Research Article
Color Difference and Acrylamide Content of Cooked Food
Deparment of Foods and Nutrition, Keimyung University, 1000, Sin-dang dong, Dal-suh gu, Dae-gu, Korea, 704-701
Ever since Acrylamide (AA) was detected in foods, the chemical mechanism for its synthesis in food has been studied. Among the many hypotheses for AA formation, the Maillard browning reaction involving free amino acids and reducing sugars under high temperature has received much attention (Mottram et al., 2002; Stadler et al., 2002; 2004). Relevant variables and parameters influencing AA formation processing have been considered (Pedreschi et al., 2004; Trystram, 2004; Matthaus et al., 2004; Levine and Smith, 2005) and raw materials containing asparagines have been proposed as a major factor influencing the formation of AA by the Maillard reaction (Mottram et al., 2002; Stadler et al., 2002; Yaylayan et al., 2003; Stadler et al., 2004; Ehling and Shibamoto, 2005). The browning color of food has also been considered as a parameter of AA content (Levine and Smith, 2005). A clear correlation between AA formation and browning color was obtained from the model system of asparagines and glucose (Ehling and Shibamoto, 2005), yeast-leavened wheat bread (Surdyk et al., 2004), rye crispbread (Mustafa et al., 2005) and potato chips (Pedreschi et al., 2005). However, the addition of asparagines increased AA content but did not affect color (Surdyk et al., 2004; Ehling and Shibamoto, 2005).
Many studies (Surdyk et al., 2004; Levine and Smith, 2005; Ehling and Shibamoto, 2005; Mustafa et al., 2005; Pedreschi et al., 2005) have found a correlation between browning color and AA formation in laboratory-processed food products. However, although commercial roasted food products, with their browning color and roasting flavor, are very popular in Korea, they have not been studied. Therefore, the aim of this study was to investigate the relation between color difference and AA content of market-purchased roasted cereals and French fries and yakwa (traditional Korean fried wheat flour dough mixed with honey) processed with a standard recipe and controlled cooking conditions.
Materials
Different brands of roasted barley and corn were purchased at the local market from November 2003 to May 2004. Raw potatoes (Sun-nong, Young Wall, Korea), wheat flour (soft wheat, Baksul Pyo, Seoul, Korea), honey (Dong Suh, Seoul, Korea), corn syrup (corn starch, over 55% maltose, Haechandle, Seoul, Korea), sesame oil (100% sesame, Haepyo, Seoul, Korea) and frying oil (100% soybean, Baksul Pyo, Seoul, Korea) were also purchased at the local market. French fries and yakwa were prepared at the laboratory as described in previous research (Koh, 2006).
Determination of Color Difference and Acrylamide Content
Color was monitored using a chromometer (CM-3500D, Minolta Co., Osaka, Japan, calibrated at L = 98.7). L, a and b are chromaticity coordinates, where L = the lightness of the color, positive a = red, negative a = green, positive b = yellow and negative b = blue. Three readings (L, a and b) were taken of the surface of each sample in five replicates. French fries and yakwa were cooled to room temperature for 2 h and then the color was measured. The AA content was determined and reported at the previous research (Koh, 2006).
Statistical Analyses of Data
All statistical analyses were performed using the SAS version 8.02 (SAS, 1990). Pearson correlation coefficient analysis was used to investigate the relation between AA content and the color difference of the samples. Generalized Linear Model (GLM) analysis was conducted to analyze the effect of each cooking factor on the amount of AA and the color of the French fries.
The data on AA contents have been cited in a previous report (Koh, 2006) to investigate the relation between color difference and AA content. Compared with roasted barley kernels, roasted barley powder for tea bags had lower Lightness (L) and AA content. For the roasted corn, a similar pattern was observed (Table 1). That is, tea bag products of both corn and barley had darker colors (low lightness and high yellowness) and lower AA contents than did the roasted whole kernel products.
The experiment was chosen to test the effect of frying time and frying temperature on AA content and color difference. The lower and upper levels in the experiment were 160°C for 4 min and 190°C for 8 min, respectively (Table 2). At the lowest temperature and shortest cooking time, a small amount of AA (73.64 μg kg-1) was detected and the highest AA content (2610.31 μg kg-1) was found in the French fries fried at 190°C for 8 min. As the frying time and temperature increased, both AA content and browning color increased. Lightness (L) was decreased and redness (a) and yellowness (b) was increased. Pedreschi et al. (2005) also observed a linear correlation between AA content and color in potato chips.
All experimental yakwa were of acceptable eating quality, but the surface color varied from rather light to dark after the addition of reducing sugar (Table 3). The sugar-added yakwa I and II were fried at the lower temperature and shorter time and the color changed significantly and the amount of AA was lower. Yakwa III, which did not contain sugar, was fried longer to develop a brown color and it contained more AA (294.49 μg kg-1).
Table 1: | Color difference of roasted barley and corn |
L: lightness, a: redness, b: yellowness; L, a and b-values are mean±standard deviation of five replicates. ND: Not Detectable |
Table 2: | Color difference of French fries |
L: lightness, a: redness, b: yellowness; L, a and b values are mean ± standard deviation of five replicates |
Table 3: | Color difference of yakwa |
I: Honey added; II: Corn syrup added; III: No sugar added.L: Lightness, a: Redness, b: Yellowness; L, a and b-values are mean±standard deviation of five replicates |
These results clearly show that, for yakwa, reducing sugars, such as honey and corn syrup, are mainly involved in the browning reaction and the frying conditions are more important for the AA content. Surdyk et al. (2004) also reported that the addition of fructose was effective in developing brown color, but it did not influence the AA content of yeasted leavened bread.
Table 5: | Pearson correlation coefficient (r) of AA contents with color (L, a and b) |
L: Lightness, a: Redness, b: Yellowness |
Table 5: | p-values of GLM analysis of time and temperature of frying and their interaction on AA content and color difference of French fries |
L: Lightness, a: Redness, b: Yellowness; L, a and b values are mean ± standard deviation of five replicates |
Correlation coefficients for AA with L, a and b values were not significant (p>0.01) for market-purchased roasted barley and yakwa, but were significant (p<0.001) for French fries (Table 4). These results indicate that the browning of market-purchased roasted corn and barley cannot be an indicator of AA content because of other influences due to differences in raw materials and cooking conditions on AA content. However, a strong correlation between color difference and AA content was found when the potatoes were fried with the same recipe but with different frying times and temperatures.
GLM analysis of French fries (Table 5) showed the significant effect of temperature and time on color difference and AA content. The interaction between time and temperature was significant for AA content but was not significant for color difference. This linear relation between time and temperature has been found in the other studies (Pedreschi et al., 2004; Surdyk et al., 2004).
The significance of the correlation between color difference and AA amount depended on the food. Under the controlled system, the correlation between color difference and the amount of AA was highly significant. However, market-purchased foods did not show a significant correlation between color and AA amount. Even under the controlled system, the addition of reducing sugar enhanced the browning color, but decreased the amount of AA because of the short frying time. Thus, the results indicate that it was hard to estimate the amount of AA from the browning color of roasted food without considering the recipe, processing conditions and raw materials.
This research was supported by the Korea Research Foundation Grant funded by the Korean government grant number R04-2003-000-10116-0.