| Degradation of starch particles from micron-level to nano-levels improves their functional properties,it expands the application range of starch and enriches the types of modified starch.Starch nanoparticles have the advantages of being natural sources,biodegradability and good biocompatibility.The application of starch nanoparticles in emulsion preparation has become a research hotspot in the food science field.In this paper,pullulanase,?-amylase and ?-amylase were used to prepare lotus seed starch nanoparticles and the best enzyme was selected;in order to further improve the physicochemical properties of lotus seed starch nanoparticles,ultrasound,ball milling and high pressure homogenization were adopted to assist in the preparation of lotus seed starch nanoparticles with enzymatic hydrolysis and was then applied to Pickering emulsion.The effects of different preparation methods on the physicochemical properties of lotus seed starch nanoparticles were studied,and the relationship between the structure of lotus seed starch nanoparticles and the physicochemical properties of Pickering emulsion was also investigated.The main research conclusions are as follows:(1)Structure and physicochemical properties of lotus seed starch nanoparticlesLotus seed starch(Lotus Seed,LS)was hydrolyzed with pullulanase,?-amylase and ?-amylase to form three different types of lotus seed starch nanoparticles(LS-SNPs),namely P-SNPs,?-SNPs and ?-SNPs.The structure and physicochemical properties of lotus seed starch nanoparticles were studied by laser particle size analysis,scanning electron microscope,X-ray diffraction,Raman spectroscopy,nuclear magnetic resonance and gel permeation chromatography system.The results of laser particle size analysis and scanning electron microscopy showed that the surface of lotus seed starch nanoparticles prepared by enzymatic hydrolysis was rough and cracked,and the particle size is: LS>?-SNPs(?-SNPs)> P-SNPs.The results of X-ray diffraction showed that lotus seed starch was C-type crystal form and lotus seed starch nanoparticles were B-type crystal form.The crystallinity was: P-SNPs(65.07 %)> ?-SNPs(60.73 %)> ?-SNPs(51.99 %)> LS(44.19 %).The results of Raman spectroscopy and NMR showed that during enzymatic hydrolysis,new functional groups were not observed in the lotus seed starch nanoparticles,the non-crystalline regions were destroyed,and the double helix structure became firm.The double helix strengths of different types of lotus seed starch nanoparticles are: P-SNPs> ?-SNPs >?-SNPs> LS.The results of gel permeation chromatography showed that lotus seed starch was decomposed into lotus seed starch nanoparticles with a low polymerization degree,and the weight average molecular weight(Mw)sizes are: LS >?-SNPs >?-SNPs >P-SNPs.In summary,thelotus seed starch nanoparticles prepared by pullulanase have the smallest particle size,weight average molecular weight and highest crystallinity,and pullulanase was the best enzyme species for preparing lotus seed starch nanoparticles.(2)Structure and physicochemical properties of lotus seed starch nanoparticles prepared by ultrasonic-assisted enzymatic hydrolysisBased on the preparation of lotus seed starch nanoparticles by pullulanase,in order to further reduce the particle size of lotus seed starch nanoparticles and increase their relative crystallinity,ultrasonic waves with different ultrasonic power,time and liquid ratio(power: 200 W,600 W,1000 W;time: 5 min,15 min,25 min;Liquid ratio: 1 %,3 %,5 %)were used to assist enzymatic hydrolysis to prepare lotus seed starch nanoparticles(U-LS-SNPs).The structure and physicochemical properties of U-LS-SNPs were studied by laser particle size analysis,scanning electron microscope,X-ray diffraction,Raman spectroscopy,nuclear magnetic resonance and gel permeation chromatography system.The results of scanning electron microscopy showed that the surface of U-LS-SNPs was cracked and uneven after ultrasonic-assisted enzymolysis,and there was no difference between U-LS-SNPs and LS-SNPs.The results of particle size analysis and gel permeation chromatography showed that the particle size of U-LS-SNPs(except 5 %treatment group)was smaller than that of LS-SNPs.With the increase ofultrasonic power and time,the weight average molecular weight gradually decreased.The results of X-ray diffraction and Raman spectroscopy showed that ultrasonic waves first acted on the amorphous region of starch granules.With the increase of ultrasonic power and time,the relative crystallinity of U-LS-SNPs increased first and then decreased.The group(600 W,15 min,3 %)had the highest relative crystallinity.The results of nuclear magnetic resonance showed that the hydrogen bond and double helix structure of starch were destroyed by ultrasound,and the double helix structure strength of U-LS-SNPs was weakened compared with that of LS-SNPs.In summary,U-LS-SNPs with the smallest particle size and the highest crystallinity can be prepared under the conditions of ultrasonic power of 600 W,time of 15 min and material-liquid ratio of3 %.(3)Structure and physicochemical properties of lotus seed starch nanoparticles prepared by ball mill assisted enzymolysisBased on the preparation of lotus seed starch nanoparticles by pullulanase,ball milling(ball milling speed: 200 r/min,400 r/min,600 r/min;time: 10 min,30 min,50 min;material-liquid ratio: 1 %,3 %,5 %)was used to assist the enzymatic preparation of lotus seed starch nanoparticles(B-LS-SNPs).The structure and physicochemical properties of B-LS-SNPs were studied by laser particle size analysis,scanning electron microscopy,X-ray diffraction,Raman spectroscopy,nuclearmagnetic resonance and gel permeation chromatography systems.Scanning electron microscope and particle size analysis results showed that with the increase of ball milling speed,time and material-liquid ratio,the particle size of B-LS-SNPs showed decrease first and then increase.The results of gel permeation chromatography showed that the excessive material-liquid ratio leads to too high degree of entanglement of starch molecular chains.With the increase of ball milling speed and time,the weight average molecular weight decreases first and then increases.The results of Raman spectroscopy and nuclear magnetic resonance showed that the Raman spectra of LS-SNPs and B-LS-SNPs are basically the same,and new functional groups were not introduced in the ball mill-assisted enzymolysis process.With the ball mill speed and time increases,the strength of the double helix structure of B-LS-SNPs first increases and then decreases.X-ray diffraction research results showed that LS-SNPs and B-LS-SNPs were both B-type crystals.Based on the above research,B-LS-SNPs with the smallest particle size and the highest crystallinity can be prepared under the conditions of ball milling speed of400 r/min,ball milling time of 30 min and material-liquid ratio of 3 %.(4)Structure and physicochemical properties of lotus seed starch nanoparticles prepared by high-pressure homogenization-assisted enzymolysisBased on the preparation of lotus seed starch nanoparticles bypullulanase,high pressure homogenization(pressure: 100 MP,150 MP,200 MP;frequency: 1 time,5 times,9 times;material-liquid ratio: 1 %,3 %,5 %)was used to assist enzymatic hydrolysis to prepare lotus seed starch nanoparticles(H-LS-SNPs).The structure and physicochemical properties of H-LS-SNPs were studied by laser particle size analysis,scanning electron microscope,X-ray diffraction,Raman spectroscopy,nuclear magnetic resonance and gel permeation chromatography system.Scanning electron microscope and particle size analysis results showed that with the increase of high-pressure homogenization pressure,time and material-liquid ratio,the particle size of H-LS-SNPs decreases first and then increases,and the treatment group(150 MP,5 times,3 %)has the smallest particle size.Regarding the molecular properties of lotus seed starch nanoparticles,LS-SNPs and H-LS-SNPs were both B-type crystals.With the increase in pressure and time of the high-pressure homogenization,the relative crystallinity of H-LS-SNPs increases first and then decreases.The strength of H-LS-SNPs double helix structure is weakened relative to LS-SNPs.The results of gel permeation chromatography indicated that high-pressure homogenization-assisted enzymolysis treatment caused the large-molecular-weight composition ratio of H-LS-SNPs to decrease rapidly and the small-molecular-weight composition ratio to increase.The molecular weight tended to decrease first and then increase.In summary,high-pressure homogenization withthe pressure of 150 MP,homogenization for 5 timesand the material-liquid ratio of 3%,H-LS-SNPs with the smallest particle size and the highest crystallinity can be prepared.(5)Preparation and research on the physicochemical properties of lotus seed starch nanoparticle Pickering emulsionBased on the above research,in this chapter,the lotus seed starch nanoparticles obtained by the four different preparation methods with the smallest particle size and the highest crystallinity were selected: LS-SNPs(prepared by pullulanase),U-LS-SNPs(600 W,15 min,3 %),B-LS-SNPs(400 r/min,30 min,3 %),H-LS-SNPs(150 MP,5 times,3 %)are used to prepare Pickering emulsion(The abbreviations of Pickering emulsion prepared by Pullanase,ultrasonic assisted enzymolysis,ball mill assisted enzymolysis and high-pressure homogenization assisted enzymolysis were: LS-SNPs+Oil,U-LS-SNPs+Oil,B-LS-SNPs+Oil,H-LS-SNPs+Oil).The physicochemical properties of Pickering emulsion were determined by the methods of emulsion appearance,laser confocal,particle size analysis,contact angle measurement and stability analysis The stability of lotus seed starch nanoparticle Pickering emulsion was also studied.Particle size analysis and laser confocal indicated that the size of different particles are: LS-SNPs+Oil > U-LS-SNPs+Oil > B-LS-SNPs+Oil>H-LS-SNPs+Oil.Starch nanoparticles were adsorbed at the oil-water interface to form a single-layer or multi-layer coating film.Contact angleanalysis showed that the closer the contact angle is to 90 °,the better the wettability of the surface of the lotus seed starch nanoparticles.The order of the contact angle is: 90°>H-LS-SNPs +Oil>B-LS-SNPs+Oil>U-LS-SNPs+Oil>LS-SNPs+Oil>0°.The results of stability analysis and emulsion appearance research indicated that the four Pickering emulsions belonged to the O/W emulsion model,and the second type of instability occurs at the bottom,which was the migration of droplets.Observing the appearance of the emulsion from 0 to 28 days,we found that milking out phenomenon occured in all types of emulsions,and the emulsions with milking intensity ranked from high to low are:LS-SNPs+Oil>U-LS-SNPs+Oil> B-LS-SNPs+Oil> H-LS-SNPs+Oil.In summary,the Pickering emulsion with the best stability can be prepared with the use of H-LS-SNPs under 150 MP,homogenization for 5 times and the material-liquid ratio of 3%. |
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