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.     

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.     

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     

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.     

Short communication: The effect of dry period duration and dietary energy density in early lactation on the rennet gelation properties of milk

This study was carried out to examine the effects of decreasing the dry period (DP) duration and altering the energy density of the diet during early lactation on the rheological characteristics of milk. Forty mature Holstein-Friesian cows were used in a completely randomized design with a 2 × 2 factorial arrangement of treatments. Cows were randomly assigned to 1 of 2 dry period treatments and 1 of 2 nutritional treatments. The DP treatments were continuous milking (CM) or an 8-wk standard dry period (SDP), and the nutritional treatments were a standard energy diet (SE) or a high energy diet (HE). Actual DP lengths were 6.3 ± 1.7 d and 62.1 ± 1.9 d for cows on the CM and SDP treatments, respectively. Milk samples were collected at 2, 6, and 10 wk postpartum. The concentrations of fat, protein, and lactose were determined in each sample. The rennet gelation properties were measured at 31°C using dynamic low-amplitude strain oscillatory rheometry. The following parameters were obtained from the resultant elastic shear modulus (G′): gelation time, maximum curd firming rate, and gel strength. Reducing the DP duration from 62 to 6 d resulted in increases in milk protein concentration (31.8 vs. 34.7 g/kg), maximum curd firming rate (2.58 vs. 3.60 Pa/min), and gel strength (69.4 vs. 90.5 Pa). Increasing the dietary energy density decreased percentage milk fat (43.1 vs. 37.7 g/kg) but otherwise had no effect. Gel strength was correlated with maximum curd firming rate (r = 0.99), and both variables were correlated with milk protein concentration (r = 0.71 and r = 0.73, respectively). The results indicate that decreasing the duration of the DP increased milk protein concentration and improved the rennet gelation properties of milk, but that dietary energy density had little effect.     

Phenotypic factors affecting coagulation properties of milk from Sarda ewes

In this study, milk-coagulation properties (MCP) were characterized in the Sarda sheep breed. Milk composition and MCP [rennet-coagulation time (RCT), curd-firming time [time to reach a curd firmness of 20 mm (k₂₀)], and curd firmness (a₃₀), (a₄₅), and (a₆₀)] were obtained extending the lactodynamographic analysis from 30 to 60 min from a population of 1,121 ewes from 23 different farms. Managerial characteristics of farms and parity, individual daily milk yields and stage of lactation of ewes were recorded. Data were analyzed using a mixed-model procedure with fixed effects of days in milk, parity, daily milk yield, and flock size and the random effect of the flock/test day nested within flock size. Sampled farms were classified as small (<300 ewes) and medium (300 to 600 ewes), and these were kept by family operations, or as large (>600 ewes), often operated through hired workers. Daily milk yield was, on average, 1.58 ± 0.79 L/d and variability for this trait was very high. The average content of fat, protein, and casein was respectively 6.41, 5.39, and 4.20%. The class of flock size had a significant effect only on curd firmness, whereas days in milk affected RCT and k₂₀. The flock test day, parity, and daily milk yield were important sources of variation for all MCP. The mean value of RCT (8.6 min) and the low occurrence of noncoagulating samples (0.44%) confirmed the excellent coagulation ability of sheep milk compared with cattle milk. A more rapid coagulation was observed in mid-lactating, primiparous, and high-yielding ewes. The k₂₀ was usually reached in less than 2 min after gelation, with the most favorable values at mid lactation. The mean value of curd firmness 30 min after rennet addition (a₃₀) was, on average, 50 mm and decreased to 46 and 42 mm respectively after 45 (a₄₅) and 60 min (a₆₀). The decreasing value of curd-firmness traits was likely to be caused by curd syneresis and whey expulsion. The correlation between RCT and a₃₀ was much lower than in dairy cows and about null for a₄₅ and a₆₀. This means that curd firmness in dairy ewes is almost independent of gelation time and this can provide specific information for this species. In conclusion, this study showed that milk from Sarda sheep is characterized by an earlier gelation, a faster increase in curd firmness with time, and greater curd firmness after 30 min compared with dairy cows. Furthermore, correlations between MCP in sheep are much lower than in bovines and some of the assumptions and interpretations related to cows cannot be applied to sheep.     

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.     

Effects of ultra-high pressure homogenization on the cheese-making properties of milk

The effects of single- or 2-stage ultra-high pressure homogenization (UHPH; 100 to 330 MPa) at an inlet temperature of 30°C on the cheese-making properties of bovine milk were investigated. Effects were compared with those from raw, heat-pasteurized (72°C for 15 s), and conventional homogenized-pasteurized (15 + 3 MPa, 72°C for 15 s) treatments. Rennet coagulation time, rate of curd firming, curd firmness, wet yield, and moisture content of curds were assessed. Results of particle size and distribution of milk, whey composition, and gel microstructure observed by confocal laser scanning microscopy were analyzed to understand the effect of UHPH. Single-stage UHPH at 200 and 300 MPa enhanced rennet coagulation properties. However, these properties were negatively affected by the use of the UHPH secondary stage. Increasing the pressure led to higher yields and moisture content of curds. The improvement in the cheese-making properties of milk by UHPH could be explained by changes to the protein-fat structures due to the combined effect of heat and homogenization.     

Grazing season and forage type influence goat milk composition and rennet coagulation properties

Two different types of pasture (cultivated and rangeland) and 2 different hay qualities (high and low quality) were examined for their effects on goat milk composition and rennet coagulation properties. Furthermore, the effect of dietary treatments in both the early and late grazing season was studied. As lactation stage is known to influence milk composition, the goats in the early and late grazing season were in the same lactation stage at the start of the experiment. The milk composition was influenced both by dietary treatment and season. Milk from goats on pasture was superior to those on hay by containing a higher content of protein and casein, and the goats on cultivated pasture had the highest milk yield. Casein composition was significantly influenced by forage treatment. Goats grazing on cultivated pasture had higher contents of α_s1-casein and also of κ-casein compared with the other treatments, whereas goats grazing on rangeland had the highest content of β-casein. Factors such as milk yield, casein micelle size, α_s2-casein, and calcium content were reduced in late compared with early season. More favorable rennet coagulation properties were achieved in milk from the early grazing season, with shorter firming time and higher curd firmness compared with milk from the late grazing season, but the firming time and curd firmness were not prominently influenced by forage treatment. The content of α_s2-casein and calcium in the milk affected the firming time and the curd firmness positively. The influence of season and forage treatment on especially milk yield, casein content, and rennet coagulation properties is of economic importance for both the dairy industry and goat milk farmers.     

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.     

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.     

Milk quality,coagulation properties,and curd firmness modeling of purebred Holsteins and first- and second-generation crossbred cows from Swedish Red,Montbéliarde,and Brown Swiss bulls

The objective of the present study was to investigate how the crossbreeding of Holstein (HO) cows with bulls from Nordic and Alpine European breeds affect milk quality traits, traditional milk coagulation properties (MCP), and curd firmness modeling obtained from individual milk samples. A total of 506 individual milk samples were collected from evening milking at 3 commercial farms located in Northern Italy. Over the past decade, the 3 farms have followed crossbreeding programs in part of their herds, whereas the remainder of the animals consisted of purebred HO. The basic scheme was a 3-breed rotation based on the use of Swedish Red (SR) semen on HO cows (SR × HO), the use of Montbéliarde (MO) semen on first-cross cows [MO × (SR × HO)], and the use of HO semen in the third cross. In all herds, a smaller proportion of purebred HO were mated to M and Brown Swiss (BS) bulls, and these first crosses were mated to SR and MO bulls, respectively. Milk samples were analyzed for milk composition and MCP, and parameters for curd firmness were modeled. Compared with purebred HO, crossbred cows produced less milk with lower lactose content, higher fat and protein content, and a tendency for higher casein content. Crossbred cows generally produced milk with a more favorable curd-firming rate (k₂₀) and curd firmness 30 min after rennet addition, among traditional MCP, and better trends of curd firmness measures as shown by model parameters: estimated rennet coagulation time, asymptotical potential value of curd firmness, and curd-firming instant rate constant. Among crossbred cows, SR × HO presented longer rennet coagulation time compared with MO × HO and BS × HO cows, and MO × HO showed shorter k₂₀ compared with BS × HO cows. Among second-generation cows, those sired by SR bulls showed a lower incidence of noncoagulated samples, higher curd firmness 30 min after rennet addition and asymptotical potential value of curd firmness, and faster curd-firming instant rate constant compared with animals sired by MO bulls. Our results revealed that different sire breeds were characterized by specific technological aptitudes, but that these were not strictly related to other milk quality traits. Furthermore, the favorable characteristics (in terms of the quality and technological properties of milk) could be maintained in the third generation of 3-way crosses without negative effects on milk yield, even though the HO heritage had been reduced from 50 to 25%. Our findings, therefore, suggest that different types of sires can be chosen (depending on the intended use of the milk) to ensure the optimization of farm crossbreeding programs.     

The effects of composition and some processing treatments on the rennet coagulation properties of milk

The effects of various processing parameters on the rennet coagulation properties of milk were assessed. Using low amplitude oscillation rheometry, the coagulation properties were monitored by measurement of the elastic shear modulus, G', as a function of time, t, from rennet addition; Gapos; was taken as a measure of curd firmness. The Scott-Blair time dependency model was fitted to the experimental G' It curves for the determination of the following coagulation parameters: gel time, maximum curd firming rate, the set-to-cut time at 20 Pa (ie, time to reach 20 Pa) and the curd firmness after a renneting time of 2400 s. The renneting properties were enhanced by increasing the levels of milk protein and fat in the ranges 0.3–7.0% (w/w) and 0.1–10% (w/w) respectively and by two stage homogenization pressure where the first stage pressure, P₁, was varied from 0 to 25 MPa and the second stage pressure, P₂, was held constant at 5 MP a. The influence of these parameters, within the range investigated, in complementing the gel forming properties decreased in the following order: protein > fat > homogenization pressure. In contrast, the coagulation properties of milk were impaired by high heat treatment, the addition of a commercial microparticulated whey protein based fat substitute and by partial replacement of protein with fat.     

Modeling of coagulation,curd firming,and syneresis of goat milk from 6 breeds

Traditional milk coagulation properties are used to predict the suitability of milk for cheese-making. In bovine and ovine species, the introduction of the concept of curd firming over time, continuously recorded by a lactodynamograph during prolonged tests, provides additional information about milk coagulation, curd-firming, and syneresis processes. The aims of present study were (1) to test the adaptability of a 4-parameter curd-firming model in the assessment of goat milk (also comparing published data of other species); (2) to describe variability of coagulation, curd firming, and syneresis processes among individual goat milk samples; (3) to quantify the effects of farm and animal factors (breed, parity, and stage of lactation); and (4) to compare 6 goat breeds for their model parameters. Milk samples from 1,272 goats reared in 35 farms were collected. Goats were of 6 breeds: Saanen and Camosciata delle Alpi for the Alpine type; and Murciano-Granadina, Maltese, Sarda, and Sarda Primitiva for the Mediterranean type. During a lactodynamographic analysis (60 min), 240 measures of curd firmness (mm) were recorded for each milk sample. The modeling of curd firming allowed us to achieve the rennet coagulation time estimated on the basis of all the data points (min); the curd firming and the curd syneresis instant rate constants; the asymptotical potential value of curd firming; the actual maximum curd firmness; and the time at which the curd firming maximum level is attained. Modeling parameter data were analyzed using a linear mixed model. Comparison with other dairy species showed several differences: goat milk coagulated later than sheep but earlier than bovine, and curd firming and curd syneresis instant rate constants were greater in small ruminants. Modeling parameters of goat milk were mostly affected by the farm effect (37% of the total variance, on average) compared with the results found for bovine and ovine samples, and this was probably attributable to the marked differences among goat farming systems. Small differences were demonstrated between Alpine and Mediterranean breeds, but the time of maximum curd firmness was lower in Murciano-Granadina compared with Maltese, Sarda, and Sarda Primitiva. Sarda and Sarda Primitiva were very similar and exhibited the most favorable coagulation properties of milk. For almost all the model parameters, the direct effect of breed was increased after correction for milk yield and composition. In conclusion, this approach allowed us to fully depict the effects of the different factors on coagulation of goat milk, and clarified the different renneting pattern among goat breeds, and with other species. Results could be used for the valorization of goat dairy products, also when these are linked to particular local breeds, and to stimulate further studies about relationships between coagulation and cheese-making traits.     

Influence of Protein and Fat Contents of Ultrafiltered Milk on Rheological Properties of Gels Formed by Chymosin

Enzymatically induced gelation of milk and milk concentrated by ultrafiltration was studied with a curd firmness tester which measured transmission of oscillatory deformations through the product. Scott Blair's relationship between pseudo modulus of rigidity (G) and time after clotting (t), G = G∞ e^—_λ/t described the gelation of ultrafiltered retentates satisfactorily. Maximum gel firmness G∞, and _λ, which is the time to reach a firmness of G∞/e, were affected substantially by protein content. The effect of protein content on rates of firming of gels were evaluated at different stages of gel formation. At the onset of gelation, rates increased up to protein content 90 g kg^—1 but decreased at higher protein concentrations. At a later stage, this decrease disappeared and rates correlated with protein content. Those observations are discussed relative to effects of protein content on proteolytic and aggregation phases of milk clotting.Times from clotting to point of cutting curd were affected significantly by slight increases in protein above that in unconcentrated milk, but clotting times were not affected. Variations in fat content had inconsequential effects on test measures.     

Composition,coagulation properties,and predicted cheesemaking traits of bulk goat milk from different farming systems,breeds, and stages of production

At the global level, the quantity of goat milk produced and its gross production value have increased considerably over the last 2 decades. Although many scientific papers on this topic have been published, few studies have been carried out on bulk goat milk samples. The aim of the present study was to investigate in the field the effects of farming system, breed type, individual flock, and stage of production on the composition, coagulation properties (MCP), curd firming over time parameters (CF_t), predicted cheese yield (CY%), and nutrient recovery traits (REC) of 432 bulk milk samples from 161 commercial goat farms in Sardinia, Italy. We found that the variance due to individual flock was of the same order as the residual variance for almost all composition and cheesemaking traits. With regard to the fixed effects, the effect of farming system on bulk milk variability was not highly significant for the majority of traits (it was lower than individual flock), whereas the effects of breed type and stage of production were much higher. More specifically, the intensive farms produced milk with the best concentrations of almost all constituents, whereas extensive farms exhibited faster rennet coagulation times, a slower rate of curd firming, lower potential curd firmness, and lower percentages of fat and energy recoveries in the fresh curd. Farms rearing the local breed, Sarda, alone or together with the Maltese breed, produced milk with the best concentrations of fat and protein, superior curd firmness, and better predicted percentage of fresh curd (CY_CURD) and recovery traits. The results show the potential of both types of breed, either for their quantitative (specialized breeds) or their qualitative (local breeds) attributes. As expected, the concentrations of fat, protein fractions, and lactose were influenced by the stage of production, with samples collected in the early stage of production (in February and March) having a greater quantity of the main constituents. Somatic cells reached the highest levels in the late stage of production, which corresponds to the goats' advanced stage of lactation (June–July), although no differences were present in the logarithmic bacterial counts between the early and late stages. Regarding cheesemaking potential, bulk milk samples of the late stage were characterized by delayed rennet coagulation and curd firming times, the lowest values of curd firmness, and a general reduction in CY%, and REC traits. In conclusion, we highlight several issues regarding the effects of the most important sources of variation on bulk goat milk, and point to some critical factors relevant for improving dairy goat farming and milk production.     

Estimation of genetic parameters for cheese-making traits in Spanish Churra sheep

The global production of sheep milk is growing, and the main industrial use of sheep milk is cheese making. The Spanish Churra sheep breed is one of the most important native dairy breeds in Spain. The present study aimed to estimate genetic parameters for a wide range of traits influencing the cheese-making ability of Churra sheep milk. Using a total of 1,049 Churra ewes, we studied the following cheese-making traits: 4 traits related to milk coagulation properties (rennet coagulation time, curd-firming time, and curd firmness at 30 and 60 min after addition of rennet), 2 traits related to cheese yield (individual laboratory cheese yield and individual laboratory dried curd yield), and 3 traits measuring curd firmness over time (maximum curd firmness, time to attain maximum curd firmness, and syneresis). In addition, a list of milk traits, including the native pH of the milk and several milk production and composition traits (milk yield; the fat, protein, and dried extract percentages; and the somatic cell count), were also analyzed for the studied animals. After discarding the noncoagulating samples (only 3.7%), data of 1,010 ewes were analyzed with multiple-trait animal models by using the restricted maximum likelihood method to estimate (co)variance components, heritabilities, and genetic correlations. In general, the heritability estimates were low to moderate, ranging from 0.08 (for the individual laboratory dried curd yield trait) to 0.42 (for the fat percentage trait). High genetic correlations were found within pairs of related traits (i.e., 0.93 between fat and dried extract percentages, ?0.93 between the log of the curd-firming time and curd firmness at 30 min, 0.70 between individual laboratory cheese yield and individual laboratory dried curd yield, and ?0.94 between time to attain maximum curd firmness and syneresis). Considering all the information provided here, we suggest that in addition to the current consideration of the protein percentage trait for improving cheese yield traits, the inclusion of the pH of milk as a measured trait in the Churra dairy breeding program would represent an efficient strategy for improving the cheese-making ability of milk from this breed.     

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.     

Evaluation of Alternative Methods to Increase Calcium Retention in Cottage Cheese Curd

The effect of several alternative methods including addition of rennet, addition of carrageenan and use of 2:1 (v/v) preconcentrated skim milk by ultrafiltration (UF) upon calcium retention, yield, composition and sensory properties of dry curd cottage cheese was investigated. Although each of the processing methods resulted in the manufacture of dry curd cottage cheese with different compositions and properties, none of them was satisfactory for increasing calcium retention. Added carrageenan bound additional whey proteins, added rennet interfered with curd syneresis and whey expulsion during cooking and use of UF preconcentrated skim milk resulted in an increase in yield, total solids and protein of the curd.     

Short communication: Fourier-transform mid-infrared spectroscopy to predict coagulation and acidity traits of sheep bulk milk

Sheep milk is mainly transformed into cheese; thus, the dairy industry seeks more rapid and cost-effective methods of analysis to determine milk coagulation and acidity traits. This study aimed to assess the feasibility of Fourier-transform mid-infrared spectroscopy to determine milk coagulation and acidity traits of sheep bulk milk and to classify milk samples according to their renneting capacity. A total of 465 bulk milk samples collected in 140 single-breed flocks of Comisana (84 samples, 24 flocks) and Sarda (381 samples, 116 flocks) breeds located in Central Italy were analyzed for coagulation properties (rennet coagulation time, curd firming time, and curd firmness) and acidity traits (pH and titratable acidity) using standard laboratory procedures. Fourier-transform mid-infrared spectroscopy prediction models for these traits were built using partial least squares regression analysis and were externally validated by randomly dividing the full data set into a calibration set (75%) and a validation set (25%). The discriminant capacity of the rennet coagulation time prediction model was determined using partial least squares discriminant analysis. Prediction models were more accurate for acidity traits than for milk coagulation properties, and the ratio of prediction to deviation ranged from 1.01 (curd firmness) to 2.14 (pH). Moreover, the discriminant analysis led to an overall accuracy of 74 and 66% for the calibration and validation sets, respectively, with greater sensitivity for samples that coagulated between 10 and 20 min and greater specificity to detect early-coagulating (<10 min) and late-coagulating (20–30 min) samples. Results suggest that Fourier-transform mid-infrared spectroscopy has the potential to help the dairy sheep industry identify milk with better coagulation ability for cheese production and thus improve milk transformation efficiency. However, further research is needed before this information can be exploited at the industry level.