Effect Of Coating Pretreatment On The Quality Of Hot Air Drying Of Zanthoxylum Bungeanum And Its Mechanism

Objective: Zanthoxylum bungeanum is a commonly used Chinese herbal medicine for both medicine and food,which has the functions of warming the middle warmer,relieving pain,aromatic invigorating the stomach,killing insects and relieving itching.Due to the short harvest period and high water content,fresh pepper is easy to deteriorate after picking,so it must be dried while fresh for storage and transportation.At present,the pepper industry is still mainly processed by hot air drying,but there are some problems such as large loss of volatile oil components,dark color and poor quality.In view of the existing problems,this study takes fresh green pepper(Zanthoxylum schinifolium sieb.et zucc.)as the research object,and adopts coating pretreatment technology to improve the drying quality of pepper,which provides a new pretreatment technology for the drying and processing of fresh pepper.Methods: Seven different coating materials(chitosan,sodium carboxymethyl cellulose,sodium alginate,pullulan,carrageenan,konjac glucomannan and xanthan gum)were used to pretreat the green pepper.The effects of different coating materials on the appearance,color,drying time and volatile oil content of pepper dried by hot air were studied,and the most suitable coating materials were screened out.On the basis of the above,the most suitable coating materials were selected,and the effects of different concentrations of coating pretreatment on the appearance quality,color,drying time,volatile oil content and components of pepper hot air drying were studied to get the best coating concentration.Finally,scanning electron microscope,Fourier transform infrared spectrum and paraffin section were used to study the film structure,microstructure and microstructure of pepper,and to explore the influence mechanism of film coating on the hot air drying quality of pepper.Results: By comparing the effects of different coating materials on the appearance,color,drying time and volatile oil content of hot air dried Zanthoxylum bungeanum,it was found that there was no obvious film residue except the dried Zanthoxylum bungeanum coated with chitosan,and the total color difference of dried Zanthoxylum bungeanum pretreated with sodium alginate was the smallest,and the volatile oil content was(3.98±0.16 m L/100 g dry basis).The effects of different concentrations of sodium alginate coating pretreatment on the appearance,color,drying time,volatile oil content and volatile components of pepper were compared.The results showed that the appearance and morphology of pepper pretreated with sodium alginate coating at 1.0%～2.5% concentration were all well maintained,and there was no significant difference between visual inspection and CK control group(no coating pretreatment),and there was no significant difference in the composition and components of volatile oil and fresh pepper.However,when the pretreatment concentration of sodium alginate coating is higher than 2.5%,the color protection effect may not be obvious;Higher than 1.5% will prolong the hot air drying time of pepper;The content of volatile oil in Zanthoxylum bungeanum increased first and then decreased with the increase of sodium alginate concentration,and the content of volatile oil in Zanthoxylum bungeanum was the highest when the concentration was1.5%.It can be seen that 1.5% sodium alginate coating pretreatment of Zanthoxylum bungeanum has good appearance quality,the highest volatile oil content and remarkable color protection effect,and does not prolong the drying time,so 1.5%sodium alginate coating is the most suitable pretreatment for hot air drying of Zanthoxylum bungeanum.Finally,scanning electron microscope,Fourier transform infrared spectrum and paraffin section were used to study the film structure,microstructure and microstructure of pepper,and to explore the influence mechanism of film coating on the hot air drying quality of pepper.It is found by scanning electron microscope that the surface of sodium alginate film is smooth,flat,compact and uniform,and no holes or cracks are observed.It has good structural integrity and can provide good barrier function.The infrared absorption spectra of pure sodium alginate powder and sodium alginate films with different concentrations were measured by Fourier transform infrared spectroscopy.It was analyzed that the mechanism of sodium alginate film formation may be that its polymer chains were reorganized into film matrix through dissolution and hydrogen bonding,and the film-forming macromolecules were rearranged into a stable structure after solvent evaporation.The infrared absorption spectra of Zanthoxylum bungeanum in CK group and Zanthoxylum bungeanum pretreated with 1.5% sodium alginate coating were measured by Fourier infrared spectroscopy.The analysis showed that there was no chemical interaction between Zanthoxylum bungeanum and sodium alginate coating pretreatment and curing,and the adhesion between the two surfaces was a physical process.The use of sodium alginate did not change the chemical structure of the surface of Zanthoxylum bungeanum,which indicated that sodium alginate coating pretreatment did not bind solute into the epidermis of Zanthoxylum bungeanum,but distributed outside the epidermis.According to the surface morphology and microstructure analysis of Zanthoxylum bungeanum,the pretreatment of sodium alginate coating is to form a dense and uniform film by physical adhesion and combining with Zanthoxylum bungeanum peel,and to reduce the contact between Zanthoxylum bungeanum and oxygen in the drying medium by covering and filling the pores of the peel,so as to reduce the adverse reactions mainly mediated by oxygen in the drying process,inhibit browning reaction,reduce the volatilization and oxidation degree of volatile oil components,and achieve the purpose of improving the color of dried Zanthoxylum bungeanum products,retaining volatile oil and effective medicinal components,and improving the quality of hot air dried Zanthoxylum bungeanum.Moreover,the pretreatment of sodium alginate coating is helpful to improve the mechanical strength of pepper epidermis,and then protect the morphological structure of oil chamber to reduce the volatilization and oxidation of volatile oil components caused by the destruction of oil chamber in the hot air drying process of pepper.Conclusion: 1.5% sodium alginate coating pretreatment can maintain the appearance color,prevent the loss of volatile oil and protect the components of volatile oil,and it is simple and convenient to operate,convenient to popularize,safe and environmentally friendly,which provides theoretical basis and data support for the development and innovation of drying and processing technology of Chinese herbal medicines.