Rheological and structural properties of tart cherry puree as affected by particle size reduction

The objective of this study was to determine the effect of micronization techniques for particle size reduction from D (v, 0.9) ≥ 800 μm to D (v, 0.9) ≤ 100 μm on the rheological and structural characteristics of tart cherry puree. Commercially frozen cherry puree was thawed and micronized using Megatron MT5000 at 15000 rpm for three cycles (M1, M2, M3; 10 min each) followed by high-pressure homogenization at 200 MPa. The particle size distribution was measured using Malvern laser particle size analyzer and total soluble and insoluble solid contents by using the AOAC official method 20.5 or 30.2. All samples were further characterized for rheology, total solids content and particle morphology. Results showed a significant reduction in apparent viscosities (2.59 Pa s to 0.0144 Pa s) with a reduction in particle size (800 μm-100 μm) and this trend was consistent in different shear rate ranges. The change in serum viscosity followed the similar trend as observed in puree viscosity with micronization. Comparative analysis of the effect of shear rate (0-1000 s−1) on viscosity showed an inverse relationship, suggesting a shear thinning behavior of the fluid. This effect on serum viscosity (0.0119 Pa s to 0.0047 Pa s) was found to be higher in the shear rate range of 0-10 s−1, as opposed to a relatively less reduction from 0.0047 Pa s to 0.0022 Pa s and 0.0015 Pa s at higher shear rates in ranges of 20-100 s−1 and 20 to 1000 s−1 respectively. The viscosity-shear rate changes were approximated by Ostwald-de Waele model (power law relation) for serum and Herschel Bulkley model for puree. The total solid content (14.20 ± 1.10 °Brix) was not significantly affected by particle size reduction. Results of this study provide useful information about the rheological properties of fruit juices/purees, which are influenced by the interactions between the dispersed phase and the serum, and are critical indicators of textural properties, mouth-feel, commercial acceptability, requirements of mixing and transport equipment.