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Natural antioxidants
There has been an international focus in the food industry on the use of natural flavours, colourants, flavour enhancers and antioxidants. Natural antioxidants are being researched intensively due to the potential toxicological long-term effects of synthetic antioxidants (Rodríguez-Rojo et al., 2012). Food companies are changing their products to adhere to the standards set by consumers. Using natural antioxidants is challenging as they do not always provide the same shelf life as a synthetic antioxidant (Chen et al., 2011).
Research showed that some phytochemical antioxidants have unpredictable effects and can act as antioxidants or pro-oxidants depending on their concentration (Choueiri et al., 2012). Pectin, a natural antioxidant, has also been reported to cause emulsion instability (Celus et al., 2018). This adds to the challenges of using natural antioxidants (Choueiri et al., 2012). Natural antioxidants often used in the food industry include proteins, tocopherols, polyphenols to name a few.
Proteins
Proteins such as β-lactoglobulin have been shown to inhibit oxidative deterioration of lipids in a wide range of food systems including oil-in-water emulsions (Faraji et al., 2004; Elias et al., 2008). Aromatic amino acids (tyrosine, tryptophan, phenyl-alanine) as well as sulphur-containing amino acids (free cysteine, methionine) can act as natural antioxidants in foods through different mechanisms such as scavenging of peroxyl radicals and chelation of transition metals (Elias et al., 2008). Proteins furthermore protect endogenous antioxidant enzymes (Elias et al., 2008) and can also act as antioxidants by reducing iron’s reactivity by means of interfering with its redox cycling capacity and oxidizing ferrous ions to the ferric state. Oxidized iron migrates to the protein’s cavity where it nucleates and aggregates to form a ferric hydroxide core that is unavailable to participate in lipid decomposition (Elias et al., 2008). Proteins can be added to the aqueous phase of oil-in-water emulsions to improve oxidative stability, however the use of proteins in oil-in-water emulsions is limited as they could impact the texture, colour and flavour of oil-in-water emulsions (Elias et al., 2008).
Tocopherols
Tocopherols and tocotrienols are important minor components in most edible oils as they are naturally present antioxidants and a source of vitamin E (Chaiyasit et al., 2007). Tocopherols found in sunflower oil are mainly in the α-tocopherol form, as illustrated in Fig 2.2. Tocopherols are naturally present in oil, but may be removed during the refining and deodorising process. According to Coppen (1999) there is very little benefit in adding natural tocopherols to a vegetable oil in addition to the natural tocopherols already present in an attempt to improve the shelf life. However, the natural tocopherols present in oil are not very heat stable and may not survive food processing operations. Therefore, the naturally occurring tocopherol may be supplemented with synthetic tocopherols. A concern often raised with the use of the tocopherols as antioxidants relates to the pro-oxidant effects at elevated levels. It has been proposed that tocopherols are not pro-oxidants, but may act synergistically with pro-oxidants in the presence of copper and ascorbate. This is probably due to the recycling of tocopherols by ascorbate through reactions with tocopheroxyl radical. Merril et al. (2008) found that the antioxidant effect of tocopherols is maintained as long as ascorbate is present.
Phenolic compounds
Phenolic compounds, naturally found in plant extracts have been reported to show good antioxidant activity (Hinnenberg et al., 2006; Jayasinghe et al., 2013), however their effect on the oxidative stability of lipid dispersions can be mixed with both antioxidant and pro-oxidant activities reported (Huang and Frankel, 1997). Jayasinghe et al. (2013) conducted extensive research on the antioxidative effect of phenolic compounds found in the fruit of Indian gooseberry fruit (Emblica officinallis) and the leaves of sweet basil (Ocimum basilicum L.) in oil-in-water emulsions and concluded that these phenolic compounds have antioxidative activity. These phenolic compounds are found in the chloroplasts in the leaves as well as the stems, roots and flowers of plants (Rodríguez-Rojo et al., 2012) and may have a bitter and astringent taste (Gomes et al., 2016). Hydro distillation removes the flavour of the herbs which enables the use of phenolic compounds as antioxidants. The number of phenolic hydroxyl groups as well as their position and the presence of other functional groups in the whole molecule plays an important role in antioxidant activity (Rice-Evans et al., 1996). The total phenolic content is expressed as gallic acid equivalents which are equal to (mg gallic acid)/(g extract) (Hinnenberg et al., 2006). The total phenolic content of rosemary extract is 162 mg gallic acid/g extract with the major active phenolic compound being rosmarinic acid (Fig 2.3) and carnosic acid (Fig 2.4) (Erkan et al., 2008). Sweet basil has 579.3 mg gallic acid/g extract equivalent (Jayasinghe et al., 2013) with the major active compounds being rosmarinic acid (Jayasinghe et al., 2003). Rosmarinic acid with four phenolic hydroxyl groups (Fig 2.3) is expected to have higher antioxidant power compared to carnosic acid, for example, with only two phenolic hydroxyl groups (Fig 2.4).
CHAPTER 1: INTRODUCTION
CHAPTER 2: LITERATURE REVIEW
2.1 OIL-IN-WATER EMULSION
2.2 OXIDATION OF OIL AND OIL-IN-WATER EMULSIONS
2.3 FACTORS AFFECTING THE DEVELOPMENT OF LIPID OXIDATION IN OIL-IN-WATER EMULSIONS
2.4 MEASURING OXIDATION
2.5 HEALTH IMPLICATIONS ASSOCIATED WITH RANCIDITY
2.5 CONCLUSION
CHAPTER 3: THE EFFECTS OF ANTIOXIDANTS AND SHELF LIFE CONDITIONS ON OXIDATION MARKERS IN A SUNFLOWER OIL SALAD DRESSING EMULSION (SOSDE)
3.1 INTRODUCTION
3.2. MATERIALS AND METHODS
3.3. RESULTS
3.4. CONCLUSIONS
CHAPTER 4: MULTIVARIATE ACCELERATED SHELF LIFE TEST OF SUNFLOWER OIL IN WATER EMULSIONS
4.1 INTRODUCTION
4.2 MATERIALS AND METHODS
4.3 RESULTS AND DISCUSSION
4.4. CONCLUSIONS
CHAPTER 5: GENERAL DISCUSSION
5.1 SHELF LIFE METHODOLOGIES
5.2 MULTIVARIATE ACCELERATED SHELF LIFE TESTING (MASLT
5.3 DISCUSSION OF RESULTS
CHAPTER 6: CONCLUSIONS
CHAPTER 7: REFERENCES
