The Effect of Frying Process on the Level of Malondialdehyde in Different Meat Products
Marzieh Hejazy; PhD^*1, Seyed Amin Khatibi; PhD² & Zahra Shamsi; DVM¹

 
¹ Department of Basic Sciences, School of Veterinary Medicine, University of Tabriz, Tabriz, Iran.
² Department of Food Hygiene and Aquatic, School of Veterinary Medicine, University of Tabriz, Tabriz, Iran.
 

 
Introduction

Meat is very important in human nutrition due to its proper content of protein, B vitamins, and trace elements. It is usually supplied in the raw form in the markets; however, consumers are highly interested in pre-cooked and ready-to-eat products (Demirezen and Uruç, 2006, Pandey et al., 2014). Among the various cooking processes, frying is one of the common methods for the preparation of food especially meat varieties and meat products that is performed using different types of oils and fats (Suomela et al., 2004). The overall quality, flavor, and texture of foods are usually improved using this process (Chukwu, 2009). Despite its popularity, this processing method has some harmful effects on food composition. During the frying process, frying oils and fats are exposed to the moisture and air, resulting in the physical and chemical deterioration of food. It also can affect the frying efficiency shelf life of fried products (Fauziah and Aini, 2000, Pandey et al., 2014). The frying oil has three main adverse reactions with food ingredients, including oxidation, hydrolysis, and thermal decomposition (Dana and Saguy, 2001). The fatty acids and triglycerides of food are oxidized during the frying process. Lipid oxidation is the major cause of chemical spoilage of meat products which results in texture modification, rancidity, loss of nutrients, production of unfavorable odors, and toxic compounds (Broncano et al., 2009, Papastergiadis et al., 2012).  Oxidation of unsaturated fatty acids results in the formation of odorless and tasteless compounds called hydroperoxides. These compounds are then decomposed to flavorful products, such as aldehydes, ketones, alcohols, acids, and hydrocarbons (de las Heras et al., 2003, Mendes et al., 2009, Nassu et al., 2003, Papastergiadis et al., 2012). These products can affect the quality of food in terms of color, texture, flavor, and odor (Mendes et al., 2009). It has also been reported that lipid oxidation products are associated with several adverse effects on the body health, such as atherosclerosis, Alzheimer’s disease, cancer, inflammation, or aging processes (Broncano et al., 2009, Nassu et al., 2003, Rahman et al., 2007).
In the products like fish and meat, determination of malondialdehyde (MDA) level is often chosen as an index of rancidity. The structure of MDA is composed of a three-carbon dialdehyde that is attached to carbonyl groups at the positions of C-1 and C-3. The mutagenic and carcinogenic effects of MDA have been reported for human body because of the formation of adducts with proteins and DNA (Papastergiadis et al., 2012, Zeb and Ullah, 2016).
In recent years, a variety of analytical methods have been proposed for determination of MDA (Bauer-Plank and Steenhorst-Slikkerveer, 2000, Zeb, 2012, Zeb and Ullah, 2016). Given the spectrophotometric method has a simple and low-cost procedure, it is usually used for the analysis of a large number of samples (Papastergiadis et al., 2012). The 2-thiobarbituric acid (TBA) test is a widely used spectrophotometric method to measure lipid oxidation in meat products which can determine MDA content (Broncano et al., 2009, de las Heras et al., 2003, Labropoulos et al., 2013, Papastergiadis et al., 2012). This method is precise, sensitive, and highly reproducible (Zeb and Ullah, 2016). At low pH and high temperature, the active methylene groups of TBA react with the monoenolic form of MDA. This reaction produces a pink-colored complex that gives a maximum absorbance at the wavelength of 532−535nm (Broncano et al., 2009, de las Heras et al., 2003, Papastergiadis et al., 2012, Seljeskog et al., 2006). The intensity of color is used for the measuring of MDA concentration (Mendes et al., 2009). Given TBA may react with some other active compounds; a more general term is used that is called thiobarbituric acid reactive substances (TBARS) (Sun et al., 2001, Zeb and Ullah, 2016).
Fried foods, such as pan kebab (Kebab Tabei), fried hamburger, sausage, chicken, fish, and beef are known as the most consumed meat products in Iran. Therefore, the purpose of present study was to determine the effect of frying process on levels of MDA in these products before and after frying.
Materials and Methods
Chemicals and materials: MDA tetrabutylammonium salt (≥ 96%) and TBA (≥ 98%) were purchased from Sigma-Aldrich (Steinheim, Germany). Glacial acetic acid (glacial) (≥ 99.8%) was also provided from Merck Company (Darmstadt, Germany). Other used chemicals had an analytical grade.
Samples of meat products: During the spring of 2020, chicken, fish and meat, commercial hamburgers (containing 30%, 55%, 70%, and 90% of meat), and sausage were bought fresh from the local markets (Tabriz city, Iran). Samples were placed into a box containing ice packs and they were immediately transferred to the laboratory.
        Breast muscles of the chicken were excised from each carcass, and cut into 1 cm-thick slices using a scalpel. Cow muscles were also cut into small pieces (1 cm thickness) and all visible fat was removed from them. Fresh rainbow trout were washed with potable water to remove the slime and blood on its surface. To yielding the fillets, the skin, head, tail, and bones were removed from each fish. Minced meat and onion were used for the preparation of pan kebab. The main ingredients of these products are presented in Table 1. The frying process was performed in a pan containing 5 mL of sunflower oil. The temperature of frying process was about 220°C. One sample from each product was used as raw control. Others were subjected to the frying process in three replicates.
Measurement of MDA in the samples: In order to prepare a stock solution (1 mM, w/v), 31.35 mg of MDA was dissolved in a flask containing glacial acetic acid (100 ml). Different concentrations of MDA (0.1, 0.2, 0.4, 0.6, and 0.8 mM) were prepared from the stock solution. Also, 11.532 mg of TBA was dissolved in 20 ml of glacial acetic acid to prepare the standard solution of TBA (4.0 mM).
One ml of the standard TBA solution was taken to a test tube containing 1 ml of MDA standards. The tubes were heated in a water bath at 95^°C for 60 min. The mixtures were cooled at room temperature and absorbance of them was measured at the wavelength of 532 nm using a UV-visible spectrophotometer (Beckman, Germany). Acetic acid was used as a blank sample. Different concentrations of MDA (ranged between 0.1 mM and 1.0 mM) were used for drawing a standard curve.
        The level of MDA in the samples was determined before and after frying. Only completely fried portions of the products were used for the analysis. One gram of each homogenized sample was completely dissolved in a test tube containing 5 ml of acetic acid. The mixture was shaken for 1 h and filtered using a filter paper. The filtrates were centrifuged for 10 min and the supernatant was used for the analysis. The supernatant (1 ml) was mixed in a tube containing TBA (1 ml).
        The tubes were placed in a water bath for 1 h at 95^°C. They were cooled at room temperature. The absorbance of mixtures was measured by a spectrophotometer in three replicates. The level of TBARS (µM MDA/g) was calculated in the extracts using the standard curve (Pandey et al., 2014).
Data analysis:          Statistical analysis of the data was carried out using SPSS 18 statistics package (IBM, SPSS, Inc.). The differences between the mean values were determined significant when the p-value was lower than 0.05. The one-way analysis of variance (ANOVA) was used for this purpose.
Results
        The calibration curve was prepared using different concentrations of MDA in the range of 0.1 to 1.0 𝜇M. The stock solution of MDA gave an accurate standard curve with high repeatability in the spectrophotometer. Linear regression in the calibration curve of MDA showed a correlation coefficient of 0.9969 (Figure 1).
        The TBARS values in the raw products were in the range of 0.03 to 0.160 µM MDA/g (Figure 2 and Figure 3). The highest and lowest levels of MDA in raw products were detected in chicken (0.03 µM/g) and pan kebab (0.16 µM/g), respectively.
        It was found that the frying process significantly (P < 0.05) increased TBARS content. The highest amount of MDA in fried products was detected in fried fish (1.24 µM/g). The results showed that the pan kebab contained a higher MDA level in comparison to other products made from cow meat.