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Feed Tech
Wednesday, April 11, 2007 3:32:23 PM
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Evaluation of analytical methods for analysis of dried distillers grains with solubles

American Feed Industry Association



Within the fuel ethanol industry, there are no guidelines or recommendations on which analytical test methods should be used for the measurement of dried distillers grains with solubles (DDGS), which can lead to much confusion over analysis and subsequent interpretation of data for moisture, protein, fat, and fibre, all of which are critical feed quality parameters for DDGS.


Most wet chemistry methods currently used for analysing DDGS in the analytical community are what would be classified as empirical methods, meaning the results are an indirect measurement of the analyte of interest and the results are in part or in whole dependent on the conditions of the assay (reagent type or concentration and assay parameters such as temperature, time and pH).


Since the analytical community has not yet come to a consensus on what empirical method is best suited for the analysis of any given analyte in DDGS, many different empirical methods are used among laboratories and even within a single laboratory.


The use of various empirical methods for a single analyte leads to results that vary significantly from lab to lab and thereby can create confusion for producers, marketers, nutritionists, regulatory bodies, and most importantly the customers/end-users.


This problem was identified by the ethanol industry and strategically addressed in the autumn of 2005. Two working group bodies were formed to collectively address the problem and cooperatively design a study which would lead to recommendations on the most applicable test methods for DDGS.


The study was designed to evaluate the efficacy, applicability, intra- laboratory variation, and inter- laboratory variation of the most commonly used test methods in the analytical community for analysing moisture/loss on drying, crude protein, crude fat and crude fibre.


Although it is widely accepted that Karl Fischer Titration provides the most accurate measurement of water in feed, the labour, reagent and instrument costs make Karl Fischer analysis an economic burden that most laboratories would not be willing to bear.


The researchers recognised these concerns and used Karl Fischer as the means of determining the gravimetric (loss on drying) method that has the least amount of bias when compared with the actual Karl Fischer method.


Using this criteria, NFTA, Lab Dry Matter (105 deg C / 3 hours), was selected as the recommended method for analysing moisture in DDGS; this method also had acceptable CV's (coefficient of variation--a method of determining repeatability) in both the intra- and inter- laboratory portions of the study.


However, all gravimetric methods should be considered and used accordingly, as "loss on drying" methods can only serve as an estimation of the "true" moisture level.


The protein methods investigated in this study were determined to be statistically equivalent, and both had acceptable coefficients of variation for both intra- and inter- laboratory portions of the study.


Three non-hydrolysis fat methods were also determined to be statistically equivalent methods for analysing DDGS.


The acid hydrolysis method was determined to be significantly different, with a bias of about +4 percent (absolute difference). It should be noted that only relative accuracy was compared and since all four methods in the investigation are empirical in nature, further study is needed to determine the most accurate method.


For more of the article, please click here.


Article made possible through the contribution of American Feed Industry Association.

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