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Thermal degradation kinetics of aqueous anthocyanins and visual color of purple corn (Zea mays L.) cob
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Innovative Food Science and Emerging Technologies 9 (2008) 341 – 347 www.elsevier.com/locate/ifset
Thermal degradation kinetics of aqueous anthocyanins and visual color of purple corn (Zea mays L.) cob Zhendong Yang, Yonbin Han, Zhenxin Gu ⁎, Gongjian Fan, Zhigang Chen College of food science and technology, Nanjing Agriculture University, Nanjing, 210095, China Received 1 July 2007; accepted 7 September 2007
Abstract Purple corn cob was the byproduct during the corn processing. Thermal degradation kinetics and Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) of aqueous anthocyanins from purple corn cob were studied at selected temperatures (70 °C, 80 °C and 90 °C) at pH 4.0. The results indicated that the thermal degradation of anthocyanin and Hunter color C⁎, a⁎ and ΔE parameters followed the first-order reaction kinetics, while Hunter color h° and b⁎ parameters followed zero-order reaction kinetics. The calculated values of activation energies (Ea) were 18.3, 35.9, 37.1, 31.6, 34.9 and 30.0 kJ/mol for anthocyanins, C⁎, a⁎, ΔE, h° and b⁎ parameters, respectively. The higher Ea indicated that greater temperature sensitivity of visual color as compared to anthocyanins content. The degradation of anthocyanins showed positive correlation with C⁎ (R2 N 0.909), a⁎ (R2 N 0.860) and ΔE (R2 N 0.940), while the degradation of anthocyanins showed negative correlation with h° (R2 N 0.828) and b⁎ (R2 N 0.735) during heating. © 2007 Elsevier Ltd. All rights reserved. Keywords: Purple corn cob; Anthocyanins; Hunter color parameters; Kinetics
Industrial relevance: Purple corn cob was the byproduct during the corn processing. Purple corn cob is dark purple to almost black color due to its high content of anthocyanins, which makes this byproduct a good source of anthocyanins. In this study, the excellent linear correlation between Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) and content of anthocyanins showed that the Hunter color parameters may also be used instead of anthocyanins content during heating. The advantage of using the visual Hunter color parameters may be measured as on-line quality control parameters during thermal processing of food industry.
1. Introduction There are various kinds of corn in the world, and the corn has various colors such as white, yellow, red, purple, brown, green and blue. Purple corn is a pigmented variety of Zea mays L., originally cultivated in Latin America and was introduced in China a long time ago. This kind of corn is mainly grown in China, especially in Shanxi and Anhui province. As the byproduct during corn processing, purple corn cob was often neglected in the production. However, it was discovered that the cob contains rich anthocyanins, such as
⁎ Corresponding author. Tel./fax: +86 25 84396293. E-mail address: [email protected] (Z. Gu). 1466-8564/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2007.09.001
cyanidin-3-glucoside, pelargonidin-3-glucoside, and peonidin3-glucoside (Pascual-Teresa, Santos-Buelga, & Rivas-Gonzalo, 2002). Obviously purple corn cobs can be used as a source of natural colorant. Recently, anthocyanins have been reported to have various biological activities, such as antioxidant, antimutagenic and anticancer activities (Bomser, Madhavi, Singletary, & Smith, 1996; Yoshimoto, Okuno, Yamaguchi & Yamakawa, 2001; Kähkönen & Heinoner, 2003; Katsube, Iwashita, Tsushida, Yamaki, & Kobori, 2003). Especially, most of the properties attributed to purple corn extracts, including coloring attributes (Duhard, Garner, & Megard, 1997; Cevallos-Casals & Cisneros-Zevallos, 2004), antioxidant (Cevallos-Casals & Cisneros-Zevallos, 2003a), antimicrobial (Cevallos-Casals & Cisneros-Zevallos, 2003b), anti-obesity activity and amelioration of hyperglycemia (Tsuda, Horio,
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Uchida, Aoki & Osawa, 2003), and anticarcinogenic properties (Hagiwara et al., 2001) were related to anthocyanins. Color is an important sensory property in determining product quality, therefore minimizing the pigment losses during processing is of primary concern to the processor (Markakis, 1982; Bridle & Timberlake, 1997). Total monomeric anthocyanin measurement is determined by pH-differential method outlined by Giusti and Worsltad (2001). This method is a spectrophotometric measurement. Results from reflectance color measurements are often correlated with the chemical measurements. Reflectance color measurements are easy and cheap, but it is usually recommended to follow the color changes during food processing. Detailed color measurements including total monomeric anthocyanin analysis together with color analysis are needed. On the other hand, the visual color, which is an indicator of pigment concentration, can be measured instantaneously using tristimulus colorimeters for on-line quality control (Rocha, Lebart & Marty-Audouin, 1993; Ahmed, Shivhare & Raghavan, 2004). The color degradation kinetics of food products are a complex phenomenon and dependable models to predict experimental color change, which can be used in engineering calculations, are limited. However, empirical mathematical modeling techniques may be used to determine the end point and kinetic effect. The kinetic parameters namely, reaction order, rate constant and activation energy provide useful information on the quality changes which occur during heating. Although color can be reported in different systems, lightness (L⁎), chroma (C⁎), and hue angle (h°) parameters were used since the most common L⁎ a⁎ b⁎ coordinates do not express hue and chroma directly and difficult to interpret independently (McGuire, 1992). So, C⁎, and h° may preferably be used as indexes of food product quality. Compared to synthetic colorants, anthocyanin pigments are more susceptible to heating. Current studies on anthocyanins from purple corn were focused on the stability structure, and physiological functionality (Hagiwara et al., 2001; Pascual-Teresa et al., 2002; Tsuda et al., 2003; Cevallos-Casals & Cisneros-Zevallos, 2004). However, studies on the degradation kinetics of anthocyanins and color of purple corn cob are lacking; the effect of the two different color systems on the evaluation of degradation kinetics of anthocyanins and the resulting changes in visual color has not been reported. The objective of this study was to investigate: (1) the degradation of anthocyanins during heating, (2) the degradation of visual color using Hunter tristimulus parameters (a⁎, b⁎, C⁎, h° and ΔE), and (3) a relationship between visual color and aqueous anthocyanins from purple corn cob during heating.
disintegrator (FSD-100A, China) and sifted through a 100 mesh sieve. All other reagents were of analytical grade. 2.2. Preparation and determination of aqueous anthocyanin extracts About 100 g (powder of purple corn cob) were macerated with 400 mL of methanol for 24 h in the dark at 4 °C. The crude extract obtained was filtered through a filter paper and the remaining residues were washed with 200 mL of methanol, and then the crude extract filtrate was collected. The methanol in the crude extract was evaporated by the rotary evaporator (0.1 MPa, 35 °C). The remaining solvents were purified by macroporous resin LSA-10 (Lanxiao Company, China). The eluate was concentrated and evaporated, and then the solvent was freezedried to obtain the red powder. The red powder was dissolved with 200 mL nanopure water, which was done as the stock solution of anthocyanins. An aliquot of the stock solution of anthocyanins was diluted with 0.2 M citrate buffer to pH 4.0. The quantification of total monomeric anthocyanins was determined by pH-differential methods (Giusti and Worsltad, 2001). Total anthocyanins were calculated as caynidin-3glucoside according to the following equation: Total anthocyanins ðmg=LÞ ¼ A MW DF 1000=ðe 1Þ ð1Þ Where A = (A510 − A700) pH 1.0 − (A510 − A700) pH 4.5; MW (molecular weight) = 449.2 g/mol for caynidin-3-glucoside; DF = dilution factor; 1 = pathlength in cm; e = 26,900 molar extinction coefficient in L/mol/cm for caynidin-3-glucoside; 1000 = conversion from g to mg all analyses were done in triplicate (n = 3). 2.3. Measurement of visual color CIELAB parameters were determined using D-65 diffused illumination of a Minolta Chrama CR-400 Colorimeter. The instrument was calibrated with a standard white plate. The measured parameters were L⁎ (lightness), a⁎ (redness), and b⁎ (yellowness). The three measured color parameters were converted into C⁎ (chroma), h° (Hue angle), and ΔE (total color change) using the following equations: h i1=2 2 2 C ⁎ ¼ ð a⁎Þ þ ð b⁎Þ ð2Þ h- ¼ arctanðb⁎=a⁎Þ
ð3Þ
2. Materials and methods
h i1=2 2 2 2 DE ¼ ð DL⁎Þ þðDa⁎Þ þðb⁎Þ :
ð4Þ
2.1. Materials
2.4. Kinetics model of aqueous anthocyanin
Purple corn cob (Z. mays L.) was generously supplied by Professor Zhong Zhang in Anhwei Technical Teachers College (Fengyang city, Anhwei province, China). The cob was dried in a heated air drier (ZT-3, China) at 50 °C, pulverized by the
After dissolving freeze-dried samples in water, the resulting extracts were heated at 70, 80 and 90 °C in a water bath. Samples were drawn at 1 h intervals, and then analyzed for color and anthocyanin content. Previous studies showed that heating
Z. Yang et al. / Innovative Food Science and Emerging Technologies 9 (2008) 341–347
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Fig. 1. The kinetics of anthocyanins and Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) of purple corn cob at pH 4.0. C = anthocyanins concentration (A); a⁎ = redness (B); C⁎ = chroma (C); ΔE = total color change (D); b⁎ = yellowness (E); h° = Hue angle (F).
degradation of anthocyanins followed at first-order reaction (Garzon & Wrolstad, 2002; Cevallos-Casals, & CisnerosZevallos, 2003a; Cevallos-Casals, & Cisneros-Zevallos, 2004). This kinetic type was expressed by the following equation: ln ðCt =C0 Þ ¼ k t
ð5Þ
t1=2 ¼ ln 0:5 k 1 :
ð6Þ
Dependence of the degradation rate constant on temperature is represented by the Arrhenius equation: Ct/C0 ln k ¼ ln k0 Ea=RT
ð7Þ
Where the C0 is the initial anthocyanin contents and the Ct is the anthocyanin contents after time t (h). t1/2 is the half life time, the k is the rate constant (min− 1), k0 is the frequency factor (min− 1), Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 J/mol K) and T the absolute temperature (Kelvin).
2.5. Kinetics model of visual color The changes in Hunter color values (a⁎, b⁎, C⁎, h° and ΔE) were modeled according to zero-order kinetics for b⁎ and h° parameters, and according to first-order degradation kinetics for the C, a⁎ and ΔE parameters (Ahmed et al., 2004; Reyes & Ciseneros-Zevallos, 2007). On this evidence, the Eq. (5) for Hunter color parameters can be rewritten as the following equation: ln ðHunter color parameterÞ=ðHunter color parameterÞ0 ¼ k t
ð8Þ b⁎orh- ¼ k t:
ð9Þ
Dependence of the Hunter color values on the anthocyanin content was described using the linear relationship as represented by the following equations: ðHunterÞ=ðHunter0 Þ ¼ k1 ð ABSt =ABS0 Þ þ k2
ð10Þ
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Table 1 Thermal degradation for anthocyanins and Hunter parameters of purple corn con at pH4.0 Index
Temperature k (h− 1) (°C)
Anthocyanins 70 80 90 C⁎ 70 80 90 a⁎ 70 80 90 ΔE 70 80 90 h° 70 80 90 b⁎ 70 80 90 a
0.0596 (0.974) 0.0773 (0.937) 0.0925 (0.962) 0.0137 (0.961) 0.0179 (0.945) 0.0325 (0.935) 0.0306 (0.982) 0.0535 (0.963) 0.0748 (0.994) 0.0153 (0.966) 0.0222 (0.945) 0.0327 (0.930) 1.694 (0.964) 2.478 (0.932) 3.925 (0.965) 0.405 (0.956) 0.491 (0.887) 0.883 (0.913)
t1/2 (h) a
11.6 9.0 7.5 50.6 38.7 21.3 22.7 13.0 9.3 45.3 31.2 21.2
Arrhenius equation Ea (kJ/mol)
k0 (h− 1)
2.60 g/kg, fresh weight) (Arozarena et al., 2002), blackberry (0.67–2.30 g/kg, fresh weight) (Wang & Xu, 2007) and Jaboticaba (0.044–0.163 g/kg, fresh weight) (Montes, Vicario, Raymundo, Fett, & Heredia 2005), the yield of anthocyanins was relatively high in purple corn cob, which made this byproduct a good source for anthocyanins.
18.3 (0.989)a 5.5E + 01
3.2. Degradation kinetics of anthocyanins 35.9 (0.984)
8.1E + 03
37.1 (0.980)
3.3E + 03
31.6 (0.999)
2.0E + 03
34.9 (0.997)
7.6E + 05
30.0 (0.933)
2.8E + 04
Numbers in parentheses are the determination coefficients (R2).
Where the Hunter0 are the initial kinds of Hunter color values and the Hunter are the final kinds of Hunter color values; the k1 and k2 are the coefficients. 2.6. Color density and polymeric color Anthocyanins color density and polymeric color content was determined during the bisulfate bleaching method (Wrolstad, 1976). Briefly, 0.4 mL of 20% K2O5S2 was added to 4.6 mL of the extract. As a control, 0.4 mL water was added to another 4.6 mL sample. Absorbance readings at 420 nm, λmax and at 700 nm (to correct for turbidity) were recorded using a spectrophotometer. This kinetic type was expressed by the following equation: Color density ðwater treated sampleÞ ¼ ½ð ABS420 ABS700 Þ þ ð ABSk max ABS700 Þ DF ð11Þ Polymeric color ðsulfite bleached sampleÞ ¼ ½ð ABS420 ABS700 Þ þ ð ABSk max ABS700 Þ DF ð12Þ
Thermal degradation of aqueous anthocyanins from purple corn cob followed first-order reaction kinetics at 70, 80 and 90 °C (Fig. 1), and Eq. (4) described well the degradation of aqueous anthocyanins over the entire temperature rage. Our results are in agreement with those from the previous studies reporting first-order reaction kinetics for degradation of anthocyanins (Culpepper, & Caldwell, 1927; Kırca, Özkan & Cemeroğlu, 2006). The coefficients of determination (R2 ) values were more than 0.937 for all cases (Table 1). The degradation rate of aqueous anthocyanins increased with increased heating temperature. As shown in Table 1, it was clear that the degradation purple corn cob anthocyanins increased as the temperature increased. The t1/2 values (Table 1) varied from 11.6 to 7.5 for pH 4.0 aqueous samples at 70, 80 and 90 °C, respectively. Wang and Xu (2007) reported that t1/2 values for anthocyanins degradation were 8.8, 4.7 and 2.9 h in blackberry at 70, 80 and 90 °C, respectively. Compared to blackberry anthocyanins, purple corn cob anthocyanins were less susceptible to high temperatures. The major anthocyanins in purple corn cob are cyanidin-3-glucoside, pelargonidin-3-glucoside, and peonidin-3-glucoside (PascualTeresa et al., 2002). However, the major anthocyanins in blackberry are caynidin-3-glucoside, and a small quantity of caynidin-3-rutinoside, caynidin-3-malonyl-glucoside (FanChiang & Wrolstad, 2005; Rommel & Wrolstad, 1992). Therefore, the different susceptibilities of natural plant anthocyanins to heating might be due to their varying anthocyanin composition. Polymeric color was calculated to determine the level of polymerization after heat treatment (Table 2). Polymeric material increased after 4 h heat at 90 °C, which increased from 20.7 to 85.1%. This result is in agreement with CevallosCasals and Cisneros-Zevallos (2004) who evaluated the polymeric color of anthocyanins from purple corn cob after 2 h heat at 98 °C. Similarly, the color density decreased from 0.775 to 0.347.
kpolymeric color ¼ ðpolymeric color=color densityÞ 100 ð13Þ Where the DF is the dilution factor. 3. Results and discussions 3.1. The content of anthocyanins in purple corn cob The content of anthocyanins from purple corn cob was calculated to be 0.680 g/kg (dry weight), expressed in cyanidin3-glucoside. Compared with other fruits, such as grape (0.25–
Table 2 Color density and polymeric color ratio of aqueous anthocyanins at pH 4.0 before and after heating Index
Anthocyanins a
Color density
Polymeric color (%)
0h
2 ha
0h
2 ha
0.775
0.347
20.7
85.1
Heating at 90 °C for 2 h.
Z. Yang et al. / Innovative Food Science and Emerging Technologies 9 (2008) 341–347 Table 3 Coefficients of Eq. (7) Index
Temperature (°C)
k1
k2
R2
C⁎
70 80 90 70 80 90 70 80 90 70 80 90 70 80 90
0.306 0.261 0.386 0.509 0.633 1.063 0.256 0.296 0.374 − 0.985 − 1.055 − 1.966 − 0.661 − 0.539 − 1.082
0.709 0.741 0.610 0.483 0.345 0.019 0.737 0.698 0.617 2.015 2.122 2.902 1.689 1.598 2.112
0.954 0.909 0.995 0.978 0.860 0.997 0.967 0.940 0.980 0.906 0.828 0.999 0.882 0.735 0.966
a⁎
ΔE
h° b⁎
3.3. Degradation kinetics of visual color Since the Hunter a⁎ and b⁎ parameters represent the redness and yellowness on the chromaticity dimension, respectively, while the C⁎ and h° parameters represent the combinations of the Hunter a⁎ and b⁎ parameters, and the ΔE represent the total color change with the white plate. There were obvious changes in Hunter color (a⁎, b⁎, C⁎, h° and ΔE) values, confirming the degradation of visual color
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attributes in the aqueous anthocyanins (Fig. 1). During heating of aqueous anthocyanins it was observed that the C⁎, a⁎ and ΔE values decreased through time following a first-order kinetics, while b⁎ and h° parameters decreased following zeroorder reaction model. Eq. (7) described adequately the degradation rate constants of the Hunter color (a⁎, C⁎ and ΔE) parameters of aqueous anthocyanins of purple corn cob over the entire temperature rage, respectively, and Eq. 9 also described adequately the rate constants for Hunter h° and a⁎ parameters (Fig. 1). The R2 values were more than 0.887 for all cases (Table 1). The change for a⁎ and h° values would be related to the degradation of redness and monomeric of anthocyanins, while the increasing b⁎ would be associated the formation of yellow chalcone species. In most degradation studies, increase in h° is used as an indicator for the degradation of anthocyanins (Cevallos-Casals & Cisneros-Zevallos, 2004; Reyes & Ciseneros-Zevallos, 2007). The t1/2 values decreased as the temperature increased, and the decrease of t1/2 values were: from 50.6 to 21.3 h, 22.7 to 9.3 h and 45.3 to 21.2 h for C⁎, a⁎ and ΔE, respectively, at temperature from 70 to 90 °C (Table 1). In this study, these Hunter color parameters (C⁎, a⁎ and ΔE) were subjected to linear regression with respect to time as represented by Eq. (8) and the coefficients were determined. Correlation coefficient values were used as the basis to select the Hunter color parameters which best described the first-order reaction for the entire temperature range. It was found that all
Fig. 2. Correlation between anthocyanins and different Hunter color parameters (ΔE, C⁎, h° and b⁎) of purple corn cob. ΔE = total color change (A); C⁎ = chroma (B); h° = Hue angle (C); b⁎ = yellowness (D).
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the three Hunter color parameters (C⁎, a⁎ and ΔE) were described closely to the first-order reaction kinetics of color degradation of aqueous anthocyanins from purple corn cob (Fig. 1). The R2 values were more than 0.930. Some researchers have reported similar observations while working on the heating of anthocyanins (Ahmed et al., 2004; Reyes, & CisenerosZevallos, 2007; Shin & Bhowmich, 1995). Based on these experimental results, all the Hunter color parameters found should have C⁎, a⁎ and ΔE as the appropriate Hunter color parameters to describe the color degradation of aqueous anthocyanins from purple corn cob during thermal processing. 3.4. Relationship between visual color and anthocyanins During heating of aqueous anthocyanins it was observed that the C⁎, a⁎ and ΔE values decreased through time, while b⁎ and h° values increased. Dependence of the Hunter color values on the anthocyanin content was described using the linear relationship as presented by Eq. (10). As shown in Table 3, the degradation of anthocyanins showed high and positive correlation with C⁎ (R2 N 0.909), a⁎ (R2 N 0.860) and ΔE (R 2 N 0.940), while the degradation of anthocyanins showed high and negative correlation with h° (R2 N 0.828) and b⁎ (R2 N 0.735). Anthocyanin pigments, being most heat sensitive, may preferably be used as an index of food product quality. The excellent linear correlation between Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) and anthocyanins (Table 3) inferred that the Hunter color parameters may also be used instead of anthocyanins. The advantage of using the visual Hunter color parameters is that it may be measured as an on-line quality control parameters during heating of food industry. 3.5. Effect of temperature on the rate constant The effects of temperature on the anthocyanins and Hunter color parameters are shown in Table 1. Dependence of the rate constant (anthocyanins and all the Hunter color parameters) on the temperature obeyed the Arrhenius relationship (Eq. (7)). The determination coefficients (R2) for the linear regression analysis were 0.984, 0.980, 0.999, 0.997 and 0.933 respectively for anthocyanins, C⁎, a⁎, ΔE, h° and b⁎ parameters. The calculated values of activation energies (Ea) were 18.3, 35.9, 37.1, 31.6, 34.9 and 30.0 kJ/mol for anthocyanins, C⁎, a⁎, ΔE, h° and b⁎ parameters respectively. The higher Ea indicated that greater temperature sensitivity of visual color as compared to anthocyanins, and the following descending order of heat sensitivity for each parameter: a⁎ ≥ C⁎ ≥ ΔE ≥ anthocyanins Fig. 2. 4. Conclusions The yield of anthocyanins in purple corn cob indicated that the purple corn cob was an important source of anthocyanin pigments. The degradation of anthocyanins during heating followed first-order reaction kinetics, while the changes in Hunter color values were modeled according to zero-order
kinetics for b⁎ and h parameters, and according to first-order degradation kinetics for the C⁎, a⁎ and ΔE parameters. The degradation rate of aqueous anthocyanins and Hunter color parameters increased as the temperature increased. The degradation of anthocyanins showed positive correlation with C⁎, a⁎ and ΔE, while the degradation of anthocyanins showed negative correlation with h° and b⁎. References Ahmed, J., Shivhare, U. S., & Raghavan, G. S. V. (2004). Thermal degradation kinetics of anthocyanin and visual colour of plum puree. European Food Research and Technology, 218, 525−528. Arozarena, I., Ayestarán, B., Cantalejo, M. J., Navarro, M., Vera, M., Abril, I., et al. (2002). Anthocyanin composition of Tempranillo, Garnacha and Cabernet Sauvignon grapes from high- and low quality vineyards over two years. European Food Research and Technology, 214(4), 303−309. Bomser, J., Madhavi, D. L., Singletary, K., & Smith, M. A. (1996). In vitro anticancer activity of fruit extracts from Vaccinium species. Planta Medica, 62, 212−216. Bridle, P., & Timberlake, C. F. (1997). Anthocyanins as natural food colours— Selected aspects. Food Chemistry, 58(1–2), 103−109. Cevallos-Casals, B. A., & Cisneros-Zevallos, L. (2003). Stoichiometric and kinetic studies of phenolic antioxidants from Andean purple corn and redfleshed sweet potato. Journal of Agricultural and Food Chemistry, 51, 3313−3319. Cevallos-Casals, B. A., & Cisneros-Zevallos, L. A. (2003). A comparative study of antimicrobial, antimutagenic and antioxidant activity of phenolic compounds from purple corn and bilberry colorant extracts. Book of Abstracts of the Institute of Food Technologists Technical Program Abstracts (pp. 29). Chicago, IL: Poster 14E-1. Cevallos-Casals, B. A., & Cisneros-Zevallos, L. (2004). Stability of anthocyanin-based aqueous extracts of Andean purple corn and redfleshed sweet potato compared to synthetic and natural colorants. Food Chemistry, 86, 69−77. Culpepper, C. W., & Caldwell, J. S. (1927). The behavior of the anthocyanin pigments in canning. Journal of Agriculture and Research, 35(2), 107−132. Duhard, V., Garner, J., & Megard, D. (1997). Comparison of the stability of selected anthocyanin colorants in drink model systems. Agro Food Industry High Technology, 8, 28−34. Fan-Chiang, H. J., & Wrolstad, R. E (2005). Anthocyanins pigment composition of blackberries. Journal of Food Science, 70(3), C198−C202. Garzon, G. A., & Wrolstad, R. E. (2002). Comparison of the stability of pelargonidin-based anthocyanins in strawberry juice and concentrate. Journal of Food Science, 67(5), 1288−1299. Giusti, M. M., & Worsltad, R. E. (2001). Characterization and measurement of anthocyanins by UV–visible spectroscopy. Current protocols in food analytical chemistry. New York: Wiley. Hagiwara, A., Miyashita, K., Nakanishi, T., Sano, M., Tamano, S., Kadota, T., et al. (2001). Pronounced inhibition by a natural anthocyanin, purple corn color, of 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP)associated colorectal carcinogenesis in male F344 rats pretreated with 1,2-dimethylhydrazine. Cancer Letters, 171, 17−25. Kähkönen, M. P., & Heinoner, M. (2003). Antioxidant activity of anthocyanins and their aglycons. Journal of Agricultural and Food Chemistry, 51, 628−633. Katsube, N., Iwashita, K., Tsushida, T., Yamaki, K., & Kobori, M. (2003). Induction of apoptosis in cancer cells by bilberry (Vaccinium myrtillus) and the anthocyanins. Journal of Agricultural and Food Chemistry, 51, 68−75. Kırca, A., Özkan, M., & Cemeroğlu, B. (2006). Stability of black carrot anthocyanins in various fruit juices and nectars. Food Chemistry, 97, 598−605. Markakis, P. (1982). Anthocyanins as food additives. In P. Markakis (Ed.), Anthocyanins as food colors (pp. 245−253). New York: Academic Press. McGuire, R. G. (1992). Reporting of objective color measurements. HortScience, 27, 1254−1255.
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Innovative Food Science and Emerging Technologies 9 (2008) 341 – 347 www.elsevier.com/locate/ifset
Thermal degradation kinetics of aqueous anthocyanins and visual color of purple corn (Zea mays L.) cob Zhendong Yang, Yonbin Han, Zhenxin Gu ⁎, Gongjian Fan, Zhigang Chen College of food science and technology, Nanjing Agriculture University, Nanjing, 210095, China Received 1 July 2007; accepted 7 September 2007
Abstract Purple corn cob was the byproduct during the corn processing. Thermal degradation kinetics and Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) of aqueous anthocyanins from purple corn cob were studied at selected temperatures (70 °C, 80 °C and 90 °C) at pH 4.0. The results indicated that the thermal degradation of anthocyanin and Hunter color C⁎, a⁎ and ΔE parameters followed the first-order reaction kinetics, while Hunter color h° and b⁎ parameters followed zero-order reaction kinetics. The calculated values of activation energies (Ea) were 18.3, 35.9, 37.1, 31.6, 34.9 and 30.0 kJ/mol for anthocyanins, C⁎, a⁎, ΔE, h° and b⁎ parameters, respectively. The higher Ea indicated that greater temperature sensitivity of visual color as compared to anthocyanins content. The degradation of anthocyanins showed positive correlation with C⁎ (R2 N 0.909), a⁎ (R2 N 0.860) and ΔE (R2 N 0.940), while the degradation of anthocyanins showed negative correlation with h° (R2 N 0.828) and b⁎ (R2 N 0.735) during heating. © 2007 Elsevier Ltd. All rights reserved. Keywords: Purple corn cob; Anthocyanins; Hunter color parameters; Kinetics
Industrial relevance: Purple corn cob was the byproduct during the corn processing. Purple corn cob is dark purple to almost black color due to its high content of anthocyanins, which makes this byproduct a good source of anthocyanins. In this study, the excellent linear correlation between Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) and content of anthocyanins showed that the Hunter color parameters may also be used instead of anthocyanins content during heating. The advantage of using the visual Hunter color parameters may be measured as on-line quality control parameters during thermal processing of food industry.
1. Introduction There are various kinds of corn in the world, and the corn has various colors such as white, yellow, red, purple, brown, green and blue. Purple corn is a pigmented variety of Zea mays L., originally cultivated in Latin America and was introduced in China a long time ago. This kind of corn is mainly grown in China, especially in Shanxi and Anhui province. As the byproduct during corn processing, purple corn cob was often neglected in the production. However, it was discovered that the cob contains rich anthocyanins, such as
⁎ Corresponding author. Tel./fax: +86 25 84396293. E-mail address: [email protected] (Z. Gu). 1466-8564/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2007.09.001
cyanidin-3-glucoside, pelargonidin-3-glucoside, and peonidin3-glucoside (Pascual-Teresa, Santos-Buelga, & Rivas-Gonzalo, 2002). Obviously purple corn cobs can be used as a source of natural colorant. Recently, anthocyanins have been reported to have various biological activities, such as antioxidant, antimutagenic and anticancer activities (Bomser, Madhavi, Singletary, & Smith, 1996; Yoshimoto, Okuno, Yamaguchi & Yamakawa, 2001; Kähkönen & Heinoner, 2003; Katsube, Iwashita, Tsushida, Yamaki, & Kobori, 2003). Especially, most of the properties attributed to purple corn extracts, including coloring attributes (Duhard, Garner, & Megard, 1997; Cevallos-Casals & Cisneros-Zevallos, 2004), antioxidant (Cevallos-Casals & Cisneros-Zevallos, 2003a), antimicrobial (Cevallos-Casals & Cisneros-Zevallos, 2003b), anti-obesity activity and amelioration of hyperglycemia (Tsuda, Horio,
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Uchida, Aoki & Osawa, 2003), and anticarcinogenic properties (Hagiwara et al., 2001) were related to anthocyanins. Color is an important sensory property in determining product quality, therefore minimizing the pigment losses during processing is of primary concern to the processor (Markakis, 1982; Bridle & Timberlake, 1997). Total monomeric anthocyanin measurement is determined by pH-differential method outlined by Giusti and Worsltad (2001). This method is a spectrophotometric measurement. Results from reflectance color measurements are often correlated with the chemical measurements. Reflectance color measurements are easy and cheap, but it is usually recommended to follow the color changes during food processing. Detailed color measurements including total monomeric anthocyanin analysis together with color analysis are needed. On the other hand, the visual color, which is an indicator of pigment concentration, can be measured instantaneously using tristimulus colorimeters for on-line quality control (Rocha, Lebart & Marty-Audouin, 1993; Ahmed, Shivhare & Raghavan, 2004). The color degradation kinetics of food products are a complex phenomenon and dependable models to predict experimental color change, which can be used in engineering calculations, are limited. However, empirical mathematical modeling techniques may be used to determine the end point and kinetic effect. The kinetic parameters namely, reaction order, rate constant and activation energy provide useful information on the quality changes which occur during heating. Although color can be reported in different systems, lightness (L⁎), chroma (C⁎), and hue angle (h°) parameters were used since the most common L⁎ a⁎ b⁎ coordinates do not express hue and chroma directly and difficult to interpret independently (McGuire, 1992). So, C⁎, and h° may preferably be used as indexes of food product quality. Compared to synthetic colorants, anthocyanin pigments are more susceptible to heating. Current studies on anthocyanins from purple corn were focused on the stability structure, and physiological functionality (Hagiwara et al., 2001; Pascual-Teresa et al., 2002; Tsuda et al., 2003; Cevallos-Casals & Cisneros-Zevallos, 2004). However, studies on the degradation kinetics of anthocyanins and color of purple corn cob are lacking; the effect of the two different color systems on the evaluation of degradation kinetics of anthocyanins and the resulting changes in visual color has not been reported. The objective of this study was to investigate: (1) the degradation of anthocyanins during heating, (2) the degradation of visual color using Hunter tristimulus parameters (a⁎, b⁎, C⁎, h° and ΔE), and (3) a relationship between visual color and aqueous anthocyanins from purple corn cob during heating.
disintegrator (FSD-100A, China) and sifted through a 100 mesh sieve. All other reagents were of analytical grade. 2.2. Preparation and determination of aqueous anthocyanin extracts About 100 g (powder of purple corn cob) were macerated with 400 mL of methanol for 24 h in the dark at 4 °C. The crude extract obtained was filtered through a filter paper and the remaining residues were washed with 200 mL of methanol, and then the crude extract filtrate was collected. The methanol in the crude extract was evaporated by the rotary evaporator (0.1 MPa, 35 °C). The remaining solvents were purified by macroporous resin LSA-10 (Lanxiao Company, China). The eluate was concentrated and evaporated, and then the solvent was freezedried to obtain the red powder. The red powder was dissolved with 200 mL nanopure water, which was done as the stock solution of anthocyanins. An aliquot of the stock solution of anthocyanins was diluted with 0.2 M citrate buffer to pH 4.0. The quantification of total monomeric anthocyanins was determined by pH-differential methods (Giusti and Worsltad, 2001). Total anthocyanins were calculated as caynidin-3glucoside according to the following equation: Total anthocyanins ðmg=LÞ ¼ A MW DF 1000=ðe 1Þ ð1Þ Where A = (A510 − A700) pH 1.0 − (A510 − A700) pH 4.5; MW (molecular weight) = 449.2 g/mol for caynidin-3-glucoside; DF = dilution factor; 1 = pathlength in cm; e = 26,900 molar extinction coefficient in L/mol/cm for caynidin-3-glucoside; 1000 = conversion from g to mg all analyses were done in triplicate (n = 3). 2.3. Measurement of visual color CIELAB parameters were determined using D-65 diffused illumination of a Minolta Chrama CR-400 Colorimeter. The instrument was calibrated with a standard white plate. The measured parameters were L⁎ (lightness), a⁎ (redness), and b⁎ (yellowness). The three measured color parameters were converted into C⁎ (chroma), h° (Hue angle), and ΔE (total color change) using the following equations: h i1=2 2 2 C ⁎ ¼ ð a⁎Þ þ ð b⁎Þ ð2Þ h- ¼ arctanðb⁎=a⁎Þ
ð3Þ
2. Materials and methods
h i1=2 2 2 2 DE ¼ ð DL⁎Þ þðDa⁎Þ þðb⁎Þ :
ð4Þ
2.1. Materials
2.4. Kinetics model of aqueous anthocyanin
Purple corn cob (Z. mays L.) was generously supplied by Professor Zhong Zhang in Anhwei Technical Teachers College (Fengyang city, Anhwei province, China). The cob was dried in a heated air drier (ZT-3, China) at 50 °C, pulverized by the
After dissolving freeze-dried samples in water, the resulting extracts were heated at 70, 80 and 90 °C in a water bath. Samples were drawn at 1 h intervals, and then analyzed for color and anthocyanin content. Previous studies showed that heating
Z. Yang et al. / Innovative Food Science and Emerging Technologies 9 (2008) 341–347
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Fig. 1. The kinetics of anthocyanins and Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) of purple corn cob at pH 4.0. C = anthocyanins concentration (A); a⁎ = redness (B); C⁎ = chroma (C); ΔE = total color change (D); b⁎ = yellowness (E); h° = Hue angle (F).
degradation of anthocyanins followed at first-order reaction (Garzon & Wrolstad, 2002; Cevallos-Casals, & CisnerosZevallos, 2003a; Cevallos-Casals, & Cisneros-Zevallos, 2004). This kinetic type was expressed by the following equation: ln ðCt =C0 Þ ¼ k t
ð5Þ
t1=2 ¼ ln 0:5 k 1 :
ð6Þ
Dependence of the degradation rate constant on temperature is represented by the Arrhenius equation: Ct/C0 ln k ¼ ln k0 Ea=RT
ð7Þ
Where the C0 is the initial anthocyanin contents and the Ct is the anthocyanin contents after time t (h). t1/2 is the half life time, the k is the rate constant (min− 1), k0 is the frequency factor (min− 1), Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 J/mol K) and T the absolute temperature (Kelvin).
2.5. Kinetics model of visual color The changes in Hunter color values (a⁎, b⁎, C⁎, h° and ΔE) were modeled according to zero-order kinetics for b⁎ and h° parameters, and according to first-order degradation kinetics for the C, a⁎ and ΔE parameters (Ahmed et al., 2004; Reyes & Ciseneros-Zevallos, 2007). On this evidence, the Eq. (5) for Hunter color parameters can be rewritten as the following equation: ln ðHunter color parameterÞ=ðHunter color parameterÞ0 ¼ k t
ð8Þ b⁎orh- ¼ k t:
ð9Þ
Dependence of the Hunter color values on the anthocyanin content was described using the linear relationship as represented by the following equations: ðHunterÞ=ðHunter0 Þ ¼ k1 ð ABSt =ABS0 Þ þ k2
ð10Þ
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Table 1 Thermal degradation for anthocyanins and Hunter parameters of purple corn con at pH4.0 Index
Temperature k (h− 1) (°C)
Anthocyanins 70 80 90 C⁎ 70 80 90 a⁎ 70 80 90 ΔE 70 80 90 h° 70 80 90 b⁎ 70 80 90 a
0.0596 (0.974) 0.0773 (0.937) 0.0925 (0.962) 0.0137 (0.961) 0.0179 (0.945) 0.0325 (0.935) 0.0306 (0.982) 0.0535 (0.963) 0.0748 (0.994) 0.0153 (0.966) 0.0222 (0.945) 0.0327 (0.930) 1.694 (0.964) 2.478 (0.932) 3.925 (0.965) 0.405 (0.956) 0.491 (0.887) 0.883 (0.913)
t1/2 (h) a
11.6 9.0 7.5 50.6 38.7 21.3 22.7 13.0 9.3 45.3 31.2 21.2
Arrhenius equation Ea (kJ/mol)
k0 (h− 1)
2.60 g/kg, fresh weight) (Arozarena et al., 2002), blackberry (0.67–2.30 g/kg, fresh weight) (Wang & Xu, 2007) and Jaboticaba (0.044–0.163 g/kg, fresh weight) (Montes, Vicario, Raymundo, Fett, & Heredia 2005), the yield of anthocyanins was relatively high in purple corn cob, which made this byproduct a good source for anthocyanins.
18.3 (0.989)a 5.5E + 01
3.2. Degradation kinetics of anthocyanins 35.9 (0.984)
8.1E + 03
37.1 (0.980)
3.3E + 03
31.6 (0.999)
2.0E + 03
34.9 (0.997)
7.6E + 05
30.0 (0.933)
2.8E + 04
Numbers in parentheses are the determination coefficients (R2).
Where the Hunter0 are the initial kinds of Hunter color values and the Hunter are the final kinds of Hunter color values; the k1 and k2 are the coefficients. 2.6. Color density and polymeric color Anthocyanins color density and polymeric color content was determined during the bisulfate bleaching method (Wrolstad, 1976). Briefly, 0.4 mL of 20% K2O5S2 was added to 4.6 mL of the extract. As a control, 0.4 mL water was added to another 4.6 mL sample. Absorbance readings at 420 nm, λmax and at 700 nm (to correct for turbidity) were recorded using a spectrophotometer. This kinetic type was expressed by the following equation: Color density ðwater treated sampleÞ ¼ ½ð ABS420 ABS700 Þ þ ð ABSk max ABS700 Þ DF ð11Þ Polymeric color ðsulfite bleached sampleÞ ¼ ½ð ABS420 ABS700 Þ þ ð ABSk max ABS700 Þ DF ð12Þ
Thermal degradation of aqueous anthocyanins from purple corn cob followed first-order reaction kinetics at 70, 80 and 90 °C (Fig. 1), and Eq. (4) described well the degradation of aqueous anthocyanins over the entire temperature rage. Our results are in agreement with those from the previous studies reporting first-order reaction kinetics for degradation of anthocyanins (Culpepper, & Caldwell, 1927; Kırca, Özkan & Cemeroğlu, 2006). The coefficients of determination (R2 ) values were more than 0.937 for all cases (Table 1). The degradation rate of aqueous anthocyanins increased with increased heating temperature. As shown in Table 1, it was clear that the degradation purple corn cob anthocyanins increased as the temperature increased. The t1/2 values (Table 1) varied from 11.6 to 7.5 for pH 4.0 aqueous samples at 70, 80 and 90 °C, respectively. Wang and Xu (2007) reported that t1/2 values for anthocyanins degradation were 8.8, 4.7 and 2.9 h in blackberry at 70, 80 and 90 °C, respectively. Compared to blackberry anthocyanins, purple corn cob anthocyanins were less susceptible to high temperatures. The major anthocyanins in purple corn cob are cyanidin-3-glucoside, pelargonidin-3-glucoside, and peonidin-3-glucoside (PascualTeresa et al., 2002). However, the major anthocyanins in blackberry are caynidin-3-glucoside, and a small quantity of caynidin-3-rutinoside, caynidin-3-malonyl-glucoside (FanChiang & Wrolstad, 2005; Rommel & Wrolstad, 1992). Therefore, the different susceptibilities of natural plant anthocyanins to heating might be due to their varying anthocyanin composition. Polymeric color was calculated to determine the level of polymerization after heat treatment (Table 2). Polymeric material increased after 4 h heat at 90 °C, which increased from 20.7 to 85.1%. This result is in agreement with CevallosCasals and Cisneros-Zevallos (2004) who evaluated the polymeric color of anthocyanins from purple corn cob after 2 h heat at 98 °C. Similarly, the color density decreased from 0.775 to 0.347.
kpolymeric color ¼ ðpolymeric color=color densityÞ 100 ð13Þ Where the DF is the dilution factor. 3. Results and discussions 3.1. The content of anthocyanins in purple corn cob The content of anthocyanins from purple corn cob was calculated to be 0.680 g/kg (dry weight), expressed in cyanidin3-glucoside. Compared with other fruits, such as grape (0.25–
Table 2 Color density and polymeric color ratio of aqueous anthocyanins at pH 4.0 before and after heating Index
Anthocyanins a
Color density
Polymeric color (%)
0h
2 ha
0h
2 ha
0.775
0.347
20.7
85.1
Heating at 90 °C for 2 h.
Z. Yang et al. / Innovative Food Science and Emerging Technologies 9 (2008) 341–347 Table 3 Coefficients of Eq. (7) Index
Temperature (°C)
k1
k2
R2
C⁎
70 80 90 70 80 90 70 80 90 70 80 90 70 80 90
0.306 0.261 0.386 0.509 0.633 1.063 0.256 0.296 0.374 − 0.985 − 1.055 − 1.966 − 0.661 − 0.539 − 1.082
0.709 0.741 0.610 0.483 0.345 0.019 0.737 0.698 0.617 2.015 2.122 2.902 1.689 1.598 2.112
0.954 0.909 0.995 0.978 0.860 0.997 0.967 0.940 0.980 0.906 0.828 0.999 0.882 0.735 0.966
a⁎
ΔE
h° b⁎
3.3. Degradation kinetics of visual color Since the Hunter a⁎ and b⁎ parameters represent the redness and yellowness on the chromaticity dimension, respectively, while the C⁎ and h° parameters represent the combinations of the Hunter a⁎ and b⁎ parameters, and the ΔE represent the total color change with the white plate. There were obvious changes in Hunter color (a⁎, b⁎, C⁎, h° and ΔE) values, confirming the degradation of visual color
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attributes in the aqueous anthocyanins (Fig. 1). During heating of aqueous anthocyanins it was observed that the C⁎, a⁎ and ΔE values decreased through time following a first-order kinetics, while b⁎ and h° parameters decreased following zeroorder reaction model. Eq. (7) described adequately the degradation rate constants of the Hunter color (a⁎, C⁎ and ΔE) parameters of aqueous anthocyanins of purple corn cob over the entire temperature rage, respectively, and Eq. 9 also described adequately the rate constants for Hunter h° and a⁎ parameters (Fig. 1). The R2 values were more than 0.887 for all cases (Table 1). The change for a⁎ and h° values would be related to the degradation of redness and monomeric of anthocyanins, while the increasing b⁎ would be associated the formation of yellow chalcone species. In most degradation studies, increase in h° is used as an indicator for the degradation of anthocyanins (Cevallos-Casals & Cisneros-Zevallos, 2004; Reyes & Ciseneros-Zevallos, 2007). The t1/2 values decreased as the temperature increased, and the decrease of t1/2 values were: from 50.6 to 21.3 h, 22.7 to 9.3 h and 45.3 to 21.2 h for C⁎, a⁎ and ΔE, respectively, at temperature from 70 to 90 °C (Table 1). In this study, these Hunter color parameters (C⁎, a⁎ and ΔE) were subjected to linear regression with respect to time as represented by Eq. (8) and the coefficients were determined. Correlation coefficient values were used as the basis to select the Hunter color parameters which best described the first-order reaction for the entire temperature range. It was found that all
Fig. 2. Correlation between anthocyanins and different Hunter color parameters (ΔE, C⁎, h° and b⁎) of purple corn cob. ΔE = total color change (A); C⁎ = chroma (B); h° = Hue angle (C); b⁎ = yellowness (D).
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the three Hunter color parameters (C⁎, a⁎ and ΔE) were described closely to the first-order reaction kinetics of color degradation of aqueous anthocyanins from purple corn cob (Fig. 1). The R2 values were more than 0.930. Some researchers have reported similar observations while working on the heating of anthocyanins (Ahmed et al., 2004; Reyes, & CisenerosZevallos, 2007; Shin & Bhowmich, 1995). Based on these experimental results, all the Hunter color parameters found should have C⁎, a⁎ and ΔE as the appropriate Hunter color parameters to describe the color degradation of aqueous anthocyanins from purple corn cob during thermal processing. 3.4. Relationship between visual color and anthocyanins During heating of aqueous anthocyanins it was observed that the C⁎, a⁎ and ΔE values decreased through time, while b⁎ and h° values increased. Dependence of the Hunter color values on the anthocyanin content was described using the linear relationship as presented by Eq. (10). As shown in Table 3, the degradation of anthocyanins showed high and positive correlation with C⁎ (R2 N 0.909), a⁎ (R2 N 0.860) and ΔE (R 2 N 0.940), while the degradation of anthocyanins showed high and negative correlation with h° (R2 N 0.828) and b⁎ (R2 N 0.735). Anthocyanin pigments, being most heat sensitive, may preferably be used as an index of food product quality. The excellent linear correlation between Hunter color parameters (a⁎, b⁎, C⁎, h° and ΔE) and anthocyanins (Table 3) inferred that the Hunter color parameters may also be used instead of anthocyanins. The advantage of using the visual Hunter color parameters is that it may be measured as an on-line quality control parameters during heating of food industry. 3.5. Effect of temperature on the rate constant The effects of temperature on the anthocyanins and Hunter color parameters are shown in Table 1. Dependence of the rate constant (anthocyanins and all the Hunter color parameters) on the temperature obeyed the Arrhenius relationship (Eq. (7)). The determination coefficients (R2) for the linear regression analysis were 0.984, 0.980, 0.999, 0.997 and 0.933 respectively for anthocyanins, C⁎, a⁎, ΔE, h° and b⁎ parameters. The calculated values of activation energies (Ea) were 18.3, 35.9, 37.1, 31.6, 34.9 and 30.0 kJ/mol for anthocyanins, C⁎, a⁎, ΔE, h° and b⁎ parameters respectively. The higher Ea indicated that greater temperature sensitivity of visual color as compared to anthocyanins, and the following descending order of heat sensitivity for each parameter: a⁎ ≥ C⁎ ≥ ΔE ≥ anthocyanins Fig. 2. 4. Conclusions The yield of anthocyanins in purple corn cob indicated that the purple corn cob was an important source of anthocyanin pigments. The degradation of anthocyanins during heating followed first-order reaction kinetics, while the changes in Hunter color values were modeled according to zero-order
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