| Food 3D/4D printing is a new food processing technology,which not only has the potential to produce"personalized customization"food,but also provides a new way of interaction between customer and food.At present,there are still some problems in the research of food 3D/4D printing,such as the extrusion process of 3D printing is not clear and the printing efficiency is low,which limits the application of 3D/4D printing in practical production and household cooking.In view of these,firstly,the mathematical model of the extrusion process of extruded food 3D printer was established by using computer fluid dynamics(CFD)based on finite volume method,to assist in understanding and studying the extrusion process of the 3D printer.Microwave heating was used as the pre-,middle and post-(4D stimulus)processing methods for 3D printing process of fruits and vegetables and their mixed gels,to realize microwave-assisted high-efficiency 3D/4D printing of fruits and vegetables and their mixed gels.The specific work is as follows:1)In current researches and reports,3D printers based on extrusion are usually screw-based or syringe-based.By establishing CFD models and 3D printing experiments of the extrusion process of these two types of printers,the fluid flow characteristics and printing performances of these two different 3D printing methods for extruded foods were compared.Through the CFD,a complex flow field was found in the screw-based 3D printer,and and there were the high shear rates and gaps between the extrusion pipe wall and the screw,which could result in backflow during the process of printing foods with high viscosity.However,a simpler flow field was found in the syringe-based 3D printer,which makes the syringe-based 3D printer easier to be applied and adjusted.Moreover,the results of printing test of these two different3D printers(syringe-based 3D printer from Shiyin Co.and screw-based 3D printer from Prorimy tech.Co.),which are widely sold in the market,showed that syringe-based 3D printer was more suitable for extruding food materials with high viscosity than screw-based 3D food printer.2)In order to explore the extrusion process of syringe-based extrusion 3D printing,Firstly,yam,purple sweet potato,carrot,yellow flesh peach,banana,mung bean,black rice,job's tears seed,brown rice,and buckwheat were used as model gel materials for 3D printing,and their rheological properties were measured.Power-law model was used to fit the viscosity of the gels and input to the CFD model of the syringe-based extrusion food 3D printer to explore the flow characteristics and differences of 10 kinds of model gels during printing process.By comparing and analyzing the flow fields of different gels during extrusion,it was found that the simulated piston pressure needed to print different gels decreased gradually:Mung bean(325.5 k Pa)>brown rice(253.9 k Pa)>buckwheat(223.6 k Pa)>yam(221.9 k Pa)>black rice(170.8 k Pa)>purple sweet potato(164.9 k Pa)>Job's tears seed(160.1 k Pa)>carrot(34.53 k Pa)>yellow flesh peach(3.655 k Pa)>banana(2.576 k Pa).On this basis,a hypothesis of using simulated piston pressure to evaluate 3D printability was proposed.This hypothesis was then verified by minimum flow stress testing and 3D printing experiment and was found to be consistent with these verification tests.Moreover,the selected 5 kinds of fruit and vegetable gels could be divided into 3 categories according to the piston pressure and printing performance:purple sweet potato and yam gel with a high starch content that could be directly printed,carrot gel with a medium starch content that could be printed but lacked mechanical strength,yellow flesh peach and banana gel with a low starch content that could not be directly printed.At the same time,the feasibility of the hypothesis was tested through the piston pressure calculation and 3D printing experiments of 5 different concentrations of polysaccharide hydrogels with a wide viscosity range.Through the polysaccharide hydrogel verification test,it was found that the piston pressure calculation based on CFD could assist in evaluating the printability in materials of syringe-based extruded food 3D printer.3)Aiming at the low starch content fruit gels which cannot be directly printed(taking yellow peach as an example),3D printing printability of fruit gels was improved by adding additives(buckwheat flour)and microwave high-efficiency pretreatment.Firstly,buckwheat starch-high methoxy pectin(BP)gel system was used as a simulation system for fruit–starch mixed gel to explore the feasibility of microwave heating combined with calcium chloride pretreatment to improve the rheological properties and printability of the mixed gel system.The plasticity of BP gels was found to be improved by microwave heating through rheological testing.The synergistic treatment significantly reduced the viscosity of the gels and reduced the minimum flow stress from 1230 Pa(steam-cooked samples,SC)to 1000 Pa(microwave heating and 1%CaCl2 addition synergistic pretreated samples,MW+1%).At the same time,through computer simulation,it was found that the pressure of the extrusion piston was reduced from141.4 k Pa(SC)to 125.0 k Pa(MW+1%)by synergistic treatment,which reduced the difficulty of printing.Fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC)and low field-nuclear magnetic resonance(LF-NMR)tests showed that microwave heating weakened the hydrogen bonds in the gel.Synergistic treatment resulted in aggregation of the gel through the crosslinking of Ca2+and amide groups in the gel.The aggregation,in turn,led to a partial compression of water from the gel matrix,thereby reducing the viscosity of the gel.The 3D printing experiment showed that the sample prapared by microwave synergistic method(MW+1%,30 s)showed significantly higher printing accuracy compared with the steamed gel sample(SC,30 min).For the practical system,50%buckwheat flour was selected to replace the yellow flesh peach powder with gel strength and 3D printing experiment.A certain mechanical property of the mixed gel of yellow flesh peach and buckwheat was obtained,and the printability of yellow flesh peach-buckwheat gel was improved by microwave synergistic pretreatment with 0.1%CaCl2 addition.Finally,the 3D print fidelity of yellow peach-buckwheat gel reached 96.40%,and the yellow flesh peach-buckwheat gel was used as a model fruit recombinant mixed gel for follow-up tests.4)To enhance the mechanical strength of 3D printed carrot gel objects,the carrot gel with medium starch content which could be printed but lacking mechanical strength was used as the research object.Microwave-assisted method was used to improve the stability of carrot gel 3D printed objects and 3D printing efficiency.First,the 3D printing experiment confirmed that 10mm high carrot gel could support its own weight.Based on this,cylindrical printing objects((?)30 mm×10 mm)were tested for drying curves of different microwave power,and carrot gel's water status and shrinkage degree at different microwave drying times.It was found that microwave heating could quickly dry the immobilized water in the carrot gel matrix and reduce the mobility of the immobilized water.However,the long microwave heating time caused 3D printed objects to shrink,which could affect printing accuracy.By analyzing the texture and rheological properties of carrot gel with different water content,the mechanical strength enhanced by dehydration,but the piston pressure increased correspondingly.Based on the above results,microwave heating(100 W)combined with 3D printing was used to improve the structural stability of 3D printed objects by microwave heating assisted drying in the 3D printing process.Finally,it was found that microwave heating(100 W,1 min)combined with3D printing(by adjusting the height of-0.5 mm per printing 10 mm height model)could improve the structural stability of carrot printed objects without affecting the appearance of printed objects.Through testing,it was found that this strategy could assist carrot gel to print objects with different heights(20,30,40,50 mm).Finally,the carrot gel added with additives(2%gellan gum,w/w)was selected as a control method.The adaptability of high speed(120mm/s)printing were tested by the microwave-assisted method and the control method through3D printing experiment.The results showed that although the control method could print the(?)30 mm×30 mm cone at a printing speed of 20 mm/s,the printing accuracy was found to be poor at the printing speed of 120 mm/s for the control method.However,the microwave-assisted 3D printing method could successfully complete the high-speed(120 mm/s)printing process of carrot gel,and the printing speed was found to be increased by 5-times compared with the control method(2%gellan gum addition and 20 mm/s printing speed).Moreover,compared with the a*value(25.79)of the sample printed by the control method,the a*value(37.44)of the sample printed by the microwave-assisted method was found to be significantly higher(p<0.05),which made the microwave-assisted sample much redder.5)The gelatin-gum Arabic-mixed oil(pepper red pigment,cinnamaldehyde essential oil and corn oil)complex coacervation microcapsules were used as stimulus-response materials to explore the feasibility of microwave-induced color change and aroma change 4D printing for yellow flesh peach mixed gels(yellow flesh peach-buckwheat gel).The shape of microcapsules embedded with oil before printing was found to be a spherical multi-core structure.The addition of microcapsules with different levels could improve the storage modulus and loss modulus of yellow flesh peach mixed gel,but it showed the limited effect on the viscosity and extrusion piston pressure of yellow flesh peach mixed gel.After 3D printing,the microcapsules in the dough suffered to high shear rates(214.8?257.4 s-1)and changed into a spindle shape.After microwave heating,the microcapsules in yellow flesh peach mixed gel were gradually destroyed and the embedded mixed oil was released.Also,after microwave heating,the a*value of the mixed gel containing microcapsules increased rapidly compared with the mixed gel without microcapsules.After microwave heating for 4 min,the content of(E)-cinnamaldehyde increased more than twice,while the original aroma in mixed gel remained unchanged.Moreover,the addition of 0.5%,1%and 5%(w/w)microcapsules were also found to significantly improve the printing performance of yellow flesh mixed gel.6)A strategy of high-efficiency synchronous 4D printing of deformation,color change and aroma change for fruits and vegetables and their recombinant mixed gels was developed after microwave postprocessing.The model yellow flesh peach mixed gel(yellow flesh peach-buckwheat gel)obtained from the above chapter was used as the material for 3D printing.The structure designed by computer aided design(CAD)that could generate microwave hot spots at specific locations was used to realize rapid deformation with microwave heating as a stimulation source.From the results of different model structures,it was found that the hot spot of microwave heating usually occurred around the“gully”of the model.As the microwave power increased,the more obvious uneven heating phenomenon occurred.Accordingly,the stronger hot spot would be found around the“gully”,so that increased the deformation degree and deformation rate.The results showed that the single-and four-petal models could bend36.7°and 55.0°in 30 s and 90 s respectively at 200 W microwave power.The results of computer simulation aided analysis showed that the interaction between microwave uneven heating and 3D printed objects to produce hot spots and form local expansion was the driving force to induce 4D deformation of 3D printed objects.At the same time,the hypothesis of 4D deformation driven by expansion was verified by hot air heating and porous structure model test.Based on the 4D deformation strategy obtained from model yellow flesh peach mixed gel,different vegetable gels obtained from chapter 3 that chould be directly printed(Chinese yam and purple sweet potato)were used to test the universality of the present 4D deformation strategy.Moreover,this strategy could overcome and realize the shaping of an unsupported suspended structure that cannot be formed by the traditional sedimentary food 3D printer.This strategy could combine with color and flavor changing microcapsules to realize the synchronous 4D printing process of deformation,color changing and aroma changing induced by microwave. |
PDF Full Text Request