Phytochemical profiles of black and yellow soybeans as affected by roasting

ABSTRACT

The current research aimed to compare the changes of phytochemicals, antioxidants, and anti-nutritional factors of yellow and black soybeans upon roasting. Yellow soybean was roasted at 210°C for 35 min and 230°C for 25 min, and black soybean was roasted at 210°C for 30 min and 230°C for 25 min. Isoflavones were significantly increased after the roasting process, while trypsin inhibitor was significantly decreased. The roasting process combined with the soaking process led to decrease in anthocyanin, soyasaponin, and antioxidant capacities. Black soybeans had more apparent variations in compounds than the yellow soybeans. For black soybeans, the processing under 230°C for 25 min presented more decrease in trypsin inhibitor and anthocyanin content. The roasting at a high temperature and short time produced products with a light color. Roasting combined with the soaking process had negative impacts on both yellow and black soybeans, but there were slight differences in the resultant compounds between different roasting temperatures.

KEYWORDS:

• Roasting
• Soybean
• Phenolic
• Isoflavones
• Antioxidants
• Saponins
• Anthocyanins
• Trypsin inhibitors

Introduction

Soybeans (Glycine max (L.) Merr) are popular not only in China but also in many other Asian countries due to their productive and nutritional properties. For instance, in America, soybean has an increased role in the diet as it contains high protein and many phytochemicals.^[1] The consumption of soybeans is widely accepted to be effective in preventing cancer, reducing osteoporosis, and preventing cardiovascular disease and renal disease.^[1,2] Antioxidant compounds, such as isoflavones and anthocyanins, can prevent ageing and cardiovascular diseases of human beings.

Soybean is a rich resource of phenolic compounds that consists of various plant secondary metabolism groups, ranging from simple molecules such as phenolic acids, flavonoids, to polymerized compounds such as lignins.^[3] Besides the phytochemical profiles and antioxidant capacities, the anti-nutritional factors have also attracted much attention recently. Trypsin inhibitor is a type of anti-nutritional substance that is related to the decrease in protein digestibility and pancreatic disease such as pancreatic hypertrophy. Although it has some benefits when used as a drug against oral and other cancers, its low content in soybean species did not reflect this benefit.^[4]

Roasting soybeans is a popular heating process, and the roasted soybeans are commonly sold as various popular snacks in China. These thermal processes can change the physical property such as color with less sensory acceptability and some chemical properties. Previous studies have shown that the antioxidant activity was enhanced after certain thermal processing due to the Maillard reaction. Moreover, thermal processing breaks the cell walls and membranes that released insoluble ester bonds to soluble phenolic substances that can also contribute to higher antioxidant property.^[5] Before the roasting process, the soybeans were soaked that helped soften the texture and shorten the cooking time. However, the soaking process can cause some of the antioxidants to leak into the water; thus, further influence the total phenolic, flavonoids, and the antioxidant capacity of soybeans.^[3]

There is limited research on the effects of roasting methods and conditions on the various quality aspects of soybeans. Hence, the aim of this research was to investigate the effects of selected roasting conditions on the phytochemical compounds (including phenolics, soyasaponins, antioxidant capacity, and anti-nutritional factors) of yellow and black soybeans.

Materials and methods

Roasting processing of soybean

Yellow soybeans (Glycine max) and black soybeans (Glycine max) were purchased from Longkeng Heifeng Technology Co., Ltd (Beijing, China). Yellow and black soybeans were rinsed three times and soaked in water overnight for 10 h. After draining the water, two batches of yellow soybeans of around 50 g were roasted in a rolling metal cylindrical basket (diameter 12 cm, length 18 cm) set in an oven(Galanz, KWS1528LQ-F2(SS), 1500W, 38L, Zhongshan, China) at 230°C for 25 min and at 210°C for 35 min, respectively. Two batches of black soybeans were also roasted in the rolling oven at 230°C for 25 min and at 210°C for 30 min, respectively. The roasting conditions were pre-experimented with different temperatures (ranging from 180 to 230°C) and varying time (10–40 min). Under the conditions chosen in this study, the soybeans should be palatable and have a roasted flavor. The final products of yellow and black soybeans are cooked with a crisp texture and a taste similar to the commercial roasted beans. All the roasted yellow and black soybeans were collected and ground into flour. Then, the roasted soybean flour was freeze dried by a freeze dyer (FreeZone Benchtop Freeze Dry System, Labaconco Corporation, USA) and stored at -80°C deep freezer for further analysis.

Chemicals and reagents

2-Diphenyl-1-picryhydrazyl (DPPH), 2, 2’-azino-bis (3-ethylbenzothiazoline-6 -sulfonic acid) (ABTS), soyasaponin Ba, butylated hydroxytoluene (BHT), catechin, phytic acid, Folin-Ciocalteu reagent, and 2,4,6-tri(2-pyridyl)-s-triazine (TPTZ) were purchased from Yuanye Biotechnical Company (Shanghai, China). Gallic acid (GA) was supplied by Damao Chemical Reagent Company (Tianjin, China). Trolox was bought from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Sulfosalicylic acid was obtained from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Trypsin from bovine pancreas, benzoyl-DL-arginine- p-nitroanilide (BAPA), and HPLC-grade acetonitrile and methanol were obtained from Sigma-Aldrich Co (St. Louis, MO, USA). The other chemicals were of analytical grade.

Preparation of phenolic extract

Soybean flours (0.5 g) were mixed with 5 mL solvent of acetone/water/acetic acid (70:29.5:0.5, v/v/v)^[6] to extract phenolic compounds. The mixed sample was extracted for 3 h using an orbital shaker, followed by 12 h in a dark place. The mixture was centrifuged at 1600 g for 10 min and collected in a new centrifuge tube. The extraction process was repeated twice, and the extracted solvents were combined and stored at 4°C in the dark until further use.

Total phenolic content

The total phenolic content (TPC) assay was done according to Folin-Ciocalteu assay with some modifications.^[6] The result was expressed as milligram of gallic acid equivalent per gram (mg of GAE/g) of raw soybeans or roasted soybeans using a calibration curve of GA with linearity range of 10–200 μg/mL (R² > 0.99).

Total flavonoids content

The total phenolic extract was used for total flavonoids content (TFC) assay as our previous report.^[6] The result was expressed as milligram of catechin equivalent per gram (mg of CAE/g) of raw soybeans or roasted soybeans using a calibration curve of GA with linearity range of 10–250 μg/mL (R² > 0.99).

Monomeric anthocyanin content

The total phenolic extract was used for monomeric anthocyanin content (MAC) assay as our previous report ^[7]. The result was calculated from the difference in absorbance in the following equation and expressed as gram of total anthocyanins per gram (mg CyE/g) of raw black soybeans or roasted black soybeans.

Ferric reducing antioxidant power assay

The total phenolic extract was used for the ferric reducing antioxidant power (FRAP) assay as our previous report ^[6]. The result was expressed as millimole of Fe²⁺ equivalent per gram (mmol of Fe²⁺E/g) of raw soybeans or roasted soybeans using a calibration curve of Fe²⁺ with linearity range of 0.1–1 mM (R² > 0.99).

ABTS radical scavenging assay

The total phenolic extract was used for ABTS radical scavenging assay, according a previous report.^[8] The result was expressed as micromole of Trolox equivalent per gram (μmol TE/g) of raw soybeans or roasted soybeans using a calibration curve of TE with linearity range of 50–1000 μg/mL (R²> 0.99).

DPPH free radical scavenging capacity assay

The total phenolic extract was used for DPPH radical scavenging assay as our previous report.^[6] The result was expressed as micromole of Trolox equivalent per gram (μmol TE/g) of raw soybeans or roasted soybeans using a calibration curve of TE with linearity range of 50–1000 μg/mL (R²>0.99).

Phytic acid with spectrophotometric method

The colorimetric Wade reagent method was used for detecting phytic acid as our previous report.^[9] The result is expressed as milligram of phytic acid equivalent per gram of raw soybeans or roasted soybeans (mg of PA/g).

Total saponin content

The extraction and determination of total saponin in soybeans were performed as our previous report.^[10] The absorbance was tested against the reagent blank under 544 nm by a UV-visible spectrophotometer (722s, Shanghai Jingmi Kexue Co., Ltd., China). The result was expressed as milligram of soyasaponin Ba equivalent per gram of raw soybeans or roasted soybeans (mg SBaE/g).

Quantification of soyasaponin Ba by HPLC: The extracts of total saponin were applied to HPLC analysis. The soyasaponin Ba quantification was performed as the conditions described in a report.^[11] The result of soyasaponin Ba content was expressed as mg of soyasaponin Ba per gram of soybeans (mg/g).

Anthocyanins

The extraction methods of anthocyanins in soybeans were referred to a previous report.^[12] Flours (0.5 g) from different temperature roasted soybeans and raw soybeans were weighed and extracted with 5 mL solvent (methanol: water: acetic acid: BHT= 85:15: 0.5: 0.2, v/v/v/v). Then, the extract was collected and dried at 45°C. The dry extract was dissolved in 5 mL of 25% of methanol. The extracted solvents were combined and stored at 4°C in the dark for further use.

Quantification of anthocyanins by HPLC

The quantification was performed as per the conditions described by a previous report.^[12] The result was expressed as milligram of peonidin-3-glucoside equivalent per gram of raw soybeans or roasted soybeans and milligram of cyanidin-3-glucocide equivalent per gram of raw soybeans or roasted soybeans.

HPLC analysis of isoflavone content

The extraction methods of isoflavone in soybeans were performed as described by a previous study.^[13] The extracts of isoflavone were applied to HPLC analysis. The isoflavone quantification was performed as the conditions described in a previous report but with some modifications.^[14] The result was expressed as microgram of daidzin, glycitin, daidzein, glycitein, and genistin equivalent per gram of raw soybeans or roasted soybeans (μg/g).

Trypsin inhibitory activity

The trypsin inhibitor assay (TIA) was performed as a previous report^[15] but with slight modification. The result was expressed as trypsin inhibitor units (TIU) equivalent per gram of raw soybeans or roasted soybeans (mg TI/g).

Maillard reaction product color value

The color value (L*, a*, b* values) was determined by handy colorimeter CR-400. L*is known as the lightness from 0 (black) to 100 (white). The other two coordinates represent redness (+a) to greenness (−a) and yellowness (+b) to blueness (-b), respectively. ΔE indicates the different color values with the target substance in the following equation.

Statistical analysis

All of the assays were conducted in triplicate except for HPLC analyses, which were conducted in duplicates, and the results were expressed in means ± standard deviations on the basis of dry weight. The significant differences were analyzed using one-way analysis of variance (ANOVA) in SPSS statistics 19 system for Windows. The differential analysis method used is Duncan.

Results and discussion

Phenolic and antioxidant profiles of raw and roasted soybean

The results of yellow and black soybeans before and after roasting have been plotted in Fig. 1A. The yellow soybeans have been detected with initial TPC value of 2.8 mg GAE/g, which decreased to 2.4 mg GAE/g after roasting at 230°C and 2.4 mg GAE/g after roasting at 210°C. The raw black soybean was 6.2 mg GAE/g, which decreased to 3.1 mg GAE/g after roasting at 230°C and 3.0 mg GAE/g after roasting at 210°C. The TPC value decreased less at higher roasting temperature with shorter time in all cases. However, the TPC value of yellow soybeans did not show significant difference before and after the roasting process. The TPC value of black soybeans had an apparent decrease after roasting, while no significant difference was observed between the roasting processes at 210°C and 230°C. The black soybeans always had a higher TPC than the yellow soybeans. This is because the soaking process can lead to the leaching out of TPC by increasing the seed coat permeability, and the thermal process can lead to the degradation of phenolics.^[3] Although there might be some increase in the TPC content when Maillard reaction occurred during the roasting process, the overall tendency was to decrease. In addition, previous studies^[16] also had a similar result that, after the thermal processing, the yellow soybeans did not have any significant differences, but the black soybeans had an apparent decrease. Only the black soybeans contained the anthocyanins that are heat sensitive compounds, which might result in the distinct decrease of TPC in black soybeans.

Figure 1. TPC (A), TFC (B), MAC (C), FRAP (D), ABTS (E), and DPPH (F) of raw and roasted soybeans. Results were expressed as mean with standard deviation from three replicates. Data marked with different letters are significant (p < 0.05) different. Yellow soybean: 210C-35M, roasted at 210°C for 35 min; 230C-25M, roasted at 230°C for 25 min. Black soybean: 210C-30M, roasted at 210°C for 30 min; 230C-25M, roasted at 230°C for 25 min.

The TFC results of yellow and black soybeans before and after roasting are presented in Fig. 1B. Raw yellow soybeans have been detected with a TFC value of 0.7 mg CAE/g, while the raw black soybeans had a TFC value of 2.4 mg CAE/g. Both yellow and black soybeans had a tendency to decrease. The TFC values of both yellow and black soybeans had an apparent decrease after roasting, while no significant difference was obtained between the roasting processes at 210°C and 230°C. The tendency of TFC was similar to the result of TPC. Moreover, the differences between yellow and black soybeans might be due to the high anthocyanin content in the seed coat of black soybeans.

As seen in Fig. 1C, there was an apparent decrease of MAC value in black soybeans after roasting. The MAC had an initial value of 0.49 mg CyE/g that decreased to 0.28 mg CyE/g under 210°C and 0.22 mg CyE/g under 230°C. There was a decreasing tendency after the thermal processes due to the heat-sensitive property of anthocyanin. Moreover, due to the heat-sensitive property, the roasting process at 230°C had a severe decrease compared to that at 210°C.

Soybeans contain multiple sources of antioxidants, such as isoflavone, anthocyanins, phenolic acid, and also the phytic acid. However, black soybeans exhibited significantly greater antioxidant capacity in these assays, around 2- to5-fold in FRAP, ABTS, and DPPH assays. The antioxidant capacity of food is considered to be important with respect to prevention of cardiovascular diseases and coronary risks. The chemical reaction gives a quick assessment of antioxidant amount, which is also an indicator of the antioxidant properties and the variation situation after roasting.

As shown in Fig. 1D, the raw black soybeans had a FRAP value of 11.05 mmol FE/100 g, and the raw yellow soybeans had a FRAP value of 1.41 mmol FE/100 g. This is consistent with the previous study.^[12] Fig. 1F showed that the raw black soybeans had a DPPH value of 22.53 μmol TE/g, and the raw yellow soybeans had a DPPH value of 3.20 μmol TE/g. The ABTS value was 52.41 μmol TE/g in raw black soybeans and 24.57 μmol TE/g in raw yellow soybeans.

Yellow soybeans showed a significant decrease tendency in antioxidant abilities of FRAP and ABTS value after the roasting process, while there was no significant decrease in the DPPH values after roasting. Compared to the raw black soybean, roasting decreased the DPPH radical scavenging capacity as well as FRAP and ABTS values. If the relationship between the distributions of TPC and antioxidant activity was considered, it was found that both yellow and black soybeans had a similar tendency with DPPH value. The yellow soybeans showed no difference after roasting, but the black soybeans showed a decreased tendency. Previous studies have shown a decreasing tendency in antioxidant properties in black soybeans compared with different species.^[3] The roasting and heating process might lead to the formation of phenolic compounds in Maillard reaction and the breakdown of insoluble phenolic compounds that can be better extracted, leading to the increase in the antioxidant tendency.^[3,5] Meanwhile, the transformation of isoflavones from aglycones and malonyl-β-glucosides to β-glucosides increased the antioxidant capacity. It has been reported that β-glucosides had around three times higher antioxidant potential than malonly-β-glucosides and aglycones.^[7] However, the leaking of TFC and TPC might lead to a decrease in antioxidant activity. Hence, the overall tendency was to decrease or to show no significant difference.

Isoflavone of raw and roasted soybeans

The accuracy of the HPLC method is satisfactory with good recovery rates of daidzin, glycitin, genistin, daidzein, and glycitein at 102%, 103%, 101%, 108%, and 108% for yellow soybeans, and 93%, 98%, 96%, 107%, and 105% for black soybean, respectively. The contents of isoflavone in daidzin, glycitin, genistin, daidzein, and glycitein are summarized in Table 1. The typical chromatograms of isoflavones in soybeans are presented in Fig. 2. In the five kinds of major isoflavones that were investigated in this study, the overall contents in yellow soybeans and black soybeans were increased except a decrease in daidzein in yellow soybeans. The conversion and loss of isoflavones during roasting significantly affected the nutritional value and antioxidant property of soybeans. The previous research has found that malonyl derivatives can be decarboxylated to acetyl derivatives, while the malonyl and acetyl derivatives can be transformed into β-gluciosides (daidzin, glycitin, genistin) due to desertification reaction.^[17] Malonyl, acetyl derivatives, and β-gluciosides can be hydrolyzed into aglycones (daidzein, glycitein). The external environment variations such as heat, acid, alkaline, and enzymes can accelerate the inter-conversion reactions of isoflavones.^[18] The daidzin, genistin, and glycitin amount in the current research was found to be increased. A previous study showed a conversion between isoflavones from malonyl derivatives to actetyl derivatives and β-glucosides after roasting. Another type of isoflavones, aglycones, has heat resistant property.^[19] Due to the prominent benefits of isoflavones, the study provided a significant finding as it shows an overall tendency to increase in the five kinds of isoflavones. The yellow soybeans roasted at 210°C for 35 min showed better results than that roasted at 230°C for 25 min with respect to the more apparent increase as follows: 1.3 times of daidzin, 0.19times of glycitin, 0.66 times of genistin, 1.14 times of glycitein, and less decrease in daidzein. In black soybeans, the roasted condition under 210°C for 30 min showed apparent increases in daidzein and glycitein, and the roasted condition under 230°C for 25 min showed apparent increases in daidzin, glycitin, and genistin. Aglycones that consisted of daidzein and glycitein can be absorbed by human intestines easily in respect to the right form structure that means a high bioavailability.^[20] In this aspect, the roasting at 210°C for 30 min is a better condition.

Table 1. Isoflavones contents (μg/g) of raw and roasted soybeans.

	Yellow soybean			Black soybean
	Raw	210C-35M	230C-25M	Raw	210C-30M	230C-25M
Daidzin	37.6	535.7	464.1	306.0	348.2	383.7
Glycitin	82.3	98.3	87.8	ND	90.8	91.22
Genistin	446.5	743.1	631.8	364.1	397.6	448.4
Daidzein	190.8	157.7	149.8	384.9	672.9	575.1
Glycitein	183.1	391.7	258.9	ND	255.1	217.2

Data were expressed as different sorts of isoflavone in raw and roasted black and yellow soybean (n = 2). Yellow soybean: 210C-35M, roasted at 210°C for 35 min; 230C-25M, roasted at 230°C for 25 min. Black soybean: 210C-30M, roasted at 210°C for 30 min; 230C-25M, roasted at 230°C for 25 min.

Figure 2. Typical HPLC chromatograms of raw and roasted soybean. (A) Raw yellow soybeans, (B) roasted yellow soybean under 210°C for 35 min, (C) roasted yellow soybean under 230°C for 25 min. Peaks: 1, daidzin, 2, glycitin, 3, genistin, 4, daidzein, 5, glycitein.

Total saponin content of raw and roasted soybeans

The total content of total soyasaponin was detected by the colorimetric method and is equivalent to that of soyasaponin Ba. The soyasaponin content was 12.96 mg/g and 11.15 mg/g in raw yellow soybeans and raw black soybeans, respectively. As shown in Fig. 3A, the soyasaponin content in the yellow soybeans had a tendency to decrease after the roasting process. There were no significant differences between raw black soybeans and roasted black soybeans under 210°C and 230°C. A few studies have focused on the sayasaponin content in soybean after the roasting process. However, during the soaking process, the saponins can dissolve in the water and be lost. In addition, the saponins are thermal sensitive products that might degrade during the thermal process of roasting.^[21] Although an optimum thermal process can stabilize the saponins content in canned bean products, as shown in a previous study that used a higher temperature and shorter time,^[2] the overall tendency observed in the study was decrease in yellow soybeans and no significant differences in the black soybeans after the roasting process.

Figure 3. Total saponin content (mg/g) (A), soyapanonin Ba content (mg/g) (B), C3G (C1) and P3G (C2) value (μg/g), and phytic acid (mg PA/g) (D) of raw and roasted soybeans. Results were expressed as mean with three replicates. Data marked with different letters are significant (p < 0.05) different. Yellow soybean: 210C-35M, roasted at 210°C for 35 min; 230C-25M, roasted at 230°C for 25 min. Black soybean: 210C-30M, roasted at 210°C for 30 min; 230C-25M, roasted at 230°C for 25 min.

Soyasaponin Ba of raw and roasted soybeans

The HPLC detected the soyasaponin Ba content in the yellow soybeans and black soybeans. The results are expressed in Fig. 3B. The soyasaponin Ba content in raw yellow soybeans was 0.29 mg/g, and in the raw black soybeans was 0.31 mg/g. The results of HPLC have similar tendencies with the total saponins content detected by a spectrophotometer. Both yellow and black soybeans showed a decrease in the total saponin content after roasting, but this decrease was not significant. The soyasaponin Ba had a tendency to increase after roasting, but this increase was not significant (Fig. 3B). Hence, after roasting, the total saponins content of yellow soybeans had a small decrease and that of the black soybeans did not show much variation before and after roasting.

Anthocyanins of raw and roasted soybeans

Anthocyanin is a blue, pink, or purple pigment substance that is detected in the coats of black soybean seeds.^[22] This is the reason anthocyanin was not detected in yellow soybeans via HPLC in the present study. In the current study, there were 1043.8 μg/g cyanidin-3-glucose and 23.51 μg/g peonidin-3-glucose in raw black soybeans. The cyanidin-3-glucose was significantly reduced after being roasted to 50.35 μg/g under 210°C for 30 min and 74.78 μg/g under 230°C for 25 min, indicating a loss of 95.2% and 92.8%, respectively (Fig. 3C). Moreover, peonidin-3-glucoside was not detected after roasting. These results might be due to the properties of anthocyanin, which is water soluble and also particularly unstable under the heating process.^[23] After the soaking process, the water had a black color which was discarded later. Though there is no direct data of anthocyanin content after roasting, the boiling and steaming processing in previous study^[12] supported the loss of 96–98% of C3G and 100% of P3G. This value is highly comparable with the current research. Hence, the roasted black soybeans that combined soaking process and roasting process might not be a good choice for consumption with respect to anthocyanin due to its tremendous loss. However, further experiments need to be conducted to confirm how much anthocyanin lost during soaking process.

Phytic acid of raw and roasted soybean

As shown in Fig. 3D, after roasting, there was a decrease in phytic acid to 41.5% and 41.0% in raw yellow soybeans after treatment under 210°C for 35 min and 230°C for 25 min, respectively. The black soybeans also decreased to 40.4% after roasting under 210°C for 30 min and 39.3% under the roasting condition at 230°C for 25 min. The two different roasting temperatures did not lead any significant differences. Previous study^[24] also reported a similar result after roasting. They reported that the yellow soybeans showed a 16.2% decreases, and the black soybeans showed a 13.9% decreases after roasting for 20 min in a conventional oven at 180°C The current experiment has a higher temperature and longer time that might lead to a more distinct phytic acid difference. The soaking and thermal process cause a reduction in phytic acid as it combines with other insoluble complex compounds that are unstable during the thermal process.^[20] Most of the phytic acid-related compounds are soluble, and the soaking process might decrease the content of these compounds.^[25]

Trypsin inhibitor of raw and roasted soybeans

Table 2 shows that both yellow soybeans and black soybeans showed an apparent decrease after roasting. After soaking and roasting processes, it has a decrease in the trypsin inhibitor. However, there was no apparent difference in trypsin inhibitor between raw soybeans and soybeans soaked for 24 h, 36 h, and 48 h from previous study.^[26] However, a decrease in trypsin inhibitor was observed after the soybeans were soaked for 72 h, which might be due to the germination processes. Hence, the thermal process can efficiently decrease trypsin activity with little effect after soaking process. There are two kinds of trypsin inhibitors, Kunitz and Brik trypsin inhibitors. When the Kunitz is thermally liable, the Brik trypsin inhibitor can have interchange of disulfide linkage (cystine residual) between inhibitors and storage proteins during the thermal process.^[13] Similar results have been reported after roasting soybeans from previous study,^[13] in which the trypsin inhibitor activity of soybeans was 27.5% compared to soybeans after roasting (10%). In addition, a previous study^[27] has reported a 4% to 10% decrease in trypsin inhibitor that gave the desired protein nutritive value; thermal treatment to achieve too low trypsin inhibitor can lead to damage to lectins, lipoxygenase, and major storage proteins as well. The residue of trypsin inhibitor had a tendency to decrease when the temperature increased even if the roasting time decreased. Hence, in order to have a lower trypsin inhibitor, the manufacturer can utilize high temperature with a short time to roast soybeans.

Table 2. Effects of different roasting conditions on trypsin inhibitor activity of soybean.

	Yellow soybean			Black soybean
Trypsin Inhibitor Activity	Raw	210C-35M	230C-25M	Raw	210C-30M	230C-25M
TIU/g	1696.5	324.8	264.5	1375.8	923.5	516.5
mg TI/g	0.89	0.1	0.13	0.72	0.48	0.27
Residue%	100	19.1	14.6	100	66	37.5

TIU: trypsin inhibitor unit, TI: trypsin inhibitors. Yellow soybean: 210C-35M, roasted at 210°C for 35 min; 230C-25M, roasted at 230°C for 25 min. Black soybean: 210C-30M, roasted at 210°C for 30 min; 230C-25M, roasted at 230°C for 25 min.

MR color value of raw and roasted soybean

According to Table 3, the roasting process had a decrease tendency of L* in both yellow soybeans and black soybeans. However, both yellow soybeans and black soybeans showed a higher decrease after roasting under 210℃. This might imply that the time has more effects than the temperature. Moreover, a* value increased after roasting both in black soybeans and yellow soybeans, which means that both soybeans had a darker color and might have a low acceptability after roasting. There were no significant difference in lightness of yellow soybeans between soybeans roasted under 230℃ for 35 min and raw soybeans. There was also no significant difference in lightness of black soybeans. The △E also implies the color differences; the higher the number, the more the variations. As shown in Table 3, the yellow and black soybeans roasted under 230℃ for 25 min had less variation than the raw yellow soybeans and black soybeans. Hence, it is recommended to roast the soybeans under 230°C for 30 min for a bright color and sensory acceptability.

Table 3. MR color value of soybean.

		L*	a*	b*	△E
Yellow soybean	Raw	61.5 a	−0.81 d	12.4 i	—
	210C-35M	54.5 b	4.79 e	12.8 i	9.07
	230C-25M	58.3 a	2.71 f	12.9 i	5.73
Black soybean	Raw	52.2 c	−2.29 g	5.47 j	—
	210C-30M	50.7 c	0.57 h	5.50 j	3.61
	230C-25M	52.0 c	0.95 h	6.10 j	3.25

MR: Maillard reaction. Yellow soybean: 210C-35M, roasted at 210°C for 35 min; 230C-25M, roasted at 230°C for 25 min. Black soybean: 210C-30M, roasted at 210°C for 30 min; 230C-25M, roasted at 230°C for 25 min.

Conclusion

The variations in compounds of yellow and black soybeans after the roasting process have attracted much attention due to the popularity of soybeans in China. In the current study, the isoflavones had apparent increases after the roasting and soaking processes, while some anti-nutritional compounds, such as phytic acid and trypsin inhibitors, had apparent decreases. However, the heating process combined with the soaking process led to a decrease in total phenolic, total flavonoids, monomeric anthocyanin, soyasaponin, and anthocyanin contents and antioxidant capacities. Black soybeans had a bigger impact than yellow soybeans that except the soyasaponin, most of its compounds have decreased half of its original contents after roasting. In summary, the roasting combination with the soaking process had a negative impact in both yellow soybeans and black soybeans. The optimal roasting condition for yellow soybeans was found to be 210°C for 35 min, and the optimal roasting condition for black soybeans was 230°C for 30 min according to the phytochemical variations in current research. However, it might need more investigations with specific aspects of black soybeans to choose the optimal roasting condition.

Funding

This research was jointly supported by two internal research grants (UICRG 201624 and UICRG 201627) from Beijing Normal University-Hong Kong Baptist University United International College, Zhuhai, Guangdong, China.

Supplemental data

Supplemental data for this article can be accessed on the publisher’s website.

Additional information

Funding

This research was jointly supported by two internal research grants (UICRG 201624 and UICRG 201627) from Beijing Normal University-Hong Kong Baptist University United International College, Zhuhai, Guangdong, China.