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PART I
PRINCIPLES AND RECENT RESEARCH DEVELOPMENTS
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CHAPTER 1
COVALENT CROSS-LINKING CONTENT, RHEOLOGICAL, AND STRUCTURAL CHARACTERISTICS OF WHEAT WATER-EXTRACTABLE AND WATER-UNEXTRACTABLE FERULATED ARABINOXYLAN GELS
ELIZABETH CARVAJAL-MILLAN, JORGE MÁRQUEZ-ESCALANTE, ANA LUISA MARTHNEZ-LÓPEZ, and AGUSTÍN RASCÓN-CHU
Research Center for Food and Development, CIAD, A.C. Carretera a La Victoria Km. 0.6, Hermosillo, Sonora, 83304, Mexico, Tel: +52-662-2892400; Fax: +52-662-2800421; E-mail: [email protected] CONTENTS
Abstract |
1.1 | Introduction |
1.2 | Experimental Part |
1.3 | Results and Discussion |
1.4 | Conclusion |
Acknowledgments |
Keywords |
References |
ABSTRACT
The aim of this research was to investigate the cross-linking content, rheological, and structural characteristics of water-extractable arabinoxylans (WEAX) and water-unextractable arabinoxylans (WUAX) gels. The intrinsic viscosity, viscosimetric mass and ferulic acid (FA) content were 3.6 and 2.1 dL/g, 440 and 74 kDa and 0.40 and 0.08 μg/mg polysaccharide, for WEAX and WUAX, respectively. The Fourier transform infrared spectrum of WEAX and WUAX presented characteristics bands (1035, 1158 and 897 cm−1) related to β(1–4) linkages. WEAX and WUAX laccase induced gels at 3% (w/v) registered a di-FA content of 0.076 and 0.008 μg/mg polysaccharide, respectively. Storage (G’) and loss (G”) moduli were 101 and 20 Pa for WEAX gel and 174 and 19 Pa for WUAX gel. The structural parameters of WEAX and WUAX gels were calculated from swelling experiments. WEAX and WUAX gels mesh size were 123 nm and 48 nm, respectively. As di-FA content was lower in WUAX gels, a more important contribution of physical interactions between polysaccharide chains and/or non-identified ferulate cross-linking structures could be responsible of these rheological and structural differences between WEAX and WUAX gels.
1.1 INTRODUCTION
Arabinoxylans are important cereal non-starch polysaccharide constituted of a linear backbone of β-(1–4)-linked D-xylopyranosyl units to which α-L-arabinofuranosyl substituents are attached through O-2 and/or O-3. Some of the arabinose residues are ester linked on (O)-5 by ferulic acid (FA) (3-methoxy, 4 hydroxy cinnamic acid) (Izydorczyk and Biliaderis, 1995). These polysaccharides have been classified as water extractable (WEAX) or water-unextractable (WUAX). WEAX and WUAX form highly viscous solutions and gel through ferulic acid covalent cross-linking upon oxidation by some chemical or enzymatic free-radicals generating agents resulting in the formation of five different di-FA structures (5–5,’ 8–5’ benzo, 8-O-4,’ 8–5’ and 8–8’ di-FA). This covalent cross-linking has been commonly considered as responsible of the polysaccharide network development even if weak interactions also contribute to the final gel properties (Vansteenkiste et al., 2004). The presence of a trimer structure of FA (tri-FA) has been reported in WEAX and WUAX (Carvajal-Millan et al., 2005a, 2007).
The utilization of water-based gels in industrial applications as texture and stability improvers or as delivery systems generates great interest (Wisniewski et al., 1976; Pothakamury and Canovas, 1995). Most of polysaccharide gels currently used are stabilized by physical interactions (hydrogen bonding and/or ionic); polysaccharide covalent networks such as WEAX or WUAX gels are not common. Covalently cross-linked gels are generally strong, form quickly, they are not temperature dependent on heating and they exhibit no syneresis after long time storage. Furthermore, WEAX and WUAX gels have interesting functional properties, which have not been exploited even though their neutral taste and odor are desirable properties for industrial applications. WEAX and WUAX networks have a high water absorption capacity (up to 100 g of water per gram of polymer) and they are not sensible to electrolytes or pH (Izydorczyk and Biliaderis, 1995).
WEAX and WUAX gels have been already studied (Izydorczyk et al., 1990; Figueroa-Espinoza and Rouau, 1998; Figueroa-Espinoza et al., 1998; Schoonevelds-Bergmans et al., 1999; Carvajal-Millan et al., 2005a, 2007) but their chemical, rheological and structural characteristics of WEAX and WUAX have not been compared elsewhere. In the present study, WEAX and WUAX from a wheat grain cultivar have been extracted, characterized and gelled and then the gel covalent cross-linking content, rheological and structural characteristics have been investigated.
1.2 EXPERIMENTAL PART
1.2.1 MATERIALS
Wheat flour and wheat bran were kindly supplied by a commercial milling industry in Northern Mexico. Laccase (benzenediol: oxygen oxidoreductase, E.C.1.10.3.2) from Trametes versicolor and all other chemical products were purchased from Sigma Chemical, Co. (St. Louis, MO, USA).
1.2.2 WEAX AND WUAX EXTRACTION AND CHARACTERIZATION
WEAX were isolated from wheat flour as previously described (Carvajal-Millan et al., 2005a). WUAX were extracted from wheat bran as reported before (Berlanga-Reyes et al., 2011). Sugars were quantified by ion-exclusion chromatography after hydrolysis process (Carvajal-Millan et al., 2005a). Sugars were eluted in a RPM-Monosaccharide Pb+2 (8%) column (300 × 7.8 mm) (Phenomenex, Torrance, US) with water at 0.6 mL/min. The column and detector temperatures were 80°C and 40°C, respectively. Mannitol was used as an internal standard. An Alliance Waters e2695 separation module with Waters 2414 RI detector and an Empower Pro Software (Waters, Milford, US) were used. Protein content was determined by the Dumas method (AOAC, 1995) using a Leco-FP 528 nitrogen analyzer. Ferulic acid was quantified by reverse phase chromatography (RP-HPLC) after a de-esterification process. Ferulic acid was eluted in a Supelcosil LC18BD (250 × 46 mm) (Supelco Inc., Bellefont, US) with water:acetic acid:methanol (59:1:40) at 0.6 mL/min and 35°C. A Waters 2998 photodiode array detector (Waters, Milford, US) was used. Detection was by UV absorbance at 320 nm.
Viscosity measurements were made by determination of the flow times of WEAX and WUAX solutions in water (from 0.06 to 0.1% w/v). An Ubbelohde capillary viscometer at 25 ± 0.1°C, immersed in a temperature controlled water bath was used. The intrinsic viscosity ([η]) was estimated from relative viscosity measurements (ηrel) of WEAX solutions by extrapolation of Kraemer and Mead and Fouss curves to “zero” concentration. The viscosimetric molecular weight (Mν) was calculated from the Mark-Houwink relationship, Mν = ([η]/k)1/α.
FT-IR spectra of dry WEAX and WEAX gel (lyophilized) powder were recorded on a Nicolet FT-IR spectrophotometer (Nicolet Instrument Corp. Madison, WI). The samples were pressed into KBr pellets (2 mg sample/200 mg KBr). A blank KBr disk was used as background. Spectra were recorded between 400 and 4000 cm−1.
1.2.3 WEAX AND WUAX GELATION
Laccase mediated cross-linking of WEAX and WUAX was performed as reported previously (Carvajal-Millan et al., 2005a). WEAX and WUAX solutions at 3.5% (w/v) were prepared in 0.1 M citrate phosphate buffer pH 5.5. WEAX and WUAX solutions were mixed with 50 μL of laccase (1.675 nkat/mg polysaccharide). Gels were allowed to form for 4 h at 25°C.
1.2.4 GELS COVALENT CROSS-LINKING CONTENT
WEAX and WUAX gels di-FA and tri-FA contents were quantified by high performance liquid chromatography (HPLC) after desertification step (Rouau et al., 2003; Vansteenkiste et al., 2004). An Alltima C18 column (250 × 4.6 mm) (Alltech Associates, Inc. Deerfield, IL) and a photodiode array detector Waters 996 (Millipore Co., Milford, MA) were used. Detection was by UV absorbance at 320 nm.
1.2.5 GELS RHEOLOGY
Small amplitude oscillatory shear was used to follow the gelation process of WEAX and WUAX solutions (3.0% w/v) by using a strain controlled rheometer (Discovery HR-2, hybrid Rheometer, TA instruments) as previously reported (Carvajal-Millan et al., 2005b). Gelation processes were studied during 90 min at 25°C, 1.0 Hz of frequency and 10% strain (linearity range of viscoelastic behavior). Frequency sweep (0.1 to 10 Hz) was carried out at the end of the network formation at 10% strain at 25°C.
1.2.6 GELS STRUCTURE
After laccase addition, WEAX and WUAX solutions were quickly transferred to a 5 ml tip-cut-off syringe (diameter 1.5 cm) and allowed to gel for 90 min at 25°C. After gelation, the gels were removed from the syringes, placed in glass vials and weighted. The gels were allowed to swell in 20 ml of 0.02% (w/v) sodium azide solution to prevent microbial contamination. During 36 h the samples were blotted and weighed. After weighing, a new aliquot of sodium azide solution was added to the gels. Gels were maintained at 25°C during the test. The equilibrium swelling was reached when the weight of the samples changed by no more than 3% (0.06 g). The swelling ratio (q) was calculated as:
where Ws is the weight of swollen gels and Wd is the weight of WEAX or WUAX in the gel.
From swelling measurements, the molecular weight between two cross-links (Mc), the cross-linking density (ρc) and the mesh size (ξ) values of the different WEAX and WUAX gels were calculated. Mc was calculated using the model of Flory and Rehner modified by Peppas and Merrill for gels where the cross-links are introduced in solution (Flory and Rehner, 1943; Peppas and Merrill, 1976). From the Mc values, the ρc and ξ in the WEAX gels were calculated as reported before (Peppas et al., 1985, 2000).
1.2.7 STATISTICAL ANALYSIS
Chemical and physico-chemical determinations were made in duplicates and the coefficients of variation were lower than 6%. Small deformation measurements were made in triplicates and the coefficients of variation were lower than 9%. Swelling and controlled release tests were made in duplicates, coefficients of variation were lower than 10%. All results ...
