Deformation and fracture of wheat, corn and rice starch gels in lubricated and bonded uniaxial compression

Wheat, corn and rice starch gels were prepared by heating 10% dispersions at 80\ub0C, 94\ub0C and 121\ub0C for 30 min. The response of these gels in uniaxial compression was determined under both bonded and lubricated conditions up to the point of fracture. For corn and wheat starch gels the stress-strain curves in lubricated compression were relatively insensitive to gel preparation temperature. For rice starch, the modulus in lubricated compression was lower than for corn and wheat. Further, the modulus of rice starch gels decreased with increasing temperature of gel preparation. Stress at fracture for all starches was strongly dependent on preparation temperature, being highest for samples prepared at 94\ub0C and lowest for the 80\ub0C gels. In bonded compression, samples become barrel shaped but the elastic modulus can be corrected for this geometric effect. When this is done, the stress-strain curves for both bonded and lubricated compression agree, but stress and strain at fracture are much lower in bonded compression. Uniaxial compression measurements such as these can provide deeper insight into the ultimate properties of gels and offer a viable alternative to purely empirical methods, such as the embedded disc, for evaluating fracture behavior of gels and other soft foods

Uniaxial compression of bonded and lubricated gels

Experimental measurements of the response of starch gels in uniaxial compression, under both lubricated and bonded conditions, are reported. With samples of initial height h//0 it was found for lubricated compression that true stress sigma versus strain plots (strain being defined as epsilon equals (h//0 minus h/h) were independent of h//0. The curves of true stress versus strain, for uniaxial compression of gels bonded to the instrument platens, lay well above the curve obtained in lubricated compression; and furthermore, the results in bonded compression were strongly dependent on h//0. The corrected curves for bonded compression were independent of h//0 and agreed with the response of the lubricated samples until near the failure point. Corrected stress for bonded compression at fracture increased with increasing h//0, while for lubricated compression, stress at fracture was constant or decreased with increasing n//0

Behavior of mozzarella, cheddar and processed cheese spread in lubricated and bonded uniaxial compression

The response of a material under uniaxial compression depends both on the bulk material properties and on the frictional effects at the sample\u2010platen interface. Three commercial cheeses (cheddar, mozzarella and processed cheese spread) were studied in uniaxial compression. Frictional effects at the sample surface were examined under three conditions: (1) lubrication of the sample\u2010platen interface; (2) bonding of the sample to the instrument platens; (3) samples neither bonded nor lubricated. The forces required to deform the cylindrical sample to a given strain level were highest for the bonded cases and lowest under lubricated conditions. The stress in lubricated compression, \u3c1L, was calculated from Fh(\u3c0R\u3c12h\u3c1)