Pineapple Wastes 2: Extraction of fiber and other leaf products
Published Date: 14th September 2020
Pineapple is cultivated primarily for its sweet fruit in many tropical, and subtropical countries. The plant consists of scaly fruit, and radiating leaves arranged spirally around the single short stem. Although in many African countries, the pineapple leaves are thrown away at farm sites and left for open burning before replanting, pineapple is one of the most important commercially grown fiber crops, since the leaves yields quality fiber for making excellent textile, and other materials. In southeast Asia and South America, pineapple leaf fiber (PALF) is one of the abundantly available waste materials being intensively explored as potential raw material for the replacement of synthetic, and non-sustainable fiber sources. Although other plant fiber sources such as banana, corn, bamboo, flax, jute, and lotus are also being explored, pineapple fiber is preferred because of the global yield of the crop, and the fineness of PALF when compared to another leaf, and bast fibers. Indeed, before the advent of cotton, and synthetic fibers, PALF was a popular material for making fabrics for centuries in the Philippines, Europe, North America, and Africa.
Recent global estimates have shown that about 1,098,705 hectares of land are under pineapple cultivation and that about 40 tons of fresh pineapple leaves could be harvested per hectare of land under cultivation. This represents an abundant year-round supply of this useful biomass, however, after the fruit harvest, the pineapple leaves are largely discarded as agro-wastes by most producers. Indeed, it has been proposed that the Perolera cultivar, which is cultivated in the Caribbean region solely for fiber could produce as much as 300 tons of leaf stubbles per hectare annually. The Philippines and Taiwan are the major producers of PALF, followed by Brazil, Hawaii, Indonesia, West Indies, and India. Pineapple leaf fiber has been utilized in the production of yarn, fabrics, footwear, bags, and paper. The proteolytic enzyme, bromelain used in food processing, medicine, and paint industry is extracted from the short stem. In addition, pineapple leaves have been used as cheap raw material for the production of ethanol, phenolic antioxidants, organic acids, and biogas.
Since the cultivation of pineapple is popular among rural farmers in the tropics, information on beneficial uses of its biomass wastes could be exploited to establish pineapple waste processing cottage industries that will improve the livelihood of these farmers. A recent report by Pandit and coworkers has shown that post-harvest handling, and marketing of pineapple fruit, and leaves are indeed empowering smallholder farmers by improving their livelihood.
Pineapple leaf fiber production
Pineapple leaf measures 55 to 75 cm long by 3.1 and 5.3 cm wide and 0.18 cm thick depending on the species or cultivar. In addition, plant spacing, and distribution of sunlight will affect the final leaf length and physical properties of its fiber. Cultivars like the Perolera that produce longer leaves are considered suitable for fiber extraction since fiber length is dependent on leaf length. Freshly collected pineapple leaf contains about 80 percent moisture, fiber, and other gummy substances. Chemical constituents of pineapple leaf are moisture (81.6 percent) cellulose (66.2 percent), holocellulose (85.7 percent), hemicellulose (19.5 percent), lignin (4.2 percent), pentosans, fat and wax, ash (4.5 percent), nitrogenous matter, and pectin. Pineapple leaf fibers can be extracted either by manual, retting, or mechanical methods using a decorticator. The process in each method however involves the separation of the fibers from leafy components, and gummy substances. After extraction, fibrous strands are split up into their coarse components.
Retting is the separation of the fiber bundles from the cortex or wood, by digesting the cementing substances between the fibers in the bundles. This is achieved by a two-stage process of physical treatment (swelling and extraction of soluble substances), and microbial (fungus or bacteria) activity. Thus, the retting method involves the utilization of microbial activities to achieve fiber extraction. The pineapple leaves are immersed in a water tank, filled with tap water, and kept for 30 - 35 days to ret without adding any chemicals. Alternatively, small bundles of scratched pineapple leaves are immersed in the tank containing a liquor of 1: 20 ratios 0.5 percent urea, or diammonium phosphate (DAP) at 28°C temperature for fast retting reactions over a period of 10 - 15 days. As the retting period increases the ease with which the fibers are extracted also improves, although this depends on the water quality because dirty water may increase the microbial population to levels that will cause damage to the fibers.
The common approach to pineapple fiber extraction is however the combination of scrapping and water retting. A broken plate or a blunt kitchen knife can be used to manually scrape the hydrophobic waxy layer on the leaf. The process is however cumbersome and requires a lot of patience, and care to avoid damaging or breaking the fibers. After manual or machine extraction, the fibers appear as long yellowish strands. Thereafter, they are washed with water and dried, followed by gentle combing in the wet condition with fine pins to separate the coarser bundles into their fine fiber components. The mechanical extraction method using a decorticator machine involves manually feeding the leaves into the machine to scrape them with the aid of revolving blades. Decorticated fibers are thereafter washed with water and sun-dried. A special-purpose machine having a metal knife scraper, roller, and the serrated roller is now commonly used to scrape out the waxy layer, before the retting process. It has been reported that PALF extracted with a decorticating machine is softer, brighter, and creamier white in color when compared to the PALF produced from the conventional method. Chemical degumming of pineapple leaves has also been reported. The procedure is usually carried out according to the stages summarized by Pandit, and coauthors as preparation (immersion in acid, H2SO4) → washing→ boiling in NaOH solution → washing → bleaching→ water extraction → oiling → drying. Usually, the leaf is not completely degummed in order not to negatively affect the spinnability of the fiber.
Fiber yield using these methods vary between 2.49 and 3.07 percent of the fresh weight. An optimum retting time of seven days has been suggested for a yield of 2.8 percent clean PALF. If, however, the pineapple leaves were dried before fiber extraction, yield becomes even smaller due to difficulties in extraction caused by breakage of the fibers, and therefore shorter products. The quality and quantity of extracted fiber is therefore influenced by the extraction method employed.
Properties and uses of pineapple leaf fiber
PALF contains 87.56 percent holocellulose, 78.11 percent alpha-cellulose, 9.45 percent hemicellulose, and 4.78 percent lignin. The fiber is characterized by its gleaming whiteness, length, and strength, Thus, making it qualitatively, and aesthetically second to none of the common leaf, and bast fibers. Pineapple leaf fiber has relatively low lignin compare to other alternative fibers, such as banana stem (18.6 percent), oil palm (20.5 percent), and coconut (32.8 percent), making amenable to bleaching and high fiber strength. The fiber can be dyed at room temperature with direct, reactive, vat, and azo dyes, with better fastness, and absorption properties than cotton. This has been attributed to the relatively high moisture content, and low reflectance value of the fiber, which are directly related to its natural greenish-yellow or cream color.
Pineapple fiber is used in making cloth, and also at times combined with silk or polyester to manufacture textile fabrics. The fiber is specifically used in the production of table linens, bags, mats, and other clothing items, which are used in different parts of the world. It has also been used in the manufacturing of conveyor belt cord, V-belt cord, and lightweight duck cloths amongst others. Pineapple leaf fiber has been used as reinforcement in thermoplastic composites, with the best fibers, and matrix miscibility being achieved at 30 weight percent treatment. Thus, PALF can be incorporated into thermoplastic materials such as polypropylene, and polyethylene to produce bio-composite products. PALF has also been blended with silk, and polyester fibers to produce composite textile materials. Because of its natural cream color, low lignin content, appropriate length, and fineness, PALF is readily blended with a cheap variety of Indian wool, which is also cream in color to produce yarns of nominal linear density. Such a PALF: Wool blended yarn (25:75) has been shown to be suitable for carpet face yarns, and home furnishing fabrics. It is however noted that the yarn quality and their spinnability improves with an increase in the pineapple fiber content.
Pineapple leaves have also been shown to be effective alternative pulping raw material, for paper production. The favorable high cellulose and low lignin contents of the fiber makes high quality pulping, and paper production from pineapple leaf possible. Researchers at the Universiti Teknologi Malaysia, Johor, Malaysia, were able to produce paper sheets by pulping pineapple leaves with 3 percent acetone to achieve the best mechanical properties, while paper strength was improved by increasing the delignification time. They were also able to reduce the delignification time by cooking the pineapple leaves at a temperature of 118◦C under an applied pressure of 80 kPa, which significantly improved the paper strength. Other studies at Universiti Tun Hussein Onn, Malaysia, have also reported 79.26 percent pulp yield from chemical pulping of pineapple leaves, with pulp properties such as tear index, tensile index, burst, and fold being of acceptable quality for paper making.
Paper produced from pineapple fiber can be reinforced, and used to manufacture biodegradable plates, which are of improved strength, and water resistance. Researchers at the Prince of Songkla University, Songkhla, Thailand, were able to achieve this by experimenting with different bio-coating solutions, which they applied as coatings on paper made of pineapple leaf pulp. They reported that bio-coatings generally increased the average grammage and thickness of the paper, although density was lower than that of uncoated paper. The longest absorbency time was achieved by paper coating with beeswax–chitosan solution, followed by alginate/gellan gum, chitosan, beeswax, and shellac in that order. The maximum tensile strength was also recorded for the paper coated with beeswax–chitosan solution, indicating that bio-coated pineapple leaf pulp can be used in biodegradable packaging to reduce the use of plastics.
Composting of pineapple leaf residue
Earthworms can consume the organic mass of the pineapple leaf residue to convert them into quality vermicompost. Researchers at the Indian Council of Agricultural Research, Kolkata, India, used the residues from PALF extraction as a substrate for vermicompost production. The pineapple leaf debris was thoroughly mixed with cattle dung at the rate of 100 kilograms per ton of the pineapple leaf waste, and transferred to a concrete tank. The set up was covered with dry grass, and leaves to serve as mulching, and allowed to decompose for 30 days, before inoculation of mature African nightcrawler earthworm species (Eudrilus engeniae) at the rate of 100 earthworms per square meter area on the compost. The set up was covered with fresh pineapple leaf residue, and allowed to decompose for another 45 days, with regular spaying of water. Thereafter, the compost was dried, ground, and sieved before packing as ready vermicompost. On analysis the vermicompost contained 1.0 - 1.2 percent nitrogen, 0.3 - 0.4 percent phosphorus, and 0.4 - 0.5 percent potassium, indicating that it is rich enough in NPK, and will be suitable for horticulture. The average moisture content in vermicompost cast was 50 percent, while the pH was 7.0.
Pineapple leaves have also been co-composted with poultry manure, in order to reduce environmental pollution arising from pineapple, and poultry production and obtain high-quality organic fertilizer. In a study conducted at the Universiti Putra, Bintulu Sarawak Campus, Malaysia, pineapple leaves were shredded, air-dried and mixed thoroughly with chicken manure slurry, and molasses in polystyrene boxes, and co-composted for 57 days at ambient temperatures. The mixing was at the rate of 3.5 kg of shredded pineapple, leaves + 350 g of chicken feed + 2.8 L of chicken manure slurry + 175 g of molasses, while the chicken manure slurry was obtained by dissolving 350 g of chicken manure in 2.8 L of water, after which solution was filtered to remove large particles. Nitrogen, and phosphorus concentrations, and cation exchange capacity increased, indicating an increase in organic material, whereas carbon content was reduced. Essentially, the final co-compost had no foul odor, low heavy metal content, and considerable amounts of nutrients, indicating that co-composting pineapple leaves, and chicken manure slurry is a potential method of reducing environmental pollution arising from these industries.
Pineapple leaves, which are generally discarded as waste by farmers can serve as a useful raw material for extracting high-quality fiber, and paper pulp production. Pineapple leaf fiber can be blended with other fibers like polyester, silk, and wool to produce composites, which have found different industrial applications. The residues from pineapple leaf fiber extraction have also been used successfully in vermicompost production. A video on pineapple leaf fiber extraction can be viewed at https://youtu.be/rJuv8KUnnrA and https://youtu.be/0pa9F49TWzA
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