| Tigernut tuber is one of the most competitive new oilseeds,which contents oil of 24-30%,with great potential for development,high yield,rich nutrition and good market prospects.Different from traditional oilseed,it has low protein content(5-8%)and high starch content(27-32%),especially after oil extraction,the starch content of meal can reach more than 45%.It is reported that the tigernut starch had physicochemical and functional properties similar to corn starch and potato starch,and was a kind of high-quality starch resource.In view of this,the rational utilization of starch in meals after oil-producing is of great significance to realize the high-value utilization of tigernut,but similar studies have not been reported.Therefore,this study first revealed the effect of conventional oil-production methods on the structural and functional properties of tigernut starch,and then used Chinese quince seed gum and dry heat treatment to improve the functional properties of starch,which may provide a theoretical basis for the high value utilization of tigernut.The main research contents and conclusions are as follows:(1)Effects of cold-pressing on structural and functional properties of tigernut starch isolated from press-cake: Tigernut tubers were pressed at 40 MPa and 60 MPa for 1h,once and three times respectively.Starch was isolated from tigernut meal and compared with untreated.As a result,the starch yield increased from 11.74% to 15.99% with increasing pressure and times,but the multi-layered structure and functional properties did not change significantly(p<0.05),only the relative crystallinity decreased from 19.43% to 18.03% and the particle size increased from 8.22 μm to 8.82 μm.The starch pasting and gel properties showed almost no difference.That is,the starches after cold-pressing pretreatment have similar properties to natural starch.(2)Effects of roasting pretreatment on structural and functional properties of tigernut starch isolated from meal after oil extracted: Tigernut tubers were roasted at 105,120,135,150°C for 30 min,and then extracted starch,raw tubers were processed in the same way as the roasted tubers.As a result,the roasting pretreatment also resulted in an increased yield of starch,similar to the cold pressing process,probably due to the disruption of cell wall cytoskeleton of tigernut tuber,which exposed more starch granules.With the increase in roasting temperature,the relative crystallinity of the starch decreased from 20.14% to 15.54%and the average chain length of amylopectin decreased from 20.03 to 19.04.These structural changes led to differences in their functional properties,mainly in the peak viscosity of the starch paste from 3668 c P to 3164 c P and the enthalpy of gelatinization from 15.6 J/g to 13.0J/g.The freeze-thaw stability was also significantly(p<0.05)improved.(3)Effects of extrusion pretreatment on structural and functional properties of tigernut starch isolated from meal after oil extracted: Tigernut tubers were extruded by controlling the barrel temperature(100°C,130°C,140°C,150°C,160°C)and the moisture(7%,11%,15%),and then extracted starch,raw tubers were processed in the same way as the extruded tubers.As a result,when the temperature at 130°C and moisture at 11%,the starch yield achieved18.18%,which was 1.5% higher than that of untreated,probably due to the porous structure of the extrudate,which allowed for greater exposure of the bound starch(mainly the irregular granules).When the barrel temperature exceeded 140°C,the starch is not separated properly at all,probably due to the higher DG(degree of gelatinization)value of the extrudate making the starch less available.For the six starches isolated,the extrusion pretreatment slightly reduced the relative proportion of A chains(from 23.83% to 23.47%)and increased the proportion of B chains(from 13.31% to 15.52%).X-RD analysis showed that the extrusion pretreatment slightly reduced the relative crystallinity of the starch granules.These changes led to differences in the functional properties of the starch,mainly in the form of a reduction in peak viscosity from 4443 c P to 4121 c P,a 25.8% reduction in swelling capacity and a facilitation of the short-term retrogradation of the starch gel.(4)Improvement of functional features of starch from defatted meal by using gum derived from Chinese quince seeds(CQSG): The addition of CQSG markedly increased the pasting temperature,peak,breakdown,final viscosities,and the setback value.The freeze-thaw stability of the gum/starch blends significantly(p<0.05)increased as the gum concentration increased,suggesting the importance of CQSG in controlling the syneresis of the gel.The addition of CQSG decreased the gelatinization enthalpy,ranging from 16.57 J/g to 12.98 J/g,and the solubility at 75 °C and 85 °C.Structure measurements revealed CQSG decreased the recrystallization of the gel,suggesting that it prevented starch gel retrogradation by likely coating the granules of starch and/or influencing the interactions of the amylose.Additionally,the relative crystallinity of starch decreased from 20.67% to 15.57% due to the addition of CQSG.Behavior index values(n)decreased from 0.46 to 0.29 as the CQSG content increased,indicating pseudoplastic flow increased.Overall,CQSG improved the properties of the tigernut starches,particularly increasing freeze-thaw stability and preventing retrogradation in the starch gel,and the most economical way of modification is to compound CQSG with starch at a mass ratio of 1:100.(5)Improvement of functional properties of starch from defatted meal by using dry heat modification and the addition of Chinese quince seed gum: According to the previous section,CQSG and starch were dry heat treated at 130°C for 2 h and 4 h at a mass ratio of 1:100,and the same dry heat treatment was carried out for tigernut starch as a reference.As a result,the starch granules will be seriously damaged after dry heat treatment for 4h,but this change will be alleviated as CQSG was added.And,the relative crystallinity of the starch granules continued to decrease as the dry heat time increased.In addition,the dry heat treatment of starch increased the peak viscosity from 3947 c P to 4594 c P,while the addition of CQSG increased the peak viscosity further to 5973 c P and also increased the setback value of the starch paste.The swelling power and solubility results showed that dry heat starch or gumstarch complexes significantly reduced the swelling power of the starch as well as the solubility of the amylose in the paste.Overall,the use of dry heat combined with CQSG can further improve the properties of starch pastes.(6)In summary,this study initially clarified the effect of conventional oil-production methods on the structural and functional properties of starch isolated from tigernut meal,and found that the high temperature process(>130°C)during the extrusion pretreatment caused severe gelatinization of the starch within the tigernut,resulting in a decrease in the separability of the starch and affecting the structural and functional properties of the starch isolated from meal;The high temperature process of roasting pretreatment(>135°C)caused an increase in the yield of starch isolated from tigernut meal,but with varying degrees of change in structural and functional properties.In contrast,the oil production processes with low temperature(cold pressing)allows not only for higher starch yields,but also for undegraded tigernut starch.In addition,the physiochemical properties of the tigernut oil produced by the above three processes are in line with industry standards.CQSG can significantly improve the freeze-thaw stability and paste properties of tigernut meal starch after oil production,and the modification of paste properties is further enhanced by dry heat treatment.Over all,this starch will have good potential for application in the food industry,such as the ingredients for frozen foods and bread,and as a food viscous agent. |
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