Ultrasound-assisted extraction of pectins from grape pomace using citric acid: A response surface methodology approach

• Pectin was obtained from grape pomace by power ultrasound and citric acid.
• Pectins were extracted at different temperature, time and pH.
• Extraction yield, molecular weight and degree of esterification of pectins were evaluated.
• Pectin extraction process was optimized by response surface methodology.
• Maximum extraction yield of pectin was achieved at 75 °C for 60 min and pH 2.0.

An ultrasound-assisted procedure for the extraction of pectins from grape pomace with citric acid as the extracting agent was established. A Box–Behnken design (BBD) was employed to optimize the extraction temperature (X1: 35–75 °C), extraction time (X2: 20–60 min) and pH (X3: 1.0–2.0) to obtain a high yield of pectins with high average molecular weight (MW) and degree of esterification (DE) from grape pomace. Analysis of variance showed that the contribution of a quadratic model was significant for the pectin extraction yield and for pectin MW whereas the DE of pectins was more influenced by a linear model. An optimization study using response surface methodology was performed and 3D response surfaces were plotted from the mathematical model. According to the RSM model, the highest pectin yield (∼32.3%) can be achieved when the UAE process is carried out at 75 °C for 60 min using a citric acid solution of pH 2.0. These pectic polysaccharides, composed mainly by galacturonic acid units (<97% of total sugars), have an average MW of 163.9 kDa and a DE of 55.2%. Close agreement between experimental and predicted values was found. These results suggest that ultrasound-assisted extraction could be a good option for the extraction of functional pectins with citric acid from grape pomace at industrial level.