| Tea is one of the three major beverages in the world and an important economic crop in China.According to the differences in harvesting time,tea is divided into spring tea,summer tea and autumn tea.Summer and autumn tea refers to the tea produced by picking fresh leaves in May to October,and a large number of fresh summer and autumn tea leaves are not effectively used due to the lower quality.Biotechnological processing,such as microbial fermentation and enzymes,has been studied to improve tea quality in the field of tea processing or deep processing.Cellulase can degrade the crude fiber in tea leaves into soluble sugar,reduce its bitterness and astringency,and promote the release of internal taste substances and aroma substances.In this study,Aspergillus niger strains with high cellulase production were isolated from Pu-erh tea and used to inoculate fermented summer and autumn black tea,and the changes of major taste substances during fermentation and the changes of volatile flavor substances before and after fermentation were examined.In addition,cellulase was added to treat the summer-autumn black tea to optimize the treatment process and detect the changes of volatile flavor substances of the main taste substances before and after the enzyme treatment.After exploring the feasibility of these two methods,a processing plant with an annual capacity of 200 tons of cold brew black tea was designed in this paper.1.A total of 25 fungal strains were isolated from Pu-erh tea,and 8 strains with strong cellulase production capacity were selected.Two strains,XO-18 and XO-19,with high cellulase activity,reached 16.58 U/m L and 16.65 U/m L after fermentation for 9 days.Both strains were identified as Aspergillus niger by morphological(colony and micromorphological observation)and molecular biological(ITS gene sequence analysis)analysis.The enzyme activity of XO-18 and XO-19 reached the maximum at 11 days of fermentation,which reached 16.87 U/m L and 17.16 U/m L,respectively.2.Two cellulase-producing Aspergillus Niger strains XO-18 and XO-19 isolated from Pu-erh tea and the acquired Aspergillus Niger strain GSICC 60106 were used for solid-state fermentation of summer and autumn black tea.The contents of soluble total sugar,free amino acids,caffeine,tea polyphenols,total flavonoids,tea pigments and water extracts were detected during the fermentation process.Volatile flavor compounds before and after fermentation were detected by HS-SPME-GS-MS,and changes in antioxidant activity were evaluated by measuring DPPH scavenging ability.The results showed that: After fermentation with three strains of Aspergillus Niger,the total soluble sugar content in black tea decreased to 4.08%,4.20% and 4.10%,the total free amino acids decreased to 2.74%,2.74% and 2.82%,and the tea polyphenols content decreased to 6.24%,6.15% and 6.46%,respectively.The contents of caffeine,total flavonoids and theafetin increased in different degrees.Among the volatile compounds,the relative contents of alkenes increased after fermentation,mainly γ-terpinene(herbaceous and fruity)and α-terpinene(lemon),indicating that fermentation promoted the formation of herbaceous and fruity notes.In addition,DPPH clearance of black tea showed a decreasing trend after fermentation.Fermentation can change the aroma characteristics of summer and autumn black tea.However,soluble sugars,amino acids and other substances that have a positive effect on the taste of tea are consumed during fermentation,and the antioxidant capacity of tea after fermentation is also reduced.Therefore,in general,the effect of A.Niger fermentation on improving the quality of summer and autumn tea is limited.3.Summer and autumn black tea was processed directly with food-grade cellulase,and the soluble total sugar content and water leachate content of black tea were used as evaluation indexes to optimize the processing process using singlefactor and orthogonal experiments.The results showed that the total soluble sugar and water leachate contents of summer and autumn black tea were increased after cellulase treatment.Based on the results of single-factor and orthogonal experiments,the optimal processing conditions were determined as follows: cellulase addition 1.2k U/g,treatment temperature 50℃,solid-liquid ratio 5:8.Black tea was processed under these conditions,and the changes in the content of the main taste substances were detected at different treatment times.After 6 h treatment with cellulase,the soluble total sugar content increased from 5.09% to 8.75%,the water extract content increased from 39.80% to 47.72%,the caffeine content increased slightly,the tea pigment content decreased slightly,and the tea polyphenol,total flavonoid and free amino acid contents did not change significantly.Among the volatile flavor substances,the relative contents of benzyl alcohol,phenethyl alcohol and methyl salicylate increased and the relative contents of D-limonene decreased after cellulase treatment,which promoted the formation of floral and sweet aroma of black tea.Regarding the cold brewing performance,the water extract content of black tea treated with cellulase increased from 28.51% to 35.73% and the total soluble sugar content increased from 5.00% to 8.68% after cold brewing,both of which were significantly increased.In summary,cellulase could significantly increase the content of total soluble sugar and water extract in summer and autumn tea,improve its aroma quality,and lay a foundation for the processing of cold tea.4.In this paper,a plant with an annual production capacity of 200 tons of cold brew black tea using fresh summer-autumn leaves as raw materials was designed based on the research results in Chapter 4,mainly including plant construction,process design,equipment selection,etc.The economic and technical analysis showed that the annual net profit of the established plant was 2.37 million yuan with a payback period of 2.21 years. |
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