- Title
- Optimized conditions for the preparation of citrus by-product aqueous extracts enriched in polyphenols and antioxidants
- Creator
- Papoutsis, Konstantinos
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- A large amount of lemon (Citrus limon L.) by-product is annually generated by the juice industry and is usually disposed to the land, constituting a severe environmental problem. Several studies have shown that citrus waste could be a good source of polyphenols which are compounds with antioxidant, antimicrobial, and antifungal properties. To date, the recovery of polyphenols from citrus by-products is performed by the use of organic solvents or their mixture with water. However, some disadvantages of the organic solvent use should be taken into consideration, including toxicity of organic solvent for human health and environment, solvent purchase and disposal costs, special storage requirements, and solvent residues in the final product. Pure water should be considered as an alternative solution, however, it leads to significantly lower recovery yields. The solution in this matter could be the application of appropriate drying conditions for high retention of polyphenols in the material before extraction, as well as the optimisation and selection of the extraction conditions employed for the recovery of polyphenols. After extraction polyphenols are prone to oxidation, and encapsulation should be employed for the protection of the extracted compounds. The encapsulation conditions, should be carefully selected, since inappropriate conditions could lead to a significant loss of polyphenols and antioxidants. According to the literature (Chapter 1) only a few studies have been conducted investigating the effect of different spray-drying conditions for the encapsulation of citrus extracts, using mainly whey protein isolate as a coating agent. Thus the current research was designed to address the following aspects: (i) investigating the effect of pre-treatment of the material before extraction on the retention of polyphenols and antioxidants, (ii) determining the optimum extraction conditions for polyphenols and antioxidants from lemon by-products, (iii) assessing the antifungal activities of the aqueous extracts against phytopathogenic fungi, and (iv) investigating the effect of different encapsulation conditions on the entrapment of polyphenols and antioxidants. The effect of three different drying techniques (freeze-drying, vacuum-drying, and hot air-drying) was investigated and the results showed that drying method significantly affects the physicochemical properties of the dried material, with hot air- and vacuum-drying techniques resulting in higher retention of polyphenols and antioxidants in the dried material compared to the freeze-drying. However, freeze-drying resulted in a dried product with significantly lower moisture content and water activity. In contrast to the freeze-drying, drying at high temperatures either by hot air- or vacuum-drying led to colour changes (browning) (Chapter 2). Two extraction techniques (hot water extraction (HWE) and aqueous ultrasound-assisted extraction (AUAE)) were optimised and compared with an organic solvent conventional extraction (OSCE) technique (Chapter 3). The results revealed that the optimum HWE resulted in extracts with the highest polyphenol content and antioxidant capacity compared to AUAE and OSCE in a considerably shorter time. These results were further supported by the application of scanning electron microscopy. Extracts obtained at the optimum HWE conditions showed high in vitro antifungal activities against Alternaria alternata, a fungus that causes losses in fruits and vegetables. The antifungal activity of the extracts was attributed to the presence of phenolic acids and ascorbic acid into the aqueous extracts. Lemon by-product extracts significantly changed the morphology of fungus hyphae, leading to a cell wall collapse and loss of linearity (Chapter 4). Two encapsulation techniques (spray-drying and freeze-drying) were investigated and compared using different coating agents (Chapter 5). Freeze-drying and a mixture of maltodextrin with soybean protein isolate were found to be the most efficient for the highest retention of polyphenols and antioxidants. Moreover, the encapsulated powder obtained by freeze-drying had significantly lower moisture content and water activity (aw) than that obtained by spray-drying. Scanning electron microscopy revealed that spray-drying resulted in the formation of spherical particles of different sizes with concavities, regardless the type of the wall material being used. Although freeze-drying resulted in powders of amorphous glassy shapes, the mixture of maltodextrin with soybean protein isolate as wall material resulted in the formation of spherical porous materials. In conclusion, in this study lemon by-products have been converted to value-added products which have the potential of being utilised by both pharmaceutical and food industries. The results presented in this thesis showed that apart from the selection of extraction conditions, sample pre-treatment (i.e. sample drying) is a crucial step and should be considered for the preparation of products from citrus wastes with high antioxidant and antifungal properties.
- Subject
- citrus; polyphenols; antioxidants; extraction; ultrasound extraction; solid solvent extraction; encapsulation; freeze-drying; Alternaria
- Identifier
- http://hdl.handle.net/1959.13/1398745
- Identifier
- uon:34483
- Rights
- Copyright 2018 Konstantinos Papoutsis
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 15 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 272 KB | Adobe Acrobat PDF | View Details Download |