This statement should be used in every training session for food testing laboratory staff. Sample preparation is the most important laboratory activity of all. Yet whenever it is necessary to investigate anomalous results, all too often the cause is simply poor sample preparation.
Generally speaking, poorly homogenised sample material will affect analytical data in two ways. Either the test result will be directly affected because a non-representative test portion has been taken, or the test material may interfere with the analytical process and thereby give rise to an anomalous result. In food analysis this can be illustrated by reference to determinations of total nitrogen and total dietary fibre.
The determination of nitrogen is often performed with combustion Dumas instruments. The most reliable of these instruments tend to be gravity fed units operating with a test portion up to 0.5g. If the test portion is not representative then widely differing results for the same sample may be obtained. This is not resolved by taking a larger test portion, but by initial satisfactory homogenisation of the test material.
Total dietary fibre analysis is a complex procedure. Challenges can arise with what might otherwise be considered homogenous samples – nuts, grains, beans and seeds – which can require specialised milling equipment. Full dispersion within buffered medium to allow for serial enzyme digestions is essential to successful method performance. Therefore, finely homogenised test material is crucial as larger particles remain untouched by this process. Such undigested material usually causes inappropriately high final results. However, in some cases, if these larger particles are then also included in the analysis for protein correction, incorrect negative values can also be achieved.
In both these examples the key to good analysis is good sample preparation – the most important activity there is within a laboratory.