FORMATION OF ACID-HEAT-INDUCED SKIM MILK GELS IN THE PH RANGE 5-CENTER-DOT-0-5-CENTER-DOT-7 - EFFECT OF THE ADDITION OF SALTS AND CALCIUM CHELATING-AGENTS

The effects of pH and added salts or chelating agents on the gel strength and dynamic theological properties of acid-heat-induced gels made from reconstituted skim milk (200 g solids/l) were investigated. Gel strength increased as pH was lowered between 5.75 and 5.15 except in the range pH 5.45-5.25 where a local maximum in gel strength was obtained at pH 5.35. Gel characteristics were affected by addition of salts or chelating agents but each of their effects was different, depending on the final pH of the milk gel. The addition of CaCl2 or chelating agents (Na2HPO4, disodium citrate or the disodium salt of EDTA) which affected micellar calcium phosphate, non-sedimentable casein and Ca2+ activity in different ways all resulted in decreased gel strength at pH 5.5. The addition of CaCl2 or MgCl2 caused a decrease in tan delta (ratio of the viscous modulus G '' to the elastic modulus G') whereas disodium citrate or the disodium salt of EDTA addition caused an increase and Na2HPO4 addition did not cause a change. The addition of NaCl (up to 50 mM), which causes an increase in ionic strength but has no effect on non-sedimentable casein and Ca2+ activity, decreased gel strength but did not change tan delta. The addition of a range of other salts (KCl, NH4Cl, NaSCN, NaNO3 or Na2SO4) also decreased gel strength at pH 5.5.