Hi,
I am getting elevated results in my starch analysis. It is about 2-3 % higher than usual. We have not changed anything. We are getting the same results over and over. I have checked the 0.309M HCL, and it is perfect. Does anyone know why it sometimes reads higher?
When we put it into a boiling shaking water bath, we agitate it constantly. I have even tried not agitating it and I get the same values.
I have made the acid up a few times and checked it and it is fine.
The water baths temp is correct.
We filter it using 5-8um filter paper as we have in the past.
Thank you.
Kind regards,
Rob
Rob,
Are you correcting for moisture level of the starch? Starch can gain or lose moisture throughout the year following changes in the relative humidity of the environment. Do you have a control starch for comparison? I am not specifically knowledgeable nor have experience with the specific method you are using.
Hi Joseph thanks for your time. Yes, we do a moisture correction. I have re-analysed the sample for moisture, and although it's changed slightly over time, it doesn't change the values to the degree I need.
I did try halving the concentration and it worked perfectly on two different samples. I did this as my old company used to have their own method and they used half the concentration of acid but heated it for 30 minutes and not 15. I just changed the acid concentration and not the time.
However, this just brings more questions as to why such a drastic change (on concentration) gives us the correct result when it was working fine before the change.
Thanks again for your reply.
According to the protocol for ISO 10520, repeatability and reproducibility limits are 1.0-2.2 % and 3.5-8.2 % respectively, for 4 different starches. So, if different operators have been working on this analysis you are still within acceptable limits.
On the other hand it appears that you are dealing with a sudden systematic increase of 2-3 % with no obvious cause. In that case I recommend to include in your tests 1 or 2 pure reference starches (assume a starch content of 99 % for these references to account for the presence of small amounts of protein, lipids and ash) and to correct your experimental results by means of these references.
From the allowed reproducibility limits it appears that the Ewers method is not very accurate. I suggest to switch over to an enzymatic method, either that of K. Brunt et al. 1998 or a testkit by Megazyme (attached). Please note that the Megazyme assay does not include the removal of soluble sugars by 40 % ethanol washing, so this treatment has to be performed prior to the actual test.
Thanks Peter. I was also in the process of recommending the Megazyme products. We've had great, reproduceable results using their offerings. Costs are about 3 euro per assay when purchasing kits containing 100.
Additional assay. You should review to determine which one best serves your needs.
Hi Peter and Joseph,
Thank you very much for the detailed replies and time you have both given me.
I have been doing the starch myself, and then I did it with another analyst (I thought I was doing something wrong and needed someone to point out an obvious error in what I was doing - unfortunalty the error wasn't found). The repeatability and reproducibilty percentages are high. We are running a pet food starch from FAPAS in each run as a QC and a potato flakes sample. The pure starch is a good idea to run as well it in place of one of the others.
However, the megazyme method might be the best option moving forward. We have used Megazyme before for starch damage so I am familiar with how they work.
I will order a kit in next week and give it a go.
To you both, thank you again. Your help and expertise are very much appreciated.
Kind regards,
Robert
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