| Wheat beer is popular with consumers due to its favorable nutrition,strong aroma,delicate foam,as well as fresh and mellow taste.Sugars are the main non-volatile component in beer,which contribute to the flavor,taste and mellow body of beer.The composition in beer depends on the type of beer,brewing cereals and production conditions.Non-starch polysaccharides(NSP)are important component of the cell wall of cereal endosperm.It is mainly composed of arabinoxylan(AX)and β-glucan,in addition to a small amount of arabinogalactan-peptides(AGP),mannose polymer(MP)and other complex polysaccharides.NSP play an important role in beer brewing,beer quality and human health.Therefore,the carbohydrates content including NSP,dextrin and oligosaccharides were analyzed and compared in three types of commercial beer based on raw materials,which included barley malt beer,wheat beer and barley malt beer with adjuncts.The correlations with physicochemical indexes of beer and their impacts on beer quality were also studied.Then the effects of wheat malt percentage and mashing parameters on the content and molecular weight distribution of NSP in wort and wheat beer were studied.NSP were purified from pure wheat beer and the molecule structure and physicochemical properties of each fraction were characterized.The effects of each fraction on beer viscosity,turbidity and foam stability were also studied.The main results were as follows:(1)The most abundant oligosaccharides in commercial beer were raffinose,followed by maltotetraose,maltotriose and maltose.The composition and concentration varied depending on the type of beer.The dextrin content in wheat beer(6.12 ～ 12.83 g/L)was significantly(p< 0.05)higher than that in barley malt beer(4.86 ～ 9.89 g/L).And the NSP content in wheat beer(1953 ～ 2923 mg/L)was significantly(p < 0.05)higher than that in barley malt beer(1442 ～ 1756 mg/L)and barley malt beer with adjuncts(619 ～ 1295 mg/L).The contents of dextrin and NSP correlated significantly(p < 0.05)with extract concentration and beer viscosity.NSP content was irrelevant to the real degree of fermentation,indicating that they are hardly used by yeast.Dextrin,malto-oligosaccharides with degree of polymerization of 2～4 and NSP were the main components of carbohydrates in beer.(2)AX was the main component of NSP in beer,followed by arabinogalactan(AG).Their contents in wheat beer were both significantly(p < 0.05)higher than that in barley malt beer and barley malt beer with adjuncts.The average content of AX in wheat beer was 1491 ±184 mg/L,which accounted for up to 63.2 ± 3.2% of total NSP.In addition,there were a small amount of β-glucan and MP.β-Glucan content was highest in barley malt beer(142 ±37 mg/L),while MP content(calculated as mannan)was comparable in wheat beer(157 ± 15mg/L)and barley malt beer(130 ± 10 mg/L).The composition and concentration of NSP in different types of beer were related to the raw materials.The contents of soluble AX and AG in wheat malt are higher than those in barley malt and other adjuncts,while β-glucan is more abundant in barley malt.(3)The substitution degree of AX(A/X)in commercial wheat beer(0.59 ± 0.02)was significantly(p < 0.05)lower than that in barley malt beer(0.70 ± 0.02)and barley malt beer with adjuncts(0.75 ± 0.06).However,its average degree of polymerization(av DP)was 54 ±12,significantly(p < 0.05)higher than that in barley malt beer(43 ± 4)and barley malt beer with adjuncts(24 ± 5).This indicated that water-soluble AX from barley malt,wheat malt and other adjuncts had different structural characteristics.There was no significant difference between av DP(36 ± 7)of MP in barley malt beer and wheat beer(36 ± 5),which was higher than that of barley beer with adjuncts(29 ± 4),indicating that there was not much difference between the molecular weight of MP extracted from barley malt and wheat malt.(4)The higher the percentage of wheat malt,the higher the NSP content in the wort.NSP contents in wort prepared with the same wheat malt percentage and different mashing processes were significantly different.The modified time at 43°C and 70°C had important effects on the degradation and dissolution of NSP from malt.The NSP content was reduced from 1084 ～ 2431 mg/L in wort to 1024 ～ 2056 mg/L in beer.Among the NSP,AX,AG andβ-glucan were reduced.However,the MP content increased from 130 ～ 158 mg/L in wort to149 ～ 186 mg/L in beer,indicating that most of the MP in beer comes from malt,and a small amount might come from the cell wall of yeast.Wheat malt percentage and mashing process also had important effects on the molecular weight and distribution percentage of NSP in wort.The fractions with molecular weight of 24.0～24.5 KDa,6.8～7.2 KDa and 76.5～86.8 KDa were the main components of NSP in wort,accounting for 40.9～46.7%,18.1～23.1%and16.1～20.8%,respectively.The molecular weight distribution of NSP in beer was roughly consistent with that in wort.(5)NSP in pure wheat beer was separated and purified.Proteins were removed by adsorption with 0.5%(w/v)bentonite.Dextrin and malto-oligosaccharides were broken down into glucose by 0.1%(v/v)amyloglucosidase at 50°C for 2h,and crude NSP were separated by precipitation with 80% ethanol.The protease and TCA method were used to deproteinize the crude polysaccharides to obtain NSP,and the NSP was fractionated and purified by Q-Sepharose Fast Flow anion exchange column chromatography to obtain fractions NSP-1and NSP-2,and the yields were 69.24% and 26.90%.The neutral polysaccharides of NSP-1and NSP-2 were 92.58 ± 1.19% and 84.03 ± 0.70%,respectively.A small amount of protein which counted for 0.54% and 0.91% were detected by both component analysis and UV scanning spectrum,and it was speculated as a glycopeptide bound to polysaccharide.(6)Scanning electron microscopy showed that the NSP was formed by a sheet-like structure through filamentous connection to form a network structure,and a small amount of small spherical particles were attached to the surface.NSP-1 had a regular and complete lamellar structure,and the surface was smooth and finely reticulated.NSP-2 had an irregular,uneven-sized fragment-like structure,with sponge-like holes on the surface.In the monosaccharide composition of NSP-1,the molar percentages of arabinose and xylose were32.97 ± 0.39% and 55.41 ± 0.27%,respectively,and the A/X value was 0.60,which were typical characteristics of AX from wheat.NSP-2 was mainly composed of 34.07 ± 0.06%arabinose,30.22 ± 0.30% xylose and 20.51 ± 0.13% galactose,in addition to 7.69 ± 0.19%mannose and 7.51 ± 0.18% glucose.The Mw of NSP-1(83.47 ± 2.48 KDa)was higher than that of NSP-2(71.78 ± 2.78 KDa).The molecular weight distribution range was wide,and the dispersion indices(Mw/Mn)were 4.35 and 3.15,respectively.The Congo red test showed that NSP in wheat beer did not have a triple helix structure,and the iodine-potassium iodide test showed that there might be more branches.FTIR spectroscopy analysis showed that NSP and its two fractions were β-configuration polysaccharides with pyran ring structure.(7)The intrinsic viscosity of the polysaccharides were NSP > NSP-1 > NSP-2.Among the concentrations of 0.25% ～ 2.0%,all three polysaccharides exhibited characteristics of non-Newtonian fluids,and the apparent viscosity decreased with decreasing concentration.At different temperatures from 25 to 65 °C,the apparent viscosity of 0.25% polysaccharides solution decreased with increasing shear rate,and the higher the temperature,the lower the viscosity.The thermal weight loss of NSP(76.81%)> NSP-2(66.01%)> NSP-1(65.07%)indicated the thermal stability of polysaccharides was improved after purification.X-ray diffraction results showed that NSP existed in crystalline and amorphous structures,and the purification was beneficial to the formation of stable crystal structure.(8)NSP,NSP-1 and NSP-2 with a concentration of 0.5 ～ 1.5g/L were added back to the beer.The turbidity,viscosity and foam stability of the beer increased with the increase of concentration.NSP-1 had a greater effect on beer viscosity,while NSP-2 had a greater effect on beer turbidity and foam stability. |
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