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Why does tobacco change texture under pressure and/or heat?

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ChinaVoodoo

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If you've ever cooked or put tobacco under significant pressure, you've noticed a certain change in texture. I used to attribute this stickiness to cells rupturing their water content w/ dissolved sugars and such. Now, I believe it is related to gelatinization.

I read an article about amylase activity during tobacco curing. There was a comment that the breakdown of starches occurs in tobacco without gelatinization. In brewing, it is considered essential for starches to be gelatinous for amylase to break them down into sugars, but I believe now that it is only essential for quick breakdown of starch. This topic is related to the glycemic index of a food: the less gelatinization, the longer it takes digestive enzymes to break down the starch, the lower the glycemic index.

From Wikipedia-
Starch gelatinization is a process of breaking down the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites (the hydroxyl hydrogen and oxygen) to engage more water. This irreversibly dissolves the starch granule in water. Water acts as a plasticizer.

Three main processes happen to the starch granule: granule swelling, crystal or double helical melting, and amylose leaching.

During heating, water is first absorbed in the amorphous space of starch, which leads to a swelling phenomenon.[1]
Water then enters via amorphous regions the tightly bound areas of double helical structures of amylopectin. At ambient temperatures these crystalline regions do not allow water to enter. Heat causes such regions to become diffuse, the amylose chains begin to dissolve, to separate into an amorphous form and the number and size of crystalline regions decreases. Under the microscope in polarized light starch loses its birefringence and its extinction cross. [2]
Penetration of water thus increases the randomness in the starch granule structure, and causes swelling, eventually soluble amylose molecules leach into the surrounding water and the granule structure disintegrates.
The gelatinization temperature of starch depends upon plant type and the amount of water present, pH, types and concentration of salt, sugar, fat and protein in the recipe, as well as starch derivatisation technology used. Some types of unmodified native starches start swelling at 55 °C, other types at 85 °C.[3] The gelatinization temperature of modified starch depends on for example on the degree of cross-linking of the amylopectin, the degree of acid treatment, acetylation. Gel temperature can also be modified by genetic manipulation of starch synthase genes.[4] Gelatinization temperature also depends on the amount of damaged starch granules. These will swell faster. Damaged starch can be produced, for example, during the wheat milling process, or when drying the starch cake in the starch plant.[5] There is an inverse correlation between gelatinization temperature and glycemic index.[4]

Gelatinization improves the availability of starch for amylase hydrolysis. So gelatinisation of starch is used constantly in cooking to make the starch digestable or to thicken/bind water in roux sauce, a soup.

As you can see from this diagram, most starches become gelatinous below boiling point. Assuming tobacco starches aren't all that different, I believe this to be the primary reason why tobacco becomes so sticky after being cooked.
Gelatinization_temperatures.gif

How pressure interacts with this process, I don't know, but it's likely related. Pressing tobacco bricks doesn't seem to work when you only do it for a few minutes. Pressing for several days under the very same pressure, however produces a more solid brick. Is this because gelatinization under pressure is time dependent?

Next experiment I have planned is to gelatinize tobacco, then kiln, comparing it to un gelatinized tobacco re-sweetness.
 
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