Acidic Deep Eutectic Solvents Characteristics Of Glucose And Its Application In Preparation Of Hesperetin From Citrus Peel Residue

Citrus peel is a by-product of citrus canned processing industry,which is rich in flavonoids such as hesperidin,and its comprehensive utilization is of great significance to the development of citrus industry.The traditional method for extracting hesperidin from citrus peel residue requires the use of organic solvents,and further utilizes strong acids such as hydrochloric acid for hydrolysis to obtain hesperetin with higher activity.Our previous research found that acidic deep eutectic solvents（Ac DES）based on choline chloride/organic acid/H₂O system had a strong dissolution effect on flavonoids and could catalyze the hydrolysis of glycosidic bonds to generate the corresponding aglycones.At present,there is no research on the determination of physical and chemical characteristics and application of Ac DES system based on glucose/organic acid/H₂O as food additive.Therefore,in this paper,we first constructed an Ac DES system based on glucose/organic acid/H₂O,perfected the ternary system phase diagram,and measured the physicochemical properties and model parameters of the system at different temperatures.Secondly,using hesperidin as the model compound,the degradation kinetics and thermodynamic model parameters of hesperidin in two Ac DESs were studied,and the hydrolysis mechanism of hesperidin and the relationship with the physicochemical properties of Ac DES were elaborated.Finally,the Ac DES was applied to the extraction and preparation of hesperetin from citrus peel residue and the conditions were optimized by single factor and response surface methodology.The results of this paper enrich the understanding of Ac DES characteristics of glucose/organic acids,and provide a new method for high-value utilization of citrus peel residue and green preparation of hesperetin.The main research and findings of this thesis are as follows:Firstly,the preparation and physical and chemical properties and models of two sets of glucose-based Ac DES systems（glucose/citric acid/H₂O,glucose/malic acid/H₂O）were investigated.The results of the ternary phase diagrams showed that the homogeneous zone of both systems was close to the region with high organic acid content at 80℃and 0.1 MPa,while the range of the homogeneous zone of the citric acid system was smaller than that of the malic acid system.The Ac DES viscosity,density,acidity and solventized color development parameters of the key components of the homogeneous zone were determined.The viscosity and density data at different temperatures（T=20,40,60,80℃）are modeled,and the parameters of temperature-viscosity model and temperature-density model are fitted.At 25℃,the H₀ ranges from2.20±0.05 to 3.01±0.26 and E_T（NR）ranges from 44.42±0.00 to 47.68±0.00 for the glucose/malic acid/H₂O（Glucose-H₂O-Malic）system,and the H₀ ranges from 2.01±0.00 to 3.01±0.00 for the glucose/H₂O-Citric system（Glucose-H₂O-Citric）.The viscosity,density and acidity of Glucose-H₂O-Citric were higher than those of Glucose-H₂O-Malic system at the same moisture content,but the hydrogen bond strength and polarity of the system were opposite.Secondly,the effects of different Ac DESs on the dissolution and hydrolysis characteristics of hesperidin were determined.The standard curves of hesperidin and hesperetin were established by HPLC,and the precision and detection limit were 0.48%and 0.20 ng/m L,respectively.The solubility of hesperidin in 9 Glucose-H₂O-Malic systems and 8 Glucose-H₂O-Citric systems was determined at 25℃,among which M5（glucose:H₂O:malic acid=2:15:8）and C6（glucose:H₂O:citric acid=3:23:7）had the largest solubility,which were 1.20±0.00 and 1.1515±0.05 mg/m L,respectively,which were higher than those in absolute ethanol.Solubility has the greatest correlation with the density and E_T（NR）of Ac DES.The solubility of all Ac DESs was higher than that of pure water,and that of malic acid system was higher than that of citric acid system.The degradation kinetics and thermodynamic parameters of hesperidin in M5and C6 at different temperatures（70,80,90 and 100℃）were determined.The results showed that the first-order reaction kinetics was observed in both solvents,and the degradation rate was M5>C6.For M5 solvent system,t_1/2 was 0.35（100℃）,1.99（90℃）,6.17（80℃）and 16.43 h（70℃）,respectively.The temperature coefficients Q₁₀at 70-100℃,70-80℃and 90-100℃were 3.61,2.66 and 5.69 respectively.The Gibbs free energy（ΔG）of the process thermodynamic parameter was in the range of 89.97-93.47 k J/mol.For C6 solvent system,t_1/2 is 0.52（100℃）,2.86（90℃）,8.06（80℃）and 29.37 h（70℃）,respectively.The temperature coefficients Q₁₀ at 70-100℃,70-80℃and 90-100℃are 3.84,3.64 and 5.51 respectively.The process thermodynamic parameter Gibbs free energy（ΔG）was in the range of 91.18-95.13 k J/mol.The degradation and transformation of hesperidin in Ac DES was an endothermic reaction that requires high activation energy,and it was an involuntary reaction that was beneficial to entropy.It is proved that the degradation and conversion efficiency of hesperidin has the greatest correlation with physical properties such as Hammett acidity and viscosity of Ac DESs,and the degradation of hesperidin in citric acid system requires higher activation energy.Finally,hesperidin was extracted from orange peel residue by using Glucose-H₂O-Malic（M5）solvent,and hesperetin was further prepared.The single factor experiment showed that the yield of Glucose-H₂O-Malic was 3.326±0.001%and 3.060±0.047%,respectively.The extraction conditions optimized by response surface methodology were as follows:solvent M5,extraction time 3 h,extraction temperature 60℃,and material-liquid ratio 1:40 g/m L.Further,the optimal conditions for heating and transforming the extract containing hesperidin were as follows:heating and transforming at 100℃for 4 h,the content of hesperetin was 13.65±0.01 mg/ml,and the conversion rate was 95.58±0.04%.Through the experiments of DPPH free radical scavenging rate and iron ion reducing ability,it was verified that the antioxidant capacity of hesperidin extract after heating conversion was better than that of hesperidin extract.