"thermal shock" when flue curing

[Puffin]

I'm setting up a kiln ahead of processing this season's crop, and will be following the recommended temperature–time profile. What I remain uncertain about is the need to ramp the temperature between the yellowing, wilting, and drying phases. It would certainly be simpler to just program the primary temperature set-points, in which case the transition between the phases of curing would each be achieved within 30 minutes if no effort was made to intentionally slow down the rate of temperature increase. Is the oft–cited rate of 2F/hour a necessity for biological reasons, or just an artifact that has made its way into the small-scale curing scene from commercial barns having a high thermal inertia for the heat input involved? I'm also interested how quickly the dry leaf may be cooled once stem–drying is completed. If biological processes have been killed off at this point, perhaps it is acceptable to just turn the power to the kiln off, in which case (pun not intended) the determining factor as to when the leaf can be handled will be the take–up of moisture? Any guidance with these questions would be much appreciated.

 

[deluxestogie]

Raising temp too rapidly between flue-curing stages (specifically transitioning from wilting to drying) can risk what is known as red puffing of the lamina. What I have always done from one stage to the next, during my flue-curing runs, is to increase the temp setting half-way to the next setting, then go to the proper set point 8 hours later. I've never had a problem doing it that way.

Once the run is complete, allow it to naturally cool. Then modestly rehydrate the leaf to low case, prior to handling and packaging.

Bob

 

[Puffin]

Thanks Bob, and just the advice I was looking for. Splitting the difference will be much easier than trying to raise the temperature incrementally.

I envisage the kiln will be fully committed to repeated curing runs for a time, and the cured leaf will need to be safely stored pending fermentation runs over Winter.
Having completed a curing run, and assuming I can then set the kiln's relative humidity once it cools to room temperature, (and may need to do this if the ambient RH is unsuitable), could I please ask for your recommendation on the relative humidity you would use to bring the leaf into that low case you mention for a combination of both ease of handling and storage? Does 60—70% sound reasonable, with >70% to be avoided for risking mold growth?

I also think I would prefer to remove the stems immediately following curing and prior to storage. Will the %RH need to be temporarily taken a bit higher, say to 80-85% to get the moisture content up so the leaves are more pliable?

 

[deluxestogie]

Commercially, flue-cured leaf is not further fermented. Properly flue-cured tobacco really does not require further fermentation. When I have experimentally kilned previously flue-cured leaf, I ended up with a redder color. The resulting smoke was less acidic than simply flue-cured leaf.

I have never had leaf or cigars develop mold at 60% RH (and stable temperature). Leaf will mold at 70%.

I do not pre-stem leaf before storage. Perhaps other forum members can advise on that question.

Bob

 

[Puffin]

Hi Bob, and thank you for your replies. I'm a little confused now though on the question of Aging/kilning/fermentation, as a further beneficial step for the "finishing" of flue–cured Virginias. The impression from the posts here,

Curing, Fermentation, Aging, Kilning? Basic Definitions

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...

forum.wholeleaftobacco.com

and in many other threads elsewhere on the forum is that such a treatment makes for a marked improvement to smokeability (in my case for pipe use) ?

 

[deluxestogie]

Try. Compare.

Kilning and aging make a dramatic difference to air-cured leaf.

Bob

 

[Puffin]

Ah, this distinction had escaped me, and on further reading prompted by your above post I see that it is pretty much a requirement for air-cured. Thanks for the clarification.