Extraction enhancing mechanism of steam exploded Radix Astragali

doi:10.1016/j.procbio.2014.08.010

Process Biochemistry

Volume 49, Issue 12, December 2014, Pages 2181-2190

https://doi.org/10.1016/j.procbio.2014.08.010 Get rights and content

Highlights

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Porous properties of steam exploded Radix Astragali were characterized.
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Steam explosion increased the area proportion of 100–100,000 nm pores from 8.25% to 91.57%.
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100–100,000 nm pores was the major factor to determine extraction performance.
•
Extraction mechanistic model of steam exploded Radix Astragali was proposed.

Abstract

Plant extraction occupies a vital status in the fine chemical industry, and steam explosion has been confirmed to be an effective pretreatment to enhance the extraction performance. In order to reveal the extraction enhancing mechanism of steam explosion, innovatively from the view of plant porous medium, we characterized porous properties of steam exploded Radix Astragali and established the correlation of porous properties and saponins extraction performance of Radix Astragali. The results indicated that average pore diameter was the most relevant to extraction parameters with R-square > 0.99 among various porous properties, which can effectively characterize the extraction performance. Area percentage of middle and large pores (100–100,000 nm) of Radix Astragali was increased from 8.25% to 91.57% after steam explosion and the increase of these pores was the major factor for enhancing extraction performance. Based on the close correlation of extraction parameters and porous properties, the extraction mechanistic model of steam exploded Radix Astragali was proposed. Altered porous properties improved the solute–solvent accessibility and internal mass transfer in the extraction process. Such findings are anticipated to prompt the enhancing effects and widespread application of steam explosion technology in plant extraction, and further identify research directions that lead to more efficient extraction.

Introduction

For centuries, plants have been utilized as both a food source and a source of active pharmaceutical agents. As of 2010, 75% of anti-bacterial compounds and 48.6% of anti-cancer compounds are either a natural product or natural product analog [1]. To aid in development and defense against adversity, plants synthesize hundreds of thousands of compounds, including saponins, alkaloids, flavonoids, terpenoids, etc. These compounds have many dietary health benefits and significant commercial value as pharmaceuticals, nutraceuticals, dyes, fragrances, flavors and pesticides [2]. Although some valuable plant natural products with simple structures are easily chemically synthesized (e.g., aspirin and ephedrine), many have complicated structures with multiple chiral centers, making chemical synthesis both difficult and commercially unfeasible [3]. A large part of sophisticated components can hence only be obtained through extraction from corresponding plants.

However, as active ingredients mainly exist in the cells of plant materials, structures of plants themselves construct multi-level mechanical barriers to the passage of constituents in and out of the cell [4], [5], including the structural heterogeneity and complexity of cell-wall constituents, the degree of lignification, the relative amount of sclerenchymatous tissue, the arrangement and density of the vascular bundles and the epidermal tissue of the plant body, particularly the cuticle and epicuticular waxes, etc. Above physical barriers inevitably restrict two controlling steps in extraction process. One is the accessibility and dissolution of solutes, the other one is the mass transfer of solutes through plant structure [6]. These existing rate-limiting steps make the extraction process low-efficiency and time-consuming. Therefore, pretreatment before extraction is necessary to decrease the extraction barriers, in order to achieve the desirable extraction performance.

Steam explosion is a widely employed and cost-effective pretreatment method for plant materials, which is defined as the steam hydrolysis at high temperature and pressure, followed by the sudden reduction of pressure for physical tearing of the hydrolyzed materials [7], [8], [9]. Typical effects of this pretreatment are substantial breakdown of the lignocellulosic structure, hydrolysis of the hemicellulosic fraction, and depolymerization of the lignin components. Due to its effective physico-chemical modification on materials, steam explosion has been employed by many researchers as an effective pretreatment process for extracting and separating bioactive molecules from plant issues [10], [11], [12], [13], [14], [15], [16]. A summary of relevant literature reports is shown in Table 1. Previous reports concentrated on the optimization of steam explosion conditions, extraction yields, kinetics and structural changes of chemical components. Few reports were available elucidating the effect mechanism of steam explosion for enhancing extraction performance. As a consequence, there exists the black box limitation between pretreatment efficiency and extraction results, hindering the determination of operation parameters and the prediction of extraction results. In addition, the lack or insufficiency of mechanism research restricts the extraction enhancing effects and application fields of steam explosion on plant extraction.

For bioactive components in plants, extraction is carried out in a natural inhomogeneous porous medium, which is the intrinsic structural feature of plants. Multi-level pore structures from lignin-carbohydrate-complex (LCC) structure and pits to vessel cells and vascular bundles, exhibit the basic porous skeleton of plants [17], [18]. Various porous properties such as quantity, geometry, distribution and connectivity of porous network, are closely related to above solute–solution accessibility barrier and mass transfer barrier [19], [20] and should have significant impact on the extraction process of active ingredients. After the high-temperature hydrolysis and explosive decompression stages of steam explosion, the chemical composition, macro- and micro-structure of plant tissues are altered, resulting in the obvious changes of porous network and the corresponding porous properties. Therefore, porous property is supposed to be an evaluation methodology of steam explosion efficiency which applies to all the plants. What is more, it may be the reason of extraction enhancement by steam explosion, which helps to reveal the extraction enhancing mechanism of steam explosion.

Thus, the aim of our study is to unveil the extraction enhancing mechanism of steam explosion from the view of plant porous medium. We investigated the effects of steam explosion on extraction performance with the example of Radix Astragali − a typical medicinal plant rich in saponins. Porous properties of steam-exploded samples were analyzed to reflect the steam explosion efficiency and predict the extraction performance. Chemical compositions and structure analysis were conducted to explain the changes of porous properties. Maximum extraction content C_∞ and diffusion coefficient D obtained from mass transfer model were considered as the key metrics for evaluation of extraction performance. Combined with the fitting model, the correlation of porous properties and extraction performance was established. On this basis, the extraction mechanistic model of steam exploded Radix Astragali was proposed in the present study.

Section snippets

Materials

Radix Astragali, the roots of Astragalus membranaceus (Fisch.) Bge. var. mongholicus (Bge.) Hsiao, were purchased from Beijing Qiancao Herbal Pieces Co., Ltd. (Beijing, China). Astragaloside IV (≥98%, lot No. 110905) was purchased from Sichuan Standard Substances Center (Sichuan, China). Acetonitrile and formic acid of HPLC grade were obtained from Aladdin Industrial Corporation (Shanghai, China). Distilled and deionized water were produced by a Milli-Q Reagent Water System (Millipore, MA,

Porous properties analyses of steam exploded Radix Astragali

To reveal the extraction enhancing mechanism of steam explosion, we firstly characterized porous properties of steam exploded samples and then established the correlation of extraction performance and porous properties. On this basis, the extraction mechanistic model of steam exploded samples was proposed to reveal the extraction enhancing mechanism of steam explosion. Porous properties of steam exploded samples were assessed by various pore parameters in the present study, including pore size

Conclusions

In the present study, the porous properties of steam exploded Radix Astragali were systematically analyzed and characterized by various porous parameters. Results indicated that steam explosion prompted the generation, amplification and interconnection of porous network. The changed porous properties improved the solute–solvent accessibility and internal mass transfer in the extraction process, which was supposed to be the extraction mechanism of steam exploded Radix Astragali.

Average pore

Acknowledgements

This work was financially supported by the National Basic Research Program of China (973 Project, No. 2011CB707401), the National High Technology Research and Development Program (863 Program, 2012AA021302) and Open Funding Project of the National Key Laboratory of Biochemical Engineering (No. 2013KF-01).

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Hull constitutes 10–13% of barley grain that provides most of the fibre with hemicellulose accounting for 30% of hull weight (Krawczyk et al., 2008). Steam explosion at high temperature and pressure, followed by a sudden release to ambient pressure (Zhang et al., 2014), disrupts structural carbohydrates in plant materials, e.g., lignin, cellulose and hemicellulose (Li et al., 2009; Han et al., 2010; Sui and Chen, 2014). Steam explosion might reduce anti-nutritional factors such as phytic acid in wheat bran and glucosinolates in canola meal (Gu et al., 2011; Guo et al., 2015).
Barley grain containing both fermentable starch and fibre might be attractive as energy source in weaned pig diets because of benefits on gut health. Rapidly fermentable carbohydrates and slowly fermentable fibre may decrease diarrhoea in weaned pigs serving as prebiotics. Steam explosion processing may disrupt the fibre matrix of cereal hulls, increasing slowly fermentable fibre of barley grain. Five diets were formulated to provide 10.0 and 9.6 MJ net energy (NE)/kg, 1.32 and 1.22 g standardised ileal digestible (SID) lysine/MJ NE for phase 1 (day 0–14) and phase 2 (day 14–28), respectively. Diets contained 593–644 g cereal grain/kg: 1) low-fermentable hulled barley (LFB); 2) LFB steam-exploded (LFB-SE; 1.2 MPa, 120 s); 3) high-β-glucan (70 g/kg) hull-less barley (HFBB); 4) high amylose (168 g/kg) hull-less barley (HFAB); or 5) low-fermentable wheat (LFW). A total of 220 pigs in 11 blocks of 5 pens each were fed 1 of 5 diets starting 1-week post-weaning for 28 days. For phase 1, the coefficient of apparent total tract digestibility (CATTD) of gross energy (GE) and crude protein (CP) of diets was greatest (P < 0.05) for LFW, intermediate for HFBB, followed by LFB-SE, LFB and lowest for HFAB. The calculated NE value was greatest (P < 0.05) for LFW, intermediate for LFB-SE, and lowest for HFBB, LFB and HFAB. For phase 2, the CATTD of GE was greater (P < 0.05) for LFW and HFBB than LFB, LFB-SE and HFAB; the CATTD of CP was greatest (P < 0.05) for LFW, intermediate for HFBB, followed by LFB, HFAB, and lowest for LFB-SE; calculated NE value was greatest (P < 0.05) for HFBB and LFW, intermediate for LFB and HFAB, and lowest for LFB-SE. For the entire trial (day 0–28), average daily feed intake (ADFI) and average daily gain (ADG) of pigs did not differ among diets, but gain-to-feed (ADG/ADFI) was lowest (P < 0.05) for the LFB-SE diet. For day 21–28, pigs fed LFB-SE diet had greater (P < 0.01) ADFI than pigs fed HFAB diet, whereas pigs fed LFW, LFB and HFBB were intermediate. Faeces consistency of pigs did not differ between LFB and LFW diets, but pigs fed LFB diet had firmer (P < 0.05) faeces than pigs fed HFBB, HFAB and LFB-SE diets. To conclude, the wheat diet had greater GE and CP digestibility than barley diets. Among barley diets, the HFBB had greater GE and CP digestibility than both LFB and LFB-SE; the HFAB diet ranked lowest. However, feeding barley to replace wheat grain in nursery diets formulated to equal NE value and SID amino acid content did not affect growth performance. Finally, dietary fermentable carbohydrates in barley made faeces less firm. Steam explosion of hulled barley grain did not increase GE or CP digestibility.
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The high-density energy generated by SE destroys the rapeseed cells (Wang et al., 2021), resulting in protein denaturation, oil accumulation and release (Yu, Guo, & Huang, 2020). Under the appropriate SE conditions, similar structural changes also appeared in soybean seed coat (Chen et al., 2020), miscanthus (Sun et al., 2020), coconut mesocarp (Inseemeesak & Areeprasert, 2020), sumac fruit (Chen & Chen, 2011), Radix Astragali (Sui & Chen, 2014), fig leaves (Qin & Chen, 2015), tea waste (Sui et al., 2019), oil palm mesocarp fibers (Marques et al., 2020), seabuckthorn pomace (Tu et al., 2018), rapeseed and rapeseed products (Yu et al., 2020), and the Chinese indigenous herb “Hangbaiju” (Gong et al., 2021). During the SE process, steam with a strong seepage force penetrates the internal cells of biomass, and explosive decompression will cause most of the steam to expand rapidly and destroy the internal structure of materials, finally forming pores on the surface when the steam rushes out.
Steam explosion (SE) with its dual effect of high temperature and high pressure has gradually been applied in food pretreatment. The process of SE treatment involves various mechanisms: acid-based hydrolysis, thermal degradation, mechanical-like disruption, hydrogen bond destruction and structure rearrangement. Phenolic compounds, as natural secondary metabolites, are found in different forms depending on their association with the food matrix, and exert significant antioxidant activity to improve health benefits.
In this review, we provide a thorough summary regarding on the working principle of SE, the forms of phenolics present in plant, the effects of SE on the concentration and compounds of phenolics, their antioxidant activity including cellular antioxidant activity, the microstructure of the plant matrix, and the application of SE.
Phenolic acids and flavonoids are common phenolic compounds. The different forms of phenolics present in the plant matrix, include free and bound phenolics, or free, esterified, glycosidic and insoluble-bound forms. During SE processing, explosion temperature/pressure and residence time were the main factors that influenced the extraction and conversion of plant phenolics. In general, the effect of SE on phenolic extraction showed a trend of increasing first and decreasing later, and finally, it reached a balanced state with the dissolution and degradation of phenolic compounds. The optimal SE conditions depend on the pretreatment strategy and physical accessibility of the raw material. Under optimal conditions, SE can increase the release of phenolics and enhance their antioxidant activity. There was a positive correlation between phenolics and antioxidant activity. SE can break down the glycosidic linkages, and ester and β-O-4 ether bonds, which leads to an increase in carboxyl and phenolic hydroxyl groups. The SE process can enhance the cellular antioxidant activity of free phenolics, but has the opposite effect on bound phenolics. Regarding the microstructure, the SE can increase the porosity and pore volume of the material, which is beneficial to solute-solvent accessibility and internal mass transfer in the phenolic extraction process. However, the migration and transformation mechanism of SE on phenolics is still not clear. More studies need to focus on the conversion mechanism of SE on bioactive components and further expand the application scope of SE.

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Extraction enhancing mechanism of steam exploded Radix Astragali

Highlights

Abstract

Introduction

Section snippets

Materials

Porous properties analyses of steam exploded Radix Astragali

Conclusions

Acknowledgements

Curr Opin Biotechnol

Curr Opin Biotechnol

Bioresour Technol

Chem Eng Sci

Process Biochem

Food Chem

Chem Eng Sci

Chem Eng J

Ind Crop Prod

Bioresour Technol

Chem Eng Sci

Natural products as sources of new drugs over the 30 years from 1981 to 2010

J Nat Prod

Mining the biodiversity of plants: a revolution in the making

Science

Biomass recalcitrance: engineering plants and enzymes for biofuels production

Science

Biomass recalcitrance. Part I: the chemical compositions and physical structures affecting the enzymatic hydrolysis of lignocellulose

Biofuels Bioprod Biorefining

Batch and continuous solvent extraction

J Am Chem Soc

Effect of steam explosion treatment on barley bran phenolic compounds and antioxidant capacity

J Agric Food Chem

Optimization of steam explosion assisted extraction of flavonoids from Ginkgo Biloba leaves by response surface methodology

J Cellulose Sci Technol

Gas explosion technology and biomass refinery