Betacyanins from red pitahaya or red dragon fruit (hylocereus polyrhizus): a potential natural red food colourant

Abstract

Red pitahaya or red dragon fruits (Hylocereus polyrhizus) are well known for their deep purple colour pulps due to the abundance of betacyanins and can be exploited as a potential source of natural food colourant. A pectinase-assisted extraction of betacyanins from pulps and peels of red pitahaya was carried out. The extraction efficiency was evaluated in terms of yield, betacyanin content (BC) and total sugar content. Without the use of pectinase, the highest yields of betacyanins from the pulps and peels of red pitahaya was obtained using 95% ethanol and 50% ethanol at a substrate to solvent ratio of 1:1 and 1:2., respectively. Using pectinase at a concentration of 1.5% and water as a solvent at a substrate to solvent ratio (w/v) of 1:1, a higher yield (17.33 0.35 %) of betacyanins with lower total sugar content (57.85 4.35 g/L) from pulps of red pitahaya was produced. Overall, the pulps of red pitahaya were found to produce higher yields of betacyanins compared to those from peels of red pitahaya. Betacyanins typically exist with high amounts of sugars (52-60 g/L) which are undesirable in the food processing industry. Therefore, a fermentation screening and optimisation study was conducted to remove the sugars and concentrate the betacyanins from the pulps of red pitahaya. Four species of lactic acid bacteria, Lactobacillus sp. (L. acidophilus, L. casei, L. rhamnosus and L. plantarum) and a yeast species, Saccharomyces cerevisiae were screened to determine the efficieny to ferment and concentrate betacyanins from red pitahaya. S. cerevisiae was found to concentrate betacyanins with lowest total sugar content (26.4 2.35 g/L). Hence, S. cerevisiae was chosen for the subsequent optimisation step. An optimisation step using response surface methodology (RSM) was performed to analyse the effects of fermentation conditions on the total sugar content, BC and yield of betacyanins from red pitahaya. This study optimised the fermentation conditions (temperature, duration, inoculum size and agitation speed) to achieve the maximum recovery of betacyanins. Temperature, time and agitation speed had a significant influence on the total sugar content and BC of fermentation-concentrated betacyanins from red pitahaya. The yield of betacyanins were significantly influenced by temperature and agitation speed. The optimum conditions obtained were as follows: temperature of 36°C, duration of 16.5 hr, inoculum size of 2.7% and agitation speed of 107 rpm. Under these fermentation conditions, at least 48% of betacyanins with a BC of 126.65 g kg-1 and 5.67% of sugars were recovered. Next, the effects of temperature (30°C, 50°C, 85°C and 100°C) and pH treatment (pH 3, pH 4, pH 5 and pH 6) on the stability of betacyanins from red pitahaya (Hylocereus polyrhizus) and red beet (Beta vulgaris) [E-162] were investigated over a 10-week period of refrigerated storage at 4°C with respect to BC and total colour changes (ΔE*). Reduction in BC was observed with increasing temperature in betalains from red pitahaya (121.50 ± 0.82 g kg-1 - 108.01 ± 1.15 g kg-1) and E-162 (120.02 ± 1.10 g kg-1 - 105.63 ± 1.43 g kg-1). Ten weeks of refrigerated storage at 4°C further increased the loss of BC (0.20 – 12.76 g kg-1) and caused a very distinct total colour changes (ΔE* > 3) for all the heat treated betacyanins from red pitahaya and E-162. The half-life (t1/2) of BC in all heat treated betacyanins from red pitahaya and E-162 ranged from 2057 to 36478 days. Immediately after pH treatment, the highest (6.98 – 8.59%) loss of betacyanin was observed at pH 3. Ten weeks of refrigerated storage at 4°C further increased the loss of BC (0.30 – 27.78 g kg-1) and caused a very distinct total color change (ΔE* > 3) for all pH treated betacyanins. The half-life (t1/2) of BC in all pH treated betacyanins from red pitahaya and E-162 ranged from 4621 to 15753 days. The application of betacyanins from red pitahaya to simulate strawberry-red colour in three model food systems (milk, yoghurt and ice cream) was evaluated in comparison to E-162. In milk, the effect of pasteurisation was found to be greater in E-162 compared to betacyanins from red pitahaya. Likewise, a greater loss of BC and total colour changes was observed in milk or yoghurt containing E-162. However, an approximate 0.15% increase of BC was observed in ice cream containing betacyanins from red pitahaya or E-162 on day-21 of storage at -18°C. Throughout the 7-days, 14-days or 21-days of cold storage, only a small difference (ΔE* < 1.5) was observed in total colour changes of milk, yoghurt or ice cream containing betacyanins from red pitahaya or E-162, respectively. The microbial viability (S. thermophilus and L. bulgaricus) in yoghurt containing betacyanins was found to be better than that of plain yoghurt (without betacyanins). Yoghurts containing betacyanins (red pitahaya or E-162) were also found to have lower syneresis than that of plain yoghurt. Ice-cream containing betacyanins from red pitahaya was found to have higher overruns compared to those of plain or E-162 containing ice-cream while the apparent viscosity of ice-creams was not affected by the addition of betacyanins from red pitahaya or E-162. The addition of betacyanins from red pitahaya or E-162 was found to enhance the antioxidant properties of milk, yoghurt and ice cream. The sensory evaluation of milk, yoghurt and ice cream containing betacyanins from red pitahaya showed a better colour acceptability compared to those containing E-162. Betacyanins from red pitahaya can therefore be used as a potential natural colourant to simulate strawberry-red colour in milk, yoghurt and ice cream