Brendan T. O'Kennedy

Effect of acidification and neutralization of milk on some physico-chemical properties of casein micelles

Abstract Acidification of milk, at low temperatures, to pH 5.0 or 4.6, followed by neutralization to pH 6.6 (reformed milk), resulted in a reduction in the buffering maximum of milk at pH ~5.1; this buffering peak is caused by the solubilization of colloidal calcium phosphate (CCP). The reduced buffering in reformed milk suggests that little reformation of CCP occurs on neutralization; reformed milks also had an elevated Ca 2+ activity. Acidification of milk to pH > 5.5, followed by neutralization to pH 6.6, hardly reduced buffering (at pH ~5.1), suggesting that either little CCP dissolved on acidification or that reformation of CCP occurred on neutralization. Acidification of milk to low pH values and neutralization resulted in improved renneting properties and a reduction in rennet coagulation time (RCT). Dialysis of reformed milk resulted in a reduction in its renneting properties which became inferior to those of control milk, possibly due to its reduced CCP content or to structural changes in the micelles caused by removal of CCP. Addition of low concentrations of CaCl 2 to milk, at a constant pH (6.6), improved its renneting properties. Electron micrographs of milk acidified to pH values ⩽5.5 prior to neutralization showed increased clustering of casein particles, presumably caused by the reduction in electrostatic repulsion between casein particles during acidification to low pH values. The original micellar appearance was not restored on neutralization or dialysis of reformed milk. It is concluded that the micellar system is not readily reversible; once disintegrated by acidification, micelles do not reform on neutralization.

Effect of mineral fortification on textural and oxidative stability of reduced-fat spreads.

The aim of this research was to investigate the effect of mineral fortification on the textural and oxidative stability of reduced-fat spreads produced using κ-carrageenan. Model systems were prepared containing varying amounts of zinc, copper, and iron. The hardness of the spreads stored at 5°C and 25°C was assessed over time using a penetrometer (20° cone), and it was observed that samples were generally weaker compared to the controls. Samples containing the transition minerals iron and copper were very susceptible to oxidation, becoming rancid rapidly. Low levels of zinc addition, between 2–10% of the recommended daily allowance (RDA), led to spreads with acceptable oxidative stability and a textural profile comparable to the controls over the storage period examined.

Effects of emulsification and microencapsulation on the oxidative stability of camelina and sunflower oils

Oil-in-water (O/W) emulsions were prepared using different concentrations of camelina or sunflower oil. Sodium caseinate was used as the emulsifier and dried glucose syrup as the wall material. Emulsions were subsequently spray dried to yield high-fat powders (71.7–85.0%). Emulsification and microencapsulation of bulk oils decreased their level of lipid oxidation (lipid hydroperoxide and p-Anisidine values, p-Avs). Sunflower oil, O/W emulsions and reconstituted powders generally had lower oxidation products than corresponding camelina oil-based products throughout storage at 15°C. p-Avs of bulk oils remained constant, whereas p-Avs of O/W emulsions and reconstituted powders decreased early in storage, and remained low thereafter. Microencapsulated omega (ω)-3 rich powders were produced, easily reconstituted and showed no signs of deterioration throughout storage. These powders provided functional properties with potential for incorporation into various food systems as a source of beneficial ω-3 fatty acids.