The physics of biltong

[IvanV]

I decided to make some biltong, following the method set out in this youtube video and accompanying recipe: How to make the best biltong by Pants Down Aprons On. I had an interesting experience in experimental physics.

Biltong is air-dried cured and flavoured meat. Typically it is briefly dry-cured with salt, then wet-cured with vinegar and flavourings, neutralised with bicarb, then given a rub of flavouring such as spices. Finally you air dry it. You aim to reduce the weight of the meat to about 50-60% of the starting weight, which tells you when it is done. And this is safe enough for bodging amateurs to do, unlike, say, salami making.

If, like us Brits, you don't live in a suitable climate for air drying meat safely, then you can do the air-drying stage in a biltong box. You put a small fan, as for a computer, and a small heater, such as a 40W light bulb in the lower part of the box. And have some ventilation holes in the upper part of the box, protected with anti-fly netting. Hang the meat, and it should be done in a few days depending on temperature, etc. You can also do it in a vegetable dehydrator, but that is typically more aggressive than is ideal and will be done in about a day, and come out a bit hard.

But Pants Down Aprons On, like some other people, notes that you can also do this by drying it in a fridge. It can take 2 weeks instead of 3 days. But in principle it is low humidity in a fridge, and so it will slowly dry. Since making a biltong box would be a faff, cost money, and then I'd have to store it, fridge seemed like a good idea.

So I hung my 3 cured meat pieces in our spare fridge in the garage, which is mainly used as expansion space in the summer. Indeed the freezer part stops working in deep winter, because it is a cheap one-thermostat fridge-freezer, and the freezer defrosts when ambient temperature falls below 5C. I removed all remaining items from the freezer into our other freezers just after starting this, as the weather finally turned cold.

After 3 days, I tested the meat, and it had lost 8% of its weight. So, extrapolating, it would be done after 15 days, more or less as PDAO suggested. After another week, I tested it again, and it had lost no further weight.

Now in a panic I order myself a 220V computer fan from a supplier to knock myself up a biltong box in a hurry. And necessity being the mother of invention, I realise that I have plenty of suitable boxes, albeit not what you usually see, made of coated cardboard rather than plastic or wood. But that will make it much easier to size holes for the 40W light bulb and fan. I scavenge a traditional light fitting from a disused home-made table lamp, and I still have plenty of 40W bulbs in the cupboard from the Old Days. I bought a 220V fan as that would be much easier to wire up than a DC fan.

3 days later, the fan has come and I have my biltong box. The meat hasn't lost any more weight in that time. I put the meat in the biltong box, and because I actually want it to be mid-20s C for drying, I do it indoors - in a room with the door closed against the cat.

After 3 days of drying in the biltong box, I test it. The smaller piece has reached 60%, so I take that out. The two larger pieces are still at about 65%, so I reckon another 8 hours overnight will do it. I test them in the morning and they are at 46%. How upon earth did that happen?

So, what's the practical physics here? Why didn't the fridge work? Why did the drying suddenly accelerate?

I think the fridge didn't work because the weather turned cold, and the fridge stopped operating. Humidity is only low inside a fridge because the cooling mechanism, as a side-effect, condenses water onto the cooling elements. That water then runs down and into an evaporation container outside the fridge. But when the fridge stopped operating, because it didn't need to, so the humidity inside the fridge can actually be higher than outside the fridge.

I think the drying accelerated because the particular trajectory of weather meant that the humidity inside my house fell. My weather station shows both interior and exterior humidity. So I can see that at precisely that overnight, the interior humidity was at its lowest in several weeks. Humidity inside my house has basically been gradually falling for about 3 weeks. If I do this again, I will actually check the interior humidity. If it is low, I'll turn the 40W bulb off. It is astonishing how much the drying accelerated, so I wonder if that is the only story.

[Martin Y]

I'm sure you're right about the fridge. Our old fridge also lives on in the garage and in this cold weather when the air is colder than the target temperature it just doesn't run, so it won't remove any moisture.

[raven]

Wow, that is interesting. I'm guessing (assuming you're in the UK?) changes in humidity have a lot to do with the sudden drying. It was super humid where I live (SE) until a week or so ago, and then it's been much drier during the recent cold snap. The other big factor in drying times is airflow, would that have been different?

I love biltong but haven't tried making it, and now I'm wondering if an airing cupboard would work. I've had some success drying herbs and chillies in there, but that didn't work so well this year which has been much cooler & damper than previous years.

[dyqik]

(Ignore.)

[IvanV]

I love biltong but haven't tried making it, and now I'm wondering if an airing cupboard would work. I've had some success drying herbs and chillies in there, but that didn't work so well this year which has been much cooler & damper than previous years.

The general instruction I have seen is that if you are attempting to air-dry your meat at ambient or warm temperatures, then you need air flow. The fan is a more important part of my biltong box than the 40W light bulb.

[hakwright]

I love biltong but haven't tried making it, and now I'm wondering if an airing cupboard would work. I've had some success drying herbs and chillies in there, but that didn't work so well this year which has been much cooler & damper than previous years.

The general instruction I have seen is that if you are attempting to air-dry your meat at ambient or warm temperatures, then you need air flow. The fan is a more important part of my biltong box than the 40W light bulb.

That was my thought too, it's probably the air flow (or lack of) that was the issue.

BTW I have also dried biltong in the oven. Set to fan on lowest temp (around 50), leave the door open a little, and check after some hours to get the level of drying you like. You can also use cocktail sticks to pierce the biltong strips, then put the cocktail sticks through gaps in the oven trays (from underneath), then rotate 90 degrees so the sticks rest across the metal grid of the tray, and the biltong hangs vertically underneath.

[Boustrophedon]

Would pulling a vacuum help?

[IvanV]

Would pulling a vacuum help?

Physics would suggest that low pressure acts as a substitute for higher temperature when dehydrating food. I read the method is sometimes used industrially for drying food.

Evidently vacpacking the meat is insufficient, we need some continuous pumping to pull the water vapour out.

I can find no mention of doing it specifically for biltong. So there is potential for an experiment to see what it does to the quality of the final product. If you like the quality, what would be the energy cost of making it that way? How much does the equipment cost? I spent £12 plus stuff I had in the garage.

A reason given for not making biltong at higher temperatures is you get a kind of surface hardening, which would not be ideal. My source recommended 25C. I have found others recommending 30C or 15C, quite a range, perhaps depending on how you prefer your biltong. The higher the temperature, the faster it goes, good for the impatient, less for the gourmet. What would a low pressure pump do?

[Martin Y]

Seems to me the downside of pulling a partial vacuum is you lose free ventilation of the container, so even if it draws out moisture you can't easily keep changing the humid air for drier ambient air. Unless, I suppose, you allow a slow air leak and run the air pump continuously, which might get expensive. (Or rather, even more expensive.)

[Boustrophedon]

Seems to me the downside of pulling a partial vacuum is you lose free ventilation of the container, so even if it draws out moisture you can't easily keep changing the humid air for drier ambient air. Unless, I suppose, you allow a slow air leak and run the air pump continuously, which might get expensive. (Or rather, even more expensive.)

In freeze drying the vacuum pump is eventually just extracting water vapour, no need for air.