Impact of milk protein genotypes on milk coagulation properties
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Evaluation of the milk coagulation properties is very important for the dairy industry, because it gives information on the processability of milk for both cheese and yoghurt/cultured milk. Milk that takes a shorter time to coagulate is more appropriate for the production of cheese with improved texture compared to the non-coagulating and poor coagulating samples (that take longer time to coagulate). Several parameters are used for studying milk coagulation properties, for example, time taken for the milk to coagulate, speed of gel formation and final gel firmness. Low amplitude oscillation rheometry (LAOR) and Formagraph (Lattodinamografo) are the most popular methods used to monitor milk coagulation properties. LAOR has been widely used in studying both the rennet and acid coagulation properties of milk, while Formagraph was designed for studying the rennet coagulation process. LAOR is limited by the fact that it measures only one sample at a time while Formagraph takes more than one sample (parallels) at the same time. An alternative method to LAOR is needed because a large throughput analysis on the acid coagulation properties of milk is needed. Differences in rennet coagulation properties of milk have been associated with the milk protein genotypes in most of the commercial dairy cattle breeds. However, limited studies are available on the effects of milk protein genotypes, salts (Ca, Mg and P) distribution, casein micelle size and milk protein composition on the acid coagulation properties of milk. Hence, the main objective of this project was to study the effects of milk protein genotypes on the rennet and acid coagulation properties in the Norwegian Red cattle / Norsk Rødt Fe (NRF). Paper I describes a comparison of LAOR and Formagraph for milk acid coagulation properties. Formagraph and LAOR obtained similar patterns for gelation time and gel firming rate. However, in some samples, the gel firmness at 60 minutes did not show similar patterns for the two methods, especially for those with weaker gels. In general, Formagraph could be used in studying acid coagulation properties of milk, especially on many samples. Paper II modeled the acid coagulation process using data retrieved from the Formagraph. Acid coagulation parameters were estimated from model equation and compared with the traditional parameters derived from the Formagraph output. MATLAB was used to fit the acid coagulation curves in four milk samples analyzed 10 times (except for one sample, which was tested 9 times). Thirty-nine model equations were fitted. Results showed good correlation between the model parameters and the traditional parameters. Less variation within parallels (replicates) was obtained for the model parameters (gel firming rate and final gel strength) than for traditional parameters. The results showed that milk acid coagulation parameters could be estimated from the model with good repeatability especially for the gel-firming rate and the final gel strength. Paper III describes the effects of milk protein polymorphism, salts distribution and casein micelle size on the rennet, and acid coagulation properties of the milk. More favorable rennet coagulation properties were obtained by αs1-CN BC, β-CN A1A2 and κ-CN BB compared to the BB, A2A2 and BE genotypes of the respective caseins, while composite genotype BC-A2A2-BB was associated with improved rennet coagulation properties compared to the rest of the composite genotypes. Surprisingly, improved acid coagulation properties were favored by κ-CN AA and composite genotype BB-A2A2-AA, which have been associated with poor rennet coagulation properties; moreover, acid coagulation properties were not significantly influenced by αs1- and β-CN genotypes. Calcium (Ca) distribution in milk was associated with variations in the rennet coagulation properties only, while phosphorus (P) content was associated with both rennet and acid coagulation properties. In brief, higher levels of total and micellar Ca were associated with improved rennet coagulation properties (shorter rennet clotting time; RCT) and shorter rennet curd firming time (k20), while soluble calcium was associated with higher rennet curd firmness at 30 minutes. Higher total phosphorus lowered the time taken for the gel formation (both rennet and acid gels). Higher soluble P favored acid coagulation properties (shorter gelation time and higher gel firmness). A higher amount of phosphorylation in αs1-CN (i.e., αs1-CN-9P) impaired rennet and acid coagulation properties of milk. Conclusively, some milk protein variants associated with improved rennet coagulation properties impaired acid coagulation properties. Whereas milk protein genotypes that improved acid coagulation properties impaired rennet coagulation properties. Paper IV investigated the effects of milk protein genotypes (αs1-CN, κ-CN and β-LG) on the physical and chemical properties of cultured skim milk on the fresh (one-day storage; D1) and stored cultured milk (fourteen days storage; D14). The particle size distribution and elastic properties of the gel (Gʹ) were not significantly influenced by the milk protein genotypes. Significant effects of κ-CN/β-LG composite genotype were observed on the yield stress and degree of syneresis in the D14 samples of cultured milk (i.e., the samples with AA/AB and BB/AB composite genotypes of κ-CN/β-LG had higher yield stress and lower degree of syneresis compared to AA/BB and BB/BB). However, the inclusion of protein content in the models reduced the effects of κ-CN/β-LG composite genotypes on the yield stress. This indicates that protein content could be the main cause of the differences in the yield stress between the samples. On the other hand, the effect of κ-CN/β-LG composite genotype combinations on the degree of syneresis were not influenced by the protein content in the model. The concentrations of lactic acid and orotic acid in the D1 cultured milks were influenced by the αs1-CN genotypes and κ-CN/β-LG composite genotypes, respectively. These effects were not observed after the inclusion of the protein content of the fresh milk in the model. Therefore, differences in the concentration of lactic acid and orotic acid are explained by the protein content in the milk rather than by the κ-CN/β-LG composite genotypes. The concentration of acetoin was influenced by the αs1/κ-CN composite genotypes both before and after the inclusion of protein content in the model as covariate. Since the protein content explained variations in the rheological properties of the samples analyzed, future research should evaluate effects of milk protein genotypes at equal protein concentration. Results could provide possibilities for improving water-holding capacity in low fat acid gels by using milk protein genomics.Evaluering av melkens koaguleringsegenskaper er svært viktig for meieriindustrien, fordi disse gir informasjon om melkens egnethet for produksjon av både ost og fermentert melk. Melk med kortere koaguleringtid er mer hensiktsmessig for produksjon av ost sammenlignet med melk som ikke koagulerer eller har dårlige koaguleringsegenskaper (melk som tar lengre tid å koagulere). Flere parametere brukes til å studere melkens koaguleringsegenskaper, dvs. tiden frem til melken starter å koagulere, geldannelsens hastighet og endelig fasthet på gelet. Low amplitude oscillation rheologi (LAOR) og Formagraph (Lattodinamografo) er de mest populære metodene som brukes til å overvåke melkens koaguleringsegenskaper. LAOR har blitt mye brukt til å studere både løpe og syre koagulering av melk, mens Formagraph opprinnelig ble designet for å studere løpekoagulering. LAOR er begrenset av det faktum at det bare måler én prøve om gangen i forhold til Formagraph, som kan måle mer enn én prøve (paralleller) samtidig. En alternativ metode til LAOR er nødvendig for å måle syrekoagulering fordi det er nødvendig å kunne analysere flere prøver samtidig. Forskjeller ved løpekoagulering av melk har vært assosiert med de ulike genotypene av melkeprotein i de fleste kommersielle raser av melkeku. Imidlertid er det begrensede studier tilgjengelig på effekter av de ulike genotypene av melkeprotein, salter (Ca, Mg og P), kaseinmicellestørrelse og melkeproteinets sammensetning på melkens syrekoaguleringsegenskaper. Hovedmålet med dette prosjektet var derfor å undersøke effektene av melkens genotyper av protein på løpe- og syrekoaguleringsegenskapene til Norsk Rødt Fe (NRF).