Transition from vegetal flavor during first crack - Page 2

[N3Roaster]

Any theories on the chemistry at work here: Trigonelline, Malic acid, etc.?

My best guess is that this is something that's present in the green coffee and that it's likely a single molecular species, but I'm not a proper chemist and I don't have what I'd need for a lab setup to properly isolate and identify that. My best guess is that the sudden change is caused by a pyrolytic reaction and if that's right it serves as a reminder that the actual temperature gradient within a coffee seed during roasting is narrower than many might guess.

[Marcelnl]

It takes quite an elaborate setup and set of experiments to identify that compound, in the end it does not matter what it is; what matters IMHO is that we know how to get rid of it.

[mikelipino]

I associate vegetal flavors with unevenness of roast in the bean, i.e. the core of the bean is more raw than the outer layers. This is easy to do on air roasters because they are so fast. To counter this, I extend the browning phase between DE and 1C to bring the entire bean up to temp, and then roast to desired level. With that I can get really light roasts without vegetal or grassy notes (still quite bright though!). I believe this is also the approach for Nordic roasts, but I also use the same approach to make lighter roasts more soluble and espresso friendly

[Marcelnl]

Can you quantify 'extend", how much time between DE and FC? and what drop temp are you referring to as lighter roast?
(just trying to understand what you are saying in relation to my own roast habits)

[GDM528]

My best guess is that this is something that's present in the green coffee and that it's likely a single molecular species, but I'm not a proper chemist and I don't have what I'd need for a lab setup to properly isolate and identify that. My best guess is that the sudden change is caused by a pyrolytic reaction and if that's right it serves as a reminder that the actual temperature gradient within a coffee seed during roasting is narrower than many might guess.

About that temperature gradient... I found a university paper on the thermal properties of a roasting coffee bean: conductivity, diffusivity, heat capacity, etc. https://www.researchgate.net/publicatio ... f_roasting

The math involved exceeded my attention span, so I got lazy and tried an experiment with my fluid-bed roaster. A small dose of greens in an aggressive airflow will approximately pin the surface temperature of the bean to the rapidly exchanging airflow. I have a fast-response thermocouple planted in the spinning bean mass to get a closer view of the local air temperature surrounding the beans. The air will be cooled by the beans, until the beans have reached thermal equilibrium with the air. The time to reach thermal equilibrium should infer the 'response time' of the beans.

Disclaimer: some of the assumptions I make, like heat exchange rate, may not port well to other roaster systems, so YMMV.

My roaster is set to inlet control (which is remarkably repeatable), so the air temperature it delivers into the chamber is largely independent of what's in the chamber. I kept to a very small dose of greens to minimize airflow resistance. I then ran the roaster with and without greens in the chamber to compare the curves:



It takes a bit over 60 seconds for the greens to reach roughly 90% of the inlet temperature for an abrupt 40C ramp in temperature. Larger batch sizes had a negligible effect on the ramp rate, which I think implies good margin from airflow resistance effects mucking up my assumptions. I observed similar delays with 3x faster thermal ramps, but for more sensible ramp rates (e.g., linear RoR) the delay time is only 30 seconds:



This has mitigated my ambitions for creating gradients inside the beans - they're actually very nimble, especially for gently ramping roasts.

[bshaw3852]

From a non-exhaustive review of literature, the mystery compound may be hexanal. See this article, https://doi.org/10.1016/j.foodchem.2016.04.124.

One method to investigate further would be to buy a standard and spike a small amount at different concentrations into a sample without this defect (test samples), and compare to the same sample without a spike (negative control) and to a sample with known defect (positive control). https://www.sigmaaldrich.com/US/en/product/sial/18109

[N3Roaster]

This looks plausible to me as well. Tempted to go ahead and do that experiment (the linked source would be shipping from the next city over from me) but I'd want to double check a couple other literature resources and it would have to wait until after I'm back in town from my upcoming guest instructor gig. If I do it in the nearish future I'll be sure to put the findings here.

[Marcelnl]

IF you try that experiment I;d go for something already diluted...I'm retty sure it's going to be a small fraction of what is cooking out of the beans.

http://www.thegoodscentscompany.com/data/rw1011071.html

[N3Roaster]

Right, it would be a lot easier to work with to test multiple (small) concentrations. I also know one of the authors of that paper so I might just run the thought past him and get some professional guidance.

[bshaw3852]

I always had good quality from Sigma Aldrich (10+ years in the lab as an analytical chemist), but whatever you choose, make sure it comes with a COA and the purity is high (hexanal is approved as a food additive but whatever impurities are in there may not be.). Also, make sure whatever you get is not expired (looks like the SA one has a 36 month expiry).

You can do serial dilutions to get close to the right concentration for spiking.