Methodology and Proximate Analysis

Opening Statement

The preparation for analysis of a small amount of material (the sample), representative of the bulk food, and normally supplied to the analyst, can be divided into the following stages:

1. The first stage for most food matrices is to prepare a weighed and calibrated aliquot – the sample for analysis – in preparation for quantitative extraction of the compounds of interest (analytes).

2. For some food samples, the material has to be rendered accessible to the extracting agent – preparation for analysis.

3. The next stage can then be either (a) removal of the analytes from the sample matrix or (b) removal of interferents from the matrix – the extraction. In each case, the analytes are in a form to be recognised and quantified unambiguously in subsequent examinations.

4. The final stage is to examine the extract, which will normally contain matrix components other than the target analytes, using various of chemical and physical methods to make qualitative or quantitative measurements of the analytes – the analysis.

One contemporary objective in the development of analytical methodology is to automate the whole assay, and there are two ways forward. The classical extraction procedure can be given over to robotic control, or the information about the chemical composition can be "extracted" directly from the sample matrix by remote sensing. Perversely, remote sensing makes extraction redundant. Thus, it is necessary at the outset to recognise that the future analysis may be an extractionless, remote sensing, robotic operation. Although considerable progress has been made already towards these goals, as a "hands on instrumental analytical chemist" the modus vivendi for this monograph was to present classical and modern experiential and methodological data in ways that may be a helpful record and also serve as a transitional reference of methodology to facilitate the advancement of the era of robotic analytical workstations.

Food Sample for Analysis

Foods (and drinks) are nutrient-containing substances that can be metabolised into body tissue and into energy to sustain body tissue. In modern parlance foods are largely solid, and drinks are largely liquid. It is convenient to refer to all nutrient sources as food – the nutrient-carrying matrix – and to consider the removal of compounds from a sample of food as an extraction. However, the English language has many words to express the idea of removing something from the whole. In analytical chemistry, for example, there is no clear distinction between a separation method and an extraction method, and it gets worse because chemists also fractionate, purify, isolate, partition, disperse and distribute components of mixtures. Here, an extraction is thought of as an operation on a sample of food that concentrates the target components, normally by removing them from the bulk of the food sample, often in preparation for further examination such as chromatographic separation. In analytical chemistry, a separation is seldom carried out on the raw material (however, see Chapter 8, Section 2, direct injection), but on an extracted or cleaned up sample for analysis. In addition, there are many procedures associated with extraction that in themselves do not actually remove anything from the sample. These processes are dealt with in Chapter 2 (Sample Preparation for Extraction), and are treated as extraction aids.

The natural origins of human foods are biologically diverse, ranging widely in texture and composition – from nutmeg to oysters. The extremely complex endogenous composition of food is made even more complex in the modern environment where so many extrinsic, additional items – additives such as antioxidants, contaminants from agriculture such as herbicides and industrial adulterants such as hydrocarbons from petroleum – may also be present. This extends the quantitative range of analyses practised by food analysts from the gram amounts encountered in proximate analysis (Section 6) to picogram and even lower amounts of highly toxic contaminants e.g. PCBs. To cover more than 12 orders of magnitude requires an enormously diverse armoury of techniques.

Analysis of Foods

It is usually a concern over the chemical composition or contamination of food and the effect this has on its value to the consumer that generates the need for analysis. The quality of food is based on the natural composition, the balance between the nutrient and the anti-nutrient composition. The health and prosperity of early civilisations depended upon their ability to refine their food supply in the short term by removing toxic materials using extraction methods, or in the long term through crop selection and plant breeding.

History of Food Extraction

Many extraction methods were invented to remove sufficient quantities of toxins (anti-nutrients) from the biological source to make the material acceptable and safe to eat. Notably, nature historically used toxins in sources of human and animal foods to maintain the balance between the survival of the browser and the browsed! These practices were incorporated into the culture of the technology employed in the early analytical laboratories.

The natural processes used to extract moisture in order to increase the "shelf-life" of food and the early uses of extraction methods to concentrate important components, e.g. essential oils, formed the bases for methods of analysis as the science of measurement began to develop. Historically, the extraction of bulk components from food made use of physical processes, such as pressure, to remove the juice or oil from the pulverised pulp. Warm air or sun drying of tomatoes or fish extracted sufficient water to reduce bacterial attack to an acceptable level in preparation for storage. Solvent extracts of essential oils from the pulverised plant, seed, or nut were concentrated by distillation in simple stills. Spicy and resinous plants were solvent refluxed in fractionation columns and valuable components separated and extracted in this way. The modern method of supercritical fluid extraction (SFE) uses ultrapure carbon dioxide as solvent, thus eliminating the fear of toxic residues in the extract. Cold-pressing methods are still used to produce high quality extracts of citrus fruits, and hydrodistillation, the steam distillation of an aqueous solution of the food matrix, was practised, especially on powders, from earliest times. There are many other examples where extraction from the bulk material was used to refine our food supplies.

Analytical data defining food quality and the methods used to obtain them have to be validated; several regulatory bodies oversee this process (FDA, FSA, AOAC, FAO, WHO, etc.). In 1963, the FAO and the...