Color Parameters Changes of Flue-cured Tobacco during Maturing and Curing and the Influence of Air-curing Before Flue-curing on Its Quality

Through the research of the changes of appearance color, intrinsic chemical compositions, structure and morphology of flue-cured tobacco leaves with different maturities during curing and the relationship between color parameter and intrinsic chemical compositions, air-curing conditions and air-curing time duration were also studied to find suitable air-curing yellowing condition that can improve equipment utilization of barn and energy saving in order to optimize curing technology operation. Results were as following.1. During bulk curing process, both L* and a* value of flue-cured tobacco leaves of different maturities showed ascending tendency. The C and b* value of flue-cured tobacco leaves with four different maturities increased before 48℃, decreased between 48℃and 54℃, and then rose again slightly, in general H value showed an descending tendency. L* value of flue-cured tobacco leaves with four different maturities showed as overripe > ripe > unripe > immature. While the a value ascended quicker as the maturity increased. b* of flue-cured tobacco leaves showed as unripe > immature > overripe > ripe, and C value of flue--cured tobacco leaves with lower maturities were relatively bigger.△L* values of flue-cured tobacco leaves with four different maturities decreased generally during bulk curing process.△E and△a * increased obviously at 48℃and△b * reduced, especially unripe and overripe flue-cured tobacco leaves.2. Color values of front and back of flue-cured tobacco leaves with different maturities had similar changing trend during bulk-curing process, color parameters of flue-curing showed positive correlation with pigment contents. Regression equation between L*, a*, b*and pigments in ripe flue-cured tobacco leaves were established during bulk-curing process, which made clear that color values could be used to judge the maturity of tobacco leaves. Main chemical components and leaf color showed certain correlation during the curing process, and regression were also applied, which aimed at exploring the relationships between intrinsic chemical compositions and color change, and ultimately providing scientific basic for proper execution of curing technology.3. Upper epidermis thickness of fresh tobacco leaves were ripe > unripe > immature > overripe, and it became thicker and thicker as the curing progress processed. Upper epidermis thickness changed greatly during late period of yellowing stage and leaf-drying stage, while lower epidermis thickness firstly increased and then decreased, and generally showed a declining tendency. Thickness of palisade tissue and tobacco leaves decreased as their maturities increased, palisade tissue thickness change greatly during yellowing stage. Spongy tissue thickness of fresh tobacco leaves were unripe > ripe > immature >overripe, tobacco leaf maturity had great influence on tissue ratio, tightness, and looseness of tobacco leaf. Vertical shrinkage of unripe leaves were bigger than leaves from the other three treatments, furthermore, horizontal and area shrinkage of ripe and overripe leaves increased earlier than unripe and immature leaves. Thickness shrinkage of leaves from all the four treatments changed dramatically.4. Treatment 1(tobacco leaves in bulk curing) and 4 (natural air-curing) were helpful to free water evaporation; after 24 hours of Air-curing condition was helpful to keep the activity of POD, PPO and PAL of tobacco leaves, these three enzyme activity of lower leaves from T2 and middle leaves from T4, upper leaves from T3 were the highest; the peak of LOX and AM activity appeared at 48 hours of Air-curing condition. Humidity conditions ,which LOX needed, was relatively higher. AM activity of lower leaves needed relatively high humidity conditions, for middle leaves, relatively high temperature was better, medium temperature and humidity would be better for AM activity of upper leaves. It indicated that internal enzymes activities of tobacco leaves were not only influenced by internal environment, but also had close relationship with external environment.5. Brightness L* of lower and middle leaves from T1 and upper leaves from T4 were higher; a* value of lower and middle leaves from T2 and upper leaves from T3 were the highest and green was lowest. b* and C value of leaves from different positions showed significant differences before 48h, however H value took on a sharp decline tendency, b* value of T1 was the highest and yellow was the strongest. Neoxanthin in tobacco leaves from different treatments degraded a lot during air-curing. Treatment 4 was helpful to degrade chlorophyll a , chlorophyll b and violaxant-hin. The better degrading variation of neoxanthin in lower leaves was T4, T1 of middle leaves had lowestβ-carotene content. Upper leaves’β-carotene declined remarkably under the condition of T3.6. Enclosed air-curing had relatively great influence on tobacco oil content and color. After 48 hours of Air-curing condition then tobacco leaves were in bulk curing, appearance quality of leaves would have a high grade. If the middle leaves were placed in fully enclosed air-curing in 48 hours, the curing time would be the lest. Tobacco leaves were palced in air-curing condition before bulk-curing. It was proved that curing process had the advantages of inner chemical components harmonization, higher medium class of flue-cured tobacco, lower curing time and curing capacity as well as efficiency improving.