Elsevier

Food Hydrocolloids

Volume 28, Issue 2, August 2012, Pages 275-283
Food Hydrocolloids

Flaxseed gum from flaxseed hulls: Extraction, fractionation, and characterization

https://doi.org/10.1016/j.foodhyd.2011.12.019Get rights and content

Abstract

Soluble dietary fibre with low viscosity has the potential to deliver acceptable mouthfeel and texture when included in the diet in a significant amount to show health benefits. Soluble flaxseed gum (SFG) was reported to have low viscosity, thus has potential in applications as a fibre fortifier. In the present work, SFG extracted from flaxseed hulls was fractionated into a neutral fraction gum (NFG) and an acidic fraction gum (AFG) using ion exchange chromatography. The protein content in SFG (11.8%) and AFG (8.1%) were completely removed by protease to obtain two more protein-free fractions, SFGnP and AFGnP; NFG contained no protein. The uronic acid content in NFG (1.8%) was mostly eliminated, whereas in AFC increased to 38.7%. NFG consisted of high molecular weight (MW) (1470 kDa) arabinoxylans and exhibited pseudoplastic flow behaviour; whereas AFG was mainly composed of rhamnogalacturonans with a higher MW fraction (1510 kDa) and a lower MW fraction (341 kDa) and showed Newtonian flow behaviour. The ranking of intrinsic viscosities (mL g−1) in decreasing order was: SFG (446.0) > NFG (377.5) > AFG (332.5). AFG was expected to have higher chain flexibility for its lower value of Huggins constant (0.16) compared to that of NFG (0.54) and SFG (0.48). In comparison with SFGnP, AFGnP and NFG, SFG and AFG showed better surface activities and emulsifying stabilities due to the presence of protein in both fractions.

Graphical abstract

The intrinsic viscosity (a) and the dependence of viscosity on concentration (b–d) of SFG, NFG and AFG.

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Highlights

► Soluble flaxseed gum (SFG) was extracted from flaxseed hulls. ► Neutral and acidic fractions were separated by ion exchange chromatography. ► Physical and functional properties were characterized.

Introduction

Flax (Linum usitatissiumum L.) was first introduced to North America as a crop for structural fibres, but its value and importance as an oil source quickly became apparent (Cunnane & Thompson, 1995). Canada is the leading country in producing and exporting oil-type flaxseed. Western Canadian flaxseed is composed of 45% oil and 23% protein (1990–2008 means) (www.grainscanada.gc.ca). The renewed interest in flaxseed as a food source is due to its health benefits attributed to its components including lignans (secoisolariciresinol diglucoside (SDG) being the predominant form), α-linolenic acid, and soluble flaxseed gum (SFG) (Hall Iii, Tulbek, & Xu, 2006). In comparison to locust bean gum, guar gum and xanthan gum at a concentration of 0.3% (w/v), SFG exhibited much lower viscosity over a range of shear rate from 10 to 1000 S−1 (Mazza & Biliaderis, 1989). Its low viscosity is favoured in dietary fibre fortification in food without leading to an over-texturization, when a significant concentration of fibre is required to show health benefits.

In vitro fermentation results showed that SFG exhibited higher bile acid binding capacity and generated higher amount of acetate and propionate compared with that of equivalent amount of flax meal, wheat and rye bran, due to its significantly higher soluble fibre content and solubility (Fodje, Chang, & Leterme, 2009). Inclusion of flaxseed in the diet of broiler chickens significantly increased the viscosity of ileal digesta and the number of lactobacilli (Alzueta et al., 2003). The high bile acid binding capacity will lead to an increased fecal excretion of bile acid, which may lower serum cholesterol (Denis et al., 2007, Theuwissen and Mensink, 2008). The high short-chain fatty acids (acetate, propionate and butyrate) productivity and selective stimulation of growth and/or activity of probiotics (Gibson, Probert, Van Loo, Rastall, & Roberfroid, 2004) of SFG also showed its potential as a good source of prebiotics. The abundance of two predominant bacteria divisions in the human gut, the Bacteroidetes and the Firmicutes, were found to have positive and negative correlation with percentage loss of body weight (Ley, Turnbaugh, Klein, & Gordon, 2006).

Flax mucilage (soluble flaxseed gum, SFG) occurs mainly at the outermost layer of hull. This fibre-rich hull is able to release mucilaginous material (soluble gum) easily when soaked in water. In earlier research analyses were based on the gum extracted from the whole seed (Cui et al., 1994a, Diederichsen et al., 2006; Mazza and Biliaderis, 1989; Muralikrishna et al., 1987, Naran et al., 2008, Oomah et al., 1995, Warrand et al., 2003, Warrand et al., 2005a, Warrand et al., 2005b) or flax meal (Fedeniuk and Biliaderis, 1994, Mueller et al., 2010). With the success of a patented dehulling process (Cui & Han, 2006), the whole flaxseed could be efficiently separated into a kernel (∼63%) fraction and a hull (∼37%) fraction (Cui, 2000) in large scale. Investigating the composition, structure, physicochemical and rheological properties of SFG from hulls will assist in exploring its potential for the food industry.

In this current study, the gum (SFG) extracted from flaxseed hulls was further separated into neutral (NFG) and acidic (AFG) fraction gums using ion exchange chromatography (IEC). The chemical (neutral monosaccharide composition, uronic acid and protein content), physical (molecular weight distribution and heat stability), rheological (viscosity, viscoelasticity, intrinsic viscosity and critical concentration,) and functional (surface tension and emulsification) characteristics of SFG and its two fractions are reported.

Section snippets

Materials

Flaxseed hull (variety Bethune) was supplied by Natunola Health Biosciences (Winchester, Ontario, Canada).

Extraction of gum

The extraction of the gum was conducted at room temperature as shown on the flow chart in Fig. 1. A batch of 500 g of flaxseed hull was soaked in 4 L of distilled water overnight under gentle stirring. A coarse filtration followed using cheesecloth to separate the hull. The filtered mucilage was collected and centrifuged (Beckman Coulter, Mississauga, Ontario) at 27,000 g and 25 °C for

Extraction, fractionation and chemical composition

The composition and yield of flaxseed gum has been reported to vary with extraction conditions, as well as culture environment and genotype. Gum yield from flaxseed increased from 4 to 9.4 % with extraction temperature increased from 25 to 80 °C (Fedeniuk & Biliaderis, 1994). Broad variations of 3.6–8.0% among 109 cultivars (80 °C, 2 h) and 5.4–7.9% among 12 genotypes (85 °C, 3 h) were also found by Cui, Kenaschuk, and Mazza (1996) and Oomah et al. (1995), respectively. Soaking flaxseed with a

Conclusions

Flaxseed gum may become a significant source of soluble fibre for both its availability and low viscosity. Two fractions, neutral (NFG) and acidic (AFG), of soluble flaxseed gum (SFG) from hulls were effectively separated using ion exchange chromatography. NFG, consisting of only one polymer with high MW, was free of protein and contained <2% of uronic acid. AFG mainly consisted of polymers with different MW and had associated 8% of protein, but not covalently linked. Both fractions were stable

Acknowledgements

The authors would like to thank Edita Verespej, Cathy Wang and Ben Huang for their technical assistance. Special thanks are also due to Prof. Shao-Ping Nie and Qing-Bin Guo for their helpful suggestions and support.

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