Does Purple Sweet Potato Powder Contain Anthocyanins?

Anthocyanins in purple sweet potato are stable in neutral pH during steaming and dehydration. Steaming and dehydration can be used to process purple sweet potato powder as food ingredient with good pleasant purple color. Although purple sweet potato possessed high content acylated anthocyanins, the anthocaynin of purple sweet potato was not stable to light. Application of anthocyanin extracts of purple sweet potato in acidic foods requires no exposure to light during storage. This research information will be useful for food industries to consider applying purple sweet potato powder or anthocyanin extract from purple sweet potato in food products.

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Anthocyanins are natural pigments alternative to synthetic colorants. Purple sweet potato (PSP) possesses high content of acylated anthocyanins, which are more stable than non-aclylated anthocyanins to pH, heat and light. The stability of anthocyanin of PSP was determined during steaming and dehydration. The PSP cultivar Terlaje were steamed for 0, 0.5, 1, 2 and 4 hours. The PSP steamed for 0.5 hour were dehydrated at 55 C for 0, 4, 8, 20, 24 hours. Anthocaynins were extracted with 1% HCl acidic water from steamed or dehydrated PSP. The total anthocyanin content was measured with the UV spectrophotometer based on monomeric anthocyanin pn 3-caf.sop-5-glc (mg/100 g fresh weight). Polymeric anthocyanins were assayed with bisulfite-treated extract. The hue angle and chroma (C) of PSP were calculated from color values L, a and b measured by a Chroma meter. Steaming PSP for 0.5 hour resulted in an increase of anthocyanin content from 42 to 60 mg/100 g and a decrease of h from -8.5 to -26.5 and C from 28.7 to 21.6, indicating an enhancement of purple color of PSP. Steaming PSP for 1 to 4 hours caused a decrease of anthocyanin content from 47 to 42 mg/100 g and of h from -25.04to -1.94and of C from 21.6 to 19.7. Although dehydration for 24 hours resulted in an increase of about 50% of polymeric color, the dehydration did not decrease anthocayanin content of steamed PSP. After 24 hour dehydration, the PSP powder exhibited a color value at h -18.1 and C 27.9. The effect of light on PSP anthocyanin extracts was also investigated during storage. Semi-purified anthocyanin extracts were prepared with C-18 Sep-Pak catridges. The anthocyanin extracts at pH 1 were treated with fluorescence light for 2 months. The stability of anthocyanin were determined with total anthocyanin content or the UV/Vis spectrum and color value (L, a, b). The stability of anthocyanin of PSP was also compared with that of red cabbage, grape skin, and synthetic colorant Red #40. the The Tinctorial strength of the colorants in pH 1 buffer was also measured. Light-treated PSP anthocyanin extracts exhibited a 5% increase in absorbance at 520 nm on the first day and then a decrease of 55% in absorbance after 2 months storage. The lightness (L) value is increased from 85 to 88. The red (a*) and blue (b) color values decreased from 9 to 3 and 4 to 2, respectively. Non-light-treated PSP extracts exhibited a 10% increase in absorbance and no significant changes in L*, a* and b* values during 2 month storage. The light stability of anthocyanin extracts and synthetic colorant was: Red #40 > red cabbage > purple sweet potato > grape skin.

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