MadSci Network: Cell Biology

Re: How does the number of beetroot cells damaged depend on pH?

Date: Thu Mar 3 07:54:52 2005
Posted By: Ian WHITE, Secondary School Teacher, Biology 11-19, Godalming College
Area of science: Cell Biology
ID: 1109697969.Cb

Mark 2 version, with a few typos removed!  Hope all's well it's 
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!

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