Ultrasonic processing influences rheological and optical properties of high-methoxyl pectin dispersions

The effect of high-intensity ultrasound on the rheological and optical properties of high-methoxyl pectin dispersions was studied. Pectin solutions (1.15 wt% pectin, 41.4 wt% sucrose) were treated with high-intensity ultrasound at intensity levels ranging from 0 to 40 W cm−2 for various times (0–60 min). Samples were adjusted to pH 1.5 to initiate gelation and their dynamic rheological properties (G′, G″) were recorded as a function of time using a rotational rheometer. Ultrasonically pretreated pectin dispersions formed weaker gels with increasing sonication power and time. After 4 h, log G′ of the pectin dispersion that was pretreated at the highest intensity level (40 W cm−2 and 30 min) was approximately five times lower than log G′ of the untreated dispersion. The change in phase angle [arctan (G′/G″)] with time indicated that the rate of gelation decreased as ultrasonic intensity and application time increased. The turbidity of ultrasonically pretreated pectin dispersions decreased by 50% yielding more transparent gels. Results were attributed to an overall reduction in the average molecular weight of pectin due to cavitational effects. A power law model was fitted to the flow curves of ultrasonically pretreated pectin dispersions to determine both flow behavior index n and consistency coefficient K. With increased sonication power and application time, n increased from 0.6 to 0.97 indicating that the flow behavior changed from viscoelastic to Newtonian.