Food analysis phenolic substances

A number of criteria could be apphed to organize this chapter, depending on the point of view by which foods are considered. In this chapter, application of HPLC to food analysis will be described considering homogeneous classes of food components lipids, carbohydrates and related substances, proteins, peptides, amino acids, biogenic amines, phenolics, vitamins, and some selected contaminants. 

I McMurrough, JR Byrne. HPLC analysis of bittering substances, phenolic compounds, and various compounds of alcoholic beverages. In LML Nollet, ed. Food Analysis by HPLC. New York Marcel Dekker, 1992, pp 579-641. 

In spite of their frequently brilliant color, it is not so much this property which is important in foods as their tendency to undergo dis-coloration. This is due to their general phenolic character which allows them to serve as effective substrates for oxidase action. They are, in fact, of all classes of phenolic substances, those most universally present in the plant kingdom. Seldom does the analysis of a plant extract fiul to reveal one or more substances of flavonoid character, and if these are absent, chlorogenic acid or one of the closely related coumarins is almost certain to be present. Thus the flavonoid compounds and coumarins are the most commonly available substrates, actual or potential, for polyphenol-oxidase or peroxidase activity. 

Inhibitors are found among food constituents. Proteins which specifically inhibit the activity of certain peptidases (cf. 16.2.3), amylases or 3-fructofuranosidase are examples. Furthermore, food contains substances which nonselectively inhibit a wide spectrum of enzymes. Phenolic constituents of food (cf. 18.1.2.5) and mustard oil (cf. 17.1.2.6.5) belong to this group. In addition, food might be contaminated with pesticides, heavy metal ions and other chemicals from a polluted environment (cf. Chapter 9) which can become inhibitors under some circumstances. These possibilities should be taken into account when enzymatic food analysis is performed. 

The electrochemical detector is extremely sensitive, but suffers from two main drawbacks. Firstly, the mobile ( ase has to be extremely pure, in particular, free of oxygen and metal ions. Secondly, by-products of the oxidation or reduction processes are often absorbed on the surface of the electrodes and thus, if quantitative activity is required, frequent calibration is necessary. Ultimately the electrodes have to be cleaned usually by mechanical abrasion and replaced in the cell. Electrochemical detection is particularly suitable for small bore columns and possibly, in the future, LC capillary columns, due to the fact that the detector can be made extremely small in size. The detector has had a fairly wide area of application. It has been used under oxidizing conditions for the detection of phenols, hydroquinone and catechols and in particular for many compounds of biological interest including, catecholamines (35). It has been used to determine substances of industrial interest, and agricultural chemicals (36). In its oxidation form, it has been used to detect amines of various types together with i enols and thiols. It has also been used in the analysis of ascorbic acid in food and biological materials, and in the pharmaceutical industry, for the analysis of multivitamin products. 

---