Measurement of Primary Lipid Oxidation Products

Lipids are important macromolecules in food. A food product s nutritional value as well as its flavor, texture, general palatability, and storage stability are affected by lipids. Therefore, both physical and chemical criteria are needed by the food processor to assess or monitor the quality of fats and oils. The basic characteristics of certain food items, such as edible oils, will be dependent upon their source. Variation from these norms can be ascertained before the oils are used in other foodstuffs. In effect, knowledge of the quality of the lipid before shipping the product to market, or use in fabricated foods, is of economic importance to the processor. 

The determination of peroxide value (PV) by an iodometric titration is described in Basic Protocol 2. Iodine is liberated by hydroperoxides in the oil in the presence of excess iodide in a stoichiometric ratio. The amount of iodine present is determined by titration with a standard sodium thiosulfate solution using a starch indicator, thereby reflecting how much peroxide is present in the oil or lipid extract. 

Another approach for the determination of PVs, as described in the Alternate Protocol, is a spectrophotometric method based on the ability of peroxides to oxidize iron(II) to iron(III). The ferric ion forms a complex with xylenol orange, whose concentration can be determined spectrophotometrically. The ferrous oxidation/xylenol orange (FOX) method is very rapid, requires little sample, and can determine PVs as low as 0.1 meq active oxygen/kg oil. 

CAUTION Chemical substances used in this unit require special handling see appendix 2B for guidelines. 

4-trimethylpentane (isooctane), ACS spectrophotometric grade (e.g., Fisher Scientific) 

---

Peroxide value (PV) is an indication of primary lipid oxidation products in oil or the level of oxidation in soybean oil. Yildiz et al. (2002) measured oxidation with the standard method (AOCS Method Cd 8-53— AOCS, 1987) and used NIRSystem 6500 spectrometer to detect PV values in soybean and corn oils. Peroxide values ranged from 0.30 to 20.85 meq/kg. They obtained a very good calibration with R = 0.99 and SEP = 0.52 meq/kg using a first derivative math treatment and 13 PLS factors. 

Measuring the content of primary oxidation products is limited due to the transitory nature of peroxides. Yet, their presence may indicate a potential for later formation of sensorially objectionable compounds. The peroxide content increases only when the rate of peroxide formation exceeds that of its destruction. In cases where peroxide breakdown is as fast as or faster than peroxide formation, monitoring lipid peroxides is not a good indicator of oxidation. This can occur in frying oils and sometimes in meat products, particularly in cooked meats where iron is very active and peroxide breakdown is quite rapid. Because the acceptability of an oil or lipid-containing food product depends on the degree to which oxidation has progressed, the simultaneous detection of primary and secondary lipid oxidation products helps to better characterize lipid quality. It is... 

Decomposition of the primary products of lipid oxidation generates a complex mixture including saturated and unsaturated aldehydes such as hexanal. Hexanal is the most commonly measured end product of lipid oxidation, and both sensory and physicochemical methods are used for its determination. Where other antioxidant activity tests may be nonspecific, physicochemical measurement of hexanal offers the advantage of analyzing a single, well-defined end product. 

Lipids are susceptible to oxidation and, as such, require analytical protocols to measure their quality. As described in vnitd2.i, autoxi-dation is one of the chief processes by which lipids degrade. The primary products from this reaction are hydroperoxides. These odorless and colorless transient species break down by various means to secondary products, which are generally odoriferous by nature. Being able to measure secondary oxidation products by simple spectrophotometric means is important for the food scientist so that he or she is able to characterize the extent of lipid oxidation. However, the researcher should be cautioned that one assay (e.g., TBA test) does not provide all the answers. To get a better picture of the story, both primary and secondary products of lipid oxidation should be assessed simultaneously by the different methods available (unitdu). 

As oxidation normally proceeds very slowly at the initial stage, the time to reach a sudden increase in oxidation rate is referred to as the induction period (6). Lipid hydroperoxides have been identified as primary products of autoxidation decomposition of hydroperoxides yields aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, known as secondary oxidation products. These compounds, together with free radicals, constitute the bases for measurement of oxidative deterioration of food lipids. This chapter aims to explore current methods for measuring lipid oxidation in food lipids. 

Analysis Measure relatively low levels of oxidation (below 1%) and include measurement of initial or primary products of lipid oxidation (e.g., hydroperoxides, conjugated dienes) as well as secondary decomposition products of lipid oxidation (e.g., carbonyls, volatiles, dialdehydes). 

Many methods have been developed to access the extent of oxidative deterioration, which are related to the measurement of the concentration of primary or secondary oxidation products or of both. The most commonly used are peroxide value (PV) that measures volumetrically the concentration of hydroperoxides, anisidine value (AV), spectrophotometric measurement in the UV region and gas chromatographic (GC) analysis for volatile compounds. Vibrational spectroscopy, because of its high content in molecular structure information, has also been considered to be useful for the fast measurement of lipid oxidation. In contrast to the time consuming chromatographic methods, modem techniques of IR and Raman spectrometry are rapid and do not require any sample preparation steps prior to analysis. These techniques have been used to monitor oil oxidation under moderate and accelerated conditions and the major band changes have been interpreted. ... 

---