Читать книгу Sustainable Practices in the Textile Industry - Группа авторов - Страница 32

Extraction of color from natural dye is one of the most important steps of dyeing. Raw materials of natural dyes are leaves, barks, fruits, flowers and roots of many plants as well as some animals and minerals as described earlier. Sources of natural dyes are carbohydrates, proteins, lipids, fats and many other cell inclusions, so only the desired coloring material requires an extraction process. In the extraction process of natural dyes, the cost of extraction and the yield of color affects the cost of dyeing. Following methods generally used for extraction of coloring materials:

1 i. Aqueous extraction: Aqueous extraction method is a traditional and most popular procedure in natural dyeing. Small pieces of fresh or dried dye material are ground in powdered form. That is soaked in water, boiled, filtered to obtain aqueous dye solution. Aqueous extraction depends on many factors such as time and temperature of the boiling, fresh or dried dye material and material to liquor ratio. Maryam et al. gave one such example i.e. extraction of color from Onion (Allium cepa) skin in aqueous condition as 5 g of dye dissolved in 100 ml water at the temperature of 100 °C for 60 min [53].

2 ii. Alkali or acid extraction: Glycosides can be hydrolyzed in acidic or alkaline condition. Mostly, natural dyes constitute glycosides, so extraction in acidic or alkaline medium can improve color yield. Some of the natural dyes are pH sensitive therefore; they destroy their dyeing properties in unwanted pH condition.

3 iii. Microwave and ultrasonic assisted extraction: The extraction of natural dye can be done by microwave and ultrasonic assistance. Microwave energy used in extraction of natural dye with a very minimum amount of solvent. Microwave increases the rate of the processes so the extraction can be completed in a shorter time with better yield. Thangabai and Kalaiarasi’s studies revealed that microwave assisted extraction of Padauk (Pterocarpus sp.) wood are more efficient as compared to conventional extraction methods [54]. Natural dye from Sorghum husk extracted with the help of ultrasound-microwave-assistance [55].

4 iv. Fermentation: Indigo extraction is the best example of fermentation method of extraction. In presence of indimuslin enzyme, glucoside indican breaks into glucose and indoxyl [56]. The enzymes produced by the microorganisms present in the atmosphere or those present in the natural resources used in fermentation for assisting the extraction process.

5 v. Solvent extraction: Natural dyes can also be extracted depending upon their solubility by using organic solvents such as acetone, petroleum ether, chloroform, ethanol, methanol, or a mixture of solvents such as ethanol and methanol, mixture of water with alcohol, and so on.

6 vi. Supercritical fluid extraction: Supercritical fluid extractions have become popular in recent years to isolate the organic compound from herbs and dyes as well as dye from natural sources. In supercritical fluid extraction a dense gas as a solvent that usually has carbon dioxide above its critical temperature (31 °C) and critical pressure (74 bar) for extraction is used [14].