# Biltong box - need to power a 100W bulb and a 12V fan

delta temp = 0.46 degrees C in one second
temp @ 1 min = 27.78 degrees C in a 1minute

Let's try to work out the steady-state temperature if it were sealed.

Work out the thermal conductance of the walls of the box. The area is about 2 m^2. If it were 10 mm plywood, the conductance would be about 0.13 W/mK * 2 m^2 / 0.01 m = 26 W/K

So your 100 W input would cause the internal temperature to reach about 4 C above ambient, with a sealed box.

But you want a temperature rise of 60 C, i.e. 15 times that.

Insulating materials like felt and polystyrene are probably about 5 times more insulating than that plywood. So maybe 30mm of polystyrene would be enough to give a steady-state sealed box temperature of 60 C.

Of course you want a fan to remove the moisture. If you want to really over-engineer the thing, make a heat exchanger so that the extracted moist air warms up the incoming dry air!

JohnW2";p="3127170 said:
You certainly would be advised to have something which would switch off the lamp before things became sufficiently hot to become a fire risk (whether due to ambient conditions, fan failure or whatever).

Kind Regards, John

I thought about, when I was still looking at a 12V fan, something like thermostat in the link:

http://tinyurl.com/q8ocb9o

I can put foil at the bottom to reflect heat upwards. Spend lots of money on some sort of thermal cut off.

I dont know how much is too much before I'm over engineering the whole thing, but I dont want to burn down my shed that i'll be housing it in either.

Let's try to work out the steady-state temperature if it were sealed.

Work out the thermal conductance of the walls of the box. The area is about 2 m^2. If it were 10 mm plywood, the conductance would be about 0.13 W/mK * 2 m^2 / 0.01 m = 26 W/K

So your 100 W input would cause the internal temperature to reach about 4 C above ambient, with a sealed box.

Good guess I am using 10mm ply for the box.

But you want a temperature rise of 60 C, i.e. 15 times that.

Insulating materials like felt and polystyrene are probably about 5 times more insulating than that plywood. So maybe 30mm of polystyrene would be enough to give a steady-state sealed box temperature of 60 C.

Well the technically I dont need to hold the air in the box and find steady state, I need to raise the temperature around the beef to dry it before pulling it out. This prevents mold (from what i read).

I'm trying to find information to back up the 60degree heat claim, i.e. I'm starting to doubt the temp needs to be kept that high. It might be that higher heat makes for quicker drying.

Tin foil will reflect the heat for long enough to ensure it's not absorbed into the walls, before it's pulled out by the fan.

Of course you want a fan to remove the moisture. If you want to really over-engineer the thing, make a heat exchanger so that the extracted moist air warms up the incoming dry air!

I do not

...I need to raise the temperature around the beef to dry it before pulling it out. This prevents mold (from what i read). ... I'm trying to find information to back up the 60degree heat claim, i.e. I'm starting to doubt the temp needs to be kept that high. It might be that higher heat makes for quicker drying.
How long is the meat going to be in this box - and is it in a 'lump', or 'sliced'? If it's long enough for the heat to penetrate it (i.e. 'a few hours', or more, for a lump, much less for slices), aren't you actually going to cook it, rather than just 'dry' it? - for beef, 55-60°C is usually regarded as 'medium rare' and 60-65°C as 'medium'.

On the other hand, if the temperature were appreciably below 60° (but still 'warm') and the amount of time significant, you could well have a 'bacterial incubator' on your hands, rather than a dryer. There are few quicker ways to get meat to 'go off' than to store it at 40° or so for a while! I guess that's why commercial practices usually use free-drying, rather than heat-assisted drying!

On yet another hand, if you used a temperature appreciably above 60°, the outside would, indeed, presumably dry more quickly, but (if it were a 'lump'), the outside would probably then be 'well cooked' before the inside dried.

... but I don't know anything much about this game, so I probably need to be educated!

Kind Regards, John

I'm trying to find information to back up the 60degree heat claim, i.e. I'm starting to doubt the temp needs to be kept that high. It might be that higher heat makes for quicker drying.

I know it gets hot down there, but not 60°.....

I'm trying to find information to back up the 60degree heat claim, i.e. I'm starting to doubt the temp needs to be kept that high. It might be that higher heat makes for quicker drying.

I know it gets hot down there, but not 60°.....

Yes, exactly what I'm thinking. So I think 100W plus fan, will get ambient temps + 10degrees or so will be fine.

The moisture is what brings on mould which is why it's needs to be drawn out (with the fan).

I'll put some tin foil at the bottom to make sure that the heat is bounced upwards.

For added measure i'll stick a battery powered smoke detector in there as well. unless anyone has a better way of cutting off the power if the bulb causes a fire?

For added measure i'll stick a battery powered smoke detector in there as well. unless anyone has a better way of cutting off the power if the bulb causes a fire?
A better idea would be a thermostat that turns the lamp OFF if the temperature in the box gets too hot thus saving meat and box from catching fire.

The moisture is what brings on mould which is why it's needs to be drawn out (with the fan).
All the more reason, I would have thought, to blow warm air directly over it.

If you have a thermostat sensing the airflow temperature, you could make a multi-purpose drier. For example, if you ever have any of that bread with olives/sundried tomatoes/walnuts/etc which has gone a bit stale, slice it very thinly and dry at 100° for about half an hour. Makes great crispbread for cheese, and keeps for weeks in an airtight container.

How long is the meat going to be in this box - and is it in a 'lump', or 'sliced'? If it's long enough for the heat to penetrate it (i.e. 'a few hours', or more, for a lump, much less for slices), aren't you actually going to cook it, rather than just 'dry' it? - for beef, 55-60°C is usually regarded as 'medium rare' and 60-65°C as 'medium'.
It should be sliced, and dried, not cooked. Slices shouldn't be too thick, or you'll never be able to chew the stuff.

On the other hand, if the temperature were appreciably below 60° (but still 'warm') and the amount of time significant, you could well have a 'bacterial incubator' on your hands, rather than a dryer. There are few quicker ways to get meat to 'go off' than to store it at 40° or so for a while!
Don't forget the meat is often first salted or brined (to draw out moisture), and then (again often) vinegar washed, and then spiced. The bacteria on beef, venison etc is on the surface, which is why cooking steaks blue is perfectly fine, but tartare possibly not, as that minces up the outside and distributes it through the rest of the meat. (And another reason apart from quality to make your own mince, rather than buy it).

There's probably not going to be many bacteria left to cause problems - the biggest problems with biltong is keeping flies off of it, and doing too much at once so it doesn't dry quickly.

It should be sliced, and dried, not cooked. Slices shouldn't be too thick, or you'll never be able to chew the stuff.
Fair enough but, as I said, raise the temperature within in to 60 degrees and many would regard it as 'cooked
Don't forget the meat is often first salted or brined (to draw out moisture), and then (again often) vinegar washed, and then spiced. The bacteria on beef, venison etc is on the surface ...
Sure, provided the surface bacteria are killed, and the surface remains uncontaminated thereafter, all should be OK, but ...
There's probably not going to be many bacteria left to cause problems - the biggest problems with biltong is keeping flies off of it ...
... and the flies could, of course, introduce new bacteria! ... and it doesn't take 'many' to cause problems. At an appropriate temperature, just one bacterium can turn into nearly 17 million (2^24, assuming they divide every 20 mins) in about 8 hours!

Having said all that, it's amazing how 'safe' such prepared foods often appear to be!

Kind Regards, John

... and the flies could, of course, introduce new bacteria!

fear not, I have fan cover gauze to prevent insects crawling in.

Having said all that, it's amazing how 'safe' such prepared foods often appear to be!
And how you can hang game birds until they are starting to rot, and they'll be fine.

Having said all that, it's amazing how 'safe' such prepared foods often appear to be!
And how you can hang game birds until they are starting to rot, and they'll be fine.
Indeed, that's the sort of thing I was thinking of. However, there is also an element of luck - I've had some nasty experiences in my time with 'well hung game' and, indeed, dried meat/fish! With the latter, I think the important thing is to get it dry as quickly as possible and keep it very dry - bacteria need water/moisture!

Kind Regards, John

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