Absorption Maximum of an IodineStarch Complex
Figure 3 indicates that the absorption maximum of a 10-3 N iodine solution was 0 between 500 nm and 900 nm, and the absorption maximum of the iodine-starch complex solution was 563 nm. The absorption of a 1% starch indicator solution was 416 nm, which does not contribute to absorption maximum of the iodine-starch complex (563 nm). The absorbance at 563 nm of each sample is proportional to the concentration of the iodine-starch complex formed as a result of the production of one mole of I2 per mole of R·OO·H presented in the sample.
Iodine Equation
Determination of PV by SPM is accomplished by the amount of iodine generated according to equation [3], and converting iodine concentration to PV according to equation [6] generated from determining the linear regression of an iodine standard curve (Figure 4A):
Y = (3477.3)(X) - 0.0131 [6]
Where:
Y = absorption at 563 nm,
X = normality of iodine solution,
b = -0.0131, and
m = 3477.3
The spectrophotometric peroxide value (meq/kg) can be calculated from the iodine standard curve (Figure 4A) and shown in Figure 4B.
Analytical Values in Lamb Tissue and Corn Oil
The lipid oxidation values of lamb tissue and corn oil determined by TBA, AOCS, and SPM are summarized in Table 1.
Lamb Tissue. Lamb tissue analyzed by AOCS and SPM had peroxide values of 1.43 and 1.52 meq/kg, respectively (P > 0.05). The CV for AOCS and SPM were 33.7 and 10.7%, respectively. The TBA value of the lamb tissue was 0.1 mg/g with a CV of 14.5%. The greater CV obtained for fresh lamb tissue by AOCS, SPM, and TBA were postulated to be due to the non-uniform variation of lamb tissue. The CV of 10.7% for SPM in contrast with 14.5% for TBA and 33.7% for AOCS PV indicates that SPM is a more reproducible method for evaluating the lipid oxidation of lamb tissue.
Corn Oil. The CV of SPM was used to compare those of TBA and AOCS methods in order to evaluate the reproducibility of these three methods. The means of peroxide value analyses by AOCS and SPM were 52.3 and 51.7 meq/kg, respectively (P < 0.05). In general, PV > 8 is considered as rancid (Ockerman, 1985). Thus, according to this criterion, the corn oil was extremely rancid. The TBA value was 0.39 mg/g. The CV for AOCS, SPM, and TBA were 0.36, 0.21, and 4.62% respectively.
The corn oil with a high PV of 51 meq/kg gave TBA of only 0.4 mg/kg. This apparent discrepancy can be explained by the fact that the TBA method is primarily based on determining the concentration of a TBA-malonaldehyde (MA) complex. The MA, a secondary oxidative product of polyunsaturated fatty acids, is not always present in oxidized systems. The interaction of MA with available amino groups in meat components has been reported to adversely affect the results of TBA analyses (Nawar, 1996).
The low temperature conditions employed during the preparation and extraction steps reduce the potential for further autoxidation in samples being analyzed by the SPM method. The comparisons of CV for AOCS and SPM indicate that SPM is the more reproducible of the two methods in analyzing PV for lamb tissue and corn oil.
It was reported that the boiling procedure in some TBA test used to speed up color development had negative impact on result due to the further breakdown of MA (Decker et al., 1998). The TBA method does not detect the oxidation of mono- and di-unsaturated fatty acids (Decker et al., 1998), while there are very large portions of these fatty acids in beef, pork, mutton and poultry (Foegeding and Lanier, 1996). Since most muscle foods consist of mono- and di-unsaturated fatty acids, peroxide value may report more accurately the lipid oxidation of foods than TBA. The color developments for AOCS and SPM for PV are instantaneous while color development for the TBA method (cold extraction) requires at least 15 hours. Therefore, peroxide value determination greatly shortens the time to analyze lipid oxidation. Titration methods for determining peroxide values have long been criticized for their lack of reproducibility because of the difficulty in determining the titration endpoint, which can be influenced by light and subjectivity (Fielder, 1974). The SPM method provides a rapid, yet more objective and more accurate method for measuring the lipid oxidation in animal tissue.