ABSTRACT
The objective of the current study was to discover how processing technology affects beany flavor of soymilk. Four different technology routes, namely soaked soybean-boiling before filtration, soaked soybean-boiling after filtration, dry soybean-boiling before filtration (DS-BBF soymilk), and dry soybean-boiling after filtration, and 26 soy cultivars were utilized to process soymilk. The nine odor compounds hexanal, hexanol, trans-2-nonenal, 1-octen-3-ol, trans,trans-2,4-decadienal, benzaldehyde, 2-pentyl furan, 1-octen-3-one, trans,trans-2,4-nonadienal were detected by headspace gas chromatography using 2-methyl 3-heptanone as the internal standard. The results showed that both soybean cultivars and the different processing technologies significantly affect the soymilk flavor. The DS-BBF soymilk and Wuxingdou 2 had the lowest total odor compounds content. Therefore, DS-BBF processing method is suggested to produce the low beany flavor of soymilk in the industry.
Introduction
Soybean is a highly nutritious food material that contains approximately 35–50% proteins and essential amino acids composition.[Citation1] Soybean food products play an important role in the current food market. Soymilk, a kind of soybean products, is abundant in protein and fatty acids. It has been widely accepted that the healthy living of Asians is attributed to a diet enriched with soybean and its active components.[Citation2] Soymilk has many potential health benefits, such as reducing the risks of heart disease, cancers, aging, and osteoporosis.[Citation3] Despite the multiple beneficial attributes of soymilk, its consumption in the Western world is limited due to the beany flavor.[Citation4,Citation5]
Beany flavor is mostly derived from the catalyzed enzymatic oxidation of linoleic acid and linolenic acid by the lipoxygenases.[Citation6] Reports from the literature have illuminated the characteristic of beany flavor.[Citation7] It was reported that hexanal was commonly associated with the grassy flavor in soymilk.[Citation8,Citation9] Other major odor compounds are hexanol, 1-octen-3-ol, 1-octen-3-one, trans,trans-2,4-decadienal, and trans,trans-2,4-nonadienal.[Citation4,Citation10] Different soybean varieties have a significant influence on the odor compounds in soymilk. The lipoxygenase activities change with the different soybean varieties.[Citation11,Citation12] In fact, the different growing locations, temperature, sun light, and other reasons may lead to the different lipoxygenase activity change in soybean cultivars[Citation13] and significantly affect the contents of odor compounds in soymilk.[Citation14]
Similarly, different processing technologies of soymilk have an influence on the odor compounds. There are several kinds of soymilk processing technologies, such as traditional method, hot water grinding,[Citation15–Citation17] and direct steam injection method.[Citation18] The heating temperature and heating time tend to affect the lipoxygenase activity of processing soymilk.[Citation19] The soybean grinding temperature is mainly responsible for the beany odor content of soymilk.[Citation1] According to the study by Yuan et al., the traditional indirect heating method and the direct steam injection method have a great impact on the odor compounds in soymilk. Furthermore, the direct steam injection method decreased the odor formation in soymilk as compared to the traditional indirect heating method. The studies also showed that flavor compounds contents in soymilk are gradually reduced with the increasing heating time.[Citation18] As described previously, soybean materials and soymilk processing technologies are considered to be the important factors that affect the flavor compounds.[Citation20] In this study, we investigated the degree how the soybean cultivars and soymilk processing technologies affect the flavor content in the soymilk. The study aims to find out a reasonable method to produce soymilk that contains less beany flavor content.
Materials and methods
Soybean materials
A total of 26 kinds of soybean cultivars were provided by the National Soybean Industry Technology Research and Development Center of China. The sample information is shown in . These soybean varieties are mainly purchased from the following areas: Heilongjiang, Henan, Inner Mongolia, Jiangsu, and Jilin.
Table 1. Description of the soybean materials.
Chemicals
Benzaldehyde, 2-methyl-3-heptanone, and 1-octen-3-one were purchased from Tokyo Chemical Industry (Tokyo, Japan). Trans,trans-2,4-decadienal, trans-2-nonenal, 2-pentyl furan, trans,tans-2,4-nonadienal and 1-octen-3-ol were purchased from Fluka (Shanghai, China). Hexanol and hexanal were purchased from Aladdin Industrial (Shanghai, China).
Processing technical routes of soy milk
Four different processing technologies were used to make soy milk. The processing technologies of soymilk are shown in . The detailed procedures are described below.
Figure 1. Processing technologies of soymilk. DS-BAF, dry soybean-boiling after filtration; DS-BBF, dry soybean-boiling before filtration; SS-BAF, soaked soybean-boiling after filtration; SS-BBF, soaked soybean-boiling before filtration.
Technology 1: dry soybean-boiling after filtration (DS-BAF soymilk)
The dry soybeans (100 g) were heated under 100℃ for 10 min and then added with 900 mL distilled water at room temperature, with a bean-to-water ratio of 1:9 (w/w). Soybeans were ground for three times at high speed using a Ladyship blender (model: LS 658, Taiwan, China). The resulting slurry was filtered through a muslin cloth to separate the soymilk material from the residue. The raw soymilk was placed in a pot and switched to the hot stove surface to heat at 98℃ for 5 min. This kind of soymilk is defined as DS-BAF soymilk.
Technology 2: dry soybean-boiling before filtration (DS-BBF soymilk)
The dry soybeans (100 g) were heated under 100℃ for 10 min and then added with 900 mL distilled water at room temperature, with a bean-to-water ratio of 1:9 (w/w). Soybeans were ground for three times at high speed using a Ladyship blender (model: LS 658, Taiwan, China). The resulting slurry in the pot was switched to the hot stove surface to heat to the boiling temperature of 100℃. It was held at this temperature for 5 min by sustaining stirring to prevent the soymilk from foaming up. The heated soymilk was filtered through a muslin cloth to separate the soymilk material from the residue. This kind of soymilk is defined as DS-BBF soymilk.
Technology 3: soaked soybean-boiling after filtration (SS-BAF soymilk)
The dry soybeans (100 g) were initially rinsed and soaked in 500 mL distilled water for 16 h at room temperature. Then was added 400 mL distilled water at room temperature, with a bean-to-water ratio of 1:9 (w/w). Soybeans were ground for three times at high speed using a Ladyship blender (model: LS 658, Taiwan, China). The following procedures are the same as technology 1. This kind of soymilk is defined as SS-BAF soymilk. It is a traditional method to prepare soy milk in China.
Technology 4: soaked soybean-boiling before filtration (SS-BBF soymilk)
The dry soybeans (100 g) are initially rinsed and soaked in 500 mL distilled water for 16 h at room temperature. Then was added 400 mL distilled water at room temperature, with a bean-to-water ratio of 1:9 (w/w). Soybeans were ground for three times at high speed using a Ladyship blender (model: LS 658, Taiwan, China). The following procedures are the same as technology 2. This kind of soymilk is defined as SS-BBF soymilk.
Flavor components in soymilk assessed by GC
Various concentrations of hexanal, hexanol, trans-2-nonenal, 1-octen-3-ol, trans,trans-2,4-decadienal, benzaldehyde, 2-pentylfuran, 1-octen-3-one, trans,tans-2,4-nonadienal, and an internal standard 2-methyl-3-heptanone were used for identification and quantification. These commercial compounds for establishing standard curves were prepared in 2% fat cow’s milk (Yili Industrial Group Co., Ltd, Inner Mongolia, China).[Citation4] Samples of soymilk made from different varieties of soybean with four different processing technologies were analyzed by using the defined procedures.
Automatic headspace sampler (PerkinElmer, MA, USA) was used for flavor extraction. Soymilk (approximately 10–15 mL) was sampled after boiling for 5 min. The soymilk was immediately placed in a small beaker and cooled in an ice bath for 20 min. Then take out 5 mL of the soymilk and place in a 20 mL headspace vial sealed with a lid containing a Teflon-lined rubber septum. A portion of the same internal standard was consistent with the concentration for each soy odor standard curve. It was injected into each sample vial using a micro-syringe. Subsequently, it was equilibrated for 30 min at 75℃ on the heating furnace. Then the sample was injected to the gas chromatography (GC) for analyzing by the GC automatic sampler.
A gas chromatograph (PerkinElmer, MA, USA) equipped with a capillary column with a polar resin of DB-Wax (Carbowax, 30 m × 0.25 mm, i.d. × 0.25 μm film thickness) was used for odor compound analysis. The following conditions were used for GC: oven temperature, 35℃ (2 min hold) to 225 ℃ (5 min hold) at 10℃/min increase rate in between; detector, flame ionization detector at 250℃; carrier gas, nitrogen, and flow rate of approximately 1 mL/min; and inlet temperature, 150℃. Results were expressed as mg/L soymilk.
Statistical analysis
Soymilk preparation was conducted in duplicate. Each sample at each processing time was performed in duplicates by using GC method. The data were subject to analysis of variance using the SPSS (18.0 IBM, Armonk, USA) package. Significant differences among treatments were analyzed using Duncan’s multiple-range tests (p < 0.05).
Results
The GC chromatograms of nine kinds of mixed standards are shown in and and one soymilk sample is shown in . Through comparing the retention time between the standards and sample, we can identify the flavor compounds and the peak area of the samples, and calculated their content based on the linear regression equation of standard. The calculation results are shown in .
Table 2. Effect of four different processing technologies on flavor compounds contents in soymilk (mg/L).
Figure 2. The GC chromatograms of standards (A) (1, internal standard 2-methyl 3-heptanone; 2, hexanal; 3, hexanol; 4, benzaldehyde; 5, 1-octen-3-one; 6, 2-pentyl furan; 7, trans-2-nonenal; 8, trans,trans-2,4-decadienal), and standards (B) (1, internal standard 2-methyl 3-heptanone; 9,1-octen-3-ol; 10, trans,tans-2,4-nonadienal), and chromatogram of a typical soymilk sample (C).
Effect of different processing technologies on flavor compounds contents
The results are presented in . Nine typical flavor compounds hexanal, hexanol, trans-2-nonenal, 1-octene-3-ol, 1-octene-3-one, 2-pentyl furan, benzaldehyde, trans,trans-2,4-nonadienal, and trans,trans-2,4-decadienal were quantified. These flavors represented the cut-grass flavor, green flavor, tallow flavor, mushroom flavor, mushroom or fish-like flavor, green flavor, bitter almond flavor, fatty flavor, and oil flavor, respectively.[Citation21]
The results showed significantly statistical differences ( and ) in terms of flavor compound contents in different soymilk, which were made from the four different processing technologies. The current study verified that hexanal and hexanol were the major flavor compounds in soymilk.[Citation9] Of the different processing technologies, the number of SS-BBF soymilk samples with high hexanal contents was less than that of SS-BAF soymilk. The hexanol contents of SS-BBF soymilk were lower than SS-BAF soymilk except for Chidou 1, Heinong 43, Nenfeng 18, Sanbaijia, and Beidou 40. The number of SS-BBF soymilk samples with high total odor compounds contents was less than that of SS-BAF soymilk. The total odor compounds contents of DS-BBF soymilk were lower than DS-BAF soymilk except for Suinong 29, Suinong 31, Zheng 196, Zheng 0045, Heihe 38, Chidou 3, Xudou 14, and Beidou 40.
The results ( and ) also showed that, of the different processing technologies, the hexanal contents of DS-BBF soymilk were lower than SS-BBF soymilk except for Suinong 29, Suinong 31, Zheng 196, Chidou 3, Lei 06, Jinong 14, and Kenfeng 16. The sample number of DS-BBF soymilk with high hexanol contents and high total odor compounds contents was less than that of SS-BBF soymilk; the hexanal contents of DS-BAF soymilk were lower than SS-BAF soymilk except for Suinong 38, Heinong 43, Heinong 48, Wuxingdou 2, Bei 8731, and Lei 06. The sample number of DS-BAF soymilk with high hexanol contents and high total odor compounds contents was less than that of SS-BAF soymilk.
Figure 3. The number of soybean varieties that produced the less beany flavor contents of soymilk. □ The number of soybean varieties that produced the lower hexanal compounds contents of soymilk. ■ The number of soybean varieties that produced the lower total odor compounds contents of soymilk.
Effect of different varieties of soybean on flavor compounds contents of soymilk
The results showed that significantly statistical differences () in flavor compounds contents existed in soymilk made from different varieties of soybean. The total odor compounds contents of SS-BAF soymilk () ranged from 8.37 to 39.25 mg/L. Among the 26 varieties of soybean materials selected in this study, there 4 four soybean varieties containing less odor compounds. They were Heinong 43 (8.24 mg/L), Wuxingdou 2 (8.37 mg/L), Beidou 40 (8.75 mg/L), and Zheng 0045 (9.70 mg/L).
The total odor compounds contents of DS-BBF soymilk () ranged from 3.77 to 35.91 mg/L. Among the 26 varieties of soybean materials selected in this study, there were 4 soybean varieties containing less odor compounds. They were Wuxingdou 2 (3.77 mg/L), Heihe 37 (5.79 mg/L), Suinong 35 (5.85 mg/L), and Hefeng 47 (5.83 mg/L).
The total odor compounds contents of SS-BBF soymilk () ranged from 5.21 to 37.44 mg/L. Among the 26 varieties of soybean materials selected in this study, there were 7 soybean varieties containing less odor compounds. They were Wuxingdou 2 (5.21 mg/L), Jinong 14 (5.84 mg/L), Kengfeng 16 (5.88 mg/L), Lei 06 (6.78 mg/L), Suinong 29 (7.56 mg/L), Zheng 196 (8.65 mg/L), and Zheng 0045 (9.12 mg/L).
The content of total odor compounds of DS-BAF soymilk () ranged from 5.14 to 31.37 mg/L. Among the 26 varieties of soybean materials selected in this study, there were 4 soybean varieties containing less odor compounds. They were Wuxingdou 2 (5.14 mg/L), Xudou 14 (6.35 mg/L), Zheng 0045 (6.61 mg/L), and Beidou 40 (7.65 mg/L).
Lowest hexanal contents and the total odor compound contents of soymilk
presents the sample number of soymilk with the lowest level of hexanal compounds contents and total odor compounds contents. The sample number of soymilk with the minimum hexanal compounds contents and the minimum total odor compounds content made from SS-BAF soymilk were 3,3; DS-BBF soymilk were 15,15; SS-BBF soymilk were 6,8; DS-BAF soymilk were 12,7; respectively.
Discussion
The flavor compounds are mainly attributed to lipoxygenase catalytic oxidation of unsaturated fatty acids such as linoleic acid.[Citation22] The contents of the nine kinds of flavor compounds are determined by using the gas chromatography-flame ionization detector. Four different processing technologies for making soymilk aim to analyze the influence of different processes on flavor compounds in soymilk and identify the process that contains the minimum total odor compounds contents. In the literature, the flavor compounds of soymilk were significantly different in the same soybean varieties with different processes.[Citation23] It shows that the different processes can greatly influence the flavor compounds contents.
Mizutani and Hashimoto[Citation1] found that the different processing method and heating process have greatly impacted the flavor profiles of soymilk. In the current study, the results show that the boiling before filtration processing (BBF) technology produced low beany flavor than the boiling after filtration processing (BAF) technology. The main reason for the low beany flavor content of soymilk is that the soybean was heated after grinding, so that the lipoxygenase has been the timely destruction.[Citation4,Citation24] The results also show that the dry processing technology is more suitable for producing the low-odor compounds soymilk than the soaking processing technology. Of the DS-BBF and DS-BAF processing methods, soybean materials were treated with a high-temperature drying process at 100℃ for 10 min, which might lead to inactivation of the lipoxygenase.[Citation25] And it also can cause the reduction of contents of beany flavor substances in soymilk as compared to the other two technologies. Many consumers shy away from soymilk, primarily due to the beany flavor.[Citation6,Citation15,Citation23] Therefore, the current study aims to find the particular production process of soymilk, which has the lowest beany flavor content in the actual production. Through the comparison of the four processes of soymilk processed with the same soybean materials, it concluded that the DS-BBF soymilk has the least total flavor contents among over half soybean cultivars. Therefore, the DS-BBF process tends to produce low beany flavor of soymilk in the actual production.
As for different soybean cultivars processed with the same processing method, the flavor compound contents have a significant difference with each other. It indicates that the different soybean varieties have a great influence on the flavor compounds in soymilk. The current study is consistent with the result.[Citation26] Soybean components have an important effect on the soymilk flavor content.[Citation27–Citation29] This study selected 26 soybean varieties to identify the lower beany flavor soybean varieties. The result can provide useful information for the industrial production of soymilk. It was found that Wuxingdou 2 has the lowest total flavor contents. The total odor compounds contents in the four processes of Wuxingdou 2 were SS-BAF (8.37 mg/L), DS-BBF (3.77 mg/L), SS-BBF (5.21 mg/L), and DS-BAF (5.14 mg/L), respectively. Wuxingdou 2 is a kind of lipoxygenase-deficient soybean cultivar that is lacking lipoxygenases 1 and 2. The current results verified a previous study that lipoxygenase-deficient soybean varieties produce soymilk with a lower off-flavor than the normal soybean varieties.[Citation4] Thus, the Wuxingdou 2 is the most suitable soybean variety for producing the lower beany flavor soymilk. In addition, the soybean varieties of Heinong 43, Beidou 40, and Zheng 0045 were also suitable for the production of the particular soymilk because of their relatively lower beany flavor.
Conclusion
Based on the above data, it was concluded that the BBF technology is better than BAF technology. Dry processing technology is more suitable for production of low-odor compound soymilk than wet processing technology. The study showed that both production processes and soybean varieties had a significant impact on the contents of flavor compounds in soymilk. Through the comparison of the same soybean varieties with the different production processes, it was found that the DS-BBF soymilk had the lowest total odor compound contents in over half soybean varieties. By the comparison of the different soybean varieties within the same processing technical route, it was found that Wuxingdou 2 had the lowest total odor compound contents. In summary, DS-BBF processing method and the Wuxingdou 2 are recommended to produce the lower beany flavor of soymilk in the industry.
Declaration of interest
The authors declare no conflicts of interest. This article does not contain any studies with human or animal subjects.
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References
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