Optimization of Extraction of Natural Pigment from Purple Sweet Potato by Response Surface Methodology and Its Stability

Purple sweet potato colour (PSPC)was a kind of natural pigment that attracted the general concern in recent years. In this paper, the response surface methodology was employed to optimize the extraction conditions of PSPC.The results showed that the extraction yield of purple colour was 11.6355mg/g at the optimum extraction conditions of extraction temperature 60C, extraction time 1 h, the ratio of solid to liquid ratio of 1 : 30, and acidified ethanol solution concentration 80%. Stability experiment showed that Fe and Al could increase the stability of PSPC, but Cu, Zn, and Pb would decrease the stability of PSPC. Ascorbic acidified could significantly increase the stability of PSPC, and Na


Introduction
The colour of foods could affect the customer's decision on purchasing behavior by causing customer's direct attention on the sensory.However, the synthetic pigment had a negative impact on human's healthy.More and more attentions are paid to the natural pigment, which could be served as a functional component.
Anthocyanin is one of the most important natural pigments.Purple sweet potato is a plant which is rich in anthocyanin.The anthocyanin from purple sweet potato could be not only used as the natural pigment but also used as the functional compound due to its obvious antioxidant, antimutation, and antineoplastic activities [1][2][3][4][5][6][7][8].In this paper, the objective was to optimize the extraction condition by response surface methodology and investigate its stability.

Materials and Methods
2.1.Materials.Fresh purple sweet potato was purchased from Tianheyuan agricultural incorporation in Suzhou.

Pretreatment of Purple Sweet
Potatoes.Purple sweet potatoes were washed and chopped into pieces, then they were dried in the oven at 50 ∘ C for 12 hours.Finally, they were smashed and kept in brown desiccator.

The Maximum Absorbance Wavelength of Purple Sweet Potatoes Pigment.
A UV-Vis spectrophotometer (UV-2102PCS) was used to determine the maximum absorbance of purple sweet potato pigment.Purple sweet potato powder was weighted and extracted in acidified ethanol aqueous solution at 60 ∘ C for one hour.Then, it was centrifuged at 5000 r/min for 15 min.The pH of supernatant was adjusted to 2, and then the supernatant was scanned from 200 nm to 700 nm.

Optimization of Extraction of Purple Sweet Potato Pigment.
A RSM was used to optimize the extraction conditions for purple sweet potato pigment.A multivariate study based on Box-Behnken design was chosen to evaluate effects of extraction parameters.The four independent variables were (2) Effect of Food Additives on PSPC Stability.Na 2 SO 3 , ascorbic acid were weighted and added in PSPC solutions with the different concentration.The solutions were kept away from light at room temperature for 2 h and then measured its absorption at 525 nm.

Measurement of the Maximum Absorption Wavelength.
As shown in Figure 1, there are three peaks at 280∼290 nm, 320∼340 nm, and 520∼530 nm.The peak at 280∼290 nm is the characteristic absorption peak of polyphenol, the peak at 320∼340 nm is the characteristic absorption peak of organic acid, and the peak located at 520∼530 nm is the characteristic absorption peak of anthocyanins.The peak at 520∼530 nm is chosen as testing wavelength.

Effect of Extractant on the PSPC Yield. As shown in
Figure 2, compared with the other extractant, acidified methanol has the highest PSPC yield, followed by acidified ethanol aqueous solution, 5% HCl.Considering its use on food, safety, and cost, the acidified ethanol aqueous solution is chosen as the appropriate extractant. (1)

Analysis of Variance (ANOVA) of Quadratic Regression
Model.ANOVA of quadratic regression model (Table 3) demonstrated that the variables were adequately fitted to the regression equation (1)   orders of the parameters for the PSPC yield in the prediction model were  4 >  1 >  2 >  3 .temperature and concentration of acidified ethanol solution and PSPC yield (Figure 4).When the extraction temperature and the concentration of acidified ethanol aqueous solution increased, the PSPC yield increased before the medium level and then decreases.The contour plot showed that the optimum extraction temperature is between 55 and 62 ∘ C and the optimum concentration of acidified ethanol aqueous solution is between 76.7% and 82.25%.According to the results of the analysis of regression model, the optimum conditions for the extraction of PSPC are at the extraction temperature of 60 ∘ C, time of 1 h, solid-liquid ratio of 1 : 30, acidified ethanol aqueous solution concentration of 80%, and the extraction yield could reach 11.6355 mg/g.

Verification Experiment.
The extraction experiments were carried out under the optimum conditions of extraction temperature 60 ∘ C, time 1 h, solid-liquid ratio 1 : 30, and acidified ethanol aqueous solution concentration of 80%.The real PSPC yield was 11.5276 mg/g, which was adjacent to the modeling value of 11.6355 mg/g with standard error of 0.93%.This result indicates that this regression model could be used to indicate the relationship between extraction condition and PSPC yield and to predict the extraction yield of PSPC.

Conclusion
In the basis of RSM experiment, the optimum extraction conditions of PSPC are the extraction temperature of 60 ∘ C, extraction time of 1 h, solid-liquid ratio of 1 : 30, and acidified ethanol aqueous solution concentration of 80%, and the

1 Figure 3 :
Figure 3: Contour plot and response surface diagram of effect between extraction temperature and time on PSPC.

Figure 4 :
Figure 4: Contour plot and response surface diagram on effect between extraction temperature and concentration of acidified ethanol on PSPC.
Condition for PSPC.Threedimensional diagram and contour plot made by full model of (1) were used to predict the relationships between the extraction temperature, extraction time, and PSPC yield (Figure3).When the extraction temperature and extraction time increased, the PSPC yield increases firstly and then decreases.At the medium value of extraction condition of temperature and time, the PSPC has the higher extraction yield (Figure3).The contour plot showed that the optimum extraction temperature is between 57.1 and 62.55 ∘ C and the optimum extraction time is between 56.6 and 62.95 min.Three-dimensional diagram and contour plot were used to predict the relationships between the extraction

Table 1 :
The coding schedule of response surface methodology.
, which were statistically acceptable at  < 0.05 level and adequate with satisfactory determination coefficients ( 2 of 0.9768 and  2 Adj of 0.9468).The PSPC yield was significantly affected by linear term  4 , quadratic terms  2 1 ,  2 2 ,  2 3 , and  2 4 , and interactions terms of  1 and  3 ,  1 and  4 ,  3 , and  4 .According to the  value, the significance

Table 2 :
Results of Box-Behnken of response surface methodology.

Table 3 :
Analysis of variance of regression model.

Table 4 :
Effects of metallic ion on PSPC stability.

Table 4 .
Compared with bank solution, the solution with Fe 3+ or Al 3+ has the higher absorbance value at 525 nm for 24 h and 48 h, which meant that Fe 3+ or Al 3+ contributed to the PSPC stability.However, the solution with Cu 2+ , Zn 2+ , or Pb 2+ has the lower absorbance value at 525 nm for 24 h and 48 h, which meant that Cu 2+ , Zn 2+ or Pb 2+ was against the PSPC stability.3.8.Effect of Ascorbic Acid and Na 2 SO 3 on PSPC Stability.Effects of ascorbic acid and Na 2 SO 3 on PSPC stability is shown in Table5.The absorbance value at 525 nm increased with the increase in the ascorbic acid concentration, which means that ascorbic acid contributed to the PSPC stability and could be used as copigment.The absorption value at 525 nm decreases with the increase in concentration of Na 2 SO 3 , which indicates that Na 2 SO 3 has a bad effect on PSPC stability.

Table 5 :
Effects of ascorbic acid and Na 2 SO 3 on PSPC stability.PSPC is 11.6355 mg/g.Fe 3+ and Al 3+ can contribute the stability of PSPC, but Cu 2+ , Zn 2+ , and Pb 2+ would decrease the stability of PSPC.Ascorbic acid can significantly increase the stability of PSPC which can be used as copigment, and Na 2 SO 3 would have bad effect on the stability of PSPC.