Research

, Volume: 19( 6)

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Evaluating the Physicochemical and Microbial Properties of Soybean Oil in Consort with Spectral and Heavy Metal Content Available in Chattogram, Bangladesh

*Correspondence:

Md. Sahab Uddin Bangladesh Council of Scientific and Industrial Research, Chattogram-4220, Bangladesh, India; E-mail: sahab_uddin@bcsir.gov.bd

Abstract

There are numerous brands of soybean oils in the local markets of South Asian region, some of which are of low quality. This study focused on the evaluation of antimicrobial and physicochemical properties of soybean oils like as, iodine value, peroxide value, free fatty acid value and heavy metal content. Comparatively, low iodine value was noticed, free fatty acid value and peroxide value was in acceptable range but heavy metal content was in alarming array. The concentration of estimated heavy metals in oil samples was found in between 6.20-12.2 for Fe, 5.0-7.5 for Cu, 0.6-3.2 for Ni, 0.31-1.30 μg/g for Pb and Cd was in below detection limit. At room temperature, FTIR spectra showed no peaks at 2166 cm-1 and 3241 cm-1 while two additional peaks appeared in this region after several times frying. The examined soybean oils showed no inhibitory activity against E. coli, S. aureus, and K. pneumoniae.

Abstract

There are numerous brands of soybean oils in the local markets of South Asian region, some of which are of low quality. This study focused on the evaluation of antimicrobial and physicochemical properties of soybean oils like as, iodine value, peroxide value, free fatty acid value and heavy metal content. Comparatively, low iodine value was noticed, free fatty acid value and peroxide value was in acceptable range but heavy metal content was in alarming array. The concentration of estimated heavy metals in oil samples was found in between 6.20-12.2 for Fe, 5.0-7.5 for Cu, 0.6-3.2 for Ni, 0.31-1.30 μg/g for Pb and Cd was in below detection limit. At room temperature, FTIR spectra showed no peaks at 2166 cm-1 and 3241 cm-1 while two additional peaks appeared in this region after several times frying. The examined soybean oils showed no inhibitory activity against E. coli,
S. aureus, and K. pneumoniae.

Keywords

Edible oils; Soybean oil; Physicochemical properties; FFA; FTIR; Heavy metals; Antibacterial activity

Introduction

Vegetable oil is a vibrant food constituent, providing strength, essential fatty acid, and a carrier of fat-soluble vitamins. These oils are commonly used in food processing and also in cosmetic manufacturing industries [1]. Both edible fats and oils are water-insoluble substances consisting primarily of fatty acid glycerol ester or triglycerides with small quantities of nonglyceridic compounds [2]. The largest integrated sources of vegetable oils are seeds of respective plants which grow in a relatively temperate area. A significant source of energy for the human diet is vegetable oil and fat, which provides 37 kJ energy from 1 g oil [3]. Among all sorts of plants, soybean oil is dug up from the seed of soybean (Glycine max) [4]. Soybean is primarily grown in South and North America (Argentina and Brazil). It is one of the major vegetable oil that is widely used in Indian subcontinent for food manufacturing purposes, particularly in Bangladesh.

Soybean is primarily imported in crude form in Bangladesh and is then processed in domestic refineries. According to the statistical (2020) report, import and domestic consumption of soybean oil in Bangladesh was 1270 metric tons and 800 metric tons respectively in 2019/20, while world production for the same period was around 56.51 million metric tons [5,6]. However, some vegetable oils are not suitable for standards that satisfy consumers with regard to their physicochemical properties or the texture and stability of food products [7,8]. It was also found that, in soybean oil the percentage of five fatty acids such as palmitic, stearic, oleic, linoleic and linolenic acids were 14 ± 0.62, 4.07 ± 0.29, 23.3 ± 2.43, 52.2 ± 2.64 and
5.63 ± 3.48, respectively [9,10]. Various factors such as oil processing, the fatty acid composition of the oil, the energy of heat or light, the concentration and type of oxygen, transition metals, thermally oxidized compounds, peroxides, pigments, and antioxidants affect the oxidation of oil [11,12]. Few nonglyceride components are counterproductive to the consistency of freshness, a shelf-life, and toxicity of edible oil by evaluating a variety of trace metals [13]. Availability of trace metals such as Cu and Ni is intended to enhance the rate of oil oxidation while other metals such as Pb and Cd are very significant due to their toxicity and metabolic activity [14]. The existence of metal in soybean oil is subject to a variety of factors, such as soil, climate, plant genotype, fertilizers and metal-containing pesticides applied during the production process or contamination from metal equipment’s [15,16]. One of the main aims of this analysis was therefore to establish concentration levels of Pb, Ni, Cd, Cu, and Fe in soybean oil.

Some vegetable oils show potent antimicrobial activity against pathogenic microorganisms. S. aureus and E. coli are opportunistic bacteria that are widely found in the environment and the human body. These microorganisms can cause life- threatening infections in the immune-compromised patient [17]. So, control of these microorganisms in the food manufacturing industries is very important. In recent times, edible oils are one of the most essential components of the diet used for cooking. One of the most common methods used for cooking food is deep frying. Repeated frying results in many oxidative and thermal reactions resulting in modifications in the oil's physicochemical, nutritional, and sensory attributes [18]. In order to determine the consistency and functionality of the oil, several researchers studied the effect of temperature on texture, stability, morphology, and numerous parameters [19-21]. In this research, we attempted to track changes in the morphological properties of oils using FTIR to assess the degree of oxidation following heating and frying.

In addition, assessment of physical and chemical parameters is also essential to ensure the quality of the oil consumed in Bangladesh. Such physicochemical parameters include moisture content, pH, density, viscosity, peroxide value, iodine value, and free fatty acid value. The desire of this study was to provide information on the quality of refined soybean oil, to compare oil quality with established standards, to increase awareness among manufacturers and to provide recommendations to the monitoring authority.

Materials and Methods

Raw materials

Hydrochloric Acid (HCl), Sodium Chloride (NaCl), Potassium di-chromate (K2Cr2O7), Starch indicator, Phenolphthalein indicator, Sodium thiosulfate (Na2S2O3), all were A.R Grade and purchased from Merck, USA and Merck, Darmstadt Germany. Seven different brands of soybean oils were purchased from the local market of Chattogram city, Bangladesh, in 2020.

Frying process

Potatoes, onion, and ginger were peeled and cut into different sizes and fried in oil at a constant temperature for three times. Frying experiment was conducted at home condition, where cooking pan and gas burner were used.

pH, density and viscosity measurement

The pH of the oil samples was measured using Universal Indicator. The indicator was exported from Thermo Fisher Scientific, USA. The density of oil samples was measured by a pick-now meter with a capacity of 25 mL using the following equation:

Density=Mass of the oil (g)/Volume of the oil (mL) [22].

The Ostwald Viscometer (ASTMAD-435, Japan) measured the viscosity of all oil samples at 25°C, and recorded the flow time of oil samples with the aid of a stopwatch.

Peroxide value

Peroxide value of oil is mainly the measure of peroxides contained in the sample. The PV value of oil is determined by measuring the iodine content released from potassium iodide [23]. 2 g of oil sample was weighted first in a 25 ml test tube then 2 g of potassium iodide and 20 ml of solvent mixture (CHCl3:CH3COOH at the ratio of 1:2) added to the solution and gently shacked. The contents were then boiled for 30 seconds in a boiling water bath. The test tube was then cooled to room temperature while placing in tap water and then transferred to a 250 ml conical flask. Then 20 ml of 5% potassium iodide and 50 ml of distilled water were also added to the flask. Finally, the solution mixture was titrated against 0.002 N sodium thiosulphate solution using a starch indicator towards the endpoint [24].

Iodine value

Oil samples with known weight were treated with an excess amount of iodobromide in Glacial acetic acid. Here, untreated IBr reacted with KI which converts the iodobromide to Iodine. Then the concentration of iodine is determined by titrating with standard sodium thiosulphate using the following equation:

Iodine Value=(b-v) × N × 126.9 × 100/w × 1000

Here, b is the volume of sodium thiosulphate used as blank, v is the amount of sodium sulfate used for the sample, N is the concentration of titrating w is the wt. of oil sample and finally, 126.9 is the molecular wt. of iodine [25].

Free fatty acid (Acid Value)

10 g of oil sample was weighed and dissolved in hot 100 ml neutralized ethanol. Then the solution was titrated using 0.001 N alkali (KOH) solution, where phenopthelin was used as an indicator. The test solution was shaken vigorously and kept warm during the whole titration process [26]. Finally the acid value of the sample oils were calculated by means of the given equation:

Acid value (As oleic acid)=N of alkali × ml of alkali × 56.1/wt of the sample (g) [22].

Heavy metal analysis

For heavy metal analysis, we used atomic absorption spectroscopy (AAS) (Thermo Scientific, UK, Model: iCE 3300 AA System). The analyses were carried out using respective hollow cathode Lamp for Pb, Cd, Cu, Ni, and Fe under standard conditions (TABLE 1).

TABLE 1.Instrumental conditions of Atomic Absorption Specstrophy (AAS)

Spectroscopic analysis

A Fourier Transform Infrared Spectroscopy (PerkinElmer, Liantrisant, UK, Model: Spectrum-II) was used to record FTIR spectra for the oil samples before and after frying. It was used to evaluate the saturation and unsaturation status of heated and normal oils at room temperature for observation of the oxidation in oils.

Antimicrobial susceptibility test

The antibacterial activity of sample soybean oils against E. coli ATCC 25922, K. pneumoniae, S. aureus ATCC 6538 were evaluated according to CLSI-2019 guidelines [27]. In this experiment, the bacterial reference strains were cultured overnight at 37°C on Nutrient agar media (Himedia Laboratories, India). The optical density of E. coli, K. pneumoniae, and S. aureus were measured at 625 nm by using UV-spectrophotometer (Shimadzu, Model: UV-1800). The final populations were adjusted to 106 cfu/ml of each bacterial strain in a sterile saline solution. An amount of 0.5 mL of the suspensions was placed on Muller Hinton Agar media (Himedia Laboratories, India) with a sterile cotton swab. Under proper aseptic condition, blank sterilized discs (OXOID, UK, 6 mm diameter) were impregnated with 20 μl of soyabean oil and DMSO (1:1 ratio). The discs were placed on the agar surface containing bacterial suspension. A broad-spectrum antibiotic, Chloramphenicol (OXOID, 30 mg/ml) was used as positive control and a paper disc containing 10% DMSO was used as a negative control. Studies were performed in triplicate. The inhibition zone was measured after 24 hours of incubation at 37°C.

Results and Discussion

The quality of soybean oils were evaluated by analyzing their physicochemical parameters like viscosity, density, free fatty acid value, peroxide value, iodine value, heavy metal content and antibacterial activity. Results associated with these tests are tabulated and interpreted below:

Viscosity, density, moisture content and pH

TABLE 2 shows the moisture content of oil samples which ranged between 0.31%-0.49%. Moisture content is a most important parameter of oil as its presence may cause rancidity or unpleasant odor in oil. According to ASTM, recommended moisture content for soybean oil is 0.2%-0.3% [28]. The test samples contained slight more moisture than recommendation level but it is more approximate to standard level. Virtually it is observed that the density of all brands of oil exist between 0.90 – 0.95 at 30°C [29]. And the oil samples exhibit a satisfactory value of density which ranged between 0.89-0.91 at 25°C. Viscosity of oil samples were observed between 33.2-43.5 cP at 25°C. In general, oil and fats possess high viscosity due to intermolecular attraction between long chains of glyceride molecule. Viscosity increase with the increase of molecular weight but decrease with temperature. According to a work by Nuruddin et al. viscosity of soybean oil at 23.9, 37.8, 48.9 oC was 54.3, 31.8, 23.3 cP [30]. At 25 oC, pH of oil samples were observed around 6.5.