Dear Charles, please see the red reply. thank you very much.
Thks data. Some first thoughts –
(1) I am not familiar with this product but IMEX with other natural items, it is normal to expect substantial variations in plate count results, eg up to 50% or more, due to natural variations within the sample and the often unavoidable limitations of the analysis procedure. The difference in yr results is however much more than usual of course.
(2) Note that sample size in method (b) may simply be too small for reliable results. I don’t know since I have always used standard method, eg BAM.
(3) Assuming the results are in units of cfu/gram and plate inoculation was 1ml, your calculations for plate count seem incorrect. Or perhaps you are not using pour plates ?
I think the results should be 30,000 cfu/gm and 500 cfu/gm respectively. Typing error ?.
(4) I guess no duplicate plates were used. This step is always highly recommended.
(5) data in method (a) looks unreliable since assuming plate of 10(-3) was read correctly, the result for plate 10(-2) should be approx. 300 count which is normally not close to tntc. This is one reason for including duplicate plates.
(6) Counts in method (b) very low for accurate result (but maybe this result well below any maximum acceptable value so you don't care ?). Even more reason to use duplicate plates if want accurate result. I hope there was no problem to read plate since IMEX, food particles can sometimes interfere at this dilution. IMO, even if procedure is proven to be working correctly, should use a larger sample to get higher counts if you are unsure of the normal bacterial level. If you want a quick, approximate method for internal use and you can validate that it is working properly, maybe useful.Comments
(A) The result from method (b) looks very low unless there is a processing step involved to lower bacterial levels. What is previous experience this product, eg typical maximum levels of total plate count. [I wondered if your reason for using 5g sample was because previous measurements with 25g always gave results in 30,000 range up so you were trying to reduce your workload by using smaller sample? ] [some people use a reduced inoculum, eg 0.5ml, for similar purpose although this can give problems also IMEX].
(B) The result from method (a) is inconclusive due mis-match between data for dilutions 10(-2) and 10(-3). IMO duplicate plates are necessary and include dilution 10(-4) for more confidence in results while you are checking procedure.
© My suggestion is for both operators to repeat the test with duplicate plates and try a standard method like BAM using 25g sample. And include dilution 10(-4). And maybe consider sending duplicate sample to a private lab if you cannot agree. Of course, if the standard is max 100,000 cfu /gram, maybe you don’t care.
IMEX, the plate count procedure can be very sensitive to method, homogenisation method, agar media / preparation, and individual operators technique. I have previously used many non-standard methods but often had customer problems, especially when my results did not match 3rd party labs. Eventually I went back to using more standard procedures which mainly solved the problem but usually take longer time.
Hope the above is useful.
Interested to hear your feedback.
Rgds / Charles.C