Abstract
In this study, the effect of dough salt content, frying temperature and time on the conjugated diene values, polymer triglyceride content, total polar material content, viscosity, and color values of the sunflower oil during frying of leavened doughs was determined using response surface methodology. Fifty repeated frying operations were applied in the same day at 160–200 °C for 1–5 min and doughs with 0–2% salt content. According to the results of the study, frying temperature, frying time and dough salt content were significantly (p < 0.05) affected total polar material content, polymer triglyceride content, viscosity and a* and b* color values of oil samples, whereas dough salt content did not affect the L* color values and conjugated diene value of oil significantly (p > 0.05). To minimize the oxidation products of frying oil, the frying process can be applied at 160 °C for 1 min using dough with 1.97% salt content.
Keywords: Sunflower oil, Color, Conjugated diene, Polymer triglycerides, Polar materials
Introduction
Deep frying is the most common food cooking method in the world (Choe and Min 2007). During deep-frying, food is immersed in hot oil at 180–200 °C and cooked in a short time. At the beginning of the frying, water removes from the capillary of food and oil penetrates into these capillaries as a counter current flow (Oyedeji et al. 2017). Deep fat frying produces a product with the desired flavor, color, and texture which make fried foods very popular (Aladedunye and Przybylski 2009). Different type of oils are used for the frying purposes. During frying, oils are exposed to several chemical processes such as hydrolysis, oxidation, polymerization, and isomerization. As a result of these processes, undesirable primary and secondary oxidation products induced (Boskou et al. 2006). The deterioration of oil is affected by the type of fryer, frying temperature, frying time, fatty acid composition, antioxidants, the turnover rate of oil, the constituents of fried foods, and the ambient oxygen concentration (Choe and Min 2007).
The effects of different frying conditions on the quality of oils are reported differently because of the various analysis methods and several experimental conditions. Response surface methodology helps to decrease the number of experimental tests, which are used to evaluate multiple parameters and their interactions. In this way, less time and less effort are required to complete the evaluation of the experimental parameters (Myers et al. 2009). There are some chemical and physical changes occurred in oil due to the repeated heating at different temperatures and durations. Therefore, estimation of the oxidative stability of oil in such a process is important in the food industry (Valantina et al. 2016).
Some researchers investigated the effects of processing conditions on the physicochemical properties of frying oil and fried foods using response surfaces methodology. Akinpelu et al. (2014) studied the effects of frying temperature, vacuum pressure and frying time during frying of plantain chips. Alvarez et al. (2000) optimized frying conditions of the potato strips using frying time and frying temperature as independent variables. Fan et al. (2005) studied the effects of frying temperature, absolute vacuum and frying time on the properties of carrot chips. However, there is little information available in the literature on the optimization of frying conditions of leavened doughs to minimize the oil oxidation.
The objective of this study was to evaluate the effect of frying conditions on the viscosity, color, conjugated diene value, the polymer triglycerides content, and the change in polar material content of frying oil during deep frying of leavened doughs using response surface methodology. Due to the oxidation products will transfer from frying oil to the leavened doughs, we also studied optimization of the frying process conditions.
Material and methods
Materials
Refined sunflower oil, instant yeast and salt were obtained from a local market (Bolu, Turkey). Isooctane and tetrahydrofuran were purchased from Merck (Darmstadt, Germany).
Preparation of dough
The dough was prepared using 1000 g refined flour, 20 g instant yeast, 650 mL water and different rates of salt (0, 1, and 2%) in a dough mixer (Kitchen Aid, Belgium) for one min. The dough was fermented in an oven at 35 °C for 45 min under the relative humidity of 80%. It rolled out (0.5 ± 0.2 mm thick) and cut into 3 cm × 3 cm pieces.
Frying process
Frying process was conducted according to Turan et al. (2019). Seven dough pieces (approximately 20–22 g) were fried in 1200 mL of refined sunflower oil for 1, 3 and 5 min at different temperatures (160, 180 and 200 °C) in a kitchen fryer without frequent oil replenishment. At the same conditions, the frying process were carried out 50 times in a day. After 50th frying, 25 mL of oil samples were cooled and put into amber colored glass bottles and kept frozen until the analysis.
Analysis of frying oil
The viscosity of the oil samples were measured at 30 °C and 200 rpm using the Brookfield Viscometer (Model DV2T-LV, Stoughton, MA) equipped with a probe numbered 31 and small sample adapter. The color values of the oil samples were measured by Minolta CR-400 (Osaka, Japan). Results are expressed as L*, a*, b*, respectively, corresponding to lightness, the green–red, and the blue–yellow component (Rossi et al. 2001).
The conjugated diene value of the oil samples was determined according to AOCS Official Methods Ch 5-91 (2000). The change in total polar materials was determined by an oil tester (Miroil Optifry, Allentown, PA, USA) at room temperature (25 °C). The equipment was calibrated with the fresh sunflower oil.
The amount of polymer triglycerides of the oil samples was determined according to Gertz (2001) and AOCS Official Method Cd 22–91 (2000) using Shimadzu Prominence HPLC system equipped with refractive index detector. Separation of the polymer triglycerides was performed with the gel permeation column (GPC, Agilent PL-Gel 100°A, 2 × 300 × 7.5 mm, 5 µm, UK) connected to a guard column (Agilent PL-Gel 50 × 7.5 mm, 5 µm, UK). The column oven temperature was 35 °C and the flow rate of the mobile phase (tetrahydrofuran) was 1 mL/min. The percentages of polymer triglycerides (dimeric and oligomeric) were determined.
Experimental design
In this study, the response surface methodology was used to determine the effect of frying temperature (A), dough salt content (B) and frying time (C) on the viscosity, color, and conjugated diene value, the polymer triglycerides, and total polar materials of frying oil. Twenty frying tests were applied as described by Turan et al. (2019) at three different levels (−1, 0, + 1) according to the central composite design. Lower and upper limits of frying temperature, frying time and dough salt contents were 160–180 °C, 1–5 min and 0–2%. Statistical evaluation and modelling with a second-order polynomial equation to determine the coefficients of the response model as well as their standard errors and significance were performed by using Design Expert 10.0.5 (Stat-Ease, Inc., Minneapolis, MN, USA) program. Response models are shown in Eq. 1.
1 |
where y is the predicted response, β0, βi, βii and βij are the regression coefficients for intercept and the linear, quadratic and interaction coefficients, respectively. Xi and Xj are independent variables and k is the number of independent variables. Stepwise deletion of terms was applied to eliminate the statistically non-significant terms (Stat-Ease 2017).
Results and discussions
Change in conjugated diene value of sunflower oil during deep frying of doughs
Conjugated dienes are created from unsaturated fatty acids during early stage of oil oxidation to obtain more stable radical (Choe and Min 2007). The conjugated diene values of the frying oil samples taken from the 50th frying operation are given in Table 1. The conjugated diene values of the frying oil were in the range of 0.67% -1.22%.
Table 1.
Some physicochemical characteristics of frying oil samples taken at the end of 50th frying
Run | Frying Temperature (°C) | Dough Salt content (%) | Frying time (min) | Conjugated dienes (%) | Total polar materials (%) | Polymer triglyceride (%) | Viscosity (cP) | L* | a* | b* |
---|---|---|---|---|---|---|---|---|---|---|
SFO-I | – | – | – | 0.29 | 0.0* | 1.11 | 44.03 | 59.93 | − 4.05 | 19.17 |
1 | 160 | 0 | 5 | 1.18 | 6.2 | 6.40 | 51.40 | 56.12 | − 3.64 | 35.95 |
2 | 160 | 1 | 3 | 0.93 | 4.4 | 5.33 | 48.65 | 56.66 | − 4.88 | 30.80 |
3 | 180 | 0 | 3 | 0.98 | 4.2 | 6.57 | 50.70 | 56.12 | − 1.69 | 38.98 |
4 | 180 | 1 | 3 | 1.14 | 7.3 | 9.15 | 51.45 | 57.37 | − 4.30 | 35.30 |
5 | 180 | 1 | 3 | 1.10 | 5.5 | 7.86 | 51.65 | 57.93 | − 4.47 | 34.10 |
6 | 200 | 2 | 5 | 1.10 | 10.1 | 11.12 | 54.85 | 49.84 | 4.61 | 41.57 |
7 | 180 | 1 | 3 | 1.05 | 4.8 | 7.82 | 52.45 | 54.66 | − 3.25 | 32.25 |
8 | 200 | 0 | 5 | 1.19 | 6.5 | 9.66 | 52.75 | 52.43 | 4.81 | 44.11 |
9 | 200 | 2 | 1 | 1.01 | 6.5 | 7.26 | 54.85 | 53.76 | 0.13 | 35.68 |
10 | 160 | 2 | 1 | 0.67 | 3.3 | 4.46 | 48.65 | 59.67 | − 5.08 | 26.62 |
11 | 200 | 1 | 3 | 1.22 | 7.1 | 9.27 | 53.30 | 51.01 | 4.05 | 44.12 |
12 | 180 | 1 | 3 | 1.12 | 6.6 | 9.53 | 50.90 | 56.52 | − 3.31 | 37.15 |
13 | 180 | 1 | 3 | 1.10 | 6.9 | 7.92 | 52.66 | 57.01 | − 3.94 | 34.36 |
14 | 160 | 0 | 1 | 0.69 | 4.3 | 4.17 | 47.20 | 58.06 | − 5.50 | 32.21 |
15 | 180 | 1 | 5 | 1.06 | 7.9 | 8.75 | 52.50 | 56.04 | − 2.57 | 40.36 |
16 | 200 | 0 | 1 | 0.92 | 3.3 | 6.29 | 51.40 | 54.00 | 2.23 | 42.43 |
17 | 180 | 1 | 3 | 1.05 | 4.6 | 8.04 | 52.35 | 57.29 | − 4.96 | 35.31 |
18 | 180 | 1 | 1 | 0.81 | 3.9 | 5.80 | 50.55 | 58.43 | − 4.56 | 32.44 |
19 | 180 | 2 | 3 | 1.07 | 5.2 | 9.05 | 51.10 | 57.70 | − 3.18 | 34.84 |
20 | 160 | 2 | 5 | 0.89 | 5.8 | 6.33 | 50.90 | 56.00 | − 5.22 | 30.99 |
SFO-I, initial values of sunflower oil before repeated frying
*During measurement polar material content of fresh oil is assumed as zero
It was observed that the change in the conjugated diene value of frying oil samples was in conformity with the reduced quadratic model generated by response surface methodology and the model was significant (p < 0.05; Table 2). The equation for the improved model with the determination coefficient (R2) of 0.9331 is shown Table 2.
Table 2.
Mathematical models for conjugated diene content, total polar material content, polymer triglyceride content, viscosity and color values of oil samples taken after 50th frying
Models | Equationsa | P values | Adequate precision | Determination coefficient (R2) | |
---|---|---|---|---|---|
Model | Lack of fit | ||||
Model 1 | CD (%) = −0.67 + 6.72 × 10−3A − 0.29B + 0.48C + 1.97 × 10−3AB – 1.1 × 10−3AC − 0.02BC -0.03C2 | < 0.0001* | 0.1666 | 16.369 | 0.9331 |
Model 2 | TPM (%) = 8.11 − 0.03A − 7.24B − 1.38C + 0.05AB + 7.5 × 10−3AC − 0.68B2 + 0.13C2 | 0.0005* | 0.9593 | 13.061 | 0.8451 |
Model 3 | Polymer TAG (%) = −54.53 + 0.61A − 1.97B − 0.07C + 0.01AB + 9.77 × 10−3AC − 1.58 × 10−3A2 − 0.16C2 | < 0.0001* | 0.7303 | 17.363 | 0.9196 |
Model 4 | Viscosity (cP) = + 27.00 + 0.12A − 3.86B + 3.56C + 0.03AB − 0.016AC−0.21BC | < 0.0001* | 0.6105 | 19.656 | 0.9199 |
Model 5 | L* = -134.32 + 2.26A + 5.65B – 1.59C – 0.03AB -0.25BC −6.55 × 10−3A2 + 0.20C2 | 0.0001* | 0.9537 | 18.273 | 0.9276 |
Model 6 | a* = + 182.46 − 2.24A − 0.52B −1.76C − 7.09 × 10−3AB + 0.02AC + 6.67*10−3A2 + 0.65B2 − 0.12C2 | < 0.0001* | 0.2227 | 19.749 | 0.9654 |
Model 7 | b* = + 65.00 − 0.59A − 3.30B + 0.88C + 0.30BC + 2.34 × 10−3A2 | < 0.0001* | 0.4874 | 21.611 | 0.9076 |
*Statistically significant at a significant level (α) of 0.05
aLetters A, B, C indicate frying temperature (°C), dough salt content (%) and frying time (min), respectively
CD, conjugated diene value, TPM, total polar material, TAG, triglyceride
Frying temperature and frying time had a significant effect on the conjugated diene values of frying oil (p < 0.05). On the other hand, the effect of dough salt content on the conjugated diene value of oil samples was not significant (p > 0.05). Tyagi and Vasishtha (1996) reported that high frying temperature caused higher quantities of conjugated dienes during deep frying of potato chips for 70 h at 170, 180 or 190 °C. Farhoosh and Tavassoli-Karani (2011) indicated that conjugated dienes of oil during repeated frying of potatoes increased as the frying duration increased. In addition, Chung et al (2004) reported the rise in conjugated dienes of sunflower oil occurred during flour dough frying.
The interaction of frying temperature and salt content of dough caused a significant increase (p < 0.05) on the conjugated diene values of the frying oil (Fig. 1a). The conjugated diene value of the frying oil samples increased as the temperature increased when the frying time was 3 min. The conjugated diene value of frying oil, which was 0.93% at 160 °C, increased to 1.22% at 200 °C during the frying of leavened doughs containing 1% salt.
Fig. 1.
Effects of frying parameters on conjugated diene content of frying oil. a Frying temperature and dough salt content. b Frying temperature and frying time. C Dough salt content and frying time
The interaction of the frying temperature and frying time caused a significant increase in the conjugated diene value of sunflower oil (p < 0.05). The conjugated diene value of oil increased as the frying temperature and frying time increased (Fig. 1b). The conjugated diene value of oil was found 0.81% during the frying of doughs at 180 °C for 1 min. However, it increased to 1.05–1.14% after 3 min frying period at the same temperature. As the frying time increases, frying oil is exposed to heat for longer periods. On the other hand, the conjugated diene value of the oil increased from 0.67% to 1.01% during frying of doughs containing 2% salt when the frying temperature was increased from 160 °C to 200 °C (Table 1).
The combined effect of dough salt content and frying time on the conjugated diene content of frying oil was significant (p < 0.05). As the frying time increased, the conjugated diene value of the oil increased whereas the conjugated diene value increased slightly as the salt content of the dough increased (Fig. 1c). When the dough containing 1% salt was fried at 180 °C for 1 min, the conjugated diene value of the oil was 0.81%. After 5 min frying, it increased to 1.06%. The conjugated diene value of the oil was 0.98% during the 3 min frying of the unsalted dough at 180 °C, while this value was 1.07% for the 2% salted dough (Table 1).
In a study, the effects of frying methods (deep fat frying and microwave frying) on the oxidative stability of various vegetable oils were investigated (Baltacıoğlu, 2016). The K232 value of sunflower oil increased due to the increasing of frying temperature from 160 °C to 190 °C. On the other hand, the K232 value decreased as a result of temperature increase in canola oil.
Change in polar materials of sunflower oil during deep frying of doughs
Total polar materials have been used as an indicator of thermal degradation of frying oils (Manral et al. 2008). The total polar materials are polymeric and cyclic nonvolatile compounds and soluble components infiltrate from the fried foods (Debnath et al. 2012). It was reported that polar compounds affected by unsaturation degree of oil, food composition, heating period, frying temperature, amount of oxygen, type of fryer, area of surface oil to volume of oil (Mlcek et al., 2015).
The change in total polar materials of frying oil samples taken from the fifty frying periods was in the range of 3.3–10.1% (Table 1). A reduced quadratic model was found significant (p < 0.05) for the variation of total polar materials of frying oil samples at the 50th frying batch and the determination coefficient of the model was 0.8451. The mathematical equation for the total polar materials of frying oil samples is also shown in Table 2.
Individual effects of frying temperature, dough salt content and frying time on the polar material content of oil samples was significant (p < 0.05). Romero et al. (1998) reported that longer frying time increases polar compounds. Also, Wang et al. (2013) reported a significant increase in total polar material of soybean oil when frying cycles of oil increased. They showed that the total polar contents of cold pressed soybean oil increased from 6.18% to 28.5% at the end of 45th of frying cycle. In another study, Tompkins and Perkins (2000) reported that the amount of polar substances of low linolenic acid soybean oil increased at 180 °C depending on the frying time during potato and fish frying.
The interactive effect of frying temperature and dough salt content led to significant (p < 0.05) increase in total polar material content of oil. When 1% salted dough was fried for 3 min at 160 °C, it had 4.4% polar material and its polar material content increased to 7.1% at 200 °C. Also, polar material content of frying oil for 3 min frying time at 180 °C increased from 4.2 to 5.2% when the dough salt content increased from 0 to 2% (Table 1). As the salt content of dough increased polar material content of oil increased slightly (Fig. 2). Pro-oxidant activity of salt was explained by Mancini et al. (2020) as releasing iron from organic molecules in food matrix. In this study, dough was prepared with flour, instant yeast, refined salt, and water. Trace metals may be present in these ingredients. Among the trace elements, iron is the most important one in lipid oxidation. It can accelerate oxidation in lipid fraction of the foods. Therefore, it is thought that salt enhance oxidation via iron released from food matrix and oxidizing elements may found in salts and other ingredients of food (Cui et al. 2016; Mancini et al. 2020; Mehta and Swinburn 2001). However, when comparing the effect of water, salt and/or sugar on the polar material content of oil, Chu and Luo (1994) reported that the rate of oil deterioration was the highest for water, followed by salt and sugar at the end of repeated frying at 180–190 °C. They claimed that the salt or sugar had water binding capacities during frying of dough.
Fig. 2.
Combined effect of the frying temperature and dough salt content on total polar materials of frying oil
Change in polymer triglycerides content of sunflower oil during deep frying of doughs
During the repeated deep-frying process, nonvolatile polar compounds, dimers, and polymer triglycerides are formed as a results of decomposition and polymerization reactions. Dimers and polymers are high molecular weight compounds, consists of carbon-to-carbon (-C–C-), carbon-to oxygen (-C-O-) and/or carbon-to oxygen- to-carbon (-C-O–O-C-) bonds between fatty acids (Choe and Min, 2007).
Polymer triglyceride content of the oil samples taken after 50th frying operations is shown in Table 1. While the polymer triglyceride content of sunflower oil was 1.11% at the beginning of the frying, it increased and varied between 4.17% and 11.12% at the end of 50th frying.
Che Man and Jaswir (2000) reported that the polymer triglyceride content of the palm oil increased during the repeated frying operation. The amount of polymer triglyceride content was reported as 0.01% at the beginning of the frying, it increased to 2.64% at the end of the 60th frying operation.
The change in the polymer triglyceride content of the frying oil was best described by the reduced quadratic model (p < 0.05) with high regression coefficient (R2 = 0.9196) and adequate precision of 17.363. The mathematical equation for the polymer triglycerides content of the frying oil samples is shown Table 2.
Individual effects of frying temperature, frying time and the salt content of the dough on the polymer triglyceride content of frying oil was significant (p < 0.05), whereas the combined effects of frying temperature and dough salt content or frying temperature and frying time or frying time and dough salt content were insignificant (p > 0.05). Polymer triglyceride content of oil at the end of 50th frying of leavened doughs containing 1% salt for 3 min at 160 °C was 5.33%, it increased to 9.27% at 200 °C during frying of doughs at the same salt content (Table 1).
Choe and Min (2007) reported that formation of dimers and polymers depends on the oil type, frying temperature, and number of frying. As the frying temperature and the number of frying increased, the amounts of polymers increased.
There was an increase in the amount of polymer triglycerides in the oil by increasing the salt content of the dough. In addition to trace metal contained in salt and other fried food ingredients, sodium chloride can also increase the activity of metals (Cui et al. 2016) and accelerate the lipid oxidation. Inversely, Chu and Luo (1994) reported that oil samples taken after repeated frying of salted doughs had lower polymer triglyceride content than oil during frying unsalted doughs.
Change in viscosity of sunflower oil during deep frying of doughs
Viscosity is defined as the resistance of a fluid to flow and shows the viscous properties of a fluid at a certain temperature and pressure. At high temperatures oil viscosity increases with polymerization and the length of the fatty acids or oil type. As a results of the polymerization high molecular weight compounds are formed. It is assumed that viscosity measurement is the one of the most suitable method monitoring oil quality (Gertz 2000).
The viscosity of the oil increased slightly during frying of leavened doughs (Table 1). While the initial viscosity of the sunflower oil was 44.03 cP, the viscosity of the oil samples varied between 47.20 cP and 54.85 cP at the end of the 50th frying operation. A quadratic model was found significant (p < 0.05) for the variation of viscosity of the frying oil samples at 50th frying batch. The determination coefficient and adequate precision value of the model were found as 0.9199 and 19.656, respectively. The mathematical equation for the model is also shown in Table 2.
Individual effects of dough salt content, frying temperature and time on the viscosity of the frying oil was found significant (p < 0.05). Also, the combined effect of frying temperature and salt content on the viscosity of the oil was significant as well (p < 0.05). As the temperature increased, the viscosity of the oil increased. Also, slight increase was observed in the viscosity of oil samples when the salt content of the dough increased (Fig. 3a). The viscosity was determined as 48.65 cP at 160 °C and it was 53.3 cP at 200 °C during the frying 1% salt containing dough for 3 min frying time (Table 1). Tyagi and Vasishtha (1996) reported that the viscosity of soybean oil increased periodically as the frying temperature increased and was highly influenced by frying temperature rather than frying medium. In addition, when the salt-free dough was fried at 180 °C for 3 min, the viscosity of the oil was 50.70 cP, this value was 51.10 cP in the oil fried 2% salt containing dough (Table 1).
Fig. 3.
Effects frying conditions on viscosity of frying oil. a Frying temperature and dough salt content. b Frying temperature and frying time
The interaction between frying temperature and frying time caused significant (p < 0.05) increase in viscosity of oil samples (Fig. 3b). While the oil viscosity was 48.65 cP at 160 °C, it was found as 54.85 cP at 200 °C during frying 2% salted doughs for 1 min. When the frying time was increased to 5 min the oil viscosity increased from 48.65 cP to 50.90 cP at 160 °C for 2% salt containing dough (Table 1).
Aydeniz and Yilmaz (2012), investigated the physicochemical changes in oil during deep frying operations and reported that the viscosity of canola oil increased from 46.27 cP to 92.7 cP after 7 days frying operations of yeasted dough at 180 °C. As the duration of oil exposure to heat increased, its viscosity increased due to the oxidation products formed. Similarly, Che Man and Tan (1999) found a noteworthy increase in viscosity with an increase in days of frying. Chatzilarazou et al. (2006) reported that there was an increase in viscosity during the use of olive oil, corn oil and blends of these oils in deep frying processes.
Change in color values (L*, a*, b*) of sunflower oil during deep frying of doughs
The color values of oil changed due to the decomposition reactions. Although color is one of the quality criteria for the oil degradation reactions during frying, the color changes can also be related with the other different chemical reactions such as Maillard browning reactions which cause darkening of oil (Takeoka et al. 1997).
Table 1 shows the change in color values (L*, a*, b*) of sunflower oil during deep fat frying. According to the results, a slight decrease in the L* value of the oil and a slight increase in the a* and b* values were observed during the frying operation. At the end of 50th frying, the L* value of the frying oil varied from 49.84 to 59.67, the a* values varied from 5.50 to 4.81, and the b* values varied from 26.6 to 44.1. Debnath et al. (2012) reported that the red and yellow color values of oil increased due to the Maillard reaction during frying of poori (an Indian traditional fried dough). Aladedunye and Przbylski (2009) observed that frying temperature had a significant effect on the formation of color components.
The changes in the color values of the frying oil samples (L*, a* and b*) were best described by the quadratic models (p < 0.05). The quadratic models showed high determination coefficients (R2) for L* (0.9276), a* (0.9654) and b* (0.9076) values. The mathematical equations for the models are shown in Table 2.
While individual effects of frying temperature and frying time on L* value of the frying oil samples were significant (p < 0.05), the effect of dough salt content was not significant (p > 0.05). The interactive effects of frying temperature and dough salt content or frying temperature and time or frying time and dough salt content on the L* color value of oil were not found significant (p > 0.05).
L* value decreased as the temperature of the frying oil increased, but the salt content of the dough was not significantly changed the L* value of the oil (p > 0.05). For 3 min frying time, L* value ranged from 54.66 to 57.93 at 180 °C during the frying of 1% salted dough. When the temperature was increased to 200 °C, this value was found as 51.01 (Table 1). It was also seen that the L* value decreased as the frying time increased. Longer frying period and higher temperature caused a decrease in L* value.
The dough salt content, frying oil temperature and time affected significantly a* value of frying oil samples (p < 0.05). The a* value of the oil was not significantly affected by the interaction between the frying temperature and dough salt content (p > 0.05), while it was significantly affected by the interaction of frying temperature and frying time (p < 0.05). As the temperature increased, a* value increased. The increasing of temperature caused more change in the oil a* value compared to the increase in frying time (Fig. 4).
Fig. 4.
Combined effects of frying temperature and frying time on a* color values of frying oil
When salt free dough was fried at 160 °C for 5-min, a* value was determined as -3.64. At 200 °C, it increased to 4.81 at the same conditions. On the other hand, when the frying time was increased from 1 to 5 min at 200 °C for unsalted doughs, the a* value of oil was found as 2.23 as shown in (Table 1).
The dough salt content, frying oil temperature and time significantly affected the b* value of frying oil (p < 0.05). The interactive effects of frying time and dough salt content, frying temperature and time or frying temperature and dough salt content on b* values of oil samples were found insignificant (p > 0.05). Although interaction of frying time and the salt content was insignificant, b* value decreased with the increasing dough salt content at 180 °C. For example, when the dough containing 2% salt was fried at 180 °C for 3 min, the b* value of the oil was found as 34.84. Whereas, when the salt-free dough was fried in the same conditions, b* value of oil was found as 38.98. On the other hand, the increase in frying time increased the b* value of the oil. During frying of 1% salted dough, oil samples at the end of 50th frying had 32.44 and 40.36 b* value for 1 min and 5 min frying time, respectively (Table 1).
Similarly, Jaswir et al. (2000) indicated the red and yellow values of palm olein increased due to oxidation products formed during deep frying of potatoes. Also, Che Man and Tan (1999) reported that the color of frying oil changed as a result of oxidation and generation of colored pigments from the potato chips during the potato frying. In another study, Aladedunye and Przbylski (2009) reported that the frying temperature affected the red and yellow colors of French fries during the seven consecutive days of frying. Also, Debnath et al. (2012) indicated that the color of the oil was found to increase with an increase in frying cycles during poori frying.
Optimization
The optimum frying conditions to minimize the oxidation products in frying oil were determined using the Design Expert 10.0.5 statistical software program and shown in Table 3. The response surface methodology program suggested different solutions for the minimization of oxidation products in frying oil. These solutions predicted 95% confidence in the range of the independent variables. The desirability was 0.972 and the optimum conditions were: frying temperature of 160 °C, frying time of 1 min and dough salt content of 1.97%. The calculated values at the optimum conditions for conjugated diene value, total polar materials, polymer triglyceride content, viscosity, color L*, a*, and b* were 0.66%, 3.14%, 4.40%, 48.38 cP, 59.85, -5.62, 26.62, respectively.
Table 3.
Optimization of frying conditions according to physicochemical characteristics of frying oil
Goal | Lower limit | Upper limit | Optimized values | |
---|---|---|---|---|
Frying temperature (°C) | In the range | 160 | 200 | 160.0 |
Dough salt content (%) | In the range | 0 | 2 | 1.97 |
Frying time (min) | In the range | 1 | 5 | 1.00 |
Total polar materials (%) | Minimize | 3.3 | 10.1 | 3.14 |
Polymer triglycerides (%) | Minimize | 4.17 | 11.12 | 4.40 |
Conjugated dienes (%) | Minimize | 0.67 | 1.22 | 0.66 |
Viscosity (cP) | Minimize | 47.2 | 54.85 | 48.38 |
Color L* | Maximize | 49.84 | 59.67 | 59.85 |
Color a* | Minimize | -5.5 | 4.81 | -5.62 |
Color b* | Minimize | 26.62 | 44.12 | 26.62 |
Desirability value: 0.972
Conclusion
In this study, the effects of dough salt content, frying temperature and time on the physicochemical properties of frying oil were investigated. In order to minimize the oxidation level in the frying oil, the leavened doughs containing 1.97% salt will be fried at 160 °C for 1 min. Primary or secondary oxidation products will transfer from frying oil to leavened doughs. Therefore, the level of the oxidation products in leavened dough will also be determined in the later stages of the study.
Acknowledgements
The authors would like to thank Bolu Abant Izzet Baysal University, Scientific Research Center (Project Number: 2015.09.04.921). The authors thank the employees of Yenigıdam Research Center of Bolu Abant Izzet Baysal University for their helps in polymer triglyceride analysis.
Footnotes
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Contributor Information
Semra Turan, Email: turan_s@ibu.edu.tr.
Sule Keskin, Email: sule.keskin@tarimorman.gov.tr.
Rukiye Solak, Email: islamoglusolakrukiye@gmail.com.
References
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