The effects of key manufacturing steps (heat treatment, evaporation and spray drying) during the manufacture of low-and high-heat skim milk powders (SMP) on the physico-chemical and processing characteristics of milk, and concentrates of varying total solids (TS) levels prepared by reconstituting the milk powders, were evaluated. Milk heat treatment had the most pronounced effect, with an increase in severity of heat treatment from 72 degrees C x 15 s to 120 degrees C x 120 s, prior to evaporation resulting in higher heat coagulation time (HCT) at pH 6.3-6.6 and ethanol stability (ES) at pH 6.2-6.6, and a marked deterioration of rennet-induced coagulability. Increasing TS of the milk on reconstitution from 9.4 to 25% reduced HCT at pH >6.3 and ES at pH 6.6-7.0, increased ES at pH 6.2-6.4, and led to partial recovery of rennet-coagulability. The results highlight how heat treatment may be used to customise the functionality of SMP to different applications. (c) 2017 Elsevier Ltd. All rights reserved.

Milk protein concentrate (MPC) powders (∼81% protein) were made from skim milk that was heat treated at 72°C for 15 s (LHMPC) or 85°C for 30 s (MHMPC). The MPC powder was manufactured by ultrafiltration and diafiltration of skim milk at 50°C followed by spray drying. The MPC dispersions (4.02% true protein) were prepared by reconstituting the LHMPC and MHMPC powders in distilled water (LHMPCw and MHMPCw, respectively) or milk permeate (LHMPCp and MHMPCp, respectively). Increasing milk heat treatment increased the level of whey protein denaturation (from ∼5 to 47% of total whey protein) and reduced the concentrations of serum protein, serum calcium, and ionic calcium. These changes were paralleled by impaired rennet-induced coagulability of the MHMPCw and MHMPCp dispersions and a reduction in the pH of maximum heat stability of MHMPCp from pH 6.9 to 6.8. For both the LHMPC and MHMPC dispersions, the use of permeate instead of water enhanced ethanol stability at pH 6.6 to 7.0, impaired rennet gelation, and changed the heat coagulation time and pH profile from type A to type B. Increasing the severity of milk heat treatment during MPC manufacture and the use of permeate instead of water led to significant reductions in the viscosity of stirred yogurt prepared by starter-induced acidification of the MPC dispersions. The current study clearly highlights how the functionality of protein dispersions prepared by reconstitution of high-protein MPC powders may be modulated by the heat treatment of the skim milk during manufacture of the MPC and the composition of the solvent used for reconstitution.

The protein content of skim milk was increased from 3.3 to 4.1% (wt/wt) by the addition of a blend of skim milk powder and sodium caseinate (NaCas), in which the weight ratio of skim milk powder to NaCas was varied from 0.8:0.0 to 0.0:0.8. Addition of NaCas increased the levels of nonsedimentable casein (from ∼6 to 18% of total casein) and calcium (from ∼36 to 43% of total calcium) and reduced the turbidity of the fortified milk, to a degree depending on level of NaCas added. Rennet gelation was adversely affected by the addition of NaCas at 0.2% (wt/wt) and completely inhibited at NaCas ≥0.4% (wt/wt). Rennet-induced hydrolysis was not affected by added NaCas. The proportion of total casein that was nonsedimentable on centrifugation (3,000 × g, 1 h, 25°C) of the rennet-treated milk after incubation for 1 h at 31°C increased significantly on addition of NaCas at ≥0.4% (wt/wt). Heat stability in the pH range 6.7 to 7.2 and ethanol stability at pH 6.4 were enhanced by the addition of NaCas. It is suggested that the negative effect of NaCas on rennet gelation is due to the increase in nonsedimentable casein, which upon hydrolysis by chymosin forms into small nonsedimentable particles that physically come between, and impede the aggregation of, rennet-altered para-casein micelles, and thereby inhibit the development of a gel network.

The study investigated the seasonal changes in the compositional, physicochemical and processing characteristics of milk from a mixed-herd of spring- and autumn-calving cows during the year 2014-2015. The volume proportion of autumn-calving milk (% of total milk) varied with season, from ~10-20 in Spring (March-May), 5-13 in Summer (June-August), 20-40 in Autumn (September-November) and 50-100 in Winter (December-February). While all characteristics varied somewhat from month to month, variation was inconsistent, showing no significant trend with progression of time (year). Consequently, season did not significantly affect many parameters including concentrations of total protein, casein, whey protein, NPN, total calcium, pH, rennet gelation properties or heat stability characteristics. However, season had a significant effect on the concentrations of total P and serum P, levels of αs1- and β-caseins as proportions of total casein, casein micelle size, zeta potential and ethanol stability. The absence of a significant effect of season for most compositional parameters, rennet gelation and heat-stability characteristics suggest that milk from a mixed-herd of spring- and autumn-calving cows is suitable for the manufacture of cheese and milk powder on a year-round basis, when the volume proportion of autumn milk, as a % of total, is similar to that of the current study.

Skim milk was pH-adjusted from 6.6 to 7.5, high heat treated (HHT, 95 °C × 2 min) or held unheated for 1 h, re-adjusted to pH 6.6, and analysed. HHT at pH 6.6 resulted in denaturation of ∼ 67% of total whey protein, partial association of denatured whey protein with the casein micelle, an increase in casein micelle size, and reductions in concentrations of serum casein, Ca and P. These changes were paralleled by a marked deterioration in rennet coagulability, higher ethanol stability in the pH range of 6.2-6.6 (P < .05), and a reduction in the pH of maximum heat coagulation time (HCT) (P < .05). Increasing the pH before heat treatment led to increases in casein dissociation and the concentrations of κ-casein and denatured whey protein in the serum, and a reduction in casein micelle size (P < .05). Simultaneously, HCT at pH 6.6-6.7 and 7.2 increased significantly.

Skim milk (SM) was fortified from 3.3 to 4.1% protein using different milk protein powders: skim milk powder (SMP), native phosphocasein (NPC), calcium-reduced phosphocasein (CaRPC), sodium caseinate (NaCas) or calcium caseinate (CaCas). Compared with SMP or NPC, fortification with NaCas and CaRPC, and to a lesser extent CaCas, resulted in milk samples having higher proportions of non-sedimentable casein and calcium, and lower- and higher-levels of κ- and αS1-casein, respectively, as a proportion of non-sedimentable casein. These changes coincided in milk samples fortified with NaCas, CaRPC or CaCas failing to undergo rennet-induced gelation, and having higher heat stability in the region 6.7–7.2 and ethanol stability at pH 6.4. The study demonstrates that the aggregation behaviour of protein-fortified milk samples is strongly influenced by the degree of mineralisation of the protein powder used in fortification, which affects the partitioning of casein and calcium in the sedimentable and non-sedimentable phases.