The last issue of the Norwegian science education magazine "Naturfag" (equivalent to "School science") has several articles on food previously posted here on fooducation. The magazine is in Norwegian and free for download.
Issue 2/2009 with mostly Christmas- and winter related content includes the following articles based on fooducation posts. Most of them are updated/revised versions and are also found as updated versions on the Norwegian Centre for Science Education "gastronomic school science" web pages www.naturfag.no/mat (Google translation here):
The effect of added sugar, salt and high temperature on microorganisms/yeast. This is not previously published on fooducation, but a time lapse video with captions in Norwegian can be seen on YouTube. It is self-explanatory, i guess. The purpose is to show an easy to set up experiment for testing conditions under which microorganisms thrive or die. Relevance is to baking (you want to promote the yeast) and preservation (you want to suppress or kill microorganisms)
The latter also made it into the news section of "Nysgjerrigper", a science knowledge project from the Norwegian Research Council.
Of course there are several other interesting topics in the issue as well, such as "Gingerbread house architecture" "Catch sight of and predict the northern light" and more.
"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).
Many a Christmas dinner, we end up with the potatoes falling apart in the dish and pale olive-green Brussel sprouts. Does it have to be like this? Using a little scientific knowledge in the kitchen can help. Part two - green vegetables Brussels sprouts and the broccoli: Do you prefer a fresh, vivid green colour, or a dull olive green? The colour in green vegetables is due to chlorophyll, which is a compound well suited to play around with. The green colour in chlorophyll is due to a magnesium atom (in fact, an ion) attached to a porphyrin ring, and acid can substitute this magnesium altering the colour. Try adding a little lemon juice or vinegar to the water next time you cook green vegetables if you want to do a “sabotage experiment” just to see what you may want to avoid. This kind of sabotage experiments are, in my opinion, just as important as the “successful” ones.
Chlorophyll molecular structure at pH = 7 (neutral/basic) to the left, and pH<7
Fruit and vegetables contain a little acid, so if we use pure water or steam the, this acid is in fact sufficient to alter the colour in a negative way. As a remedy, try adding a couple of teaspoons of (sodium) bicarbonate/natron per litre of water. This makes the water slightly basic. The water will turn green as well, but there is more than enough chlorophyl left for the vegetables. Short cooking times is also recommended, as chemical reactions take time, and the replacement of magnesium is no exception. This is probably the reason that the colour change is more visible in Brussels sprouts than broccoli, the sprouts cook longer and thus more of the chlorophyll is degraded.
Left: cooked with a little bicarbonate (pH ca. 9), right with a little lemon juice or vinegar (pH ca. 4.5)
So, in the two posts conclusion: treat the potatoes and vegetables the opposite way.
Happy New Year
Erik
Background info: McGee, H. (2004): McGee on Food and Cooking – An Encyclopedia of Kitchen Science, History and Culture. London: Hodder and Stoughton. Belitz, Grosch og Schieberle (2004): Food Chemistry (3. utg.). Berlin: Springer.
PS: have any idea why the water turns green on adding bicarbonate? Please let me know.
The last issue of the Norwegian science education magazine "Naturfag" (equivalent to "School science") has several articles on food previously posted here on fooducation. The magazine is in Norwegian and free for download.
