Cooking pit revisited - temperature logging

Spring and summer time equals cooking pit time. This time we did some more serious temperature measurements, showing interesting results. A brief report follows...

Every May/June we take our food culture students out for some primitive cooking, ref. the previous post Primitive food, heat transfer and a day out. This time we did some more serious temperature logging with the help of Type K 4-Port Temperature Sensor connected to a Pasco datalogger.* This enabled us to monitor temperatures automatically at four different places in the cooking pit at one time:

• inside of the pit, at the bottom
• inside of the pit, at the top
• inside the trout cooking
• 10-15 cm outside the pit, ca. 25 cm deep (to monitor the heat loss through the soil)

Temperature plot. Click for large version

Most noteworthy is the temperature difference between the bottom and top, since the food and rocks are laid in layers. This is very interesting in terms of where to place food the next time we'll use this method. One might also exploit this to cook different foods in the same pit (i.e. meat and chicken or fish); place the meat at the bottom and place the fish directly on top of the meat. Also, note that the fish, being wrapped in foil, levels off at 110 °C. I guess this is due to the large water content in a closed package. Hence, this isn't the method if you aim at sous-vide type results. The flavour and texture is however still very good, not at all mushy.

This time we dug two pits

• Pit 1: lamb's leg and potatoes, somewhat less than 3 hr cooking time
• Pit 2: Trout and chicken, 1 hr 10 min. cooking time (see temp. plot)

Data logger with 4-port K-type sensor. More pictures in previous post

We're on our way to publish a web based teaching plan on this topic, including historical, physical science, and food related information at www.naturfag.no/mat (Norwegian national school science web pages) and www.natursekken.no. All in Norwegian (but google and babelfish make increasingly good translations). I'll post a note when it's out.


Reference:
Wandsnider, L. "The Roasted and the Boiled: Food Composition and Heat Treatment with Special Emphasis on Pit-Hearth Cooking." J. Anthr. Arch. 1997, 16, 1-48. (This ref. is most relevant for indigenous American traditional pit-hearth cooking, using rather different foods. For Scandinavian prehistoric methods, see the previous post and coming teaching plan)

www.naturfag.no/mat, English translation
www.natursekken.no, English translation



* The K type thermocouples measure a range of -200 - 1000 °C! Although the probe sleeves are limited to 482 °C, this is sufficient for use inside the pit

Primitive food, heat transfer and a day out

What did people do when there were many to feed and ovens weren't though of yet? To start digging a hole in the ground was maybe not a bad idea. At least, that was what we did with 40 twelve-year old kids. This is the Stone age way of cooking, and we ended up feeding 130 persons.

Cooking in a cooking pit (nice animation at steinalder.no) is in fact quite a simple thing as long as you've got enough time. At a school trip for seventh-graders at Bratteberg skule (primary school), this was one of the points during a day of many such activities. Groups of five kids spent ca. 45 minutes working at the pit. We started at 11.30 and dug up the food at 18.00, feeding the kids, teachers, and families for a real feast of lamb's legs, salmon and potatoes. The work was guided by a parent (secondary school teacher) and myself.

Menu for 130 persons (might be scaled down, of course)
six legs of lamb (2.3-2.7 kg each)
seven salmons (ca. 3 kg each)
130 potatoes (preferable baking potatoes)
salt, pepper, garlic, herbs (thyme, oregano, rosemary)

Equipment
2-3 shovels
bar (lever for removing rocks stuck in the ground while digging)
firewood (a couple of 80 litre sacks for each pit)
loads of aluminium foil, good quality
thick, heat-resistant working gloves (at least two pairs, non-plastic)
terracotta pot with lid (optional)
cooking thermometers (optional, but a lot to be learned from using. We used six in parallel)

What to do

The day in beforehand
rub lamb's legs with salt, pepper, herbs and garlic. Pack thoroughly in foil, preferably four layers. Keeping or removing the bone is a matter of taste (and skill), optional. Salmons might be done two different ways. In our case, five were filleted, rubbed with salt, pepper and herbs and packed as single fillets in foil. The other two were gutted and cleaned, but kept as round fish. Rubbed with salt, pepper and herbs, and placed in terracotta pots. Stick in thermometer probes into the food if you've brought them. Potatoes are wrapped singly in two layers of foil, but this might be done on the day as a parallel activity to digging.

On the day, for one pit (we made two)

1. cut out rectangular pieces of the turf, in total approximately the size of a coffee table, ca. one by two metres. Be sure to keep the turf on whole pieces, and turn them over to each side
   
   
2. dig the pit where the turf is removed. Depending on the size of the stones, the pit needs to be 0.4-0.7 metres deep. Round stones require a somewhat deeper pit compared to flat ones. At the same time, collect loads of stones/rocks. Size may vary, but minimum is the size of an open hand. Maximum size is what you can carry, both to the pit and back (we don't want to leave too many marks in the nature). You need enough stones to cover the area of the pit to at least three layers.
   
   
3. line the bottom and sides of the pit with stones.
   
   
4. build and light a fire in the pit. Use plenty of firewood. The fire is burning steadily, add more stones to the fire (you need at least 50% more than you think, so don't be modest with the stones). Let the fire burn down (takes at least 1-1.5 hours)
   
   
5. remove the loose stones and charcoal from the pit with shovel or gloves, leaving the ones lining the pit. Layer the food and hot stones, making sure that all the food is surrounded by hot stones. Turn the turf back over the stones/food, earth side down. Stick one thermometer probe directly into the pit (if you've got one)
   
   
6. leave for at least 2.5 hours (lamb or fish in pot) or one hour (fish fillets in foil). Potatoes are ok after one hour if they're well surrounded by hot stones (in our case, the ones in the middle of the pit were good, the ones out on the sides were not ready).
   
   
7. Carefully lift of the turf, remove the food, unwrap, and serve.

Experiences
In our case, this was a highly successful activity, and was perfect for a day out with class/school, be it the last day before holiday, school trip, or just a day in the garden with friends (if you've got a garden that allows for digging). With only adults, calculate 5-6 hours from start to serving, add one hour for a school activity. I've done this a number of times with university college students, but this is the first time with primary school kids. No problems, but one needs to be at least two adults.

The temperature in the pit was surprisingly high. We started out with almost 320 °C in the closed pit(!) at 15.00, ending up at 140 °C 3.5 hours later. It was really fun recording the temperatures, seeing how the temperature in the pit fell and the food heated up. I regret not putting a thermometer in one of the potatoes, though. We also recorded the temperature in the fish and soil outside the pit (omitted in the plot). The temperature in the fire was recorded with an IR thermometer, going well above 500 °C.
In our case, this was a highly successful activity, and was perfect for a day out with class/school, be it the last day before holiday, school trip, or just a day in the garden with friends (if you've got a garden that allows for digging). With only adults, calculate 5-6 hours from start to serving, add one hour for a school activity.

The food is extremely tender and flavourful due to the long cooking (we left the lamb 3.5 hrs and fish fillets 1 hr 20 min). However, I wouldn't recommend aiming at medium rare done meat, but rather go all the way to the pulled pork-type texture (where the meat just falls apart). The fish might be somewhat overcooked, but who cares? This is supposed to be primitive cooking!

If you open the pit too early, there is no going back - the heat is gone. In that case, you better have a fire or an oven at hand. That's maybe the drawback with the method, and the best reason to accept somewhat overcooked food.


What might be learned

• Cooking time vs. type of food (lamb takes long, fish fillets short)
• heat transfer (stones and pit cools, the food warms)
  
• heat capacity (the stones store the heat that is used for cooking)
• data logging (temperature vs. time)
  

...and as a bonus comes that this is a nice outdoor activity with lots of physical exercise. Also, the large span of activities allows for a number of personalities to attend. In our case, those with lots of energy to spare dug and carried stones, whereas those with less energy of motivation could deal with the firewood, wrap potatoes or log the temperature.


Other comments
We did the temperature logging manually, recording time and temperature with pen and paper. Automatic dataloggers with computer interface are of course a possibility, but we went for the manual method.

I've not included safety matters here, but heat, open flame, the use of sharp and heavy tools etc. are all matters that carry a certain amount of risk. However, it should not put anyone off as long as the work is well organised and adults are present (this applies to the Norwegian school regime, at least).

The ideal type of ground for a cooking pit is slightly moist, not too sandy, and with a good layer of turf. That way, the pit keeps its shape, and the turf works as a tight lid.

How to afford this with a tight school budget? In our case, the guests that attended the meal in the evening (parents, families) payed an entrance fee. This covered most of, or all, the expenses.

Make sure that you leave as few marks as possible. Keep the turf whole, don't leave hot stones directly on the turf (leave them on the heaps of earth you've dug up), carry most of the stones back to where they were found. And, by all means, ask for permission to dig and light an open fire.

Erik