What Curing Actually Does to a Tobacco Leaf

A green tobacco leaf has almost no smell. It is stiff, it is wet, and it is packed with starch.

What comes out of a curing barn is a dry, brown, fragrant leaf. Nothing was added to make that happen. The leaf was changed, deliberately, by heat and time.

This article follows the chemistry of that change. It ends with a decade-long try to breed a higher-nicotine tobacco that the FDA could never prove reached a cigarette.

What Curing Has to Fix

Three problems have to be solved before a leaf is worth smoking, and each needs a different reaction.

The first is bulk. A living leaf is full of starch, a storage sugar built from glucose. Starch burns badly and it makes smoke harsh.

The second is color. The leaf is green because of chlorophyll, and green does not stay green through a week of heat. Something else has to take its place.

The third is character. Breaking starch down is not enough on its own. The leaf still has to acquire an aroma, and the only route to one runs through reactions with amino acids at heat.

Harvested leaves strung onto poles inside a curing barn in Brisbane, 1932. The gaps left between leaves are the point: air and heat have to pass evenly through them, or the ones in the middle cure differently from the ones on the outside. Queensland Department of Agriculture and Stock, public domain, via Wikimedia Commons

The engineering is mostly about evenness. Leaves are spaced on poles, poles are stacked in tiers, and heat is moved past them deliberately rather than left to the weather.

Even spacing matters more than it looks. Heat has to pass between the leaves rather than around them, so a dense row cures into something different from a loose one.

The four methods, and why they differ

Bitzer and colleagues list the methods as fire, flue, sun and air, each associated with its own flavor profile.

The reason is carbohydrate chemistry. Different methods hold the leaf at different temperatures, for different lengths of time, and sometimes in direct smoke.

So the sugar that survives differs, and so does the Maillard chemistry that sugar goes on to drive. Air-cured and fire-cured leaf keep more protein and amino acid and much less sugar than flue-cured leaf.

Choosing a method is therefore choosing which reactions run. The flue is not a more efficient version of the same idea. It is a different recipe.

An open-sided log curing barn, built with gaps between the slats so air moves freely through hanging leaves. Queensland State Archives, public domain, via Wikimedia Commons

Yellowing: Turning Green Into Aroma

The first stage of flue-curing is called yellowing, and it is the stage where the chemistry that matters actually happens.

Fan and colleagues sampled cured tobacco at fixed stages, at 38, 42 and 48 degrees Celsius, to track what changes inside the barn.

What they found is that chlorophyll and carotenoid pigments break down, and the products of that breakdown become the aroma.

The number that makes the point is this. Pigment degradation products make up 85 to 96 percent of the neutral volatile aromas in cured tobacco.

Carotenoids degrade into around a hundred different aroma compounds, and the chlorophyll route produces one molecule so dominant it accounts for most of the volatile fraction on its own.

That molecule is neophytadiene. It is worth naming, because almost all the aroma people describe in cured tobacco comes from one breakdown product of the pigment that made the leaf green.

So the green does not disappear from the leaf. It is converted into the smell. Chlorophyll breaks down into molecules that are fragrant at concentrations where the parent pigment was not.

The intermediate that betrays a bad cure

If the job is not finished properly, the intermediates pile up instead of the aroma compounds.

The first products of chlorophyll breakdown are molecules called pheophytin a and pheophorbide a. In low-quality leaf, Fan and colleagues found these accumulating at 30 to 40 percent higher levels, while the finished aroma products fell.

Under the microscope the cause was visible. Leaf that cured badly showed delayed chloroplast disassembly, with starch granules still packed inside and the thylakoid membranes still tightly stacked.

The organelles never came apart, so the enzymes that finish the job were never released. The leaf did not fail at chemistry. It failed at dismantling.

Starch, and Why It Has to Go

The second conversion is simpler and more mechanical. Starch gets broken down into sugar.

During yellowing, enzymes hydrolyze the starch into soluble sugars. Those sugars are the substrate for everything that comes next.

Gong and colleagues explain why leaving it is so bad. High levels of starch, pectin, cellulose and protein in tobacco leaf produce a burnt smell and rough smoke when the leaf is smoked.

Starch that burns incompletely generates acetaldehyde and acrolein, two irritating gases. Cellulose roughens the structure of the smoke and causes choking irritation.

Pectin is worse in a different way. Its pyrolysis produces methanol, which becomes formaldehyde and formic acid, and it also worsens combustibility so more tar forms.

And protein degrades into quinoline and hydrogen cyanide, while raising the bitterness of the smoke and adding a smell like burning feathers.

None of that is a curing fault in the usual sense. It is the leaf arriving with the wrong compounds still in it.

Bacteria finish the job

Heat alone does not finish the job. Microorganisms living on the leaf surface do a large part of it.

Gong and colleagues report that the microbial community changes in order as the leaf ages. Sacchariferous bacteria come first, then starch-degrading bacteria, then cellulosic bacteria.

That succession tracks the chemistry exactly. Each group arrives when the compound it eats is at its most abundant.

In surveys of tobacco from 14 grades, the dominant genera were Variovorax, Sphingomonas and Bacillus. Bacillus correlated with the volatile compounds that give flue-cured tobacco its sweet and aromatic character.

The researchers went further than describing this. They screened Paenibacillus amylolyticus, which makes amylase, pectinase and cellulase, and used its enzymes on cured leaf.

Cured leaf wetted and mixed with fresh green leaf. Adding moisture to a dry leaf restarts microbial activity, which is why cured tobacco is often slacked before it is worked. South Australian government photographer, CC0, via Wikimedia Commons

Starch, pectin and cellulose all fell. Total soluble sugar and reducing sugar rose. Volatile aroma compounds rose with them, and panelists reported less irritation and a better aftertaste.

The Reaction That Gives Flavor and Trouble

Once sugars and amino acids are both present, they react. The reaction is named for the French chemist who studied it, and it is the same chemistry that browns bread crust and roasts coffee.

Bitzer and colleagues put curing plainly. It is a form of mild fermentation, and several enzymatic and Maillard reactions occur, producing a wide variety of products.

This is the stage that creates the aroma, and it is also the stage that creates the problems.

Many of the products of curing are precursors of harmful compounds in smoke. Bitzer and colleagues found that gas-phase free radicals in smoke varied nearly eightfold across tobacco types, and that they were most affected by the curing method.

Perique tobacco, which is fermented rather than cured, produced the highest radical levels at 42 nanomoles per gram. Canadian Virginia flue-cured leaf produced the lowest, at 5.

Leaf reduced to a fine powder after air drying. Getting to this state means the starch and fibre have already been broken down, because neither survives combustion cleanly. Aworuth, CC BY-SA 4.0, via Wikimedia Commons

The radicals tracked the carcinogen NNK closely, at a correlation of 0.92. Across the fifteen tobaccos tested, NNK varied 192-fold and nicotine 14-fold.

So the choice of curing method is not a cosmetic decision. It measurably changes what is in the smoke.

That is the honest double edge of this chemistry. Nothing in it is a flaw or a trick. The same reactions that build a pleasant aroma also build much of what makes the smoke dangerous.

A cigar rolled by hand from a cured wrapper leaf, long after the curing chemistry that gave the leaf its flavor was finished. kuhnmi, CC BY 2.0, via Wikimedia Commons

The Nicotine Experiment That Cannot Be Shown

Nicotine is concentrated in tobacco leaves, and it is the reason the leaf is worth processing at all. Attempts to raise its content have a documented history.

In 1994 the US Food and Drug Administration investigated a tobacco breeding project called Y-1, run by Brown & Williamson with a Brazilian partner.

The investigation began with an anonymous tip telling investigators to search the patent databases. A librarian found a citation, and the document behind it was in Portuguese.

The filing described a genetically stable tobacco variety with nicotine at 6 percent or higher. An FDA investigator recalled that the highest figure anyone had achieved in flue-cured tobacco was 3.4 percent.

Processing equipment from the tobacco industry on display, showing the scale of the barns and vats that curing depends on. Kritzolina, CC BY-SA 4.0, via Wikimedia Commons

So the breeding target was real, and it was roughly double anything in the commercial record.

The leaf reached the United States too. Customs invoices showed more than half a million pounds of Y-1 shipped to Brown & Williamson in Louisville on 21 September 1992.

Then the investigation stalled. The FDA could not establish that any cigarette actually sold contained Y-1, and the company denied that it had withheld anything.

This is where the original post went wrong, in a way worth correcting. It stated that the higher-nicotine leaf persisted in the company's cigarettes from 1991 until 1999, as established fact.

What the contemporaneous reporting supports is narrower and more interesting. The leaf was bred, it was shipped in bulk, and it was documented. Whether it ever entered a marketed cigarette was never established either way.

What the record does say

Federal law prohibited exporting tobacco seed before 1991, apart from small research quantities. A scientist named on the patent told investigators she had shipped seed to Brazil, and could not recall how much.

Her job, she said, was not to raise nicotine. Y-1 already had a high-nicotine gene in its seed when her company began. Her work was to make the plant male sterile, so competitors could not grow it from saved seed.

That is the detail most retellings leave out. The interesting part of the project was not only the chemistry but the control of the seed.

A correction to the plant name

The original post carried a botanical error worth fixing, since it appears in the tags as well as the text.

It is Nicotiana tabacum, one word and one b. Tobacco belongs to the Solanaceae, the nightshade family, alongside potato, tomato, eggplant and pepper.

The compounds in tobacco leaf include alkaloids built on the same skeleton as those in other nightshades. That shared chemistry is why the family is both medicinal and toxic.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means here
Curing	The controlled drying that converts a green leaf into flavored tobacco.
Flue-curing	Curing with indirect heated air, for cigarette tobacco.
Air-curing	Curing in open barns without direct heat.
Fire-curing	Curing over open fires, using smoke as well as heat.
Yellowing	The early stage where the leaf loses green and the aroma forms.
Chlorophyll	The green pigment broken down during yellowing.
Carotenoid	A yellow pigment that breaks down into aroma compounds.
Neophytadiene	One chlorophyll breakdown product, and most of the leaf's aroma.
Starch	A storage sugar in the green leaf that curing removes.
Maillard reaction	A reaction between sugars and amino acids at heat.
Hydrolysis	Enzymatic splitting, here of starch into sugar.
Pectin	A plant polysaccharide that turns to methanol when burnt.
Alkaloid	A nitrogenous base; nicotine is one.

The words in this article that would otherwise send you to a dictionary.

Key Takeaways

• A green tobacco leaf has almost no smell. Curing builds the aroma from nothing that is added.
• Cured leaf's aroma is not added to it. It comes from the leaf's own green pigment being broken down.
• Almost all of that aroma fraction is a single chlorophyll breakdown product, neophytadiene.
• Starch is broken into sugar during yellowing, because burnt starch yields irritating acetaldehyde and acrolein.
• Bacteria finish the job in order: sugar-eaters first, then starch-eaters, then cellulose-eaters.
• Curing is mild fermentation, and Maillard chemistry runs in it. The same reactions that build aroma also build smoke toxicants.
• Gas-phase free radicals in smoke varied nearly eightfold by tobacco type and tracked NNK at r = 0.92.
• Y-1 was bred to 6 percent nicotine and half a million pounds were shipped in 1992, but no cigarette was ever shown to contain it.
• Flue-cured leaf keeps more sugar than air- or fire-cured leaf, so the curing method genuinely changes the chemistry.

Frequently Asked Questions

Is curing just drying?

No. Drying would remove the water and stop there. Curing is a set of chemical conversions that happen while the leaf is drying, and the aroma is the evidence that they ran.

Why does the leaf have to lose its green color?

Because the pigment is the raw material. Chlorophyll and carotenoids break down into fragrant molecules, so the green is what the aroma is made from.

Do the bacteria come from the leaf itself?

Largely, yes. Gong and colleagues found a succession of sugar-, starch- and cellulose-degrading bacteria already living on leaf surfaces, arriving in the order the compounds they eat appear.

Which curing method produces the most sugar?

Flue-cured leaf, by a wide margin. Air-cured and fire-cured leaf retain more protein and amino acid and far less sugar, which is why they taste different and blend differently.

Is curing the same as fermentation?

Overlapping but not identical. Bitzer and colleagues call curing a form of mild fermentation. Fermented tobaccos such as Perique go further, and produce a markedly different smoke chemistry.

Did cigarettes ever contain the Y-1 tobacco?

It was never established. Investigators traced half a million pounds of the leaf to Louisville but could not show it reached any cigarette on sale, and the company denied concealing it.

Key references
• Fan et al., pigment degradation
• Gong et al., starch and microbes
• Bitzer et al., curing and radicals
• Los Angeles Times, Y-1 investigation

Images: Wikimedia Commons, with the author and license named in each caption. Figures and dates follow the four sources above.