I tried different roast profiles ranging from 250-325F 4-28 minutes. I am trying not waste many beans by putting only a cup full of beans in at a time. I realize that the roast wouldn't be representative of a full oven, but my goal was to try to find a good dwell temperature and time. Once I found my dwell time and temp I would increase total roast time to account for the increased ramp time of a loaded oven. Do you think this approach is acceptable or should I always experiment with what will be my typical roasting weight?
Roasting. Some of you will love this. Some of you will hate it. Some will eat it up and suddenly everything will make sense. Some will fall into despair that you will never get it and you are doomed to wander the wasteland of uncontrolled roasting. I really hope the former outweigh the later.
I understand your reasoning about roasting just a cup. Unfortunately, the reality is that to effectively use that technique you will probably end up using more beans in the long run as you scale up. The reason is this. You learn best from an experiment by changing one variable at a time. Otherwise the permutations get too great and you can't tell really what effect your changes had. You are changing two (or possible three things) as you progress up in weight. Weight, time and temperature/profile. It's possible to glean information from this but it actually takes more runs and more cocoa unless you get lucky. I know you said your plan was to increase roast time. Experience tells me that won’t work. Fundamentally because by changing the time, you are changing the roast profile which means you are in the apples and oranges situation. They are not comparable. More on profiles in a little bit.
The main problem is that by roasting only a cup of beans, the data you get isn't transferable to a larger load expect to say you will know you will need some combination of more time and heat. But you already know that, right? In essence you used a cup of beans and have no more useful information that you did before. If your 1 cup of beans took 8 minutes at 325 F you have nowhere to go with that information if you try and roast 2 cups. It's not linear. It won't take 16 minutes @ 325 F, nor will it take 8 minutes at 650 F. It will be somewhere in the middle....which you already knew most likely.
On the other hand, if you roast 1 lb for 16 minutes @ 300 F, and determine they are under roasted, you have somewhere to go. You should either increase the roast temperature (my first choice) OR time. If 1 lb is still under roasted at 16 @ 350 F, then for your next roast you increase the temperature to 350 F for 16 minutes. If that is still under roasted, you can start to increase time as you don’t want the oven much hotter, generally speaking. In short, yes, it will take a few runs, but you will get useful data from the methodical approach.
So to directly answer your question, I think you should test roast the weight what you will be roasting in your standard batches. Here is what I hope is a good analogy.
You will never master baking a 2 lb loaf of artisan bread by baking 2 oz bread rolls. Rolls need a cooler temperature or you will burn them. And they need a shorter time. 350-400 for 10-12 minutes will do the trick. A 2 lb loaf could well take 60 minutes at 350 F. To compensate for that large mass, you can put it in at 500 F for 15 minutes. It won't burn since there is so much there that has to heat up and soak in. But you can't keep it there or is will burn. After 15 minutes, you turn it down to 425-450 for another 15-20 minutes and it continues to cook without burning the outside. Sounding a little familiar?
In short, you have to learn your over for a given batch size of beans. You have to actually load the oven enough so it's doing some work. And you have to find the sweet spot, batch size wise, for your oven. All ovens have a setting for 350 F. But they all don't apply the heat at the same rate. Some might take 5 minutes to get an empty oven up to temperature, and another might take 15 minutes because it is lower wattage or has less BTU/hr (electric vs gas). That right there is the crux why I don't and simply can't give people exact temperature 'profiles' to roast in an oven. We have zero knowledge that your oven matches my oven and without that, a temperature is useless.
I'm going to keep repeating this different ways. Let's say you need to go 1 mile in your car. 1 mile is well roasted beans. How am I to tell you (without an odometer) how long to push on the gas pedal to the floor when you only have a speedometer? I should make the note that we can't talk about partly pushing it down. We have to push it to the floor as that is how ovens work. They apply full power until they hit temperature, then turn off until the oven cools some amount below temperature. In short, I think you can see I can't give you that information.
My car might make it to 60 mph in 5 seconds. If that is the case I need somewhere around 55 seconds to go 1 mile. But if your takes 25 seconds, you clearly are going to take well over a minute total to get there. And without that exact data, and pulling out some algebra equations, I can't tell you how long it will take.
But let us assume we have that data for both cars. What can we predict if we hook up a 1 ton full trailer to the cars (i.e. roasting more beans). I think you can see that the answer is nothing at all. Maybe my car is light and can accelerate well under a light load but has no extra capacity so takes 60 seconds to get to speed under load. Yours on the other hand is huge and that is why it took so long to get to speed the first time. Yours, because it has plenty of extra capacity hardly notices the extra load and takes only an additional 5 seconds and you are up to speed in 30 seconds. Clearly yours will make the mile in just about the same time, whereas mine will take 50% longer.
At the end, what I am trying to get across is that AMBIENT oven temperature (i.e. the dial setting) is a terrible gauge to describe roasting profiles....but it is all we have!! By preference I would like to say this to describe a roasting profile.
For 6 lbs of beans, apply 2000 watts of power for 12 minutes. Then apply 1200 watts of power for 3 minutes. Finally, apply 1000 watts of power until the beans either smell done or the beans are at 265 F. Why? Because it is scalable! If you have 2 lbs of beans, then you need to apply 1000 watts for 12 minutes, 600 watts for 3 minutes and 500 watts until you hit 265 F. Half the beans, half the power. Because THAT is a roasting profile. Energy input in a given time. Not temperature settings.
And just so I don't skip the detail, that is for a well insulated system, neglecting heat loss. The reality (based off a given roaster's insulation - see, more variables again) is that I may need only 900 watts for the 2 lbs of beans in the first leg of the profile, or I might need 1100 watts. And how do you know which it is? NOW temperature comes into play. Because we are getting to how I roast day to day. For the above profile, for me, this is what is actually going on.
Apply enough energy so my surface bean temperature goes from room temperature to 210 F in 12 minutes. For MY ROASTER this is 2000 watts.
Turn down the energy input so the bean temperature goes to 245 F in 3 minutes. For my roaster this is 1200 watts.
Turn down the energy again so I reach final target temperature of 265 F (which I have predetermined by SMELL and TASTE of previous roasts) in 3 minutes.
That now is an honest to goodness roasting profile that anyone can take and apply to their own roasting situation and with any amount of beans. Assuming they have a way to control the power (energy input) of the roaster AND know the bean temperatures at different stages along the way. Without all three of those items, my profile is useless to you. Which is why I don’t offer them up as a matter of rote. It is also why I give the very generic ‘profiles’ I do give.
From my Roasting page:
In general, if you try oven roasting, you will start hot (350-400) for a short amount of time and slowly lower it to you target temperature (300-320 F). The more you are roasting, the higher your initial temperature can and has to be.
Remember, you want to roast the cocoa beans, not bake them. This is how this looks: Whole cocoa beans 375-400 5 minutes 350 5 minutes 325 5 minutes 300 until done. Look for the aroma of baking brownies and/or pops. Both are good indicators you are there.
I’ve tried the best I could to give something usable based on what I know and what the limitations are to oven roasting.
So, there. What does that get you? Hopefully a peak into my thought processes about roasting, profiles, what they are and what they are not. Hopefully there is a nugget or two in there on what you need to develop your own profiles, or at least an understanding the limitations of your tools (your oven). And why I’m not being obstinate about sharing my profiles. To rift off of A Few Good Men, “You want the truth? You can’t handle the truth!”.
I’m not withholding the truth because you can’t handle it. Or maybe I am. I’m withholding it because in most of the cases out there, it’s useless information because you don’t have the background and equipment to use it.
This is my first concerted effort to get you the background so you can handle the truth.