| With the development of bio-technology, the application of xylanase has progressivelyexpanded. Nowadays, xylanase is successfully applied in the paper-making, food andpharmaceautical industries. By virtue of its application in the food industry, the need for it to bestudied extensively. This paper reports studies on partial characterization of neutral xylanase and itsapplication in bread making and instant baked barley beverage making.Initially, the activities of neutral xylanase, pectinase and cellulase were determined in theenzyme preparations. The activity of neutral xylanase as determined by DNS method was foundto be 4460±65 U/g at pH 6.4 and 50℃ with birch xylan as substrate;however, the activities ofboth pectinase and cellulase were less. Next, partial properties of neutral xylanase such as itsoptimal temperature, optimal pH, temperature stability and pH stability were investigated. Theoptimal temperature and pH of the neutral xylanase were 55℃ and pH 6.4, respectively. Xylanasewas stable in the range of pH 3.2～6.4 at 25℃ over a 10hr period. However, in the pH range from7.2 to 10.4, its inactivation rate increased slowly, although all the inactivation rates were small.Neutral xylanase was heat-stable in the temperature range 30 to 50℃ but was completelyinactivated at 70℃ in 5min. Moreover, the rate of enzyme denaturation increased rapidly withcorresponding increase in temperature above 50℃.Based on its partial characterization, the feasibility of application of neutral xylanase as bakingadditive was studied. The farinograph showed that as baking additive its use could improve mixingand enhance the velocity of mixing since dough forming time was reduced by about 50% with30ml/100kg flour addition. Moreover, neutral xylanase not only markedly improved the quality ofbaking and the baked bread, it also led to an increase in the volume and specific volume of bread.Additionally, its use led to a decrease in the hardness and springiness of crumb as well assignificantly reducing retrogradation and staling during the storage period thus prolonging the shelflife of the baked product. The added amount ranged from 5 to 42 ml/100kg wheat flour amongwhich the optimal addition was 30ml/100kg wheat flour.Neutral xylanase combined with amylase and proteinase was employed to liquefy baked barleyfor instant beverage. Initially, the barley containing 10±0.20% water was baked by the modifiedbarley-baking equipment;indicating that the optimum condition of baked barley was 178℃ for24min. Next neutral xylanase compounded by amylase and proteinase was added to liquefy bakedbarley. The optimal doses of amylase, xylanase, proteinase were determined to be 1.0%, 1.0% and1.5% ([E]/[S]), respectively;whereas the ratio of sample to water 1:9, pH 6.5, 60℃ and 60minwere the optimal processing parameters. The extraction yield of soluble components of bakedbarley rose to 64～66% based on these optimal processing conditions. After that, instant bakedbarley beverage was prepared by concentration and centrifugal spray drying in order. The dryingconditions were: atomizer air pressure of 0.25Mpa, inlet temperature of 180℃ with an outlettemperature of 80℃. The product had good instantizing property. The wetness, dispersibility, bulkdensity and solubility of the instant beverage were 53.50±2.17S, 12.33±0.58S, 0.4279±0.0034g/ml and 93.66±0.03%, respectively.During the enzymatic degradation of baked barley, the relative viscosity markedly decreased inthe first 10min, the average size of suspended particles in enzymatic degradation system decreasedmore quickly in the first 30min and less quickly in the final 30min. Moreover, the relativemolecular weight of the soluble ingredients evidently decreased. In the soluble ingredients afterenzymatic degradation, the relative molecular weight of most peptides was less than 1,500Da andthe mean relative molecular weight of soluble sugar was 1,582Da. Compared with xylan contents oftwo other samples with/without use of neutral xylanase, the results indicated that the contents ofwater-extractable xylan in beverage of baked barley could be enhanced more than 3 times afterliquefaction when it (neutral xylanase) was employed giving the beverage more functionality.The flavor profile of instant baked barley beverage was caramel and sweet-like aroma. The keyvolatile compounds in the instant beverage were aldehyde (hexanal, nonanal etc.), ketone(3-hydroxy-2-butanone, 1-hydroxy-2-butanone, 3-methyl-1,2-cyclopentanedione etc.), acid (aceticacid, 2-methyl-propanoic acid, 3-methyl-butanoic acid, etc.), pyrazine (methyl pyrazine,2,5-dimethyl pyrazine, 2,3-dimethyl pyrazine, etc.), furan (5-methyl-2(3H)-furanone,2-furan-methanol, 2(5H)-furanone, 5-methyl-furfural, etc.), pyrrole (5-methyl-1H-pyrrole-2-carbox-aldehyde, etc.). The volatile compounds of three samples, including baked barley,enzyme-extracted solution and instant beverage, were compared. It was found that flavor precursors,for example aldehyde, ketone, pyrazine and furan produced in the course of baking of the barleyand that aroma could increase in the course of enzymatic degradation of baked barley since theconcentration of heterocyclic compounds increased markedly. Lastly, the pleasant instant beverageflavor was formed after spray drying of enzyme-extracted solution. Owing to evidence of therelease of xylose and arabinose when xylan was degraded by xylanase, xylanase could enhance theflavor of instant baked barley beverage. |
