How soluble is soluble starch?

Peter Sigsworth

COMMITTEE
Just coming to the end of my 'soluble starch' which I make up as 2g dissolved in 100ml distilled water heated to produce a clear liquid.
I use this in amylase practicals and it gives a clear (in both meanings of the word) complete end result.

Made up some 2% starch today using the same recipe but with a new bottle of 'soluble starch' - wouldn't clear fully at all when heated - so although it cleared a bit it remained cloudy.

Did a test run with - usual concentrations - 5ml 0.1% Pancreatin added to 5ml if the new starch solution. Samples every 30s with iodine solution.

It took 4 minutes for the starch to start to disappear but then (unlike my old stuff) it never completely stopped testing positive. I could see small dots of starch which produced a slight positive result which went on for 20 minutes - so not as clear end result as there was with the old stuff. I added the iodine into the reacting tube and it produced a very dark brown solution ie a positive result all be it not completely blue/black. So still starch present after 30 minutes.

I can only explain this by questioning the purity of the new 'soluble starch' - Because it will not clear there must be a proportion of the starch which is not soluble, I guess must have some longer chain starch molecules which stay suspended (making it cloudy) and take much longer to break down.

Does anyone know where I can get proper 'soluble starch' from. What supplier do you use that give you go clear end results.
 

CovTech

COMMITTEE
You might get better results making the solution the other way around - that is to say add boiled water to the starch rather than heating it afterwards
We usually make a very stodgy paste with a few drops of cold water, add half volume of boiling water and then make up to full volume with cold

Never had an issue
 
I bought soluble starch from SLS not that long ago - works fine. It was the amylase that caused me issues until I switched to liquid a few years back. Always add the starch to boiling distilled water (or vise versa) and put it on the stirrer for 5 mins.
 

Peter Sigsworth

COMMITTEE
I bought soluble starch from SLS not that long ago - works fine. It was the amylase that caused me issues until I switched to liquid a few years back. Always add the starch to boiling distilled water (or vise versa) and put it on the stirrer for 5 mins.
I'm pretty sure it's the starch because, as all good scientists do, everything is the same except the starch so any difference must be due to the one variable you changed.

I don't have any difficulty with dissolving the old stuff so I am doing it right.
I'll have a play and post some images tomorrow
 

clairelucas

COMMITTEE
You might get better results making the solution the other way around - that is to say add boiled water to the starch rather than heating it afterwards
We usually make a very stodgy paste with a few drops of cold water, add half volume of boiling water and then make up to full volume with cold

Never had an issue
Which is CLEAPSS way of doing it- Recipe book 49
 

Peter Sigsworth

COMMITTEE
OK folks

This is it with the two batches new on left old on right
1605800008030.png
Here they are after bringing to boil on stirrer
1605800061250.png

Clearly a difference the new one says extra pure on it. I even continued to boil the new one for several minutes but it didn't clear.
The new one is SciChem exp JUl 2024 the old one is Timstar, about 5 years old at least
 
Well its slightly strange it says soluble on the bottle! I'm slightly interested it says Ex Potato on the bottle - I had a bottle labelled Potato Starch a while ago - but it definitely did not say anything about it being soluble. Perhaps that's the clue. Very strange.
 
I've always made it by putting a beaker on hotplate to heat with about 2/3 the amount of water allow it to start to boil. Make a paste with (usually 1g starch). Add to boiled water ASAP but remove from hotplate or it goes brownish. Cool make up to amount (usually 100ml).

Not seen the potato part on bottle before.
 
I've always made it by putting a beaker on hotplate to heat with about 2/3 the amount of water allow it to start to boil. Make a paste with (usually 1g starch). Add to boiled water ASAP but remove from hotplate or it goes brownish. Cool make up to amount (usually 100ml).

Not seen the potato part on bottle before.
Looks like Peter's method is sound - its SciChem that seems to have gone a bit wonky donkey by the looks of it!
 

Peter Sigsworth

COMMITTEE
Ok I seem to have sorted it - or at least made it usable - I boiled it for at least 5 minutes and that seemed to clear it to a level I can cope with. SciChem have just started with a new supplier LOBA CHEMIE PVT LTD their containers are covered in phrases like 'Extra Pure' and 'For Reasearch and Development' - that worries me as these are not standard terms for levels of purity.
 
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