Recent developments in Fourier transform infrared spectroscopy-partial least squares (FTIR-PLSs) extend the application of this strategy to the field of the edible oils and fats research. In this work, FT-IR spectroscopy was used as an effective analytical tool to determine the peroxide value of virgin walnut oil (VWO) samples undergone during heating. The spectra were recorded from a film of pure oil between two disks of KBr for each sample at frequency regions of 4000–650 cm−1. Changes in the values of the frequency of most of the bands of the spectra were observed and used to build the calibration model. PLS model correlates the actual and FT-IR estimated value of peroxide value with a correlation coefficient of 0.99, and the root mean square error of the calibration (RMSEC) value is 0.4838. The methodology has potential as a fast and accurate way for the quantification of peroxide value of the edible oils.
The production of walnut constitutes a significant proportion of the income of farmers in many countries. In the recent years, the peroxide value of virgin walnut oil (VWO) has received great attention because of its sensory qualities and biological activities. Epidemiological studies show that VWO not only reduces serum cholesterol but also has nutritional cranial nerve cells which can adjust plant nerve function. Other experts discover that walnut oil does not only act as an officinal but also can be used as the “old man and the infant nutrition oil” as well as aerial work and flight personnel senior’s health care oil [
In this way, VWO production is a promising factor to the viability of ecological economy. Nevertheless, VWO can be a target of adulteration with cheaper oils or even metamorphic oils because of its higher prices in the global market. The adulteration comprises a great hazard not only for the economic development and prosperity of those communities but also for the health and safety of VWO consumers. Therefore, the development of rapid and accurate analytical techniques capable of detecting the quality of VWO is currently highly demanded.
In fact, the quality of oils depends on its chemical composition that changes qualitatively and quantitatively; one of the most important indicators of performance and shelf-life is the oxidative stability of oils, that is, their resistance to the oxidation process. Previous study showed that the unsaturated fatty acid content of oils is higher; that is, the unsaturated level is higher, and the oxidation is easier [
Generally, the sample of oil is oxidized when subjected to air or oxygen flow, heating, exposure to light, catalysers, and so forth. Although the mechanism of the oil degradation process was influenced by the oxidative conditions, it has normally been established as being a free radical mechanism yielding hydroperoxides, also called primary oxidation products [
In the past years, the traditional method of establishing the PV of edible oils is titration, and the method has been standardized by ISO (3960:1977) or GB/T 5538-1955, ISO (3960:2001) or GB/T 5538-2005. The determination process of titration is susceptible to a variety of external factors, and the measured value has poor reproducibility and low sensitivity, only can be applied to the large concentration of hydroperoxide determination.
The recent developments in FT-IR spectroscopy instrumentation and the application of this technique expand in food research, facilitating particularly the studies on edible oils and fats. FT-IR methods have demonstrated themselves to be rapid and nondestructive analytical tools with minimum sample preparation necessary. Due to the fact that the intensities of the bands in the spectrum are proportional to concentration (i.e., Beer’s law is obeyed) and coupled with the chemometric techniques, FT-IR spectroscopy becomes an excellent tool for quantitative analysis. Several applications have been performed using this analytical approach together with chemometric methods, to detect walnut oil adulteration [
Although a lot of work has already been published on the determination of PV by FTIR spectroscopy, different articles have different analysis methods. A primary FTIR spectroscopic method for the determination of PV in edible oils, based on the stoichiometric reaction of triphenylphosphine (TPP) with the hydroperoxides present in edible oils to produce triphenylphosphine oxide (TPPO), accurate quantitation of the TPPO by the measurement of the absorption band at 542 cm−1, provides a simple means of determining PV [
In this paper, FT-IR spectroscopy is used for the oxidation and PV evaluation of virgin walnut oil. The oxidation experiments were carried out in all cases by heating the samples in a convection oven at 70°C at different time, the PV of each sample were determined by chemical and FT-IR spectroscopy method. PLS model correlates the actual and FT-IR estimated values of PV with a coefficient of determination (
Virgin walnut oil (VWO) was purchased from the local market. The samples were placed in a convection oven in uncovered polystyrene Petri dishes and heated at 70°C for 1, 2, 3, 4, 5, 6, 8, and 10 d, respectively.
PV determination was performed according to the method proposed by ISO (3960:2001) or GB/T 5538-2005. 2.00–3.00 g of VWO sample was placed in iodine flask and dissolved in a blended solution of 50 mL isooctane-glacial acetic acid (2 : 3, v/v). A saturated solution of KI (0.5 mL) was added. The mixture was shaken by hand for 0.5 min and kept in the dark for another 3 min. After the addition of 30 mL distilled water, the mixture was titrated against sodium thiosulphate (0.01 M) until the yellow colour almost disappeared. Then, about 0.5 mL of starch indicator (0.05%) solution was added. Titration was sustained until the blue colour just disappeared. A blank was also determined under similar conditions. PV (meq/kg) was calculated as follows:
FT-IR spectra of samples were obtained using Shimadzu IRPrestige-21 equipped with DLATGS as detector and potassium bromide (KBr) as beam splitter and controlled with the IRsolution software. A small quantity (~20
The chemometric analyses were performed using the software SIMCA-P. Quantification of the PV of VWO was carried out using partial least square (PLS) [
The peroxide value (PV) of each sample used in both calibration and validation.
Samples | Peroxide value (meq/kg) | |
---|---|---|
Calibration | Validation | |
1 | 4.1432 | 4.2356 |
2 | 9.0373 | 18.4758 |
3 | 10.8598 | 28.5217 |
4 | 15.2708 | 36.1313 |
5 | 17.5795 | 46.6111 |
6 | 29.3056 | 57.3778 |
7 | 29.5217 | 66.158 |
8 | 33.9717 | 75.0164 |
9 | 43.2461 | 81.4029 |
10 | 47.2113 | 98.0299 |
11 | 58.135 | |
12 | 62.3264 | |
13 | 65.2184 | |
14 | 77.2294 | |
15 | 80.0891 | |
16 | 94.3897 | |
17 | 98.3299 | |
18 | 120.144 |
As a descriptive example a virgin walnut oil FT-IR spectrum is presented from 4000 to 650 cm−1 in Figure
IR spectra of virgin walnut oil (VWO) sample before oxidation.
Figure
FTIR spectra of the virgin walnut oil (VWO) sample after 0, 6, 8, and 10 days, respectively, under oxidative condition at region of 4000–1030 cm−1. (A) 0 day. (B) 6 days. (C) 8 days. (D) 10 days.
The calibration model of PV was carried out by using PLS algorithm. The PV of each sample used in calibration is presented in Table
Figure
Iodometric and estimated values of peroxide value (PV) for calibration and validation sets (▲: calibration set; ■: validation set).
In order to estimate the prediction ability of the developed model, PLS calibration model was used to predict the values of PV of 10 VWO samples as validation data sets (Table
PLS performance for analysis of calibration and validation model.
Model | Principal components |
|
Std. dev. | RMSEC (meq/kg) | RMSEP (meq/kg) |
---|---|---|---|---|---|
Calibration | 10 | 0.9918 | 0.236 | 0.4838 | |
Validation | 10 | 0.9903 | 0.257 | 0.3545 |
From the results here obtained it can be concluded that FT-IR spectroscopy is an adequate technique for detecting the changes produced in virgin walnut oil sample during an oxidation process. The changes observed in the frequency values of most of the bands of the spectra throughout the oxidation experiment provide accurate information about the different stages of the process and for this reason, FTIR-PLS could be useful to evaluate the oxidative state of VWO and to determine the PV, one of the most important indicators of performance and shelf-life, in a simple, accurate, and fast way.
The authors gratefully acknowledge the financial support of State Science and Technology Support Program (2011BAD46B00) and Yunnan Science and Technology Program (2011AB006).