Prepared fruits and vegetables were subjected to high pressure processing at 100–400 MPa for 5–60 min in an isostatic press and their influence on product texture was evaluated. Pressure had a dual effect on product texture characterized by an initial loss in texture, ascribed to the instantaneous pulse action of pressure, followed by a more gradual change as a result of pressure‐hold. The extent of the initial loss and the subsequent partial recovery were pressure dependent with the former more prominent at higher pressures and the latter at lower pressures. The pressure treated samples were generally brighter in color somewhat resembling the appearance of mildly heat treated samples. For all vegetables pressure treated at 100 MPa for 60 min, the initial loss in texture was totally recovered during the pressure hold yielding an overall texture firmer than that of the raw product. There was no recovery of texture during the standing period at atmospheric pressure after the pressure treatment.
Rheological characteristics of 2 commercial brands of stirred yogurt were evaluated using a Haake Rotovisco Model RV 20 with an M‐5‐Osc measuring head and MV‐1 rotor assembly from measured shear stress values under a programmed 3‐cycle up‐ and down‐shear rate at 100 s−1/min from 0–500 s−1 (5 min). The upward shear‐rate flow behavior of the yogurt samples could be described by a Herschel‐Bulkley model while the downward shear‐rate curves were essentially linear. Both upward and downward shear rate curves demonstrated progressive structural degradation with repeated shearing. The dependency of rheological parameters on temperature in the range 10–25°C followed both Arrhenius and Turian models.
The influence of pectin (O-O-0.5%) and raspberry concentrate (64"B; O-10%) on the rheological characteristics of commercial stirred yogurt was evaluated using a computer-controlled rotational viscometer. All upward flow cutves (shear rate vs shear stress) followed the HerschelBulkley model while downward curves showed a linear relationship. The consistency coefficient (upward flow) and apparent viscosity (downward flow) showed a continuously increasing trend with increasing pectin and raspberry concentrate. The rheology of flavored yogurt could be fabricated by post-incubation mixing of stirred yogurt with pectin and raspberry concentrate.
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