Potent Natural Dyestuffs from Onion Skin to Fabrics Using or Lacking Bio-Mordants

Abstract

Natural dyes yield a remarkable range of items with complex colouration that go well together. Skin that constitutes the outermost part of onion contains certain phytochemicals, such as; cardiac glycosides, tannins, flavonoids (quercetin, etc.), anthocyanidins (pelargonidin, etc.), natural dyestuffs, and reducing sugars in aqueous medium. Among the various phytochemicals, tannins, anthraquinones, and flavonoids are crucial for giving cotton fabric its shade and vibrancy. Wool has been dyed in the past and present using onion peel. It has historically been utilised in rugs and carpets in Anatolia, Turkey. It can be applied both with and without bio-mordants to silk fabric. Tea leaves, tamarind, aloe vera, and ash from acacia bark are utilised as bio-mordants. Furthermore, colour measurement values, such as Tristimulus characteristics, Lab values, and munsell notations, are applied to dyed materials.

1. Introduction

Colourants called dyes and pigments are used in food, medicine, textiles, and other essential items that are necessary for human survival [1]. Because dyes are more soluble and contain finer particles than pigments, they vary from pigments in this regard [2]. The use of colourants dates back to the Stone Age, and many ancient civilizations expanded their use of dyes to textiles once weaving techniques advanced [3]. Evidence demonstrates that the use of pigments and dyes dates back to the dawn of human civilization; natural pigments were employed by the ancient Egyptian, Roman, Greek, and Indian civilizations [4]. Among the popular ancient colours were madder, blue indigo, and yellow made from turmeric or saffron; crucial pigments included ochre, limestone, and charcoal [5]. Animals, minerals, and plants were the main sources of natural colourants. Almost every component of the plant, including the leaves, roots, fruit, flowers, bark, wood, and seeds, was used to create a variety of colours and their combinations [6, 7]. Until the middle of the nineteenth century, when William Henry Perkin created the first synthetic dye, mauve or aniline purple, in 1856, there was no interruption to the use of natural pigments and dyes [8, 9]. Dyes have a variety of uses; for example, fluorescent natural dyes can be successfully employed in beetroot cell imaging [10]. The use of natural dyes is a promising advancement in the field of dye-sensitised solar cells (DSSCs). Biosensors (indigo), dye-sensitised solar cells, extract used for cancer treatment (Punica granatum), and corrosion inhibitors (curcumin, lawson, and berberine derived from plant materials) contain nitrogen (N), sulphur (S), and O heteroatoms in their structure [1].

Using a mordant is usually required throughout the natural dyeing process. Metallic salts including sulphates, chlorides, hydroxides, and oxides can be used as a source of mordant. Alum and iron are two of the metal mordants used in natural dyes that are harmless to the environment and are not subject to any ecoregulations [11]. Numerous of these mordants have the potential to contaminate soil and produce waste in the form of water [12]. The incompatibility of these mordants with natural dyeing’s eco-friendly tenets presents another issue. Widespread use would result in significant environmental issues [13, 14].

However, bio-mordants, which have enough colouring and firmness properties, are said to be environmentally friendly and sustainable substitutes for metal mordants [15]. Sources of bio-mordant materials include hyperaccumulative metal plants and plants high in tannin. A number of bio-mordants, including pomegranate peel, rosemary, and thuja leaves, have been suggested as viable substitutes for potassium dichromate, stannous chloride, iron sulphate II, copper sulphate II, and aluminium [16]. Plant parts that contain natural polyphenols, commonly referred to as tannins, include bark, wood, fruits, fruit peels, leaves, roots, and plant galls [17].

2. Onion skin as a natural dyestuff

One cuisine item that is typically seen on tables in Asian nations is onions. In addition to having high-quality ingredients, it can be claimed that this product remains a fixture in both dinners and salads [18]. The onion skins are wasted after use. At the grocery shop, onions (Allium cepa) come in three primary varieties based on their colour; white, yellow, and red [19], as shown in (Figure 1). However, appropriate names such as Red Zeppelins or Australian Browns are seen at farmers’ markets. Red is usually the sharpest, yellow is in the middle, and white is typically the mildest in terms of pungency [20].

Figure 1.

Different types of onions.

The edible part of an onion (Allium cepa) is the swelling bases of the leaves, which comprise the flesh [21, 22]. A well-ripened onion has dried out and lost much of its greenery, leaving just a little amount of shrivelled tissue at the ends of the outer fleshy scales [23]. The world’s dry onion production region is in an exceptional position.

2.1 Chemical composition of onion skin

A variety of onion waste components, including ash, protein, carbohydrates, and total dietary fibre, are excellent sources of several minerals, including potassium, calcium, magnesium, iron, zinc, manganese, selenium, total phenolics, flavonoids, and flavonols [22, 24]. Additionally, the powdered onion skin revealed that it had a considerable amount of ash, total dietary fibre, soluble dietary fibre, and insoluble dietary fibre but less protein and crude fat [25]. Onion outer scales were found to include ash, moisture, carbohydrates, crude protein, crude fibre, and crude fat [26]. Additionally, the study identified 12 distinct fatty acids from the onion peel’s outer layer, of which half were unsaturated and the other half saturated [27]. The fatty acid profile showed that the total amount of saturated fatty acids was 21.42%, while the total amount of unsaturated fatty acids was 76.79%. Lauric acid (0.94%), myristic acid (1.28%), palmitic acid (9.80%), palmitoleic acid (2.84%), stearic acid (8.81%), oleic acid (17.57%), linoleic acid (52.87%), linolenic acid (2.88%), arachidic acid (0.59%), behenic acid (1.23%), erucic acid (0.63%), and lignoceric acid (0.54%) were among the fatty acid profile [28, 29]. Research has also shown that the peel of onions, which mostly consists of the phenolic component quercetin, is a more concentrated source of phytochemicals than the edible flesh. Furthermore, onion skin has been found to contain quercetin 3,4′-diglycoside, protocatechuic acid, kaempferol, 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy- 3(2H)-benzofuranone, isorhamnetin, quercetin-7,4′-diglycoside, isorhamnetin-3,4’diglycoside, quercetin-3-glycoside, quercetin-4′-glucoside, isorhamnetin-4′-glycoside, protocatechoyl quercetin, quercetin dimer 4′-glycoside, quercetin dimer hexoside, and quercetin trimer [30]. India produces over 22.43 million tonnes of onions annually, making it the second-largest producer of onions worldwide. Since onion peels are basically removed before being used, the yield of usable peeled onions varies from 73.5 to 81.6% depending on the onion’s size, which ranges from 2.5 to 4.5 inches. As a result, on average, onion peels make up between 10 and 25% (%w/w basis) of the onion’s total weight, producing a sizable amount of vegetable waste material. Based on a further extrapolation of the data, India produces between 2.3 and 4.5 million tonnes of onion peels annually. This would lower the economic worth of the precious onion material as an agricultural commodity, in addition to creating a significant environmental issue with trash management. Because of this, developing nations like India have a lot of opportunity to develop some value-added and environmentally benign products, which are essential for creating jobs and economic growth, in addition to developing environmentally benign products.

This research examines the practical application of onion peels for advantageous purposes, including the extraction of natural dyes. The plant’s outer skins have long been employed in Turkey’s textile industry as a natural colouring agent. Because of this, natural dyes made from these vegetable wastes can provide economic benefit and create jobs. Therefore, hand-woven carpets and rugs could benefit from using these wastes for dyeing [31].

2.2 Extraction methods for obtaining dye

The extraction of dyes from onion skin has been studied in various ways; measured amounts of ground-up samples (10 g) were wrapped in calico and put inside the Soxhlet extractor’s thimble; 250 mL of absolute ethanol was added as a solvent to a round-bottom flask that was attached to an isomantle-mounted Soxhlet extractor and condenser; the crushed plant material was loaded into the Soxhlet extractor’s thimble. The side arm had glass wool lagged on it. Using the isomantle, which is used to evaporate, the solvent was heated before passing through the apparatus and into the condenser. Put another way, the condenser unit of the apparatus contains the evaporated ethanol, and the Soxhlet extraction procedure heats the solvent (ethanol) to boiling temperature (>78°C). After that, the condensate is poured into the reservoir holding the thimble. The cycle restarts when the solvent level hits the syphon and flows back into the flask. For 3 hours, the dyestuff extraction was refluxed. In order to acquire the crude solid dyestuff extracts using a water bath, the extract phases produced by the various extraction operations were first distilled to recover some of the solvent and then allowed to evaporate to dryness. To create pure dye samples for the physical and chemical analyses, the crude dye stuff extracts were further air-dried at room temperature [32]. Water and acetic acid are used as solvents to remove the colour. In order to remove the onion peels, 0.5 g of them was extracted, and 45 ml of distilled water was mixed with 5 ml of concentrated acetic acid. As a result, a 1:19 ratio of acetic acid to water was used. A magnetic stirrer was used to continuously swirl the mixture. Throughout the whole experimental time, the procedure was constantly observed. Five millilitre samples of the combination were taken at regular intervals. A 4 millilitre sample was extracted for gravimetric examination. The material was cut up into 1 millilitre for ultraviolet (UV) examination. Every experiment was carried out in batches [33]. The common yellow onion, Allium cepa, is used to make Shepherd Textiles Onion Peel Extract, as shown in (Figure 2). The dark colours of onion skins are caused by a variety of anthocyanin chemicals, which can be boiled away to create a potent fabric dye. Moreover, onion skins contain a lot of natural tannins, which make them perfect for dyeing cellulose materials like linen and cotton. For pleasing mustard tones, use them at 5% weight-of-fabric on unmordanted fibre. For exceptionally deep, rich, golden oranges, dye again at 5% weight-of-fabric and mordant with alum. Approved by GOTS (Global Organic Textile Standard) and appropriate for use on fibres that will bear the “organic” label [34].

Figure 2.

Yellow onion skin extract.

Yellow onion skin dye imparts golden/yellow, orange, and brown tones. When considering alternative natural dyes, onion skin dye is comparatively colourfast. It works rather well to dye animal-based fibres like silk and wool, but less so on cotton and linen [31, 35]. The colour of red onion skins is deeper than that of yellow, which can be either peachy yellow or even orange, as shown in (Figure 3). Thus, make sure to experiment with several onion varieties if you are going for a particular colour [36, 37]. The onion skins were allowed to dry in the sun before being ground into tiny pieces using a grinding machine. After removing the leftovers with a fine sieve, the weight was recorded. Following the processes of drying, crushing, and straining, 805 grammes of skins weighing 1 kg was discovered. Figure 2 displays the crushed and dried skins. Using an aqueous extraction method, the colour component was removed from the skins. A set amount of crushed skins was extracted at pH 5 using a material-to-liquor ratio of 1:10 (crushed skin weight in grammes; water volume in millilitres) at a boil for 60 minutes. The mixture was chilled after each extraction step, and the dye extracts were then precisely filtered [38].

Figure 3.

Red onion skin extract.

3. Dyeing process

Dyeing process is based on different steps that are described here.

3.1 Preparing fabrics for dyeing

At pH 9, an aqueous soap solution (15 g/L soap) was used to degum the raw silk fabric. Throughout the treatment, the material-to-liquor ratio was kept constant at 1:50. For 60 minutes, the temperature was gradually increased to 80°C. After degumming, the cloth was cleaned for 10 minutes at 65°C using 2 g/L of detergent. After that, the cloth was treated for 60 minutes at 60°C with a 35% hydrogen peroxide (H₂O₂) (3 mL/L) solution, keeping a material-to-liquor ratio of 1:50 at pH 9 and 60°C. After that, it was washed for 10 minutes at 65°C using 2 g/L detergent [38]. Wool and nylon textiles were placed in a bath with a material-to-liquor ratio of 1:50, 2 g/l nonionic detergent, and 5 g/l sodium sulphate. After that, the bath’s overall temperature was raised to 45 degrees Celsius, and it stayed there for 30 minutes. After rinsing with warm water, wool and nylon were allowed to air-dry. A bath of 5 g/l nonionic detergent and 2 g/L sodium carbonate (Na₂CO₃) was added to the cotton fabric at a material-to-liquor ratio of 1:50. After that, the whole bath was brought to a boil and kept there for an hour. After that, it was rinsed and allowed to air-dry [36].

3.2 Bio-mordants in natural dyeing

Numerous advancements in textile material colouring have been developed, particularly in recent times. The application of natural dyes derived from plants in dyeing procedures is the first of these advances. Historically, natural dyeing techniques have made extensive use of metal salts, often known as mordants. The word “mordant” comes from the Latin word “mordere”, which means “to bite”. Something that can be attached to a fibre and reacts chemically with natural colourants is called a mordant. Natural dyes fall into two categories: substantive dyes and nonsubstantive dyes, depending on how they are utilised. It is not necessary to pretreat the fabric before applying the substantial dyes (such as turmeric, indigo, and orchil). On the other hand, nonsubstantive dyes (such as fustic, cochineal, madder/alizarin, and logwood) can only colour or work with materials that have previously been mordanted if a mordant is added to the dyebath [15].

The widely recognised vat dyeing technique was used to dye the scrubbed cotton garments. Each batch, which consisted of six pieces of scrubbed cotton cloth, was dyed in turn. Cupric sulphate, ferrous sulphate, potassium dichromate, alum, lemon juice, and aloe vera were used as mordanters for the six cloth samples. One batch of cotton was utilised for each of the three distinct mordanting procedures, such as pre-mordanting, simultaneous mordanting, and post-mordanting, which were performed. Another piece of cloth was likewise dyed without a mordant [39]. The materials need to be dried after being scrubbed and bleached. After that, it is taken to be mordant. Both synthetic and natural mordants were utilised for mordanting. Iron water, myrobalan fruit, and eucalyptus bark were utilised as natural mordants. Alum and ferrous sulphate were utilised as synthetic mordants [14, 40].

Using 5% (on weight-of-fabric) of each of the following mordants: ferrous sulphate, alum, stannous chloride, tannic acid, and tartaric acid; four distinct combinations of mordants were also used: ferrous sulphate-alum (2.5 + 2.5%), ferrous sulphate-alum-tin (2 + 2 + 1%), alum-tin (2.5 + 2.5%), and alum-tin-tannic acid (2 + 2 + 1%) at 60 C for 60 minutes while maintaining a material-to-liquor ratio of 1:50. Once more, alum and stannous chloride were utilised independently, with cream of tartar (CT) acting as a mordant helper [41].

3.3 Fastness of the onion skin dye on fabrics

Regarding the colour yellow for 3 days, 500 grammes of linden, 1 kg of onion skin, and the equal quantity of sage plants are stored in cold water [42]. A vivid chromatic colour known as lemon yellow is created when wool that has been mordanted with alum is cooked in the same water for an hour. Tones that are coppery yellow can be created by boiling wool that has been mordanted with chrome in this water [31]. The fastness of onion skin, which is generally considered to be low, varies based on the fastness of the other dyestuffs used in combination with it. Dried quince leaves are coloured red by soaking them in water for a week along with red onion skins [43]. As much as 500–700 grammes of wool is needed for every kilogramme of pre-mordanted (alum) wool in order to produce 1 kilogramme (kg) of the material. An hour of boiling produces a brilliant peach red colour [44].

3.4 Examples of successful natural dye projects using onion skin

Onion peels are a rich source of plant components known as flavonoids, and they can also be used to make a dye called pelargonidin. Additionally, a suitable piece of leather was stained using pelargonidin dye, which was produced from onion peels using acetic acid. In light of this, the current study examines the practical application of onion peels for advantageous purposes, such as the extraction of natural dyes, as a feasible method of developing environmentally safe goods from agricultural waste [33]. Excellent ultraviolet protection factors (UPFs) (50+) can be achieved when dyeing fabrics with or without mordants, with the exception of nylon, which has a low UPF when dyed with onion. It is advised to avoid skin cancer by using these naturally coloured materials, and the textile colouring business is encouraged to use their colourants [36]. Using a dyeing process based on natural components taken from the outer shell colourant (quercetin) of onions using water and organic solvents, coloured silk fabric was created for textiles [45].

4. Conclusion

In this chapter, we concluded that onions (Allium cepa) are of different types and their skin (peel) is not wasted but used as dyes due to having different phytochemicals, quercetin, pelargonidin, etc. Onion skin gives different colours with and without different mordants. These dyes are used for not only fibres but also leather and other substances. Excellent UPFs (50+) can be achieved when dyeing fabrics with or without mordants, with the exception of nylon, which has a low UPF when dyed with onion. It is advised to avoid skin cancer by using these naturally coloured materials, and the textile colouring business is encouraged to use their colourants.