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Afternoon. Does anyone know the specific heat of green coffee? Could only find brewed coffee (same as water) on the web. Does anyone understand the effect of mass on the specific heat formula? For example, if it takes 1000 joules to heat a 1/4 of coffee to 1st crack in 13 minutes, does it take 2000 joules to heat a 1/2 lb to 1st crack in 13 minutes? Any thoughts on how to figure out the relationship of joules or calories in the formula to watts in an electric heating element? Thanks, TIM |

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On Thu, Jun 21, 2007 at 10:35:25AM -0700, thirddayhomeroaster wrote: < |

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Nah - any ol mechanical engineer who works with heat transfer will do. Specific heat is the amount of heat required to change the temperature of a unit mass of substance by1 degree. Since we like to think of watts (power), it is convenient to thing of specific heat in terms of SI units. Just fyi, In SI units, specific heat is given in joules per kilogram degree C. As mentioned before, power is energy per unit time - watts is joules per second. Specific heat has no time component in it, so we're just talking about energy required to raise the temperature of the coffee by a certain amount. Your example is correct. It takes twice the amount of energy to heat twice the amount of substance to the same temperature. Since you are interested in doing so in the same time interval it will take twice the power TRANSMITTED TO THE COFFEE. The answer to your question is simple, but the real world question that i think you might be asking (Do I need twice as big of a heating element to roast twice as big a load of coffee in the same time?) has a more complicated answer that needs to take into account changes in the efficiency of heat transfer from the heating element to the coffee. Some areas that muddy the waters are the amount of coffee that is in physical contact with the drum (if drum roasting), changes in heat transfer to a larger or smaller mass from air convection in the drum, and others. -Greg At 02:06 PM 6/21/2007, you wrote: < |

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Oh, I forgot to mention that you might be able to actually find the specific heat of coffee if you search for it on the net (I didn't try). A first approximation could be to use the specific heat of cellulose, if you can't find a real value. Then you need to know the amount of temperature rise from the initial temperature of the coffee (prolly room) to first crack, the mass of the coffee (use kg and Degrees C so you don't screw it up), and the time. Power required is the specific heat multiplied by the temperature difference, divided by the time in seconds. Then look at the heating element specification. The efficiency of the system is the ratio of the power consumed by the coffee divided by the power produced by the heating element. I think that is the relationship you asked for in your second paragraph. I should point out that that relationship holds only for that amount of coffee in the roaster for the reasons I mentioned in my other email. -Greg At 02:06 PM 6/21/2007, you wrote: < |

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I am relatively new here, but have a lot of experience involving water and heating. So... I am going to go out on a limb and say that water content is going to be the largest influence on the heat capacity of coffee. Assuming cellulose and coffee are fairly similar the heat capacity can be up to 1.3 times that of pure water. Taking into account that water removal is a key to roasting and the specific heat of the phase transition between water and steam is higher than just raising the temp. one degree it starts to get pretty sticky. Here is a reference if you want to read the ugly details:http://www.nature.com/nature/journal/v176/n4471/pdf/176083a0.pdfJust for the record I love the idea of trying to calculate roast parameters theoretically, but as a working scientist I can tell you there is a lot of empirical observation that also needs to be done. Let me know off list if the PDF does not work and you would still like to see it. Stan |

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I'm new too-2 days now. I appreciate the response. The link takes me to a screen that asks me to log in/buy the article. Are you able to select adobe as your printer and email it that way? I'd love to see what it has to say. I'm trying to beef up my roaster to the most that can be roasted on 120v. Thanks, TIM ---- Stan Klonowski wrote: < |

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Stan, You'll be interested in the articles from Association Scientifique Internationale du Café. http://www.asic-cafe.org/Dan < |

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Basically, yes. Heat is an "extensive" parameter, meaning if you double the system the heat content, or heat required to accomplish a reaction or change in temperature (both of which are happening in raising beans to first crack, as well as vaporizing water) also doubles. Specific heat is an "intensive" parameter, expressed in joules per kilogram, joules per mole, etc. Practically speaking, though, the heat transfer mechanism is way too complicated to calculate, meaning it is probably hopeless to try to correlate the rate at which heat is produced by the element with the heat required to roast your beans. Often in fluid bed roasters, adding more bean mass makes the temperature rise faster, due to slower air flow. Therefore, the specific heat of the beans is not likely to be of any practical use, which is probably why you're not finding it anywhere. If you were really curious it's fairly easy to measure: just heat a known amount of beans to a known temperature, add them to a known mass of water in an insulated container like a Styrofoam cup, then measure the temperature when it stops changing. This does not take into account the heat required to vaporize the water in the beans. Once you're into cracks, of course, you have an endothermic reaction, further complicating things. thirddayhomeroaster wrote: < |

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The problem will be solved soon with robots blasting fast moving beans arranged single file with laser beams, so I wouldn't worry about it. On 6/22/07, David Liguori wrote: < |