Effect Of Different Drying Methods On Quality And Crude Polysaccharides Physicochemical Properties Of Lichi Pulp

Litchi is a typical fruit in southern China. Dried litchi fruit is a product with high economic value in the domestic market, but poor quality had seriously hampered the development of it. In recent years, the study of dried litchi fruit focused on the optimization of new drying technology and method, while the report for quality of drtied litchi from different varieties was rare and effect of different drying methods on quality and polysaccharide physicochemical properties of litchi fruit is not clear. Quality of dried litchi from different varieties was evaluated by means of sensory evaluation combined with physicochemical properties in this paper.Quality of litchi dried by different drying methods was compared and physicochemical properties of crude polysaccharide extracted from litchi was investigated. This paper aimed at providing theory and technical support for lychee further processing. The main research results are as follows:(1) Comparison of quality of dried litchi from different varieties:Sensory evaluation combined with physicochemical properties was used to evaluate quality of dried litchi from22different varieties. The results showed sensory evaluation and physicochemical properties had significant difference between different varieties (P<0.05). And there was a significant correlation between sensory evaluation and physicochemical properties. Sensory hardness, sensory color, sensory taste, sensory aroma showed a significant positive correlation with overall sensory evaluation (p<0.01). Highly negative significant correlation was showen between overall sensory evaluation and TPA hardness, browning, titratable acidity (p<0.01), while overall sensory evaluation had a significant positive correlation with total soluble sugar, reducing sugar, but no correlation with TPA adhesion.Sensory adhesion and TPA adhesion, browning and sensory color had a significant positive correlation, resectively. Sensory taste showed a significant correlation with total soluble sugar and titratable acidity.(2) Comparison of volatile components of dried litchi from different varieties: Means of gas chromatography-mass spectrometry combined with headspace solid phase microextraction was used to determine the contents of volatile components of dried litchi from22different varieties. A total of105volatiles were detected, of which12were in all the samples.Volatile substances mainly included hydrocarbons, alcohols, aldehydes, ketones, esters, and the average percentage contribution of hydrocarbons was the biggest, followed by alcohol.The result of principal components analysis showed that ethyl alcohol, acetic acid,3-hydroxy-2-butanone,2,3-butanedol, furfural, D-limonene, benzyl alcohol, cubebene, copaene were the main volatile components. Three clusters had different volatile components according to cluster analysis.(3) Effect of different drying methods on quality of litchi:Effects of drying methods of FVD, MVD, HPD and HAD on quality of litchi was studied. Comparison results showed that there was significant difference between effect of different methods. Porosity of lychee dried by FVD is high, followed by lychi dried by MVD, lychi dried by HPD and HAD is lowest.The rehydration and hardness also showed the same sequence. In addition, fresh litchi pulp showed no viscosity, while dried processing resulted in increase of viscosity. L*value of lithi dried by FVD, MVD, HPD and HAD decreased sequentially, while a*value, b*value and browning increased sequentially. Browning showed a significant correlation with L*value, a*value, and b*value (p<0.01). Methods of FVD, MVD, HPD and HAD made total soluble sugar reduced by3.18%,10.27%,9.55%,9.46%, respectively and titratable acidity increased by11.06%,28.76%,53.10%,132.30%. MVD made reducing sugar decreased by12.08%, while the other methods of FVD, HPD and HAD caused increase of7.87%,8.46%,8.25%, respectively, but there was no significant difference (p<0.05).(4) Effect of different drying methods on volatile components of litchi:Effect of drying methods of FVD, MVD, HPD and HAD on volatile components of litchi was studied. The results showed that total volatile components of lithi dried by FVD, MVD, HPD and HAD was933.17,2167.05,1837.97,2162.58,2671.09μg/100g DW, resectively and kinds of volatile components was39,50,30,43,46, resectively.68volatiles were detected, of which18were in all the samples. Four methods made the content and kind of hydrocarbons decrease and the content and kind of alcohols and aldehyde sincrease sequentially. The content and kind of aldehydes and ketones of lithi dried by HPD and HAD than lithi dried by FVD and MVD.(5) Effect of different drying methods on physicochemical properties of crude polysaccharides extracted from litchi pulp:Effect of drying methods of FVD, MVD, HPD and HAD on physicochemical properties of crude polysaccharides from lithi pulp was studied. Five samples were polysaccharides combined with protein. Arabinose, mannose, glucose and galactose mainly composed polysaccharides. FVD had no effect on monosaccharide composition, but MVD made glucose increase, wehile made Arabinose, mannose, and galactose decrease. HPD and HAD made galactose and arabinose increase, while made mannose and glucose decrease. All the methods made total amino acid content decrease. Total amino acid content of crude polysaccharides from lithi pulp dried by FVD is highest, followed by HPD and HAD, MVD is lowest.MVD and HAD could make litchi polysaccharides degradation. All the methods had no effect on O-Glycosidic bond. Crude polysaccharides of litchi deied by FVD and MVD contained furanos and a-D-galactopyra-nose, while HPA and HAD made the structure of furanose and a-D-galactopyranose disappear.