If it smells rancid and tastes rancid, it is rancid. The Peroxide Value (PV) test is useful for predicting shelf life when used together with other tests during a shelf life study. In contrast, it is not always useful in quality control, especially in the absence of sensory evaluation and other tests. So why test for Peroxide Value?
There are several known mechanisms and pathways for the decomposition of lipids and the subsequent production of a rancid flavor-aroma. Although, there are various methods that an analytical chemist can use to look for chemical indicators of rancidity, a single test is not necessarily conclusive.
Each of the peer reviewed chemical tests for rancidity target specific indicators, but it is important to understand that there are errors associated with the tests. It has become a standard quality control practice to test certain foods for Free Fatty Acids (FFA) and Peroxide Value (PV). The former is a shelf life indicator test for hydrolytic rancidity and the latter an indicator test for autoxidation. The tests for FFA and PV look at unrelated lipid decomposition pathways. Of the two, the most frequently misused, misrepresented and misinterpreted test is PV. Suppliers will stubbornly argue that a product cannot be
rancid because laboratory tests indicate a low PV. They will question how the PV can be high if the FFA is low. There is clearly a need to have a better understanding of the pathways that lead to rancidity and the limitations and purpose of each test method. Let us begin with PV.
Lipid oxidation is a dynamic three-stage process and peroxide value changes over time. The PV is low during the beginning of a food’s “shelf life.” Lipid oxidation is controlled for a time by various methods including oxygen barrier packaging, absence of light, modified atmosphere packaging and temperature-controlled storage. At some point during the food’s life PV peaks and then drops down again. Picture a graph, where PV is on the Y-axis and time is on the X-axis. The line is slightly inclined during an induction period during which rancidity is not detected. With initiation of lipid oxidation and the start of the free
radical chain reaction the process enters the monomolecular stage. The third stage is the bimolecular period during which rapid uptake of oxygen and accumulation of hydroperoxides causes a peak in PV. As PV increases so does the generation of aldehydes, ketones and other non-radical compounds, after which the PV peak drops down.
The PV test is used in a shelf life study to detect the initiation stage of the autoxidation process. A shelf life study is a series of regularly schedule tests of product held under controlled packaging and storage conditions. Alone, the PV test is not reliable for controlling incoming raw material, because you might not know what side of the PV peak you are on without sensory evaluation and other tests. Where do we go from here?
Consider that aldehydes are most responsible for the flavor-aroma that we call rancid when autoxidation has occurred. Therefore, one approach to controlling raw material is to look at the Total Oxidation (TOTOX) value when attempting to confirm rancidity by analytical chemistry. Using this strategy, in addition to testing for PV, also test for P-Anisidine Value (AOAC Method Cd 18-90). The P-Anisidine Value (AnV) is a test for aldehydes. The TOTOX value considers both PV and aldehyde production and is
2 x PV + AnV
Although the PV might not be high, an elevated AnV suggests the product is in some incipient or advanced state of rancidity. Knowing the typical AnV of the product you can establish a tolerance for acceptance.
There are many other useful laboratory tests for rancidity, but each has limitations. Ultimately, the key is to always employ several types of rancidity tests, understand their limitations and stand firm on sensory evaluation. The bottom line is if it tastes rancid and smells rancid, it is rancid.