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lwt 41 (2008) 14121416 composition and colour stability of anthocyanins extracted from fermented purple sweet potato culture gongjian fan, yongbin han, zhenxin gu?, feirong gu college of food science and technology, nanjing agricultural university, nanjing 210095, peoples republic of china received 28 may 2007; received in revised form 2 september 2007; accepted 6 september 2007 abstract the components and colour stability of anthocyanins in purple sweet potato were investigated. anthocyanins were extracted from fermented purple sweet potato culture fermented by suzhou wine starter (rhizopus 3.851, 3.866 and saccharomyces cerevisiae). in these purple sweet potato anthocyanins (pspas), fi ve major anthocyanins were detected by high performance liquid chromatography (hplc). cyanidin and peonidin were found to be the major anthocyanidins in pspas by acid hydrolysis of anthocyanins. pspas were more stable under the acid conditions (ph 2.04.0) than the subacid conditions (ph 5.06.0) as per uvvis absorption spectra and cielab colour coordinates. r 2007 swiss society of food science and technology. published by elsevier ltd. all rights reserved. keywords: purple sweet potato; anthocyanin; fermentation; composition; colour stability 1. introduction public concern about the safety of synthetic colourants in food has led to increasing attention to natural colourants as alternatives. anthocyanins, as a group of phenolic compounds widely existing in the plant kingdom, present a spectrum from orange to blue in colour in the natural world, satisfying consumers demand for food colours. therefore, anthocyanins have been be used instead of synthetic pigments (mazza smith, marley, seigler, singletary, wang, wang, lin, chu, chou, wrolstad, 2000). therefore, hydrochloric acid or formic acid is often added to the solvents to prevent degradationof theanthocyanins(brigita,mirko, glenda, julia n, jose , gonnet kjell fax: +8602584396293. e-mail address: guzx (z. gu). andacetates.thesesecondarymetabolitesmight combine anthocyanins and form pyroanthocyanins which are more stable than anthocyanins (pretorius, 2000). however, there was no mention of pyroanthocyanins detected in pspas fermented by the japanese scientists (kousukeetal.,1990),thoughitisreportedthat pyroanthocyanins could be detected in red wine (morate, go mez-cordove s, caldero n, 95% ethanol, the ratio 15:85, v/v), and then put in thermostatic water at 501c for 5h. after centrifugation at 1766?g for 15min, the supernatant was concentrated to 50ml by the rotatory evaporator (451c) and stored at 41c in a refrigerator for later analysis 2.3. acid hydrolysis of pspas one millilitre of concentrated pspas solution was dissolved in 10ml of hydrochloric acid (1.0mol/l) in a screw-cap test tube. the solution was hydrolyzed at 981c for 1h, cooled in an ice bath (luigia prodanov, domnguez, bla zquez, salinas, mazza & brouillard, 1987). in this research, the lmax shifted from 527 to 548nm long as the ph value increased from2.0to7.0,whichwasvisually confi rmedby differences in the colour of the solution. the increase in ph values brought about browning at 420nm which then led to an increase in absorbance of the pspas, and the formation of less carbinol bases at 520nm which led to a decrease in the absorbance. these reactions suggest that the pspas were more stable at lower ph values than at higher ph values (table 2). the colour stabilities of pspas at different ph values could be confi rmed by the uniform cielab colour space parameters of l*(lightness), c*(chroma), h (hue) (table 3). the c*represents distance away from the grey tone (i.e., degree of colour saturation) in a scale from 0 to 100. results showed that c*increased when ph values decreased, and the highest c*was found at ph 2.0, while the lowest c*was around ph 7.0. the low c*values indicate that the coloured material in the model solution decreased. the anthocyanins changed from red to colour- less at the subacid condition. the h values indicate colour varieties at different ph values. our results showed that h decreasedfrom0.621to?15.161.theanthocyanins solution was red at ph 2.0 and gradually turned to more red-lilac by raising ph to 7.0. it is believed that at low ph, anthocyanin exists as fl avylium cation which is the most stable form. this bright red form transforms into blue quinonoidal bases or colourless carbinol pseudobases as the ph value increases. in the alkaline condition, the antho- cyanin exists as yellow chalcone species (brouillard, 1982). the l*had no relationship with the alteration of ph values. 4. conclusions to sum up, anthocyanins extracted from fermented purple sweet potato were more pure than chemistry extraction. more than 80.7% of the pspas are acylated anthocyanins. cyanidin and peonidin were the major anthocyanidins in pspas. the determination of pyr- article in press min 05101520 mau 0 50 10 15 20 25 30 1 2 fig. 2. hplc profi le of purple sweet potato anthocyanins after acid hydrolysis. table 2 colour characterization of pspas at different ph values ph valuecidicdctlmax 2.00.42470.0020.21470.0000.51170.0020.22170.000527.070.0 3.00.35170.0000.23070.0010.42670.0000.23870.001530.070.0 4.00.25370.0030.27970.0020.32470.0050.29370.002531.570.5 5.00.15670.0000.37970.0000.22270.0000.40570.000535.570.5 6.00.11070.0000.49470.0050.17870.0010.54170.004545.070.0 7.00.09970.0010.63170.0000.18770.0020.71770.001548.071.0 table 3 cielab values of pspas at different ph values ph valuescielab l*c*h 2.018.7070.013.0270.050.6270.00 3.018.8370.072.8070.19?1.0470.01 4.018.7570.092.6970.02?2.5670.02 5.018.5370.082.3570.17?3.5170.01 6.018.9870.042.3170.10?12.9670.01 7.019.2370.082.3170.02?15.1670.01 g. fan et al. / lwt - food science and technology 41 (2008) 141214161415 oanthocyanins requires further research. pspas extracted from fermented purple sweet potato were found to be more stable at low ph values. the research result has a positive implication for using pspas as natural colourants in food industry. references bolvar, a. c.-c., & luis, c.-z. 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