Espresso Coffee: The Science Of Quality
> Written by leading coffee technology specialists in consultation with some of the world's biggest coffee manufacturers, the second edition of the successful Espresso Coffee will once again comprehensively cover the current status of the chemistry and technology of espresso coffee. It comprehensively covers topics such as agronomy, green coffee processing, roasting/grinding, packaging, percolating and decaffeination techniques. It provides a comprehensive resource for those interested in the fundamental notions of coffee quality; with a point of reference given in the form of a detailed bibliography to provide direction to the wider literature.
I believe OP is roasting coffee to try to improve taste, not to test pressure limits of a jar, and the reason he asked this question :0
All roasted bean off-gasses, regardless of quantity. After roasting, coffee continues to undergo both chemical and physical changes that affect quality. "The pressure within the bean pores should be greater than the atmospheric pressure... as the external pressure becomes higher than the partial pressure of volatiles present in the beans, the degassing rate is reduced allowing a larger quantity of volatiles to be dissolved in the lipid fraction or absorbed on the active sites." (Clarke, 1987a, Labuza et al. 2001, reference from page 231, Espresso Coffee, The Science of Quality
If you throw beans into a sealed jar, the off gassing creates atmospheric pressure thats higher than the that within the bean pore. Evident when you open the jar lid and it makes the whooshing sound. It's worth testing yourself, I'm always pleasantly surprised when I can improve flavor in any way.
From what I've read, some acids present in the green coffee interact and convert to aromatic compounds, and these in turn the flavors. Also, some of the acids are present in the roasted bean, but they don't produce flavor, but more taste, the acidity, or brightness that some people refer to.
Composition of raw arabica coffee bean, according to this book, pages 149 and 201, includes the following acids:
Chrlorogenic (highest at 7%), alaphatic, quinic, malic, citric, and phosphoric, glutamic, hydrophobic, asparatic, phenolic, and feruloyl.
Page 198 states that the aroma impact compounds, the chemicals that contribute to aroma and flavor, "originate" from some of these acids. I read this to mean that the acids interact with other chemicals during the roasting process to produce new flavor compounds. So, it doesn't look like the acids themselves produce actual flavor, per say. Then on page 201, there's a list of about 40 of these aromatic compounds, and their correlating flavors.
+1 for Sweet Marias library, that's a gold mine.
Also, OP should check out the most comprehensive coffee textbook, and don't be thrown by the title, its all about coffee.
This book is excellent in terms of depth. It's a pretty comprehensive guide to coffee. However, it reads the same as a chem engineering/organic chem textbook, so fair warning, it's heavy going.
One more for Scott Rao's book. I thought it was very good. But it's by far not the most detailed. This one is basically a chemical engineering textbook about coffee and coffee roasting. It's the best one I've come across.
I don't think anyone who wants to be an expert can skip this one.
Scott Rao's books aren't really scientific, they'll help you get grounded on the practical side.
There's a book ("Espresso coffee. The science of quality") written by Andrea Illy and Rinantonio Viani. You'll find very interesting details there. Available at Amazon
Rob Hoos' Modulating the Flavor of Coffee is one I'd recommend. Willem Boot has some free stuff that has been invaluable, though modern wisdom goes against some of his advice. Tim Wendelboe has a few videos out there that I've found worth revisiting.
I'd actually advise to stay away from Rao stuff at least at first. He has some good info, but he also teaches some hocus-pocus stuff as "Commandments." If he said something to the effect of "when you're starting out, here's some general rules you should try to follow" I'd be all for it, but instead he teaches them as Absolute Truths and they simply are wrong in that context. It may be that the spirit of his advice is right for newbies, but the way he delivers it is definitely all wrong. He's the reason so many people think they can look at a profile and have some idea what a coffee tastes like (they can't, profiles are useful relative to other profiles on the same equipment, period).
If you're really into the chemistry and science of coffee, Illy's book is probably the most comprehensive piece of modern literature on the subject. It's pretty freaking dense though.
I’m writing this response with OP in mind, so bear with me while I lay the groundwork.
People talk about caffeine as if it's separate from all the other good stuff, but it’s really not. Inside the bean cellulose structure are the hydropholic substances, meaning they enter into a charged interaction with water molecules. The ones we’re after are acids, sugars, salts, and caffeine, a nitrogenous compound. They all extract at slightly different rates but, without pressure, they can’t extract fast, because they’re trapped in the cells. So, they come out through diffusion, ie the movement of these substances out of the cells, and into the water, aided by temperature. This is our brew. A great way to measure this rate of movement is an organic chemistry method called “fractioned extraction”, it’s similar to distillation except it uses chromatography, a way of separating organic and inorganic compounds. The primary function of fractional extraction is to quantify the extraction rate of one material from another, through time.
A food scientist, M. Petracco, did just this in 1989 with caffeine in coffee, concluding that it's not extracted quickly in the beginning of the brew, but instead more steadily, through time: “Data suggest that caffeine extraction from coffee grounds is incomplete…due to the short time available…to extract caffeine from the cellular structure, supported here by results from his fractionated caffeine extraction tests. And here’s the link to the original article, scroll down to Nitrogen Compounds.
Three other scientists, Spiro, Cammenga and Eligehausen, also concluded in 1993 that, “…its solubility in water displays a marked dependence on temperature, where quantitative extraction takes place at brewing provided enough time is allowed...” And also, two other scientists, Proctor and Merriitt, who found “a higher extraction efficiency for caffeine...with prolonged extraction time and higher extraction temperature.” I pulled those excerpts from pages 265 and 298 of probably the best written book on the topic of coffee science, “Espresso Coffee, Second Edition: The Science of Quality, by Andrea Illy and Rinantonio Viani where they also reference A. Peters, published in 1977, by The Association Scientifique Internationale du Café. I found his articles, but I don't want to pay to download them.
Two years ago, The European Food Research and Technology Organization published a white paper called, Comparison of nine common coffee extraction methods: instrumental and sensory analysis, with more supporting evidence on this topic, “Caffeine extraction may be obtained only by coffee preparation methods that permit a long contact period between water and ground coffee...caffeine is extracted from within the cell, by diffusion, and not simply by washout kinetics”
I hope this clarifies things.
Rao focuses on application. This is the bible on coffee science