Effect Of Drying Methods On The Physicochemical And Functional Properties Of Longan Pulp Polysaccharides

Longan(Dimocarpus longana Lour)which belongs to the genus Sapindus,which is a homologous fruit with a variety of physiological functions,and polysaccharide is one of the most important functional substance.The effect of dryness on the quality of pulp has attracted wide attention as it is the main method of processing longan,but studies involving polysaccharides are rare.The effects of different processing methods on the physicochemical properties,in vitro functional activities and digestive properties of polysaccharides in longan pulp were investigated in this study,providing a theoretical basis for the product to improve quality and efficiency.The main research contents and results are as follows:(1)The effect of drying method on the browning of longan pulp.Maillard reaction is one of the important mechanisms of the change of polysaccharide structure.The free amino content,total amino acid content,browning degree and color were used as indicators to analysis the degree of browning of longan pulp after hot air,vacuum,infrared and freeze drying.The water content of dried flesh in different ways ranged from 15.45%to 27.57%.Except for freeze-drying,the free amino content and total amino acid of the pulp after drying in other ways were significantly decreased(p<0.05),and the degree of browning was higher,a significant decrease can be seen,in the L value and the significant increase in a~+value experienced a significant increase(p<0.05).The free amino content of pulp was positively correlated with L value,indicating that Maillard reaction is the main cause of browning of pulp.(2)The effect of dry method on the physical and chemical properties of longan pulp polysaccharides extracts.The crude polysaccharides extracts,namely FLP,HDLP,VDLP,IDLP and FDLP which,were extracted from fresh pulp,were treated by and hot air,vacuum,infrared and freeze drying by normal temperature weak alkaline water extraction,isoelectric precipitation and ethanol precipitation.The content of polysaccharides extracts in these five crude polysaccharides ranged from 46.80%to 63.13%,which were mainly composed of glucose and containing a small amount of uronic acid(1.56%~6.72%)and the content of protein content ranged from 27.56%to 33.28%.The five crude polysaccharides extracts exhibited infrared spectrum characteristics of polysaccharides and proteins.Compared with FLP,the amide characteristic absorption intensity and C-H stretching vibration absorption intensity of HDLP,VDLP and IDLP were significantly weakened.In addition,the drying method significantly affects the molecular weight distribution of polysaccharides extracts in the pulp,which was manifested as follows:the average molecular weight was reduced to a different extent;the relative content of component(>10~7D)was significantly reduced;the component(>10~7D)disappeared in FDLP,and a large number of components with molecular weight(<10~5D)are produced.This showed that the drying method had a significant effect on the physical and chemical properties of polysaccharides extracts in longan pulp,especially the obvious difference between the effect of"cold method"and"thermal method".(3)Comparison of in vitro activities of polysaccharides extracts in dried longan pulp by different methods.The antioxidant,antitumor and immunostimulatory activities of the polysaccharide extracts were evaluated in vitro.The free radical scavenging ability of the five polysaccharides was different.The DPPH free radical scavenging abilities of HDLP were significantly lower than other polysaccharides extracts at 1~5 mg/mL concentration,while the hydroxyl free radical scavenging ability of FDLP was the highest(p<0.05).Among the five polysaccharides,FDLP had the strongest total antioxidant capacity of FRAP(p<0.05).Among the five polysaccharides extracts,FDLP had the strongest total antioxidant capacity of FRAP,and the growth of tumor cells(HepG2 and SGC7901)had no significant inhibitory effect.When the inhibition rate was between 200~800μg/mL ranged from-9.82%to 14.01%.After drying,the ability of pulp polysaccharide(100~400μg/mL)to stimulate NO production in macrophages was significantly enhanced(p<0.05),but the ability to stimulate the secretion of TNF-αfrom macrophages was slightly reduced(except HDLP).It can be seen that the effect of the dry method on the polysaccharide activity in the pulp is different.The freeze-dried method is beneficial to enhance the antioxidant activity,and the hot air drying is beneficial to enhance the immunostimulating activity.(4)Comparison of in vitro digestion characteristics of polysaccharides extracts in dried longan pulp by different methods.Combined with the analysis of reducing sugar content and the analysis of free monosaccharide composition,the stability of different longan pulp polysaccharides extracts in vitro digestive system was investigated.The results showed that compared with the small intestine digestive solution,the oral digestive solution and gastric digestive solution were simulated after 2~6 h of digestion experienced a significant increase in the content of reducing sugar,(p<0.05),and the difference in content of different digestion time was small;compared with other polysaccharides extracts,IDLP was more resistant to oral digestion but more susceptible to gastric so;ution digestion.The composition of free monosaccharide in the digestive juice was analyzed by HPLC,the peak area of free monosaccharide in different digestive juices was consistent with the content of reducing sugar,showed most of the free monosaccharide was glucose.In summary,the dry method has certain influence on the basic composition,physicochemical characteristics,functional activity and digestive characteristics of polysaccharides extracts in longan pulp.It is feasible to change the characteristics of components in foods and improve their quality through processing conditions.The results can provide reference for the regulation of the quality of dried meat products.