Preparation And Properties Of A Water-Soluble Dietary Fibre By Enzymatic Hydrolysis Of Guar Gum

Guar gum was hydrolysed using Novoferm 43TM (a commercial enzyme containing galactoman-nanase activity) in order to reduce its viscosity and enhance its functionality. One unit of galac-tomannanase activity (GMA) was de?ned as the quantity of enzyme (ml) that decreased to halfthe initial viscosity per minute of substrate under the conditions of the assay (pH 4.5 at 40oC).Novoferm 43TM was found to contain 3.6 0.03 U GMA/ml and it was used at a rate of 0.003 U ?GMA/ml to hydrolyse a 1% solution of guar gum at pH 4.5, at 40oC . Three batches of guar hydrolysates were obtained corresponding to 4, 6, and 8 hours ofhydrolysis. Their reducing powers – measured as D-mannose equivalent – were, 13.56 0.11, ?16.1 0.13, and 22.74 0.07 %, respectively; indicating that signi?cant cleavage of the guar ? ?molecule had occurred which exposed many of its reducing ends. These values were higherthan the reducing value of a commercial sample of partially hydrolysed guar gum (PHGG)known as Sun?berTM (10.64 0.29). ? The hydrolysis process was monitored by determining the intrinsic viscosity of hourlyaliquots, and it was observed to progress very rapidly in the very ?rst hour. Intrinsic viscositydropped from 13.03 (native guar gum) to 2.36 dl/g (82% drop) in the ?rst hour, and then taperedoff (dropping at <10% every hour) in the subsequent hours, to 0.44 dl/g at 8 h. An in vitro analysis of the biological activity of the hydrolysates in terms of their effects onthe growth of Bi?dobacterium infantis CGMCCC 1.505 and Escherichia coli revealed a signif-icant increase (p<0.05) in bi?dogenesis with increasing degree of hydrolysis. The hydrolysateswere superior to the commercial PHGG sample, but less effective than fructooligosaccharides(FOS), which are recognised bi?dogenic substrates. All the hydrolysates did not support thegrowth of non-pathogenic Escherichia coli. On this basis, therefore, a mild-to-moderate prebi-otic property was assigned to the hydrolysates produced in this study. The molecular weight of guar gum, and the molecular weight and molecular weight distri-butions of the hydrolysates were estimated using capillary viscometry and high performancegel ?ltration chromatography (HPGFC), respectively. Guar gum molecular weight was reducedfrom about 1493 76.57 kDa (native), to a heterogenous distribution of molecular species rang- ?ing from about 53 kDa to 300 Da among the hydrolysates. High molecular weight fractions(10,000 – >500,000 Da) were also present, eluting at shorter retention times and having abun-dances of between 54 to 43%. Guar gum was further hydrolysed for 10, 12 and 24 h and the HPGFC elution pro?les viAbstractdetermined to see the effect of prolonged or enhanced hydrolysis. No great improvement in theelution pro?les was realised. The high molecular weight species persisted in their abundance(42 – 44%; a decrease of about 10% as compared to the abundance in GH4), with a moderateincrease in the abundance of the lower molecular weight species (up to about 31% after 24h). A rheological investigationwas carried out on the hydrolysates and compared with the nativeguar gum and Sun?berTM PHGG. Solutions of 10 mg/ml were investigated at 25oC over 1 – 100s-1 shear rate using an AR 1000 Rheometer (TA Instruments Ltd., Surrey, England). Only guargum showed pseudoplasticity because its solutions' viscosities dropped with increasing shearrate. The rest of the solutions had ?ow characteristics similar to Newtonian ?uids in that theirviscosities did not change signi?cantly with increasing shear rate. Effects of pH (2.4 – 8) and ionic concentration (0 – 1 mol/l NaCl) on the shear viscosity,?ow behaviour index (n), and consistency index (K) were also studied. Viscosity (at 50s-1 shearrate), n and K for the hydrolysates were not signi?cantly affected by pH and...