Developing a Thermochromic Material and Study of Kinetics of Degradation of Anthocyanin from Local Grapes

Abstract

Thermochromic materials undergo a colour change with temperature which finds numerous practical applications in food and medical industry. Since most thermochromic materials contain at least one toxic compound, their application in medical and food industry is limited. The present study intended to develop a thermochromic material based on natural pigment, anthocyanin, extracted from local grapes. Different biodegradable polymers were incorporated into anthocyanins and thermochromic effect was monitored. A kinetic study was conducted to identify conditions which enhance anthocyanin stability. Thermochromic ability of anthocyanin from local grapes was investigated using agar, agarose and chitosan biopolymers as matrices, while kinetic study was carried out for different solvents, temperatures and pH values. The concentration of anthocyanin was measured using pH differential method. Although for agar and agarose biopolymers, a considerable thermochromic effect was not observed, for chitosan – acetic acid – anthocyanin mixture, positive results were observed. At isothermal conditions, chitosan film which was originally wine red changed gradually to violet, then brownish yellow and finally to brown. Depending on temperature, pH and the solvent, colour of anthocyanin changed. At low pH values, low temperatures and with polar solvents, high colour retention was observed than at high pH values, high temperatures and with less polar solvents. Colour retention of samples had a direct relationship with degradation rate of anthocyanin. The kinetic results proved that acidified ethanol extracts of anthocyanin from local grapes are stable at low temperatures. Results indicated that chitosan – acetic acid – anthocyanin mixture has potency for use as thermochromic material. Further, by enhancing the conditions identified in the kinetic study, anthocyanin from local grapes can be used as a food colorant in other food items