|MadSci Network: Cell Biology|
Mark 2 version, with a few typos removed! Hope all's well it's f..ing freezing here and has been for the past 2 weeks - NOT a good time to break your woodburning stove (don't ask, it's too embarassing!). Despite LOADS (for us) of snow in SE England, we have had only the odd flake here, so College continues as usual and no sledging :( The pigment in beetroot is called betalain, and, like most plant pigments, it is an indicator. We think of litmus paper which changes from red to blue at about pH 7.0, but litmus is aactually a plant pigment! Other pigments change colour at different pH's - so there are plant dyes which can be used to indicate a whole range of pH's. Dark red/purple pigments are common (blackberry, elderberry, red wine, red cabbage) and all lose their 'blueness' as the pH rises (i.e. they become pink or green). For that reason, most cooks are instructed to add lemon juice or vinegar when cooking such vegetables, to ensure that the dark red pigment remains. (Similarly, green vegetables, such as peas and beans, stay greener in alkaline solution (add sodium bicarbonate or baking powder); however, this destroys much of the Vitamin C, so that piece of 'advice' is usually ommitted from modern cookery books, in favour of shorter cooking times! Sadly, undercooked veg may look and taste nicer, but 'crunchy' veg is harder to digest, so most of the extra vitamins are not absorbed....... To return to your question - as you change the pH, the H-bonds in the proteins in the cell membranes change (amino-acids are Zwitterions, so act as both acid and base, opposing any external pH change). In short, proteins are buffers. Since membranes are up to 70% protein, changing the pH GREATLY from the cell's natural pH will cause these proteins to GREATLY change their shape. This causes 'holes' to appear in the membrane, allowing the betalain to leak out - hence the colour of the external solution changes. In summary, you have TWO factors here. 1. Changing the pH (generally, much more alkaline) will make more dye leak out 2. Changes in pH will cause the colour of the dye to change too - particularly above the critical value (in litmus, pH 7.0; for betalain there are 3 colours - red (acid); dark red (neutral) and purple (basic) "Betalains are unaffected by pH in the range 3.5 to 7.0 (acid to neutral). Beetroot extracts in most foods will therefore not discolour as a direct result of pH. The optimum pH for both betacyanin and betaxanthin pigments occurs in the slightly acidic 5.0-6.0 range. The colour of red beetroot extract changes from red towards blue as pH increases above 7.0. Root tissue exposed to high or alkaline pH (7.5-8.5) becomes discoloured. Cut beetroot retains its purple-red colour well in acidic solutions such as malt vinegar (acetic acid)". In short, you should ensure that the pH of the EXTRACTED solutions are the same BEFORE you measure their intensity (normally with a colorimeter set to 540nm - green) NOTE: A condition known as 'beeturia' occurs when your 'pee' turns red/purple after eating beetroot. A fairly consistent 12-14% (i.e. 1 in 7) of the population seem to show this - BUT it is not interited and some, unknown, environmental factor is at work here (identical twins show no correlation). In some parts of Hungary 40 years ago, studies found 100% beeturia - so something strange was happening in their local environment! Betalain is unstable at high tempertaures, but it is harmless, so is often added as a food dye in cold foods like yoghurt and ice-lollies etc which claim to be 'blackcurrant' or 'plum' flavoured! Hope that little lot helps! ian
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