Extractability and chemical and enzymic degradation of psyllium (Plantago ovata Forsk) seed husk arabinoxylans Valerie Van Craeyveld, Jan A. Delcour,Christophe M.Courtin * Laboratory of Food Chemistry and Biochemistry, Leuven Food Science and Nutrition Research Centre (LFoRCe), Katholieke Universiteit Leuven, Kasteelpark Arenberg 20 bus 2463, B-3001 Leuven, Belgium a r t i c l e i n f o Article history: Received 10 January 2008 Received in revised form 8 May 2008 Accepted 18 June 2008 Keywords: Psyllium seed husk Plantago ovata Arabinoxylans Extractability Acid hydrolysis Enzymic hydrolysis a b s t r a c t To assess the potential of arabinoxylan (AX)-rich psyllium (Plantago ovata Forsk) seed husk (PSH) as a source for production of arabinoxylo-oligosaccharides (AXOS), the parameters determining PSH AX extraction yield and the chemical and enzymic hydrolysis of PSH AX were investigated. The seed husk material had a high content of AX (62.5%) with an arabinose to xylose ratio of 0.41. The water extract- ability of PSH AX was affected by suspension concentration rather than by temperature. Maximally, 27% of all AX could be extracted, even when using very dilute suspensions (0.1% w/v). When subjected to alkaline conditions, a pH of at least 12 was needed to significantly increase extractability (up to 77% of all AX). The extractability-increasing effectof alkali was, however, reversible,as extractability decreased again when lowering the pH. Acid hydrolysis at high temperature drastically increased AX extraction yields (up to 97% ofall AX), released monomeric arabinose (up to 38%), and lowered the average degree of polymerisation (DP avg ) of the AX (down to 31).The presence of substituents (arabi- nose,xylose,rhamnose and galacturonic acid) on the xylan backbone was an important factor limiting degradation by xylanases.Enzyme preparations containing substituent-removing enzymes were far better for enhancing the extractability ofa large portion of the AX population (up to 82%) and for degrading the extracted fragments (down to DP avg 14) than preparations lacking such activities. The above results show that PSH is a good source for the production ofAXOS,since both chemicaland enzymic treatments significantly increase the extractability of PSH AX and convert PSH AX molecules into small fragments. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Psyllium (Plantago ovata Forsk) is an annual plant, grown pri- marily in India (Dhar,Kaul, Sareen,& Koul, 2005; Iqbal,1993).It has a long history of use in both conventional and traditional sys- tems of medicine.Psyllium seed husk (PSH) is mechanically re- moved from the seed (Anderson, Allgood, Turner, Oeltgen, & Daggy, 1999 ). It is a well-known laxative (Marlett & Fischer, 2002; Marlett,Kajs, & Fischer,2000) and lowers serum total and LDL cholesterol concentrationsin hypercholesterolemic adults (Anderson et al.,1999; Arjmandi,Sohn, Juma, Murthy, & Daggy, 1997; Olson et al.,1997),as well as all-day and post-lunch post- prandial glucose concentrations in subjects with type 2 diabetes (Anderson et al., 1999). It is not clear to what extent these proper- ties can be ascribed to the most abundant polysaccharide in PSH, i.e.a complex heteroxylan with, as main monosaccharides, arabi- nose and xylose, and which is further referred to as arabinoxylan (AX) (Fischer et al.,2004; Kennedy,Sandhu,& Southgate,1979; Marlett & Fischer,2002,2003),or possible degradation products. PSH AX are highly branched non-starch polysaccharides with a main chain of densely substituted b-(1,4)-linked xylopyranose res- idues.Single arabinofuranose and xylopyranose residues, or short side chains consisting of these monosaccharides, are attached at positions 2 and/or 3 of the main chain xylopyranose residues (Ed- wards,Chaplin,Blackwood,& Dettmar,2003; Fischer et al., 2004). In addition,PSH AX contain additional residues such as rhamnose (3–5%) and galacturonic acid (5–8%) (Edwards et al., 2003; Ken- nedy et al.,1979; Marlett & Fischer, 2002).A schematic represen- tation of the PSH AX structure is given in Fig. 1. Due to their complex structure,PSH AX resist enzymic degradation (Edwards et al., 2003; Fischer et al., 2004). Fischer et al. (2004) suggested that the resistance of the polysaccharide to the action of glycoside hydrolases is due to an unusualinkage pattern in the molecule and/or the high density ofits branching.Only 9% ofall PSH AX was found to be water-extractable (Marlett & Fischer, 2002).Un- like wheat bran AX,PSH AX form strong gels in water (Gelissen, Brodie, & Eastwood, 1994; Sandhu,Hudson, & Kennedy, 1981). The gel dissociates almost completely in the presence of sodium 0308-8146/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2008.06.035 * Corresponding author.Tel.: +32 16 321917; fax: +32 16 321997. E-mail address: christophe.courtin@biw.kuleuven.be (C.M. Courtin). 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