Curing in china

Curing of tobacco is a process by which the harvested leaves are made ready for marketing. In more technical terms it may be stated that curing is a process of drying , decomposition of chlorophyll until the green colour disappears from the harvested leaves, hydrolysis of starch into sugars and respiration or fermentation of sugars there after.

• In curing the mineral salts also crystallize out producing the grain of the leaves.
• The sole object of curing is to dry the green leaf in such a way that the final has required colour, texture and aroma.
• Thus curing is probably the most important operation in the production of tobacco and the method and efficiency of curing determines, to a large extent , the quality of the product.

• In air curing the source of energy is the atmospheric temperature carried through air.
• The leaves are spread in the shade either in a barn or in a shed for curing.
• A proper regulation of atmospheric temperature, oxygen and humidity results in a better quality cure. There are two stages of air curing.

1. Yellowing of leaves
2. Development of brown colour.

• As such this method has very little value for curing a high grade tobacco as the leaves give up their moisture slowly and turn finally to a brown colour.
• The methods adopted for curing differ from place to place as explained below
• Air-curing is commonly adopted in West Bengal, Bihar, U.P., M.P., Punjab, Karnataka, Telangana, Andhra Pradesh and Tamil Nadu. There are three main methods for air-curing viz.

• Ground-curing
• Rack-curing
• Pit-curing.

Ground-curing

• In U.P. the popularly grown hookah tobacco is slowly cured in heaps on the ground. At Kampil in Faraukhabad district, which is famous for its hookah tobacco, the wilted plants are cut and collected into heaps and are left in the field for two days after which the leaves are separated and tied into a bundle of two or three leaves.
• These bundles are again heaped up and stored on a farmstead and turned occasionally till finally cured.
• The chewing tobacco of Karnataka, after whole plants are cut, is allowed to remain on the ground for six days after which they are turned over in the early morning to expose the other side.
• These fully exposed plants are taken to the curing-shed where they are bulked and rebulked for 15 to 20 days. The leaves are then separated from the plants tied into bundles according to their length or size.

Rack-curing

In this method the matured leaves are harvested by priming method and they are tied into bundles containing 4 to 6 leaves according to their size. These bundles are then strung on a jute string, which are fixed on specially erected bamboo poles in a barn.

• A required atmospheric temperature and relative humidity of 70-80 per cent is maintained by either closing or opening the ventilators or sprinkling water inside the barn. The curing process takes about 5 to 6 weeks time.
• The rack-curing is usually adopted in following areas
• Tamil Nadu for country cigarette, cigar, cheroot, chewing and snuff, tobacco.
• Maharashtra for bidi and chewing tobacco.
• Andhra Pradesh for cheroot and bidi tobacco.
• Assam for hookah tobacco.
• Lanka tobacco leaves in Telangana and Andhra Pradesh are strung on ropes in shed for 30-45 days for curing and developing yellowish-brown colour of the leaves.

Pit-curing

• The pits, prepared in the ground, are used for curing the tobacco leaves however, the method is not very common. Pit curing is mainly adopted in Punjab, Tamil Nadu, Maharashtra, Telangana and Andhra Pradesh.
• The tobacco grown in Jullundhar and Firozepur districts of Punjab is pit cured.
• In this method the pits are lined with tree leaves or dried straw in order to prevent mixing of leaves with soil. Then the wilted plants are arranged in layers into small heaps.
• In Firozepur a layer of Aak (Calotropis sp.) leaves are placed between the layers of tobacco leaves for increasing the pungency of cured leaves.
• The top of the heap is covered with a layer of straw and then with soil which should be about 10 to 15 cm above the ground level so that rain water does not percolate into the pit.
• It takes 6-8 days for curing after which they are twisted into ropes or made into bundles.
• The pits must be opened at right time as any delay in opening causes overheating and the leaves will get spoiled.
• Sometimes leaves are placed in the first pit for 24 hour and then transferred to the second pit 48 hours and then back again to the first for another 24-48 hours. Thus a quick fermentation takes place and the leaves attain a dark brown colour and fruity smell.

• Fire is used as the source of energy in this method and the important leaf which is cured is Jaffna tobacco (a chewing type) in Ceylon and in Tamil Nadu.
• In this method the harvested leaves are wilted for few hours in the field, tied into bundles of 3 to 4 leaves and hung on racks in a smoke-hut.
• They are then smoked for 12 hours by burning coconut husk, leaf, stalks, etc. and stocked for 3 days and again smoked.
• During the smoking treatment the creosotic substances produced from the smoke are absorbed by the tobacco that imparts a particular taste and antiseptic properties in the leaves.
• The leaves are fermented into bulks for 3 to 4 weeks after smoke treatment and then treated with salt water from the lagoons or with jaggery for a special or peculiar taste of this type.

• In sun-curing the source of energy is solar-heat. Sun curing is commonly adopted in Tamil Nadu, Telangana, Andhra Pradesh, Bombay, Bihar, U.P., Punjab and West Bengal.

In this method of curing

• Whole plant is strung on string or rack (as in case of chewing and cigar tobacco in Tamil Nadu) or
• Whole plant is spread on the ground (as in case of bidi tobacco in Bombay, hookah and chewing tobacco in U.P. and Punjab) or
• Otherwise primed leaves or separated leaves with pieces of stalks are tied into bundles and strung on ropes (as in case of Natu tobacco in Telangana and Andhra Pradesh) or
• These leaves are spread on the ground (as in case of chewing tobacco in U.P. and hookah tobacco in W. Bengal).
• The method differs from place to place and type to type of tobacco viz. cigar and chewing tobacco in Telangana and Andhra Pradesh takes about 15 to 20 days and Natu tobacco takes around 45 to 60 days for curing.
• The leaves are separated from the stalk after the plants are initially wilted in the field and the leaves are then strung on bamboo poles for curing in the sun.
• The hookah and chewing tobacco in Bihar are harvested and left in the field itself upto 4 to 6 days for drying after which they are heaped for two days.
• Again they are spread and dried for 6 days then heaped for two days and spread thereafter upto 4 days for drying.
• Thus after 45 days the leaves attain dark brown colour after which they are stripped and tied into bundles of 25 to 30 leaves for fermenting in bulks.

• Flue-curing is an improved method of curing. The curing consists of drying green leaves under artificial atmospheric conditions by adopting a process which does not allow the green leaf to come in direct contact with smoke or flames of the fuel and which permits the regulation of temperature and humidity.
• The main feature of flue curing is the drying of leaf under controlled conditions, where the starch gets converted into sugars and the green tobacco becomes bright, aromatic and fine textured.
• This method is adopted in almost entire Virginia cigarette tobacco areas like Guntur, Godavari, Mysore and other areas at Hyderabad, Madras, U.P, M.P. and Orissa.
• The flue-cured tobacco was first used for chewing purpose but later, its mildness and low nicotine content made it popular as a smoking tobacco.
• The cigarette smoking came into existence since 1880-1890 and it became increasingly popular with its manufacture from flue-cured Virginia tobacco.
• Now every priced cigarette is prepared with flue-cured tobacco having a certain proportion of other types of tobacco got from sun-curing like Natu tobacco of Telangana and Andhra Pradesh.
• The production technique of flue-cured tobacco is little different from other types
• It is raised with low nitrogen supply
• Harvested at an advanced stage of maturity so that the tobacco might become rich in starch and poor in nitrogenous substances like nicotine at the time of harvesting.
• The leaves are always harvested by priming 3-4 leaves at a time.

The harvested leaves are strung on sticks and then stacked into a specially constructed flue- cured barn which is artificially heated with gradual increase in temperature until the leaves dry.

In the production of FCV tobacco nearly 27 percent of the total cost of production is being spent towards curing. in order to reduce the cost of production, especially in curing, improved technologies have been developed

Low profile Barn

Solar Barn

• Adoption of low profiel barn which is an improved version over conventional barn (7.32 m X 4.88 m X 3.2 m ) will help in accomodating 120 sticks more (about 250 kg of green leaf)in the barn. about 185 kg of coal and 20 hours of time per charge can be saved by adopting this improved design
• Solar energy is designed to utilise solar energy for curing of tobacco to save non renewable source of energy

Stages Of Curing

These stages are

• Yellowing of leaf,
• Fixing of colour,
• Drying of leaf.

Yellowing of leaves

• In the yellowing process of leaves, which usually requires 24-36 hours or slightly longer, only gentle heating is done with a temperature ranging between 90 to 100°F and relative humidity between 85 to 90 per cent.
• It is a slow starvation process and the object of slow heating is to give optimum humidity and retain, as much moisture in the leaves as is possible to keep them alive for 30-36 hours.
• At the end of this stage the leaves attain a bright lemon yellow colour and the starchy matter in them gets converted into sugars.

Fixing of colour

• The fixing of colour takes another 24 to 30 hours for completion. The stage, being very critical, should be attended to with utmost care because slight change in temperature deteriorates the quality, which lowers the market price.
• The temperature of the barn should be gradually increased when yellowing is over with a decrease in the humidity by opening the ventilators.

Any delay in drying of leaves, after yellowing is over, results in a development of a brownish tinge on the yellow leaves due to oxidation of tannin like substances that is called "sponging".

But, on the other hand, a sudden rise in the temperature, when the leaves are still wet, results in a bluish back discoloration called "scalding".

• The fixation of colour merely involves drying of the tissues to the point where oxidation of colour producing matter cannot take place.
• Ample ventilation is provided so that relative humidity of the barn is effectively reduced and the temperature is advanced at the rate of about 4-6 degrees per hour untill it reaches to 140°F which is maintained until the leaf blade becomes practically dry.
• At this stage about 80 per cent of the leaf moisture is driven off.

Drying of midrib of the leaf

• This process consists of nothing but drying of midrib of the leaves. During this process the ventilators are closed for conservation of heat.
• Temperature of the barn is advanced at the rate of 5 degrees per hour until it reaches to 170 to 180°F and the temperature is maintained for 18-24 hours for completion of drying process of midrib in all parts of the barn.
• This way the entire process of flue curing usually requires about 84-96 hours.
• After drying, the leaves are generally left in the barn overnight with doors and ventilators opened so that the leaves may absorb moisture from the atmosphere and permit their handling next morning otherwise they will remain very brittle and break into pieces while handling.
• To achieve softness in the leaves, sometimes water is sprinkled over the floor of the barn and doors and ventilators are closed at night.
• The cured leaves are reduced to about 10 to 20 per cent of their green weight which possess a moisture ranging between 8 to 18 per cent after curing.
• A nicely cured leaf should be of bright lemon yellow colour, fine and silky texture, no blemish, mild strength, slow, regular and continuous burning character, white ash, and pleasant and neutral aroma etc.

• There are some factors, which are responsible for the quality of the finished product. The important ones of these are mentioned below

Temperature

• Curing is a vital process in production of quality tobacco, and it is expected to proceed normally within the prescribed limit of temperature.
• A temperature below or above the normal limit will inhibit or slow down the curing process but the drying will not necessarily be stopped and in fact it may be accelerated.
• Temperature should be within 60 to 180°F depending upon the stage of curing.

Relative humidity

• Relative humidity is highly important influencing the rate of drying.
• It is observed that within normal curing temperature the actual rate of drying in the air curing process is relatively slow, requiring several week for completion whereas, in the flue curing, the whole process is completed within 3 to 4 days.
• On an average the relative humidity should be 85 per cent at beginning but after the leaves begin to turn yellow a lower humidity that permits a rapid drying is advised.

Air supply

• Curing is largely a process of oxidation in which an adequate air supply is required.
• Normally the air supply is met with when ventilators are opened to regulate the humidity of the chamber.
• Opening of ventilators also governs the exposure of leaves to the light.
• Total darkness is not a necessary condition for successful curing whereas bright light may be injurious in some types, especially it affects the colour of the cured products.

Condition of the leaf

• The capacity of tobacco to cure and the extent of the process depend on the type of leaf involved, the condition under which it was produced and the amount of tobacco mass, its degree of maturity and other physical characteristics.