Abstract
Synthetic dyes are by far the most widely applied colourants in industry. However, environmental and sustainability considerations have led to an increasing efforts to substitute them with safer and more sustainable equivalents. One promising class of alternatives is the natural quinones; these are class of cyclic organic compounds characterized by a saturated (C6) ring that contains two oxygen atoms that are bonded to carbonyls and have sufficient conjugation to show color. Therefore, this study looks at the potential of isolating and applying quinone dye molecules from a sustainable source as a possible replacement for synthetic dyes. It presents an in-depth description of the three main classes of quinoid compounds in terms of their structure, occurrence biogenesis and toxicology. Extraction and purification strategies, as well as analytical methods, are then discussed. Finally, current dyeing applications are summarised. The literature review shows that natural quinone dye compounds are ubiquitous, albeit in moderate quantities, but all have a possibility of enhanced production. They also display better dyeability, stability, brightness and fastness compared to other alternative natural dyes, such as anthocyanins and carotenoids. Furthermore, they are safer for the environment than are many synthetic counterparts. Their extraction, purification and analysis are simple and fast, making them potential substitutes for their synthetic equivalents.Graphic
Highlights
Dyes can generally be categorised as either synthetic or natural
While synthetic dyes are typically synthesised from petrochemical sources, natural colourants are abundantly available in nature and can be obtained from plants, microorganisms, animals or minerals
Soxhlet extraction was the most efficient technique, as it gave the highest yield of total quinone content at 11.5 mg/g dw, compared to 5 mg/g dw for dynamic maceration, 4.5 mg/g dw for ultrasound-assisted and 3.75 mg/g dw for static maceration methods
Summary
Dyes can generally be categorised as either synthetic or natural. While synthetic dyes are typically synthesised from petrochemical sources, natural colourants are abundantly available in nature and can be obtained from plants, microorganisms, animals or minerals. Natural quinones constitute a large class of aromatic compounds that are prevalent in nature and can be found in several families of algae, fungi (including lichens), bacteria, flowering plants and arthropods [21, 24] They are found in insects like cochineal (Dactylopius coccus) [27], kermes (Kermes vermilio) [28] and lac (Kerria lacca) [25]. The aglycone forms are converted into glycoside anthraquinones by β-glycosidase activity or through an oxidative process [26] This process occurs when one or more sugar molecules such as rhamnose or glucose are bound to the hydroxyl group at position R-6 or R-8 (Fig. 1) [30]. Anthraquinone pigments have been isolated in concentrations in excess of 30 mg/(g dw) (dw: dry weight) from Morinda citrifolia, R. emodi, Rheum undulatum, Rubia cardigolia and Tectona grandis [38, 41, 42]
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