Fundamental Requirements of Lipid Oxidation Analyses

Students learn about monitoring reactions in the context of straightforward reactions with fixed stable products that form in high concentrations; for example,

+B→C (2.1)

+B→C+D (2.1a)

This situation directly contrasts with lipid oxidation, which has multiple possible pathways that change with conditions and over time. In these reactions, products are actually intermediates that degrade or transform into other compounds, as shown in Fig. 1.2 of Chapter 1, and product yields are quite low; a total reaction of less than 1% renders foods inedible. Thus, lipid oxidation is generally considered to be the greatest analytical challenge in food science. Similar arguments could be made for cosmetics and many personal care products, as well as for biological tissues in which lipid oxidation and its effects are complex, even when total oxidation levels are low.

One question that persists in lipid oxidation analysis is which product is best to monitor? The answer is strongly influenced by the either quantitative or qualitative end goals of analyses. Industry rejects batches and makes decisions about formulations based on quantitation, which requires accuracy and reproducibility, and product class analysis (that is, for all hydroperoxides or all epoxides, regardless of structure) is usually adequate. In contrast, basic research seeks to elucidate reaction mechanisms, and oxidation sequences need as much quantitative and qualitative detail about individual products as possible. Oxygen consumption is often preferred for determining oxidation kinetics when starting from fresh products because it is independent of product transformation, but it cannot be used for spot analyses of products off the production line or storage shelf. For the latter, the question becomes, does one measure hydroperoxides because they form first (even though they decompose) or secondary products to detect hydroperoxide breakdown and because consumers can smell and taste them? Does a researcher analyze only volatiles because gas chromatographs are readily available, only non-volatile products because they remain in the product, or both? The picture of lipid oxidation that we construct depends on the analytical strategy used.

A wide variety of assays for various lipid oxidation products has been developed; many methods have been standardized, and more are used simply by following (and modifying) procedures published in research papers. Some of the most common assays for various lipid oxidation products are listed in Table 2.A. A number of excellent reviews and books have presented details of individual methods (McDonald & Mossoba, 1997; Shahidi, 1998; Dobarganes & Velasco, 2002; Yildiz et al., 2003; Kamal-Eldin & Pokorny, 2005). Therefore, it is not the goal of this chapter to provide protocols for analyses or to recommend specific assays. Rather, this chapter argues for new approaches and seeks to change how lipid oxidation analyses are viewed and used to attain more complete and accurate information. First and foremost, the author challenges readers to “think chemistry” in all lipid oxidation analyses, rather than seeing black boxes in which procedures are accepted and applied blindly; instead, they should look beyond peroxide values to track multiple products and attempt mass balance between products.