Abstract
Vegetable oils usually undergo a series of processing steps prior to being brought to market in food and pharmaceutical products. Bleaching is an important step in this sequence and uses clay mineral adsorbents to remove lipid oxidation products and plant pigments. This improves the shelf-life, palatability and appearance of the oil. Despite the bleaching process being over 100 years old, knowledge of its effect on the minor components of vegetable oil is incomplete. Also, a full understanding of the mechanism by which specific compounds are removed, such as the plant pigment β-carotene, has not been determined. This thesis presents an investigation into the bleaching of a sesame oil. The interaction of β-carotene with an acid-activated clay (AAC) adsorbent is then studied in detail. Lastly, a variety of clay adsorbents have their physicochemical properties and interaction with β-carotene characterised, with the view of better understanding what makes an efficacious adsorbent.An investigation into a crude and refined sesame oil using UPLC-MS and an in-house developed data analytics method revealed large differences in their composition. The triacylyglycerol (TAG) and diacylglycerol (DAG) profile of the oil was assigned. DAGs were shown to be removed extensively by refining. The levels of lipid oxidation products of all varieties were shown to be significantly reduced. A positive correlation between the extent of removal and the number of lipid C=C bonds was observed. Levels of the antioxidants sesamin, sesamolin and tocopherols were also reduced drastically.
Studying the interaction of β-carotene with an AAC revealed that upon adsorption β-carotene is oxidised to a variety of carotenoid oxidation products (COPs). UPLC-MS/MS analysis showed that mostly C40 carotenoids but also many shorter chain, C10-C30 scission products are produced. Through interpretation of the UPLC-MS/MS data and use of a chemical standard, canthaxanthin and 3’,4’-didehydro-β-caroten-4-one were identified as some of the major oxidation products. Other short-chain carotenoids were tentatively identified by accurate mass and retention time measurements in combination with literature searching. A similar analysis of the COPs produced by a clay which had its native surface water molecules exchanged for H218O revealed uptake of 18O by the COPs. This indicated that surface water molecules are the source of the oxygen.
Characterisation of the structure, surface area, porosity and acidity of five clay adsorbents, and relation of these to their activity in adsorbing and degrading β-carotene, revealed material properties which facilitate β-carotene removal. High surface areas, a significant contribution of mesopores of radius greater than 2 nm, and a high concentration of Brønsted acid sites in particular were shown to enhance β-carotene adsorption and oxidation.
Bleaching was therefore shown to significantly decrease the levels of vegetable oil minor components which are both detrimental to oil quality, as well as potentially protective to oil shelf-life. The mechanism of β-carotene removal was shown to be partly chemical in nature, and to be sensitive to adsorbent moisture, specific surface area (SSA), porosity and acidity.
| Date of Award | 5 Dec 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Paul J Gates (Supervisor) & Jeroen S Van Duijneveldt (Supervisor) |
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