, and acidity in Sarda dairy sheep

Sheep milk is an important source of food, especially in Mediterranean countries, and is used in large part for cheese production. Milk technological traits are important for the sheep dairy industry, but research is lacking into the genetic variation of such traits. Therefore the aim of this study was to estimate the heritability of traditional milk coagulation properties and curd firmness modeled on time t (CF_t) parameters, and their genetic relationships with test-day milk yield, composition (fat, protein, and casein content), and acidity in Sarda dairy sheep. Milk samples from 1,121 Sarda ewes from 23 flocks were analyzed for 5 traditional coagulation properties by lactodynamographic tests conducted for up to 60 min: rennet coagulation time (min), curd-firming time (k₂₀, min), and 3 measures of curd firmness (a₃₀, a₄₅, and a₆₀, mm). The 240 curd firmness observations (1 every 15 s) from each milk sample were recorded, and 4 parameters for each individual sample equation were estimated: rennet coagulation time estimated from the equation (RCT_eq), the asymptotic potential curd firmness (CF_P), the curd firming instant rate constant (k_CF), and the syneresis instant rate constant (k_SR). Two other derived traits were also calculated (CF_max, the maximum curd firmness value; and t_max, the attainment time). Multivariate analyses using Bayesian methodology were performed to estimate the genetic relationships of milk coagulation properties and CF_t with the other traits; statistical inference was based on the marginal posterior distributions of the parameters of concern. The marginal posterior distribution of heritability estimates of milk yield (0.16 ± 0.07) and composition (0.21 ± 0.11 to 0.28 ± 0.10) of Sarda ewes was similar to those often obtained for bovine species. The heritability of rennet coagulation time as a single point trait was also similar to that frequently obtained for cow milk (0.19 ± 0.09), whereas the same trait calculated as an individual equation parameter exhibited larger genetic variation and a higher heritability estimate (0.32 ± 0.11). The other curd firming and syneresis traits, whether as traditional single point observations or as individual equation parameters and derived traits, were characterized by heritability estimates lower than for coagulation time and for the corresponding bovine milk traits (0.06 to 0.14). Phenotypic and additive genetic correlations among the 11 technological traits contribute to describing the interdependencies and meanings of different traits. The additive genetic relationships of these technological traits with the single test-day milk yield and composition were variable and showed milk yield to have unfavorable effects on all measures of curd firmness (a₃₀, a₄₅, a₆₀, CF_P, and CF_max) and t_max, but favorable effects on both instant rate constants (k_CF and k_SR). Milk fat content had a positive effect on curd firmness traits, especially on those obtained from CF_t equations, whereas the negative effects on both coagulation time traits were attributed to the milk protein and casein contents. Finally, in view of the estimated heritabilities and additive genetic correlations, enhancement of technological traits of sheep milk through selective breeding could be feasible in this population.     

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.     

Breed of cow and herd productivity affect milk composition and modeling of coagulation,curd firming,and syneresis

Milk coagulation properties (MCP) have been widely investigated in the past using milk collected from different cattle breeds and herds. However, to our knowledge, no previous studies have assessed MCP in individual milk samples from several multi-breed herds characterized by either high or low milk productivity, thereby allowing the effects of herd and cow breed to be evaluated independently. Multi-breed herds (n = 41) were classified into 2 categories based on milk productivity (high vs. low), defined according to the average milk net energy yielded daily by lactating cows. Milk samples were taken from 1,508 cows of 6 different breeds: 3 specialized dairy (Holstein-Friesian, Brown Swiss, Jersey) and 3 dual-purpose (Simmental, Rendena, Alpine Grey) breeds, and analyzed in duplicate (3,016 tests) using 2 lactodynamographs to obtain 240 curd firming (CF) measurements over 60 min (1 every 15 s) for each duplicate. The 5 traditional single-point MCP (RCT, k₂₀, a₃₀, a₄₅, and a₆₀) were yielded directly by the instrument from the available CF measures. All 240 CF measures of each replicate were also used to estimate 4 individual equation parameters: RCT estimated according to curd firm change over time modeling (RCT_eq), asymptotic potential curd firmness (CF_P), curd firming instant rate constant (k_CF), and syneresis instant rate constant (k_SR) and 2 derived traits: maximum curd firmness achieved within 45 min (CF_max) and time at achievement of CF_max (t_max) by curvilinear regression using a nonlinear procedure. Results showed that the effect of herd-date on traditional and modeled MCP was modest, ranging from 6.1% of total variance for k₂₀ to 10.7% for RCT, whereas individual animal variance was the highest, ranging from 32.0% for t_max to 82.5% for RCT_eq. The repeatability of MCP was high (>80%) for all traits except those associated with the last part of the lactodynamographic curve (i.e., a₆₀, k_SR, k_CF, and t_max: 57 to 71%). Reproducibility, taking into account the effect of instrument, was equal to or slightly lower than repeatability. Milk samples collected in farms characterized by high productivity exhibited delayed coagulation (RCT_eq: 18.6 vs. 16.3 min) but greater potential curd firmness (CF_P: 76.8 vs. 71.9 mm) compared with milk samples collected from low-productivity herds. Parity and days in milk influenced almost all MCP. Large differences in all MCP traits were observed among breeds, both between specialized and dual-purpose breeds and within these 2 groups of breeds, even after adjusting for milk quality and yield. Milk quality and MCP of samples from Jersey cows, and coagulation time of samples from Rendena cows were better than in milk from Holstein-Friesian cows, and intermediate results were found with the other breeds of Alpine origin. The results of this study, taking into account the intrinsic limitation of this technique, show that the effects of breed on traditional and modeled MCP are much greater than the effects of herd productivity class, parity, and DIM. Moreover, the variance in individual animals is much greater than the variance in individual herds within herd productivity class. It seems that improvement in MCP depends more on genetics (e.g., breed, selection) than on environmental and management factors.     

Content of alpha S1-casein and coagulation properties in goat milk

Samples of goat milk with low and high alpha S1-casein content collected from animals of Alpine and Saanen breeds in the same stage of lactation were compared for coagulation properties (coagulation time, rate of curd formation, curd firmness) and chemical composition (total solids, ash, total protein, total casein, whey protein, fat, Ca, P, pH). Milk with low alpha S1-casein had a faster coagulation time, whereas milk with high levels produced the firmer curd associated with a better chemical composition. Within high alpha S1-casein milk, comparison between breeds showed milk from the Alpine breed had significantly better coagulation properties than that from the Saanen breed. Milk composition accounted for 27% of the variation in coagulation time, 21% of variation in cured formation rate, and 54% of variation in curd firmness.     

Effect of genetic polymorphism of milk proteins on rheology of acid-induced milk gels

Individual milk samples from 80 cows in mid-lactation of the Swedish Red and Swedish Holstein breeds with known protein genotypes of β- and κ-casein and β-lactoglobulin were analysed for acid coagulation properties. Glucono-δ-lactone (1.5%) was added to defatted, heated (90–95 °C) samples and rheological properties of the gels were measured using a Bohlin VOR Rheometer. Coagulation time (CT) and curd firmness after 4, 8 and 10 h (G₄′, G₈′, and G₁₀′ were registered for each sample. Milk protein composition was analysed by reversed phase HPLC. Concentration of β-lactoglobulin in milk was found to be an important factor for the variation in CT and G′. The A allele of β-lactoglobulin was associated with higher concentrations of β-lactoglobulin in milk compared with B. When no adjustment for β-lactoglobulin concentration was made, there was a significant overall effect of β-lactoglobulin genotype on acid coagulation, where the AA and AB genotypes were associated with better curd firmness compared with BB, whereas at equal β-lactoglobulin concentrations a tendency in the opposite direction was found with a significant and positive effect of BB compared with AB. Lactose concentration of milk had a positive effect on acid coagulation and was shown to improve G′ in milk with low β-lactoglobulin concentrations.     

Estimation of Residual Pepsin and Chymosin Activity in Curd,

Pasteurized milk (225 g) adjusted to pH 6.2 was set with 3.5 milk clotting units of chymosin (EC 3.4.23.4). The same amount of milk at pH 5.8 was set with 3.5 milk clotting units of porcine pepsin (EC 3.4.23.1). Fifteen minutes after clotting, the curd was broken, and curd and whey were separated by centrifugation at 3500 × g for 20 min. The curd (30 g) was extracted at pH 6.8 in 450 ml water or at pH 6.2 (chymosin) or 5.8 (pepsin) in 450 ml 1 M sodium chloride.Chymosin was completely released from the curd and accounted for by both methods of extraction. Pepsin was completely released and accounted for after extraction in 1 M sodium chloride at pH 5.8 but was partly inactivated during extraction at pH 6.8.Assay of curd extracts and whey by a linear agar diffusion test accounted for 102 ± 6% of the pepsin activity added to milk when the curd was extracted in 1 M sodium chloride. Extraction at pH 6.8 allowed recovery of only 63% of the activity. Chymosin recovery was 100 ± 5% by both methods of curd extraction.     

Genetic parameters for milk coagulation properties in Estonian Holstein cows

The objective of this study was to estimate heritabilities and repeatabilities for milk coagulation traits [milk coagulation time (RCT) and curd firmness (E₃₀)] and genetic and phenotypic correlations between milk yield and composition traits (milk fat percentage and protein percentage, urea, somatic cell count, pH) in first-lactation Estonian Holstein dairy cattle. A total of 17,577 test-day records from 4,191 Estonian Holstein cows in 73 herds across the country were collected during routine milk recordings. Measurements of RCT and E₃₀ determined with the Optigraph (Ysebaert, Frepillon, France) are based on an optical signal in the near-infrared region. The cows had at least 3 measurements taken during the period from April 2005 to January 2009. Data were analyzed using a repeatability animal model. There was substantial variation in milk coagulation traits with a coefficient of variation of 27% for E₃₀ and 9% for the log-transformed RCT. The percentage of variation explained by herd was 3% for E₃₀ and 4% for RCT, suggesting that milk coagulation traits are not strongly affected by herd conditions (e.g., feeding). Heritability was 0.28 for RCT and 0.41 for E₃₀, and repeatability estimates were 0.45 and 0.50, respectively. Genetic correlation between both milk coagulation traits was negligible, suggesting that RCT and E₃₀ have genetically different foundations. Milk coagulation time had a moderately high positive genetic (0.69) and phenotypic (0.61) correlation with milk pH indicating that a high pH is related to a less favorable RCT. Curd firmness had a moderate positive genetic (0.48) and phenotypic (0.45) correlation with the protein percentage. Therefore, a high protein percentage is associated with favorable curd firmness. All reported genetic parameters were statistically significantly different from zero. Additional univariate random regression analysis for milk coagulation traits yielded slightly higher average heritabilities of 0.38 and 0.47 for RCT and E₃₀ compared with the heritabilities of the repeatability model.     

Effect of milk fat concentration and gel firmness on syneresis during curd stirring in cheese-making

An experiment was undertaken to investigate the effect of milk fat level (0%, 2.5% and 5.0% w/w) and gel firmness level at cutting (5, 35 and 65 Pa) on indices of syneresis, while curd was undergoing stirring. The curd moisture content, yield of whey, fat in whey and casein fines in whey were measured at fixed intervals between 5 and 75 min after cutting the gel. The casein level in milk and clotting conditions was kept constant in all trials. The trials were carried out using recombined whole milk in an 11 L cheese vat. The fat level in milk had a large negative effect on the yield of whey. A clear effect of gel firmness on casein fines was observed. The best overall prediction, in terms of coefficient of determination, was for curd moisture content using milk fat concentration, time after gel cutting and set-to-cut time (R² = 0.95).     

Repeatability estimates for milk coagulation traits and non-coagulation of milk in Finnish Ayrshire cows

Effects of systematic environmental factors and milk production and quality traits on milk coagulation properties (MCP), and on repeatability of those traits were estimated from 979 milk samples collected once a month over a period of 2 years from 83 Finnish Ayrshire cows. Estimation was based on a multitrait animal model and REML methodology. In addition, persistence of non-coagulation of milk in individual cows, and factors associated with it were established from a sub sample of 24 cows producing non-coagulating (NC) milk at least once. MCP were at their best during the first lactation, at the beginning and at the end of lactation, and during grazing seasons. Variation in MCP with systematic environmental factors was partly due to variation in composition and quality of milk, especially in pH and ln (somatic cell count, SCC). Coefficients of repeatability for milk coagulation time and curd firmness were 0.65 and 0.68. These estimates were of the same magnitude as those for protein content, but were higher than those for daily milk yield, fat content, pH, and SCC. Based on the repeatability estimates for the milk coagulation traits and effects of the environmental factors, cows should be sampled at least three times during a lactation to estimate reliably breeding values for the milk coagulation traits. A total of 10% of the milk samples did not coagulate in 30 min after addition of rennet. Cows that produced NC milk at least once (30% of the cows) could be classified into those that produced NC milk only a few times during a lactation and those that produced NC milk at almost every sampling. Based on logistic regression analyses, peak and mid-lactation, high milk yield, low protein and fat content and high pH increased the risk of non-coagulation of milk.     

Milk Coagulation Properties and Milk Protein Genetic Variants of Three Cattle Breeds/Types in Sri Lanka

Milk coagulation is the primary step in the development of most dairy products. Raw milk from individual cows and different breeds exhibit distinct coagulation capacities. This variation is largely influenced by milk protein genetic variants. The aim of the present study was to evaluate differences in coagulation properties between milk obtained from three cattle breeds/types found in Sri Lanka. A total of 90 milk samples (400 mL from each individual) were collected from two Sri Lankan cattle breeds/types (Thamankaduwa White/TW and Local/“Batu” cattle) and one European cattle breed (Friesian). Collected samples were subjected to enzymatic coagulation using commercial rennet source (Chymax®, Christian Hansen Standard, Denmark) and lactic acid bacteria (LAB) coagulation using a commercial starter culture(YFL 8 12,Christian Hansen Standard, Denmark) to determine milk coagulation properties. Different properties of milk coagulum such as yield, curd firmness, syneresis and rheological properties were evaluated. The biochemical composition (lactose, protein, fat, solid-non-fat) of milk samples were determined. Capillary Zone Electrophoresis (CZE) method was used to determine milk protein genetic variants. Experimental design was Nested Completely Randomized Design with three treatments. Milk coagulation time and curd firmness after enzymatic-coagulation were not significantly different (p>0.05) among the breeds. Coagulum yield was significantly higher (p<0.05) for the TW type than that of other breeds. Coagulum yield was negatively correlated with β-caseinA1 and α-lactalbumin in both enzymatic (-0.58) and LAB coagulation (-0.69). Coagulum yield was positively correlated (p<0.05) with β-casein B variant (0.70), protein (0.34) and lactose (0.36) contents. Meltability value was weakly and positively (p<0.05) correlated (0.34) with fat content of milk. Overall results indicate that there is a significant correlation between milk coagulation properties and milk protein genetic variants in three cattle breeds/types considered in the current study. TW type is the unique among studied breeds in terms of coagulation properties and milk protein genetic variants.     

Short communication: Factors affecting coagulation properties of Mediterranean buffalo milk

The aim of this study was to investigate sources of variation of milk coagulation properties (MCP) of buffalo cows. Individual milk samples were collected from 200 animals in 5 herds located in northern Italy from January to March 2010. Rennet coagulation time (RCT, min) and curd firmness after 30 min from rennet addition (a₃₀, mm) were measured using the Formagraph instrument (Foss Electric, Hillerød, Denmark). In addition to MCP, information on milk yield, fat, protein, and casein contents, pH, and somatic cell count (SCC) was available. Sources of variation of RCT and a₃₀ were investigated using a linear model that included fixed effects of herd, days in milk (DIM), parity, fat content, casein content (only for a₃₀), and pH. The coefficient of determination was 51% for RCT and 48% for a₃₀. The most important sources of variation of MCP were the herd and pH effects, followed by DIM and fat content for RCT, and casein content for a₃₀. The relevance of acidity in explaining the variation of both RCT and a₃₀, and of casein content in explaining that of a₃₀, confirmed previous studies on dairy cows. Although future research is needed to investigate the effect of these sources of variation on cheese yield, findings from the present study suggest that casein content and acidity may be used as indicator traits to improve technological properties of buffalo milk.     

Effect of frequency of milking on yield,composition and processing quality of milk

The objective was to determine the effect of once-daily milking (ODM) and omitting one evening milking each week (13TWM), in late lactation on milk production, composition and processability. Seventy-two cows were assigned to three treatments (ODM, 13TWM and twice-daily milking [TDM]) from 4 October to 12 December. Cows were on average 218 d into lactation at the start of the trial, and all cows were managed similarly throughout the trial. Milk yields and gross milk composition of cows on all treatments were measured, and milk samples for detailed compositional and processability analysis were collected from TDM and ODM treatments at two consecutive milkings and at one milking each week, respectively. Milk yield was significantly reduced (P < 0.001) and milk fat and protein concentrations were increased (P < 0.01) with ODM compared with TDM. Milk yield and fat and protein concentrations of milk from TDM and 13TWM herds were similar. Casein concentrations in ODM and TDM milks were similar, but ODM milk had a higher (P < 0.05) whey protein content. Somatic cell count of ODM and TDM milks was similar. Rennet coagulation time (RCT) and curd firmness (A60) of milk were not affected by milking frequency. However, rate of curd aggregation (K20) of ODM milk was reduced (P < 0.05) compared with that of TDM milk. Plasmin activity in ODM milk was numerically higher than in TDM milk, but the effect was not significant. ODM milk had higher NAGase activity than TDM milk (P < 0.01). In conclusion, once daily milking reduced milk yield by 29% and did not adversely affect the processability of milk. Moreover, one evening milking per week could be eliminated without adverse effects on milk yield or composition.     

Processing characteristics of dairy cow milk are moderately heritable

Milk processing attributes represent a group of milk quality traits that are important to the dairy industry to inform product portfolio. However, because of the resources required to routinely measure such quality traits, precise genetic parameter estimates from a large population of animals are lacking for these traits. Milk processing characteristics considered in the present study—rennet coagulation time, curd-firming time, curd firmness at 30 and 60 min after rennet addition, heat coagulation time, casein micelle size, and milk pH—were all estimated using mid-infrared spectroscopy prediction equations. Variance components for these traits were estimated using 136,807 test-day records from 5 to 305 d in milk (DIM) from 9,824 cows using random regressions to model the additive genetic and within-lactation permanent environmental variances. Heritability estimates ranged from 0.18 ± 0.01 (26 DIM) to 0.38 ± 0.02 (180 DIM) for rennet coagulation time; from 0.26 ± 0.02 (5 DIM) to 0.57 ± 0.02 (174 DIM) for curd-firming time; from 0.16 ± 0.01 (30 DIM) to 0.56 ± 0.02 (271 DIM) for curd firmness at 30 min; from 0.13 ± 0.01 (30 DIM) to 0.48 ± 0.02 (271 DIM) for curd firmness at 60 min; from 0.08 ± 0.01 (17 DIM) to 0.24 ± 0.01 (180 DIM) for heat coagulation time; from 0.23 ± 0.02 (30 DIM) to 0.43 ± 0.02 (261 DIM) for casein micelle size; and from 0.20 ± 0.01 (30 DIM) to 0.36 ± 0.02 (151 DIM) for milk pH. Within-trait genetic correlations across DIM weakened as the number of days between compared intervals increased but were mostly >0.4 except between the peripheries of the lactation. Eigenvalues and associated eigenfunctions of the additive genetic covariance matrix for all traits revealed that at least the 80% of the genetic variation among animals in lactation profiles was associated with the height of the lactation profile. Curd-firming time and curd firmness at 30 min were weakly to moderately genetically correlated with milk yield (from 0.33 ± 0.05 to 0.59 ± 0.05 for curd-firming time, and from ?0.62 ± 0.03 to ?0.21 ± 0.06 for curd firmness at 30 min). Milk protein concentration was strongly genetically correlated with curd firmness at 30 min (0.84 ± 0.02 to 0.94 ± 0.01) but only weakly genetically correlated with milk heat coagulation time (?0.27 ± 0.07 to 0.19 ± 0.06). Results from the present study indicate the existence of exploitable genetic variation for milk processing characteristics. Because of possible indirect deterioration in milk processing characteristics due to selection for greater milk yield, emphasis on milk processing characteristics is advised.     

Acid coagulation properties and suitability for yogurt production of cows’ milk treated by high-pressure homogenisation

The effects of high-pressure homogenisation (HPH) of cows’ milk were investigated for suitability for yogurt manufacture, compared with the processes currently applied in industry. Milk at different inlet temperatures (30 °C or 40 °C) was subjected to HPH treatment at 100, 200 or 300 MPa (one stage) and 130, 230 or 330 MPa (two-stage). HPH-treated milk was compared with milk heat-treated (90 °C for 90 s) and homogenised at 15 MPa, and with milk treated under the same thermal conditions and also fortified with 3% skim milk powder. Milk treated at 300 or 200 MPa showed higher gel strengths on coagulation, higher gel firmness in texture analysis, less syneresis and lower titratable acidity compared with conventionally treated milk fortified with 3% skim milk powder.     

Forage Growth and Performance of Grazing Dairy Cows Supplemented with Concentrate and Chopped or Long Hay

Holstein and Brown Swiss cows were in three groups of eight in a 3 × 3 Latin square design with 4-wk periods. Cows averaged 88 d in lactation at start of trial. All cows simultaneously grazed grass and clover pastures on a rotational basis. Supplement treatments were concentrate (A), concentrate with 10% added chopped hay (B), and concentrate plus 1 kg long hay/cow daily (C). Concentrate was offered at 1 kg/3 kg 4% fat-corrected milk to cows on A and C, and 1 kg/2.7 kg to those on B. Forage growth, amounts available for grazing, and average dry matter intake were measured by clipping subsamples prior to and after each grazing and by use of cages placed in the field.Milk yields and milk fat percentages were 27.1, 3.34; 27.9, 3.32; and 28.3, 3.20 for treatments A, B, and C, respectively. Differences between A and C were significant. Milk fat percentage decreased moderately when cows first went to pasture.There were no treatment differences in fat yield, milk protein percentage, yield of fat-corrected milk, rumen fluid composition, or body weight gain.Available forage per cow exceeded 22 kg dry matter/cow daily except during one 10-d period toward the end of the trial when it amounted to 16.6 kg. Intake of dry matter from pasturage, measured by the sward cutting technique, averaged 14.5 kg./cow daily over the 12 wk.