The effects of pH and ionic strength on the thermal gelation of brown trout myo®brils from white and red muscles were analysed by thermal scanning rheometry. The highest gelation ability was obtained at low pH (around 5.6) whatever the ionic strength. No effect of ionic strength was observed at pH 5.6; however, at pH 6.0, lowering the salt (KCl) concentration to 0.3 M or less improved the characteristics of the gels formed. The effects of pH and ionic strength on myo®brils from both muscle types appeared to be similar, but red muscle proteins were less sensitive to changes in their physicochemical environment. Consequently, the differences between muscle types appeared to be dependent on pH and ionic strength. Solubility measurements revealed large differences between muscle types and between different pH values. Ultrastructural observations con®rmed that different kinds of gels were formed depending on the physicochemical conditions and muscle type origin.
INTRODUCTIONThe thermal gelation properties of myo®brillar proteins have been studied in a large variety of animals. 1 The capacity to produce gels at relatively low temperature is speci®cally observed in aquatic organisms, and this property has been used for processing ®sh myo®brils in surimi-like products. Thermal gelation properties based on interactions between proteins are very dependent on the physicochemical environment, especially pH and ionic strength. 1 Both ionic strength and pH act on myo®bril dissociation prior to heating and on protein charges, which in turn affect protein±protein and protein±solvent interactions during heating. Susceptibility of thermal gelation to physicochemical conditions varies between animal species. Proteins from ®sh are much more sensitive to changes in pH and ionic strength than those from mammals and birds and can form a gel only in a narrow range of pH and salt concentration. 2 Amongst ®sh species, myosin from cold-water ®sh is more sensitive to changes in the physicochemical environment than myosin from warm-water ®sh. 3 Fish muscle proteins, especially myosin, show an optimal thermal gelation around pH 6.0. 3±6 As the pH increases above this value, the transition temperature of myosin decreases, 5 and this is associated with protein destabilisation and consequently a lower gel-