Universitet Cereal ( 1 — > 3 , 1 — > 4 )-ß-D-glucans – functional properties and molecular interactions

In the current study we provide applicable knowledge on the elemental food functional properties exhibited by barley (GlucagelTM) and oat (PromOatTM) β-glucan products used as additives in foods. Rheological properties of β-glucans and molecular interaction with aroma compounds have been investigated in order to exploit the matrix properties of β-glucan in foods. Additionally, we have determined purity, size, and the structural characteristics of the two β-glucan products. INTRODUCTION Health benefits and functional properties of dietary fibres like mixed linkage (1→3,1→4)-β-D-glucans from cereals make them interesting food constituents. Due to their viscosity enhancing effect and the fact that they are able to form gels, they are applicable as food hydrocolloids and have among others been suggested as ingredients in health promoting functional foods. High levels (i.e. 3-10 %) of β-glucans are present in the aleuronic and starchy endosperm cell walls of barley and oat. The polymers consist of long, linear chains of glucosyl residues linked via (1→3)and (1→4)-β-glucosidic linkages (Fig. 1). Structure, molecular weight and concentration of the β-glucan influence the functional effect in foods . Figure 1. Primary structure of mixed linkage (1→3, 1→4)-β-D-glucan. MATERIALS AND METHODS GlucagelTM is a soluble barley β-glucan product supplied by GraceLinc Ltd. (Christchurch, New Zealand). PromOatTM, supplied by Biovelop (Kimstad, Sweeden) is a soluble β-glucan product derived from oat bran. Total β-glucan content was determined in triplicates using the MegazymeTM mixed linkage β-glucan assay kit (Megazyme International Ltd., Bray, Ireland). Molecular weights were determined by gel permeation size exclusion chromatography (GPSEC) on a series of three columns; TSKgel GMPWxl (Tosoh Bioscience LLC, Montgomeryville, USA) Shodex B-806 HQ, and SB-806M HQ (Showa Denka K.K., Tokyo, Japan). The samples were calibrated against β-glucan standards (Megazyme International Ltd., Bray, Ireland) and Pullulans (Shodex Cereal (1→3, 1→4)-β-D-glucans – functional properties and molecular interactions Mette Skau Nielsen1, Anders Ola Karlsson2 and Søren Balling Engelsen1 1 Quality & Technology and 2 Dairy Technology Department of Food Science, University of Copenhagen, Faculty of Life Sciences Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark Standard P-82, Showa Denko K.K. , Tokyo, Japan). The H NMR spectra were acquired on a Bruker Avance 400 (9.4 T) spectrometer (Bruker Biospin, Rheinstetten, Germany), operating at a Larmor frequency of 400.13 MHz for protons, using a High-Resolution Magic-Angle Spinning (HR-MAS) probe equipped with a 4 mm (o.d.) rotor. Experiments were performed at 75 ̊C using the water pulse sequence, a recycle delay of 4 s, 64 scans, a spin-rate of 7 kHz and a dwell time of 60.4 μs for acquisition of 32 k data points. Viscosity measurements were conducted using a Stresstech rheometer (Reologica Instruments AB, Lund, Sweden), where temperature was regulated by a circulating water bath. Measurements were performed at shear rates between 1 and 30 s for every 10 ̊C (10-80 ̊C). Activation energy was calculated from the Arrhenius equation. Equilibrium dialysis was performed on mixtures of GlucagelTM (2.5% w/v), and various aroma compounds, that is vanillin derivatives (2 mM) (Sigma-Aldrich Co., Copenhagen, Denmark) using dialysis membranes passing 6-8 kDa globular molecules (Spectrum Laboratories Inc., Breda, The Netherlands) in 1 ml in-line equilibrium dialysis cells (Bel-Art products, Pequannock, U.S.A.). The dialysis cells were kept in a thermostat-regulated water bath (37 °C) from where sampling (10 μl) at times 15, 30, 45, 60, 75, 90, 120, 150, 180, 240, and 300 min were performed. Absorbance of the dialysate samples was measured at 280 nm on a Spectra Max 190 microplate reader (Molecular Devices Corporation, Sunnyvale, U.S.A.). A curve derived from the exponential equation: Absorbance = A (1-e) (1) was fitted to data from each individual dialysis run, where A is the asymptotic or equilibrium value, t is the dialysis time and k is the dialysis rate constant. Asymptotic values were compared between dialyses of pure aroma compound and the mixture of βglucan and aroma compound to quantitatively determine the level of dialysate retention (∆A) by β-glucan. All samples were tested four times, that is, two replicates were dialysed 12 hours (day 1) after mixing and further two replicates were run after 36 hours (day 2). RESULTS Product characterisation Total β-glucan content in GlucagelTM was 80.0 (±8.2) %, whereas the result for PromOatTM was 33.3 (±0.9) %. These figures approach the values stated by the suppliers, 75% and 30-40% respectively. The molecular weight of β-glucan from GlucagelTM and PromOatTM determined as equivalent to pullulan standards were 0.191 × 10 and 1.12 × 10, respectively. This gives a relative difference of 1:6. Figure 2. Viscosity of GlucagelTM solutions A: 1% and B: 5% (w/v) determined at 10-80 ̊C and shear rates of 1-30 s. Values represent means of two replicates. Proton nuclear magnetic resonance (H NMR) analysis showed significant differences in the ratio of β-(1→3) to β(1→4) linkages between GlucagelTM and PromOatTM, 41:59 (±1) % and 30:70 (±3) %, respectively. Furthermore, additional 0 2 4 6 8 10 12 0 5 10 15 20 25 30 Vi sc os ity (m Pa s ) 10 C