1. Introduction
Laser heating has been recognized since the 1970s, focusing on the CO2 laser. Utilizing CO2 laser allows non-metallic materials to be heated directly. CO2 laser with a wavelength of 10.6 µm equates to the photon energy of 943 cm−1, which is of the same order as lattice phonons in covalent crystals and is absorbed by existing lattice vibrations. This absorption technique also provides the advantage of more uniform heating for the materials that are generally transparent to the wavelength of 10.6 µm, meaning that heating is provided via this process throughout the entire thickness of the sample, contrarily in metallic materials where the laser beam is absorbed primarily at the surface [1].
Several studies were done in different applications of the laser heating techniques such as in thermoluminescence imaging of spatial dose distributions of ionizing radiation and the measurement of two-dimensional temperature distributions in high-power laser dielectric coating [2]. Some researchers studied laser-heating in the combustion of agricultural wastes [3] [4] [5]. Other researchers used it in enhancing the ceramic mechanical properties [6] [7]. Numerous studies were done by using lasers in food irradiations such as milk pasteurization [8] [9], production of yogurt [10], oil irradiation [11] and irradiation of bee honey [12]. Very promising results for applications involving non-destructive detection and classification of materials were obtained by Kassu et al. 2021 [13].
Edible oil is a vital component of our daily diet. It provides energy and essential fatty acids, and serves as a carrier of fat-soluble vitamins. Edible oils such as sunflower (Helianthus annuus) oil are continuously used for deep-fat frying. Usage of the same frying oil repeatedly is a common practice in the household or the restaurants to save costs. The methods used in assessing the quality of cooking oil and discarding it are observing foam, color turning dark or when it emits a bad odor [14]. Chemical reactions such as thermal polymerization, hydrolysis and oxidation occur when cooking oil is heated during the deep-frying process [15]. Using reheated edible oil is unhealthy. Edible oil is subjected to high temperatures for long periods in the process of frying food; this practice yields lipid peroxidation products, which is harmful to human health [16].
In this work, we tried to determine the effect of carbon dioxide laser (10,600 nm) heating and reheating on the chemical and physical characteristics of sunflower oil compared with the effect of electrical heater heating and reheating, in order to determine the most healthy heating method for cooking.
2. Materials and Methods
2.1. Material
Sunflower oil was obtained from the local market in Khartoum, Sudan. Five samples of sunflower oil were taken with amount of 50 ml for each sample. Then the following processes were done.
2.2. Heating Process
One of the five sunflower oil samples was heated to 250˚C using electrical heater. The second sample was heated twice to the same degree (250˚C) using electrical heater; it was left to cools to the room temperature then reheated; it takes two minutes in heating and in reheating. The third sample was heated to 50˚C using laser irradiation. The fourth sample was heated twice to the same degree (50˚C) using laser irradiation; it was left to cools to the room temperature then reheated. The irradiation process was done using carbon dioxide laser (CO2) (Model IB-601B, Beijing Innobri Technology, China) with wavelength 10,600 nm and output power 30 Watt, with duration time of exposure equal to 30 minutes. The sunflower oil sample was placed in open beaker with a capacity of 100 ml on magnetic stirring at room temperature. The distance between the oil and the end of the laser was 1 cm. The fifth one was left to be control sample.
2.3. Physicochemical Properties Characterization
Physicochemical properties such as acid value, ester value, free fatty acids, peroxide value, density, refractive index, viscosity, and moisture of the five sunflower oil samples were characterized according to the methods described in the A.O.A.C. 1990 [17]; All tests were performed in triplicate.
2.4. Determination of Oil Color
Color measurement of the oil samples was determined using a Lovibond Tintometer (Model E AF900 (The Tintometer Ltd.)) as units of red, yellow and blue according to the AOAC method [17]. Each visual measurement of every sample was taken in triplicate.
2.5. FT-IR Characterization
FT-IR spectra of the five sunflower oil samples were carried out using a Fourier Transform Infra-Red Spectrometer (Shimadzu, Japan). It is used to compare the chemical structure of the different five sunflower oils.
3. Results and Dissections
Due to repeated heating, the quality, color, smell and taste of edible oil changes due to the formation of polymers and polar compounds [18].
In this study; the effects of heating and reheating of the sunflower oil samples using electrical heater and carbon dioxide laser were investigated for the changes in the physicochemical properties like acid value, ester value, free fatty acids, peroxide value, density, refractive index, viscosity, and moisture content. The results of these properties were compared with the unheated sample.
3.1. Physicochemical Properties
The effects of heating and reheating on the physical and chemical characteristics of sunflower oil using laser and electrical heater compared with the unheated sunflower oil are presented in Tables 1-3.
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Table 1. Values of the chemical properties of the five sunflower oil samples.
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Table 2. Color measurements of the five sunflower oil samples.
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Table 3. The other physical properties of the five sunflower oil samples.
3.1.1. Results of the Chemical Properties
The obtained results of heating processes of the sunflower oil in Table 1 showed increasing in the oil chemical properties. Reheating processes caused in more increasing in the oil chemical properties; these results agree with the results of Adriana Abdul Aziz1 et al. [19]. It was observed that laser-heating process increased the oil chemical properties greater than the increasing of reheating processes by electrical heater.
This obtained results showed that the trend of chemical properties was increased crossover all tested samples starting from the once-heated electrically sample undergoing with the twice-heated electrically then once-laser-heated undergoing with twice-laser-heated sample. Chemical properties had increased with the increasing number of heating sessions. The increment of chemical properties of the oils is due to the development of hydroperoxides of unsaturated fatty acids because of the lipid oxidation process. Oxidation of oils is influenced by means of different factors which include the degree of unsaturation, heat, light, oil processing, antioxidants and transition metals [19].
3.1.2. Results of the Physical Properties
The chemical reactions in the oil leads to changing in some physical properties such as viscosity, density, darken color, or production of foam [20]; these changes clearly showed in this study in the results presented in Table 2 and Table 3.
Table 2 showed the changing of the oil color for the control, heated, and reheated samples. The blue color does not appear by heating and reheating using the electrical heater; while it was increased from 0 up to 0.2. The red color increased from 1.2 up to 1.8 by heating and reheating using the electrical heater; while it was increased from 1.2 up to 2.5. The yellow color increased from 10.2 up to 11.5 by heating and reheating using the electrical heater; while it was increased from 10.2 up to 11.9. It was observed that oil has darkened its color and emanated smoke during heating sessions, which indicates oil degradation. Chemical reactions, which influenced by heating processes; results in foaming production, darkening of oil color, and off-flavor. Increase in color of oil by heating was reported by Sonia and Badereldeen [21], and was found also by Augustin et al. [22] and Mudawi et al. [23].
Table 3 shows changes in moisture, density, viscosity and refractive index (RI) of all oil samples during heating and reheating. Moisture increased from 0.34% in control sample to 0.63% after reheating process using electrical heater, while it increased up to 0.91% after reheating process using laser. The density increased from 0.9150 cm3 in control sample to 0.9178 cm3 after heating and it decreased to 0.9173 cm3 after reheating process using electrical heater, while it increased up to the same value 0.9176 cm3 after heating and reheating process using laser. While viscosity increased from 60.58 Cp in control sample up to 60.85 Cp after reheating process using electrical heater, whereas it increased up to 61.47 Cp after reheating process using laser. The refractive index increased from 1.4698 in control sample to 1.4786 after reheating process using electrical heater, while it increased up to 1.4866 after reheating process using laser. It is clear that all physical properties of the five sunflower oil samples increased by heating and reheating at 50˚C. Physical properties temperature dependence is agree with the study of Wiege et al. in 2020 [24].
3.2. FTIR Results
FT-IR spectroscopy is a very good technique for analysis, as the intensities of the bands in the spectrum are proportional to concentration. FT-IR spectra of the control, heated and reheated sunflower oil samples are presented in Figure 1.
Each sample contains C=O (1745.46 cm−1) and C-O (1163.00 cm−1) which may indicate possible existence of COO, ester functionality (oils), samples were rich in C-H (sp3) (2923.88 cm−1 and 2854.45 cm−1) indicating presence of CH3 and CH2 as reinforced by bands in (1463.87 cm−1 and 1377.08 cm−1). Weak band at 1650 cm−1 C=C with C=C-H 3006 cm−1 but not conjugated with C=O.
As shown in Table 4, the changed band assignments of sunflower oil samples were: 3469.70 cm−1, 2677.01 cm−1 2731.02 cm−1, 2360.71 cm−1, 1654.81 cm−1 and
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Table 4. Band assignments of sunflower oil samples.
585 - 595 cm−1. It showed a notable difference depend on heating and reheating processes using laser and electrical heater.
4. Conclusions
In this paper, the effect of carbon dioxide laser (10,600 nm) heating and reheating on the chemical and physical characteristics of sunflower oil compared with the effect of electrical heater heating and reheating were investigated. As a result, when the same cooking oil is reheated, the chemical reactions enhance foaming, darkening of oil color, increased viscosity, and off-flavor. Hence, repeated heating of the oil can lead to degradation of the cooking oil, both chemically and physically. It was found that the long time of heating using laser rising temperature up to 50˚C catalyzed chemical reactions that resulted in effects in the oil samples characteristics greater than the effects of the electrical heater in a few minutes with temperature 250˚C.
Acknowledgements
The authors would like to thank the Institute of Laser at Sudan University of Science and Technology (SUST) for providing equipment used in this work.