Testing can yield no better results than the quality of the sampling. Often a small sample must represent tons of a non-uniform material. Specific regulations cover sample collection and preparation for a number of commodities.
Generally, to obtain a representative sample, one should collect several sizeable portions and mix thoroughly. Divide the large sample half, remix, divide in half again, etc. until the desired sample size is obtained. This principle should be used whether or the sample is in large particles, small particles or a non-homogenous semi-solid such as meat. The division into smaller portions should include grinding, if necessary, to reduce the particle size. When the initial particle size is large and the amount of sample is very large, the sample may be ground progressively smaller as the amount of sample is deceased until the final particle size is very small.
UDY methods generally give recommendations for sample preparation to help ensure quality results. For example, homogenate suspensions are recommended for meat and a number of dairy products. When homogenates or other fluid samples are stored prior to analysis, a preservative such as UDY-Pol should be added to the sample. Storage in a refrigerator is often advisable. Some materials should not be exposed to light for extended periods prior to testing.
If samples are sent to other laboratories for comparison of results, the samples should be ground and mixed before distribution to ensure that all laboratories receive identical samples. Alfalfa and other forages are especially difficult to sample. They require that relatively large portions be ground and mixed prior to testing. Grinding as fine as possible is advisable.
When solid materials must be ground to obtain a homogeneous and representative sample, the Cyclone Sample Mill is often ideal. It meets USDA specifications for grinding grains and forages and does not require cleanout between samples. Sample heating is minimal, therefore, sample degradation is negligible. Generally, samples may contain up to 15& moisture or 20% oil without special handling. A Cyclone Sample Mill retrofit and Mill is also available through UDY Corporation for higher moisture or oil fat content. An optional Forage Cover with its larger opening is used for stems, leaves, and forages. Samples such as wheat may be fed into the Mill manually or by an optional Sample Feed Controller. A tank-type vacuum can also be attached to give higher velocity air flow and improved performance when samples tend to build up in the Mill.
Most fluid samples do not require sample preparation. Occasionally, dilution is used to increase fluidity and simplify handling. When dilution is required, it is often simply weighing out a portion of the sample with a known weight or volume of water or other diluent and mixing until homogeneous. The Syringe-Pipet permits rapid and easy collection of a consistent sample volume for fluid samples. When the density of samples is uniform, the consistent sample volume eliminates the need to weigh samples.
Semi-solid materials vary considerably in uniformity. Those that are homogeneous can often be tested without special preparation. For example, cheese can be tested directly or blended into a homogenate and an aliquot taken using the Syringe-Pipet. Non-homogeneous materials require blending with water or other solutions to break them down into a homogenate or suspension of small particles. Once a homogenate has been prepared, the Syringe-Pipet may be used for additional mixing and collection of a consistent sample volume. Preparation of homogenates greatly improves sample quality when the original material is non-uniform, such as in most meat samples.
When samples must be held for an extended period before testing, or if additional testing may be needed later, a preservative should be added for fluid or semi-fluid samples. For short periods, refrigeration may be adequate.
UDY-Pol is an excellent preservative for liquid samples. It contains bronopol which is an effective anti-microbial agent and does not interfere with UDY protein tests or other common tests. It may, however, lower results for Fat Measurement of dairy products unless precautions are taken. When UDY-Pol is used at the recommended dose of 0.2 ml (2 drops) per 100 ml of sample, the concentration of bronopol is 0.018%. UDY-Pol includes a dye in order that the preserved samples can be readily identified.
UDY-Pol can be added directly to fluid samples such as milk using the dropper that is included. When meat samples must be held, the homogenate can be preserved with UDY-Pol. UDY-Pol can be added to the citric acid and methanol fixative solution to eliminate separate addition.
Preparing meat homogenates using a solution containing methanol and citric acid, rather than just citric acid solution, leads to much greater stability of the homogenate. Problems of fat separation and foaming are virtually eliminated, making sampling of the homogenate much easier for both immediate and later testing.
In some cases, such as for raw milk, the usual warming of the sample is needed to speed the blending so that homogeneous representative samples are obtained. This usually shortens the time the sample will remain stable. If periodic retesting is anticipated, it is generally best to spilt the sample initially, thus eliminating heating those portions which are not to be tested immediately. UDY Milk Check Samples are prepared from homogenized milk. They should not be warmed as this will cause subsequent rapid deteriorization.