Physicochemical Properties And Molecular Structure Of Maca Starch

Maca（Lepidium meyenii Walp.）belongs to the family Brassicaceae.As a traditional natural and edible medicinal plant,it has been taken for thousands of years by local people.Recently,it has become popular in Europe,United States,Japan and other countries for its unique nutritional value and pharmacological effects.Previous studies mostly focused on the extraction and characterization of its bioactive components.Starch,as the main component of the maca dried fruit and residue after extraction of bioactive compounds,can be used as a food matrix to provide intrinsic viscosity,viscous texture and other features.However,its physicochemical properties and molecular structure are largely unknown.Moreover,there is a lack of comprehensive evaluation of how maca products could be affected by the fine structure of amylopectin.Therefore,it is necessary to systematically study the physicochemical properties of maca starch.The relationship between the fine structure of amylopectin and the characteristics of modified maca starch as well as maca starch-based food products were also waiting to be revealed.This research will provide the theoretical and practical basis for the development and utilization of maca resources.In this study,three varieties of maca（yellow,purple and black）in Yunnan Province of China were used as raw materials to conduct the following research:1.The physicochemical properties of three varieties of maca starch have been investigated.Maca starch granules were found in irregular shape with several fissures on the surface.The Maltese cross was obvious under polarization microscope.Particle size of maca starch showed a mono-modal distribution.The mean particle size was in the range of 9.0-9.6μm,smaller than that of maize,cassava and potato starch.The maca starch showed typical B-type X-ray diffraction pattern.The apparent amylose content,crystallinity,and gelatinization temperature were in the range of 21.0%-21.3%,22.2%-24.3%,and 47.1-47.5°C,respectively.Maca starch has low gelatinization temperatures,high swelling power and water solubility.It also showed high gelatinization viscosity,low shear resistance and fast retrogradation.Rheological experiments revealed that the viscosity of three types of maca starch pastes became smaller with increased shear rate,indicating shear thinning behavior.The gel of yellow maca starch had the highest yield stress.Characteristic thixotropic behaviors were found among all three maca starch samples.The shear resistance of the yellow maca starch paste was lower than that of the purple and black maca starch;the storage modulus（G’）of the three maca starches was greater than the loss modulus（G’’）,indicating that the maca starch was both viscous and elastic,and elasticity occupies a relatively dominant position.2.The amylopectin chain length distributions and internal chain length profiles of maca starch have been revealed.No significant difference were found in average chain length between these three kinds of maca amylopectin.However,the chain length distributions of three kinds of maca amylopectin were significantly different from those of maize amylopectin.The chain length profiles of maca and maize amylopectin reached peaks at DP（Degree of polymerization）12 and DP 14,respectively,while all of the distributions showed shoulders between DP 18 and DP 21.The internal chain length distribution varied significantly among three maca amylopectins.The molar content of A chain ofφ,β-limit dextrin ranged from58.02%to 61.91%.DP 25 was the cut-off point between short B chains（BS）and long B chains（BL）.The ratio between short B chains and long B chains（BS:BL）varied from 5.16 to5.92 while the ratio of short unit chains to long unit chains(S_ap:L_ap)ranged from 13.76 to15.95.3.Correlation between structure and physicochemical properties of maca starch has been discussed.The gelatinization temperatures of maca starch were positively correlated with the amount of A chains（P<0.01）.The amount of fingerprint A chains(A_fp,DP 6-8)were found negatively correlated with the swelling power and water solubility at various temperatures（P<0.05）.Compared with the structure of internal chain,the rheological properties of maca starch were more related to the unit chain length distribution of amylopectin.The consistency coefficient（K）was negatively correlated with average amylopectin chain length(CL_ap)（r=–0.923,P<0.01）,and the yield stress（σ₀）is negatively correlated with A_fp（r=–0.786,P<0.05）.Temperature when storage modulus（G’）reached a peak(T_G’max)had a negative correlation with amylopectin fraction fb1（DP 13-24,r=–0.948,P<0.01）.Positive correlations between T_G’max’max and fraction fb2（DP 25-36）and fraction fb3（DP≥37）（r=0.952,P<0.01;r=0.821,P<0.01）have also been found.4.HPP（high hydrostatic pressure）treatment of maca starch has been prepared and investigated for its physicochemical properties.It was found that the volume average diameter,surface area average diameter,and the proportion of total particles in the maca starch granules increased significantly after treated by a pressure above 300 MPa（P<0.05）.Water solubility and swelling power of HHP treated maca starch were lower those of the original starch.After HHP treatment at 500 MPa for 10 min,most of the maca starch granules collapsed and lost their original shape.The unbroken starch granules were almost unable to find.Ultra-high pressure treatment could enhance the in vitro digestibility of maca starch.The digestibility of maca starch was significantly accelerated when the high pressure increased to 500 MPa and600 MPa,where 80%of the starch was digested within the first 3 hours.The break down value of maca starch after ultra-high pressure treatment was slightly higher than that of the original starch,but the setback value did not change significantly.This suggests that the ultra-high pressure treatment could decreased the stability of the starch paste.Ultra-high pressure treatment increased the peak viscosity of maca starch,thus it could be used as thickening agent in applications such as soups,sauces,and dairy products for desired properties.5.The influence of amylopectin structure on the properties of yogurt with addition of HPP treated maca starch was studied.The ultra-high pressure treated maca starch was applied to stirred yogurt,and the correlation between the water holding capacity,apparent viscosity and the structure of maca starch amylopectin was analyzed.The main structural factors affecting the quality of yoghurt were:proportion of A chain in amylopectin>amount of fingerprint B chain(B_fp,DP 3–7)>proportion of internal short B chain in amylopectin.This project could expand the application prospects of maca starch,provide a theoretical basis and original data for the development of novel applications based on maca starch,and play a role in promoting the development of maca planting and processing industries.