What is curing?
There seem to be a number of ‘myth-like’ stories about curing tobacco. A favourite is that seamen, carrying tobacco from America, noticed that bales of tobacco leaf fermented during the course of crossing the Atlantic.
The result of the fermentation was that the tobacco leaves emitted a sweet smell (because starches in the leaves underwent a chemical change and became sugars). Smokers of this fermented tobacco found that the taste was much to their liking and better than unfermented tobacco. Thus the idea of deliberately fermenting tobacco leaves came to be. But there are variations on this story. It seems that there was a practice of transporting tobacco leaves inside used rum casks and that the leaves fermented in the casks and, at the same time, absorbed rum flavouring. It is important to understand that tobacco is very ‘hydrophilic’. Hydrophilic means ‘water loving’ – tobacco absorbs water readily. If there is also rum flavouring in the water, the tobacco will absorb the rum flavouring along with the water which carries the flavouring. Thus the idea of not only fermenting tobacco but also flavouring tobacco came to be.
Whatever the history might be, it became the practice to ferment the tobacco leaves deliberately at the source rather than leaving it to chance.
Thus, tobacco growers in America built special barns with temperature and humidity controls which created the optimum conditions for the fermentation of the leaves to occur. Once cured, the tobacco can be packaged, stored and transported with ease. We would have great difficulty in emulating these methods, since we do not have the same climactic conditions. It is no accident that the best places to grow and ferment tobacco leaves occur in a belt around a position a little north or south of the equator, including the Southern States of America, Zimbabwe, India, China, and many more.
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So the Tobacco Company processes of curing tobacco are difficult for us to emulate, so we would be better off using our own methods. Here is a quote from a site which illustrates the methods that Tobacco
Companies use – be sure to note the way in which the temperature in increased in a controlled manner over a few days:
In general, curing can be divided into three distinct stages:
yellowing, leaf drying, and stem drying. The first stage can be described as a period of major chemical conversions and color development. Air temperature in the
barn is maintained between 30 and 40°C, with relative humidity of 80 to 95%, (5 (
https://boltonsmokersclub.wordpress...avouring-and-finishing-tobacco-forcigarettes/ popRef2('i0082-4623-44-1-51-b5')),12 (
https://boltonsmokersclub.wordpress.com/growing-curingflavouring- and-finishing-tobacco-for-cigarettes/popRef2('i0082-4623-44-1-51-b12')))
for about 48 h or until the leaves turn yellow. In the second stage, air temperature in the barn is increased gradually to 50 or 60°C, while relative
humidity is lowered to allow more rapid moisture removal. This stage lasts for 36 to 72 h (12
(
https://boltonsmokersclub.wordpress...avouring-and-finishing-tobacco-forcigarettes/ popRef2('i0082-4623-44-1-51-b12'))).
The last stage (stem drying) generally requires 36 to 48 h. Air temperature is increased to 74°C with further decrease of relative humidity to permit rapid drying of the midrib.
[Source:
http://www.tobaccoscienceonline.org/doi/full/10.3381/0082-4623-44.1.51
(
http://www.tobaccoscienceonline.org/doi/full/10.3381/0082-4623-44.1.51)]
Do we have the resources and staff to control the temperature, ventilation and humidity in that way? Can we even create such temperatures? The only alternative is to take a longer time at lower temperatures etc, but it is all a bit hit and miss.
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Tobacco can be cured through several methods, including:
Air cured (
Curing of tobacco - Wikipedia) tobacco is hung in well-ventilated barns and allowed to dry over a period of four to eight weeks. Air-cured tobacco is low in sugar, which gives the tobacco smoke a light, mild flavor, and high in nicotine. Cigar and burley tobaccos are air cured.
Fire cured (
Curing of tobacco - Wikipedia) tobacco is hung in large barns where fires of hardwoods are kept on continuous or intermittent low smoulder and takes between three days and ten weeks, depending on the process and the tobacco. . Fire curing produces a tobacco low in sugar and high in nicotine. Pipe tobacco, chewing tobacco, and snuff are fire cured.
Flue cured (
Curing of tobacco - Wikipedia) tobacco was originally strung onto tobacco sticks, which were hung from tier-poles in curing barns (Aus: kilns (
Kiln - Wikipedia), also traditionally called Oasts (
http://education.yahoo.com/reference/dictionary/entry/oast)). These barns have flues run from externally-fed fire boxes, heat-curing the tobacco without exposing it to smoke, slowly raising the temperature over the course of the curing. The process generally takes about a week. This method produces cigarette tobacco that is high in sugar and has medium to high levels of nicotine.
Sun-cured (
Curing of tobacco - Wikipedia) tobacco dries uncovered in the sun. This method is used in Turkey,Greece and other Mediterranean countries to produce oriental tobacco. Sun-cured tobacco is low in sugar and nicotine and is used in cigarettes.
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The intention is to hang the leaves for as long as is necessary for them to cure themselves. But note that
there is a fan running constantly! The fan is required to circulate air to avoid mould; mould can occur very
easily in stagnant conditions. Mould spores float about in the air and, given warm, damp, stagnant conditions, can gobble up leaves in no time.
plantpath.ifas.ufl.edu
Here is a quote from Tobacco Science:
Chemical changes mediated by enzymatic activity during the yellowing stage lead to the formation of desired compounds in the cured tobacco. Starch is converted into reducing sugars during yellowing and early leaf drying. As starch degrades, reducing sugar concentration increases and reaches its peak by the end of the yellowing stage. It then declines due to respiration, which oxidizes reducing sugar into carbon dioxide and water. Reducing sugars contribute up to approximately 22% of flue-cured leaf dry weight and are major components of cured leaf quality.
Chlorophyll degradation, noted by the disappearance of green and the emergence of yellow colors, is widely used to judge the curing progress. Full development of yellow color is often used to mark the end of the yellowing process, which is usually associated with completion of certain chemical reactions, especially starch to sugar conversions.
This is possible because the degradation of starch and chlorophyll occur at about the same rate, although the reactions are independent. However, in some cases tobacco leaves appear yellow before desirable chemical changes have been completed. This might lead to low quality tobacco and a poor smoke taste.
The bold bit [my bold] suggests that you can have yellow leaves which have not fermented or fermented
leaves which have not turned yellow!
Easy solution? Make sure that the leaves ferment!