1. Introduction
Buckwheat has high nutritional value, with a higher content of dietary fiber, vitamins, phosphorus, calcium, iron, lysine, linoleic acid, niacin and rutin. It also has high medicinal value, exhibiting hypocholesterolemic, antihypertensive, anticancer, anti-inflammatory, antidiabetic, and neuroprotective activity [
1]. Therefore, buckwheat has great potential as a staple food for different types of consumers [
2].
The remarkable health benefits of buckwheat products contribute to their widespread popularity in most East Asian countries and parts of European countries [
3]. However, the development of whole buckwheat noodles has been limited so far, because of the high breaking rate and cooking loss. The protein composition of buckwheat is different from that of wheat, and so it struggles to form effective protein networks like those seen in a gluten network [
4]. The addition of wheat flour or exogenous gluten is usually used to compensate for this deficiency. It was reported that buckwheat flour could not be used in large quantities (basically 30% or less) during the conventional noodle processing process, and 20% addition showed the most acceptable sensory qualities [
2]. Extrusion processing is a physical technology which is characterized by no pollution, high efficiency and energy savings. During extrusion, starch granules are gelatinized and partly degraded by heat and shear effects. The starch network is well-formed, replacing the protein network to some extent [
5]. In our previous studies, extrusion processing instead of conventional noodle processing techniques was successfully utilized to produce whole buckwheat noodles [
6].
Recently, the development and application of fine or superfine grinding, one of the typical techniques of which is jet milling, in whole grain-based food research have been widely studied [
7]. The decrease in the particle size of materials to a micro size, which causes some changes in physicochemical properties, such as structure and surface area, and brings about some unexpected characteristics. The physical and processing properties of whole grain flours can be improved by fine or superfine grinding, thus leading to a higher quality of the final products. For example, Wang et al. [
8] reported that the decrease in whole wheat flour particle size obviously enhanced the tensile resistance and extensibility of dough. Niu et al. [
9] also showed that milled materials with finer particle sizes presented positive effects on the quality improvement of whole wheat noodles.
Hence, the purposes of this research were to investigate the effects of jet milling on the physicochemical properties of buckwheat flours and the cooking quality, textural properties, and proximate compositions of extruded whole buckwheat noodles. This study is expected to provide meaningful information on exploiting whole buckwheat noodle production for commercial practice.
2. Materials and Methods
2.1. Materials
Dehulled buckwheat grain (
Fagopyrum esculentum Moench) was bought from Yanzhifang Food Co., Ltd. (Anhui, China). The cultivar of buckwheat above is widely grown in Liaoning, China. The grains were ground by an ultra-speed centrifugal pulverizer (ZM 200, Retsch, Haan, Germany), of which the electrode speed was set to 14000 rpm, and passed through a 0.5 mm sieve to obtain coarse buckwheat flour (BF1). The chemical components of BF1 (dry basis) were analyzed according to the AACC standard methods 46-11.02, 30-25.01, 08-01.01 and 76-13.01 (AACC, 2000) [
10], of which the protein content was 15.07 ± 0.07%, that of crude fat was 2.57 ± 0.03%, that of total ash was 2.26 ± 0.03%, and the total starch content was 75.85 ± 0.32%. The Total Dietary Fiber Kit was bought from Megazyme International Ireland Ltd. (Wicklow, Ireland). All other chemical reagents utilized in this research were of analytical grade
2.2. Preparation of Buckwheat Fine Powder by Jet Milling
The resulting coarse buckwheat flour was pulverized by means of J-50 jet milling (TECNOLOGIA MECCANICA, Bergamo, Italy). The crushing pressure was 12 MPa, and the feeding pressure was 10 MPa. Three treatments with different feeding speeds of 145 r/min, 200 r/min and 250 r/min were applied to obtain fine buckwheat flour with varying particle sizes, defined as BF2, BF3 and BF4, respectively.
2.3. Particle Size
The particle size distribution of samples was tested in wet method mode using a S3500 Particle Size Analyzer (Microtrac Co., Ltd., Montgomeryville, PA, USA). Referring to Yu et al. [
11], ultra-pure water was used as a dispersant, and the refractive index of the samples was set to 1.434. The data were analyzed by the system software FLEX 10.5.3.
2.4. Damaged Starch (DS)
Damaged starch (DS) of buckwheat flour samples was measured according to the AACC Method 76-30A (AACC, 2010) [
12]. A 1.0 g sample and 50 mg of alpha-amylase were added into 45 mL of acetate buffer, and incubated at 30 °C for 15 min. The above suspension was mixed evenly with 3 mL of H
2SO
4 solution and 2 mL of sodium tungstate solution, and then filtered. The results were quantitated by titrating to measure reducing sugars.
2.5. Color
Color analysis (L*, a* and b*) was performed by using a CM-5 chroma meter (Konica Minolta, Osaka, Japan). A white standard plate was used for calibration, and then the samples were placed and measured in a sample holder. The data were expressed as the mean of 10 measurements taken at random locations in the sample.
2.6. Hydration Properties
The hydration properties of buckwheat flour were measured on the basis of the method of Anderson et al. [
13] with minor adjustment. A 1.0 g sample (W
0, dry basis) was put into a centrifuge tube (W
1) filled with 25 mL of distilled water and vibrated to disperse it completely. The dispersion was incubated in a water bath at 100 °C for 30 min and shaken for 30 s every 10 min, followed by centrifugation at 4200 r/min for 15 min. The supernatant was decanted into a pre-weighed 500 mL beaker (W
2) and drained at 105 °C until reaching the constant weight (W
3). The dry residue in the centrifugal tube was weighed (W
4). The water solubility index (WSI), water absorption index (WAI) and swelling power (SP) were calculated as follows:
2.7. Pasting Properties
Pasting properties were tested by a Rapid Visco Analyser (RVA, Model Super-3, Newport Scientific, Warriewood, NSW, Australia), according to the AACC Method 76-21.01 (AACC, 2000) [
14]. The values of peak viscosity, trough viscosity, breakdown, final viscosity, and setback were calculated.
2.8. Rheological Properties
The paste samples were obtained after the RVA measurement, then transferred to the rheological test by a dynamic rheometer (MCR 301, Anton Paar, Graz, Austria). The measurement parameters were as follows: plate diameter, 50 mm (rotor: PP50); strain, 0.5%; temperature, 25 °C; frequency, 0.1–20 Hz; and gap, 1 mm. The storage modulus (G′), loss modulus (G″) and tan δ were recorded.
2.9. Gel texture Properties
The paste samples by RVA were put into a sealed plastic mold (the sample size was 2 cm × 2 cm × 2 cm cube) and then held at 4 °C for 24 h to form a stable gel system. The textural properties of samples were determined using a texture analyzer (TA-XT 2i Stable Micro Systems, Surrey, UK). The texture profile analysis (TPA) mode was selected and the test conditions were as follows: pretest speed, 5.00 mm/s; test and posttest speed, 2.00 mm/s; strain, 65%; trigger force, 5.0 g; interval time, 2 s and data acquisition rate, 200 PPS. Hardness, springiness, cohesiveness, chewiness and resilience were calculated. Six replicates were performed.
2.10. Preparation of Extruded Whole Buckwheat Noodles (WBN)
Based on our previous work [
5,
6], extrusion runs were performed by a DSE-20/40D twin screw extruder (Bradender, Germany), with a screw length to diameter ratio (L/D) of 40:1 and a die size of 2 mm. The screw of the extruder was divided into 6 zones, with temperatures set at 40:60:110:90:80:80 °C from zone 1 to zone 6. The screw speed was set constant at 120 rpm and the moisture content was adjusted to 38%. The extruded whole buckwheat noodles prepared by BF1, BF2, BF3 and BF4 were defined as WBN1, WBN2, WBN3 and WBN4, respectively.
2.11. Scanning Electron Microscopy (SEM)
Buckwheat flours (BF), WBN and cooked WBN at the optimal cooking time were freeze-dried for further SEM observation. Detailed SEM test methods referred to Liu et al. [
15].
2.12. Cooking Properties
The cooking properties of WBN were determined based on AACC Method 66-50 (AACC, 2000) [
16] and were described in detail in our previous work [
6].
2.13. Texture Properties of WBN
The texture properties of WBN were analyzed according to Liu et al. [
11] with slight modifications. The textural profile analysis (TPA) mode was applied by a TA-XT2i Texture Analyser (Stable MicroSystems, Surrey, UK). The noodles were cooked at the optimal cooking time and the measurements were carried out with six replicates. The test conditions were as follows: cylindrical probe (P/36R); pretest speed, 5.00 mm/sec; test and posttest speed, 2.00 mm/s; strain, 75%; trigger force, 5.0 g; interval time, 5 s; and data acquisition rate, 400 PPS.
2.14. Determination of Dietary Fiber Content
Referring to AOAC Method 991.43 (AOAC, 1999) [
17], the insoluble dietary fiber (IDF), soluble dietary fiber (SDF) and total dietary fiber (TDF) in flour (BF) before extrusion and in noodles (WBN) after extrusion were tested by a FOSS fiber analyzer (Fibertec 8000, Hoganas, Sweden) with Megazyme K-TDFR kit.
2.15. Statistical Analysis
All measurements were carried out in triplicate unless mentioned specifically. Data analysis was accomplished using the statistical software Origin 8.0 (OriginLab, Northampton, MA, USA) and SPSS 18.0 (IBM, USA). ANOVA and Duncan tests were utilized to assess statistical differences between the mean values (p < 0.05).