31 Dec 2008

TGRWT #13 roundup: caraway and cocoa

It's time to sum up the entered dishes for They go really well together #13. From the entries, it seems like caraway and cocoa/chocolate indeed do go well together.
However, for starters it should be mentioned that this combination has been discussed on khymos at several instances already. Also, I was made aware of the fact that pumpernickel contains both of these, see discussion and links at Medellitin.

What fascinates me with this month's entries is that you can literally build a whole multi-course meal on caraway and cocoa (in order of submission):

Caraway cocoa chili
by Brian at The Food Geek
[...] the chili turned out great, [...]all the flavors were balanced quite well. I could certainly taste the caraway, [...] and it definitely adds a new note to the chili. Probably some sesame would have rounded it out nicely.

Caraway chocolate cake
by M
[...] mixed 150 gram self raising flour, 1 tablespoon cocoa, 1 teaspoon caraway fruits, 1/4 teaspoon salt, 150 gram brown sugar, 75 gram walnuts, 75 gram pine nuts, 75 gram raisins and 75 gram chopped chocolate. I added 300 gram Turkish yoghurt, mixed everything gentle but thoroughly, spooned the mixture into a silicon cake pan and baked the cake in a preheated oven, 170 degrees Celsius, for about 75 minutes.

Caraway cocoa egg cream
by Rob at The Curious Blogquat
I was able to get the caraway flavor to stick with the chocolate [...] The chocolate is already subdued in an egg cream due to the seltzer, so it was nicely balanced with the caraway. We both enjoyed the drink and found it to be a nice pairing. [...] Interestingly I wouldn't do this combination again, but only because its a bit too "normal" for my liking. It was too good together, and not enough "wow" factor.

Chocolate cookies with caraway
by Martin at Khymos
In the stripped down version with only cocoa, caraway and some bitter lemon peel there you first notice the cocoa, then there is an aftertaste of caraway. I like caraway, so I was very pleased with the result. [...] Anyhow I think the 10:1 ratio of cocoa to caraway worked very fine. [...] With all the other spices present I wasn’t able to single out the caraway flavor, but it (probably) added to the overall complexity. I really recommend the recipe - even if you decide to omit caraway!

Chocolate-caraway mousse/caraway caramel tree surrounded by coconut water gelee pond, white chocolate-caraway-rye bread puree and dehydrated chocolate mousse rock (!)
by Larry at tri-2-cook
I like the flavor combo. It's not a powerful statement and it wasn't particularly easy to keep the caraway at the top of the flavor profile with the chocolate but they really do "go well together". Fun stuff.

Chocolate - caraway - peanut - Kumquat dessert
by Sølve at strezzafoods
I was quite happy with the result. A rich creamy dessert, and the caraway brought a spicy dimension that was both new but also very harmonious. The kumquat added much needed acid and bitterness. All in all a very different and amusing dessert, almost savory with nuts, spice, bitter chocolate and salt.

Beignets with Duck, caraway-chocolate and juniper sauce

by Alessio (alessiofangano [at] msn.com)

For 15 beignets:
15 Beignets
245 g Duck breast
40 g dark chocolate (60%)
1 tsp Caraway
100 g Butter
60 ml Red Wine like Shiraz
5 Juniper berries
½ tbsp Sugar
0.5 g Guar gum

Put the red wine with the juniper berries in a small saucepan and reduce over low heat with the lid on. When reduced of half, add the sugar and let simmer for some minutes. Strain trough a fine sieve and add guar gum while still warm. Brown butter on a skillet over medium fire. Strain and reserve. Grind caraway seeds in a mortar or spice grinder. Melt chocolate, caraway powder and 1 tsp of brown butter over a bain marie. Pour it on a shallow container so that the chocolate layer will be around 4mm thick. Let cool down at room temperature, unmold and cut into cubes. Refrigerate until ready to use. Take away the skin from the duck breast. Cut the meat in small dices. Render the duck skin in a warm skillet under a weight, drained the excess fat from time to time. Put the brown butter on a pan and when well warm, sauté rapidly the duck dices. Mix duck and chocolate dices in a bowl. Open in two the beignets, put in the lower part some of the duck chocolate mixture. Spoon over a teaspoon of the red wine sauce. Grind over some black pepper and close with the upper part and serve.

Verdict: When eating the beignet, the first aroma to be perceived is that of the juniper. The crunchiness of the beignet is followed by the juiciness of the duck meat and the barely melting chocolate with its caraway notes. In the whole the dish works very well, the tricky part is in the lightness of the beignet. The flavors come out very harmoniously and fulfilling. Chocolate and caraway are a perfect combination especially with red meat or game.

Chocolate sauerkraut
by Ole Eivind at Helt naturlig
As the sauce coated the cabbage shreds, the initial taste was very chocolatey with a nice caraway aftertaste. I could clearly taste that the two ingredients go really well together. The texture wasn't too bad either, but I wouldn't call the dish a success. The cabbage itself didn't work well, and for a dish consisting of little more, having the cabbage work against you can't be described as much other than a failure. It was an interesting failure however [...]

Almond-caraway coated chocolate truffles
by yours truly at fooducation
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. [...] the chocolate and caraway work very well together.

Some comments on caraway aroma and flavour pairing with cocoa
Interestingly (at least to those of us with chemistry-oriented brains), the compound responsible for the intrinsic caraway aroma is (S)-carvone. The other stereoisomer, (R)-carvone, has a spearmint aroma. A striking example of the effect of chirality/handedness in chemistry: the two compounds are identical apart from the fact that they're mirror images, resulting in very different aromas.

(S)-carvone is also an important aroma component in dill seeds, and caraway is suggested to function as substitute for dill aroma (R. Zawirska-Wojtasiak: "Oils obtained from caraway and dill seeds are almost identical in composition").

(S)-carvone (left) has caraway aroma whereas (R)-carvone on the right smells of spearmint

Aroma components in caraway
Surprisingly, I could not find any literature reports on aroma components in caraway, but The Good Scents Company has listed aroma components for both caraway and chocolate/cocoa:

Aroma components in caraway
cuminyl alcohol
isobutyl angelate

Also, Flavornet has information on flavour components in caraway, only including carvone and carveol isomers, though. Compared to chocolate and cocoa, caraway seems to be a relatively uncomplicated mixture of aromatic substances. Is this possibly due to roasting of the cocoa beans resulting in very complex mixtures of aroma substances in these (from Maillard reactions etc.)?

I was quite surprised when I didn't find any direct matches between caraway and cocoa/chocolate. Maybe are any common substances in low concentrations? A caraway+cocoa google cross-search at The good Scents company returns a lot of hits, but none(?) with natural occurrence in both ingredients. The matches are mostly in the "odor and/or flavor blends with" category. This is of course uplifting, but gives no definite answers in terms of common components. If anyone can come up with such matches, I'd be happy to have a comment.

Finally, I can't resist taking a somewhat amusing detour by the 1927 Nature article Fluctuations in Affective Reactions to the Odour of Caraway Oil by J. H. Kenneth:

"Fluctuations in the affective reactions to caraway oil claimed attention [...the preference being] higher after lunch than that before. In the case of camphor, rosemary oil, menthol, sassafras oil, fennel oil, and a few other odours, no consistent fluctuations of this nature wore [sic] observed."

So, according to Nature, it's not indifferent what time of day you have your caraway. The paper also includes a plot of preference of caraway odour vs. time of day and moon phases :)

29 Dec 2008

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.

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

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.

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.

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

27 Dec 2008

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.

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

20 Dec 2008

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.

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.*

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


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).

8 Dec 2008

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

4 Dec 2008

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.

24 Nov 2008

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).

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.

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).


6 Nov 2008

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 :)


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

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.


4 Aug 2008

Deciphering an old preserves recipe

Teaching food preservation methods combined with science often results in the classical why-questions; why does the recipe tell us to do things this way, and why is that so important (...and is it really that important?)

Autumn means ripe fruit, berries and vegetables, and many of us look forward to harvesting for the coming year. Only a few areas of the world have the benefit of continuous supply of fruit and vegetables (especially we up here in the north), and this has lead to loads of ways to preserve food. Nowadays, however, many of these methods are used for culinary purposes rather than survival. Recently, Martin wrote about cherry jam (I have given a few on his post as well). Also, Hervé This has written about culinary proverbs and old wives tales, "culinary precisions" as he terms them, some being sound advice from a scientific viewpoint, some being directly misleading, and others probably not making that a big difference.

So, I thought this might be a good time to take a recipe for sugar-preserved pears and have a closer look at it, step by step. It is in fact a quite fun exercise.

Traditional sugar-preserved pears «the old fashioned way» (generic recipe)
  1. Heat jar in oven or water by slowly increasing the temperature to above 100 °C and keeping it at that temperature for a certain amount of time (i.e. 15 minutes). Cool.
  2. Peel pears. Cut in half and remove seeds and core
  3. Make syrup from water and sugar by boiling the syrup until the sugar is dissolved. Leave to cool
  4. Fill a jar with the pears. Fill up with syrup, cold or lukewarm, but not hot
  5. Put the lid loosely on, do not tighten
  6. Place jar in a pot with cold water almost up to the rim of the jar. Heat slowly and keep at boiling point for a certain amount of time (i.e. 8 minutes)
  7. Remove jar from pot and
    Either: leave to cool for a certain amount of time (i.e. 15 minutes). Then tighten the lid
    Or: Tighten the lid while boiling hot
  8. Turn jar upside down and
    Either: store upside down
    Or: leave upside down for a certain amount of time (i.e. 15 minutes)
Taking a closer look
Firstly, the main reason for all these operations is to keep the pears edible for a long time, more specific until the next time ripe pears are at hand (a year, usually...). The two main ways that fresh fruit is spoilt are chemical (enzymes and reaction with oxygen from the air) and biological (microorganisms: bacteria, moulds and yeasts). Even though the methods were developed before the discovery of microorganisms and enzymes, the results of these were evidently clear. The source of enzymes is the fruit itself, whereas microorganisms are ubiquitous: the fruit itself, hands, tools, jar and in the air. So a perfectly sterile fruit would over some time be contaminated by just sitting in the air.
Two important facts: microorganisms thrive and multiply at temperatures between 10 and 40 °C, and die at high temperatures. The trick is thus to avoid the 10-40 °C window. Also, microorganisms need water to thrive and multiply.

So, how much of this procedure makes sense from a scientific point of view? I've marked the steps being most «fishy» with a red asterisk:
  1. Heat jar and lid in oven or water by slowly increasing the temperature to above 100 °C and keeping it at that temperature for a certain amount of time (i.e. 15 minutes)
    Makes sense, but cooling leaves the jar and lid ready for infection. However, they're dry and not prone to being infested

  2. Peel pears. Cut in half and remove seeds and core
    No effect other than possibly infecting the pears from hands, tools and surrounding air

  3. Make syrup from water and sugar by boiling the syrup until the sugar is dissolved. Leave to cool
    Sugar has a preserving function due to its dehydrating effect on microorganisms (a later post will deal with this). Sugar won't kill microorganisms, but inhibit growth. If the syrup is thoroughly boiled, microorganisms in the water and sugar are killed. Cooling the syrup is not recommended if it can be avoided (10-40 °C window)

  4. Fill a cold jar with the pears. Fill up with cold or lukewarm, but not hot, syrup
    Warning: we are in the 10-40 °C window. From a microbiological view, the best would be to add hot/boiling syrup. However, this might damage the fruit, such as turning the surface mushy (in the case of plums, the skin would most likely break).

  5. Put the lid loosely on, do not tighten
    Makes sense. During heating, the contents expand and trapped and dissolved air is expelled (gas solubility is lower at higher temperature). The air above the fruit expands when heated and needs to go somewhere

  6. * Place jar in a pot with cold water almost up to the rim of the jar. Heat slowly and keep at boiling point for a certain amount of time (i.e. 8 minutes)
    We are in the 10-40 °C window for quite some time during slow heating. The historic reason is most likely that old glass types had tensions/stress that would result in the jar cracking from shock heating or cooling. Modern glass production methods solve this by i.e. annealing. My experiences with ordinary jam jars is that they manage shock heating and cooling quite well. If the fruit survives, rapid heating is preferable.

  7. * Remove jar from pot and either leave to cool for a certain amount of time (i.e. 15 minutes). Then tighten the lid, or tighten the lid while boiling hot Leaving the jar open at this point is certainly not a good idea. Cooling results in the air above the fruit contracting, sucking in air from the surroundings (see note). Even though microorganisms might die when entering, thermally stable spores might enter which might develop at a later point. Tightening the lid right away is by far preferable. If you use modern jars with aluminium lids that pop down due to reduced pressure inside the jar, this should happen when the jar cools (if you buy a jar of jam and the lid doesn't pop when you open it, return it and get another). If the lid pops up before or during storage, consume immediately («oh, what a disappointment» ;)).

  8. Turn jar upside down and either store upside down or leave upside down for a certain amount of time (i.e. 15 minutes)
    Makes sense. Getting the lid sterile is always a difficult task. Turning the jar lets the hot contents come in contact with the lid, sterilising it getting rid of a majority of the microorganisms present. One might speculate whether keeping the lid seals moist might also be a reason (why wine is stored lying), but a biologist friend of mine meant that the atmosphere above the fruit would result in a moist enough atmosphere for that purpose, and that the sterilising effect of killing microorganisms is the point here. If that is true, it should not make a difference which way the jars are stored.

Remember that operations conducted in a kitchen are far from sterile procedures. For this reason, many such recipes rightfully ask for two and sometimes three actions with apparently the same purpose.

Finally, while hard cheeses with unwanted mould often are safe to eat if the mould is removed by cutting away a layer of the cheese, stored preserves such as jams, sugar preserved fruit and syrups should be discarded if the seals are broken or visible mould is seen. In the cheese, the microorganisms cannot travel due to the dry and solid structure, whereas diffusion is jams etc. occurs rather easily and the whole jar might very well be contaminated even though the visible mould is removed.

What might be learned/taught
Loads of microbiology. However, since I'm primarily a chemist, I won't venture too far into this. To me, taken that microorganisms are everywhere and that they die when they are heated, the most important things to teach would be (again, from the top of my head):
  • temperature-volume relationships of gases and gas-liquid relationships (popping lids, see note below)
  • critical thinking, don't always believe what you read
  • working systematically, always questioning why should I do this? (see Five cardinal rules in cooking)
In fact, this experiment/recipe might be among the few cases where one can test and predict something rather complicated based on a very limited amount of knowledge, but are often stated as among the most important treats in enquiry based science teaching (quite a paradox, really). Such situations are scarce, and I'm thrilled every time I stumble upon one.


Note: Heat expanding air is easily illustrated by putting a blown-up balloon into the freezer; it contracts. Take it out, and it expands back to (almost, at least) original size. However, when the jar contents are boiling, the headspace will in fact be filled by mostly steam from the preserves. When this water condenses (in the closed jar), the volume of the steam (now liquid water) is reduced by a factor of ca. 1300(!), resulting in a considerably lowered pressure.

Addendum (6. August)
some statements about sterilisation are modified as common canning does not result in sterile product (hence the need for preservatives such as sugar, salt, acid/vinegar etc.). This way of canning/preserving more resembles high pasteurization which is common in some milk products.

30 Jul 2008

Can dry ice be food?

During our summer holiday in the United States this summer, I found over-the-counter dry ice in the supermarket. A truly marvelous thing! This opens up a host of possibilities, both in the home kitchen and as a science teacher.

Dry ice is in fact frozen, solid carbon dioxide (CO2), the «carbonated» part of carbonated drinks. One fascinating thing about CO2 is that it sublimes rather than melts – solid CO2 does not melt to give liquid CO2, but goes directly over to become gas. Hence, the argument of the producer that dry ice can be used as coolant without getting everything wet. Further, the freezing point of CO2 is -78 °C (-108 °F), and since this is far colder than that of water/ice the cooling power is larger, but you might get frost burns, so be careful (see precautions below).

The producer gives a number of suggestions for how to use dry ice, such as:
  • Cooling: no wet ice – no water, but note that the low temperature results in water condensing on the outside of the container, especially in humid weather. Stays cold longer than water-based ice
  • Freezing: ordinary freezing or flash-freezing (very low temperature gives rapid freezing. Stays frozen longer than water-based ice
  • Make carbonated beverages: add dry ice into still beverages to get a fizzy drink (they have a recipe for home made root beer on their web site)
  • Special effects – fog or witches brew-like effects
The product
The dry ice from this producer comes in solid blocks packed in plain plastic bags. You need to bring some insulated box or bag. Size will obviously vary since the dry ice is stored in insulated containers (using freezer is not of any great help since the ice is far colder than the freezer). Hence, the bag is weighed upon purchase, and the older the dry ice, the greater loss from the bag. This is no problem, as expiry date is no question (as long as it is kept pure and away from other products). As it is meant for use in carbonated drinks, it must obviously be pure enough to ingest (not in solid form however, see below).

My experiences
  • Dry ice in blocks is quite hard, but brittle. Use a short and hard knife (I used an oyster knife), a screwdriver or similar to chop off pieces. It should also be possible to shave off to get a more powdery dry ice.
  • Dry ice curling: chop off a flat piece and leave it on a table or the floor. After a short while it will start floating with almost no friction against the surface, and you can play around with it. Use gloves or touch is only very briefly

Carbonated drinks (the dry ice must be completely dissolved before drinking)
  • Using dry ice for carbonating drinks results in a quite different effect from using SodaStream-type carbonated drinks. One tablespoon of dry ice in a large glass gave a slight tingling sensation on the tongue. Using too much, results in a somewhat bitter taste. Using shavings is better than large chunks, as the latter take a long time to dissolve. The dry ice was either added and stirred with a spoon (i.e. juice) or added after the liquids in the blender for icy drinks, smoothies etc.
  • I tried making icy drinks (slush) using larger amounts of dry ice, but did not succeed. My approach was to crush dry ice together with ordinary ice. This resulted in a layer of rock-hard ice in the bottom of the blender. In addition, I guess this might damage the plastic container of the blender due to over-cooling.
  • Dry ice might very well be combined with ordinary ice. In crushed-ice drinks, add the dry ice after the liquid contents. When using whole ice cubes, mix dry ice with the liquid and add ice cubes. In all cases, stir or blend well after adding the dry ice.
  • Previously, I've tried using a CO2-charged dessert whip for making carbonated apple juice. Squirting the juice out of the nozzle gave a carbonated juice quite similar to the dry ice version (much of the CO2 bubbles out in the process). Martin’s carbonated fruit seem to be quite different, but that may simply be due to the longer waiting time. Obviously, the dessert whip is not meant to make carbonated drinks as there are dedicated soda siphons on the market. Note that Martin also have a few posts that mention the use of dry ice.
  • The cooling effect of dry ice is low, slow and very local when added to a liquid. In fact, water ice seems much more effective for cooling drinks. This is probably due to the escaping CO2 gas whereas water ice adds cold water to the mixture as it melts.
Tested recipes:
  • Dry iced ice tea: works ok, but the bitterness in the tea might be enhanced by the dry ice
  • Carbonated juices (pina colada mix, cherry mix): works ok
  • Smoothies: I tried a strawberry/banana/coconut drink («Coconut Dream») below and it was good with a slight tingling sensation on the tongue

What might be learned (or rather, taught)
There are lots and lots that might be learned in connection with dry ice. A few topics from the top of my head:
  • Gases, liquids and solids
  • Freezing, melting and evaporation/boiling
  • Friction and force (dry ice curling)
  • Acid-base equilibria (see also "naked egg"-resource which is relevant, CO2 is produced)
  • Solubility of gases in liquids
  • Questions about CO2 itself
See also the Q/A section below.


Main point: dry ice is harmless if you avoid ingesting it in frozen state and if you’re careful with skin/eye contact.

Ingestion of solid dry ice may result in severe frost damage due to the low temperature of the dry ice. The same might happen from direct skin contact, but it may be handled if small pieces are thrown back and forth between the hands, minimising the contact time. Textile gloves work well. Upon sublimation the
CO2 increases its volume by 540 times. Ingesting a tea spoon of dry ice would then produce 2.7 l gas in the stomach, which might result in internal injury. Dry ice might damage certain plastic items, varnished material etc., due to the low temperature.


In my vocabulary dry ice might very well fall into the category of food, but this depends on how it is used. Use it as a cooland or reagent in a lab, and it is a chemical. Use it for a somewhat more tingling smoothie, and it’s food.

Finally, I have only explored a few possibilities. I expect that many others are more proficient in finding even more tempting culinary applications than these simple icy drinks and smoothies (as have already been suggested in Martin's and other blogs, follow links and comments at his relevant entries mentioned above).


A few questions and answers
Q: What is the «smoke» coming from dry ice, either pure or in drinks?
A: The fog is not dry ice, but condensed or frozen water from the air (or the drink). The air above the dry ice is cooled to an extent that water in the air condenses or freezes; miniature snow crystals.

Q: Why is expiry date not
a relevant issue?
CO2 is among the most stable compounds around. Hence, it will not deteriorate apart from sublime («disappear»). However, if other products that might give off odour etc. are in the vicinity, these might condense on the surface and the dry ice is increasingly contaminated.

Q: Why do pieces of dry ice float freely on hard surfaces?
A: In contact with a warmer surface, gaseous CO2 is liberated creating a «gas cushion» between the table and dry ice. The same effect being used in hydrofoil boats and what you see if you spill drops of water onto a hot cooking range. As long as gas is produced in the interface, this «gas cushion» results in almost no friction.

Q: Does dry ice float or sink?
A: In water (or other beverages), dry ice will float due to the porous, and thus gas-filled, structure.
CO2, however, is more dense than air and sinks in air (the CO2 will escape only slowly in a glass filled with CO2 gas if it is not stirred)

Q: Is it possible for private persons to get dry ice in Norway?
A: Yes, from producers AGA (retailers, choose «industry dealer» when prompted) and Yara Praxair (retailers) through their network of retailers. However, it seems to be quite expensive, at least compared to the $0.99/lb in US supermarkets.