Continuous and pulsed ultrasound pectin extraction from navel orange peels

Pectin is a valuable product (up to 30 $kg^-1) that makes-up 20-30% of an orange's peel. The commercial extraction is lengthy (up to 6h) and energy intensive as it requires heating aqueous solutions (60-100 °C). Ultrasound speeds up the extraction process reducing processing time by macroscopic and microscopic mixing by acoustic cavitation. We adopted an ultrasonic horn to deliver a rated power of 500W at amplitudes of 20%, 40%, and 60% with and without pulsation to extract pectin from waste orange peels. These correspond to power densities of 0.08Wml^-1, 0.16Wml^-1 and 0.24Wml^-1, respectively. The extractions operated at a pH of either 2 or 3. The experimental data agree with the fitted values from the statistical model (R²=95.5%). The model confirms our predictions that yield increases with amplitude/power density and decreasing pH. The highest yield was (11%) at a pH of 2 and with continuous ultrasonic irradiation at a power density of 0.24Wml^-1. There is only a 1.3% difference between this datum and pulse ultrasound mode (1 s on/1 s off) at the same conditions - a Student's t test confirmed that there was no significant difference in yield between continuous and pulse mode. However, pulsing is more efficient in that it consumes less than half the energy of continuous operation (80kJ vs. 190kJ).