Evaluation of ascorbic acid effect as enzymatic browning inhibitor in pink pulp anona fruit (Annona diversifolia) and on the polyphenol oxidase kinetics

Abstract

The study was executed at Agricultural Science Faculty of El Salvador University, from August 2016 to February 2017; it was developed on two stages. In the first part, the anona pulp was extracted to make a puree, which was used as a representation of the pulp exposed when the shell is naturally cracked. Ascorbic acid solutions at 0.50%, 0.70% and 0.90% (w/v) were used as treatments and a treatment with no ascorbic acid application used as control. The experimental units were stored at 12 ±1 °C for 13 days while titulable acidity, pH and organoleptic characteristics of anona puree were measured; the shelf life of the anona puree without antioxidant was 8 days, meanwhile the three treatments with ascorbic acid extended the shelf life to 11 days. The second part comprised the kinetic study of anona polyphenoloxidase and the effect of three solutions of ascorbic acid (0.5, 0.7 and 0.9 mM) on it by means of spectrophotometry, using catechol as substrate; the kinetic parameters of polyphenoloxidase without ascorbic acid were Km=3.64 mM/L, Vmax=0.160 mM*L-1*min-1 and its catalytic efficiency (Vmax/Km)=0.044 at 14ºC and pH=6.1. Ascorbic acid levels did not show any differences (F2,71=3.05; P=0.0536), but there were significant differences between catechol concentrations (F7,71=33.26; P<0.0001) tested with a P=0.05, showing the highest percentage in the lowest concentration of catechol (1 mM), being 51.45% (EE=3.33) expressed as adjusted average of the three treatments