Quality characteristics of silver carp surimi gels as affected by okara

ABSTRACT The objective of this study was to investigate the effects of okara on the quality of silver carp surimi gels. The characteristics of surimi gels without and with okara (0%, 2%, 4%, 6%, 8%, and 10% w/w) were evaluated by measuring water-holding capacity (WHC), color, sensory, textural, and rheological properties. With further addition of okara, WHC, breaking force, textural properties, and whiteness were decreased (P < 0.05), while springiness of surimi meatballs had no significant difference in sensory evaluation. Storage modulus (G′) and loss modulus (G″) decreased along with increasing okara concentration. However, the sensory evaluation showed it was acceptable for surimi meatballs with 6% okara or less. Among different particle sizes (375, 805, 509, 387, 190, and 34 μm) of okara, surimi gels with 6% okara of larger particle sizes had higher values of hardness, gumminess, chewiness, and breaking force, while those with smaller particle sizes showed higher whiteness, but there was no significant difference on WHC of surimi gels. The storage modulus (G′) and loss modulus (G″) of surimi pastes also decreased with increasing particle sizes of okara. However, sensory evaluation showed no difference on surimi meatballs with different particle sizes of okara. Results demonstrated that okara could be used as an ingredient to improve the quality and nutritional value of surimi-based products.


Introduction
Okara or soybean residue is the yellowish-white fibrous residue of soybeans produced during the production of soy milk or tofu production processes, and the most common utilization is used as animal feed or fertilizer. [1][3] Okara also contains high levels of soy-specific phytochemicals, such as isoflavones and soy saponins.Both are metabolized by the gut microbiome, with the former reducing menopausal symptoms and the latter connecting to antioxidant and cardiovascular protective effects. [4]Okara can be used as an ingredient in food processing for calorie intake reduction and dietary fiber supplements. [5]With the development of the soybean industry, the accumulation of okara is considered a major challenge for Asian countries (China, Japan, Korea, and Singapore). [6]tudies have shown that dietary fiber from okara has many health-promoting properties, such as antioxidant activity, anti-obesity activity, low fat, and hypoglycemic effects.9][10] Nowadays, interest has been taken to powered okara for its advantageous technological properties, such as high water-holding capacity (WHC) and fat-binding capacity.Furthermore, DF contributes to strengthen the structure of food system. [11,12]The parameters of tofu gel texture profile analysis (TPA) decreased with increasing okara insoluble dietary fiber (IDF) concentration, due to the inhomogeneous and unstable network structure caused by the anionic group (-OH) on the DF surface. [13]The addition of okara (60-mesh) significantly increased hardness, chewiness, and breaking force of pork gels, because of the function of okara in absorbing water and filling (as a filler) in the gel matrix.Thus, the addition of okara might have different effects on the textural properties of foods made from various protein sources, due to different nonspecific interactions between fiber, protein, water, and other components in the gel system. [13,14][17] However, few studies on surimi with IDF.For example, there were studies about effect of cellulose on surimi-based shellfish analog [18] and effect of wheat dietary fiber on Alaska pollock surimi products. [19]Results indicated that wheat dietary fiber was very important for functional characteristics of fishery products.Some studies investigated the effect of fiber particle size on the products, such as the size of sugarcane dietary fiber that could affect the softness and sensory properties of bread and the size and addition ratio of okara fiber-affected tofu properties. [13,20]The addition of okara caused an increase in hardness and breaking force of pork meat gels. [21]The addition of nanosized okara IDF (0-0.8 g/100 g) could enhance the gel strength of silver carp surimi and affect the moisture distribution of the gel. [22]However, there are fewer studies on the effects of okara concentration and particle size on the characteristics of surimi gels.
Surimi has attracted global attention for its good gel-forming ability and rich nutritional value.With a global production of 4.9 million tons in 2020, silver carp (Hypophthalmichthys molitrix) is one of the most important freshwater fish species in the world. [23]Silver carp is mainly used for surimibased food production in China, due to its rich nutrition, low breeding costs, and fast growth rate.The objective of this study was to investigate the influence of concentration and particle size of okara on the properties of surimi gels from silver carp.It would potentially improve the quality and nutritional value of surimi-based products with the utilization of the by-product from tofu/soymilk production.

Materials
Frozen silver carp surimi was purchased from Honghu Jingli Aquacultural Food Ltd (Jingzhou, Hubei, China) and stored at −24°C.The moisture content of surimi was 76.44%.Fresh okara was obtained from a local soybean shop in the local market.

Preparation of okara powder
Fresh okara was placed evenly on stainless steel plate and dried at an electricity heat drum wind drying oven (DHG-9240A, Jinghong Experiment Equipment Industry Co., Ltd., Shanghai, China) at 60°C for about 20 h.It was 375 μm of average particle diameter for okara powder.Then, okara powder was stored in polyethylene bags in a desiccator (φ 40 cm).The desiccative okara was ground and then pass different size screens (20-mesh, 40-mesh, 60-mesh, 80-mesh, and 100-mesh), and the average particle diameters were 805, 509, 387, 190, and 34 μm successively.

Preparation of surimi gels
Frozen silver carp surimi was thawed at 4°C for 12 h, cut into small pieces, and then chopped for 1 min in food processor (HR7625, Philips, Hong Kong, China).Salt (2.0%) was added into surimi and mixed for 1 min.Then, okara was added by two different procedures and mixed for 2 min.The first procedure was to add different concentrations of okara (0, 2%, 4%, 6%, 8%, and 10% of the total weight), and the sample without added okara was used as the control.The second procedure was to add 6% different particle sizes of okara (375, 805, 509, 387, 190, and 34 μm) and 375 μm okara as the control, which was raw and non-screen-separated okara.The final moisture content was adjusted to 80% in surimi paste with iced water.Prepared paste was stuffed into a polyethylene sausage casing (diameter 20 mm).The prepared samples were heated into a water bath (HH-4, Changzhou Guohua Electric Co., Ltd., Jintan, Jiangsu, China) at 90°C for 30 min, cooled in an ice-bath for 30 min, and stored at 4°C for further analysis.

Preparation of surimi meatballs
For the preparation of surimi meatballs, thawed surimi was weighted and ground with food processor (HR7625, Philips, Hong Kong, China) at speed of 1 for 1 min.Then, salt (2% of the total weight) was added and mixed for 1 min.Ingredients (6% starch, 0.5% onion powder, 0.5% monosodium glutamate, 2% sugar, 9% egg white, 1% cooking wine, 5% soya-bean oil of total weight), water, and okara (2%, 4%, 6%, 8%, 10% of the total weight, or 6% okara with different particle sizes) were added and mixed for 2 min.The final moisture of all surimi pastes was adjusted to 80%.Surimi meatballs (diameter 20 mm) were shaped by hand and cooked in boiling water for 6 min.The cooked surimi meatballs were cooled at room temperature for sensory evaluation.

Proximate analyses
Total crude fiber content of okara was determined by the Chinese National Standard (GB/T 5009.10-2003).Crude fat was determined by Soxhlet extractor method, and crude protein content was measured by the Kjeldahl method.

Okara functional properties
Water retention capacity (WRC): WRC was measured the capacity of water retained by the sample in a certain amount of water. [24]Sample (1.0 ± 0.05 g) was mixed with distilled water (25 mL) in a 50 mL centrifuge tube.The sample was stirred and left at room temperature for 24 h, centrifuged for 20 min at 4,500 × g, and then removed residual water by filter paper.WRC was expressed as g water/g dry sample.
Swelling capacity (SC): SC was measured as bed volume after equilibration in excess solvent. [25]kara (1.5 ± 0.05 g) was mixed with 20 mL distilled water in a 50 mL graduated cylinder, stirred gently to eliminate trapped air bubbles, and then left it on a level surface at room temperature for 6 h.Finally, the volume occupied by the sample was measured.SC was expressed as mL water/g dry sample.
Fat adsorption capacity (FAC): FAC was measured as the quality of oil retained by the sample. [26]ample (1.0 ± 0.05 g) was mixed with 20 mL soybean oil (Yihai Kerry Oils & Grains Co., Ltd., Wuhan, China) in a 50 mL centrifuge tube.The sample was stirred and left at room temperature.After 24 h, the sample was centrifuged for 20 min at 4,500 × g.The excess supernatant was decanted, and the dissociative oil was removed by filter paper.FAC was expressed as grams of soybean oil retained per gram of dry sample.
WHC: WHC was measured following the method of Cardoso et al. [27] Surimi gel (2.0 g) was weighed and placed them between two layers of filter paper.The sample was placed at the bottom of a 50 mL centrifuge tube and centrifuged in an Avanti J-E high speed refrigerated centrifuge (Beckman Coulter, Brea, CA, USA) at 3,000 × g for 20 min at 4°C.After centrifugation, the samples were weighed again.

Instrumental textural properties
For textural property analysis, the gels were placed at room temperature before measurement, cut into cylinders (20 mm in diameter, 20 mm in height), and measured by TA.XT Plus Texture Analyzer (Texture Technologies Corp., Scarsdale, NY, USA).The conditions of texture analysis were as follows: 5.0 mm/s pretest speed, 1.0 mm/s test speed, 1.0 mm/s posttest speed, compression 50% of initial height, 5 s pause time between two compressions, and 5.0 g trigger force with a flat plunger (P/36 R).Cylinder-shaped samples with a length of 20 mm were prepared and subjected to measurements at room temperature. [28]

Puncture test
The parameters of the puncture test were 5.0 mm/s pretest speed, 1.0 mm/s test speed, 1.0 mm/s posttest speed, 10 mm puncture distance, and 50 g trigger force with a spherical-ended stainless steel plunger (P/0.25S).Breaking force and breaking deformation of gels were obtained. [29]

Color evaluation
The color values of gel samples were determined by a Hunter Lab Ultra Scan XE colorimeter (Hunter Lab Co., Ltd., Reston, VA, USA).Usually, lightness (L*), redness (a*), and yellowness (b*) values were recorded.The whiteness (W) was calculated using the following equation: [30,31] W ¼ 100 À ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ð100 À L�Þ � ð100 À L�Þ þ a � �a � þb � �b� p

Dynamic rheological properties
Dynamic rheological properties of samples were determined according to the method by Wang et al. [14] with slight modification.Dynamic rheological properties of surimi pastes were conducted by an AR2000ex Rheometer (TA Instruments, Newcastle, DE, USA) equipped with a 40 mm parallel steel plate.A gap of 1 mm was set, and silicone oil was used to prevent water evaporation.Surimi paste was loaded between the cone and plate.During the temperature sweep, the sample was heated from 20°C to 90°C at a heating rate of 1°C/min with 5 Pa oscillation stress and 1 Hz frequency.Storage modulus (G', representing elastic properties) and loss modulus (G", representing viscous characteristics) were recorded.

Sensory evaluation
Acceptance test was done with 7-hedonic scale range from 1 (dislike extremely) to 7 (like extremely).In this study, untrained consumers were recruited from the campus of Huazhong Agricultural University (Wuhan, China).Samples were placed on the paper plates.Five attributes were taste, appearance color, springiness, texture, and overall likeness of surimi meatballs.Each sample was coded with a randomly selected three-digit number.Panelists were instructed to use water to clean their palates between the samples.Surimi meatballs were considered acceptable if their mean scores for overall likeness were above 4 (neither like nor dislike).All samples were evaluated under the same conditions.

Statistical analysis
Analysis of variance was performed using the SPSS package (SPSS 17.0 for Windows, SPSS Inc, Chicago, IL, US).Differences among the mean values of various treatments were measured by Duncan's multiple-range test (P < 0.05).All sample conditions were replicated in triplicates with three repeat measurements for each replication.

Proximate analyses
Table 1 shows the basal component of okara with different particle size.Okara powder contained protein 23.03 g/100 g dry basis, lipid 20.54 g/100 g dry basis, crude fiber 43.93 g/100 g dry basis.
Content of crude protein increased with okara particle size decreased, while crude fat and fiber decreased, which might be related to the damage to the cell walls caused by grinding, fat freed and increased content.Protein content in black soybeans increased with decreasing particle size, while lipid and DF content showed an opposite trend. [32]nctional properties of okara Hydration properties (WRC and SC) of okara are related to a matrix retaining water capacity. [33]WRC is an important property of fibrous materials in both technological and physiological terms.Okara retained approximately 5.97 times its weight of water and 1.35 times its weight of oil separately, while its SC was about 4.85 mL/g.WRC of okara was lower than other fibrous materials, like fruit and vegetable fiber concentrates, [34] but within the same range as reported in white-grape. [35]The FAC of okara was consistent with wheat bran (1.6 g oil/g) [26] and fruit and vegetables (less than 2 g⁄g), [36] but it was much lower than Laminaria digitata (15.6 g oil/g). [37]The SC of okara was lower than wheat fiber. [19]Many factors could influence the hydration properties of DF, such as the chemical structure of component polysaccharides, and processing method upon DF, including particle size, ionic form, pH, temperature, ionic strength, the type of ions in solution, and processing stresses. [38]Furthermore, the source of DF played an important role in the capacity of holding water and oil.For example, WRC of DF from lemon (12.50 g water/g) [39] were quite different from citrus fruits (10.66 g water/g) [40] and grapefruit (9.77 g water/g). [41]able 2 shows the functional properties of okara with different particle sizes.Hydration properties (WRC and SW) and FAC decreased significantly (P < 0.05) with the reduction of particle sizes.The WRC of the largest particle size (805 μm) okara was the highest, while there were significant differences (P < 0.05) comparing with others.Particle size and structure of okara had an effect on the hydration properties, while the grinding might cause collapsing of the DF matrix and weaken the ability of entrapping water. [42,43]The FAC is an important property of DF. [44] Okara FAC decreased significantly (P < 0.05) with the reduction of particle sizes.Different particle sizes of DF had different hydrophobic properties and overall charge density, which might be the reason for the FAC changes. [2,26]With the increase in particle size, the SC of okara increased and then decreased.The SC of okara was the highest at a particle size of 509 μm, while the changed trend was consistent with WRC and FAC.The results suggested that larger particle sizes of okara exhibited higher functional properties in a range of particle sizes (Table 2), which could be reinterpreted as DF bulk density.It could absorb more water with more open structure of a larger particle size of okara.

Water-holding capacity
The WHC is the ability to effectively immobilize water molecules through the capillary effect of the gel matrix and is an important property of food systems that reflect spatial protein arrangement. [45]yosin and actin interact during the gelling process could produce a continuous three-dimensional network to keep water. [46]The WHC was expressed by measuring the expressible moisture in surimi gels.The WHC of surimi gels significantly decreased (P < 0.05) with the increasing concentration of okara (Figure 1a).Surimi gel with 8% okara resulted in the lowest WHC (70.60%).There was no significant difference for the WHC of surimi gels with 8% and 10% okara (P > 0.05).The increasing concentration of okara significantly reduced the WHC of surimi gel (P < 0.05), which was consistent with the trend for surimi containing wheat fiber as the moisture constant was held constant. [47]The reduced WHC of surimi gels with okara was probably related to the high content DF in okara. [18,48]On the other hand, composition ratio of insoluble/soluble DF in okara was actually up to 50.77/4.71%, [49]o IDF filled into the protein dimensional network and weakened its ability of entrapping water.The relative content of protein in surimi decreased with the increase of okara addition.This could cause the DF to be inserted into the three-dimensional protein network of surimi and have a certain destructive effect on the protein network structure, thus weakening its ability to retain water in the gel system.Figure 1b shows WHC of surimi gels with different particle sizes of okara.In an okara/water system, larger particle size okara could be retained more water as a result of the higher internal pore volume. [50]The particle sizes of okara did not significantly affect the WHC of surimi gel (P > 0.05), which was consistent with restructured fish products with wheat fiber of different particle sizes. [47]It might not be so critical for differences in dietary fiber characteristics, as DF and protein competed to bind water at the same time. [51]Results indicated that the amount of okara added had a significant effect on the WHC of surimi gels, but the particle size of okara did not significantly influence its WHC.

Textural properties
TPA is an empirical method for assessing texture and allows the determination of different parameters. [13]Table 3 shows the textural properties of surimi gels with okara.Hardness, cohesiveness, gumminess, and chewiness decreased significantly (P < 0.05) with the addition of okara, compared with the control.It might be due to an increase in okara content that led to a decrease in the protein content of surimi.Surimi with higher protein concentration was more likely to have the gelation. [52]he addition of DF could relatively reduce the protein content in the system. [53]It could result in the disruption effect of DF on surimi gel when interacted with the surimi protein dimensional network.Meanwhile, lower proportion of protein also decreased hardness. [54]The springiness decreased in surimi gels with more addition of okara (>4% w/w).The decreasing effect of textural properties of surimi gels with the addition of okara could be related to the increased interactions in DF and water, which replaced the protein-water interactions. [51]These results were consistent with that of adding wheat fiber to horse mackerel [47] and adding cellulose to surimi gel in which hardness and cohesiveness reduced significantly. [18]However, controversial results were reported on textural properties when various DF added to diverse meat products. [55]The variation of texture was probably due to the "filling effect" and "dehydration (water absorption) effect" of DF. [21,22,[56][57][58] The particle sizes of okara significantly influenced that of surimi gels (Table 4).Hardness, gumminess, and chewiness of surimi gels significantly decreased with the reduction of particle sizes of okara (P < 0.05), but springiness and cohesiveness slightly increased.There was no significant difference (P > 0.05) on springiness as okara particle size decreased from 387 μm to 190 μm.The WRC and FAC of okara decreased with decreasing particle size (Table 2).This was consistent with the trends in hardness, gumminess, and chewiness of surimi gels, indicating that WRC and FAC of okara with different particle sizes might influence the textural properties of surimi gels.Soy protein isolate showed strong effects of rigidity on medium-grade Alaska pollock surimi gel. [31]Wheat dietary fiber could weaken the gel by acting as a filler and decrease the integrity of the protein network, while smaller particle size revealed lower hardness and chewiness of giant squid surimi gel. [51]The particle size of deacetylated Konjac Glucomannan (KGM) had a significant effect on the gel properties of surimi gels from silver carp. [59]Hardness, gumminess, and chewiness decreased with reducing particle sizes of okara might be due to the decrease in surimi or fish protein and the increase in crude fiber in the gel network.Results indicated that the increase of okara addition negatively affected the textural properties of surimi gels, while the increase in the particle size of okara had a positive effect on hardness, gumminess, and chewiness of surimi gel but a negative effect on its springiness and cohesiveness.Puncture test: Gel strength is an indispensable indicator of the quality of surimi-based products. [60]Figure 2a-c shows breaking force, deformation, and gel strength of surimi gels with different levels of okara.It was the lowest breaking force of surimi gels with more than 6% okara.Breaking force of surimi gels decreased significantly (P < 0.05) with increasing okara because okara fibers might weaken the gel structure of surimi gels.The proteins of okara might have acted as fillers and formed a secondary gel network, which was presumably not associated with the myofibrillar gel matrix.Soy isolate protein could hinder its cross-linking action, which might have some negative impact on the gel strength. [61]As a "non-gelling" component, okara fibers might bind to the main network in a randomly interacting manner and weaken the gel matrix. [62]Breaking distance and gel strength showed the same trend as breaking force.When okara concentration was higher than 6% (w/w), there was no significant difference in the performance of surimi gels (P > 0.05).The WHC of surimi gel tended to decrease with the increase of okara addition (Figure 1a).Such reduction could be related to the capacity of water retention and mass expansion during heating, probably because addition of some ingredients into meat products might produce less rigid and more easily broken structure. [63]he particle size of dietary fiber could affect the textural properties of meat products. [64]Figure 2d-f shows breaking force, deformation, and gel strength of surimi gels with different particle sizes of okara.Breaking force of surimi gel was significantly decreased as the particle size of okara decreased (P < 0.05), probably due to the higher surface activity caused by smaller size of okara particles. [2,14]t consisted of the pattern of hardness of surimi gels with different particle sizes of okara (Table 4).As the filler, different particle sizes of okara might result in different denser structures of surimi gels.The deformation, which characterized the springiness of surimi gel (Table 4), increased slightly as the particle sizes of okara decreased (Figure 2e).The particle sizes of okara had no significant effect on gel strength of surimi gels (P > 0.05).It was similar with the effect of wheat dietary fiber with short-particle DF (SS3) and long-particle DF (SL3) on gel strength of giant squid surimi gels. [51]Therefore, the addition of okara affected the network structure, resulting in changes in the characteristics of surimi gel.

Color measurement
Color is an important attribute of food that affects consumer acceptability. [13]Color parameters were detected to evaluate whether okara concentration can cause changes of surimi gels.Generally, surimi gels demand higher lightness (L*), lower yellowness (+ b*), and higher whiteness. [65]Table 5 shows the effect of different levels of okara on the color of surimi gel.The highest L* value was obtained in the control or surimi gel without okara (P < 0.05).The L* value of surimi gels decreased significantly (P < 0.05) as okara content increased.The lower L* values with okara might be related to the lower WHC of surimi gel.The a* value of surimi gels increased significantly (P < 0.05) as more addition of okara.The increase of a* value should be related to okara color.Like a* values, the b* values of surimi gels increased significantly (P < 0.05) as more addition of okara.Therefore, the increase of okara caused a significant decrease in the whiteness of surimi gels (P < 0.05), mainly due to the color of okara itself.The whiteness of tofu decreased due to more addition of okara. [13]Although okara slightly decreased the whiteness of surimi gels, the color of silver carp surimi gels was still accepted when compared with the premium quality surimi of most species. [66]able 6 displays the color changes of surimi gels with different particle sizes of okara.The L*, a*, and b* values increased significantly (P < 0.05) as the particle size of okara decreased.Therefore, its whiteness has also increased significantly (P < 0.05).The enhancement in whiteness of the nanosized DF added tofu gel could be attributed to the milky white color of nanosized DF. [2] The whitening effect of smaller okara powder was due to okara color itself, because of okara became whiter as the decrease in its particle size. [2,13]The results demonstrated that more addition of okara decreased the whiteness of surimi gels (Table 5) but the smaller particle size of okara could increase the whiteness (Table 6).

Rheological properties
Dynamic rheology is used to determine the nonfracture gel properties of surimi.69] Figure 3aandb displays dynamic viscoelastic behavior of surimi pastes with or without okara during heating from 20°C to 90°C.G′ values first showed a slight increase before temperature reached 40°C, which indicated proteins formed gel networks.Then, G′ decreased drastically with the heating temperature from 40°C to 47.7°C, possibly due to the dissociation of actin-myosin complex and the denaturation of myosin tail. [72]G′ values increased when the temperature over 48.4°C because of the aggregation of unfolded proteins and the formation of irreversible gel networks. [73]During heating, myosin head began to denature and aggregate as the temperature increased, due to the formation of disulfide bonds at 50-60°C. [28]The G′ value increased slightly as the temperature continued to increase.The pattern of G′ values depended on okara content of the surimi paste during heating.The higher content of okara in the paste caused the lower G′ values during the temperature-rise period, because of the weakening effect of okara on the formation of protein gel network.The denaturation temperature of surimi pastes delayed as more addition of okara.In general, the G″ curves exhibited a similar pattern to G′ curves, but G″ values were much lower than G′ values over the entire temperature range.G″ increased slightly from 20°C to approximately 40°C and then decreased sharply from 40°C for disorderly gelling process.Compared with the control, surimi paste with okara showed lower G″, and the G″ was inversely proportional to okara concentration, indicating that okara had a negative influence on surimi protein rigidity structure.

Sensory evaluation
Table 7 shows the sensory properties of surimi meatballs with different levels of okara.There were significant differences for all sensory properties of surimi meatballs with okara (P < 0.05).The ratings of taste, appearance color, texture, and overall likeness were reduced with the increase of okara in surimi meatballs.There was no significant difference on springiness among surimi meatballs with or without okara (P > 0.05), except with 10% okara.However, it was above 4 (neither like nor dislike) for all sensory ratings for surimi meatballs with 6% okara.Results indicated that consumers might accept surimi meatballs with a small quantity of okara (≤6%).It might increase the overall favorability or acceptance of the product if it was clearly stated that it was DF-enriched [74] and increase the attention of DF-enriched products. [75]ote:Different letters in the same column indicate significant differences (P < 0.05).
Table 8 shows sensory evaluation for surimi meatballs with different particle sizes of okara.It was significantly higher for the springiness of surimi meatballs with the smallest particle size (34 μm) of okara than others (P < 0.05).The springiness increased with the reduction of the particle sizes of okara, while all scores of the springiness were more than 5.There were no significant differences in taste, appearance color, texture, and overall likeness among meatballs with different particle sizes of okara.All sensory ratings were more than 4, indicating that different particle sizes of okara did not have a negative effect on surimi meatballs.Therefore, the particle size of okara did not significantly affect the sensory properties of surimi-based  c and d) shows the rheological behaviors of surimi pastes with different particle sizes of okara during heating from 20°C to 90°C.The patterns of the G′ were similar for surimi pastes with different particle sizes of okara.The initial (20°C) G′ of surimi paste with 34 μm okara was higher than that with other particle sizes of okara and kept top during temperature sweep process.G′ increased with the reduction of particle sizes of okara.It was the lower G′ for surimi pastes with larger particle size of okara, probably due to its higher WRC that could absorb water and reduce lubrication of surimi pastes. [70]n general, the G″ curves exhibited a similar pattern to G′ curves, but G″ values were much lower than G′ values over the entire temperature range.G′ and G′′ were paralleled for a typical gel system, while the former was higher than the latter during thermal gel formation. [71]Unlike G′, surimi pastes showed an initial slightly increase in G″ at 20-38°C and decreased sharply at 38-42°C.It was the highest G″ for surimi pastes with the smallest particle size of okara (34 μm) during heating from 20-90°C.Results demonstrated that G' decreased in surimi with the increase of okara addition during heating, but increased with the reduction of okara particle size.The particle size variation (34-509 μm) had no effect on the dissociation and denaturation temperature.G" decreased with increasing okara addition and particle size.products.Okara could be used in surimi products with the addition of appropriate amount for improving the sensory properties of surimi products.Okara particle size had no significant effect on the sensory properties and did not affect the organoleptic properties of surimi products.

Conclusion
The addition of okara into surimi products resulted in significant changes in texture, WHC, color, and rheological properties (P < 0.05).The addition of okara reduced textural properties and gel strength, and WHC and color decreased as okara concentration increased.The oscillatory test indicated that okara addition delayed the denaturation of surimi protein and decreased the G′ and G″.Moreover, sensory evaluation showed that surimi meatballs with small amounts of okara (≤6%) had higher acceptability.The particle sizes of okara affected textural properties of surimi gels but not the WHC.L* and whiteness were increased significantly in surimi gels with the reduction of particle sizes of okara.G′ and G″ were decreased in surimi with larger particle size of okara during heating.However, the sensory evaluation showed no significant difference on surimi meatballs with different particle sizes of okara.Thus, there was a potential apply of okara to new high-DF surimi-based products.It could be very useful in the improvement of product quality and nutritional value by adding proper amount of okara.

CRediT authorship contribution statement
Yudong Wang contributed to conceptualization, methodology, validation, formal analysis, writing-original draft.Xiaoqin Tu contributed to investigation, software, and data Curation.Liu Shi contributed to methodology, validation, and software.Hong Yang contributed to funding acquisition, conceptualization, investigation, methodology, resources, aupervision, validation, visualization, andwriting-review and editing.

Figure 1 .
Figure 1.Effects of okara on the WHC of surimi gels.(a) Different addition levels; (b) different particle sizes.Different letters indicate significant differences.

Figure 2 .
Figure 2. Effects okara on the breaking force, breaking deformation, and gel strength of surimi gels.(a, b, c) are different addition levels; (d, e, f) are different particle sizes.Different letters indicate significant differences.

Figure 3 .
Figure 3. Effects of okara on the storage modulus (G′) and loss modulus(G") of surimi pastes during heating.(a and b) are different addition levels; (c and d) are different particle sizes.(c and d) shows the rheological behaviors of surimi pastes with different particle sizes of okara during heating from 20°C to 90°C.The patterns of the G′ were similar for surimi pastes with different particle sizes of okara.The initial (20°C) G′ of surimi paste with 34 μm okara was higher than that with other particle sizes of okara and kept top during temperature sweep process.G′ increased with the reduction of particle sizes of okara.It was the lower G′ for surimi pastes with larger particle size of okara, probably due to its higher WRC that could absorb water and reduce lubrication of surimi pastes.[70]In general, the G″ curves exhibited a similar pattern to G′ curves, but G″ values were much lower than G′ values over the entire temperature range.G′ and G′′ were paralleled for a typical gel system, while the former was higher than the latter during thermal gel formation.[71]Unlike G′, surimi pastes showed an initial slightly increase in G″ at 20-38°C and decreased sharply at 38-42°C.It was the highest G″ for surimi pastes with the smallest particle size of okara (34 μm) during heating from 20-90°C.Results demonstrated that G' decreased in surimi with the increase of okara addition during heating, but increased with the reduction of okara particle size.The particle size variation (34-509 μm) had no effect on the dissociation and denaturation temperature.G" decreased with increasing okara addition and particle size.

Table 2 .
The functional properties of okara with different particle sizes.
Note: Different letters in the same column indicate significant differences (P < 0.05).Abbreviations: WRC: water retention capacity; FAC: fat adsorption capacity; SC: swelling capacity.

Table 3 .
Effects of different levels of okara on the textural parameters of surimi gels.

Table 4 .
Effect of different particle sizes of okara on the textural properties of surimi gels.
Note: Different letters in the same column indicate significant differences (P < 0.05).

Table 5 .
Effects of different levels of okara on the color of surimi gels.

Table 6 .
Effects of different particle sizes of okara on the color of surimi gels.

Table 7 .
Effects of different levels of okara on the sensory properties of surimi meatballs.Note: Different letters in the same column indicate significant differences (P < 0.05).