Ultrasonic pretreatment and drying temperature-induced modifications of three pectin fractions affect the microstructure and textural properties of dried grapes.

The effect of ultrasonic pretreatment on grapes' microstructure, texture, characteristics of three pectin fractions were investigated over vacuum drying temperature ranging from 65 to 85 °C. Ultrasonic treated samples exhibited a better microstructure and texture quality than with vacuum drying alone. High drying temperature led to increased water-soluble pectin (WSP, from 23.85 to 25.66 mg/g AIR) and chelate-soluble pectin (CSP, from 1.92 to 2.87 mg/g AIR) content, decreased sodium carbonate-soluble pectin (NSP, from 12.11 to 4.43 mg/g AIR) content. Within pectin fractions, monosaccharide content and molecular weight both decreased as the drying temperature increased after ultrasonic pretreatment. Characterization by Fourier transform infrared spectroscopy, X-ray diffraction, circular dichroism, and scanning electron microscopy revealed differences between pectin fractions in terms of structures. Principal component analysis and correlation analysis indicated that the changes in galacturonic acid content and sugar ratio of WSP, NSP fractions influenced the texture properties of dried grapes.