Almond-caraway coated chocolate truffles

As a part of "They go really well together no. 13" (TGRWT #13), I wanted to try on making chocolate truffles. This was a first-timer for me, but I must (not very humbly) say that this time I had a lucky strike.

I think this might be the first time I've made something that gives me that experience you get when you go to a restaurant and they serve something you've never tasted before which leaves you both happy and amazed at the same time. Maybe for the first time, I felt that I'd produced such an experience. It's somewhat like the first time I was able to ride my bike without the support wheels.

Being a rookie in the field of confectionery, Shirley O'Corriher was my crutch (Cookwise, I haven't got around to buying Bakewise, yet). For basis recipe for chocolate truffles, I chose the "Smoothest-Ever Truffles" which worked well. Since I had already made salty almonds and caraway schnaps for Christmas, the table was laid.

Ingredients
5 egg yolks
300 g dark chocolate (70% cocoa, tried two different with similar results)
170 g milk chocolate ("lys kokesjokolade")
60 ml heavy cream
50 g butter (salted)
60 ml caraway schnaps
260 g salty almonds
13 g caraway seeds

Directions
1. Almonds were chopped in food processor and mixed with caraway seeds
2. Chocolate was finely chopped in food processor
3. Cream and egg yolks were heated gently in a pan, stirring constantly until slightly thickened. Butter added and stirred over low heat until melted. Chocolate added and stirred over low heat until melted. Stirred in the caraway schnaps
4. Left to cool in the pan (4-10 °C for a few hours or overnight)
5. Scooped up and made ca. 2-3 cm (1 in) diameter rough spheres. Rolled the spheres in my hands (warm hands make them melt slightly on the surface, helping ground almonds to stick)
6. Rolled in the ground almonds/caraway-mixture and left to cool


The experience
Firstly, a burst of crunchy, salt, roasted almonds followed by rich, dark chocolate. Second, while the chocolate melts and the cocoa flavour still dominates, the caraway comes through and lingers on together with the chocolate. Balance and contrast at the same time, both in flavour and texture, taking me through various phases of experience. Also, the chocolate and caraway work very well together.

The experience is different whether I let it melt in the mouth or if I chew it. Melting in the mouth gives a stepwise flavour experience since the caraway takes some time to come through. Chewing results in all the flavours coming out at the same time.

Comments
The size of the balls seem to make a difference. Too small, and all the tastes come out at once, and the amount of salt/almond/caraway is somewhat overpowering. Making them ca. 2-3 cm (1 in) diameter gives a creamy interior and a somewhat crunchy exterior which, to me, gives the best result.

The caraway schnaps does not seem to cut through the chocolate on its own, but gives that extra bite. I tried covering with cocoa powder rather than the almond/caraway mixture, resulting in no discernible caraway flavour. Hence, the caraway schnaps on its own does not give sufficient caraway flavour.

The amount of salt and caraway is of course a matter of taste preference. Some might want to reduce the amount of salt (i.e. use more lightly salted almonds), and caraway (less caraway vs. chopped almonds). To me the given amounts work well, though.

Reference
O’Corriher, S.: Cookwise. New York: William Morrow 1997.

Salty roasted almonds and caraway schnaps

Christmas time is making your own stuff from the basics. This year brought, amongst other things, salty almonds and caraway schnaps (aquavit).

Salty roasted almonds

• ca. 1 l concentrated brine
• 0.5-1 kg almonds
  

A very approximate, but still fool proof, method: Make a concentrated brine by dissolving as much salt as possible in ca. 1 l boiling water (takes approx 500 g or so, maybe). Dump in almonds (0.5-1 kg), as much so that the almonds are just covered. Put on lid and leave cool/cold overnight. The lower the temperature, the saltier the almonds, I usually keep them on the porch in wintertime but the fridge works well. Next day, strain and spread on a baking tray with parchment. Roast in the oven, fan turned on, at 150 °C to taste (I usually prefer 15-20 minues).

Principles governing the crystallisation of salt on the almonds

• Rapid cooling gives small crystals. If you want larger salt crystals that crunch, cool slowly (start with placing in a warm room), and use less saturated brine.
• A lower end temperature results in precipitation of a larger proportion of the salt, probably giving saltier almonds. For less salty almonds, leave in warmer place to cool and/or strain earlier.
  

The good thing about these is that no fat is involved, avoiding greasy almonds. Also, the method allows for easy production of rather large amounts (0.5-1 kg at a time) with a minimum of labour.


Caraway schnaps (aquavit)
The caraway schnaps/aquavit/snaps is inevitable at many Norwegian Christmas dinners. Making one's own (in Norway and Denmark, the term would be "dram" or "krydderdram") is in fact rather easy. Although the flavour might not be as complex as the commercial ones (sherry cask aging is not an option in our home...), I believe that the ones you make at home might be more concentrated in the spice/herb flavour. The directions follow the recipe from Rolf Øvrum:

• 25 ml caraway seeds
• 50 + 450 ml 40% unflavoured alcohol/ethanol (80 proof)
  

Mix caraway seed and 50 ml ethanol in a lidded jar and leave in room temperature for 5-6 days to extract (longer storage times results, according to the literature, in unpleasant turpentine-like flavour). Make sure that the jar is perfectly clean as the alcohol might extract remaining flavour from plastic-lined lids of jars previously used for other foods. Filter through filter paper (paper coffee filter works fine), giving a highly concentrated caraway extract. Dilute with the 450 ml ethanol, rinsing the caraway seeds and filter paper with some of the ethanol in order to recover all the extract. It is perfectly drinkable right away, but will round of well if left to mature, preferably for 3-5 months. Goes well with strong flavoured meats such as mutton or game.

Reference
Øvrum, R.: Akevitt av egen avl. Cappelen Damm 1999.

Leavens in cookies - theory and practice

Recently, I published a popular science article in Norwegian. Title might be translated "Christmas cookie chemistry, ...and some physics" on www.naturfag.no/mat. Focus is testing the effect of using different chemical leavening agents on the same cookies (yeast is hence not the issue here). A summary follows.

All photos (unless otherwise stated): Erlend Krumsvik

Why do some recipes require baking powder, others ask for baking soda, and yet others want hartshorn (baker's ammonia). For some cookies, various recipes for the same cookie even ask for different leavening agents. Puzzling...

(In writing this, I've realised that hartshorn is a rather rare ingredient in English speaking countries(?), even though it's still common in Scandinavia, and possibly also in central Europe. See references in the end for some information)

The chemistry, in short

• Baking soda = sodium bicarbonate = NaHCO3. Requires acidic ingredient in order to produce CO2 (but see below)
• Baking powder = baking soda plus (most commonly) two solid acids, one reacting at room temperature whereas the other doesn't react before heated. Does not require acid but does require water in order for the reagents to react.
  
• Hartshorn = bakers ammonia = ammonium bicarbonate = (NH4)(HCO3). Does neither require acid nor water (but water may be used if required by other reasons). Reacts at higher temperatures only.

The two former produce CO2, whereas the latter produces both CO2 and ammonia (NH3). In all cases, gas production results in bubble expansion, giving porous and levened cookies.

Physics, in short
Either whipping eggs with sugar or creaming butter + sugar generates bubbles. When placed in the oven, the air in the bubbles expand and (more importantly) water evaporates expanding the bubbles.

Discussion
Both baking powder and hartshorn do not require any other ingredient to work properly.

Baking soda, however, needs some acid (i.e. lactic acid from sour cream or buttermilk, syrup etc.). Heating pure baking soda releases some gas, but half of the baking soda remains as sodium carbonate (Na2CO3, soda). This might give a soapy bitter taste. Also, baking soda alone results in a dough in the basic pH range (opposite of acidic). In short, higher pH promotes Maillard reactions, resulting in darker cookies and more pronounced baking/caramel/nutty flavour. Also higher pH retards gluten formation, resulting in shorter texture (or more correctly, low pH promotes gluten formation).

Hartshorn liberates CO2 and ammonia. Ammonia is quite a basic substance, and seems to give rather marked different results compared to the rest (see below). Also, harthorn gives crisp, brittle and porous cookies. I haven't found any sources say exactly why, however (frustrating...). One reason might simply be of physical nature: cookies made with hartshorn are perforated due to bubble formation in a rather dry dough, resulting in cookies with lots of small holes.*

Results
Made "tyske skiver" (shortbread cookies) and "sirupssnipper" (variety of gingerbread cookies).

(Click for table in pdf format or here for table in html format)

Of course, there are lots of variables to take into account in addition to the ingredients; baking temperature, baking time, inhomogenous temperature in the oven, using more than one tray, craftsmanship etc. The experiments above were conducted in identical fashions as far as possible. The various versions were not baked on the same tray, however, which would have given the most comparable results.

Some rules of thumb

• For soft cookies: use baking powder, alternatively baking soda plus and acidic ingredient
• Extra brown/dark cookies: use (more) baking soda. Not recommended if the cookies have a mild flavour, strong flavour might/will mask the taste of sodium carbonate (soda). Hartshorn might work, but has other side effects.
• Brittle/crisp: hartshorn
  
• Short texture: avoid working dough with water. Crumble flour and butter thoroughly. Keep pH above neutral by using (or adding a little) baking soda, or use hartshorn.

Skipping the leavening agent altogether is also an alternative that should be considered.


General quantities and conversions

• 1 ts baking powder = 1/2 ts hartshorn = 1/4 ts baking soda
• amount of acid for 1/2 ts baking soda ↔ ca. 250 ml buttermilk or 1 ts lemon juice/vinegar
• 1 ts baking powder ↔ 250 ml (150 g) flour
  

References
Belitz, Grosch & Schieberle: Food Chemistry (3. ed.). Berlin: Springer 2004.
Gardiner & Wilson: The Inquisitive Cook. New York: Henry Holt & co 1998.
McGee, H.: McGee on Food and Cooking. London: Hodder and Stoughton 2004.
O’Corriher, S.: Cookwise. New York: William Morrow 1997.
Olver, L.: The Food Timeline


* Harold McGee has an interesting entry on hartshorn, however erraneously stating that ammonium bicarbonate does not release water. Indeed, it does as seen from the reaction scheme below. I gues, the main point is that hartshorn does not require water to funtion, allowing for rather dry doughs (flour - sugar - butter).

The food timeline

A visit by The Food Geek made me aware of this treasure of a web site. The food timeline gives a comprehensive overview on the history of various foods and dishes. When teaching food topics, historic facts (or myths) are often excellent starting points. The problem is that such information isn't really very easy to get to. Here, loads of fascinating information is organised in a straightforward and highly effective way.


The site is far more than a timeline with information on foods and dishes, though:

• historic cookbooks
• economics & historic prices
• Christmas food
• meal times
• references to various literature
• free e-mail answer service promising answer withing 24 hours! (I haven't tested it yet, however)
• tips and strategies in searching for historic information on food
  

Finally, citing the pages: "Information is checked against standard reference tools for accuracy". Also their book shelf is quite impressive.

Highly recommended


Late addition: I submitted a question (on hartshorn) and received an answer less than two days later. Not bad at all, taken that the service is free and done by a professional

TGRWT #13: caraway and cocoa/chocolate

"They go really well together" round no. 13 is hereby announced. In taking on the task of hosting this somewhat sinister-sounding round, I thought a whiff of Christmas might be a proper safeguard, announcing caraway and cocoa/chocolate as flavour partners.

For newcomers: TGRWT (They Go Really Well Together) is a monthly(?) open invitation to come up with dishes that combine certain given ingredients that are supposed to match well. The hypothesis is that foods containing one or more common major volatile compound will work well in combination. The result of this might thus be a range of new and, sometimes unlikely, good combinations of ingredients. For further introductions to the topic, see the foodpairing web site and the introduction to the topic at khymos.


This is how you can participate in TGRWT #13:

1. Prepare a dish that combines chocolate and caraway. You can either
use an existing recipe (if there is any) or come up with your own.

2. Take a picture of the dish and write an entry in your blog by
January 1st with TGRWT #13 in the title. Readers will be particularly
interested in how the flavour pairing worked out, so make an attempt
at describing the taste and aroma and whether you liked it or not.

3. A round-up will be posted here (with pictures). Please send an
email to erik_at_fooducation.org with the following details: Your
name, URL of blog, URL of the TGRWT #13 post and a picture for your
entry in the round-up. If you don't have a blog, email me your name,
location, recipe and a brief description of how it worked out and I'll
be glad to include it in the final round-up.

Late addition: deadline 31. December

I'll have a look in the literature, and in the round-up I hope to be able to say something about what makes these two ingredients match, at least according to the favour pairing hypothesis.

Chanterelle + Apricot (TGRWT #12)

The "They go really well together" relay rolls on, and it's time for #12 - matching chanterelle and apricot. Host for this round is Tri2Cook. I've been rather reluctant to try on mushrooms in cooking, but there has to be a first time for everything. So, I strayed into the kingdom of fungi.

Since we had bought a lamb earlier this autumn and had planned on a dinner (literally) on parts of this for some time, I thought this might be a good opportunity. At a wedding dinner many years ago, we were served a wonderful creamed chanterelle side dish with some meat that I now have forgotten (but the chanterelles etched into my memory). We were going to be eight at the table, of which two were children.

The meal

• Oven-roasted lamb shoulders (rosemary, thyme, cellery, carrots, onion, salt, pepper)
  
• Creamed chanterelles with apricots
• Mashed potatoes & carrots (mixed)
• Brussels sprouts and green peas
  
  
• Blue cheese (Royal blue, rather mild)
• Sugared ("sweet-pickled") pumpkin
• Lingonberry jam/jelly (Scandinavian replacement for cranberry jelly)
  

Creamed chanterelles with apricot

• canned chanterelles, 3 tins each containing 110 g mushrooms
• 7-8 dried apricots
• 150-200 ml double cream
• 150 ml skimmed milk or water
  
• 1-2 T butter
• thyme, salt, pepper

The day in beforehand, apricots were chopped in small squares and soaked in the double cream. The mixture was warmed in the microwave until lukewarm and then left to infuse in the fridge overnight, resulting in a yellow apricot-flavoured cream, in which time it had almost solidified. Thinned with skimmed milk and some water until fluid.

The chanterelles were drained, rinsed in cold water and dried on a paper towel. The stock was reduced (very salty as the canning stock was salted). The chanterelles were lightly fried in butter, apricot-cream (with the apricot bits) and spice was added and brought to a light boil. Thickened with corn flour and some of the reduced stock was added.

Lamb shoulders
The day in beforehand, two shoulders from a small free-range lamb (12 kg after slaughtering) were rubbed with a mixture of thyme, rosemary and ground black pepper. Slashed and half garlic cloves inserted. Left to marinate overnight. Next day rubbed with salt and placed in cooking bag. Added one onion cut in four, 3-4 coarsly chopped carrots and a one coarsly chopped cellery. Left in 110-120 °C oven for 4.5 hrs. Since the meat was home-slaughtered free-range, I turned down the idea of a sous-vide approach and went for meat cooked to the brink of disintegration (interior temperature was 100 °C for 1.5-2 hrs).

Gravy
The juice from the lamb shoulders was used as base for a standard gravy made with corn flour, adding the rest of the reduced chanterelle stock.

Verdict
The lamb was great, cooked greens and mashed potatoes/carrots don't need further comments. Two shoulders from such a small lamb was sufficient, but not too much for eight people.

The creamed chanterelles/apricots (chanterots, maybe?) were good, but by no means exceptional. I realise that my lack of formal food education might play a role here, combined with my lack of experience with cooking mushrooms in general. The creamed chanterelles were based on a recipe, but felt somewhat insipid and lacked some sort of freshness. It was almost like chanterelles and apricots were too similar (doubly insipid, in a way). Adding blue cheese made a difference, however, cutting through the rather bland, mildly sweet taste. Maybe the "chanterots" lacked acid, or perhaps a little soy sauce? Or was lamb not the perfect match? Pork or turkey might work even better with this one.

A few thoughts on the pairings in this meal
In planning the meal, I browsed through the main ingredients over at the food pairing site. Starting with the main flavour contributors lamb, thyme, rosemary, garlic and apricots. There were no separate scheme for chanterelle, but one might maybe use mushroom, or...? Not surprising, several matches came up, resulting in the inclusion of blue cheese on-the-side as well as choosing carrots as part of the vegetables. I wanted to add chopped almonds as well, but unfortunately I forgot. The resulting food pairing scheme of the ingredients used is shown below (created using the free CmapTools mind map software).

Erik

Non-stick chewing gum

A great article in the last issue of Science in School: "Easily removable chewing gum". Secondly: is there a culinary potential in restaurant/home-made chewing gum?

The article by Halina Stanley in the last issue of the Science in school (free journal) is fun reading, describing why gum sticks and also referring to recent research at Bristol University (UK) on making non-stick chewing gum.

Taking this one step further, it came to me that I've never been served home-made chewing gum at any restaurant. Wouldn't this be an interesting as palate-cleanser, say as an alternative to sorbets, granitas etc.? I googled "make your own chewing gum" and came up with loads of hits. Some of these were ready-made kits (going at $10-20), but even more interesting were some of the more general recipes that allow for leeway in flavour addition. However, I guess a ready-made kit might be a good starting point. It seems that the most commonly available products are based on natural gum (chicle), whereas most commercial brands nowadays use synthetic polymer mixtures to achieve the ideal properties, more on this in the Science in school- article.

Teaching potential
I suppose there are numerous possibilities in teaching polymer chemistry using chewing gum, testing various properties etc. A number of relevant links in the article for experiments and activities.

I'd really like some gum base for Christmas present this year :)

Erik

PS: Science in school is highly recommended reading in general, not only for teachers. On top of all, it's free :)

Green vegetables and chlorophyll revisited

"I am an imbecile! I see only half of the picture!"

...is one of my favourite quotes of Agatha Christie's famous detective, Hercule Poirot. After pondering for months about why the broccoli cooking water turns green when using slightly basic and not when the water is slightly acidic the answer was right beneath my nose all the time, and I felt exactly like beloved Hercule (see the posting "Christmas dinner trimmings - a hot potato? Part two").

The trick to cooking wonderfully green vegetables is using a pinch of baking soda (sodium bicarbonate) in the cooking water. Because the water then is slightly basic, the magnesium ion is retained in the chlorophyll, and the colour is a vivid green, see the above mentioned posting. Deliberately using some acid (vinegar) renders the vegetables dull olive green.

What puzzled me was that the cooking water turns green when the vegetables are the most green, whereas the water is completely colourless when the vegetables are dull. How come? For a long time my hypothesis was that the chlorophyll, or some of its derivatives, is extracted to the water when using baking soda, but not when using vinegar.

Earlier this autumn, during a kitchen lab lesson, it suddenly struck me that the chlorophyll (or a chlorophyll derivative) might be there all the time, but that it's invisible in the acidic water, and that seems indeed to be the case. In fact, it retains it's colour, being green in basic water, and colourless in acidic water (see Martin's comment in the Christmas dinner trimming post).

The ultimate test is to look for chlorophyll colour in the acidic water, and the most straightforward experiment was to add some base to the colourless cooking water, and voila: the water took colour! Rendering the solution acidic again by adding some more vinegar resulted in colourless solution, as seen from the video below.

So, this is an example of chemical reversibility: adding one ingredient (i.e. acid/vinegar) you push the situation one way, adding another (baking soda/ammonium chloride, neutralising the acid), you pull it back to towards the starting point.

What might be learned/taught
In my opinion, this adds some chemistry to the kitchen trick of cooking green vegetables with bicarbonate. Also, it provides a meaningful arena for teaching acid/base equilibria and naturally occurring indicators.

Some details
To be honest, in this case it's slightly more complicated than going straight forward and back, and the colour diminishes in going back and forth. Acid and base is added consecutively, whereas the magnesium ions that are responsible for the colour are constantly diluted. Also, adding acid/base introduces other ingredients (acetate/vinegar and sodium/ammonium ions from the base). Thus it's not an entirely pure back and forth situation. I guess, if I'd added magnesium ions together with the baking soda there should might have been a more distinct colour change . One of these days I'll have to do just that.

Finally, the world is usually more complicated than meets the eye. I might very well have missed a point or two somewhere along the way. But anyway; I'm content with this explanation, and the observation of reversibility adds another dimension to using this experiment with students.

Late addition
When chlorophyll (either structure, a or b) reacts with an acid, pheophytin is formed. This is also coloured, but more olive-green or yellowish , depending on whether it's the a or b form. It might very well be these, or derivatives thereof, that are seen in the water solution. There are loads of scientific publications on chlorophyll, of course. A paper of relevance to science education is found in J. Chem. Ed. (This, Valverde, & Vignolle).

Banana and clove milkshake (TGRWT #11)

Martin at khymos.org has posted another invitation to join in on "They go really well together" (TGRWT). The idea is that foods with one or more flavour compounds in common will taste well in combination; the concept of flavour pairing. This is my first attempt to participate, and the focus is what might be work together with kids (i.e. in a school situation).

My first idea was to keep things very simple, and don't add too many ingredients. That way, the flavour pairing experience might also be easier to perceive and evaluate.

Banana and clove milkshake
200 ml milk
500 ml vanilla ice cream
1 banana
4 or 10 whole cloves, each cut in two (for more efficient extraction)

Simmer milk with cloves for five minutes, cool to room temperature. Remove the cloves, cut banana in pieces. Run milk, banana and ice cream in a blender. Serve.

Result: using 10 cloves gives a milk shake with a marked, but not dominant, clove flavour. Using 3-4 cloves gives a milk shake with just a hint of clove flavour. Hence, the latter left me with the question: "there is something different about this, but I cannot really put my finger on it". Quite fascinating. In my opinion, banana milk shake is on the brink to being insipid. The cloves made a difference, adding another note to the drink. Conclusion: I find the banana-clove combination to be successful.

Chocolate stuffed banana
1 banana
3-4 squares milk chocolate (preferably chopped hazelnut-type)
6-7 whole cloves
optional: ice cream and berries/fruit to serve

Cut each banana lengthways, through the skin, making sure you don't cut all the way through the bottom layer of skin. Distribute the cloves by piercing/inserting them along the length of the banana on both (in)sides. Place chocolate squares in the cut of the banana, wrap in aluminium foil and cook for 15-20 minutes at 225 °C (or on barbecue for ca. 10 min). To serve, unwrap the bananas and place them on serving plates, skin and all.

Result: A typical barbecue or hiking dessert, rather heavy on the sweet side. I thought maybe the cloves would make a difference. The cloves did not cut through the heavy, cloying feeling. However, it added a layer of complexity and variation as the different spoonfuls tasted somewhat differently (one spoon had clove flavour, the other hadn't). Serving together with ice cream and slightly tart berries (i.e. redcurrants) compensates for some of the heaviness.

Finally, adding cloves to banana bread would be an interesting variation of this recipe. I've not tried this, however.

Erik

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