Effect of pH on the gelation properties of skim milk gels made from plant coagulants and chymosin.

The effect of three milk pH values, 6.0, 6.3 and 6.7, on gelation properties was monitored by small amplitude oscillatory rheology as well as a large deformation (yield) test for gels induced by the plant coagulants, Cynara cardunculus L. and Cynara humilis L., and chymosin. Gel microstructure was studied using confocal scanning laser microscopy. For each coagulant, a decrease in pH of milk resulted in a decrease in gelation time (tg), and an increase in the rate of increase in storage modulus (G'). At pH 6.0 and 6.3, plant coagulant-induced gels reached a maximum value in G' (G'max) followed by a decrease in G' values during the rest of the experiment, reflecting higher proteolytic activity of plant coagulants towards caseins as determined by SDS-PAGE. Gels produced at pH 6.0 and 6.3, exhibited a minimum in loss tangent (tan delta) followed by slight increase in tan delta during gel aging, that may have been associated with faster rearrangements of the gel network structure. In gels aged for approximately 6 h, the values for G', tan delta at low frequency (0.006 Hz) and yield stress were higher for chymosin than for plant-induced gels. For gels with the same pH value, no major differences were observed in microstructure between coagulants. Gels made at low pH values (6.3 and 6.0) appeared to have a denser or more interconnected structure than gels made at pH 6.7. Our results suggest that, at a low pH, the type of coagulant used in gelation is likely to have a considerably impact on gel/cheese structure.