RHEOLOGICAL INDEXES OF FRUIT CONTENT IN JAMS - EFFECT OF FORMULATION ON FLOW PLASTICITY OF SHEARED STRAWBERRY AND PEACH JAMS

The effect of formulation factors on Casson yield values measured at low shear rates (0.08 less-than-or-equal-to gamma less-than-or-equal-to 1.01s-1) (sigma01) and at medium shear rates (2.58 less-than-or-equal-to gamma less-than-or-equal-to 387.30s-1) (sigma02) was analyzed in previously sheared strawberry and peach jams. Twenty three samples of each fruit jam were prepared according to a second order composite rotatable design. Composition ranges were: fruit content, 25-55 %; soluble solids content, 60-70-degrees-Brix; added pectin in strawberry jams, 0.3-0.7 % and in peach jams, 0.1-0.5 %. Variation of sigma01 values in strawberry jams depended mainly on the interactions between fruit and soluble solids and between fruit and pectin, while in peach jams, it depended on fruit-soluble solids and soluble solids-pectin interactions. Patterns of change of sigma02 values with composition were similar to those observed for sigma01 in both strawberry and peach jams. Predictive power of sigma01 and sigma02 values for estimation of fruit content was low, but taken in conjunction with soluble solids content and total pectin values, 79.5 % of the variability of fruit content in strawberry jams and 91.1 % of same in peach jams could be explained.