Identification, characterization and application of a natural food colourant from Hibiscus sabdariffa

Abstract

Hibiscus sabdariffa is an under-utilised plant that has been reported to have great potential in the pharmaceutical and nutraceutical industries. The vibrant red pigment indicates a source of anthocyanins that could be produced into a food colourant with additional nutritional benefits however stability is a hindering factor. The crude anthocyanins were extracted from dried calyces by means of four different acidified ethanol and methanol solvent systems to determine the maximum crude anthocyanin yield. The crude extracts were analysed under the following parameters; heat, light, pH stability and degradation kinetics, which included thermal degradation and DPPH radical scavenging ability. Two synthetic colourants were analysed based on the stability parameters; heat, light and pH and compared with the natural H. sabdariffa crude extracts. Each of the four crude extracts were analysed for the total phenolic content using Folin Ciocalteu’s method. The DPPH and FRAP assays were used to determine the radical scavenging activity of the extract with the highest yield. The identification and quantification of the crude anthocyanins were carried out using HPLC-DAD. The highest crude anthocyanin yield of 19.92% was observed by HCl acidified ethanol extract Acetic acid/water/methanol extract produced the lowest yield of 8.72%. The stability results showed that pigment retention of samples heated at 80˚C had a greater decrease over time than those heated at 50˚C. The pH stability of samples incubated for 7 days indicated that crude anthocyanins degraded slower at acidic pH, which is in keeping with reported literature therefore this extract, should be added to foods with lower pH. Light stability showed slower degradation in dark incubated samples resulting in 84% pigment retention after a 10 day period. Synthetic colourants proved to be superior, as they had showed better stability than the natural colourant under the same conditions. Half-life of thermally treated samples showed a decrease upon heating, colour was also affected as

samples became dull and murky. DPPH of thermal treated samples showed a decline in radical scavenging activity from 70 to 85˚C and thereafter an increase was observed between 85 and 90˚C, this could be due to the release of degradation products that have antioxidant capability. Solvent systems did not have an effect on the total phenolic content of crude extracts as no significant difference was observed by each of the H. sabdariffa crude extracts contained an average of 54.67 mg/ml GAE. The radical scavenging ability assessed by the DPPH and FRAP assays showed 53.75% and 57.51% radical scavenging ability respectively. Although the synthetic colourants showed better stability, a natural food colourant from H. sabdariffa can still be beneficial as it has potential to be applied into foods that contain low pH such as jelly and yoghurt. The additional benefits that natural food colourants possess aid in the marketability of the product.