The most common requirement for "well fermented" that I am aware of is 80%. Much more than that and there is real risk of over-fermentation generating off flavors.

Here is a device used in Nicaragua for measuring pH and temperature of pulp at collection:

Here is a link to the kind of device shown above. It's not the same one. I have not used the one I link to, it's just an example of the kind of device that can be used.

http://hannainst.com/hi98107-phep-ph-tester.html

To measure the pH of the cotyledon you need a lab pH meter. You take a couple of seeds from the pile when you turn, peel them, grind them, and then add a measured amount to a buffer solution to the vessel. The sensor then measures the pH. Here is the kind of device I am talking about. I have no experience using this one, it is just an example of the kind of device I am talking about.

http://hannainst.com/hi5221-research-grade-ph-orp-meter-with-cal-check.html

If price is a real concern, here is another:

http://hannainst.com/edge-dedicated-ph-orp-meter.html

Digital refractometers (to measure sugar content) are also available:

http://hannainst.com/hi96801-digital-refractometer-for-brix-analysis.html

although optical refractometers are far more commonly used.

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@DiscoverChoc

Thanks Clay for the advice. We're in East Africa. Nearest laboratory is a couple days a way and I think very unlikely we'd be able to use a mechanical device to pressing beans.

I was going for 90% because this seemed like the minimum acceptable quality for an FCC contract and I thought that would be a good place to start. I did not realize all of the flavour would be lost.

We are using thermometers to monitor temp each day. I do not know pH or sugar content. Do you have any recommendations for tools that would allow us to regularly measure pH of the cotyledon?

Why are you looking for 90% fermentation? (By this I am assuming you mean 90% well fermented where well fermented equals brown color in dry beans.)

By the time the beans hit this level of fermentation the chemicals contributing to flavors other than cocoa have been cooked out.

The best indicator that fermentation is complete is to measure the pH of the cotyledon. You need the starting pH (average - and different beans and growing areas have different starting pH range) and the finished pH is around 1.3-1.5 below that. You need to measure how low the pH goes and then when it starts creeping up again fermentation is over.

Some residual acidity is not necessarily a bad thing, but too much is not good. You don't need to bring the beans in overnight but you may need to cover them depending on weather. (Where are you located?) Consistency is important. If you change the drying method much based on changing local conditions you will get different results every time. Less is known about the science of flavor development due to drying than fermentation.

Keep in mind that when the beans are in the bags for 2 days they are actually fermenting so you need to keep this in mind. Better to have some mechanical means to remove the extra pulp quickly (a press or centrifuge) or have the first box designed in such a way that it enables the juice to drain more quickly. 

Do you know the starting pH, sugar content, and temperature of the bean mass? You should.

Are you putting thermometers in the boxes so you know temperatures during the process? You should.

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@DiscoverChoc

Hi,

Obviously these questions are dependent on the local situation, weather, harvest specs, etc. but just hoping for some general advice which may shorten my learning curve.

1. What is the best indication of complete fermentation for a box of forastero beans? (Drop in temperature below 45C? Purple/brown color inside? Absence of acid smell? Time in box?). Assume we are looking to acheive 90% or more. Is some remaining acid after fermentation good? Would it reduce if I lengthened drying time by transferring beans to the drying beds at night?

2. Generally speaking, How easy is it to over-ferment beans? Can this happen if beans are left for an extra 0-12hrs? 12-24hrs? or does it require more like 48-72hrs? I have read that longer fermentation time is generally correlated with better fermentation rate, but I've also read that fermenting up to 100% can cause a vinegar taste and formation of bad bacteria.

Background:

Our beans are all smallholder sourced, mostly forastero. Farmers cut pods themselves and deliver 'fresh beans' to us same day as harvest. There is a lot of mucilage so we stack the beans on pallets in pp bags for 2 days before putting them in hard wood boxes 3'x3'x3' covered with banana leaves. We shift boxes every 48hrs. We are experimenting with soaking/washing the beans after fermentation. Beans are sun-dried on raised beds for 5-7days.

For this recent batch, I have found that by day 6 the beans are still quite warm in the center >45C with an acid/alcohol smell to them. When cut open many are dark purple with brown juice others pale. (see attached @ 2 days on pallets + 6 days in boxes).