where cp is the heatcapacity of the solvent (inJkg−1 K−1) and m
is the mass of solvent used (inkg).The solvent used in all experiments was citric acid. Thus,the ultrasonic bath was working at a
frequency of 37 kHz, power output of 140 W and power density of
0.05 W mL−1.
The extraction was carried out according to the methods proposed by Canteri-Schemin et al.(2005) and Panchev, Kirtchev,and
Kratchanov (1994), with slight modifications.Ten grams of grape
pomace were mixed with 100 mL of citric acid solution. The flask
was placed in the center of the bath using the conditions of experimental design.The hot acid extract was filtered through a screen
of 1 mm mesh,created by two layers of cheesecloth,then the
filtrate was cooled down to 4 ◦C. Following that,the filtrate(con-
taining pectin) was precipitated with ethanol 95% (1:2,v/v) and
stirred for 10 min. Next,the mixture was centrifuged at 1750×g
for 30 min.The precipitated pectin was separated by filtration
(#40 glass fiber) and rinsed twice with ethanol 95% and acetone.
The pectin extracted ultrasonically was dried at room temperature for 12 h. Finally,the resulting material was milled to obtained
powdered pectin.The yield was calculated as grams of product
obtained per 100 g of grape pomace (fresh weight).Uronic acids
were determined by colorimetry (Blumenkrantz &Asboe-Hansen,
1973), using samples hydrolyzed for 1 h at 100 ◦C in 1 M H2SO4.
Results showed that uronic acid accounted for 96–99% of the pectin
extracted in all samples.