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ANTIOXIDANT ACTIVITY OF RAMBUTAN (NEPHELIUM LAPPACEUM) PEEL EXTRACTS IN SOYBEAN OIL DURING ACCELERATED STORAGE AND FRYING Number of words: 20.392 Minh Viet Thao Nguyen 0150047 Promotor: Prof. dr. ir. Katleen Raes Master’s Dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master of Science in Food Technology Academic Year: 2016 - 2017 Deze pagina is niet beschikbaar omdat ze persoonsgegevens bevat. Universiteitsbibliotheek Gent, 2021. This page is not available because it contains personal information. Ghent University, Library, 2021. Preface The completion of this thesis has required the help and support of numerous people. First of all, I would like to express my gratitude towards Prof. dr. ir. Katleen Raes for all of her help and constructive advice throughout my research. Secondly, I would like to thank my supervisor – Nhat Minh Phuong Nguyen for her valuable experience in laboratory, for detailed explanation and for her enthusiastic guidance. I have learned so much from them. I would also like to thank members the Laboratory of Food Microbiology and Biotechnology (UGent, Campus Kortrijk) for all of their help and support. Finally, I would like to thank my parents, who supported me with infinite love and understanding, my dearest friends and my boyfriend for their support and encouragement. I feel extraordinarily blessed to have such a network of wonderful people in my life. Thank you all for believing in me and helping me reach my goals. Table of contents Abstract ................................................................................................................................................ 1 1. INTRODUCTION ....................................................................................................................... 2 2. LITERATURE REVIEW ............................................................................................................ 4 2.1. Lipid oxidation ...................................................................................................................... 4 2.1.1 Overview ........................................................................................................................ 4 2.1.2 Mechanism of lipid oxidation ........................................................................................ 4 2.1.2.1 Autoxidation .......................................................................................................................... 4 2.1.2.2 Photosensitized oxidation ...................................................................................................... 6 2.1.3 Lipid oxidation decomposition products ....................................................................... 7 2.1.4 Factors influencing oxidative stability ........................................................................... 8 2.1.4.1 Fatty acid composition ........................................................................................................... 8 2.1.4.2 Temperature ........................................................................................................................... 9 2.1.4.3 Light ..................................................................................................................................... 11 2.1.4.4 Impact of storage on lipid oxidation .................................................................................... 11 2.1.4.5 Minor compounds ................................................................................................................ 11 2.1.4.6 Enzymes ............................................................................................................................... 12 2.2. Antioxidants: control of lipid oxidation .............................................................................. 12 2.3.1 Overview ...................................................................................................................... 12 2.3.2 Classification and mechanism ...................................................................................... 13 2.3.2.1 Primary antioxidants ............................................................................................................ 13 2.3.2.2 Secondary antioxidants ........................................................................................................ 13 2.3.3 Natural antioxidants ..................................................................................................... 16 2.3.4 Impact of high heat treatment on antioxidants ............................................................. 19 2.3. Rambutan peel, a natural source of antioxidants................................................................. 20 2.3.1. Overview ...................................................................................................................... 20 2.3.2. Cultivation and processing of rambutan in Southeast Asia ......................................... 21 2.3.3. Phenolic compounds in rambutan peel ........................................................................ 23 2.4. Soybean oil .......................................................................................................................... 24 2.4.1. Introduction .................................................................................................................. 24 2.4.2. Composition of soybean oil ......................................................................................... 26 2.4.3. Factors influencing oxidative stability, evaluation and control ................................... 27 3. MATERIALS AND METHODS ............................................................................................... 29 3.1. Materials .............................................................................................................................. 29 3.1.1. Reagents ....................................................................................................................... 29 3.1.2. Rambutan peel ............................................................................................................. 29 3.1.3. Soybean oil................................................................................................................... 29 3.1.4. Potato ........................................................................................................................... 30 3.2. Methods ............................................................................................................................... 30 3.2.1. Preparation of rambutan peel extract ........................................................................... 30 3.2.2. Experiments ................................................................................................................. 31 3.2.2.1. Accelerated storage conditions ........................................................................................ 31 3.2.2.2. Heating test ...................................................................................................................... 32 3.2.2.3. Frying process: Application on deep frying with potatoes .............................................. 32 3.2.3. Analysis of oil and potato ............................................................................................ 32 3.2.3.1. Conjugated dienes and trienes .......................................................................................... 32 3.2.3.2. Determination of peroxide value (PV) ............................................................................. 33 3.2.3.3. Para-anisidine value (p-AnV) .......................................................................................... 33 3.2.3.4. Determination of total phenolic content (TPC) ................................................................ 34 3.2.3.5. Determination of malondialdehyde (MDA) in oil............................................................ 34 3.2.3.6. Determination of malondialdehyde in fried potatoes ....................................................... 35 3.2.3.7. Color measurement .......................................................................................................... 36 3.2.4. Statistical analysis ........................................................................................................ 37 4. RESULT AND DISCUSSION .................................................................................................. 38 4.1. Accelerated storage conditions............................................................................................ 38 4.1.1. Color of oils ..................................................................................................................... 38 4.1.2. Conjugated dienes and trienes ......................................................................................... 40 4.1.3. Peroxide value (PV)......................................................................................................... 40 4.1.4. p-anisidine value (p-AnV) ............................................................................................... 44 4.2. Heating test .......................................................................................................................... 49 4.3. Frying process ..................................................................................................................... 52 4.3.1. Conjugated dienes and trienes ..................................................................................... 53 4.3.2. Peroxide value (PV) ..................................................................................................... 53 4.3.3. Para-anisidine value (p-AnV) ...................................................................................... 54 4.3.4. Malondialdehyde (MDA) in oil ................................................................................... 56 4.3.5. Color of potatoes .......................................................................................................... 57 4.3.6. Color measurement ...................................................................................................... 59 4.3.7. Malondialdehyde (MDA) in potato ............................................................................. 62 5. CONCLUSIONS AND PROSPECTS ....................................................................................... 65 References .......................................................................................................................................... 67 List of abbreviations BHA butylated hydroxyanisole BHT butylated hydroxytoluene CD conjugated dienes CT conjugated trienes DHHDP dehexahydroxydiphenoyl FC Folin–Ciocalteu GAE gallic acid equivalents HHDP exahydroxydiphenoyl KI potassium iodine MDA malondialdehyde p-AnV para-anisidine PC phosphatidylcholine PI phosphatidylinositols PV peroxide value RP rambutan peel Sen photosensitizers TAG triacylglycerols TBA thiobarbituric acid TBHQ tert-butylhydroquinone TCA trichloroacetic acid TEP tetraethoxypropane TPC total phenolic content Abstract Rambutan peel is a by-product which has been proven to possess a high amount of phenolic compounds, and antioxidant potential. The antioxidant efficacy of rambutan peel extract in stabilization of soybean oil under different conditions i.e. storage, frying with (frying test) and without food matrix – potatoes (heating test) conditions was investigated. To evaluate the antioxidant effect of rambutan peel on the oxidative stabilization of soybean oil during 160 days of storage periods under different storage conditions (dark/light, 4 and 30˚C), conjugated dienes (CD), conjugated trienes (CT), peroxide value (PV), p-anisidine (p-AnV) and total phenolic content (TPC) of oils were analyzed for every 20-day interval. The changes in the PV, p-AnV and TPC of oils indicate that rambutan peel extract seemingly exhibit antioxidant potentials in soybean oil during storage. However, phenolic compounds in rambutan peel extract are not stable in light condition compared to TBHQ. The results from peroxide value (PV), p-anisidine (p-AnV), of soybean oils under frying with and without potatoes illustrate that both TBHQ and rambutan peel extract have no effect on retardation of oil oxidation. However, the results from malondialdehyde (MDA) levels of oil samples and fried potatoes indicate that rambutan peel extract can prevent the formation of MDA – a secondary oxidation product. Besides this, the tremendous decrease in TPC of 100ppm TBHQ and 1000ppm RP treatments under heating condition indicate that both TBHQ and phenolic compounds in rambutan peel are not heat stable. The fluctuation of CD and CT parameters in all tests shows that they are not reliable to evaluate the oxidation level of soybean oil in this study. The most attractive appearance of fried potatoes was obtained from rambutan peel extract (apart from batch 1). The results from color measurement showed that both rambutan peel extract and TBHQ treatments had insignificant effect on the total color attributes of fried potatoes compared to the control. The overall results suggest that rambutan peel can be considered as a promising source of antioxidants for retardation of the lipid oxidation, especially in vegetable oils. 1 1. INTRODUCTION Vegetable oils are well-known as the ideal cooking medium due to a high amount of polyunsaturated fatty acids which are known for their cholesterol lowering effect. However, these fatty acids are more prone to be oxidized compared to animal fats which contain mainly saturated fatty acids (Ruxton, Reed et al. 2004). Lipid oxidation is a serious issue in food science, causing off-flavors, and decreasing nutritional quality and safety. Oxidation can be prevented by means of methods such as vacuum packaging, modified atmosphere packaging, and freezing. Yet, the utilization of antioxidants seems to be the most preferable way due to its effectiveness, convenience, and economy (Naz, Siddiqi et al. 2005). Synthetic antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ) are applied to retard the oxidative degradation. However, synthetic antioxidants are known to have the potential mutagenicity and other health problems (Sehwag and Das 2013). The carcinogenic effect of synthetic antioxidants was reported by Ito, Hirose et al. (1986). Also, synthetic antioxidants, may cause liver enlargement and interfere with the liver enzyme system (Martin and Gilbert 1968). These reasons induce the search for their replacement with natural antioxidants. In recent decades, much attention has been paid to phenolic compounds due to their multiple biological effects such as inhibition of low-density protein oxidation, reducing the heart disease risks, anticarcinogenic properties and antioxidant activity (Williams and Elliot 1997, Miyake, Murakami et al. 1999). Phenolic compounds work as antioxidants to scavenge reactive oxygen species and to chelate metals (Khatun, Eguchi et al. 2006). Top sources containing phenolic compounds are fruits and vegetables (Dimitrios 2006). Spices, herbs and their extracts have been proved to demonstrated strong antioxidant activities (Sehwag and Das 2013). Recently, worldwide emerging great sources of natural antioxidants are agricultural by-products which are peels and seeds. Some studies have reported the effectiveness of by-product extracts in stabilization of vegetable oils. For instance, the strong effectiveness of potato peel extract as a natural antioxidant in soybean oil during storage was evaluated by Habib and Shah (2004). Pomegranate peel extracts also exhibited the antioxidant activity in stabilization of sunflower oil under accelerated conditions (Iqbal, Haleem et al. 2008). Shaker (2006) reported that red grape seed and peel extracts can prevent oxidation in sunflower oil. Besides, many published studies which focus on the 2 evaluation and exploitation of waste products such as citrus peels and pomace, apple pomace, grape seeds and pomace, carrot pulp waste, mango seed kernel, and others were summarized by (Dimitrios 2006, Maisuthisakul 2009). Rambutan (Nephelium lappaceum L.) is a tropical fruit which is a member of the family Sapindaceae. Rambutan is popular in South-East Asia such as Vietnam, Philippines, Indonesia, Thailand, Burma, Sri Lanka and India (Tindall 1994). This kind of fruit can be consumed in fresh or as processed fruit. During processing, a huge amount of peels is discarded. Rambutan peels have been proven to be a rich source of phenolic compounds with great antioxidant activity, low pro-oxidant and non-toxic capacity (Okonogi, Duangrat et al. 2007, Palanisamy, Cheng et al. 2008, Thitilertdecha, Teerawutgulrag et al. 2010). The effect of rambutan peel extract on stabilizing sunflower oils during 24 days of storage at 60oC was reported by Mei, Ismail et al. (2014). However, the effect of the rambutan peel extract as a rich source of antioxidants in oils under different storage conditions (light – dark, low – high temperature) as well as under frying condition with food matrix the has not yet been reported. Within this framework, the objective of this thesis is to investigate antioxidant activity of rambutan peel extract in soybean oil under different storage conditions i.e. dark – 4°C, dark – 30°C and light – 30°C. Furthermore, the effectiveness of rambutan peel towards oxidative stability of soybean oil under frying conditions without and with a food matrix (e.g. potatoes) was assessed in this thesis. 3

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Katleen Raes. Master's Dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master of Science in Food Technology. Academic Year: 2016 - 2017 .. To evaluate the antioxidant effect of rambutan peel on the oxidative stabilization of soybean oil during
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