Content originally authored by Vaughan Langford, Syft Technologies. Quantum Analytics is an authorized distributor for Syft Technologies in the United States.
Edible oils are used in a wide range of culinary applications. Oils containing unsaturated fatty acids have been demonstrated to offer health benefits over their saturated counterparts but are unfortunately significantly susceptible to oxidation. This oxidation is a significant cause of reduced shelf-life in these oils and can be a good indicator of the efficacy of their packaging.
In this white paper we briefly reviewed the chemistry of edible oils and their oxidation. Then we describe how oxidation can be detected early and with high specificity in order to diagnose leaks and inefficiencies in the packaging, using Selected Ion Flow Tube Mass Spectrometry (SIFT-MS).
Edible oils are composed of fatty acids (high molecular weight carboxylic acids) and glycerol. Each glycerol molecule is bonded to three fatty acids via ester linkages, forming triglycerides (triacylglycerols), as shown in Figure 1.
Figure 1. A triglyceride composed of glycerol and three fatty acids. Here three eicosanoic acid (arachidic acid) units, each having 20 carbon atoms, are shown. This acid is found in peanut oil, for example.
Vegetable oils normally contain fatty acids between 12 and 26 carbons in length. A fatty acid with only single carbon-carbon bonds (as shown in Figure 1) is a saturated fatty acid because the carbons are bonded to the maximum number of hydrogen atoms. Fatty acids that contain carbon-carbon double bonds are described as unsaturated because more hydrogen atoms could be added to them (this is sometimes done industrially to create so-called hydrogenated vegetable oils from unsaturated oils).
The presence of a double bond in the fatty acid significantly changes its chemical behavior. In particular, increasing the number of double bonds causes the rate of oxidation of the fatty acid to increase. This is illustrated in Table 1, which lists some of the commercially significant fatty acids found in vegetable oils. When fatty acids oxidize, the oxidation products adversely affect the taste and smell of the oil.
However, fatty acids with multiple double bonds (“polyunsaturated” fatty acids) have a beneficial effect on health, helping to protect against heart disease when consumed.1 So the most desirable situation is to use oils which have a high percentage of unsaturated fatty acids, but to prevent oxidation as much as possible.
Table 1. Structures and relative oxidation rates (25 °C) of several important fatty acids found in vegetable oils.2
Oxidation of Edible Oils
Oil oxidation is a chain reaction involving free radicals. It starts when the ever-present radical, oxygen, is excited (often catalytically by another species) and becomes more reactive. Oil oxidation is more readily initiated under high light levels and all reactions proceed faster at elevated temperatures. Oil oxidation occurs in two stages called primary and secondary oxidation.
Primary oxidation consists of oxygen adding to fatty acids at the double bond position to form peroxides. This is more likely to occur to fatty acids with more double bonds. The peroxides have no taste or smell, so are very insidious. An oil can have a large percentage of peroxides (measured in milliequivalents of oxygen per kilogram, or meqO2/kg) but still smell and taste the same as when it was fresh. The change to the oil’s sensory attributes occurs when the peroxides break down in the process of secondary oxidation.
Secondary oxidation produces volatile compounds, which affect the taste and smell of the oil. Table 2 shows the two most abundant volatile oxidation products for three important fatty acids. Each fatty acid yields quite different volatile oxidation products, all of which differ from natural, desirable volatile compounds in the oils. These compounds give oxidized oil its distinctive aroma, which trained oil tasters describe as “rancid”, but it can also be likened to the smell of varnish.
Table 2. Three fatty acids commonly found in vegetable oils and their first and second highest concentration volatile secondary oxidation products.
Unfortunately a direct correlation does not exist between the proportions of fatty acids in the oil and the proportions of volatile oxidation products in the headspace. However, the degree of oxidation can be determined by comparing the volatile profile of the test oil with reference standards. This is the method used in sensory analysis.
For many oils (e.g. fish, canola, sunflower and peanut oil) the volatile oxidation products constitute the entire oil aroma because the refining process removes all original volatile aroma compounds from the oil. Hence oxidation can quickly produce a noticeable, undesirable change in the flavor of these oils.
Refined oils are more susceptible to oxidation because refining removes their natural antioxidants. Antioxidants scavenge radicals, preventing them from oxidizing the fatty acids. Vitamin E (tocopherol), a naturally occurring antioxidant, is often added after refining to slow the oxidation process.