Chemical reagents and materials
Dried black beans [Phaseolus vulgaris L.], mung beans [Vigna radiata (L.) R. Wilczek], and soybeans [Glycine max (L.) Merr.] were harvested and dried at Dongguk University Farm located in Goyang, Republic of Korea, in 2020. Beans were sown in June and harvested in October. The growing conditions of beans followed the farm’s protocol. Beans were dried using a convection oven (Jinsung Co., Seoul, Korea) at 40 °C for 48 h. The variety of plants was identified in the department of biological science of Dongguk University, Goyang, Republic of Korea. The beans were ground for 20 s by a grinder (SMKANB-4000; Poongnyun Co., Ltd., Korea), and the ground beans were passed through three different standard testing sieves with sizes of 355 μm, 500 μm, and 710 μm for 2 min using a sieve shaker (CG-211–8; Chunggye Co., Korea). The selection of the sieve size was based on the previous report [22]. The moisture contents of mung bean, soy bean, and black bean are 12%, 8.5%, and 8%, respectively. All beans were stored at room temperature until the analysis.
Ethanol (30% v/v) was purchased from J. T. Baker (Center Valley, PA, USA), and stored at room temperature until the analysis. HPLC-grade water, methanol (> 99.8%), hexane (> 97.0%), and dichloromethane (DCM, > 99.8%) were purchased from J. T. Baker (Center Valley, PA, USA). Anhydrous sodium sulphate (> 95%) was purchased from Junsei Chemical (Tokyo, Japan). Sodium chloride (99.0%) was purchased from Samchun Co. (Seoul, Korea). Divinylbenzene/carboxen/polydimethylsiloxane (50 μm DVB/CAR/PDMS) solid-phase microextraction (SPME) fibre was purchased from Supelco Inc. (Bellefonte, PA, USA). Methyl cinnamate (> 99.0%) and an alkane standard (C7–C30, > 98.0%) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, Mo, USA). Seventeen standard compounds were purchased from different suppliers as follows: hexanal (> 95.0%), 2-methyl-1-butanol (> 98.0%), 2-heptanone (> 99.0%), 3-octanone (> 98.0%), 1-hexanol (> 99.9%), 3-octanol (> 99.5%), benzaldehyde (> 99.5%), phenethyl alcohol (> 98.0%) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, Mo, USA), while cis-3-hexen-1-ol (> 97.0%), 1-pentanol (> 99.0%), octanol (> 98.0%), nonanal (> 95.0%), 1-octen-3-ol (> 98.0%), 1-octanol (> 98%), γ-hexalactone (> 99.0%), 1-nonanol (> 99.0%), and benzyl alcohol (> 99.0%) were obtained from Tokyo Chemical Industry Co, (Tokyo, Japan).
Isolation of volatile compounds by DRP and continuous LLE
Volatile compounds from beans (black bean, mung bean, and soybean) were extracted using a slight modification of a previously reported DRP method [24]. The extraction of beans was carried out triplicate replication for the analysis.
In this method, 100 g of beans, 400 mL of 30% ethanol, a magnetic stirrer bar, and boiling chips were filled into a 1L round-bottomed flask. Next, the solution was spiked with 50 μL of methyl cinnamate as an internal standard (100 μg/mL) and 20 μL of n-alkane standard (100 μg/mL). The mixture was steam-distilled at designated temperatures (50 °C, 60 °C, and 70 °C) under reduced pressures (50 mmHg) until the volume of the distillate reached 200 mL. The distillate (200 mL) was extracted with 200 mL of dichloromethane (DCM) using continuous LLE for 6 h at 60 °C. After extraction, anhydrous sodium sulphate was added to the collected DCM for dehydration, and the solution was stored in a deep freezer overnight. Subsequently, anhydrous sodium sulphate was separated from the solution using filter paper (No. 1, 110 mm, Whatman). The solution was concentrated with a rotary evaporator to 1 mL, and the residual solution was further concentrated under a purified nitrogen steam to 0.4 mL. This extract sample was used for gas chromatography–mass spectrometer detector (GC–MSD) analysis.
Isolation of volatile compounds by HWE and HS-SPME
Volatile compounds from beans (black bean, mung bean, and soybean) were extracted using a slightly modified version of a method described in a previous study [25]. HWE and HS-SPME followed by GC–MSD analysis were performed to assess the volatile compounds of beans. For HWE, 30 g of beans and 150 mL of distilled water were filled into a 250 mL Duran laboratory bottle. The mixture was extracted overnight in a water bath (70 °C), followed by filtration using filter paper (No. 1, 110 mm, Whatman).
For HS-SPME, 10 mL of the extracted solution, 1 g of sodium chloride, a magnetic stirrer bar, and boiling chips were filled into a 20 mL headspace vial. The solution was spiked with 3 μL of methyl cinnamate as an internal standard (10 μg/mL) and 20 μL of n-alkane standard (100 μg/mL). The headspace vial was incubated for 10 min in a beaker filled with distilled water maintained at a designated temperature (70 °C, 80 °C, and 90 °C) by a hot plate. After equilibrium, the SPME fiber, divinylbenzene/carboxen/poly-dimethylsiloxane (DVB/CAR/PDMS) for 40 min at 70 °C, 80 °C, and 90 °C, respectively. After absorption, the SPME fibre was inserted into the injection port of GC at 230 °C for 10 min for desorption of volatile compounds.
Instrumental analysis
Analysis of volatile compounds in beans (black bean, mung bean, soy bean) was performed using GC–MSD (Agilent 7820A gas chromatography with Agilent 5977E mass spectrometry detector). A DB-WAX column (60 m × 0.32 mm I.D. × 0.5 μm film thickness, J&W Scientific, Folsom, CA) was used to separate volatile compounds. The helium carrier gas flow was maintained at 1.0 mL/min with a constant flow. The injector was used in the splitless mode at 230 °C. The oven temperature was set to 44 °C for 5 min. Subsequently, the temperature was raised to 170 °C at 3 °C/min and held for 10 min, and finally raised to 240 °C at 8 °C/min for 5 min. The ionization energy was 70 eV and scan range was 50–550 m/z.
Identification and quantification of volatile compounds
Volatile compounds of beans (black bean, mung bean, and soybean) were analysed in relation to various particle sizes and extraction temperatures. Bean extracts were analysed by two types of extraction methods: (a) DRP and continuous LLE, (b) HWE and HS-SPME.
Volatile compounds of bean extracts were analysed qualitatively and quantitatively, and the 10 most abundant volatile compounds in each extract were identified. Identification of volatile compounds was performed on the basis of three parameters: (A) mass spectrum (MS) in Wiley Library, (B) Kovats retention index (KI) on the DB-WAX column in the NIST database, and (C) co-injection (CO). For quantification of volatile compounds, the samples were run in triplicate. The peak area ratio (PAR) was calculated by dividing the integrated areas based on the total ion chromatograms with the areas of the internal standard (methyl cinnamate).
Statistical analysis
Each experiment was repeated three times and the mean ± standard deviation (SD) were presented. To compare significant differences, data were analysed with one-way ANOVA and Duncan’s multiple-range tests (p < 0.05). Statistical analysis was performed by IBM SPS Statistics 25 (IBM, Chicago, USA).