Tobacco roasting is a complex physiological and biochemical process which is accompanied by physical changes. Physical change means that the moisture in the tobacco leaves is continuously vaporized and discharged from the tissues, which makes the tobacco leaves dried out gradually; physiological and biochemical changes are the basic and most important changes, and these changes are extremely complicated. These changes occurred under the influence of respiratory enzymes, hydrolases, and reduced enzyme, and it also include the whole process of decomposing, transforming and consuming of tobacco leaves. The principle of drying tobacco leaves is to study the basic laws of this process, so as to formulate artificial control measures, and to lead various changes of tobacco leaves develop in the direction that is most conducive to industrial availability. The basic theory for drying tobacco leaves can be summarized as the following picture:
Technical index and general process of tobacco three-step drying method
Toasting is a crucial technical link in flue-cured tobacco production. The three-step drying process of flue-cured tobacco is based on the basic principles of tobacco leaf curing. It is processed according to tobacco leaves characteristics and the condition of the curing equipment, which has bring the essence of traditional domestic curing technology into the typical simplified baking process. This drying process optimizes and simplifies the controlling indicators of the temperature, humidity, duration and tobacco leaf changes in the baking environment, and ensures that the appearance and internal changes of the tobacco leaf are coordinated during the baking process, and that the internal moisture dynamics and substance transformation are coordinated as well. The tobacco leaves will finally be roasted and turned in yellow by this 3-step drying process, and it will achieve the unity of internal quality and appearance quality of tobacco leaves.
Schematic diagram of three-step curing technology for flue-cured tobacco
Step 1: becoming yellow
Target: 1. the tobacco leaves become yellow with slightly green stem;
2. the leaves become fully withered and collapsed;
3. the main veins are soft
Step 2: fixing the color
Target: 1. the leaves become totally dried;
2.the leaves becomes a big roll
Step 3: cured-leaf
Target:the tobacco stem becomes totally dried
The basic idea of three-step curing process of flue-cured tobacco is paying attention to the intrinsic quality of the tobacco leaf and its consistency with the appearance quality, and striving to simplify the technology qualification and strengthen the operability. The core of the technology is to provide the most suitable temperature and humidity conditions for the coordinated development of the apparent changes and internal changes of the tobacco leaves during the curing period. It particularly emphasizes how to become full yellowing, and especially emphasizes the effect of the environmental humidity (wet bulb temperature) at each stage of the curing, which is the main elements for tobacco leaf quality. Several key points must be grasped in the curing application according to the different tobacco qualities :
The first thing is that the temperature for turning the tobacco leaves into yellow should be controlled at 35-38℃, and the temperature difference between the dry and wet bulbs should be kept at 2～3℃, so that the tobacco leaves will turn into medium yellow and the leaves will become soft.
The second is to control the temperature below 40～42℃ in order to make it become complete yellow, keep the wet bulb temperature at 35～37℃, afterwards, the tobacco leaves is basically become full yellow, fully withered and collapsed, and the main stems are soft, so that the tobacco leaves are fully transformed and form more basic substances of aroma. Therefore, it is necessary to appropriately extend the time for keeping the temperature at 41～42℃, so that the tobacco leaves could reach yellow patches with slightly green stems, and the leaves are fully withered and collapsed, and the main stems are soft.
The third is the color-fixing stage, the temperature should be raised at an appropriate speed according to the quality of the tobacco leaves, and the appropriate humidity should be mastered to ensure that the tobacco leaves completely turn yellow and smoothly fix the color. When the dry bulb temperature is at 46～48℃ and the wet bulb temperature is at 37～38℃, the tobacco leave stem turns into yellow and the leaf becomes a small roll of yellow flake.
The fourth is to keep the wet bulb temperature at 38~39°C and keep the dry bulb temperature at 54°C. It is better to keep such temperature for a long time to make the tobacco leaf becomes a big roll, which promotes the formation of more aroma substances.
At the stage for drying the tobacco stems, the highest temperature should be controlled at 65~68℃, and the wet bulb temperature should be controlled at 40~43℃ to improve the color and chromaticity of the tobacco leaf, meanwhile, it should be noticed to avoid the volatilization and loss of tobacco leaf aroma substances.
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