Effects of ultrasound and/or heating on the extraction of pectin from grapefruit peel

• Pectin was affected by many parameters of ultrasound-assisted heating extraction.
• Swelling behavior of vegetative material was closely correlated with pectin yield.
• Both heating and ultrasound had significant effects on the extraction kinetics.
• Heating and ultrasound showed a synergistic effect on pectin extraction.
• It was more efficient in pectin extraction when combining heating with ultrasound.

Effects of ultrasound and/or heating on the yield of pectin, swelling behavior of material, and kinetics of pectin extraction from grapefruit peel were investigated. Several extraction parameters significantly affected the yield of pectin and swelling index (SI) of vegetal tissue. The optimal extraction conditions for ultrasound-assisted heating extraction (UAHE) selected through single-factor experiments were as follows: ultrasound power density 0.40 W/mL, duty cycle 50%, temperature 60 °C, solid–liquid ratio 1/50 g/mL. Image studies showed that UAHE disrupted the vegetal tissue and significantly improved its swelling behavior. There existed significantly high correlations between tissue SI and pectin yield, indicating that the improvement of pectin extractability via disrupting vegetal tissue was the main mechanism for ultrasonic enhancement of extraction. A theoretical model, which could simultaneously describe the extractability, dissolution and degradation rates of pectin, and predict the maximal yield and the optimal time, was used to study the extraction kinetics when ultrasound and/or heating were applied. Yields of pectin extracted using UAHE (0.40 W/mL and 60 °C), ultrasound-assisted extraction (UAE, 0.40 W/mL and 30 °C), heating extraction (HE, 60 °C), room temperature extraction (RE, 30 °C) and conventional heating extraction (CHE, 80 °C) within 60 min were monitored and analyzed by the model respectively. The kinetics study showed that both heating and ultrasound could significantly facilitate the extractability, dissolution and degradation of pectin, and there existed a synergistic effect between them. Compared with CHE, UAHE significantly improved the extractability and extraction rate of pectin, leading to higher yield (26.74%) with shorter extraction time (51.79 min) and reduced temperature (60 °C). These results suggested that UAHE could be an efficient technique for the extraction of pectin from plant materials.