Abstract/Summary

Blackcurrant skins is a by-product from the blackcurrant pressing process during the production of the juice, jams, jellies and nutraceutical ingredients. Blackcurrant skins are rich in polyphenolic compounds, primarily anthocyanins, which can be potentially utilised by the food industry as alternatives to synthetic colourants and as natural ingredients that can exert health promoting properties. Overall, the aim of the work presented in this thesis was to address key parts of a potential valorisation strategy for blackcurrant skins, focusing from the efficient drying process to the extraction of anthocyanins, and finally the application of anthocyanins as natural colourants. More specifically, the work aimed to address a number of objectives including comparing different approaches for drying the skins at laboratory and commercial scale, understanding the key factors influencing anthocyanin extraction from dried blackcurrant skins (DBS) (i.e. solvent, temperature, time), and evaluating different copigmentation methods for the stabilisation of the extracted anthocyanins and their colour. To address the first objective, different drying processes of blackcurrant pressed residues including industrial rotary drum drying, laboratory oven drying and freeze drying were investigated. Furthermore, in the case of industrial rotary drum drying the effect of different drying parameters such as temperatures, residence time, the ratio of air speed to drum rotor speed and particle sizes were also evaluated. The results demonstrated that industrial rotary drying showed significantly (p ≤ 0.05) higher total free anthocyanins (17.9 ± 0.4 mg/g) compared to freeze and oven drying, with the key factors influencing these identified as the hot air speed with gradient temperatures. This work indicated that industrial drum drying under optimised conditions prevents overheating of the blackcurrant skins which may lead to degradation of anthocyanins especially in the case of smaller particle sizes. 4 To address the second objective, a representative DBS sample from industrial rotary drum drying was extracted using alternative extraction methods to the conventional solvent-based extraction methods (i.e. with ethanol, methanol and methanol/water mixtures), including using food grade acetic acid and hot water at different temperatures and extraction times in order to identify the conditions that provided high yields of anthocyanins and other phenolic compounds such as hydroxycinnamic acids and flavonols. The composition, antioxidant activities and colour profiles of the obtained anthocyanins-containing extracts were analysed with the aim primarily to understand the influence of pH, temperature and solvent on colour formation and anthocyanin stability. Acetic acid/water mixtures (pH 1.5) resulted in extracts with the highest free anthocyanins (consisting primarily of delphinidin-3-O-glucoside, delphinidin-3-O-rutinoside, cyanidin-3-O-rutinoside and cyanidin-3-O-glucoside) and total phenolic content [16.6 mg/g and 37.0 mg gallic acid equivalent (GAE)/g], respectively amongst all solvents after 2 h extraction, which also demonstrated high antioxidant activities and high colour intensity and stability. To address the third and fourth objectives, the anthocyanin stability and colour stability of the extracts were enhanced through an intermolecular copigmentation approach involving non-covalent interactions between anthocyanins and various phenolic acids. This phenomenon is known as charge-transfer complex formation, or π–π interactions. The phenolic acids used as copigments were ferulic, caffeic, chlorogenic, tannic and rosmarinic acids, at pH 3.0 and 6.0 in buffers and solvent/buffer solutions. The results indicated that the anthocyanins/ferulic acid complexes at pH 3.0 exhibited the strongest interaction and resulted in the highest colour and anthocyanin stability during storage at 4 °C for 140 days, most likely because ferulic acid contains methoxylated derivatives that induce a stronger copigmentation effect than phenolic acids with hydroxyl substituents such as caffeic, chlorogenic, tannic and rosmarinic acids. Furthermore, the effect of copigmentation between anthocyanins and copigment through non-covalent bond in the intermolecular 5 copigmentation and covalent bond in the enzymatic acylation were compared. In terms of the enzymatic acylation approach using lipase as the biocatalyst, acylated anthocyanins were successfully synthesised using cinnamic acid methyl ester as the acyl donor, with the highest conversion yields obtained for cyanidin 3-(6″- cinnamoyl)-glucoside (~15.1%) and delphinidin 3-(6″- cinnamoyl)-glucoside (~10.1%). Interestingly, the acylated anthocyanins were more stable and demonstrated better stability and better colour stability during storage compared to the non-acylated anthocyanins, indicating that the acyl group which is covalently bound to the basic anthocyanin structure protects the anthocyanin molecules from hydration. Overall, the work presented in this thesis demonstrated that a valorisation process for DBS targeting the extraction of natural anthocyanins is potentially feasible without the need for large amounts of solvents. Moreover, the stability of the extracted anthocyanins can be enhanced through a number of copigmentation approaches, rendering them suitable alternatives to synthetic colourants for food applications.