Evaluation and utilization of plant pigments as natuaral food colourants

Abstract

Employing natural colours is the current marketing trend because of consumers’ concern about the safety of artificial food dyes, reinforced by possible health benefits of the natural pigments. Replacement of synthetics with natural colourants, however, is challenging because natural colourants are usually less stable, more costly, not as easily utilized as artificial colours, requires more material to achieve equivalent colour strength and have limited range of hues. With this background, the present study was formulated to evaluate the efficacy of different eco-friendly solvents in the extraction of anthocyanin, betalain and carotenoid pigments from plant sources and subsequently, application of the extracted pigment in different categories of food products to study their stability with the title “Evaluation and utilization of plant pigments as natural food colourants” which was carried out at Department of Postharvest Management, College of Agriculture, Vellanikkara, Kerala Agricultural University, Thrissur, Kerala during 2019-22. The main objectives of the study were to standardize the process for extraction of plant pigments from selected fruits (mangosteen), vegetables (red amaranthus, basella, gac fruit), and flowers (butterfly pea, marigold) and to assess application of extracted pigments in food products (guava squash, guava jelly and ash gourd candy). The present study was focused on the extraction of anthocyanin, betalain and carotenoid pigments. Two crops for each pigment group were taken viz., butterfly pea flowers and mangosteen fruit rind for anthocyanin pigment; basella berries and red amaranthus leaves for betalain pigment; gac fruit seed aril and marigold flower petals for carotenoid pigment. Clitoria ternatea L. produces edible flowers that are rich sources of brilliant blue-coloured anthocyanins called "ternatins". Among the different extraction methods used, acidified aqueous extraction method gave significantly higher recovery percentage of pigment concentrate (74.03±0.84 %) and aqueous extraction method resulted in higher total monomeric anthocyanin content (TMAC) (7925.29±36.07 mg/L), whereas instrumental colour values indicating brilliant-blue colour viz.., a* (46.90±0.77), b* (-63.25±0.13), chroma (78.74±0.38) and hue angle (306.56±0.50), higher antioxidant activity viz., DPPH (3.49±0.59 µl/ml), FRAP (3.99±1.10 µl/ml) and ABTS ((2.42±0.01 µl/ml), total phenolics (29.78±1.79 mg GAE/100 g), total flavonoids (20.13±0.40 mg QE/100 g) were recorded in microwave assisted extraction (MAE) method with aqueous medium. The inedible dark purple rind of mangosteen (Garcinia mangostana L.) fruit is a rich source of anthocyanin pigment that can be used for food application if extracted by using eco-friendly solvents. Microwave assisted extraction method with acidified solvent recorded significantly higher recovery percentage of pigment concentrate (55.03±2.98 %), instrumental colour values indicating purple-red colour viz.., L* (3.17±0.38), a* (59.53±0.85), b* (5.09±0.52), chroma (60.80±1.00) and hue angle (28.06±0.33°), higher antioxidant activity viz., DPPH (2.29±0.24 µl/ml), FRAP (2.83±0.21 µl/ml) and ABTS (2.02±0.03 µl/ml), total phenolics (32.25±0.30 mg GAE/100 g) and total flavonoids (40.02±3.52 mg QE/100 g), whereas acidified solvent extraction method resulted in higher TMAC (17652.54±139.47 mg/L). Basella (Basella alba var. rubra L.) berries with dark purple skin and deep red violet flesh are potential sources of betalain. Among the different extraction methods used, MAE (50 % ethanol) method recorded significantly higher betacyanin (605.83±4.10 mg/g), betaxanthin content (86.35±1.67 mg/g) and total betalain content (692.18±2.52 mg/g), instrumental colour values viz.., a* (52.27±0.24), b* (23.24±0.16), chroma (58.75±0.19) and hue angle (27.40±0.17), higher antioxidant activity viz., DPPH (3.40±0.04 µl/ml), FRAP (3.69±0.19 µl/ml) and ABTS (2.99±0.04 µl/ml), total phenolics (211.37±0.29 mg GAE/100 g) and total flavonoids (124.07±2.53 mg QE/100 g), whereas higher recovery percentage of pigment concentrate (53.13±0.41 %) was recorded in acidified aqueous extraction method (1 % citric acid). Red amaranthus (Amaranthus tricolor L.) plants contain large amounts of betalain, including betaxanthin and betacyanin along with flavonoids, alkaloids, and other elements. Among the different extraction methods used, MAE (50 % ethanol) method recorded significantly higher betacyanin (601.15±2.25 mg/g), betaxanthin content (75.63±0.55 mg/g) and total betalain content (676.78±2.79 mg/g), instrumental colour values viz.., L* (41.94±0.06), a* (33.37±0.22), chroma (63.37±0.17) and hue angle (58.41±0.02), higher antioxidant activity viz., DPPH (1.34±0.01 µl/ml), FRAP (3.69±0.19 µl/ml) and ABTS (0.81±0.01 µl/ml), total phenolics (190.03±0.22 mg GAE/100 g) and total flavonoids (179.07±0.49 mg QE/100 g), whereas higher recovery percentage of pigment concentrate (58.73±0.43 %) was recorded in acidified aqueous extraction method (1 % citric acid). Gac fruit (Momordica cochinchinensis Spreng) seed aril has been reported as a rich source of bioactive compounds which include carotenoid, phenolic and flavonoid compounds. Among the different extraction methods used, ethyl lactate extraction method gave significantly higher total carotenoid content (2069.83±4.38 µg/g), betacarotene (689.43±1.04 µg/g), lycopene (1052.31±0.87 µg/g) and lutein content (216.96±0.22 µg/g), instrumental colour values viz.., L* (60.44±0.12), a* (61.35±0.20), b* (43.24±0.10), chroma (65.62±0.10) and hue angle (36.64±0.18), higher antioxidant activity viz., DPPH (0.59±0.05 µl/ml), FRAP (0.63±0.004 µl/ml), ABTS (0.58±0.005 µl/ml) and total phenolics (211.57±0.13 mg GAE/100 g), whereas higher recovery percentage of pigment concentrate (89.53±0.13 %) was recorded in MAE with ethanol extraction method. Marigold (Tagates erecta L.) flowers are the most concentrated common sources of carotenoid, with lutein, a dihydroxylated compound, accounting for 85 per cent of the total carotenoid present in the flower and are one of the most important sources of carotenoid for application in the food industry. Among the different extraction methods used, ethyl lactate extraction method recorded significantly higher recovery percentage of pigment concentrate (93.33±0.27 %), total carotenoid content (2276.93±3.61 µg/g), beta-carotene (491.03±0.85 µg/g), lycopene (236.96±0.10 µg/g) and lutein content (1310.02±0.27 µg/g), higher antioxidant activity viz., DPPH (0.355±0.03 µl/ml), FRAP (0.557±0.003 µl/ml), ABTS (0.201±0.002 µl/ml) and total phenolics (337.61±0.23 mg GAE/100 g), whereas higher instrumental colour values viz., L* (70.20±0.12), a* (10.48±0.14), b* (76.49±0.15) and hue angle (82.44±0.19) were recorded in ethanol extraction method. Anthocyanin, betalain and carotenoid pigments’ stability in processed products viz., guava squash, guava jelly and ash gourd candy coloured with anthocyanin, betalain and carotenoid pigments were evaluated for a period of three months under ambient and refrigerated storage conditions. The retention of the pigment content (TMAC, total betalain and total carotenoid content), antioxidant properties viz., DPPH, FRAP and ABTS, instrumental colour properties and sensory scores of processed products was higher in the coloured guava squash, guava jelly and ash gourd candy stored under refrigerated condition as compared to ambient condition. The findings of the study indicated higher recovery percentage of pigment concentrates having highest pigment content and antioxidant properties with bright hues of their respective pigments. The proposed extraction methods have been shown to be safer, economic, convenient and eco-friendly. They serve as foundational techniques for the commercial extraction of anthocyanin, betalain and carotenoid pigments that can be up-scaled as green-extraction techniques at the industrial level. Further, the products viz., guava squash, guava jelly and ash gourd candy added with the extracted pigments (anthocyanin, betalain and carotenoid) not only increase the aesthetic value but also the nutritive properties with other qualities intact