Foods are an extremely diverse group of products that are often supplemented with additional items such as preservatives, vitamins and antioxidants, and sometimes contaminated with harmful substances such as pesticides. Food analysis, promoted by industry, academia and government institutions, pushes research towards more advanced, sensitive and environmental-friendly methodologies. Sample preparation is a crucial part of the analysis of foodstuffs. Current sample preparation techniques used in food analysis are reviewed and the advantages and drawbacks of each one are discussed.
The concept of "food analysis" has grown in complexity as a result of international regulations and legislation, consumer demand and globalization of the food market. It is absolutely necessary to trace a detailed description of a food, in terms of its nutritional value, chemical composition, bioactivity and toxicological aspects.
The entire process, from the production, through to packaging and up to the introduction into the market, must be carefully monitored, and this is where food analysis plays a major role. In the last few years, a considerable variety of analytical methods applied to food analysis have been made available at the same rate as the increasing concern about food safety.The choice of the analytical method depends upon various factors, such as nature of sample, goal of the analysis and the availability of resources (human and instrumental). Official analytical methods (i.e. AOAC International) are characterized by a high level of accuracy, precision and ruggedness, as a result of the validation of methods.
Food safety is an issue of topical interest, when considering the up-to-date consumer, attracted by healthy, dietetic and functional foods. This implies a deeper investigation that goes further than the mere determination of constituents, requiring a careful observation of the biological activities, if any, and their exploitation in the so-called "nutraceuticals". The latter designate a group of foods ("nutritional + pharmaceutical") with demonstrated health benefits, such as omega 3 fatty acids in milk. The final result of such a food screening leads to the creation of a comprehensive label, reporting a quali-quantitative description of the food product, along with nutritional indices and traceability.
Above all, it must be emphasized that a successful food analysis cannot be achieved without an appropriate, convenient and reliable sample preparation methodology.
The reliability of the conclusions drawn from a food analysis greatly depends upon the procedures used for sampling and sample preparation, which can be the most common source of errors. An ideal sample should be representative, in every part, of the bulk material from where it was taken. Grinding, chopping, digestion and centrifugation are some of the procedures utilized when performing sampling. Once the criterion of homogeneous sampling has been fulfilled, the sample must be made suitable for subsequent analysis.
Schematically, sample preparation can be performed in two basic steps: (i) extraction of target analytes; (ii) removal of interfering substances. Only occasionally can food samples be analysed directly: in most cases they need a sample clean-up step, necessary to remove interfering substances. Sometimes, this becomes a necessary step to make the analysis itself possible, as in the case of samples that need to be treated with derivatizing agents (e.g. methylation of free fatty acids prior to GC analysis). For example, if considering the headspace evaluation of a coffee aroma, there is no need to perform any sample preparation procedure, because the objective of the investigation is the evaluation of a property possessed by the sample in its original form.
However, the growing concern over food safety and its implications on human health has led to an increasing number of samples being tested and analyses being performed. Traditional techniques used for food sample preparation are often time-consuming and require consistent amounts of solvents and reagents. One of the objectives of analytical research in the last decade has been the miniaturization of the techniques used. Therefore, as will be described later, a series of "microextraction" techniques have been introduced, along with automated, fast, cheap and solventless sample preparation methods.