Rheological changes during isothermal holding of salted beef homogenates

The gelation of lean meat homogenates was examined in a thermal scanning rheology monitor, a device that non-destructively measures rigidity and elasticity, as response to a repetitive small movement. Salted (NaCl and pyrophosphate) homogenates were scanned from 10°C to 85°C at 1°C/min, continuously or with an isothermal hold period at 25, 35, 45, 50 or 55°C. The intention of isothermal holding was to visualize key features of the gelation process. Whatever the thermal path, the final properties of the gel were substantially the same. This indicates that heat-mediated gelation follows only one path and is complete by 85°C. The hold at 55°C yielded the most information on gelation. During the hold, gelation was very nearly complete while rigidity steadily increased. This showed that transition from the sol state to the gel and rigidity development are separate events. Samples held at 25°C underwent pH-dependent transitions in rheological properties but at higher hold temperatures these transitions were often not observed. They were caused by a progressive loss of pyrophosphate due to an endogenous pyrophosphatase. The pH and temperature dependencies of the transitions were easily explained by classic enzymology. The enzyme activity could be inhibited by 10 mm NaF. Under these conditions the onset of final gelation occurred at a slightly, but significantly, lower temperature. Models to explain this and another subtle effect of pyrophosphate are presented. At a given pH, although gelation is subtly affected by pyrophosphate, the final rigidity and elasticity are much the same in its presence or absence. Pyrophosphate is in near universal use in salted meat products. Presumably its reported advantages, better water-holding capacity and greater gel strength, are not revealed by the thermal scanning technique.