Optimization of acid-extraction of pectic fraction from grape (Vitis vinifera cv. Chardonnay) pomace, a Winery Waste

Chardonnay grape pomace was evaluated as a source of pectin. A central composite design was used in order to determine the effect of pH, extraction time (Et) and liquid: solid ratio (LS) on the yield and uronic acid (UA) content of the pectins extracted using boiling HNO3 solution. The optimized extraction condition to reach the maximum yield and UA was pH = 2.08, Et = 135.23 min and LS = 35.11 ml/g, resulting in theoretical yield of 12.8% and UA of 64.4%. The experimental yield of the pectic fraction obtained under the optimized conditions (GPOP) was 11.1% and the UA was 56.8%. GPOP had ~25% glucose. It was treated with α-amylase and amyloglucosidase, resulting in the fraction α-GPOP. The starch-free pectic fraction was composed of 63.5% UA, 7.8% rhamnose, 6.0% arabinose, 13.6% galactose and minor amounts of other neutral monosaccharides. It contained a low-methoxyl pectin (degree of methyl-esterification 18.1%) and had an average molar mass of 154,100 g/mol. It consisted of 55.7% homogalacturonan and 35.2% rhamnogalacturonan I (RG-I). NMR analyses suggest that RG-I portion of α-GPOP is highly branched by short chains or single residues of arabinose and galactose.