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Curing, Fermentation, Aging, Kilning? Basic Definitions

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deluxestogie

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Curing Tobacco
When you begin with leaf from the field (primed or stalk-cut), and then allow it to convert from a living leaf to a dead leaf (of yellow or brown), you have cured the leaf. Often in this forum, we refer to this as "color-curing".

There are a number of methods for color-curing, each of which is more suitable for specific classes of tobacco.

Curing Methods
  • air-curing
  • sun-curing
  • flue-curing
  • fire-curing
  • rajangan
  • other, uncommon methods
Air-curing can be used for all classes of tobacco. Hang the leaf or whole stalks in a ventilated, shaded place, and wait for it all to go from green to yellow to brown. The process depends on ambient temperature and humidity (the weather), and may require a few weeks to several months. Air-cured leaf usually needs at least another few weeks (up to many months or even years) of "aging" to reach its best quality for smoking.

Sun-curing can be used for (flue-cure) Virginia varieties as well as most Oriental varieties. Green leaf or entire stalks are hung in the sun to cure (and brought inside if rain threatens, then returned to hang in the sun). Sun-curing typically requires two to four weeks. Again, subsequent "aging" improves the quality. Sun-curing results in more retained sugars, and a brighter color in the cured leaf, compared to air-curing. Most non-Virginia, non-Oriental tobacco varieties do not cure properly with sun-curing.

Flue-curing is exclusively for (flue-cure) Virginia varieties, though it can also be used for most Oriental varieties. Green leaf is placed within a closed chamber, and subjected to the heating regimen specified in this chart.

Flue Cure Chart.jpg

Flue-Curing Chart


The purpose of flue-curing is to start with primed, green, mature leaf, fix the maximum sugars within the living leaf, then rapidly wilt and dry (kill) the leaf. The process yields the golden tobacco commonly associated with cigarettes, though it is also commonly used for pipe-blending. The only flexible part of the timing shown in the above chart is the "yellowing" phase. Different varieties of tobacco require greater or lesser time to adequately yellow (most of the lamina, but not the veins and central stem). Leaf from the upper stalk positions require more time than leaf from lower stalk positions. Humidity can be artificially maintained during the yellowing phase, and you can peek into the flue-cure chamber as often as you like during the yellowing phase. Once you progress beyond the yellowing phase, you should not open the chamber for any reason, until the flue-curing is complete. Flue-curing can sometimes be accomplished using the same container that might otherwise be used for kilning. These are not the same method.

Fire-curing begins with either green leaf or yellow leaf, and subjects the hanging leaf (or whole stalks) to low heat and smoke from combusting wood or sawdust (or in the making of Latakia, smoke from specific woods and herbs of the Mediterranean basin). The result is a dark tobacco that carries the intense aroma of the smoke used to cure it.

Rajangan is an Indonesian curing method in which green leaf is formed into a roll, then shredded. The green, shredded leaf is then spread out in the sun to sun-cure. The range of varieties for which this method works is unclear. This method is similar to "dashboard curing" of shredded, green leaf.

There are other niche methods used in some parts of the world, and these are not discussed here.

Finishing Tobacco

Beginning with color-cured leaf, there are numerous methods for finishing tobacco.

Finishing Methods
  • Aging (mellows with age)
  • Kilning (>122°F to 128°F for 1 to 2 months, with humidity)
  • Cavendish (steamed or pressure-cooked)
  • Perique pressure-processing (anaerobically pressed beneath a liquid seal)
  • Pressing (subjected to clamping pressure in aerobic conditions, with our without casings)
  • Processing for various smokeless uses
  • Other uncommon methods
These methods are amply discussed in their own threads on the forum. Worth noting is that kilning is not color-curing, even though the same container may be used to accomplish either desired goal. The conditions within the container differ dramatically for each method.

Fermentation

Fermentation is what happens when dead (color-cured) tobacco leaf is acted upon by its own oxidizing enzymes. Microbes are not a part of that process. Lengthy aging (in case) will eventually ferment leaf completely, but it takes a long, long, time. Cigar tobacco factories and plantations often create massive piles (pilones) of tobacco, which acts as its own insulation, and allows the temperature of the center of the pile to increase, which accelerates its fermentation. Since only the center of a pile reaches temperatures above the ambient temp, the piles are repeatedly taken down and rebuilt. For the home grower, kilning in a kiln (122°F to 128°F for 1 to 2 months, with humidity) replicates the internal temp of the center of a massive pile, but does so simultaneously for all of the tobacco within the kiln. Aging = kilning = fermentation. The only differences among those three methods are the quantity of work you must do, and the time required to mostly ferment the tobacco.

Bob
 
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deluxestogie

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Wet-bulb vs. Dry-bulb: not important

You don't need to care about a web-bulb thermometer for flue-curing. But prior to the availability of inexpensive hygrometers to measure relative humidity, the comparison of dry-bulb to wet bulb temperatures could provide a way to calculate it. This was invented during the early 19th Century.

Here is a diagram of how a wet-bulb thermometer is constructed. It is just a standard thermometer with the bulb (at the bottom) kept wet. How much this will lower the temperature of the wet-bulb thermometer depends directly on how rapidly water evaporates from the surface. Commercially (yes, they are still made and sold), a detailed chart is usually attached, in order to look up the dry-bulb temp vs. the wet-bulb temp, and determine the relative humidity as a percent.

WetBulb_DryBulb_thermometer.jpg



Bob
 

deluxestogie

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What is "Case"?

This is a quite old tobacco term that vexes most people unfamiliar with its use. With regard to tobacco, it is a statement of its moisture content. Out of case describes tobacco that is so dry that it crumbles to fragments and dust when handled in any way. In case describes tobacco that contains a high enough moisture content that it can be easily handled, without damaging the leaf. With regard to users of whole leaf tobacco, and for those who grow, cure and finish tobacco, there are four general levels of case:
  • out of case: very noisy, like dried autumn leaves, and crumbles when handled
  • low case: much quieter, is mostly flexible, though it may crack slightly when folded
  • medium case: sounds like thick vinyl, is entirely flexible, and has a moderate stretch
  • high case: silent, feels somewhat moist, though not wet, is flexible and fully stretchy
In this usage, "case" should not be confused with "casing", which refers to the addition of liquids (often flavorings) to tobacco. It also has nothing to do with the concept of a container, as in, "packed in a case".

Tobacco can be stored out of case (totally dry), so long as it is protected from physical damage. When out of case, stored tobacco ages very little, if at all. Tobacco stored in low case will age properly, and not mold. Tobacco stored in medium or high case will promptly (within a few days to a few weeks) mold, and be ruined. (Above 122°F [50°C], mold cannot grow.)

Different varieties of tobacco, as well as different stalk levels of the same variety exhibit differing tendencies to draw in and hold moisture. This tendency to draw and hold moisture is its hygroscopic tendency. Upper leaf or thicker leaf is usually more hygroscopic than lower or thinner leaf. Stems are usually more hygroscopic than the leaf lamina.

For storage of leaf, its sound and feel should guide the determination of case. While 60% relative humidity (RH) is usually safe, and usually maintains low case, this is not true of all leaf. Also, RH changes significantly with a change in temperature. Just get accustomed to touching and listening to the leaf to determine case. Always store tobacco in relatively stable temperatures.

The level of case can be lowered by exposing the leaf to drier conditions, then sealing it when it seems to be in low case. The level of case can be raised by exposing it to more humid conditions, or by lightly misting it with non-chlorinated water. As an example of misting, a one pound bag of leaf that is out of case can easily absorb 4 to 6 sprays of water from a typical spray bottle. Then it should be sealed overnight or for a few days, in order to evenly distribute the moisture.

With minimal practice, determining and adjusting the case of tobacco leaf becomes intuitive. No instrumentation required.

Bob
 
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