Interactive effects of casein micelle size and calcium and citrate content on rennet‐induced coagulation in bovine milk

Abstract

Interactive effects of casein micelle size and milk calcium and citrate content on rennet‐induced coagulation were investigated. Milk samples containing small (SM) and large (LM) micelles, obtained from individual Holstein cows, were modified by addition of calcium and/or citrate and milk coagulation properties were evaluated in a full factorial design. The results showed that LM milk had a higher relative proportion of casein, coagulated faster, and resulted in a stronger gel than SM milk. Addition of calcium slightly decreased casein micelle size, while addition of citrate slightly increased micelle size. Calcium addition resulted in a shorter coagulation time and the strongest gels, while citrate addition increased the coagulation time and resulted in the weakest gels. Addition of calcium and citrate in combination resulted in intermediate coagulation properties. The interactive effect of micelle size and citrate was significant for gel strength. Microstructural differences between the milk gels were consistent with the rheological properties, for example, the micrographs revealed that a more homogeneous network was formed when calcium was added, resulting in a stronger gel. A more inhomogeneous network structure was formed when citrate was added, resulting in a weaker gel. Thus, variations in casein micelle size and in calcium and citrate content influence rennet‐induced coagulation in bovine milk. The calcium and citrate contents in Swedish milk have changed over time, whereby calcium content has increased and citrate content has decreased. In practical cheese making, calcium is added to cheese milk, most likely altering the role of inherent citrate and possibly influencing casein micelle size. The observed interaction effect between casein micelle size and citrate in this study, suggests that larger micelles with moderate citrate level will result in firmer gels, whereas a higher citrate content reduced gel strength more in case of large than SM. Since firmer gels are likely to retain more protein and fat than less firmer gels, this interaction effect could have implications in practical cheese production.

Composition and properties of bovine milk: A study from dairy farms in northern Sweden; Part I. Effect of dairy farming system

“…Rennet coagulation time and G20 are generally inversely related; milk that coagulates rapidly (low RCT) has higher gel strength (high G20). This is visualized in the PCA in Figure 3B, where RCT and G20 are located on opposite sides in the loading plot, previously also reported by Priyashantha et al (2019). Hallén et al (2007) investigated the rheological properties of individual milk samples from SRB and SH and observed that protein concentration was positively associated with G20, but not with RCT, also confirming our observations.…”

“…Casein micelle size was determined by nanoparticle tracking analysis (NTA) using NanoSight NS500 (Malvern Instruments) according to the method described by Priyashantha et al (2019). The system was coupled to a temperature sensor, stage controller, and syringe pump (Harvard Apparatus) fitted with a 1-mL syringe filled with 2,000-fold diluted skim milk.…”

“…Milk coagulation properties are reported to be correlated with several milk quality attributes (Glantz et al, 2010;Priyashantha et al, 2019), which in turn are known to be influenced by breed and different on-farm factors (e.g., the type of milking system). The higher milking frequency associated with automated milking systems (AMS), has been suggested to influence several milk compositional parameters compared with milking parlor or tiestall milking (e.g., lower fat and protein content; Løvendahl and Chagunda, 2011), longer rennet coagulation time (RCT) and higher levels of free fatty acids (FFA;De Marchi et al, 2017;Wiking et al, 2019), as well as lower plasmin and plasminogenderived proteolytic activity (Johansson et al, 2017).…”

Composition and properties of bovine milk: A study from dairy farms in northern Sweden; Part II. Effect of monthly variation

“…Despite the interaction effect, the shortest RCT was also recorded with milk from December (361 s), and the longest was 529 s for milk collected in February. The highest G20 was associated with the lowest RCT value, which is in agreement with results reported by Priyashantha et al (2019). The observed differences in G20 could likely to be an effect of variation in protein content (e.g., higher protein resulted in stronger gel compared with lower protein content; Panthi et al, 2019).…”

“…The pH of milk samples was measured using a pH meter (Seven Compact S210) at room temperature. Average casein micelle size was determined by nanoparticle tracking analysis using NanoSight NS500 (Malvern Instruments) according to the method described by Priyashantha et al (2019). Rennet-induced coagulation properties of skim milk were evaluated using a Bohlin CVOR-150-900 rheometer (Malvern Instruments Nordic AB) according to Johansson et al (2015) using calf rennet (75/25 chymosin/bovine pepsin, 180 international milk clotting units, Kemikalia) at a concentration of 0.18 international milk clotting units per milliter.…”