Quality characteristics and antioxidant activity of roasted yakgwa according to the addition ratio of mealworm

Mealworms (100%, Jeonnam Jangseong) were purchased from Chamgosoae (Jeonnam, Korea). Medium wheat flour (Gompyo, Korea), starch syrup (Chungjungone, Seoul, Korea), brown rice oil (Kasisuri, Busan, Korea), refined rice wine (Lotte Chilsung Beverage, Gunsan, Korea), salt (Cheil Jedang, Seoul, Korea), soybean oil (Sajo, Seoul, Korea), pepper (Shinsegae, Pocheon, Korea), and traditional rice syrup (Chungjungone) were purchased from a mart in Pocheon and an online mall.

Gallic acid, DPPH (α,α-diphenyl-β-picrylhydrazyl) and ABTS [2,2’-azino-bis (3-ethylbenzthiazoline-6-sulphonic acid)] were purchased from Sigma Chemical Co. (St. Louis, MO, USA). All other chemicals and solvents were of commercial analytical grade.

Table 1 lists the MY components and their mixing ratios. The mixing ratio used for yakgwa dough preparation was determined in preliminary experiments according to previously reported methods (Kwon et al., 2023). MP contents of 0-12 wt% (with respect to flour) were used. All other materials (starch syrup, refined rice wine, rice bran oil, salt, pepper) were used in identical contents. The mealworms were pulverized for 30 s using a grinder (FM-909T, Hanil Elec., Bupyeong, Korea). Medium wheat flour, MP, salt, and pepper were mixed and sieved, and the mixture was supplemented with rice bran oil. The resulting dough was mixed using a motion that resembles cutting (50 times), supplemented with refined rice wine and starch syrup, mixed using the same motion (50 times), left in a refrigerator for 30 min, rolled out to a uniform thickness of 1 cm, cut to dimensions of 3 cm width×3 cm length, and shaped by making four holes. The shaped dough was rapidly dipped in cooking oil, immediately placed on the pan of a preheated electric oven (DHC10-II, Softmill, Gwangju, Korea), and baked at 120°C for 15 min and then at 170°C for 10 min. The resulting yakgwa (Fig. 1) was cooled at room temperature for 1 h without applying any syrup coating for subsequent analysis. For sensory evaluation, the raters were given yakgwa that had been dipped in syrup for 1.5 h and dried for 30 min. The syrup was prepared by boiling a 1:1:0.2 (w/w/w) mixture of traditional rice syrup, starch syrup, and water for 15 min.

Fig. 1. Appearance of yakgwa added with mealworm powder. FM0, flour added with 0% mealworm powder; FM1, flour added with 3% mealworm powder; FM2, flour added with 6% mealworm powder; FM3, flour added with 9% mealworm powder; FM4, flour added with 12% mealworm powder.

Download Original Figure

The proximate compositions of MP and MY were determined using the AOAC method (2000). Moisture content was determined using an infrared moisture analyzer (MB45, Ohaus Corporation, Switzerland). Crude ash content was determined using the dry ashing method (KD210-B-1054 electric furnace, OPSIS, Furulund, Sweden). Crude protein content was determined using the micro-Kjeldahl method (KD210-B-1054 automatic distillation system, OPSIS). Crude fat content was determined using the Soxhlet method (SX-360-A2 extractor, OPSIS). Carbohydrate content was calculated as 100% minus the sum of moisture, crude ash, crude protein, and crude fat contents (Choi et al., 2016).

The volume of the MY dough was measured by placing a 5 g sample in a graduated cylinder with 30 mL of distilled water and used to calculate the MY density (= weight/ volume).

For pH and sugar content measurements, a 3 g sample (MP or MY dough) was mixed with 27 mL of distilled water, and the mixture was homogenized and filtered. The pH and sugar content of filtrate were measured by a pH meter (STARA2115, Thermo Fisher Scientific Inc., Waltham, MA, USA) and Brix meter (Atago PR-101α, Atago Co., Tokyo, Japan), respectively.

The MY expansion degree was determined from pre-/postcooking widths, lengths, and heights, and cooking loss was calculated from pre-/postcooking weights.

MY hardness was measured using a texture analyzer (TAXT Plus, Stable Micro System Ltd., Godalming, UK) with a 2 mm cylinder as the probe (pretest speed=3.0 mm/s, test speed=1.0 mm/s, posttest speed=5.0 mm/s, test distance=5.0 mm/s, trigger force=5.0 g).

For MP color measurement, an experimental cell was completely filled with MP. MY color was measured using a Hunter system (JX 777, Juki, Tokyo, Japan) and expressed as the surface L (lightness), a (redness), and b (yellowness) values. The standard plate featured L=98.20, a=-0.02, b=-0.27.

MP or MY (30 g) was mixed with 70% ethanol (70 mL), and the mixture was agitated in a shaking incubator (C-SK-2, Changshin Science Co., Seoul, Korea) at 20°C and 150 rpm for 12 h.

For total polyphenol content measurement, the agitated mixture (0.1 mL) was supplemented with 2 N phenol (0.2 mL) and distilled water (2 mL), shaken, and left to react for 3 min at room temperature. The resulting solution was mixed with 10 wt% Na₂CO₃ (2 mL) and left in the dark for 1 h, and the absorbance measured at 765 nm (EMC-18PC-UV, EMCLAB GmbH, Duisburg, Germany) was converted into total polyphenol content, which was expressed as mg gallic acid equivalents (mg GAE)/100 g. For total flavonoid content measurement, the agitated mixture (0.1 mL) was supplemented with 1 N NaOH (0.3 mL) and diethylene glycol (2 mL) and left to react (37°C, 1 h). Subsequently, the absorbance measured at 420 nm was converted into total flavonoid content, which was expressed as mg naringin equivalent (mg NE)/100 g.

MP and MY samples were prepared as described in Section 2.9. For the measurements of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) scavenging activity, the sample (0.5 mL) was supplemented with a 0.15 mM DPPH solution (2 mL) and left in the dark for 30 min, after which the absorbance was measured at 517 nm and converted into DPPH scavenging activity as

For the measurements of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) cation radical (ABTS^•+) scavenging activity, 7 mM ABTS and 2.45 mM potassium persulfate solutions were mixed in a 14:1 (v/v) ratio, and the mixture was left to react for 20 h. The resulting solution (1.6 mL) and sample of interest (0.1 mL) were mixed and left to react for 5 min, after which the absorbance was measured at 734 nm and converted into ABTS^•+ scavenging activity as

For reducing power measurements, the sample (0.5 mL) was supplemented with 0.2 M sodium phosphate buffer (0.5 mL) and 1 wt% potassium ferricyanide (0.5 mL), and the mixture was left to stand at 50°C for 20 min. Next, 10 wt% trichloroacetic acid (0.5 mL) was added, and the mixture was centrifuged. The supernatant (1 mL) was supplemented with distilled water (1 mL) and 0.1 wt% FeCl₃ (0.2 mL), and the absorbance was measured at 700 nm.

Sixty panelists (17 male and 43 female undergraduates in their 20s) were recruited from Daejin University (Daejin University Institutional Review Board (IRB) No. 1040656-202309-HR-01-05) and fully familiarized with the sensory evaluation method. The samples were cut to an identical size (width 1.5 cm×length 1.5 cm×height 3 cm), placed on a white plate, and evaluated by panelists provided with lukewarm water for mouth rinsing. Evaluation was performed on a seven-point scale (one: strongly dislike, seven: strongly like) in terms of overall preference, appearance, color, flavor, texture, and taste.

All measurements were performed in triplicate, and the results were expressed as the corresponding means±standard deviations. The SPSS Statistics software (ver. 27, IBM Co., Armonk, NY, USA) was used. The significance of the differences across samples was tested using the analysis of variance. In the case of significance, Duncan’s multiple range test was performed (p<0.05).

Table 2 presents the proximate compositions of MP and MY, revealing that MP contained 2.69 wt% moisture, 55.70 wt% crude protein, 35.96 wt% crude fat, 3.70 wt% crude ash, and 2.43 wt% carbohydrate. This composition almost agreed with those previously reported for freeze-dried mealworms by Lee et al. (2016) (0.11 wt% moisture, 51.38 wt% crude protein, 30.25 wt% crude fat, 3.47 wt% crude ash) and Jin et al. (2021) (0.80 wt% moisture, 54.07 wt% crude protein, 36.78 wt% crude fat, 3.29 wt% crude ash) with the exceptions of moisture and carbohydrate contents. Beom et al. (2007) reported that the moisture content of hot air-dried Acorus calamus leaves considerably exceeded that of the corresponding freeze-dried leaves. Thus, the lower moisture content of MP observed herein was ascribed to the use of hot air-drying in the present study and freeze-drying in previous studies. The proximate composition of Gryllus bimaculatus was previously determined as 6.20 wt% moisture, 65.02 wt% crude protein, 18.25 wt% crude fat, and 5.22 wt% crude ash (Baik et al., 2022), i.e., the crude protein content exceeded that observed herein for MP. The high crude protein content of mealworms (≥50 wt%) makes them a promising food supplement for increasing protein intake (Lee et al., 2023).

The moisture content was 6.14 wt% in the FM0 group, significantly decreasing to 4.44 wt% in the FM4 group (p<0.05). The crude protein/fat contents in the FM0 and FM4 groups were 8.37/20.87 wt% and 11.70/28.45 wt%, respectively, significantly increasing with the increasing MP content (p<0.05). The crude ash content did not significantly vary across the FM0-FM3 groups (1.29-1.33 wt%) but significantly increased to 1.45 wt% in the FM4 group (p<0.05). The carbohydrate content ranged from 53.96 to 63.30 wt% and decreased with the increasing MP content (p<0.05). In previous studies on MP-supplemented pan bread (Lee et al., 2023) and muffins (Yoon and Shin, 2023) and rice cookies supplemented with Tenebrio molitor larvae, Protaetia cataphracta larvae, and Gryllus bimaculatus powder (Jang et al., 2022), the moisture and carbohydrate contents decreased with the increasing additive content, while the crude protein, fat, and ash contents concomitantly increased, in agreement with the results of the present study. These findings verify the effects of the differences in proximate composition between flour and edible insects on the foods they are applied to (Lee et al., 2023). Hence, the addition of 3-12 wt% MP during the production of yakgwa was anticipated to increase its protein and fat contents but decrease the carbohydrate content and thus enhance nutritional functions.

Table 3 presents the measured density of MY dough. Koh and Noh (1997) showed that cookie density can serve as a quality indicator, with low density indicating low cookie quality and facilitating crumbling. The density of MY dough in the present study was 0.17 g/mL across all samples without any significant variation, which agreed with the results of a previous study on rice cookies supplemented with Tenebrio molitor larvae, Protaetia cataphracta larvae, and Gryllus bimaculatus powder (Jang et al., 2022). Park and Joo (2021) also showed that sea buckthorn leaf powder content had no effect on sample density, which was attributed to the fact that this content was low and indicated that it did not significantly impact cookie quality. The lack of density change observed herein was therefore ascribed to the relatively low MP content.

The results of pH and sugar content analyses are listed in Table 3. The pH and sugar content of MP were 6.93 and 1.70 °Brix, respectively. The pH reported by Nam and Sim (2021) for MP was 6.82 and agreed with that obtained herein.

pH values of 5.72, 5.97, 6.17, 6.32, and 6.42 were obtained for the FM0, FM1, FM2, FM3, and FM4 groups, respectively, significantly increasing with the increasing MP content (p<0.001). In a previous study on rice cakes supplemented with MP (Nam and Sim, 2021), pH increased with the increasing MP content, in line with the results presented herein. Park and Joo (2021) reported that the pH of sea buckthorn leaf powder was lower than that of flour and thus rationalized the decrease in dough pH with the increasing MP content. Thus, the increase in the dough pH with the increasing MP content observed herein was ascribed to the lower pH of flour (5.99) compared with that of MP (6.93).

The sugar content of FM0-FM4 groups ranged from 1.70 to 1.80 °Brix and was insignificantly higher in the FM0 and FM4 groups (p<0.05). Park and Joo (2021) reported that the sugar content of sea buckthorn leaf powder-supplemented samples exceeded that of flour and showed that the dough pH increased with the increasing sea buckthorn leaf powder content. On the contrary, the sugar content of our MP (1.70 °Brix) was low to not significantly affect the sugar content of MY.

Table 4 lists the expansion degrees and cooking losses of MY. Expansion degrees of 20.93%, 14.81%, 15.24%, 15.49%, and 12.83% were observed in the FM0, FM1, FM2, FM3, and FM4 groups, respectively, significantly decreasing with the increasing MP content (p<0.001). Oh and Sim (2017) reported that samples with moisture contents below that of the medium wheat flour featured low expansion degrees because of the low amounts of moisture available for gluten formation. The moisture content of MP in the present study (5.83 wt%) was lower than that of the medium wheat flour (11.72 wt%); hence, the insufficient amount of moisture for gluten formation led to low expansion degrees. Cho et al. (2017) reported that mealworm proteins created a network structure through fusion during high-temperature extrusion, whereby compactness increased and the expansion degree deceased. Hence, the increase in protein content with the increasing content of MP in MY resulted in increased compactness and thereby decreased the expansion degree.

Cooking loss is largely due to the loss of moisture during the heating of dough in the baking process (Chung et al., 2014). For cookies, which are generally produced similarly to yakgwa, the MP added to the dough is known to generate bound water through physicochemical reactions and thus reduce cooking loss (Oh et al., 2022). The cooking loss in the FM0-FM4 groups equaled 7.77-9.70% and was not significantly affected by the MP content. Thus, the MP content had a negligible effect on the cooking loss of MY.

The hardness of yakgwa depends on the raw materials used, production methods, and sample characteristics (Kwon et al., 2023). Herein, hardness of 1.18, 1.09, 1.08, 1.03, and 0.85 kg were observed for groups FM0, FM1, FM2, FM3, and FM4, respectively, decreasing with the increasing MP content (p<0.05; Table 4). In a study on MP-supplemented pasta (Kim et al., 2014), a decrease in hardness with the increasing MP content was observed and ascribed to the concomitant reduction in gluten formation. Chabot (1979) and Shin (2015) reported that cookie hardness depended on the content of moisture, specific weight, and pore development and decreased with the decreasing expansion degree and water loss as a result of the development of abundant pores or reduced gluten formation. Hence, the negative correlation between MY hardness and MP content observed herein was attributed to the prevention of gluten formation in yakgwa, which, in turn, reduced the expansion degree and cooking loss.

Table 4 presents the color values of MP and MY based on the Hunter system. The L, a, and b values of MP equaled 43.93, 5.83, and 12.95, respectively, and agreed with those reported by Nam and Sim (2021) (44.84, 2.33, and 10.14, respectively). In multiple previous studies, MP was reported to be dark brown, with MP addition therefore causing food darkening and reddening (Kim et al., 2014; Nam and Sim, 2021; Lee et al., 2023; Yoon and Shin, 2023). Thus, we expected MP addition to reduce lightness and increase redness.

The L value equaled 74.17 in the FM0 group, corresponding to the highest lightness, and 56.11-62.60 in the FM1-FM4 groups, indicating darkening with the increasing MP content (p<0.001). The a value of MY was the lowest (0.47) in the FM0 group, increasing with the increasing MP content (p<0.001). The b value of MY decreased with the increasing MP content, with the exception of the FM0 group (p<0.001). In a study on MP-supplemented cookies (Min et al., 2016), the L and b values decreased, whereas the a value increased with the increasing MP content, which agreed with the trend observed herein. Shin (2019) reported that food color was greatly influenced by the color of the main and added materials. The MP proteins were shown to facilitate the Maillard reaction and, hence, the browning of MP-containing rice cakes. Thus, the color change of MY due to MP addition was ascribed to the influence of the Maillard reaction of proteins and sample color.

Table 5 presents the total polyphenol and flavonoid contents of MP and MY. The total polyphenol and flavonoid contents of MP were 406.52 mg GAE/100 g and 21.18 mg NE/100 g, respectively, deviating from the values (335.75 mg GAE/100 g and 8.44 mg GAE/100 g, respectively) previously reported by Jin et al. (2021). This deviation was ascribed to the variations in the mealworm breeding conditions/duration and employed drying method (Jin et al., 2021).

The total polyphenol content of MY ranged from 77.00 to 167.54 mg GAE/100 g, significantly increasing with the increasing MP content (e.g., by 118% upon going from FM0 to FM4). The total flavonoid content of MY increased by as low as 38% and as high as 114% in proportion to the added amount of the sample compared to the control group (p<0.001). Similar results, namely an increase in the total flavonoid and polyphenol contents with the increasing additive content, were obtained for muffins supplemented with MP (Yoon and Shin, 2023), yakgwa supplemented with tartary buckwheat flour (Kim et al., 2023), and yakgwa supplemented with kumquat (Kwon et al., 2023). In addition, as high total polyphenol and flavonoid contents were reported to result in excellent antioxidant activities, the addition of MP was anticipated to enhance antioxidant activity (Jin et al., 2021; Yoon and Shin, 2023).

Table 5 lists the antioxidant activities of MP and MY. The DPPH and ABTS^•+ scavenging activities and reducing power of MP were 90.25%, 44.06%, and 1.74 OD, respectively, deviating from the values reported by Jin et al. (2021) (82.60%, 66.39%, and 1.02 OD, respectively). As in the case of the total polyphenol and flavonoid contents, this deviation was ascribed to the variations in the mealworm breeding conditions and drying method.

For MY, the DPPH scavenging activity was the lowest at 28.22% (FM0), significantly increasing with the increasing MP content (p<0.001). The ABTS^•+ scavenging activity was also lowest in the FM0 group, increasing 2.31-4.24-fold upon MP addition. The reducing power increased with the increasing MP content, equaling 0.18, 0.39, 0.43, 0.54, and 0.62 OD in the FM0, FM1, FM2, FM3, and FM4 groups, respectively. In studies on rice cakes supplemented with MP (Nam and Sim, 2021) and yakgwa supplemented with mulberry (Shin et al., 2014), the DPPH and ABTS^•+ scavenging activities and reducing power significantly increased with the increasing additive content. In their study on rice cookies supplemented with Tenebrio molitor larvae, Protaetia cataphracta larvae, and Gryllus bimaculatus powder, Jang et al. (2022) reported that antioxidant activity (and hence, bioactivity) increased with the increasing insect content. Cho et al. (2017) reported that the presence of selenium in mealworms could be responsible for their beneficial effect on antioxidant activity, and Lee et al. (2014) reported that the hydrolysis of mealworm proteins at high temperatures released certain substances with antioxidant activities. Hence, the increase in the antioxidant activity of MY with the increasing MP content was ascribed to the effects of the selenium, polyphenols, and flavonoids contained in MP as well as the products of mealworm protein hydrolysis during yakgwa production. The addition of MP was therefore concluded to effectively increase the antioxidant activity of yakgwa.

Table 6 presents the sensory evaluation results for MY, revealing that the overall preference score showed no significant differences across the samples in groups FM0-FM3 (5.30-5.53) and was lowest for the FM4 group (4.50). The appearance and color scores were 6.10 and 5.73 in the FM0 group and 6.10 and 5.89 in the FM1 group, respectively, significantly decreasing with the increasing MP content (p<0.001). The flavor and texture scores did not vary significantly across the samples. The taste score was 5.48, 5.47, 5.35, and 5.33 in the FM0, FM1, FM2, and FM3 groups, respectively, showing no significant differences across the samples, and was lowest in the FM4 group (4.43, p<0.001). In a study on rice cakes (Nam and Sim, 2021), the addition of >6 wt% MP with respect to rice flour decreased the preference scores. In a study on pan bread (Lee et al., 2023), the addition of >5 wt% MP with respect to the bread mix powder also decreased the preference scores. In a study on rice cookies supplemented with Tenebrio molitor larvae, Protaetia cataphracta larvae, and Gryllus bimaculatus powder (Jang et al., 2022), most preference scores were higher in the control group than in the groups with added insect powder. In a study on fishcakes supplemented with MP (Seo et al., 2020), the scores of all five items tested in the sensory evaluation were higher in the control group. Although edible insects have attracted growing interest with the steadily decreasing negative consumer perceptions (Jeong, 2017), low preference scores were reported for foods containing edible insects in previously conducted sensory evaluations. Herein, the FM4 group featured the lowest preference scores of the six tested items (4.10-5.07); however, these scores were still above the moderate level of 4.0. The overall preference, flavor, texture, and taste scores of the FM3 group (MP content=9 wt%,) were not significantly different from those of the control group. Thus, yakgwa was determined to be well suited for supplementation with MP to enhance the consumer preference for mealworms.

The results of proximate composition, quality characteristics, antioxidant activity, and consumer preference analyses collectively suggested that 6-9 wt% MP with respect to flour is the most suitable level for increasing the protein content and antioxidant activity of yakgwa without markedly decreasing consumer preference. Additional studies aiming to increase the MY preference scores should be carried out to develop a healthy, tasty, and stylish traditional Korean dessert with a high consumer preference.