Sampling and sample preparation for food analysis

The term “food” refers to the broad range of edible materials that comprise the essential body nutrients required for life and growth, such as proteins, carbohydrates, fats, vitamins, or minerals. Foodstuffs are described variously as “lic&id” or “solid”, and “wet” or “dry”, depending on the amounts of water and fat they contain. Samples of plant origin are classified for analytical purposes as having a high or medium water content and a lower content of saccharides (from 5% to 15%), very low water content (dry), or a high content of oils [l]. Similarly, food samples can be divided into four main groups based on water and fat content [2]. Food samples of biological origin (liquid or solid) have been divided generally into the five categories described in Table 25.1. This coarse division is important when considering the choice of isolation technique, extraction solvent, and sample clean-up method during an analytical procedure [3]. Moisture content is an important consideration during sampling procedures, in part because it affects the extent of sample heterogeneity. Virtually all foods are heterogeneous, and the analyst should be familiar with their variability in composition and structure. In general, fresh foods of plant origin are more variable in composition than fresh foods of animal origin. The analyst should be also aware of the postmortem or postharvest physiological changes that can occur after a fresh food is sampled and which can affect sample heterogeneity. A combination of cold storage and chemical preservation may be required to maintain sample integrity in the event of prolonged storage. Although the chemical and physical properties of foods are inherently variable, even between samples that originate from the same breed or strain, the variability in composition of a single food sample can be minimized with proper sampling and sample pretreatment techniques. Two approaches can be used for sampling a food mass that is larger than the amount required for analysis in the