Buckwheat noodles: processing and quality enhancement

Abstract

In recent years, buckwheat noodles have gained increased importance because of their functional properties. These qualities are attributed to the abundance of bioactive compounds (e.g., rutin, quercetin) and nutraceuticals (e.g., B vitamins, unsaturated fatty acids). Buckwheat noodle consumption has been shown to be associated with improved metabolic health. Buckwheat flour exhibits properties similar to those of common cereal flours in food processing, but devoid of gluten. However, the maintenance of good textural properties and high sensory acceptability are key challenges in the development of gluten-free products, and these limitations prevented widespread application of buckwheat in the food industry. Nevertheless, continuous technological developments related to raw materials processing, noodle processing, and noodle quality enhancement have contributed to the growing popularity and acceptability of buckwheat noodles in recent times. These improvements could render buckwheat noodles a healthy gluten-free alternative to wheat noodles.

Introduction

Noodles constitute a significant part of Asian diet. Noodles made from white wheat flour are popularly consumed, however, noodles can also be made from buckwheat, rice, and starches derived from pulses and potatoes (Fu, 2008; Wang et al., 2016). However, a specific disadvantage with noodles from refined popular flours (such as wheat, corn, and rice) is that they can swiftly increase blood glucose levels following the consumption; this is particularly due to the presence of rapidly digestible and absorbable carbohydrates (Wang et al., 2016). The rapid rise in blood glucose levels is detrimental to human health, especially for people with diabetes. To redeem the negative effect, health-conscious consumers are increasingly preferring several alternatives, including buckwheat.

Buckwheat (Fagopyrum esculentum Moench) is a pseudo-cereal and it doesn’t belong to the grass family (Poaceae) as cereals do. It is cultivated in many countries and most of the buckwheat is used for food (Christa and Soral-Smietana, 2008). In many European countries, buckwheat is used as a basic food ingredient in soups and porridges (Ma et al., 2013). Buckwheat is generally classified into common and Tartary buckwheat. Both common buckwheat (F. esculentum) and Tartary buckwheat (F. tartaricum) are widely used to prepare various cultural foods such as kasha, pizzoccheri, and naengmyeon as well as bread, cakes, instant noodles, dried noodles or vermicelli, pasta, cookies, crackers, muffins, pancakes (Mota et al., 2016; Yilmaz et al., 2020).

Buckwheat contains starch (65–75%), protein (10–12.5%), lipid (4.7%), minerals, and vitamins (Yalcin, 2021). It is also a good source of fiber, gluten-free, and rich in various bioactive compounds (e.g., rutin, quercetin, kaempferol‐3‐rutinoside); and therefore increasingly considered a potential functional food (Yilmaz et al., 2020). It contains a high quality protein, i.e., well‐balanced amino acids with a high biological value. Predominant minerals of buckwheat include potassium, magnesium, phosphorus, calcium, iron, copper, zinc, manganese; and vitamins include A (β-carotene), B₁ (thiamine), B₂ (riboflavin), B₃ (niacin), B₅ (pantothenic acid), B₆ (pyridoxine), C (ascorbic acid), and E (tocopherols) (Unander, 2002; Ahmed et al., 2014). Particularly, as buckwheat is free from gluten, it can readily be included in the diet for people with gluten intolerance.

Buckwheat noodles have been the most popular food in East Asia region including Japan, Korea, and China. Especially in Japan, various types of buckwheat noodles (called as ‘soba’) are formulated and popularly consumed (e.g., Jinenjo, Mugwort, Cha, and Ito soba) (Ikeda and Asami, 2000). Buckwheat noodles have been shown to influence the metabolism beneficially compared to wheat noodles (Kreft and Skrabanja, 2002). A significantly reduced rate of in vitro amylolysis of buckwheat noodles compared with white wheat bread was observed. Noodles prepared with wheat flours partially substituted with buckwheat flour (in the range of 5 to 30%) exhibited improved total polyphenol content and antioxidant capacity. In addition, the cooking quality of noodles was unaffected by the fortification (Kiss et al., 2019). Wang et al. (2016) prepared buckwheat noodle with glucose sustained-release function. In this review, details about buckwheat noodle processing, noodle quality improvement strategies including significance of extrusion, quality criteria of the noodles, and factors affecting quality are discussed. Additionally, developmental aspects related to decontamination, shelf-life, and buckwheat allergy as well as bioactive compounds, namely rutin and polyphenols, are included.

Processing technology of buckwheat noodle

Buckwheat noodles can be prepared without any additives (some handmade soba noodles contain 100% buckwheat flour) or with additives such as wheat flour (most of soba noodles possess at least 60% of buckwheat, i.e., buckwheat flour and wheat flour blend at a ratio of 60:40) (Giménez-Bastida et al., 2015; Hatcher et al., 2008). According to Japanese Food Agency regulations, soba noodles must contain a minimum of 35% buckwheat in their composition (Hatcher et al., 2011). For gluten-free noodle production, common buckwheat has been shown to be a better material than Tartary buckwheat (Ma et al., 2013). Some characteristics of noodles prepared using common and Tartary buckwheat are given in Table 1. Different flour fractions of buckwheat (e.g., inner-layer flour alone) or whole flour can be used for noodle preparation (Ikeda and Asami, 2000).

Table 1.

Some characteristics of noodles prepared using common and Tartary buckwheat

Parameter	Common buckwheat noodles	Tartary buckwheat noodles	Reference
Textural characteristics	Higher tensility (22–25 g force), relatively low adhesiveness (27–33 g force)	Relatively low tensility (13–16 g force), increased adhesiveness (100–176 g force)
Water uptake	45–48%	48–65%
Sensorial characteristics	Light grey color (desirable), better sensory properties	Faint yellow, bitter taste	Ma et al. (2013)
Antioxidant properties	Relatively low reducing power [96–114 mg vit. C equivalent/100 g DW], DPPH radical scavenging [0.8–1.0 mmol Trolox equivalent/100 g DW], and ABTS + scavenging [3–4 mmol Trolox equivalent/100 g DW] capacities	Higher antioxidant capacity (relatively high reducing power [425–576 mg vit. C eq./100 g DW], DPPH radical scavenging [7.0–8.7 mmol Trolox eq./100 g DW], and ABTS + scavenging [31–40 mmol Trolox eq./100 g DW] capacities)
Phenolics & flavonoids	Relatively low total phenolics (182–221 mg GAE/100 g DW) and flavonoids (183–218 mg rutin equivalent/100 g DW)	Relatively high concentrations of total phenolics (969–1362 mg GAE/100 g DW) and flavonoids (502–600 mg rutin eq./100 g DW)

Van Hung et al. (2007) developed gradual milling (of whole buckwheat grains) method to improve noodle-making quality and nutritive values of buckwheat flour. The starch-rich inner fractions are much whiter than the outer fractions (dark‐grey color), which contain high concentrations of dietary fiber, protein, and phenolic compounds. The ash and protein contents of the flour fractions have been shown to be increased in the order from the inner to the outer fractions (Van Hung et al., 2007). A study on the effects of different buckwheat flour refinements on the appearance, composition, and texture of buckwheat soba noodles showed that noodles made with white flour exhibited superior springiness, chewiness, and recovery parameters (Hatcher et al., 2008). In addition, dark flours-based cooked noodles were thicker with higher cutting stress and high resistance to compression. Ohisa et al. (2002) showed superior noodle-making property of buckwheat flour made from cold counter-jet-mill (CJM) in comparison to roller-milled flour. In excess water, CJM flour expanded to 2.8-fold of original volume and CJM flour-based noodles (cooked) exhibited a long and unbroken line.

Buckwheat noodles are made in two ways-by sheeting/slitting and extrusion. In Japan, hand-made buckwheat noodles are made from a blend of 7–8 parts of buckwheat flour to 2–3 parts of wheat flour, whereas machine-made noodles require a higher proportion of wheat flour (40–80%) to enhance the binding power of the mixture (Fu, 2008). The process of noodle preparation consists of three key steps, namely mixing of hydrated buckwheat flour, spreading of the dough, and cutting (Horigane et al., 2004). The main objectives of mixing are to rapidly and uniformly distribute the ingredients and to hydrate the flour particles (Liu et al., 2015). Typically, raw/extruded buckwheat flour along with other flours (e.g., wheat), salt (1–2% based on flour weight), and water were mixed and kneaded into dough using a mixer. Different types of mixers, including vertical mixer, horizontal mixer, low-speed super mixer, continuous high-speed mixer, and vacuum mixer, are commonly used in the noodle industry (Gulia et al., 2014). Mixing is generally followed by dough resting, which accelerates further hydration of flour particles and redistribute water in the dough system. In a study, dough resting was allowed for 30 min (Wang et al., 2019). Following mixing and resting, the dough is sheeted using a sheeting roller. The purpose of sheeting is to achieve a smooth dough sheet with desired thickness, and a continuous and homogeneous dough sheet. Dough sheets are cut into noodles as per requirement (e.g., 2.0 mm width and 1.0 mm thickness). For dried or semi-dried buckwheat noodles preparation, fresh noodles can be dried using a dehydration apparatus.

Quality criteria of buckwheat noodle

Color

The color of fresh noodles is an important optical property for consumers (Jia et al., 2019). In general, color is measured by spectrocolorimeter (like HunterLab colorimeter) in terms of L* (represents brightness or lightness from 0 to 100), a* (redness from –128 to + 127), and b* (yellowness from –128 to + 127) color scale. Soba noodles are light to dark grey in color. With the increase of Tartary buckwheat flour (TBF) levels in noodles, the color of dough sheet become darker; mean L*, a*, b* values of 60% Tartary buckwheat noodles and wheat noodles (control) were 64.02, 0.41, 27.78 and 78.01, 1.22, 19.57, respectively (Jia et al., 2019). Ikeda and Asami (2000) showed that buckwheat noodles made from straight flour exhibited high a* and b*, whereas those made from inner layer flour exhibited low a* and b*. On the other hand, no significant difference in L* was noted among the samples tested. The application of superheated steam (SHS) for decontamination of buckwheat flour did not alter its color characteristics negatively (Ono et al., 2007). The color of dried buckwheat noodles (DBN) has been shown to be darker than that of fresh buckwheat noodles (FBN); buckwheat noodles with 20% and 80% of extruded buckwheat flour exhibited considerably lower b* values than that with 50% extruded buckwheat flour (Wang et al., 2021a).

Cooking quality

The parameters reflecting the cooking quality of buckwheat noodles include cooking loss, optimal cooking time (OCT), water absorption, swelling index, and cooking breakage ratio. Cooking loss and broken rate during the cooking process are primarily due to the dissolution of loosely bound gelatinized starch on the surface of noodles, which depends predominantly on the degree of gelatinization and the strength of the retrograded starch network around the gelatinized starch (Resmini and Pagani, 1983). Compared with DBN, FBN exhibited a shorter OCT (320–330 s and 140–180 s, respectively), a lower cooking loss (16.60–44.40% and 11.56–22.31%, respectively), and lower breakage ratio (37.5% and 21.3%, respectively, when the addition ratio of extruded buckwheat flour was 80%) (Wang et al., 2021a). They also showed that the cooking loss of buckwheat noodles was increased significantly with increase in the amount of extruded buckwheat flour from 20 to 80%. The addition of sodium alginate and xanthan gum (1.0–1.5%) to composite flours (wheat + buckwheat in different ratios) has been shown to improve the cooking quality of the noodles (Kim and Kim, 1983). Corn or potato starches can improve the cooking properties of buckwheat noodles. Noodles containing corn starch exhibited relatively low cooking loss, higher weight increase and swelling volume values compared with noodles containing potato starch (Yalcin, 2021).

Texture

Texture is an important quality parameter that affects consumer preference and acceptance of a particular food product (Hatcher et al., 2011). The attainment of good palatability, an important quality factor of food, in buckwheat noodles largely depends on the texture of the product (Ikeda and Kishida, 1992). Elasticity and hardness are the most critical texture parameters for consumers (Wang et al., 2021a). Other instrumental textural attributes include cohesiveness, chewiness, and resilience. Buckwheat noodles exhibited a significantly weaker texture or lower measurements of hardness, cohesiveness, chewiness, and resilience (after cooking) compared with white salted noodles and egg noodles (Liu et al., 2016). This could be due to the fact that buckwheat proteins primarily contain water-soluble and salt-soluble albumins and globulins which are not suitable as contributors to the wheat gluten network (Liu et al., 2016). In addition, the presence of fiber particles in buckwheat flour can disrupt the continuity of the protein-starch network, resulting in noodles with weaker texture (i.e., lower firmness) (Choy et al., 2013). It has been shown that the incorporation of wheat flour into buckwheat dough (buckwheat-to-wheat ratio of 8:2) led to significantly (P < 0.05) higher springiness (0.929 texturometer units [TU] vs. 0.292 TU for buckwheat dough without wheat flour), cohesiveness (0.420 TU vs. 0.099 TU), and chewiness (0.076 TU vs. 0.012 TU) of the dough, and to significantly (P < 0.05) lower hardness (0.197 TU vs. 0.432 TU) and adhesiveness (0.018 TU vs. 0.292 TU) (Ikeda and Kishida, 1992). In terms of palatability, noodles made from a mixture of buckwheat flour and wheat flour were significantly superior than noodles made from buckwheat flour alone (Ikeda and Kishida, 1992).

Different strategies have been applied to improve the textural properties (e.g., appropriately low hardness and high springiness) of buckwheat noodles. To increase the textural properties of buckwheat/wheat dough/noodles, processing treatments, including pre-gelatinization, high temperature or steam puffing, fermentation, and high hydrostatic pressure, have been used (Wang et al., 2019). Han et al. (2012) showed that the textural properties of cooked buckwheat noodles can be improved by the addition of Ca(OH)₂ to buckwheat flour. They observed an improvement in hot-water swelling power of the flour with Ca(OH)₂ addition (up to 0.2%); and cooked noodles exhibited the highest levels of tensile strength and cutting force when Ca(OH)₂ was added at 0.4% level. The improvements are due to the formation of a compact and homogenous gel network under alkaline conditions by starch-Ca²⁺ interactions. In another study, Han et al. (2014) showed that the addition of 0.4% Ca(OH)₂ and 1.5% konjac glucomannan to buckwheat flour significantly improved its thermomechanical properties. They noted a marked improvement in the tensile strength and firmness of cooked noodles because of the addition, and the additives acted synergistically. The degree of protein polymerization has been shown to be increased by alkali addition and this could enhance the rheological properties of dough and texture properties of buckwheat noodles (Guo et al., 2017). It is known that noodle texture is primarily a function of protein content. However, Hatcher et al. (2011) showed that gluten strength (a function of protein quality, not of quantity) of flour plays a significant role in textural attributes of soba (a blend of buckwheat and common wheat flour) noodles.

Factors affecting quality of buckwheat noodle

In general, the quality of noodles depends on the flour characteristics, proportions of ingredients, and conditions used during preparation or processing variables (Karim and Sultan, 2014). Strong gelling properties of buckwheat starches are likely to contribute substantially to the texture of soba noodle (Hatcher et al., 2008). Yoo et al. (2007) showed that the incorporation of buckwheat flour into wheat flour reduced the farinograph water absorption (the composite flours of buckwheat 60% + wheat 40%, buckwheat 90% + wheat 10%, and buckwheat 100% + wheat 0% exhibited water absorption of 62.4%, 58.0%, and 55.9%, respectively) and enhanced the initial pasting temperature (63.6 °C, 64.6 °C, and 65 °C, respectively). Reduced dough water absorption and stability of wheat flour as well as diminished noodle firmness were also shown by the addition of TBF to wheat flour (Han et al., 2013). However, noodles formulated with 10–20% of fraction A or 10% of fraction B of TBF (both the fractions contain relatively low protein, fiber, lipid, and ash contents and relatively high starch contents) exhibited no significant differences in terms of cooking loss or tensile strength of cooked noodles as well as sensorial properties compared with the values of control, i.e., wheat flour noodles (Han et al., 2013). On the contrary, higher water absorption and gelatinization temperature have been reported for dough containing buckwheat flour (Nikolic et al., 2011). In addition, they showed that buckwheat flour incorporation may contribute to a relatively low degree of softening and extended dough stability compared with wheat flour only. Since the main dough component responsible for water absorption is gluten and as the buckwheat flour is gluten-free, the good water absorption could be due to other proteins such as albumin and globulin (Nikolic et al., 2011).

Decreased pasting parameters were noted for composite flour containing wheat and hydrothermally-treated buckwheat flours (7:3, w/w) compared with composite flour containing native buckwheat (Yoo et al., 2012). This could be due to partial disruption of starch granules under elevated temperatures of hydrothermal treatments (steaming over boiling water for 10 min and autoclaving at 120 °C for 10 min), where the temperatures are higher than that needed for starch gelatinization. Compared to steaming, starch gelatinization was more sensitive to autoclaving treatment as the lowest pasting parameters were observed for autoclaved samples. In that study, the highest peak (due to the higher degree of starch gelatinization) and final viscosities were observed for the sample containing native buckwheat flour. Handoyo et al. (2006) showed that pasting properties of fermented buckwheat (FeB)-substituted wheat flour were lower than those of the control (without FeB flours) except for gelatinization temperature and breakdown viscosity. The 30% of FeB substitution was the best choice for noodle manufacturing since it had lower peak viscosity (~ 440 BU), which was suitable for Japanese noodle such as soba. The cooked noodles with the FeB flours exhibited more elasticity and smoothness, and the colors were more brownish than those of the control.

Pre-harvest sprouting of buckwheat has been shown to decrease the processing suitability of buckwheat flour. Sprouting decreases pasting viscosity, and therefore the mechanical properties of boiled noodle may be affected by flour prepared from sprouting grains (Hara et al., 2007). The length of buckwheat storage has been shown to be inversely related to water absorption, pasting viscosities, and development time of its corresponding dough (Wang et al., 2021b).

Rheological studies indicated that buckwheat dough possessed more elastic character than viscous character (Siwatch et al., 2017). In a study, frequency sweep curves of native and processed buckwheat flour showed that storage modulus G′ (elastic property) was greater than loss modulus G″ (viscous property) for all the samples during whole range of frequency, indicating solid elastic-like behavior of native and processed buckwheat dough (Siwatch et al., 2017). Among the flours, cooked buckwheat flour exhibited weak viscoelastic properties. In another study, hydrothermally-treated (steamed or autoclaved) buckwheat flour exhibited relatively low elastic properties (reduced elongation stress) compared with native buckwheat, indicating that hydrothermally-treated buckwheat flour did not contribute to strengthening the gluten network of dough made from a mixture of wheat and buckwheat flours (Yoo et al., 2012). Jet-cooking has been shown to damage buckwheat flour structure, thereby decreasing viscoelasticity (Inglett et al., 2009). The addition of buckwheat flour (husked and unhusked) to wheat flour caused the weakening of protein structure because of incapability of buckwheat flour protein to form a network in dough system like gluten does in wheat dough systems (Hadnađev et al., 2008). In addition, the rate of gelatinization and maximum torque (viscosity) were shown to be decreased by the addition of both types of buckwheat flour. Among different flour fractions obtained by gradual milling of buckwheat, the middle fractions exhibited significantly higher peak viscosity than the other fractions (Van Hung et al., 2007). In addition, noodles made from the outer flour fractions (high in protein, low in starch) were softer and more elastic compared with those made from the inner flour fractions.

Quality enhancement of buckwheat noodle

The maintenance of good textural properties and a high sensory acceptability score are key challenges in the development of gluten-free products (Starowicz et al., 2018). Ma et al. (2013) reported best overall quality score (the sum of scores for sensory attributes including color, taste, odor, hardness, slipperiness, chewiness) in the range of 5.57–6.71 (on a 1–9 sensory scale) for noodles made from common buckwheat. For noodles made from Tartary buckwheat, the score range was 4.57–4.71. Buckwheat’s unique flavor is one of the most important quality characteristics of boiled buckwheat noodles; lipase and peroxidase (enzyme activity) in buckwheat flour are important for flavor generation of boiled buckwheat noodles (Suzuki et al., 2010). Due to the absence of gluten in buckwheat and the lack of binding between hydrophilic globulin and albumin, buckwheat flour cannot develop its own viscosity and noodles are generally made from a mixture of buckwheat flour and wheat flour, yam or egg to decrease their fragility (Tsutsumi et al., 1990; Ikeda, 2002). For sheeted noodles, buckwheat noodles are made from 30% buckwheat flour and 70% wheat flour (to develop gluten network). Highest quality (better color and texture) buckwheat-enriched instant noodles have been obtained through the incorporation of common buckwheat flour (20%) to a recipe of instant noodles (Choy et al., 2013).

Potential approaches to improve quality

Hydrocolloids or gluten have been added to buckwheat noodles in order to increase the content of buckwheat flour and to modify noodle quality (to enhance the appearance and texture) (Han et al., 2012; Wang et al., 2019). Bilgicli (2008) showed that gluten-free noodle with 20% buckwheat flour (prepared with buckwheat flour: rice flour: corn starch at a ratio of 20:40:40 and with 3% xanthan gum, 20% whole egg, and 0.5% salt) exhibited the highest taste score and similar overall acceptability scores to that of control noodles (prepared with wheat flour, egg, and salt). They also showed relatively higher cooking loss in noodles containing buckwheat flour; and ash, phytic acid, and crude fat contents as well as redness values were increased with buckwheat flour addition. On the other hand, binder addition decreased rutin level and compromised pure and clean taste of buckwheat (Horigane et al., 2004).

Kim and Kim (1983) showed that the addition of sodium alginate and xanthan gum (1.0–1.5%) to composite flours (wheat + buckwheat in different ratios) can improve their dough-making properties. They also shown that dried noodles can be made from the composite flour of wheat 60% + buckwheat 40%, and the kneading property of composite flour was inversely related to the quantity of buckwheat flour. Liang et al. (2017) showed that the addition of Pueraria root starch (PRS) to the mixture of wheat flour and buckwheat flour improved the quality of buckwheat noodles as well as their nutritional function. The improvements are due to the fact that PRS possess low amylose content, fewer branches, and shorter amylopectin branch lengths compared with cereal, bean, and rhizome starches. Yalcin (2021) showed that buckwheat noodle containing 50% corn starch possessed optimum quality characteristics and beneficial components. Wang et al. (2016) developed a sustained-release (glucose) buckwheat noodle by adding xanthan gum/konjac glucomannan mixed gum and cornstarch.

The supplementation of L-ascorbic acid (L-AA) has been shown to enhance the quality of buckwheat noodles (Wei et al., 2017). They observed reduced water absorption and cooking loss of noodles and enhanced hardness as well as tensile force by the addition of L-AA. In addition, a direct relationship between the concentration of L-AA and viscosity properties, final viscosity, retrogradation value of buckwheat flour was shown. Their results also demonstrated that L-AA promoted protein cross-linking in the buckwheat noodles.

The freshness of buckwheat plays a vital role in the qualities of noodles. Quality deterioration of noodles made from stored buckwheat grains has been reported (Wang et al., 2021b). They showed that superheated steam treatment of buckwheat grains can inactivate lipid oxidation enzymes (lipase, peroxidase, and lipoxygenase) and therefore flavor deterioration of noodles made from stored buckwheat can be retarded. In addition, the treatment aided in maintaining the pasting properties of starch stable and increased the strength and elasticity of the dough, leading to the enhancement of cooking qualities and texture profiles of buckwheat noodles.

Pre-gelatinization treatments (including steaming, roasting, boiling, microwave, and extrusion) of buckwheat flours have been shown to improve their processing quality (Sun et al., 2018). They noted that pre-gelatinized buckwheat flours-based noodles exhibited relatively better textural property and sensory quality compared with the noodles made from native buckwheat flour. The addition of different pre-gelatinized starches, including extrusion-cooked maize starch (ECMS), drum-dried tapioca starch (DDTS), and drum-dried maize starch (DDMS), have been shown to improve the tensile strength of Tartary buckwheat dough sheet significantly (Obadi et al., 2020). Their results showed that DDTS exhibited a relatively high viscosity and water absorption index (due to a higher proportion of long amylopectin and higher average chain length of amylopectin) than the others. They concluded that DDTS was better than the others in improving the processing quality of Tartary buckwheat noodles by enhancing starch gel formation and its cross-links with the gluten network. Chen et al. (2020) showed that the processing quality of noodle dough with high (70%) TBF can be improved by the addition of wheat flour and wheat gluten with high gluten index and shown to be better than pre-gelatinized TBF addition. The most likely mechanism is through forming a compact gluten network in the dough sheets.

Functional ingredients, namely vitamin D₂-enriched Shiitake mushroom and seaweed-derived calcium (Aquacal), were added for quality enhancement of cold buckwheat noodles (Chung et al., 2007). They showed that microbiological safety and sensory characteristics (bitterness, chewiness, gloss, elasticity, and mushroom flavor) can be maintained up to 6 days at 5 °C and 8 weeks at –18 °C.

Starch retrogradation is beneficial for starch noodles since the process increases hardness, decreases the cooking loss, and lowers the glycemic index. Sun et al. (2021) optimized retrogradation process for time-saving and improving qualities of extruded whole buckwheat noodles. They showed that 4 °C water pre-cooling for 10 s facilitated the formation of the initial starch gel network structure and retention of water molecules in extruded whole buckwheat noodles. In addition, 4 °C water pre-cooling for 10 s followed by 4 °C refrigerator retrogradation increased starch retrogradation; and improved the cooking quality and textural characteristics of cooked noodles.

Significance of extrusion processing

Extrusion cooking can be applied to physically modify (alteration of molecular structure) cereal-based expansible starchy and proteinaceous materials (Wu et al., 2017). The process unifies starch gelatinization and formation steps, and allows starch itself to form a starch network in place of protein network. Extrusion is more likely to improve the functionality and noodle-making quality of buckwheat flour. From the nutritional and quality perspective, appropriate extrusion processing variables are crucial to produce whole buckwheat noodles. Sun et al. (2019) showed that, with the increase of extrusion temperature (T = 100–160 °C), both the total flavonoid content (TFC) and total phenolic content (TPC) were decreased and the highest retention of the contents were observed when extruded at a moisture content of 40%. Extrusion at severe conditions of high shear and high temperature significantly increased the soluble dietary fiber (SDF) content. The cooking qualities of buckwheat noodles, such as the cooking loss, broken rate, and texture characteristics, were positively related to the degree of gelatinization (DG). However, when the DG was higher than 87.96%, noodles became over-cooked, resulting in the deterioration of cooking quality. Wang et al. (2019) showed that buckwheat noodles with extruded buckwheat flours exhibited higher hardness, elasticity, and total sensory score as well as lower breakage ratio than those of buckwheat noodles with raw buckwheat flour. They also showed that extruded buckwheat flour with dynamic viscosity higher than 7.37 Pa·s functioned as an adhesive, which can hold starch granules and other components within flours together. Wang et al., (2020) prepared extruded buckwheat flours (EBF) and these were used for the fortification of wheat flour. They showed that fortification with EBF (30% feeding moisture) significantly extended the stability time of the composite dough and decreased attenuation time. Further, the fortification resulted in remarkably higher levels of TFC, TPC, kaempferol, rutin, and antioxidant capacity of cooked noodles. Also, cohesion, hardness, springiness, and chewiness were significantly improved with the EBF (30% feeding moisture) fortification.

Wu et al. (2017) showed that Tartary buckwheat flour mixture (TBFM, micronized Tartary buckwheat flour and extruded-micronized Tartary buckwheat flour [1:1, w/w]) can be used as a partial substitute (10–60% content of TBFM) of wheat flour in noodle formulation. With the increase of TBFM content, hardness of the noodles improved and internal networks became more compact. Noodles substituted with TBFM (10–60%) exhibited significantly higher antioxidant activities and similar or better cooking properties (cooking loss, cooked breaking rate, optimal cooking time, and water adsorption rate) than that of wheat noodles.

Fu et al. (2020) showed that the substitution of rice flour with buckwheat flour at 30 g/100 g remarkably enhanced the nutritional (high retention of flavonoids and polyphenols, increased levels of resistant starch and dietary fiber) and cooking characteristics (low cooking loss) of extruded gluten-free rice noodles.

Additional developments related to buckwheat noodles

Decontamination and shelf-life

Buckwheat flour is known to contain a high microbial load because of lack of disinfection in the milling process. Microbial contamination in commercial buckwheat flour has been shown to be relatively high (5 log CFU/g) compared to wheat flour (2.6 log CFU/g) (Ono et al., 2007). Ono et al. (2007) used superheated steam (SHS) for decontaminating buckwheat flour; viable counts of total bacteria and coliforms on buckwheat grains were reduced to < 2 log CFU/g following treatment with SHS. In addition, viable microbial cell counts of buckwheat flour milled from SHS-treated grain were relatively low (< 2 log CFU/g). The SHS treatment significantly improved the preservation of buckwheat noodles, without negatively affecting the sensory characteristics of buckwheat noodles.

Aqueous ozone mixing (AOM) has been shown to reduce the initial total plate count (TPC) of semi-dried buckwheat noodles significantly, with improved microbial stability during storage (Bai and Zhou, 2021). In addition, AOM enhanced the cooking quality as well as textural properties (increased the protein cross-linking) and decreased the cooking loss of the noodles. Bai et al. (2017a) showed that the combination of aqueous ozone treatment and modified atmosphere packaging was effective in prolonging the shelf-life of semi-dried buckwheat noodles. In their study, noodles prepared with aqueous ozone (2.21 ppm) exhibited relative low microbial load (47% reduction), the shelf-life of noodles was extended to 9 days when packed under N₂: CO₂ = 30:70, and textural and sensorial characteristics were unaffected during the storage. In another study, Bai et al. (2017b) showed that active packaging approaches, especially the combination of oxygen scavenger + ethanol emitter + polyvinylidene chloride packaging material, can significantly prolong the shelf-life of semi-dried buckwheat noodles, without negatively affecting the quality.

The lowering of water activity (a_w) of semi-dried buckwheat noodles using different types of a_w lowering agents has been shown to prolong their shelf-life (Bai et al., 2018). Hu et al. (2020) showed that gaseous ozone treatment (2.4 g/h for 15 min) of buckwheat-based composite flour significantly decreased the microbial TPC of the flour and extended shelf-life of fresh noodles prepared from the ozone-treated flour was observed. In addition, the treatment improved the lightness, water absorption, development time, stability time, and farinograph quality number of the flour. Furthermore, swelling capacity, solubility, peak viscosity, final viscosity, and setback of the flour were increased by ozone treatment.

Functional compounds: Polyphenols

Buckwheat noodles are rich source of dietary polyphenols. Especially, Tartary buckwheat noodles have been shown to contain relatively higher levels of flavonoids (183.33–218.69 mg rutin equivalent/100 g dry weight (DW)) and total phenolics (182.65–221.27 mg gallic acid equivalent (GAE)/100 g DW) compared with common buckwheat noodles (Ma et al., 2013). Therefore, higher antioxidant capacity was observed for Tartary buckwheat noodles. In Tartary buckwheat noodles, the levels of phenolic compounds namely rutin, quercetin, p-hydroxybenzoic acid, and protocatechuic acid were higher; and a high concentration of chlorogenic acid was observed in common buckwheat noodles. These functional components have the potential to prevent chronic diseases linked to oxidative stress.

Buckwheat is a rich source of dietary rutin (quercetin-3-rutinoside), a polyphenolic compound. Cho and Lee (2015) extracted rutin from buckwheat milling fractions by ultrasonic-assisted ethanol extraction, and the rutin-enriched material (REM, 31.8% rutin) was incorporated (1–2% by weight) into the formulation of wheat-based instant fried noodles. They observed a remarkable rutin loss in noodles by cooking in hot water compared with frying. The oxidative rate of instant fried noodles during storage was retarded by the application of REM. Rutin enrichment in the TBF (by rutin migration from the bran fraction) by hydrothermal treatments of steaming, boiling, and autoclaving has been shown (Oh et al., 2019). Especially, autoclaving was the most effective in rutin enrichment.

Rutin loss is usually observed when flour is prepared from buckwheat seeds by grinding. Also, when water is added to buckwheat flour, the enzymatic degradation of rutin into quercetin with a bitter taste has been shown (Li et al., 2008). Steaming, boiling of buckwheat grains, and extrusion of the flour (but not dry heating or microwaving) have been shown to result in the retention of > 85% of original rutin level, with no bitterness in the hydrated flours (Li et al., 2008). Yoo et al., (2012) employed hydrothermal treatments (steaming and autoclaving) to inactivate rutin-degrading enzymes in Tartary buckwheat. They showed that noodle samples possessing hydrothermally-treated buckwheat flour (prepared with wheat and buckwheat flours in 7:3, w/w) exhibited higher concentrations of rutin (> 0.83 g/100 g) compared with control noodle having native buckwheat flour (0.27 g/100 g). Suzuki et al. (2019) used ‘Manten-Kirari’, a new Tartary buckwheat variety, to prepare rutin-rich noodles with minimal bitterness. The variety exhibited trace rutinosidase activity, with no bitterness, and ~ 90% of rutin remained in noodles made from the new buckwheat.

Buckwheat allergy

Buckwheat has been shown to contain potent allergens (e.g., BW24KD, a 24-kDa protein) and the allergens trigger a type I, IgE-mediated, immediate-type hypersensitivity reaction (Wieslander, 1996). Fermentation could aid in the development of hypoallergic buckwheat. Handoyo et al. (2006) showed that Rhizopus oligosporus was effective to decrease allergenic proteins of buckwheat (through assimilation of proteins as an energy source for its own growth). In their study, steamed buckwheat was inoculated with the spores of the fungus and incubated for different periods, and 48-h-fermented buckwheat was found to contain higher amounts of amino acids and minerals, with low levels of allergenic proteins and anti-nutritional agent (phytic acid), than buckwheat sample without fermentation.

In conclusion, the quality of buckwheat flour, other ingredients, and process parameters play a crucial role in determining the quality of buckwheat noodles. Although additives (e.g., gluten, hydrocolloids) aided to increase the content of buckwheat flour and modify noodle quality, they may increase the cost of buckwheat noodles and it is also against the trend of ‘clean label’. Therefore, physical treatments like extrusion could be beneficial. The mixing of noodle dough in vacuum could help improve the quality of noodles since the process aid in thorough absorption of water and therefore the formation of a more homogenous water–flour mixture. Advanced technologies (e.g., confocal scanning laser microscopy) could be used to understand the microstructure changes in dough and noodles. In addition, near infrared (NIR) spectroscopy can be used as a low-cost, rapid, and accurate method for chemical analysis.

Author’s Contribution

Conceptualization, investigation, writing—original draft, P.P., writing—review and editing, S.L. All authors have read and agreed to the published version of the manuscript.

Declarations

Conflict of interest

There is no conflict of interest.