27 Oct 2008

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).

9 Oct 2008

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.


6 Oct 2008

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)

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.
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.