Elsevier

Journal of Food Engineering

Volume 213, November 2017, Pages 38-46
Journal of Food Engineering

Biorefining of buckwheat (Fagopyrum esculentum) hulls by using supercritical fluid, Soxhlet, pressurized liquid and enzyme-assisted extraction methods

https://doi.org/10.1016/j.jfoodeng.2017.04.029Get rights and content

Highlights

  • Multistep biorefining processes were developed and tested for buckwheat hulls.

  • Supercritical carbon dioxide extraction for lipophilic fraction was optimized.

  • Pressurized liquid extraction with higher polarity solvents used for phenolics.

  • Enzyme assisted extraction increase soluble extracts from hulls and residues.

  • Buckwheat hull extracts showed strong antioxidant activity in various assays.

Abstract

Dehulling of buckwheat seeds produces high amounts of processing by-product, the hulls. Soxhlet, supercritical carbon dioxide (SFE-CO2), pressurized liquid (PLE) and enzyme assisted extraction (EAE) schemes were applied for the biorefining of hulls into higher added value soluble fractions. The highest total yield (on dry hull weight basis) of soluble solids in Soxhlet extraction using hexane, acetone and methanol was 5.2%, while at optimal SFE-CO2 parameters (55 MPa, 60 °C, 90 min) in the selected interval of independent variables 1.47% of lipophilic constituents were isolated. Addition of 5% co-solvent ethanol in SFE-CO2 increased the yield to 1.86%. Extract yields were highly dependent on powder particle size. Extraction process was continued by applying to SFE-CO2 residues PLE (at 10.3 MPa) and EAE using ‘green’ solvents, ethanol ant water and commercial xylanolytic enzyme preparation Viscozyme L. These methods enabled to increase the yields of soluble fractions 4–5-fold. The extracts and residues of extractions were evaluated using antioxidant activity and total phenolic content assays. The study shows that buckwheat hulls may be biorefined into strong antioxidants and other functional ingredients, which may find various applications, primarily in foods and nutraceuticals.

Introduction

Buckwheat grain-like seeds are popular and widely used food raw materials in many countries. Buckwheat is a Polygonaceae family gluten–free pseudocereal, which has been grown and used for food and medicine since ancient times. Common buckwheat (Fagopyrum esculentum Moench.) and Tartary buckwheat (F. tataricum Gaertn.) are nowadays the most important commercially species: in 2013 buckwheat cultivation area was over 2.2 million hectares and global production was over 2.3 million tons (Food and Agriculture Organization of the United Nations, 2013).

The main product of buckwheat are the groats, which are obtained by cleaning and dehulling the seeds. Depending on cultivar and moisture content dehulling recovery of buckwheat groats were reported in the range of 32.5–69.2% (Mazza, 1993). Consequently, large amounts of primary seed processing by-products, mainly hulls, are produced. Currently they are mainly used as pillow fillers. Buckwheat groat components and their functional properties have been comprehensively reviewed: high quality proteins, resistant starch, dietary fiber, lignans, flavonoids, phytosterols, fagopyrins, fagopyritols, phenolic compounds, vitamins, minerals and antioxidants were reported as the vital constituents in buckwheat (Ahmed et al., 2014, Gimenez–Bastida and Zielinski, 2015, Zhang et al., 2012). However, the studies of buckwheat hulls focused both on their composition and processing are rather scarce. Phenolic compounds and antioxidant activity of buckwheat hulls, brans and flours were reported by Peng et al. (2004) and Li et al. (2013). Most recently it was reported that buckwheat hulls, depending on the plant cultivar, contained 1.88–2.06% minerals, 5.13–5.68% proteins, 0.50–0.81% crude fat, 91.72–92.19% total carbohydrates and 0.15–2.26% starch (Dziadek et al., 2016). Comparing to the whole buckwheat seeds, the hulls contained higher amount of dietary fiber (76.52–80.73%) and polyphenolic compounds (434–525 mg chlorogenic acid equivalents/100 g); they also demonstrated stronger antioxidant capacity in ABTS•+ scavenging assay (63.20–66.50 μmol trolox equivalents per g) (Dziadek et al., 2016). Lipids extracted from roasted buckwheat hulls were richer in phytosterols than lipids from other buckwheat products (Dziedzic et al., 2016). The studies of technological process of buckwheat groats production on dietary fibre and its fractions content showed that the hulls contained the highest amount of total dietary fibre, lignin and cellulose fractions (Dziedzic et al., 2012). Buckwheat contained an average of 387 and 1314 mg/100 g of flavonoid and 47 and 77 mg/100 g of rutin in the seed and hull, respectively (Oomah and Mazza, 1996). Protocatechuic acid (13.4 mg/100 g of dried hulls), 3,4-dihydroxybenzaldehyde (6.1 mg/100 g), hyperin (5.0 mg/100 g), rutin (4.3 mg/100 g), and quercetin (2.5 mg/100 g) were determined in the fractions of ethanolic extracts of buckwheat hulls almost 20 years ago (Watanabe et al., 1997), while more recently catechin, 1-O-kaffeoyl-6-O-α-rhamnopyranozyl-β-glycopyranoside (swertiamacroside), procianidin B2-3-O-gallate, orientin/isorientin, quercetrin, rutin, vitexin, hiperin, isoquercitrin, epiafzelchin-epicatechin-O-dimethyl gallate and epicatechin-O-3,4-dimethylgallate were identified without quantitative determination (Dziadek et al., 2016). The compounds extracted from buckwheat hulls demonstrated antibacterial effects, which might be explored for protecting foods against various bacterial strains (Čabarkapa et al., 2008). Buckwheat hull fractions were also shown to possess anticancer properties against a variety of different cancer cell lines (Kim et al., 2007). Buckwheat hull hemicelluloses were suggested for improving the quality of bread prepared from medium-quality wheat flours (Hromadkova et al., 2007). Considering these findings it may be suggested that buckwheat hulls may be processed into higher added value products.

In general, valuable constituents may be isolated from the plant matrix by using various extraction methods. Maceration in various solvents and percolation, followed by filtration and solvent removal are conventional methods used for this purpose. However, such methods possess several disadvantages, particularly when hazardous organic solvents requiring remarkable energy inputs for their evaporation are used. Several process variables should be optimized for the most effective isolation of target constituents as it was recently shown in case of recovery of phenolics, flavonoids and antioxidants from palm kernel by-product (Wong et al., 2015). More modern extraction techniques include supercritical fluid (SFE), pressurized liquid, ultrasound, microwave and enzyme assisted extractions. However, application of only one of these methods and one selected solvent can provide only one soluble fraction, whereas the residues usually are discarded as a waste or used as fertilizers, for composting, animal feeding, etc. a sustainable processing of biomass into a spectrum of bio-based products (food, feed, chemicals, materials) and bioenergy (biofuels, power and/or heat). Therefore, development of complex multistep processing methods, which include conventional and novel techniques for the recovery of high added value constituents may be considered as a promising trend in biorefining the by-products of agro-food industry (Galanakis, 2012). Emerging technologies were reported as particularly promising for the production of nutraceuticals from agricultural by-products (Galanakis, 2013).

The goal of this work was to evaluate various extraction processes, including conventional Soxhlet extraction, SFE, PLE and enzyme assisted extraction, on the possibility to process buckwheat hulls into higher added value ingredients. Fraction yields were considered as the main process output, antioxidant capacity was assessed for Soxhlet and SFE extracts. In general, the main target of this study was to obtain several functional ingredients by using multistep extraction processes. It is expected that the results obtained will serve in further development of biorefining technologies of buckwheat hulls until their final valorization, industrial implementation and wider applications.

Section snippets

Plant material and chemicals

Buckwheat (Fagopyrum esculentum) hulls were purchased from the local company Ekofrisa (Prienai r., Lithuania). Carbon dioxide (99.9%) and nitrogen were purchased from Gaschema (Jonava r., Lithuania). Folin-Ciocalteu phenol reagent, 2,2’-azinobis (3-ethylbenzothiazoline 6-sulfonate) (ABTS), AAPH (2, 2’-azobis(2-amidino-propane) dihydrochloride), sodium acetate were from Lach Ner (Czech Republic), potassium iodide (KI) from Chempur (Piekary Śląskie, Poland) and sodium sulphate (Na2SO4) from

Optimization of SFE-CO2 conditions and comparison of the yields with Soxhlet extraction

The main target of applying SFE-CO2 was to optimize this process for the isolation of lipophilic substances from buckwheat hulls. Such substances were reported to contain valuable plant phytosterols (Dziedzic et al., 2016). Experimentally obtained extract yields form hulls were rather small, from 0.36 to 1.47% (w/w) (Table 1). Optimal extraction conditions for obtaining the highest oil yield (1.47%) were 90 min, 60 °C and 55.0 MPa. It should be noted that these parameters are on the edge of the

Conclusion

Buckwheat hulls extracts isolated by various polarity solvents contained up to 144 mg GAE/g of phenolic compounds and demonstrate strong antioxidant capacity, however conventional extraction methods provide rather low yields of soluble fractions (in total up to 6.5%). Application of RSM enabled to optimize SFE-CO2 parameters for obtaining the highest extract yield from buckwheat hulls, however, the material should be finely ground (<0.23 mm) for obtaining the highest, 1.47% yield of lipophilic

Acknowledgements

This study was funded by Research Council of Lithuania, grant no. SVE06/2011.

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