Effects of feeding whole-cracked rapeseeds, nitrate, and 3-nitrooxypropanol on protein composition, minerals, and vitamin B in milk from Danish Holstein cows

The present study was conducted to assess the individual or combined effects of feeding dietary fat (whole-cracked rapeseed), nitrate, and 3-nitrooxypropanol (3-NOP) on protein profile, mineral composition, B vitamins, and nitrate residues in milk from dairy cows. A total of 48 Danish Holstein cows used in an 8 × 8 incomplete Latin square design were fed 8 factorially arranged diets: (30 or 63 g crude fat/kg DM) × (0 or 10 g nitrate/kg DM) × (0 or 80 mg 3-NOP/kg DM) over 6 periods of 21 d each. In each period, milk samples were collected from individual cows during the third week by pooling milk obtained from 4 consecutive milkings and analyzed for protein profile, including protein modifications, mineral composition, riboflavin, cobalamin, and presence of nitrate residues. Fat supplementation led to an increase in the phosphorylation degree of αS1-CN by 8.5% due to a decreased relative proportion of αS1-CN 8P and an increased relative proportion of αS1-CN 9P and further to a decrease in the relative proportion of αS2-CN by 2.4%. Additionally, fat supplementation decreased the relative proportions of glycosylated and unglycosylated forms of κ-CN, consequently leading to a 3.6% decrease in total κ-CN. In skim milk, K, Ca, P, and Mg concentrations were altered by individual use of fat, nitrate, and 3-NOP. Feeding nitrate resulted in a 5.4% increase in riboflavin concentration in milk, whereas supplementing 3-NOP increased the cobalamin concentration in milk by 21.1%. The nitrate concentration in milk was increased upon feeding nitrate, but this increased concentration was well below the maximum permissible limit of nitrate in milk (<50 mg/L). Overall, no major changes were observed in milk protein, and mineral compositions by feeding fat, nitrate, and 3-NOP to dairy cows, but the increased riboflavin and cobalamin concentrations by nitrate and 3-NOP, respectively, could be of beneficial nutritional value for milk consumers.

Effects of protein genetic variants on their phosphorylation levels, milk composition, milk proteome, and milk coagulation ability in Chinese Holstein bovine milk

Milk samples were collected from 3625 Chinese Holstein cows to assess the effects of κ-casein (κ-CN) and β-lactoglobulin (β-LG) genetic variants on its milk coagulation properties. The results show that Chinese Holstein cows have a higher frequency of the κ-CN AA and AB variants, and β-LG of the AB and AA variants. Of these, κ-CN B variants, the β-LG AA and BB variants were more frequent in milk showing good coagulation. The effects of the genetic variants on milk composition, milk proteome, and protein phosphorylation sites were studied. The results showed that higher concentrations of protein and dry matter were found in κ-CN BE variant. Moreover, large variations in milk proteome among different κ-CN and β-LG variants were observed. Highly phosphorylated for κ-CN, especially Ser97, was observed in cows with the κ-CN BE variant, but no effect of β-LG variants on phosphorylation site was found. Of the various factors examined, variation of κ-CN phosphorylation sites Ser97 may be the most important in affecting casein structure and milk coagulation ability. Some milk protein contents were found to be negative factors for milk coagulation. In summary, this study showed that κ-CN genetic variants contained different milk compositions and phosphorylation site Ser97 influenced milk coagulation.

Effect of composition, casein genetic variants and glycosylation degree on bovine milk whipping properties

This study investigated the individual and combined effects of ĸ-Casein (ĸ-CN; AA, AB, BB), β-Casein (β-CN; A1A1, A1A2, A2A2) and high and low ratios of glycosylated ĸ-CN to total ĸ-CN, referred to as the glycosylation degree (GD), on bovine cream whipping properties. The genetic variants of individual cows were identified using reversed-phase high-performance liquid chromatography (RP-HPLC) and verified through liquid chromatography-mass spectrometry (LC-MS). A previously discovered relationship between days-in-milk and GD was validated and used to obtain high and low GD milk. Whipped creams were created through the mechanical agitation of fat standardised cream from milk of different ĸ-CN, β-CN, and GD combinations, and whipping properties (the ability to whip, overrun, whipping time and firmness) were evaluated. No significant correlation was measured in whipping properties for cream samples from milks with different ĸ-CN and β-CN genetic variants. However, 80 % of samples exhibiting good whipping properties (i.e., the production of a stiffened peak) were from milk with low GD suggesting a correlation between whipping properties and levels of glycosylation. Moreover, cream separated from skim milk of larger casein micelle size showed superior whipping properties with shorter whipping times (<5 min), and higher firmness and overrun. Milk fat globule (MFG) size, on the other hand, did not affect whipping properties. Results indicate that the GD of κ-CN and casein micelle size may play a role in MFG adsorption at the protein and air interface of air bubbles formed during whipping; hence, they govern the dynamics of fat network formation and influencing whipping properties.

Effect of CSN3 Gene Polymorphism on the Formation of Milk Gels Induced by Physical, Chemical, and Biotechnological Factors

During the last decade, research into genetic markers in the casein gene cluster has been actively introduced in cattle breeding programs. A special interest has been paid to the polymorphism of the CSN3 gene, responsible for the expression of the k-casein, playing a key role in protein coagulation, interaction with whey proteins, stabilization, and aggregation of casein micelles. This paper aimed to determine the effect of CSN3 genetic polymorphism on acid; rennet; acid-rennet; heat- and acid-induced as well as heat- and calcium-induced coagulation in skimmed milk; and protein-standardized milk systems (UF, NF, RO, VE). The influence of polymorphic variants of the CSN3 gene on the coagulation ability of milk proteins was assessed by the particle size of casein micelles, protein retention factor in the clot, and coagulation ability (duration of induction period, mass coagulation period, dynamic viscosity in gel point). The correlation between CSN3 gene polymorphism and protein coagulation was revealed. Milk systems obtained from CSN3 BB milk were found to have the shortest duration of coagulation, formation of better gel strength values, and increased yield compared to CSN3 AA. This study will improve the efficiency of milk processing and optimize the technology of dairy product production.

The role of glycosylation in amyloid fibril formation of bovine κ-casein

In order to explore the functions of glycosylation of κ-Casein (κ-CN) in bovine milk, unglycosylated (UG) and twice glycosylated (2G) forms of κ-CN B were purified by selective precipitation followed by anion exchange chromatography from κ-CN BB milk and tested for their amyloid fibril formation and morphology, oligomerisation states and protein structure. The diameter of self-assembled κ-CN B aggregates of both glyco-form were shown for the first time to be in the same 26.0-28.7 nm range for a 1 mg mL^-1 solution. The presence of two bound glycans in the protein structure of 2G κ-CN B led to a greater increase in the maximum amyloid fibril formation rate with increasing protein concentration and a difference in both length (82.0 ± 29.9 vs 50.3 ± 13.7 nm) and width (8.6 ± 2.1 vs 13.9 ± 2.5 nm) for fibril morphology compared to UG κ-CN B. The present results suggest that amyloid fibril formation proceeds at a slow but steady rate via the self-assembly of dissociated, monomeric κ-CN B proteins at concentrations of 0.22-0.44 mg mL^-1. However amyloid fibril formation proceeds more rapidly via the assembly of either aggregated κ-CN present in a micelle-like form or dissociated monomeric κ-CN, packed into reorganised formational structures above the critical micellar concentration to form fibrils of differing width. The degree of glycosylation has no effect on the polarity of the adjacent environment, nor non-covalent and disulphide interactions between protein molecules when in the native form. Yet glycosylation can influence protein folding patterns of κ-CN B leading to a reduced tryptophan intrinsic fluorescence intensity for 2G compared to UG κ-CN B. These results demonstrate that glycosylation plays an important role in the modulation of aggregation states of κ-CN and contributes to a better understanding of the role of glycosylation in the formation of amyloid fibrils from intrinsically disordered proteins.

Functionality of glycomacropeptide glycated with lactose and maltodextrin

The Maillard reaction (MR), under proper environmental conditions, has been used to improve protein functionality. In the present work, 2 high temperatures (50-80°C) and water activity (Aw; 0.45-0.67) were used to promote exogenous glycosylation of glycomacropeptide (GMP) while minimizing processing times (0, 8, 24, 48, and 96 h at 50°C; 0, 2, 4, 8, and 24 h at 80°C). Maltodextrin, a polysaccharide commonly used in the food industry as a functional ingredient, was used as a reducing sugar, and compared with lactose, a native milk sugar. The progression of MR was evaluated by tracking changes in molecular weight using SDS-PAGE, the formation of Amadori compounds, and browning. Aqueous glycosylated GMP solutions (5 to 20% wt/vol) were tested for solubility, rheological properties, and foam formation. As expected, MR progression was faster with Aw = 0.67 and 80°C. Glycosylated GMP powders showed no change in their solubility after MR. However, the apparent viscosity ( γ˙ = 30 s^-1) of the 20% wt/vol suspensions exhibited a slight increase when GMP was glycosylated with maltodextrin for 24 h at 80°C, and a 2-log increase when GMP was glycosylated with lactose, with a high browning development in both cases. The foam expansion index of the resuspended glycosylated powders was increased by between 25 and 66% compared with the nonglycosylated powders. Better foam stability (approximately 2 h) and no browning development were observed for GMP glycosylated with maltodextrin for 2 h at Aw = 0.67 and 80°C. The results show that GMP has undergone further glycosylation by means of controlled MR, which improves viscosity and foaming index without negatively affecting solubility. These preliminary studies provide a basis for the future creation of a new ingredient with GMP and reducing sugars.

Targeted Bottom-Up Mass Spectrometry Approach for the Relative Quantification of Post-Translational Modification of Bovine κ-Casein during Milk Fermentation

κ-casein (κ-CN) is one of the key components in bovine milk, playing a unique role in the structuration of casein micelles. It contains in its chemical structure up to sixteen amino acid residues (mainly serine and threonine) susceptible to modifications, including glycosylation and phosphorylation, which may further be formed during milk processing. In this study, changes in post-translational modification (PTM) of κ-CN during bovine milk fermentation were investigated. One-to-five-day fermented milk samples were produced. A traditional bottom−up proteomics approach was used to establish a multiple-reaction monitoring (MRM) method for relative quantification of κ-CN PTM. Endoproteinase Glu-C was found to efficiently digest the κ-CN molecule. The developed LC-MS method was validated by performing assessments of linearity, precision, repeatability, reproducibility, limit of detection (LOD), and limit of quantification (LOQ). Among the yielded peptides, four of them containing serine and threonine residues were identified and the unmodified as well as the modified variants of each of them were relatively quantified. These peptides were (1) IPTINTIASGEPTSTTE [140, 158], (2) STVATLE [162, 168], (3) DSPE [169, 172], and (4) INTVQVTSTAV [180, 190]. Distribution analysis between unmodified and modified peptides revealed that over 50% of κ-CN was found in one of its modified forms in milk. The fermentation process further significantly altered the composition between unmodified/modified κ-CN, with glycoslaytion being predominant compared to phosphorylation (p < 0.01). Further method development towards α and β-CN fractions and their PTM behavior would be an asset to better understand the changes undergone by milk proteins and the micellar structure during fermentation.

Phosphorylation and glycosylation isoforms of bovine κ-casein variant E in homozygous Swedish Red cow milk detected by liquid chromatography-electrospray ionization mass spectrometry

Variations in the phosphorylation and glycosylation patterns of the common κ-casein (CN) variants A and B have been explored, whereas studies on variant E heterogeneity are scarce. This study reports for the first time the detailed phosphorylation and glycosylation pattern of the κ-CN variant E in comparison with variants A and B. Individual cow milk samples representing κ-CN genotype EE (n = 12) were obtained from Swedish Red cows, and the natural posttranslational modifications of its κ-CN were identified and quantified by liquid chromatography-electrospray mass spectrometry. In total, 12 unique isoform masses of κ-CN variant E were identified. In comparison, AA and BB milk consisted of 14 and 17 unique isoform masses, respectively. The most abundant κ-CN E isoform detected in the EE milk was the monophosphorylated, unglycosylated [1P 0G, ∼70%; where P indicates phosphorylation from single to triple phosphorylation (1-3P), and G indicates glycosylation from single to triple glycosylation (1-3G)] form, followed by diphosphorylated, unglycosylated (2P 0G, ∼12%) form, resembling known patterns from variants A and B. However, a clear distinction was the presence of the rare triphosphorylated, nonglycosylated (3P 0G, ∼0.05%) κ-CN isoform in the EE milk. All isoforms detected in variant E were phosphorylated, giving a phosphorylation degree of 100%. This is comparable with the phosphorylation degree of variants A and B, being also almost 100%, though with very small amounts of nonphosphorylated, glycosylated isoforms detected. The glycosylation degree of variant E was found to be around 17%, a bit higher than observed for variant B (around 14%), and higher than variant A (around 7%). Among glycosylation, the glycan e was the most common type identified for all 3 variants, followed by c/d (straight and branched chain trisaccharides, respectively), and b. In contrast to κ-CN variants A and B, no glycan of type a was found in variant E. Taken together, this study shows that the posttranslational modification pattern of variant E resembles that of known variants to a large extent, but with subtle differences.

Milk-clotting properties on bovine caseins of a novel cysteine peptidase from germinated Moringa oleifera seeds

A cysteine peptidase was previously identified from germinated Moringa oleifera seeds, but its milk-clotting properties on bovine caseins was still unclear. In this study, this novel cysteine peptidase (MoCP) showed preferential activity on κ-casein (κ-CN), with greater hydrolytic activity compared with calf rennet, whereas weak hydrolysis of α-casein and β-casein made MoCP suitable for application in cheesemaking and may yield various functional peptides. All 3 evaluated caseins were hydrolyzed to form relatively stable peptide bands within 3 h of proteolysis with MoCP. Cleavage sites were determined by gel electrophoresis, liquid chromatography mass spectrometry/mass spectrometry, and peptide sequencing, which revealed that cleavage of κ-CN by MoCP occurred at residue Ile129-Pro130 and generated a 14,895.37-Da peptide. The flocculation reaction between MoCP and κ-CN determined by 3-dimensional microscopy with super-depth of field revealed that the initial 30 min of reaction were key for milk coagulation, which may affect curd yield. Overall, the findings presented herein suggest that the cysteine peptidase from germinated M. oleifera seeds can be considered a promising plant-derived rennet alternative for use in cheese manufacture.

Kinetics of pepsin-induced hydrolysis and the coagulation of milk proteins

Hydrolysis-induced coagulation of casein micelles by pepsin occurs during the digestion of milk. In this study, the effect of pH (6.7-5.3) and pepsin concentration (0.110-2.75 U/mL) on the hydrolysis of κ-casein and the coagulation of the casein micelles in bovine skim milk was investigated at 37°C using reverse-phase HPLC, oscillatory rheology, and confocal laser scanning microscopy. The hydrolysis of κ-casein followed a combined kinetic model of first-order hydrolysis and putative pepsin denaturation. The hydrolysis rate increased with increasing pepsin concentration at a given pH, was pH dependent, and reached a maximum at pH ∼6.0. Both the increase in pepsin concentration and decrease in pH resulted in a shorter coagulation time. The extent of κ-casein hydrolysis required for coagulation was independent of the pepsin concentration at a given pH and, because of the lower electrostatic repulsion between para-casein micelles at lower pH, decreased markedly from ∼73% to ∼33% when pH decreased from 6.3 to 5.3. In addition, the rheological properties and the microstructures of the coagulum were markedly affected by the pH and the pepsin concentration. The knowledge obtained from this study provides further understanding on the mechanism of milk coagulation, occurring at the initial stage of transiting into gastric conditions with high pH and low pepsin concentration.

Biological, functional and nutritional properties of caseinomacropeptide from sweet whey

Bioactive peptides derived from bovine milk proteins have gained much attention due to their health promoting functions. All over the world, cheese industry generates high volumes of sweet whey that could be used as an alternative source of bioactive peptide in nutraceuticals and food industry. Caseinomacropeptide (CMP) is a bioactive peptide derived from κ-casein by the action of chymosin during cheese manufacturing. CMP consist of two forms which are glycosylated (gCMP) and non-glycosylated (aCMP). The predominant carbohydrate in gCMP is N-acetylneuraminic (sialic acid) which gives functional and biological properties to gCMP. Due to its unique composition and technological characteristics such as wide pH range solubility, emulsifying, gelling, and foaming ability, CMP has received special attention. Therefore, there is an increased interest in researches for isolation and concentration of CMP. However, the isolation and purification methods are not cost-effective. It would be easier to optimize the conditions for isolation, purification, and utilization of CMP in nutraceuticals and food industry through deeper understanding of the effective factors. In this review, the structure of CMP, biological activities, isolation, and purification methods, the factors affecting functional properties and application areas of CMP in food industry are discussed.

Naturally Occurring Glycosidases in Milk from Native Cattle Breeds: Activity and Consequences on Free and Protein Bound-Glycans

Little is known about the extent of variation and activity of naturally occurring milk glycosidases and their potential to degrade milk glycans. A multi-omics approach was used to investigate the relationship between glycosidases and important bioactive compounds such as free oligosaccharides and O-linked glycans in bovine milk. Using 4-methylumbelliferone (4-MU) assays activities of eight indigenous glycosidases were determined, and by mass spectrometry and ¹H NMR spectroscopy various substrates and metabolite products were quantified in a subset of milk samples from eight native North European cattle breeds. The results showed a clear variation in glycosidase activities among the native breeds. Interestingly, negative correlations between some glycosidases including β-galactosidase, N-acetyl-β-d-glucosaminidase, certain oligosaccharide isomers as well as O-linked glycans of κ-casein were revealed. Further, a positive correlation was found for free fucose content and α-fucosidase activity (r = 0.37, p-value < 0.001) indicating cleavage of fucosylated glycans in milk at room temperature. The results obtained suggest that milk glycosidases might partially degrade valuable glycans, which would result in lower recovery of glycans and thus represent a loss for the dairy ingredients industry if these activities are pronounced.

Structural Analysis of a Novel Aspartic-Type Endopeptidase from Moringa oleifera Seeds and Its Milk-Clotting Properties

A novel aspartic-type endopeptidase was previously obtained from Moringa oleifera seeds; however, its specific milk-clotting properties have remained unclear. Here, we used various biophysical and molecular simulation approaches for characterizing the structure and function of the aspartic-type endopeptidase. The endopeptidase was preferentially active toward κ-casein (CN) and hydrolyzed it more than calf rennet; however, its ability to hydrolyze α-CN and β-CN was weaker than that of calf rennet. The endopeptidase cleaved κ-CN at Gln135-Asp136 and generated a 15 588.18 Da peptide with 135 amino acids. We further simulated the docking complex of the endopeptidase and κ-CN and found out that they possibly combined with each other via hydrogen bonds. The flocculation reaction between the endopeptidase and κ-CN indicated that milk coagulation occurred within 60 min. Overall, our observations suggest that the aspartic-type endopeptidase can be a potential rennet alternative for cheese making.

Differential in vitro digestion rates in gastric phase of bovine milk with different κ-casein phenotypes

Casein (CN) micelles will coagulate in the stomach after ingestion, which is similar to the cheesemaking process. Although genetic variants of bovine proteins, especially κ-CN, have been confirmed to influence the coagulation properties of the CN micelle, its influence on milk digestibility has not been revealed yet. This study aimed to investigate how genetic variants, glycosylation degree of κ-CN, and CN micelle size influence digestion rates during in vitro gastrointestinal digestion. Three milk pools, representing κ-CN phenotypes of either AA, BB, or AB composition were prepared from milk of individual Danish Holstein cows representing these different genotypes. In vitro digestion of the 3 milk pools, AA, BB, or AB, was investigated by sodium dodecyl sulfate-PAGE, liquid chromatography-mass spectrometry, and degree of hydrolysis. The results showed that κ-CN AA milk had faster digestion rate in the gastric phase compared with BB and AB milks, whereas only small differences were apparent in the intestinal digestion phase. The results further documented that the milk pools representing κ-CN phenotypes BB and AB had comparable overall glycosylation degrees (50.9% and 50.0%, respectively) and higher than that of the AA milk pool (46.9%). Further, the AA milk pool was associated with larger CN micelles. These differences in CN micelle sizes and glycosylation degrees can be part of underlying explanations for the differential in vitro digestion rates observed between the AA, BB, and AB κ-CN milk pools.

Variations in N-linked glycosylation of glycosylation-dependent cell adhesion molecule 1 (GlyCAM-1) whey protein: Intercow differences and dietary effects

In bovine milk serum, the whey proteins with the highest N-glycan contribution are lactoferrin, IgG, and glycosylation-dependent cellular adhesion molecule 1 (GlyCAM-1); GlyCAM-1 is the dominant N-linked glycoprotein in bovine whey protein products. Whey proteins are base ingredients in a range of food products, including infant formulas. Glycan monosaccharide composition and variation thereof may affect functionality, such as the interaction of glycans with the immune system via recognition receptors. It is therefore highly relevant to understand whether and how the glycosylation of whey proteins (and their functionality) can be modulated. We recently showed that the glycoprofile of GlyCAM-1 varies between cows and during early lactation, whereas the glycoprofile of lactoferrin was highly constant. In the current study, we evaluated intercow differences and the effects of macronutrient supply on the N-linked glycosylation profiles of the major whey proteins in milk samples of Holstein-Friesian cows. Overall, approximately 60% of the N-glycan pool in milk protein was sialylated, or fucosylated, or both; GlyCAM-1 contributed approximately 78% of the total number of glycans in the overall whey protein N-linked glycan pool. The degree of fucosylation ranged from 44.8 to 73.3% between cows, and this variation was mainly attributed to the glycans of GlyCAM-1. Dietary supplementation with fat or protein did not influence the overall milk serum glycoprofile. Postruminal infusion of palm olein, glucose, and essential AA resulted in shifts in the degree of GlyCAM-1 fucosylation within individual cows, ranging in some cases from 50 to 71% difference in degree of fucosylation, regardless of treatment. Overall, these data demonstrate that the glycosylation, and particularly fucosylation, of GlyCAM-1 was variable, although these shifts appear to be related more to individual cow variation than to nutrient supply. To our knowledge, this is the first report of variation in glycosylation of a milk glycoprotein in mature, noncolostral milk. The functional implications of variable GlyCAM-1 fucosylation remain to be investigated.

Investigation of Age Gelation in UHT Milk

Milk samples with twelve combinations of κ- and β-casein (CN) and β-lactoglobulin (β-Lg) variants were obtained to investigate the effect of protein variant on the mechanism/s of age gelation in ultra-high temperature (UHT) skim milk. Only milk groups with κ-CN/β-CN/β-Lg combinations AB/A1A2/AB and AB/A2A2/AB suffered from the expected age gelation over nine months storage, although this could not be attributed to the milk protein genetic variants. Top-down proteomics revealed three general trends across the twelve milk groups: (1) the abundance of intact native proteins decreases over storage time; (2) lactosylated proteoforms appear immediately post-UHT treatment; and (3) protein degradation products accumulate over storage time. Of the 151 identified degradation products, 106 (70.2%) arose from β-CN, 33 (21.9%) from αs1-CN, 4 (2.7%) from β-Lg, 4 (2.7%) from α-La, 3 (2%) from κ-CN and 1 (0.7%) from αs2-CN. There was a positive correlation between milk viscosity and 47 short peptides and four intact proteoforms, while 20 longer polypeptides and 21 intact proteoforms were negatively correlated. Age gelation was associated with specific patterns of proteolytic degradation and also with the absence of the families Bacillaceae, Aerococcaceae, Planococcaceae, Staphylococcaceae and Enterobacteriaceae, present in all the non-gelling milk groups pre-UHT.

Estimation of genetic parameters and detection of chromosomal regions affecting the major milk proteins and their post translational modifications in Danish Holstein and Danish Jersey cattle

Background

In the Western world bovine milk products are an important protein source in human diet. The major proteins in bovine milk are the four caseins (CN), α_S1-, α_S2-, β-, and k-CN and the two whey proteins, β-LG and α-LA. It has been shown that both the amount of specific CN and their isoforms including post-translational modifications (PTM) influence technological properties of milk. Therefore, the aim of this study was to 1) estimate genetic parameters for individual proteins in Danish Holstein (DH) (n = 371) and Danish Jersey (DJ) (n = 321) milk, and 2) detect genomic regions associated with specific milk protein and their different PTM forms using a genome-wide association study (GWAS) approach.

Results

For DH, high heritability estimates were found for protein percentage (0.47), casein percentage (0.43), k-CN (0.77), β-LG (0.58), and α-LA (0.40). For DJ, high heritability estimates were found for protein percentage (0.70), casein percentage (0.52), and α-LA (0.44). The heritability for G-k-CN, U-k-CN and GD was higher in the DH compared to the DJ, whereas the heritability for the PD of α_S1-CN was lower in DH compared to DJ, whereas the PD for α_S2-CN was higher in DH compared to DJ. The GWAS results for the main milk proteins were in line what has been earlier published. However, we showed that there were SNPs specifically regulating G-k-CN in DH. Some of these SNPs were assigned to casein protein kinase genes (CSNK1G3, PRKCQ).

Conclusion

The genetic analysis of the major milk proteins and their PTM forms revealed that these were heritable in both DH and DJ. In DH, genomic regions specific for glycosylation of k-CN were detected. Furthermore, genomic regions for the major milk proteins confirmed the regions on BTA6 (casein cluster), BTA11 (PEAP), and BTA14 (DGAT1) as important regions influencing protein composition in milk. The results from this study provide confidence that it is possible to breed for specific milk protein including the different PTM forms.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0421-2) contains supplementary material, which is available to authorized users.