Which of the following statements regarding the rheological behavior of fluid food products are CORRECT?
Options:
(A) Tomato purees, apple sauce, and fruit concentrates generally follow the Herschel-Bulkley model ($\tau = \tau_0 + K \dot{\gamma}^n$) with flow behavior index $n < 1$ (shear-thinning with a yield stress).
(B) Pure honey and clarified vegetable oils exhibit Newtonian behavior where shear stress is directly proportional to shear rate and viscosity is independent of shear rate.
(C) Concentrated corn starch suspensions can exhibit dilatant (shear-thickening) behavior ($n > 1$) at high shear rates.
(D) Thixotropic food materials exhibit an irreversible permanent decrease in viscosity that does not recover upon standing.
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
Analysis of statements:
- Option A is CORRECT: High-pectin/pulp fruit purees require a finite yield stress ($\tau_0$) to initiate flow and thin at higher shear rates ($n < 1$).
- Option B is CORRECT: Clarified oils and true sugar solutions (honey, sucrose syrups) lack internal entangled microstructure and behave as Newtonian fluids.
- Option C is CORRECT: Dense suspensions of rigid starch granules show shear-thickening ($n > 1$) because close packing expands (dilates) under rapid shear, forcing liquid into newly created voids.
- Option D is INCORRECT: Thixotropy is strictly defined as a time-dependent, *reversible* decrease in apparent viscosity under constant shear, which fully recovers when left at rest.