General size reduction considerations
After receiving the sample at the laboratory, usually the sample will be too large for analysis. It must be reduced in bulk size and / or particle size. The best way to prepare a smaller sample from a bulk sample of small, solid particle size is to pile the sample even on a clean surface, flatten the pile, and divide it into quarters and then combine two opposite quarters and discard the other two. Repeat this operation until the sample is reduced to a manageable size. A similar process can be done with homogeneous liquids and four containers. The process may be automated. The objective is to prepare a sample homogeneous enough to ensure that there is negligible difference between the repeated test portions. Grinders, blenders, and food processors are useful equipment to reduce particle size.
Various types of mills are available to reduce sample particle size and provides a homogeneous mass from a heterogeneous mixture. Bowl cutter, meat mincers, tissue grinders, mortars and pestles, or blenders are most useful for moist samples. Mortars and pestles or mills are best for dry samples. Mills are classified according to their mode of action. Particle size is controlled in some mills by adjusting the distance between Burrs or blades and / or by screen mesh size.
Some foods are better ground after drying in a desiccator or vacuum oven. Grinding wet samples may cause significant losses of moisture and/or chemical changes. Grinding foods when they are frozen reduces undesirable chemical changes. Grinding should not heat the product. This can be controlled by not overloading the grinder. Bare-metal mills should be avoided and must not be used if the sample is to be analyzed for trace metals.
Enzymatic inactivation Food materials are rich in enzymes. Under many conditions of analysis, the enzymes are still active and may interfere with final results. If the action of the enzyme alters the compound being analyzed, it must be controlled or inactivated. The control method varies widely depending on the size, consistency, and composition of the enzymes present; and the intended analysis. Some enzymes are particularly heat labile and can be inactivated with minimal heating. Storage at low temperatures (-20 to -30c) protects many, but not all, foods from the action of enzymes. Some enzymes are inactivated by inorganic compounds, by a shift in pH, or by salting out. Oxidative enzymes may be controlled by reducing agents.
Lipid protection Lipids create a particular problem in sample preparation. Foods high in fat are difficult to grind at room temperature. These foods may need to be ground in a frozen state. Unsaturated lipid components may be altered by oxygen exposure. Samples should be protected by storage under nitrogen. Antioxidants may be helpful if they do not interfere with final analysis. Light can initiate oxidation of unsaturated fatty acids. Lipid materials are more stable during frozen storage in intact tissue rather than in extracted tissue. Low- temperature storage under nitrogen is usually recommended to protect most foods.
Microbial growth and contamination
Microorganisms are present in nearly all foods and if not controlled can alter the composition of the sample. Likewise, they are present on all but sterilized surfaces and can easily contaminate samples; the latter is more of a problem in samples destined for microbiological examination. Freezing, drying, and chemical preservatives can control microbial growth. Frequently, a combination is employed. The method of preservation depends on the nature of the food, the expected contamination, the storage period and conditions, and the analyses that are to be performed.