Effects of mineral salts and calcium chelating agents on the gelation of renneted skim milk

The effects of adding CaCl2, orthophosphate, citrate, EDTA, or a mixture of these, to reconstituted skim milk (90 g of solids/kg solution) on the gelation of renneted milk were mediated by changes in Ca2+ activity and the casein micelle. At pH 6.65, the addition of citrate or EDTA, which removed more than 33% of the original colloidal calcium phosphate with the accompanying release of 20% casein from the micelle, completely inhibited gelation. Reformation of the depleted colloidal calcium phosphate and casein in the micelle, by the addition of CaCl2, removed this inhibition. When the minimum requirements for colloidal calcium phosphate and casein in the micelle were met, the coagulation time decreased with increasing Ca2+ activity, leveling off at high Ca2+ activity. The storage modulus of renneted gels, measured at 3 h, increased with increasing colloidal calcium phosphate content of micelles up to a level at which it was approximately 130% of the original colloidal calcium phosphate in the micelles. Further increases in colloidal calcium phosphate by the addition of CaCl2, orthophosphate, or mixtures of these, which did not change the proportion of casein in the micelle, decreased the storage modulus. The gelation of the renneted milk was influenced by Ca2+ activity, the amounts of colloidal calcium phosphate, and casein within the micelle, with the effects of colloidal calcium phosphate and casein within the micelle clearly dominating the storage modulus. These results are consistent with the model of Horne (Int. Dairy J. 8:171-177, 1998) which postulates that, following cleavage of the stabilizing K-casein hairs by rennet, the properties of the rennet gel are determined by the balance between the electrostatic and hydrophobic forces between casein micelles.     

Characterization of fish oil-in-water emulsions using light-scattering,nuclear magnetic resonance,and gas chromatography-headspace analyses

The effect of the molecular environment on the physical and oxidative properties of homogenized or microfluidized fish oil-in-water emulsions (5% w/w tuna oil in pH 7 phosphate buffer) stabilized by whey protein isolate (WPI, 1 or 5% w/w) or lecithin (2.5% w/w) was examined. Laser light-scattering measurements showed that WPI-stabilized emulsions had smaller particle sizes than lecithin-stabilized emulsions, and that higher pressures reduced the particle size. WPI afforded more protection against oil oxidation than did lecithin, as evidenced by the lower headspace propanal of emulsions as measured by GC-headspace analysis, despite the larger interface in WPI-stabilized emulsions. Reducing the concentration of WPI in emulsions from 5 to 1% decreased the oxidative stability of WPI-stabilized emulsions. The ¹H NMR transverse relaxation times (T ₂) of FA chains in emulsion droplets stabilized by the same surfactants made by homogenization or microfluidization were different and not always related to particle size. The higher mobility (i.e., longer T ₂) of the unsaturated parts of the FA chains within an oil droplet, compared with the saturated parts, suggests that the unsaturated components tended to stay in the core of the oil droplets. This experimental result supports the hypothesis reported in other literature that the more unsaturated FA are buried in the oil core of oil-in-water emulsions. The lack of a universal correlation between particle size and oxidation suggests that the mobility of particles in an emulsion has an influence on the rate of oxidation.     

Microencapsulated Fish Oil Powder Formulation with Improved Resistance to Oil Leakage During Powder Compression

The effects of the addition of pectin to a primary fish oil emulsion stabilized by a heated milk protein–carbohydrate mixture, and heat treatment of emulsion at low pH (with or without the addition of pectin) on the properties of final emulsions (pH 7) and powders (~42 % w/w/fat) prepared from these emulsions were examined. Either the addition of pectin (without the additional heat treatment at pH 3), or additional heat treatment step at pH 3 (with or without pectin) reduced the surface oil and free oil of spray dried powders, and also reduced oil leakage on powder compression. However, the extra heat treatment step at low pH to the emulsion (without pectin) resulted in a greater susceptibility to oxidation of EPA and DHA in the powder. A judicious choice of ingredients for formulating and processing emulsions prior to spray drying enables the application of food‐grade microencapsulated fish oil powders in directly compressible tablet applications.     

Effects of pH,calcium-complexing agents and milk solids concentration on formation of soluble protein aggregates in heated reconstituted skim milk

The proportion of protein present in the supernatant after centrifugation and the formation of soluble protein aggregates in heated (90 °C for 10 min) reconstituted skim milk were investigated as a function of (a) pH, (b) milk concentration 9–21%, w/w (milk solids non-fat, MSNF) and (c) addition of calcium-complexing agents (orthophosphate or EDTA). Compositional changes in milk, resulting from pH adjustment or salt addition, altered the distribution of caseins between the serum and micellar phases of milk prior to heating, and influenced the amount and composition of soluble aggregates formed during heat treatment. Although milk pH was a good predictor of the aggregation behaviour of the proteins during heating of milk of different solids concentrations, neither the proportion of protein in the supernatant prior to heating nor the pH alone could predict the aggregation behaviour when the composition was adjusted with mineral salts.     

Mineral and casein equilibria in milk: effects of added salts and calcium-chelating agents

We have investigated the effects of adding a range of mineral salts and calcium-chelating agents on the distribution of casein and minerals between the non-pelleted and pelleted phases of milk obtained upon centrifugation at 78000 g for 90 min. Adding CaCl2 or mixtures of NaH2PO4 and Na2HPO4 to reconstituted skim milk (90 g milk solids/kg) at pH 6.65 increased both pelleted casein and pelleted calcium phosphate. Opposite effects were obtained by adding citrate or EDTA. The change in pelleted calcium phosphate was not simply related to casein release from the micelle. Upon adding 5 mmol EDTA/kg milk, 20% of the pelleted Ca, 22% of the pelleted phosphate and 5% of the micellar casein were removed. Increasing the concentration of EDTA to 10 mmol/kg milk decreased the pelleted Ca by 44% and the pelleted phosphate by 46%, and caused 30% of the micellar casein to be released. The effects of adding phosphate, citrate or EDTA at pH 6.65, followed by the addition of CaCl2, demonstrated the reversibility of the dissolution and formation of the micellar calcium phosphate. There were limits to this reversibility that were related to the amount of colloidal calcium phosphate removed from the casein micelles. Adding CaCl2 to milk containing > or = 20 mmol EDTA or > or = 30 mmol citrate/kg milk did not result in complete reformation of casein micelles. Light-scattering experiments confirmed that the dissolution of moderate amounts of colloidal calcium phosphate had little effect on micellar size and were reversible, while the dissolution of larger amounts of colloidal calcium phosphate resulted in large reductions in micellar size and was irreversible.     

The use of sedimentation field flow fractionation and photon correlation spectroscopy in the characterization of casein micelles

Sedimentation Field Flow Fractionation (SdFFF) was combined with Photon Correlation Spectroscopy (PCS), to characterize changes in the structure of the colloidal particles of reconstituted skim milk of diameter >50 nm (aggregates of casein and calcium phosphate known as casein micelles) with the changes in partitioning (with the addition of salt) of calcium (Ca), inorganic phosphate (Pi) and casein between the serum and colloidal phases of the milk. The number weighted particle size distributions are determined. These are well represented by a log-normal distribution. Methods are presented for estimating the relative contributions of scattering and absorbance to the SdFFF detector signal and for taking both into account when analysing SdFFF data. The values found for the effective density of the casein micelles were in good agreement with the literature and ranged from (1.06-1.08 g cm(-3)) according to the composition of micelles. The changes in the scattering intensity as determined by PCS correlated with the changes in the particle composition. Although the concentrations of colloidal calcium phosphate (CCP) (1.1-3.5 g/kg milk) and micellar casein (18.1-27.2 g/kg milk) varied considerably only small changes in the size distribution of particles >50 nm diameter were observed except for milk to which 30 mmol Pi+10 mmol Ca/kg milk had been added where the particle size distribution shows a swelling of the particles consistent with a lower than expected value for the particle density. These observations suggest that the micelles have the ability to both lose (depleted micelles) and accommodate (enriched micelles) more casein, calcium and inorganic phosphate in their interior, thus confirming the model of the micelles which postulates an open structure allowing freedom of movement of casein and small ions.     

Properties of low-fat stirred yoghurts made from high-pressure-processed skim milk

Physical properties of stirred yoghurt made from reconstituted skim milk that was high-pressure (HP)-treated at 100, 250 or 400 MPa, at 25, 70 or 90 °C, for 10 min, prior to inoculation with yoghurt cultures, were studied; portions of milk HP-treated at 25 °C were also heat-treated at 90 °C for 10 min before or after pressure treatment. Control yoghurts were made from skim milk given a heat treatment at 90 °C for 10 min. Fermentation time was not affected by treatment applied to the milk. HP treatment of skim milk at 25 °C, before or after heat treatment, gave stirred yoghurts of similar viscosities to that made from conventionally heat-treated milk. Lower viscosities were obtained when stirred yoghurts were made with milk HP-treated at elevated temperatures. A model is proposed to correlate properties of yoghurt with HP/heat-induced changes in interactions and structures of protein in the milk samples.Industrial relevanceTo meet end user expectations, the dairy industry needs to diversify its product range by tailoring specific functionalities. To meet these expectations, new processing methods such as high-pressure processing are of interest for their potential to achieve specific and/or novel functionalities and/or improve efficiencies, including reduced chemical and water use. In this paper, an investigation of the use simultaneous pressurization and heating of milk before the manufacture of stirred yoghurt is presented.