Study On Preparation Of Licorice Sweetener By Steam Explosion

Licorice sweetener is an important class of glycoside sweeteners with high sweetness,high physiological activity and high bioavailability.In this paper,the steam explosion(SE)was applied to treat the licorice(Glycyrrhiza uralensis Fisch.)pieces to efficiently destroy the dense structure of licorice.At the same time,the sweet components were released and dissolved,and glycyrrhizic acid(GL)was deglycosylated and converted into high-intensity sweetener glycyrrhetinic acid 3-O-mono-?-D-glucuronide(GAMG)and high-value product 18?-glycyrrhetinic acid(GA).The main research contents and results are as follows:The SE treatment of licorice was carried out with different maintenance temperatures and time.The conversion rules of GL and the production rules of GAMG and GA in SE process of licorice were determined.On the basis of above rules,the kinetics and thermodynamic models of GL conversion reaction were established to reveal the mechanism of the deglycosylation reaction of GL by SE.The results showed that the high temperature and thermal acidic environment of SE caused the in situ hydrolysis of GL in licorice,resulting in the conversion of GL into GAMG and GA.The highest conversion rate of GL was 30.71%,and the highest production yields of GA and GAMG were 21.47%and 5.24%.The conversion mechanism of GL may be that the proton ion dissociated from the licorice structural carbohydrate attacks the ?-1,2 glycosidic bond between the GL aglycone or the ?-1,3 glycosidic bond between the aglycone and the aglycon.So the aglycone of GL was removed to form GAMG or GA.The reaction kinetics and thermodynamic results showed that the GL conversion reactions during SE process were considered to be first-order reactions with endothermic,entropic reduction and non-spontaneous features.In order to further improve the conversion efficiency of GL,licorice was preimpregnated with acid catalyst(HCl),alkali catalyst(NH3·H2O)and salt catalyst(NH4Cl)before SE.The results showed that the highest conversion rates of GL after HCl and NH3·H2O and NH4Cl SE were 60.05%,36.90%and 28.92%,respectively,which were 38.70%,15.55%and 7.57%higher than SE licorice.HCl would be used to promote GL degradation and transformation.NH3·H2O would be combined with GL to form glycyrrhizinate to promote GL dissolution.NH4Cl would be decomposed in SE high-temperature environment and thus have features of both strong acid and weak alkali.Therefore,NH4Cl is suggested to be used when licorice sweetener or GAMG is the target product,and HCl can be used when GA is the target product.The physicochemical properties of licorice including macroscopic and microscopic porous structures,chemical components,functional group structure and thermodynamic properties were studied before and after SE treatment.The results showed that the SE had both the thermal and mechanical modification effects,which caused the break of dense structure of cell wall,cell and tissue of licorice,resulted in the increase of water solution content of licorice by 31.29%and the decrease of hemicellulose by 64.03%,and the increase of acid insoluble lignin by 1.59 times.Catalytic SE further enhanced the above tendency,and the sequence of promotion effect was as follows:HCl>NH4C1>NH3·H2O.After SE treatment with HCl,NH4Cl and NH3·H2O,the removal rate of hemicellulose reached 85.29%,68.12%and 63.08%,respectively.And the acid insoluble lignin was increased by 3.57 times,2.22 times and 1.45 times.The weak acid environment formed by the degradation of hemicellulose promoted further deconstruction of the licorice structure,increased the fiber crystallinity,and reduced the thermal stability of licorice.The thermal acidic environment induced by above changes should accelerate the possibility of hydrogen protons to attack GL glycosidic bonds and thus promote GL conversion.Meanwhile,the destroy of the dense structure of licorice should promote the dissolution and diffusion of GL and its conversion products.The extraction kinetics of GL and its conversion products from conventional and catalytic SE licorice were studied,and the extraction promoting mechanism of steam explosion on the GL and its transformation products was elucidated.The results showed that the extraction yield of GL and its conversion products reached the maximum value within the extraction time of 2 h after steam explosion.Compared with the raw materials,the extraction equilibrium time was shortened by over 80%and the diffusion coefficient D value increased by around 2 times,indicating that the extraction efficiency of GL and its transformation products was significantly enhanced after SE.GL and its conversion products in SE licorice were purified by acid precipitation and macroporous resin dynamic elution.The crude licorice sweetener was obtained,and its purity was increased from 2.98%to 35.97%after above purification.