Contribution of proteolytic activity associated with somatic cells in milk to cheese ripening

Prediction of fresh and ripened cheese yield using detailed milk composition and udder health indicators from individual Brown Swiss cows

The composition of raw milk is of major importance for dairy products, especially fat, protein, and casein (CN) contents, which are used worldwide in breeding programs for dairy species because of their role in human nutrition and in determining cheese yield (%CY). The aim of the study was to develop formulas based on detailed milk composition to disentangle the role of each milk component on %CY traits. To this end, 1,271 individual milk samples (1.5 L/cow) from Brown Swiss cows were processed according to a laboratory model cheese-making procedure. Fresh %CY (%CY_CURD), total solids and water retained in the fresh cheese (%CY_SOLIDS and %CY_WATER), and 60-days ripened cheese (%CY_RIPENED) were the reference traits and were used as response variables. Training-testing linear regression modeling was performed: 80% of observations were randomly assigned to the training set, 20% to the validation set, and the procedure was repeated 10 times. Four groups of predictive equations were identified, in which different combinations of predictors were tested separately to predict %CY traits: (i) basic composition, i.e., fat, protein, and CN, tested individually and in combination; (ii) udder health indicators (UHI), i.e., fat + protein or CN + lactose and/or somatic cell score (SCS); (iii) detailed protein profile, i.e., fat + protein fractions [CN fractions, whey proteins, and nonprotein nitrogen (NPN) compounds]; (iv) detailed protein profile + UHI, i.e., fat + protein fractions + NPN compounds and/or UHI. Aside from the positive effect of fat, protein, and total casein on %CY, our results allowed us to disentangle the role of each casein fraction and whey protein, confirming the central role of β-CN and κ-CN, but also showing α-lactalbumin (α-LA) to have a favorable effect, and β-lactoglobulin (β-LG) a negative effect. Replacing protein or casein with individual milk protein and NPN fractions in the statistical models appreciably increased the validation accuracy of the equations. The cheese industry would benefit from an improvement, through genetic selection, of traits related to cheese yield and this study offers new insights into the quantification of the influence of milk components in composite selection indices with the aim of directly enhancing cheese production.

Influence of Zinc Feeding on Nutritional Quality, Oxidative Stability and Volatile Profile of Fresh and Ripened Ewes' Milk Cheese

Zinc represents a ubiquitous element in cells with relevant roles in the metabolism of essential nutrients in animals. The aim of this study was to investigate the effect of dietary zinc supplementation on nutritional and aromatic properties of milk and Pecorino cheeses obtained from lactating ewes. Fifty-two commercial ewes were randomly assigned to two groups. The control group was fed with a conventional complete diet, while the experimental group received a daily supplementation of 375 mg/head of zinc oxide. At the end of the trial, which lasted 30 days, samples of milk and related cheese were collected in order to obtain information about the chemical composition and volatile profile. The experimental feeding strategy induced a significant increase in zinc concentration in milk. Furthermore, both in milk and cheese, was observed an increase in vaccenic, rumenic and total polyunsaturated fatty acids, with the consequent significant reduction of atherogenic and thrombogenic indices. The volatile profile of dairy products was also positively affected by dietary zinc intake, with an increase in concentration of hexanoic acid and ethyl esters. The present study suggests interesting possible effects of dietary zinc supplementation of ewes in improving the nutritional characteristics of fresh and ripened dairy products, although more specific and in-depth assessments should be performed on these new products, in order to characterize potential variations on consumers acceptability.

Comparative transcriptome analysis of mammary epithelial cells at different stages of lactation reveals wide differences in gene expression and pathways regulating milk synthesis between Jersey and Kashmiri cattle

Jersey and Kashmiri cattle are important dairy breeds that contribute significantly to the total milk production of the Indian northern state of Jammu and Kashmir. The Kashmiri cattle germplasm has been extensively diluted through crossbreeding with Jersey cattle with the goal of enhancing its milk production ability. However, crossbred animals are prone to diseases resulting to unsustainable milk production. This study aimed to provide a comprehensive transcriptome profile of mammary gland epithelial cells at different stages of lactation and to find key differences in genes and pathways regulating milk traits between Jersey and Kashmiri cattle. Mammary epithelial cells (MEC) isolated from milk obtained from six lactating cows (three Jersey and three Kashmiri cattle) on day 15 (D15), D90 and D250 in milk, representing early, mid and late lactation, respectively were used. RNA isolated from MEC was subjected to next-generation RNA sequencing and bioinformatics processing. Casein and whey protein genes were found to be highly expressed throughout the lactation stages in both breeds. Largest differences in differentially expressed genes (DEG) were between D15 vs D90 (1,805 genes) in Kashmiri cattle and, D15 vs D250 (3,392 genes) in Jersey cattle. A total of 1,103, 1,356 and 1,397 genes were differentially expressed between Kashmiri and Jersey cattle on D15, D90 and D250, respectively. Antioxidant genes like RPLPO and RPS28 were highly expressed in Kashmiri cattle. Differentially expressed genes in both Kashmiri and Jersey were enriched for multicellular organismal process, receptor activity, catalytic activity, signal transducer activity, macromolecular complex and developmental process gene ontology terms. Whereas, biological regulation, endopeptidase activity and response to stimulus were enriched in Kashmiri cattle and, reproduction and immune system process were enriched in Jersey cattle. Most of the pathways responsible for regulation of milk production like JAK-STAT, p38 MAPK pathway, PI3 kinase pathway were enriched by DEG in Jersey cattle only. Although Kashmiri has poor milk production efficiency, the present study suggests possible physicochemical and antioxidant properties of Kashmiri cattle milk that needs to be further explored.

Development of a laboratory technique for the evaluation of protease enzymes activity in goat and sheep milk

The shelf life of milk and its products is strongly influenced by the quality of raw milk. Several bacteria present in raw milk can produce proteases: among them, those from psychotropic microorganisms (e.g. Bacillus subtilis), which produce enzymes under refrigeration, cause the most serious problem. The interaction of these enzymes with the milk proteins and the main proteolytic enzymes becomes important in determining the quality of the milk. An artificial substrate (azocasein) was used to quantify the enzyme activity through the release of a chromogenic product that was measured spectrophotometrically after clarifying the sample by the addition of trichloroacetic acid. The detection limit and quantification limit were 1.43 and 4.77mg/ml respectively for sheep milk and, 2.25 and 7.5mg/ml respectively for goat milk. This method is useful for determining the proteolytic activity in different media and its effectiveness depends on chemical-nutritional characteristics of the sample.

The nonlinear effect of somatic cell count on milk composition, coagulation properties, curd firmness modeling, cheese yield, and curd nutrient recovery

The aim of this study was to investigate the relationships between somatic cell count (SCC) in milk and several milk technological traits at the individual cow level. In particular, we determined the effects of very low to very high SCC on traits related to (1) milk yield and composition; (2) coagulation properties, including the traditional milk coagulation properties (MCP) and the new curd firming model parameters; and (3) cheese yield and recovery of milk nutrients in the curd (or loss in the whey). Milk samples from 1,271 Brown Swiss cows from 85 herds were used. Nine coagulation traits were measured: 3 traditional MCP [rennet coagulation time (RCT, min), curd firming rate (k20, min), and curd firmness after 30 min (a30, mm)] and 6 new curd firming and syneresis traits [potential asymptotic curd firmness at infinite time (CFP, mm), curd firming instant rate constant (kCF, % × min(-1)), syneresis instant rate constant (kSR, % × min(-1)), rennet coagulation time estimated using the equation (RCTeq, min), maximum curd firmness achieved within 45 min (CFmax, mm), and time at achievement of CFmax (tmax, min)]. The observed cheese-making traits included 3 cheese yield traits (%CYCURD, %CYSOLIDS, and %CYWATER, which represented the weights of curd, total solids, and water, respectively, as a percentage of the weight of the processed milk) and 4 nutrient recoveries in the curd (RECFAT, RECPROTEIN, RECSOLIDS, and RECENERGY, which each represented the percentage ratio between the nutrient in the curd and milk). Data were analyzed using a linear mixed model with the fixed effects of days in milk, parity, and somatic cell score (SCS), and the random effect of herd-date. Somatic cell score had strong influences on casein number and lactose, and also affected pH; these were traits characterized by a quadratic pattern of the data. The results also showed a negative linear relationship between SCS and milk yield. Somatic cell score influenced almost all of the tested coagulation traits (both traditional and modeled), with the exceptions of k20, CFP, and kSR. Gelation was delayed when the SCS decreased (slightly) and when it increased (strongly) with respect to a value of 2, as confirmed by the quadratic patterns observed for both RCT and RCTeq. The SCS effect on a30 showed a quadratic pattern almost opposite to that observed for RCT. With respect to the CFt parameters, kCF decreased linearly as SCS increased, resulting in a linear decrease of CFmax and a quadratic pattern for tmax. Milk SCS attained significance for %CYCURD, %CYWATER, and RECPROTEIN. As the SCS increased beyond 3, we observed a progressive quadratic decrease of the water retained in the curd (%CYWATER), which caused a parallel decrease in %CYCURD. With respect to RECPROTEIN, the negative effect of SCS was almost linear. Recovery of fat and (consequently) RECENERGY was characterized by a more evident quadratic trend, with the most favorable values associated with an intermediate SCS. Together, our results confirmed that high SCS has a negative effect on milk composition and technological traits, highlighting the nonlinear trends of some traits across the different classes of SCS. Moreover, we report that a very low SCS has a negative effect on some technological traits of milk.

Characterization and Comparative Analysis of the Milk Transcriptome in Two Dairy Sheep Breeds using RNA Sequencing

This study presents a dynamic characterization of the sheep milk transcriptome aiming at achieving a better understanding of the sheep lactating mammary gland. Transcriptome sequencing (RNA-seq) was performed on total RNA extracted from milk somatic cells from ewes on days 10, 50, 120 and 150 after lambing. The experiment was performed in Spanish Churra and Assaf breeds, which differ in their milk production traits. Nearly 67% of the annotated genes in the reference genome (Oar_v3.1) were expressed in ovine milk somatic cells. For the two breeds and across the four lactation stages studied, the most highly expressed genes encoded caseins and whey proteins. We detected 573 differentially expressed genes (DEGs) across lactation points, with the largest differences being found, between day 10 and day 150. Upregulated GO terms at late lactation stages were linked mainly to developmental processes linked to extracellular matrix remodeling. A total of 256 annotated DEGs were detected in the Assaf and Churra comparison. Some genes selectively upregulated in the Churra breed grouped under the endopeptidase and channel activity GO terms. These genes could be related to the higher cheese yield of this breed. Overall, this study provides the first integrated overview on sheep milk gene expression.

Proton transfer reaction time-of-flight mass spectrometry: A high-throughput and innovative method to study the influence of dairy system and cow characteristics on the volatile compound fingerprint of cheeses

The aim of this work was to study the effect of dairy system and individual cow-related factors on the volatile fingerprint of a large number of individual model cheeses analyzed by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). A total of 1,075 model cheeses were produced using milk samples collected from individual Brown Swiss cows reared in 72 herds located in mountainous areas of Trento province (Italy). The herds belonged to 5 main dairy systems ranging from traditional to modern and the cows presented different daily milk yields (24.6±7.9kg × d(-1)), stages of lactation (199±138 d in milk), and parities (2.7±1.8). The PTR-ToF-MS revealed 619 peaks, of which the 240 most intense were analyzed, and 61 of these were tentatively attributed to relevant volatile organic compounds on the basis of their fragmentation patterns and data from the literature. Principal component analysis was used to convert the multiple responses characterizing the PTR-ToF-MS spectra into 5 synthetic variables representing 62% of the total information. These principal components were related to groups of volatile compounds tentatively attributed to different peaks and used to investigate the relationship of the volatile compound profile obtained by PTR-ToF-MS to animal and farm characteristics. Lactation stage is related to 4 principal components which brought together 52.9% of the total variance and 57.9% of the area of analyzed peaks. In particular, 2 principal components were positively related to peaks tentatively attributed to aldehydes and ketones and negatively related to alcohols, esters, and acids, which displayed a linear increase during lactation. The second principal component was affected by dairy system; it was higher in the modern system in which cows received total mixed rations. The third principal component was positively related to daily milk production. In summary, we report the first application of this innovative, high-throughput technique to study the effects of dairy system and individual animal factors on volatile organic compounds of model cheeses. Individual cheesemaking procedures together with this spectrometric technique open new avenues for genetic selection of dairy species with respect to both milk and cheese quality.

Influence of milk somatic cell content on Parmigiano-Reggiano cheese yield

The aim of this study was to determine the influence of the somatic cell content (SCC) of milk on Parmigiano-Reggiano cheese yield, produced in commercial cheese factories under field conditions. The study was carried out following the production of 56 batches of Parmigiano-Reggiano in 13 commercial cheese factories by processing milk collected from Italian Friesian cattle herds. The vat-milk (V-milk) used for making each cheese batch was obtained by mixing evening milk (partially skimmed following spontaneous separation of fat overnight, natural creaming) and morning milk. The batches of cheese produced were divided into 5 classes according to the SCC value of the evening milk determined prior to natural creaming (class 1, from 0 to 200,000; 2, 201,000-300,000; 3, 301,000-400,000; 4, 401,000-500,000; 5, over 501,000 cells/ml). The cheese yield was calculated as the amount of 24-h cheese, expressed in kilograms, obtained from 100 kg of V-milk (24 h ACY). The values of fat, crude protein, true protein, casein and 24 h ACY of V-milk were negatively correlated with the somatic cell score (SCS) of the evening milk. Conversely, a positive correlation was observed between chloride and SCS. Fat, protein fractions (crude protein, casein and whey proteins), P and titratable acidity of V-milk were positively correlated with its 24 h ACY, while chloride, pH and SCS showed a negative correlation. A significant drop in 24 h ACY was observed in classes 3, 4 and 5, therefore when the SCC of the evening milk exceeded 300,000 cells/ml. Finally a lower recovery of milk fat in cheese was observed as SCC of evening milk increase.

Role of somatic cells on dairy processes and products: a review

Somatic cells are an important component naturally present in milk, and somatic cell count is used as an indicator of udder health and milk quality. The role of somatic cells in dairy processes and products is ill-defined in most studies because the role of these cells combines also the concomitance of physicochemical modifications of milk, bacterial count, and the udder inflammation in the presence of high somatic cell count. The aim of this review is to focus on the role of somatic cells themselves and of endogenous enzymes from somatic cells in milk, in dairy transformation processes, and in characteristics of final products overcoming biases due to other factors. The immune function of somatic cells in the udder defense and their protective role in milk will be primarily considered. Different characteristics of milk induced by various somatic cell counts, types, and their endogenous enzymes influencing directly the technological properties of milk and the final quality of dairy products will be discussed as well. By comparing methods used in other studies and eliminating biases due to other factors not considered in these studies, a new approach has been suggested to evaluate the effective role of somatic cells on dairy processes and products. In addition, this new approach allows the characterization of somatic cells and their endogenous enzymes and, in future research, will allow the clarification of mechanisms involved in the release of these components from somatic cells during dairy processes, particularly in cheese technologies.

Transcriptional profiling of bovine milk using RNA sequencing

BACKGROUND

Cow milk is a complex bioactive fluid consumed by humans beyond infancy. Even though the chemical and physical properties of cow milk are well characterized, very limited research has been done on characterizing the milk transcriptome. This study performs a comprehensive expression profiling of genes expressed in milk somatic cells of transition (day 15), peak (day 90) and late (day 250) lactation Holstein cows by RNA sequencing. Milk samples were collected from Holstein cows at 15, 90 and 250 days of lactation, and RNA was extracted from the pelleted milk cells. Gene expression analysis was conducted by Illumina RNA sequencing. Sequence reads were assembled and analyzed in CLC Genomics Workbench. Gene Ontology (GO) and pathway analysis were performed using the Blast2GO program and GeneGo application of MetaCore program.

RESULTS

A total of 16,892 genes were expressed in transition lactation, 19,094 genes were expressed in peak lactation and 18,070 genes were expressed in late lactation. Regardless of the lactation stage approximately 9,000 genes showed ubiquitous expression. Genes encoding caseins, whey proteins and enzymes in lactose synthesis pathway showed higher expression in early lactation. The majority of genes in the fat metabolism pathway had high expression in transition and peak lactation milk. Most of the genes encoding for endogenous proteases and enzymes in ubiquitin-proteasome pathway showed higher expression along the course of lactation.

CONCLUSIONS

This is the first study to describe the comprehensive bovine milk transcriptome in Holstein cows. The results revealed that 69% of NCBI Btau 4.0 annotated genes are expressed in bovine milk somatic cells. Most of the genes were ubiquitously expressed in all three stages of lactation. However, a fraction of the milk transcriptome has genes devoted to specific functions unique to the lactation stage. This indicates the ability of milk somatic cells to adapt to different molecular functions according to the biological need of the animal. This study provides a valuable insight into the biology of lactation in the cow, as well as many avenues for future research on the bovine lactome.

Effects of somatic cells on the protein profile of hard ovine cheese produced from different breeds

Bulk tank ewe's milks with low (<500,000 ml−1), medium (1,000,000–1,500,000 ml−1) and high (>2,500,000 ml−1) somatic cell counts (SCC) from three breeds were used to manufacture hard ewes’-milk cheese. Physico-chemical analysis and capillary electrophoresis of fresh cheeses and cheeses that had been ripened for 1, 2, 3 and 6 months were carried out. The results showed that high SCC levels in milk affected the moisture content of only freshly made cheeses and the pH, fat content and fat acidity of ripened cheeses. Regarding proteolysis, the levels of all β-CNs in freshly made cheeses were significantly lower as the SCC values increased and the Castellana breed was the most affected by SCC levels because a significant decrease in all α-CNs was also observed as SCC levels rose. Analysis of the casein profile by principal component analysis (PCA) revealed that there were no clear differences according the SCC up to the third month. However in the third and sixth months cheeses with low levels of SC were closely grouped and characterised by the highest levels of intact caseins. Regarding the effect of breed, the results point to a more intense proteolytic activity in the Assaf breed, whose more matured cheeses showed the highest content of casein proteolytic fragments.

Influence of somatic cell counts and breed on physico-chemical and sensory characteristics of hard ewes'-milk cheeses

The aim of the present work was to perform a physico-chemical, descriptive quantitative and consumer-preference analysis of hard ewes'-milk cheeses that had been matured for one year and to determine the correlations between the variables studied. The cheeses were elaborated with milk from three breeds of sheep (Castellana, Churra and Assaf) with different somatic cell counts (lower than 500,000 cells ml-1; between 1,000,000 and 1,500,000 cells ml-1, and more than 2,500,000 cells ml-1). The results show that the cheeses elaborated with milk with high SCC had lower values of dry extract and fat and high values of pH and fat acidity and were described as pungent, granulose and less creamy. Regarding the effect of breed, the cheeses made with milk from the Churra breed had lower values for fat and those made with Assaf breed milk were significantly more rancid. The study of correlations showed that creaminess was positively correlated with the dry extract and total fat content and negatively correlated with ash and fat acidity, indeed grainy texture and pungency had the opposite sign in their correlation with these latter variables. The yellow colour was positively correlated with ash and negatively with protein. Finally, the consumer preferences reveals that the less accepted cheeses showed the higher values for rancidness and pungency and they were less likely to accept the cheeses made with Assaf breed milk.

Effect of somatic cell counts on ewes' milk protein profile and cheese-making properties in different sheep breeds reared in Spain

Bulk tank ewe's milks from Assaf, Castellana and Churra breeds categorized within three different Somatic Cell Count (SCC) groups (LSCC: <500 000; MSCC: 1 000 000 to 1 500 000; and HSCC: 2 500 000 to 3 000 000 cells ml−1) were used to investigate changes in capillary electrophoresis protein profiles and cheese-making properties. The results do not reveal a significant effect of SCC on total casein contents, because the sum of β-caseins decreased as SCC increased; no statistically significant differences were observed for the sum of α-caseins, and the values of κ-casein were higher in the HSCC milk. However, the soluble proteins other than α-lactalbumin and β-lactoglobulin increased with SCC. Regarding the effect of breed, the Assaf breed had the lowest contents of κ-CN, αs1-I-CN, αs1-II-CN 1-CN, αs1-III-CN, β1-CN and β2-CN. The protein profile was significantly correlated with curd textural properties. αs1-I-CN was the most influential variant because it was positively correlated with a large number of textural parameters. Cheese yield was positively correlated with all casein variants except αs1-III-CN, showing that the milk from local breeds were more suitable for cheese-making due to their higher contents of all the casein variants. Regarding curd texture properties LSCC milk curds showed more cohesiveness, associated with its lower content of αs1-III-CN and Castellana milk curds showed the highest values for firmness owing to their higher content of αs1-I-CN.

Identification of peptides in milk as a result of proteolysis at different levels of somatic cell counts using LC MALDI MS/MS detection

The somatic cell count (SCC) in milk is associated with increasing proteolytic degradation of caseins and it has been suggested that enzymes derived from somatic cells contribute to a lower yield and poorer quality of cheese. It is essential to increase the knowledge on naturally occurring milk proteinase activities to better understand how to improve the technological quality of milk. The aim of this work was to identify peptides actually present in milk as a result of proteolysis at different levels of SCC and to assign these peptides to potential responsible proteases where possible. Peptide fractions were prepared from acid whey by ultrafiltration at a molecular cut-off value of 10 000 Da. The peptides were separated using capillary reversed phase high performance liquid chromatography (RP-HPLC) and identified by matrix-assisted laser desorption/ionization-time of flight tandem mass spectrometry (MALDI-TOF MS/MS). Peptides identified ranged in mass from 1023 to 2000 Da, and originated from αS1-, αS2- or β-casein. Possible responsible proteases that could be suggested when examining the peptide cleavage sites included plasmin, cathepsin B, D and leukocyte elastase. The results indicated that plasmin was primarily responsible for the observed proteolysis in milk at low cell count, whereas the cathepsins and elastase became implicated at elevated cell count. Specificity and activity of cathepsins and elastase has earlier mainly been studied in model systems, whereas less is known about their activities in milk itself. This is also the first indication of involvement of elastase in milk proteolysis through the unequivocal determination of cleavage sites.

Exploitation of endogenous protease activity in raw mastitic milk by MALDI-TOF/TOF

The proteic profiling of bovine milk produced by cows with subclinical mastitis was obtained by MALDI mass spectrometry. A simple procedure of chemical fractionation of raw milk was developed, whereby less complex fractions of proteins were obtained prior to mass spectrometric and SDS-PAGE analysis. The profiles of milk proteins thus obtained could allow the identification of either early markers of the acute phase of mastitis or endogenous peptide of innate immune response. The activity of the endogenous proteases in raw milk produced from each quarter of healthy and mastic cows was therefore assayed over 24-, 48-, 96-, and 216-h incubation at 37 degrees C at both physiological and acid pH. Sequence-specific peptides were identified for each fraction by MS/MS experiments, and all tandem mass spectra were evaluated using MASCOT database searching. The results show a specific proteolytic activity of endogenous enzyme toward beta-casein precursor (P02666), alpha-S2-casein (P02663), alpha-S1-casein (P02662), and kappa-casein (P02668).