Effects of Grinding Methods of Tartary Buckwheat Leaf Powder on the Characteristics and Micromorphology of Wheat Dough


The functional components in tartary buckwheat leaf powder can give flour products higher nutritional value. To comprehensively realize the high-value utilization of tartary buckwheat and its by-products, electric stone mill powder (EMP), ultra-fine mill powder (UMP), steel mill powder (SMP), and grain mill powder (GMP) from tartary buckwheat leaves were used in the preparation of wheat dough, and this was used to explore their effects on dough properties and protein microstructure.

With an increase in tartary buckwheat leaf powder, the hydration characteristics, protein weakening rate, and starch gelatinization characteristics of the dough changed, and the water holding capacity and swelling capacity decreased.

The retrogradation value increased, which could prolong the shelf life of related products. The water solubility of the dough showed an upward trend and was the lowest at 10% UMP. The addition of UMP produced a more uniform dough stability time and the lowest degree of protein weakening, which made the dough more resistant to kneading.

An increasing amount of tartary buckwheat leaf powder augmented the free sulfhydryl content of the dough and decreased the disulfide bond content.

The disulfide bond content of the dough containing UMP was higher than that of the other doughs, and the stability of the dough was better.

The peaks of the infrared spectrum of the dough changed after adding 10% UMP and 20% EMP. The content of α-helical structures was the highest at 10% UMP, and the content of ordered structures was enhanced. The polymerization of low molecular weight proteins to form macromolecular polymers led to a reduction in surface hydrophobic regions and the aggregation of hydrophobic groups. The SEM results also demonstrated that at 10% tartary buckwheat leaf powder, the addition of UMP was significantly different from that of the other three leaf powders, and at 20%, the addition of EMP substantially altered the structure of the dough proteins.

Considering the effects of different milling methods and different added amounts of tartary buckwheat leaf powder on various characteristics of dough, 10% UMP is the most suitable amount to add to the dough.

tartary buckwheat dough leaf powder hydration characteristics rheological properties microstructure

1. Introduction

Tartary buckwheat has high nutritional value, containing protein, bioflavonoids, multiple vitamins, cellulose, 18 different amino acids, chlorophyll, magnesium, potassium, calcium, iron, manganese, zinc, chromium, copper, selenium, and other ingredients [1]. Tartary buckwheat leaves are rich in macroelements such as calcium, magnesium, and phosphorus, as well as essential trace elements such as iron, manganese, potassium, and tin, and they also contain a large number of flavonoids such as rutin, which have physiological activities and can effectively prevent cardiovascular diseases and capillary embrittlement [2,3]. At present, the research on tartary buckwheat has mainly focused on its seeds.

Researchers have also studied the polyphenols in tartary buckwheat flowers, leaves, stems, and roots and found the highest rutin content in leaves [4]. Although tartary buckwheat leaves contain a large number of functional components such as rutin, carotenoids, lutein [5], and d-inositol [6], all of which have high nutritional and health-promoting value, the use of the leaves is still infrequent and underdeveloped. Tartary buckwheat cake made using tartary buckwheat leaf powder as the raw material was found to have an antioxidant capacity 2.27–2.99 times higher than that of wheat cake [7]. Researchers added bitter wheat leaves, hawthorn leaves, and ginkgo leaves to flour to make bread and peach crisp and found that adding three percent plant leaf powder to flour had no adverse effect on the sensory quality of the baked goods, and the loss of flavonoids in processing and baking was small [8]. Other researchers have developed a detoxifying health meal replacement powder [9]. However, the utilization of tartary buckwheat leaf powder is still infrequent, and there are fewer foods developed with tartary buckwheat leaves than with other raw materials. The reason that tartary buckwheat leaves are not used often in food could be that they are not well known to the public. Adding them to noodles could not only improve the nutritional value of the noodles, but also expand the applications of tartary buckwheat leaves in food. Different grinding methods can affect the physical and chemical properties and pasting properties of flour, resulting in different degrees of impact on the processed products.

The grinding treatment of the sample can not only improve the utilization of raw materials, but it is also a necessary process for the production of products. Different grinding methods will have different degrees of influence on the physical and chemical properties of grain powder. Different grinding methods will change due to the speed and heat production, and the powder processing characteristics of the grain powder will be affected by the mechanical strength.

This study selected four types of commonly used grinding equipment to grind tartary buckwheat leaf powder. From there, we will enrich tartary buckwheat leaf products and introduce them into staple foods. In this study, tartary buckwheat leaf powder prepared by different milling methods was added to wheat flour to improve the nutritional value of wheat flour. The addition of tartary buckwheat leaf powder will change the network structure of dough. By analyzing the hydration characteristics, rheological properties, internal disulfide bond content, and protein structure of the dough, the effects of adding tartary buckwheat l eaf powder on the processing characteristics and microstructure of wheat dough were explored.

The aim was to provide data and a theoretical basis for tartary buckwheat leaf powder noodles.

2. Materials and Equipment

2.1. Materials

Tartary buckwheat leaves were obtained from the Shanxi Agricultural University Functional Food Research Institute (Taiyuan, China). Wheat flour (protein: 12.20%, fat: 1.60%, starch: 73.00%, ash: 0.70%) was obtained from Wudeli Food Flour Co., Ltd. (Handan, China). Rutin, γ-aminobutyric acid (GABA), and gallic acid were purchased from Yuanye Biology Co., Ltd. (Shanghai, China). All chemicals used were of analytical grade.

2.2. Preparation of Mixed Powder, Dough, and Protein

2.2.1. Preparation of Mixed Powder

The tartary buckwheat leaves were dried at 60 °C, ground by four different milling methods to form: electric stone mill powder (EMP), (Bluestone 45 × 65, Quanzhou Grinding Edge Electric Stone Mill Co., Ltd., Quanzhou, China), where the rotation speed was 24 r/min; ultra-fine mill powder (UMP) (RT-UF26, Rongcong Precision Technology Co., Ltd., Taiwan, China), where the rotation speed was 25,000 r/min; steel mill powder (SMP), (LG30, Tianjin Taisite Instrument Co., Ltd., Tianjin, China), where the rotation speed was 24,000 r/min; and grain mill powder (GMP), (HK-860, Guangzhou Xulang Machinery Equipment Co., Ltd., Guangzhou, China), where the rotation speed was 1420 r/min.

The tartary buckwheat leaves were separately fed into different devices for grinding, and the samples were taken out from the discharge port, until all the powder was passed through an 80-mesh sieve. Tartary buckwheat leaf powder prepared by different milling methods was added to wheat flour, and the proportion of buckwheat leaf powder was 0%, 10%, 20%, and 30%. The two powders were mixed evenly and stored at 4 °C.

2.2.2. Preparation of Dough

One hundred grams of each flour mix (0%, 10%, 20%, or 30% tartary buckwheat leaf powder) was added to the mixer (Little Bear Electric Co., Ltd., Foshan, China) with 60 mL of distilled water and mixed until the dough was formed. The dough was placed in a dough press at a roll pitch of 5 mm and rolled 3–4 times until a dense dough sheet without holes was formed.

2.2.3. Extraction of Dough Protein

A portion of the dough was subjected to alkali extraction and acid precipitation to obtain gluten protein. The dough and extracted protein were then lyophilized, ground through an 80-mesh sieve, and stored at 4 °C, which were used to determine its microstructure.

2.3. Determination of Particle Size of Tartary Buckwheat Leaf Powder

The powder was dispersed in ethanol at a ratio of 1:300 (w/v) and analyzed using a laser diffraction particle size analyzer (BT-2001, Dandong Baiter Instrument Co., Ltd., Dandong, China).

The suspension was measured at a temperature of 20 ± 0.5 °C, with the laser obscuration range set at 10–20%. Each sample was measured at least three times. The cell rupture rate is a percentage that measures the degree of particle fragmentation or crushing, typically used to evaluate the effectiveness of the crushing process. The calculation formula is as follows.

Cell rupture% = [1−(1− 10 D50 )]×100 1−1−10D50×100; D50 is the median particle size, indicating the corresponding particle size when the cumulative particle size distribution percentage reaches 50%.

2.4. Determination of Dough Rheological Properties

The torque value of the kneaded dough after adding water was measured by two heating processes, and the rheological properties of the dough with different compositions were measured using a Mixolab 2 (automatic comprehensive powder analyzer; Chopin Technologies, Paris, France). The Chopin + 80 g program was used for assessment. During the experiment, if the target torque C1 value was not within 1.10 ± 0.05 N·m, the estimated water absorption rate was adjusted by altering the amount of buckwheat leaf powder and water until the desired torque range was obtained.

2.5. Determination of Hydration Properties

The water-holding capacity (WHC), water solubility (WS), and swelling capacity (SC) were determined according to methods outlined by Zhang et al. [10].

2.6. Free Sulfhydryl (-SH) Content

The content of free sulfhydryl groups in the samples was determined by the method of Liu et al. [11], with some modifications using Ellman’s reagent. The lyophilized sample (50 mg) was dissolved in 2 mL Tris-glycine buffer (pH 7.2), mixed with guanidine hydrochloride (4.7 g), diluted to 10 mL with Tris-glycine buffer, shaken at 37 °C for 1 h, and then centrifuged at 13,000× g for 10 min. One milliliter of the supernatant was mixed with 3 mL of Tris-glycine buffer and 0.1 mL of Ellman’s reagent (5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB), 4 mg/mL). The reaction was carried out at room temperature for 20 min. The absorbance was measured at 412 nm using an ultraviolet spectrophotometer (A380, Shanghai, China).

2.7. Fourier Transform Infrared (FTIR) Spectroscopy Analysis

The samples and spectral-pure KBr (Kermal, SP, CAS: 7758-02-3) were dried at 40 °C for 8 h, mixed at a ratio of 1:100, and ground in an agate mortar before pressing into 20 mg thin disks at 10 lbs. The background was scanned 32 times from 400–4000 cm−1 at a resolution of 4 cm−1 (Nicolex is5, Thermo Scientific, Waltham, MA, USA). Omnic (version 8.0, Thermo Nicolet Inc., Waltham, MA, USA) and PeakFit (version 4.12, SPSS Inc., Chicago, IL, USA) were used to analyze the Fourier infrared spectra [12].

2.8. Fluorescence Spectrum Analysis

The surface hydrophobicity of the gluten protein samples was determined using the 8-anilino-1-naphthalenesulfonic acid (ANS) fluorescent probe method as described in a previous study by Han et al. [13] with simple modifications. Briefly, 2 mg of a lyophilized sample was mixed with 1 mL of 50 mM acetic acid solution and shaken for 1 h at 37 °C. The supernatant was diluted to 1 mg/mL with an acetic acid solution. Twenty microliters of ANS (8.0 mM in the same buffer) was added to 4 mL of sample, and the fluorescence intensity was measured at 280 nm (excitation wavelength) and 410 nm (emission wavelength) using a Hitachi F4500 fluorescence spectrometer (Tokyo, Japan) with the slit width set at 5 nm.

2.9. Scanning Electron Microscope (SEM) Analysis

Following the method reported by Sun et al. [14], lyophilized dough protein was selected for microstructure analysis. The samples were attached to the sample stage with conductive glue, sprayed with gold for 90 s, and cross-sections were observed with a cold-field SEM at 5.0 kV. Observations were made and photographed at magnifications of ×1000 and ×800.

2.10. Statistical Analysis

All tests were performed in triplicate. Two-way analysis of variance (ANOVA) was performed utilizing SPSS statistical software (19.0, SPSS Inc., Chicago, IL, USA). Differences in the means were determined by Duncan’s test at a significance level of 0.05.

3. Results and Discussion

3.1. Analysis of Properties of Tartary Buckwheat Leaf Powder

3.1.1. Analysis of Basic Components of Tartary Buckwheat Leaf Powder

The basic components of the tartary buckwheat leaf powder were significantly different among the milling methods used (p < 0.05, Table 1). GMP had the highest ash content, followed by EMP, UMP, and SMP. The protein content of UMP was slightly higher than that of the other three milled powders, and there was no significant difference with GMP.

Table 1. Basic components of tartary buckwheat leaf powder obtained by different milling methods.