RS and free asparagine in SP roots
Table 1 shows that the moisture content in sweet potatoes was the highest, followed by carbohydrates, crude protein, crude fiber, ash, and crude fat. The results imply that sweet potatoes are another carbohydrate-rich food source (Lebot 2010). A preliminary sugars analysis of this study variety has been done and we observed no existence of maltose in raw SP roots. Hence, maltose was used as the internal standard for SP sugars analysis. Furthermore, literature reported that maltose was formed in cooked SP roots and not found in raw SP roots (Picha 1985). Sucrose, glucose, and fructose are the carbohydrates that occur naturally in sweet potatoes (Picha 1986). The sucrose concentration in raw sweet potatoes was 52 mg/g, whereas those of glucose and fructose were 4.17 and 5.05 mg/g (on a dry weight basis), respectively. The high concentrations of RS (glucose and fructose) are generally found in SP roots. A report from 21 SP cultivars showed that RS was varied from 16% to 45% (dry weight basis) in SP roots (Aina and others 2009). The sucrose concentration in SP roots is approximately 10 times higher than the sucrose concentration in potato roots (4.80 mg/g), approximately 5 times higher than in rye (11.5 mg/g) and wheat flour (9.76 mg/g) (Elmore and others 2005; Viklund and others 2008). The total RS, that is, glucose and fructose, in the SP root is similar to that in potatoes (9.58 mg/g) but higher than in rye flour (4.55 mg/g) and wheat flour (0.97 mg/g) (Elmore and others 2005; Viklund and others 2008).
Table 1. Chemical composition of sweet potato roots (% on a fresh weight basis)a
|Moisture content||65.3 ± 0.97|
|Carbohydrates||24.9 ± 0.66|
|Crude protein||4.51 ± 0.249|
|Crude fiber||2.29 ± 0.086|
|Ash||2.11 ± 0.032|
|Crude fat||0.814 ± 0.0729|
Table 2 shows that the highest amino acid concentration in sweet potatoes is glutamic acid, followed by asparagine and aspartic acid. These 3 of AA contributed more than 60% of the total free AA in SP roots. Different authors showed that both asparagine and aspartic acid are among the highest concentrations of free AA in potato, rye, and wheat samples (Elmore and others 2005; Viklund and others 2008). The relative ratio of free asparagine to the total free AA for SP root was 0.21. This ratio is similar to that of rye flour (0.26), higher than that of wheat flour (0.16), but lower than that of potato flakes (0.38) (Elmore and others 2005).
Table 2. Free AA composition of sweet potato roots (mg/g on a dry weight basis)a
|Glutamic acid||2.399 ± 0.004||25.6|
|Asparagine||1.976 ± 0.006||21.1|
|Aspartic acid||1.633 ± 0.001||17.4|
|Alanine||0.601 ± 0.00||6.43|
|Serine||0.483 ± 0.002||5.17|
|Phenylalanine||0.312 ± 0.001||3.35|
|Threonine||0.282 ± 0.001||3.03|
|Valine||0.261 ± 0.002||2.79|
|4-Hydroxyproline||0.225 ± 0.001||2.42|
|Tryptophan||0.180 ± 0.001||1.93|
|Leucine||0.173 ± 0.001||1.85|
|allo-Isoleucine||0.165 ± 0.001||1.77|
|Lysine||0.143 ± 0.001||1.53|
|Proline||0.137 ± 0.001||1.47|
|Isoleucine||0.103 ± 0.002||1.1|
|α-Aminoadipic acid||0.102 ± 0.004||1.09|
|Tyrosine||0.094 ± 0.001||1.01|
|Glycine||0.073 ± 0.002||0.78|
The comparison of RS and free asparagine content implies that the acrylamide concentration in SP chips, which ranged from 296 to 2019 μg/kg (Table 3), was within the range of the acrylamide concentration in products made from normal potatoes (1800 to 7600 μg/kg) (Viklund and others 2008) and from rye (59 to 3166 μg/kg) (Elmore and others 2005). The presence of RS and free asparagine in SP roots, as precursors of acrylamide formation, suggests that acrylamide formation in SP chips is associated with the Maillard reaction.
Table 3. Acrylamide concentration (μg/kg) of deep-fried sweet potato chips using 4 different types of vegetable oils in 10 consecutive frying sessionsa
| ||Type of Frying Oil|
|No of Frying||PO||COO||CAO||SBO|
|1||296 ± 44.9b||886 ± 22.4b||374 ± 98.2b||1060 ± 183b|
|2||810 ± 73.7c||1113 ± 58.2bc||1185 ± 68.9c||1356 ± 38.9c|
|3||1072 ± 98.2cd||1090 ± 38.1bc||1054± 146c||1754 ± 115d|
|4||1098 ± 152cd||1352 ± 84.9cd||1736 ± 11.4d||1859 ± 170de|
|5||1349 ± 39.7de||1469 ± 44.2d||1955 ± 62.4de||2036 ± 72ef|
|6||1524 ± 153e||1808 ± 59.7e||1975 ± 115def||2180 ± 92.6fg|
|7||1591 ± 36.5e||2155 ± 180f||1925 ± 223def||2279 ± 90.6fgh|
|8||2167 ± 373f||2243 ± 13.0fg||2121 ± 36ef||2337 ± 91.7gh|
|9||1992 ± 225f||2460 ± 281gh||2208 ± 274f||2488 ± 61.2h|
|10||2530 ± 96.5fg||2643 ± 31.1h||2574 ± 148g||2849 ± 100.2i|
|Mean||1443 ± 668j||1722 ± 626k||1711 ± 652k||2019 ± 534l|
Influence of the degree of unsaturation of VO on acrylamide formation
All VOs showed that the temperature dropped from initial 180 ± 5 °C to 155 ± 5 °C after SP slices loading for frying, while the temperature increased to 172 ± 5 °C when SP chips were removed from deep fryer. No significant difference (P < 0.05) in those temperatures change among the VOs used in this study. The acrylamide concentration in the SP chips in each batch of frying, using 4 different types of VO were measured. Table 3 shows that there was a significant difference (P < 0.05) in the acrylamide concentration among the frying oils tested. The mean acrylamide concentration in SP chips from 10 fried batches, was the highest for SBO (2019 μg/kg) and the lowest for PO (1443 μg/kg). The mean acrylamide concentrations for COO and CAO are at 1722 and 1711 μg/kg, respectively. The difference among the 4 types of VO is the degree of unsaturation, and the relative degree of unsaturation can be measured by determining the FA composition and IV in the oils (Table 4) (Lalas 2009). Table 4 shows that COO contained the highest percentage of saturated FAs (>80%), whereas SBO and CAO contained more than 80% unsaturated FAs. Table 4 also shows that PO contained 43.3% saturated FAs and 55.9% unsaturated FAs. However, SBO contained higher polyunsaturated FAs than PO, which was dominated by linoleic acid (53.1%). In contrast, CAO contained a higher level of unsaturated FAs, which were dominated by oleic acid (63.7%). Moreover, Table 4 shows that before 10 consecutive frying sessions, the IV for SBO was the highest, followed by CAO, and lastly PO while the IV for COO was the lowest. A high IV indicates a high unsaturation level (Lalas 2009). Therefore, the relative degrees of unsaturation in the VOs used in the present study the lowest to the highest, are as follows that is, COO < PO < CAO < SBO. Oils containing a higher degree of polyunsaturated FAs are more susceptible to oxidative reactions than more saturated oils (Choe and Min 2006; Choe and Min 2007). Various authors found that lipids play a role in the formation of acrylamide in foods and reported that the higher the degree of unsaturation of the lipid, the higher the lipid oxidation rate and thus the higher the formation of acrylamide (Lingnert and others 2002; Ehling and others 2005; Capuano and others 2010). For example, Ehling and others (2005) revealed that oleic and linoleic acids formed a 10-fold higher acrylamide level in a heating model system compared with stearic acid when they were heated with asparagine at 180 °C. This correlation explains our results, which showed a higher acrylamide concentration in SP chips that were fried using SBO and CAO, compared with PO. This is most likely due to the higher content of oleic acid and linoleic acid in these 2 oil types when compared against PO. In addition, linoleic acid is more reactive to oxidation than oleic acid due to the presence of conjugated dienes in linoleic acid (Choe and Min 2006; Lalas 2009).
Table 4. Quality characteristics of vegetable oils before and after 10 consecutive frying sessionsa
|Oil properties||Before frying||After frying|
|POV (meq/kg oil)||1.39 (0.004)||3.99 (0.197)||4.62 (0.056)||4.39 (0.006)||8.08 (0.09)b||9.65 (0.129)b||9.74 (0.056)b||10.0 (0.056)b|
|IV (g of I2/100 g oil)||57.5 (0.93)||14.9 (0.41)||108 (0.96)||125 (0.74)||56.7 (0.64)||14 (0.46)||107 (0.31)||122 (0.28)b|
|PAV||1.26 (0.157)||3.17 (0.103)||3.67 (0.186)||3.01 (0.091)||15.4 (0.176)b||11.9 (1.52)b||15.5 (1.49)b||20.4 (1.57)b|
|FFAs (%)||0.09 (0.001)||3.33 (0.002)||0.06 (0.002)||0.10 (0.001)||0.11 (0.008)b||4.30 (0.003)b||0.07 (0.003)b||0.15 (0.01)b|
|FAs distribution (%)|| || || || || || || || |
|SFA||43.3 (0.05)||86.1 (0.30)||7.16 (0.015)||15.8 (0.17)||43.4 (0.19)||86.2 (0.70)||6.72 (0.064)b||14.3 (0.34)b|
|MUFA||45.9 (0.01)||11.1 (0.12)||64.4 (0.48)||24.6 (0.42)||45.8 (0.17)||10.5 (0.03)b||64.9 (0.22)||30.8 (0.14)b|
|PUFA||10.6 (0.03)||2.79 (0.017)||27.4 (0.51)||58.7 (0.30)||10.5 (0.04)||2.58 (0.014)b||27.0 (0.36)||53.7 (0.35)b|
Our findings are consistent with those of Gertz and others (2002), who reported that the lowest concentration of acrylamide was formed in French fries fried at 180 °C with PO (722 μg/kg), followed by sunflower oil (852 μg/kg), and rapeseed/CAO (1060 μg/kg). In addition, Capuano and others (2010) reported that a lower concentration of acrylamide was formed in a model system containing palm oil (approximately 750 μg/kg) compared with sunflower oil (approximately 1000 μg/kg). They concluded that a heating medium with saturated fats is less prone to thermal oxidation and hence produces less acrylamide (Capuano and others 2010).
Although COO contained the highest saturated FA concentration (86.1%) while CAO contained the lowest saturated FA concentration (7.16%, Table 4), no significant difference was found in the mean concentration of acrylamide in SP chips that were deep-fried in these 2 respective oils (P > 0.05). This result is possibly due to the higher FFA content in COO compared with CAO (Table 4). The high FFA content is well known in tropical copra extracts due to the poor refining process of the oil. Lingnert and others (2002) suggested that the higher glycerol and FFA levels were generated during frying using COO compared with palm oil due to the difference in the smoke point between these 2 oils. The remaining oxidized compounds (that is, FFAs) upon oxidation produced three-carbon-unit carbonyl compounds (that is, glycerols) that later form acrolein, which reacts with the ammonia from asparagine to form acrylamide (Yasuhara and others 2003; Ehling and others 2005).
Influence of consecutive frying sessions on acrylamide formation
In the present study, we found that consecutive frying significantly (P < 0.05) influenced the formation of acrylamide for all types of VOs. The Pearson correlation was used to study the relationship between the increase of consecutive frying sessions to acrylamide concentration from Table 3 and found a strong and significant (P < 0.05) correlation for all 4 types of VOs for the increase of consecutive frying sessions and acrylamide concentration. The Pearson correlation coefficient (r) was determined to be 0.941 for PO, 0.976 for COO, 0.92 for CAO, and 0.96 for SBO. The acrylamide concentrations were gradually increased as the number of consecutive frying sessions increased (Table 3). Table 3 shows that the initial concentration of acrylamide in SP chips that were fried using PO was significantly increased from 296 to 810 μg/kg in the second frying session, and increased to 1072 μg/kg in the third frying session. However, the concentration of acrylamide from the third to the 4th frying sessions was not significantly different (P > 0.05) (Table 3). Then a significant (P <0.05) increase of acrylamide concentration from the 4th (1098 μg/kg) to the 5th (1349 μg/kg) frying sessions. Following, there was an acrylamide increment from the 6th to the 9th frying sessions. Then the acrylamide concentration increased again from the 9th (1992 μg/kg) to the 10th frying session (2530 μg/kg). In comparison, the initial concentration of acrylamide in SP chips that were fried using SBO significantly (P < 0.05) increased from 1060 to 1754 μg/kg in the third frying session. Subsequently, the concentration of acrylamide gradually, but significantly (P < 0.05) increased from the 4th to the 10th frying sessions (Table 3). Additionally, CAO performed a similar trend when compared to SBO. These results demonstrate that PO is an oxidative stable VO as suggested by Matthäus (2007). Furthermore, Matthäus (2007) highlighted that palm oil has the highest oxidative stability when compared with other animal fats and highly unsaturated oils. Our results are in agreement with those of Capuano and others (2010), who reported that the thermal stability of palm oil, contributed to the lower formation of acrylamide in a sugar-free model system compared with a sunflower oil model.
Table 3 shows that the mean concentrations of acrylamide in SP chips, which were fried using COO, were not significantly (P > 0.05) different from the first (886 μg/kg), second (1113 μg/kg), and third (1091 μg/kg) frying sessions. There is an increase of acrylamide concentration from the third to the 4th (1352 μg/kg) frying sessions. Additionally, the mean concentrations of acrylamide between the 4th, and the 5th (1469 μg/kg) frying sessions were not significantly different (P > 0.05). The acrylamide formation appears to be slowing down, with a declining trend being observed in the first 5 frying sessions. The high composition of saturated FAs (86.1%, Table 4) in COO, possibly contributed to the lower or less prone thermal oxidation during the above mentioned period. However, the concentration of acrylamide slowly increased from the 6th to 10th frying sessions (Table 3). The increased acrylamide concentration could be due to the increased FFA composition of COO (4.30% after the 10 consecutive frying sessions compared with 3.33% before frying, Table 4). As discussed previously, these oxidized compounds (FFAs) or the precursor of three-carbon-unit carbonyl compounds (glycerols) upon oxidation will generate acrylamide formation (Yasuhara and others 2003; Ehling and others 2005).
Table 4 also shows that there was a significant difference in the mean POV and PAV before and after 10 consecutive frying sessions for all types of VOs (P < 0.05). Increased POVs indicate that the level of the primary lipid oxidation products increased, which resulted in the formation of hydroperoxides (Lalas 2009). Meanwhile, increased PAVs imply that the level of secondary lipid oxidation increased and resulted in the formation of aldehydes, generally 2-alkenals and 2,4-alkadienals (Shahidi and Zhong 2005). Zamora and Hidalgo (2008) revealed that different lipid oxidation products are able to convert asparagine to acrylamide. The reactivity of the top 3 assayed compounds was in the order of 2,4-decadienal, 4,5-epoxy-2-decenal, and methyl 13-oxooctadeca-9,11-dienoate, which were all derived from linoleic acid. They claimed that α,β,γ,δ-diunsaturated carbonyl compounds (that is, products of secondary lipid oxidation) were relatively more reactive than hydroperoxide compounds (that is, products of primary lipid oxidation) to form acrylamide (Zamora and Hidalgo 2008). These reports again explained why SBO produced higher acrylamide than other oils, probably due to linoleic acid as the dominant FA in SBO.
In addition to the POV and PAVs, Table 4 shows that there was a significant difference in the mean of FFA content before and after 10 consecutive frying sessions for all types of VOs (P < 0.05). The mean FFA contents after 10 consecutive frying sessions for PO, COO, CAO, and SBO were significantly (P < 0.05) higher than those before the 10 consecutive frying sessions. These results are in agreement with those of Chung and others (2004), who reported that the FFAs content in VO increased (from 0.017% to 0.062%) as the number of frying sessions increased (from the 1st to the 13th frying session). The FFA contents are expected to increase because water is introduced into the frying system by the raw SP chips, which accelerates the rate of hydrolysis. It is well understood that water can promote the hydrolysis of triacylglycerols to form a mixture of mono- and diacylglycerols, glycerol and FFAs (Lalas 2009). The dehydration of glycerol generates acrolein, which has a 3 carbon unit, which later forms acrylamide (Lingnert and others 2002; Yasuhara and others 2003). Moreover, repeated hydrolysis occurs during consecutive frying sessions. The water content gradually increased during consecutive frying sessions and enhanced the heat transfer of the deep frying oil; therefore, more interaction among the acrylamide precursors could occur to generate more acrylamide (Gertz and others 2003). Our findings are in similar agreement with those of Gertz and others (2003), who found a higher concentration of acrylamide in partially hydrogenated rapeseed oil upon the consecutive frying of potato French fries. They suggested that lipid oxidation upon prolonged heating could promote the formation of acrylamide. However, Mestdagh and others (2007) reported that acrylamide formation is independent of oil oxidation in a consecutive frying experiment, in which French fries were deep fried in corn oil and in palm fat.
Correlation of acrylamide concentration with color
Pearson correlation analysis was used to study the relationship of the color parameter (L*, a*, and b*) with the formation of acrylamide. Although the CIE color space is composed of 3 parameters, L*, a*, and b*, that are linked to each other, only b* showed a strong correlation with the acrylamide concentration (r = 0.642, n = 160, P < 0.05) (Figure 1). The color parameters L* and a* showed no significant correlation (P > 0.05) with the acrylamide concentration. Different studies have shown a good correlation between the L*, a*, and b* color parameters and acrylamide concentration in potato chips (Serpen and Gökmen 2009; Mulla and others 2011). However, literatures which indicated that SP chips’ color is correlated with acrylamide formation have not been found.
Figure 1. Acrylamide content versa color parameters b* (yellowness) for deep fried sweet potato chips for all treatments tested.
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