Several authors have combined UAE with other extraction techniques. Pectin-enriched materials
obtained from sugar beet pulp wastes were extracted by UAE combined with subcritical water by using
the RSM methodology [48]. The highest extraction yields (24.6%) were obtained at pressure 10.70 MPa,
120.7 °C, 30.5 min and S:L ratio 44.0 g·mL−1.
The effect of the extraction temperature and sonication time is essential for UAE, but not much
published work has focused on the optimization of UAE conditions and exploration of the combined
effects of these variables in the pectins extraction. Wang et al. used RSM to study the extraction of
grapefruit peel pectins by UAE, evaluating the combined effects of ultrasound power and extraction time
and temperature on the extraction yield in HCl aqueous solutions (pH = 1.5) [49]. This study also
compared UAE results with those obtained with conventional extraction methods using the same solvent
at 80 °C for 90 min. UAE provided extraction yields of 16.3% at low temperatures and short times.
In summary, UAE could be considered as an efficient and economic technique to extract pectins from
plant resources.