Abstract
The purpose of this study was to clarify the effect of surfactant-added oils on the glass transition as a function of water content, fracture behavior, color, and crude oil content of the fried coatings (post-fried wheat flour-water mixture) obtained under various frying conditions (at 150-180 °C for 1.0-4.5 min). Polyglycerol oleic acid esters having hydrophile-lipophile balances of 7.4 (hydrophobic) and 13.3 (hydrophilic) were employed, and canola oils with and without 0.5 % (w/w) surfactants were used for frying. The samples obtained at 170 °C became glassy after frying times of 1.4 min, 1.9 min, and 2.4 min in the hydrophilic surfactant-added oil, hydrophobic surfactant-added oil, and surfactant-free oil, respectively. The glassy samples showed brittle fracture behavior, and the maximum fracture force for the glassy samples obtained using the surfactant-added oils was lower than that obtained using surfactant-free oil. The frying time to obtain glassy samples decreased with increasing frying temperature, and the frying time in the hydrophilic surfactant-added oil was reduced by 60-80 % compared to the surfactant-free oil. When the browning color of the glassy samples obtained for the shortest frying time was compared at each temperature, the samples fried in the hydrophilic surfactant-added oil showed less browning than those fried in the surfactant-free oil due to the reduction of frying time. There were no significant differences in the crude oil content between surfactant-free oil (69.9-105.7 g/100 g-defatted DM, dry matter) and the hydrophilic surfactant-added oil (78.6-115.5 g/100 g-defatted DM) at each frying temperature (except for 150 °C).
Keywords: glass transition, texture, oil sorption, browning, batter
Abbreviations
Wc, critical water content; DM, dry matter; Tg, glass transition temperature; HLB, hydrophile-lipophile balance; HSD, honestly significant difference.
INTRODUCTION
Deep-frying is a cooking method in which foodstuff is heated in a large amount of frying oil preset to high temperature (150-190 °C) [1, 2]. Some deep-fried foods are coated in batter (a wheat flour-water mixture) in advance, and the coating takes on a brittle texture after frying (e.g., tempura). The physical change of batter to fried coating during deep-frying is understood through a state diagram [3], as schematically shown in Fig. 1(a). In the initial stage of frying, the temperature of the batter rapidly increases and starch granules in the batter gelatinize above the starch gelatinization temperature. The temperature of the batter remains at around 100 °C during the evaporation of bulk water, and then the temperature and the water content increases and decreases, respectively, as the frying time increases. After frying, the fried coating is cooled down to room temperature, and then it takes on a glassy state below the glass transition temperature (Tg). This process is described as pathway A in Fig. 1(a). The glassy fried coating has a brittle texture [4, 5, 6], which is commonly desirable in deep-fried foods. If frying is stopped at an insufficient frying condition, the fried coating stays at a temperature above Tg after cooling. That is, the fried coating becomes rubbery. This process is described as pathway B in Fig. 1(a). Since the molecular mobility of glassy materials dramatically increases by the glass to rubber transition [4, 5, 6], the rubbery fried coating has a ductile texture, which is commonly not preferred. The dotted area in Fig. 1(a) is enlarged and quantitively shown in Fig. 1(b). In our previous study [5], the mechanical Tg for a model fried coating (particles of fried batter) was investigated using a thermomechanical analysis, and the Tg curve was determined (Fig. 1(b)). In addition, the water content at which Tg becomes room temperature (typically 25 °C) was determined to be 4.36 g/100 g-DM, dry matter; this water content has been commonly described as the critical water content (Wc). The fracture behavior was confirmed to change from ductile to brittle when the water content was lower than the Wc [5].
Fig. 1. Schematic state diagram for batter frying (a). Frying pathways A and B illustrate the process by which batter turns into glassy and rubbery fried coatings, respectively, by deep frying in oil. The dotted area in Fig. 1(a) is enlarged and quantitively shown in Fig. 1(b). The Tg values were taken from the Reference [5]. Room temperature (dashed line) was set to 25 °C. The water content at which Tg becomes 25 °C (Wc) was 4.36 g/100 g-DM.
It is not easy to dry low-hydrated materials to a water content below Wc (i.e., drying from pathway B to A in Fig. 1(a)), because the water evaporation rate decreases in the low-hydrated condition. This process is commonly described as the “falling rate period” [7]. The interfacial tension between oil and water is one of the important factors for deep-frying. Liu, et al. [8] investigated that the water content of deep-fried foods obtained with two types of frying oil with different interfacial tensions. From the results, it was demonstrated that oil with lower surface tension tended to promote water evaporation of the samples. The reason why the effect of the interfacial tension on the water evaporation was not so large may be that there was not much difference in interfacial tension between the two types of oil used. Taking the fact that surfactants reduce the interfacial tension between oil and water, it is expected that surfactant-added oil strongly promotes the evaporation of water during frying. Surfactants such as monoglyceride and diglyceride are known to be produced by the decomposition of frying oil during frying, and thus the interfacial tension decreases with increase in frying time [9]. It is suggested that the produced surfactants promote oil sorption of fried foods due to the reduction of the interfacial tension between oil and food [9, 10]. In addition, the produced surfactants elevate heat transfer at the interface between oil and food due to the improved contact among them [9, 11]. The type and amount of the produced surfactants, however, are highly limited, and the effect of surfactant-added oils on the glass transition and fracture properties of a fried coating was not quantitatively understood.
The purpose of this study was to clarify the effect of surfactant-added oils on the glass transition as a function of water content, fracture behavior, color, and crude oil content of the fried coating samples obtained under various frying conditions. To compare effect of hydrophile-lipophile balance (HLB), two types of surfactants with different HLB were employed. When the experiment on the fracture properties of fried coatings was started, we were faced with the problem of sample preparation; because batter is a viscous liquid, the structure of fried batter (fried coating) dramatically changes every time. The large difference in the sample structure caused a larger deviation in water content and fracture behavior. To diminish the impact of the structural factor on the experimental data, batter was loosely solidified using gelatin (batter gel), and the batter gel having a constant structure was employed as the raw material for fried coating samples.
MATERIALS AND METHODS
Materials
Wheat flour (Nisshin Seifun Welna Inc., Tokyo, Japan), gelatin from bovine skin (Sigma-Aldrich Corp., St. Louis, USA), and canola oil (The Nisshin OilliO Group, Ltd., Tokyo, Japan) were employed. The wheat flour contained 8.7 % protein and 1.5 % lipid according to the product information. The water content of the wheat flour and gelatin was gravimetrically determined to be 11.7 % and 10.9 %, respectively, by oven drying at 105 °C for 16 h. Polyglycerol (mainly decaglycerol) oleic acid esters having HLB of 7.4 and 13.3 were provided by Mitsubishi Chemical Corp. (Tokyo, Japan). The former and latter are denoted as hydrophobic and hydrophilic surfactants, respectively.
Sample preparation
Gelatin (5.26 g) was added to tap water (129 g), and the mixture was held at approximately 25 °C for 10 min to let the gelatin swell. The gelatin was dissolved at 45 °C for 5 min with magnetic stirring, and then cooled to 40 °C. Wheat flour (100 g) was added to the gelatin solution, mixed with a whisk, poured into an aluminum tray to a depth of 2 mm, and then stored at 5 °C overnight to obtain a loosely solidified batter (batter gel); the gelatin content was 2 % (w/w) against total weight, and the weight ratio of dry wheat flour to water was 1:1.6 [5, 6]. It was preliminarily confirmed that the gelatin content was the minimum needed to obtain a loosely solidified batter; 1 % and 3 % gelatin formed a liquid-like batter and a closely solidified batter, respectively. The batter gel was molded into circular form using a 40-mm-diameter die cutter, and then cooled at approximately 0 °C.
Canola oils with and without 0.5 % (w/w) hydrophobic and hydrophilic surfactants were prepared as the frying oils. It was preliminarily observed that the hydrophilic surfactant-added oil became remarkably cloudy when more surfactant was added to the oil. A sufficient volume of frying oil was place into a fryer (EFK-A10, Zojirushi Corp., Osaka, Japan) with an attached temperature controller (TXN-700, AS ONE Corp., Osaka, Japan), and the frying oil was heated to 150, 160, 170, and 180 °C. The batter gel was fried for 1.0 to 4.5 min, depending on the frying temperature. Approximately 0.25 min after deep-frying began, the batter gel folded and shrunk. To maintain as constant a sample structure as possible, the folded batter gel was spread manually using a stick. After that, the batter gel was completely submerged in the frying oil using a metal net during frying. The fried batter gel (fried coating sample) was removed from the frying oil using the metal net, and then cooled at approximately 25 °C for 2 min.
Fracture properties
The fracture properties of the fried coating samples were investigated using a texture analyzer (combination of ZTA-20N and MX2-500N, IMADA Corp., Aichi, Japan). The sample was placed on a hollow cylindrical stage (outer diameter 38 mm and inner diameter 32 mm), and then compressed at 0.5 mm/s using a spherical plunger (diameter 10 mm). This approach can give a clear fracture or puncture peak even for thin samples [12]. The maximum fracture force was obtained from the force-displacement curve. The measurements were carried out in triplicate, and the values were averaged.
Water content
The fried coating samples were manually crushed into small pieces in a mortar with a pestle, and then the water content was measured gravimetrically by vacuum-drying at 110 °C (shelf temperature) for 7 h [4, 5, 6, 13]. The measurements were carried out in triplicate, and the results were averaged.
Crude oil content
The fried coating samples were crushed, and then vacuum-dried at 110 °C (shelf temperature) for 7 h, similar to the case for the water content measurement. The crude oil content of the fried samples was evaluated by the Soxhlet extraction method. The crude oil was extracted using diethyl ether at 60 °C for 8 h [5, 6]. The measurements were carried out in triplicate, and the results were averaged.
Statistical analysis
Statistical analysis was performed with a t-test or Tukey’s honestly significant difference (HSD) test at p < 0.05 or < 0.10 using KaleidaGraph software (Version 3.6, Hulinks Inc., Tokyo, Japan).
RESULTS AND DISCUSSION
Fracture behavior of fried coating samples
Representative force-displacement curves for fried coating samples are shown in Fig. 2. The samples can be classified according to their fracture behavior into three types. In the first type, a large number of small sharp peaks were observed (Fig. 2(a)). This is a characteristic fracture behavior for dry foods having a brittle texture, and they commonly are a glassy state [8]. In the second type, a small number of large smooth peaks were observed (Fig. 2(b)). This is a characteristic fracture behavior for dry foods having a ductile texture, and they commonly are a rubbery state [9]. In the final type, sharp peaks were observed first, followed by smooth peaks (Fig. 2(c)). This is thought to be because the surface of the sample was brittle (glassy state), but the interior was ductile (rubbery state). Because this study focused on the frying condition that produces a fried coating having a brittle texture, the samples were finally divided into brittle (similar to Fig. 2(a)) and ductile including partially ductile (similar to Figs. 2(b) and (c)), and the results discussed below are based on this classification.
Fig. 2. Representative force-displacement curves for fried coating samples.
The fracture behavior was classified as (a) brittle, (b) ductile, and (c) partially ductile.
Effects of surfactants and frying time on water content and fracture properties of fried coating samples
The effect of the frying time on the water content of fried coating samples obtained at 170 °C (a widely accepted frying temperature) is shown in Fig. 3(a). Surfactant-free oil, hydrophobic surfactant-added oil, and hydrophilic surfactant-added oil were employed as the frying oil. For each frying oil, the water content rapidly decreased in the early stages of frying, and then gradually decreased. The deviation in the data was too large in the early stage of frying due to rapid evaporation of a huge volume of water, but then it decreased. The horizontal dashed line indicates Wc (4.36 g/100 g-DM) for fried coating determined previously [5]. At a water content lower than Wc, the fried coating samples are expected to be a glassy state when they are cooled down to 25 °C. The results for the brittle and ductile samples are plotted as open and closed symbols, respectively, in Fig. 3(a). As expected, it was found that the samples with a water content lower than Wc showed brittle fracture behavior, and those with a water content higher than Wc showed ductile fracture behavior. From these results, the samples having brittle and ductile behavior are hereafter described as glassy and rubbery samples, respectively. Although the fried coating samples contained gelatin to obtain a constant batter structure, gelatin is known to have a minor effect on Tg for carbohydrate materials [14]. Fracture force and oil content of fried coating samples, on the other hand, may have been affected by the addition of protein [15].
Fig. 3. Effect of frying time on (a) water content and (b) maximum fracture force for fried coating samples obtained at 170 °C.
The values are mean ± SD (n = 3). The open and closed symbols represent samples having brittle fracture behavior (glassy sample) and ductile behavior, including partially ductile (rubbery sample), respectively. In Fig. 3(a), the horizontal dashed line indicates Wc reported in a previous study [5]. The dotted lines indicate a linear approximation between the water contents above and below Wc for each sample. The frying time to obtain a glassy state was determined from the crossover point between the dotted and dashed lines.
The frying time at which the water content became lower than Wc (frying time to create a glassy state) was evaluated by a linear approximation between the water contents above and below Wc (dotted lines in Fig. 3(a)). The frying time for a glassy coating was lower in the order of hydrophilic surfactant-added oil (1.4 min) < hydrophobic surfactant-added oil (1.9 min) < surfactant-free oil (2.4 min). This means that the surfactants promoted the evaporation of water from the fried coating during frying, and the hydrophilic surfactant had a greater effect than the hydrophobic surfactant.
Effect of surfactants on the water evaporation from the fried coating during frying is discussed with reference to model drawings (Fig. 4). The water content of fried coating rapidly decreased in the early stages of frying (Fig. 3(a)). That is, water vapor bubbles phase is generated around the surface of fried coating (Fig. 4(a)). Since the water vapor bubbles phase insulates the surface from the frying oil, heat flux is reduced [11] and subsequent water release from the fried coating is diminished. The presence of surfactants in the frying oil reduces the interfacial tension between the oil and water, allowing the departure of water vapor bubbles from the surface of fried coating (Fig. 4(b)). To efficiently interact with the large amount of water molecules that are generated rapidly in the frying oil, many hydrophilic groups are required for the surfactant. In this viewpoint, the hydrophilic surfactant will be more effective than the hydrophobic surfactant.
Fig. 4. Model drawings of fried coating samples obtained by the surfactant-free oil (a) and surfactant-added oil (b).
The arrows indicate the expected movement of water molecules.
The effect of frying time on the maximum fracture force of fried coating samples obtained at 170 °C is shown in Fig. 3(b). As in Fig. 3(a), the results for the glassy and rubbery samples are plotted as open and closed symbols, respectively. The maximum fracture force for the glassy fried coating obtained by surfactant-added oil (open square and open triangle) was always lower than that for the glassy fried coating obtained by surfactant-free oil (open circle). As stated earlier, the surfactant promoted water evaporation due to the reduction of the interfacial tension between frying oil and water. The surfactant also reduces the interfacial tension between frying oil and wet fried coating [11], and thus the expansion of the fried coating during frying will be promoted (Fig. 4(b)). The effect of surfactant-added oils is similar as the effect achieved by increasing frying temperature; the higher frying temperature, the higher water vapor pressure, and thus the greater expansion is expected. In fact, it is reported that the crust of foods fried at high temperatures was higher porosity [16].
Effect of frying temperature on frying time for glassy samples and their appearance
Fried coating samples obtained by surfactant-free and hydrophilic surfactant-added oils were employed to also investigate the effect of frying temperature on the frying time for glassy samples and how it affected their appearance.
The effect of the frying time on the water content of fried coating samples obtained at 150, 160, and 180 °C is shown in Fig. 5. As shown in Fig. 3(a), the results for the glassy and rubbery samples are plotted as open and closed symbols, respectively. The relationship between the water content and frying time was similar behavior to that observed at 170 °C (Fig. 3(a)), and the frying time required to obtain a glassy state was evaluated at each frying temperature.
Fig. 5. Effect of frying time on water content of fried coating samples obtained at 150, 160, and 180 °C. The explanations of these figures are the same as those in Fig. 3(a).
The effect of frying temperature on the frying time for glass is shown in Fig. 6. The frying time for glassy samples decreased with increasing frying temperature, but there was little difference between 170 and 180 °C. Water molecules located at the surface of the materials evaporate in the initial frying stage; the water evaporation is strongly affected by the frying temperature because the water molecules and frying oil are in direct contact. For further water evaporation, water molecules located inside the materials have to diffuse to the surface. This process is not sensitive to the frying temperature because the inside of the materials is not exposed to frying oil. Because the water evaporation rate will be much higher than the water diffusion rate at high temperature (i.e., water diffusion will not keep up with water evaporation), there will have been little difference in the frying time required to obtain a glassy state between 170 and 180 °C.
Fig. 6. Effect of frying temperature on frying time to obtain a glassy state for each fried coating sample.
In the temperature range between 150 and 170 °C, a glassy fried coating was obtained using the hydrophilic surfactant-added oil faster (1.0-1.5 min) compared to the surfactant-free oil. This result (Fig. 6) is useful for controlling the texture of a fried coating according to the time-temperature-transformation concept. For example, surfactant-free oil set to 170 °C requires 2.4 min of frying time to form a glassy fried coating. For the same frying time, it is expected that the hydrophilic surfactant-added oil enables the formation of a glassy fried coating by frying at 156 °C. Frying at a lower temperature contributes to energy saving in the food industry.
The effects of frying temperature and time on the appearance (digital images) of fried coating samples obtained by the oils with and without the hydrophilic surfactant are shown in Fig. 7. The higher the frying temperature and the longer the frying time, the browner the coating color due to the progress of the non-enzymatic browning reaction [17]. It is known that this browning reaction is promoted by the reduction of water content in an intermediate water content range due to the solute-concentration effect [18]. Because the surfactant-added oil enhances water evaporation, the fried coating samples obtained by the surfactant-added oil were browner than those obtained by the surfactant-free oil under the same frying conditions.
Fig. 7. Effects of frying temperature and time on appearance of fried coating samples.
The black thick frames highlight the glassy samples obtained using the shortest frying time.
To achieve a brittle texture of fried coating, it is necessary to obtain a glassy fried coating. In a practical viewing, a shorter frying time is preferable. Thus, glassy fried coating samples obtained using the shortest frying time were treated as optimum fried coating samples and are emphasized by black thick frames in Fig. 7. Specifically, these were the samples fried by surfactant-free oil at 150 °C for 4.5 min, at 160 °C for 3.5 min, at 170 °C for 2.5 min, and at 180 °C for 2.5 min and fried by the hydrophilic surfactant-added oil at 150 °C for 3.5 min, at 160 °C for 2.0 min, at 170 °C for 1.5 min, and at 180 °C for 1.5 min. When the browning reaction color of glassy fried coating samples obtained using the shortest frying time was compared at each temperature, the glassy samples obtained by the surfactant-added oil were less brown than those obtained by the surfactant-free oil due to the reduction of frying time to obtain a glassy state. Although a browner glassy fried coating can be briefly obtained by the prolonged frying time, it is difficult for surfactant-free oil to obtain the opposite result. That is, the surfactant-added oil can increase the variation in browning for the glassy fried coating.
Effect of frying temperature on maximum fracture force and crude oil content of glassy samples
Glassy fried coating samples obtained using the shortest frying time (as shown by black thick frames in Fig. 7) were employed, and the maximum fracture force and crude oil content were investigated.
The effect of the frying temperature on the maximum fracture force for the glassy samples obtained by oils with and without the hydrophilic surfactant is shown in Fig. 8. There was a minor effect of frying temperature on the maximum fracture force of each sample. This result agreed with those reported previously; there were no significant differences in the maximum breaking force among the deep-fried foods obtained at 150-180 °C [19].
Fig. 8. Effect of frying temperature on maximum fracture force for glassy fried coating samples obtained using shortest frying time.
The values are mean ± SD (n = 3). Asterisks indicate a significant difference between surfactant-free oil and hydrophilic surfactant-added oil at p < 0.05 (t-test). The dashed lines (linear approximation) are guides for the eyes.
The values for the glassy samples obtained using the hydrophilic surfactant-added oil tended to be lower than those obtained using the surfactant-free oil at each frying temperature. As discussed earlier (Fig. 4(b)), it is thought that the reduction in the maximum fracture force induced by the surfactant-added oil is caused by the reduction of the interfacial tension between frying oil and wet fried coating.
The effect of frying temperature on the crude oil content of the glassy samples obtained by oils with and without the hydrophilic surfactant is shown in Fig. 9. The crude oil content of glassy samples tended to increase with increasing frying temperature; there were significant differences between 150 and 160-180 °C for surfactant-free oil (Fig. 9(a)) and between 170 and 180 °C for surfactant-added oil (Fig. 9(b)) at p < 0.05 (Tukey’s HSD test). It is known that the oil content of fried foods increases with increasing frying temperature and frying time [20]. As the mechanisms involved in oil sorption into the fried foods, water-oil replacement, surfactant, and cooling-phase effects are suggested [7] as follows. The porous layer is formed at the surface of fried foods due to water evaporation and then filled with the frying oil (water-oil replacement effect). The water-oil replacement effect acts during frying, but oil sorption also occurs in the cooling process after the frying; steam condensation associated by the cooling promotes oil sorption (cooling-phase effect). These effects will be enhanced by the increased frying temperature, because the higher frying temperature, the higher water evaporation rate, and thus the thicker and crispier (more porous) layer is expected [7, 21]. Surfactant effect, on the other hand, originates from the frying oil; surfactants produced by the thermal decomposition of frying oil (mono- and di-glycerides) reduce the interfacial tension and contact angle between the oil and foods, and thus oil sorption is promoted [22]. This effect will be also enhanced by the increased frying temperature.
Fig. 9. Effect of frying temperature on crude oil content of the glassy fried coating samples obtained using shortest frying time by surfactant-free oil (a) and hydrophilic surfactant-added oil (b).
The values are mean ± SD (n = 3). Different letters are significantly different at p < 0.05 (Tukey's HSD test).
According to the surfactant effect explained earlier, there is a possibility that the surfactant added into the oil reduces the interfacial tension and contact angle between the oil and fried coating, allowing it to absorb more frying oil. In fact, the crude oil content of glassy samples obtained using the surfactant-added oil was significantly higher than that obtained using the surfactant-free oil at p < 0.05 (t-test), as shown in Fig. S1 (see J. Appl. Glycosci. Web site). However, there are no significant differences at p < 0.05 (t-test) in the crude oil content between surfactant-free oil and the surfactant-added oil at other frying temperatures. This was because the surfactant added to the frying oil was a hydrophilic type; even if the surfactant is absorbed into the fried coating, there is little effect to cause the coating to also absorb the frying oil. According to this interpretation, there is a possibility that hydrophobic surfactant-added oil elevates the crude oil content of glassy fried coating. For confirmation, the crude oil content of a glassy sample obtained by the hydrophobic surfactant-added oil at 170 °C for the shortest frying time (2.0 min) was investigated, and 106.0 ± 10.4 g/100 g-defatted DM (mean ± SD, n = 3) was obtained. This value was not significant at p < 0.05 (Tukey’s HSD test), but tended to be higher than those obtained by surfactant-free oil (88.5 ± 8.4 g/100 g-defatted DM) and hydrophilic surfactant-added oil (78.6 ± 15.0 g/100 g-defatted DM); the tendency is supported by a significant difference at p < 0.10 between hydrophobic surfactant-added oil and hydrophilic surfactant-added oil.
CONCLUSION
This study clarified the effect of surfactant-added oil on the physical properties of the fried coating. The surfactants promoted the evaporation of water from the fried coating, and the hydrophilic surfactant had a greater effect than the hydrophobic surfactant. The results showed that the frying time required to obtain a glassy fried coating was reduced by the surfactant-added oils. According to the time-temperature-transformation concept, it is expected that the surfactant-added oils enable the formation of a glassy fried coating by frying at a lower temperature than surfactant-free oil does. Frying at a lower temperature or for a shorter time contributes to energy saving in the food industry. The maximum fracture force for the glassy fried coating obtained at 160 and 170 °C using the hydrophilic surfactant-added oil was significantly lower than those using the surfactant-free oil (p < 0.05). When the browning color of glassy fried coating samples obtained using the shortest frying time was compared at each temperature, the samples fried in the hydrophilic surfactant-added oil had less browning than those fried in the surfactant-free oil due to the reduction of the frying time to reach a glassy state. There were no significant differences at p < 0.05 in crude oil content between the surfactant-free and hydrophilic surfactant-added oils at each frying temperature (except for 150 °C). Hydrophobic surfactant-added oil, however, tended to elevate the crude oil content of the glassy fried coating. This study employed the fried coating samples as model deep-fried foods. For the quality control of fried coatings for deep-fried foods, foodstuff covered by a fried coating should be employed as samples in future work.
STATEMENTS AND DECLARATIONS
This study was funded by The Nisshin OilliO Group, Ltd. Kanji Aoyagi, Tomonari Otsuki, and Hidetaka Uehara are employees of The Nisshin OilliO Group, Ltd.
Supplementary Material
Fig. S1. Effect of frying temperature on crude oil content of the glassy fried-coating samples obtained using shortest frying time. The values are mean ± SD (n = 3). The asterisk indicates a significant difference at p < 0.05.
ACKNOWLEDGEMENT
The authors thank FORTE Science Communications (https://www.forte-science.co.jp/) for English language editing.
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Supplementary Materials
Fig. S1. Effect of frying temperature on crude oil content of the glassy fried-coating samples obtained using shortest frying time. The values are mean ± SD (n = 3). The asterisk indicates a significant difference at p < 0.05.