Pineapple ( Ananas comosus L. Merr.), Waste Streams, Characterisation and Valorisation: An Overview

Processing pineapple industry produces huge amounts of waste thus contrib-uting to worsen the global environmental problem. Valorising pineapple waste through further processing until it is transformed into valuable products using environmentally friendly techniques is both, a challenge, and an opportu-nity. The aim of this review is to characterize and highlight the phytochemical constituents of pineapple peel, pharmacological therapeutical in the form of processed food and instant drinks; its potent natural antimicrobial properties may be applied for food conserva-tion and as potential leads to discover new drugs to control some infectious microbial. Pineapple waste is a promising source of metabolites for therapeu-tics, functional foods, and cosmeceutical applications.


Introduction
Pineapple (Ananas comosus L. Merr.) is a tropical fruit belonging to the family of Bromeliaceae [1], having a short stem and slender hard leaves that grow to medium to large-sized fruit. It is the third most important tropical fruit produced globally after bananas and mangos. It is native to South America in Brazil and Paraguay, and often referred to as pineapple. The fruit was spread from native to South and Central America, when the Guarani Indians took A. comosus on sea voyages as provisions and to prevent scurvy. When Columbus found the fruit in 1493 called it pina, because he thought it looked like a pin cone. The hybrids known today first appear, when the Dutch improved the fruit by crossbreeding, around 1700. The cuttings of the plants were sold to English, who grew them as pot-house plants [2] [3].
Pineapple is a medium tall (1 -1.5 m) herbaceous perennial plant with 30 or more trough-shaped and pointed leaves 30 -100 cm long, surrounding a thick stem. A. comosus is an example of a multiple fruit: multiple, spirally arranged flowers along the axis each produce a fresh fruit that becomes pressed against the fruits of adjacent flowers, forming what appears to be a single fleshy fruit.
The fruit lets of A. comosus are arranged in two interlocking spirals, eight spirals direction, thirteen in the other; each being a Fibonacci number. Pollination is required for seed formation; the presence of seeds negatively affects the quality of the fruit [2] [3]. Pineapple is not a single fruit but a composite of many, originated from flowers whose individual fruit fuse together around a central core.
Each fruit let can be identified by the rough spiny marking on the A. comosus surface. Pineapple has a wide cylindrical shape, a scaly green, brown, or yellow skin and a regal crown of spiny, blue-green leaves. The fibrous flesh of A. comosus is yellow in colour and has a vibrant tropical flavour that balances the testes of sweet and tart. The area closer to the base of the fruit has more sugar content and therefore a sweeter taste and more tender texture [2] [3]. Table 1  The pineapple has a world production of 25.4 million metric tonnes. The consumption of the pineapple is wide due to its sweet and acidic flavour and varies as it is consumed fresh, cooked, juiced, or preserved. The tropical fruit is an ex-cellent source of vitamins, minerals, and nutrients and is rich in antioxidants. Pineapples are made up of a cluster of multiple berries that grow within the second year of cultivation and are harvested one year later. Pineapples best grow in subtropical and tropical areas where the weather is warm, but cool at night [1] [4].  The pineapple has a world production of 25.4 million metric tonnes. The consumption of the pineapple is wide due to its sweet and acidic flavour and varies as it is consumed fresh, cooked, juiced, or preserved. The tropical fruit is an excellent source of vitamins, minerals, and nutrients and is rich in antioxidants. Pineapples are made up of a cluster of multiple berries that grow within the second year of cultivation and are harvested one year later. Pineapples best grow in subtropical and tropical areas where the weather is warm, but cool at night [1] [4].
There are four varieties of Pineapple available worldwide: Smooth Cayenne, Red Spanish, Queen, and Abacaxi [5] [6]. Pineapples are cultivated on more than a million hectares of soil, resulting in US$ 9 billion to the global economy annually [7] [8]. The main pineapple varieties for commercial use are Smooth Cayenne, Singapore Spanish, Red Spanish and Selangor Green [9]. In Brazil, the most popular varieties are Perola and Smooth Cayenne [10]. Cost Arica, India, Thailand, Philippines, Brazil, China, Nigeria, Mexico, Indonesia, Columbia are the leading producers ( Figure 2).  Approximately, 60% of the pineapple fruits are edible, thus processing residuals range between 45% to 65% [11]. The crown and stem of the pineapple are cut off before peeling and the centre is eliminated during processing. All these wastes like core, peel, stem, crown, and leaves account for about 50% (w/w) of general pineapple weight. Two types of waste may be labelled from pineapple production: POFW (pineapple on farm waste) and PPW (pineapple peel waste). POFW normally involves leaves, roots, and stem leftovers at the farm. Pineapple is processed in many methods inside the pineapple product zone, together with slicing, pulping, and juicing. Before the pineapple product changed into processed, exclusive steps were taken to get rid of the crown, then put off the peels and core.
A one-of-a-kind volume of PPW is generated through every step. Researchers located that 9.12% of middle, 13.48% of peels, 14.4% of pulp, 14.87% of the top, and 48.04% of completed products have been produced by using pineapple. Therefore, with increasing pineapple production, pineapple waste increases proportionally. Waste disposal is a growing issue because it is usually prone to microbial spoilage and causes serious environmental problems. It directly represents the enormous challenge of pineapple waste disposal, which will further lead to environmental pollution if not successfully used [11].
Due to their pleasant aroma and its attractive sweet flavour, pineapple is widely consumed fresh, canned, in processed juices, and as an ingredient in exotic foods [12], and its derived products are popular. Besides its sweet flavour, pineapple is very wealthy in essential nutrients consisting of potassium, calcium, phosphorus and iron, vitamin C, copper, folate, glycans, fibers, and different crucial factors.
All these substances make pineapple an excellent candidate for part of a balanced dietary weight-reduction plan. One of the most favourable aspects of its composition is that it consists of a minimum amount of fat and sodium, but incorporates high quantities of carbohydrates [13] [14].
Pineapple canning industries produce large quantity of solid waste. The dry matter content of pineapple waste is around 10%, composed of about 96% organic matter and 4% inorganic matter [15]. According to Conesa et al. [16], in the elaboration of pineapple-based food products, this value can be 50%. Other authors reported a waste production from pineapples processing in the range of 25% -35% of the weight of the fruit [17]. This discrepancy can be due to pineapple variety and the type of processing used. Worldwide, agricultural industries generate large amounts of bio-mass residues which can cause environmental and pollution concerns [18]. Wastes derived from food processing are identified as major resources for the bio-based processes development [19] [20] and a sustainable use of these food by-products to produce value-added products (as chemicals, materials, and fuels) could contribute to reduce environmental concerns and improvement of economic growth. Taking this in consideration, strategies are needed to recover other added value compounds available in pineapple wastes.

Bioactive Components in Pineapple Waste
Pineapple waste is a rich source of bioactive components. Pineapple primarily has its waste in the form of crown, peel, stem, and core. There is a different bioactive compound present in each portion. For example, citric acid in pineapple leaves, bromelain in the leaves, stem, and peel, ferulic acid in the leaves, ascorbic acid in the core [4]. Moreover, phenolic compounds also found in pineapple residues are of great interest in the pharmaceutical and food industry due to their biological properties with application on human health, examples as myricetin, salicylic acid, tannic acid, trans-cinnamic acid and p-coumaric acid identified in a high dietary fiber powder from pineapple shell which is a part of waste and these compounds were reported as potent antioxidants [21]. On the other hand, it has been shown that the polyphenols found from pineapple wastes such as ferulic acid and syringic acid, are responsible for antioxidant and antimicrobial activity [22]. Also, glycosides obtained from these residues have various applications as food additives, prebiotics, and bio preservatives [23]. Also, pineapple waste is an enriched raw material, as insoluble fibers, pectins, sugars, protein, vitamins, minerals and phenolic compounds [24] [25] [26] [27], and bromelain, a group of proteases with different applications in the food, textile, cosmetics and others [28]. Pineapple fruit is rich in vitamins A and C, flavonoids, tannins, and other polyphenolic compounds, organic acids, and soluble monosaccharides and disaccharides (up to 15%). Carotenoids are the source of characteristic fruit colour, while the flavour comes from a very complex mix of substances where oxygenated aliphatic compounds have the main role. Stem and full-grown fruit contain proteolytic enzyme that breaks down protein, bromelain, which belongs to the group of endopeptidases [11] [29]. Pineapple fruit is also low in sodium and rich in potassium [2] [3]. Pineapple residue contains high concentrations of vitamins, sugars, fibers, and other constituents which can be used for human consumption [30]. Previous scientific studies have reported the use of pineapple processing residue for citric acid extraction [31], phenolic compounds production [31], bromelain from stem and rind [31], hydrogel from peel cellulose [31], nanocellulose from leaf fibres [32].
The canning industry is producing large quantities of solid and liquid wastes, could be a potential source for the extraction of beneficial bioactive compounds including bromelain [33] [34], fibers, protein, carotenoid, mineral composition, total polyphenol content [35] [36]. Gallic acid (31.76 mg/100g dry extracts), catechin (58.51 mg/100g), epicatechin (50.00 mg/100 g), and ferulic acid (19.50 mg/100g) were found to be the main polyphenolics in pineapple peels [37], vitamin C, carotenoid, phenolic compounds, flavonoids [38] [39] [40] [41]. Arsyada et al. [42], reported that pineapple peel extract contains chemical compounds namely: flavonoids, saponins, tannins, and enzymes bromelain and flavonoids were the highest concentration of chemical compounds in pineapple peel extract. Also, pineapple is a rich source of manganese, which helps build and maintain bone strength and it also has plenty of vitamin C. It also contains bromelain, that can reduce swelling, bruising, healing time and pain associated with injury and surgical intervention [43]. The phytochemical analysis of the pineapple peel showed that, alkaloids, flavonoids and saponin had the highest abundance of the phytochemical properties; and phenolic, tannins and terpenoids were present as moderately as well as glycoside and steroids as trace amount [44]. Chlorogenic acid and ferulic acid in aqueous pineapple extract is considered having an important role as reducing agent or antioxidant [45].
Wei et al. [51] identified 44 volatile compounds from pineapple pulp and core.
However, the ones with the highest odor activity values were ethyl hexanoate, nonanal, and decanal. Morais and Silva [46] emphasized that ethyl hexanoate is one of the most important compounds related to the pineapple flavor. Also, they found ethyl hexanoate is an important pineapple aroma volatile compound with decanal, ethyl octanoate, acetic acid, 1-hexanol, and many ketones such as γ hexalactone, γ-octalactone, δ-octalactone, γ-decalactone and γ-dodecalactone. According to Facundo [47], the compound ethyl hexanoate is related to the aroma note described as "pineapple". This compound was also identified in the pineapple processing residue distillate obtained by simple hydrodistillation technique, beside Ethyl octanoate, 2-heptanone, and γ-hexalactone [47]. Tokitomo et al. [52] detected 29 odor-active compounds, some of them were γ-octalactone, δ-octalactone, γ-decalactone, and γ-dodecalactone. Also, Elss et al. [48] reported the presence of the following volatile compounds of fresh pineapple juice and its water phase extracts: 2-methyl-3-buten-2-ol, methyl pentanoate, butyl acetate, hexanal, 2-pentanol, 1-butanol, ethyl hexanoate, limonene, z-ocimene, linalool, furfural, acetic acid, α-terpineol, geraniol, and γ-octalactone. As well as, Spanier et al. [49] analyzed volatile components present in fresh-cut pineapple and the effect of its storage such as acetic acid, 1-hexanol, and nonanal. Zhang et al. [53], reported that The number of publications for the term, Pineapple peel waste on ScienceDirect clearly shows that scientists started intensive research only 6 years ago, but it has been particularly pronounced in the last two years. Figure 4 shows the graphical number of reported publications in the last sixteen years.

Antioxidant Capacity of Pineapple Peels
de Oliveira et al. [54] have concentrated on the antioxidant capacity of pineapples. They studied the total phenolic content and antioxidant activities of methanolic extracts of pineapple residues (including pulp, seeds, and peels from a local juice factory) using DPPH and superoxide anion scavenging activity. Hossain et al. [37] extracted total polyphenols of pineapple with different solvents, and the results showed that the polyphenolic contents of the extracts were found to be highest in methanol corresponding to the highest antioxidant properties. Gallic acid, catechin, epicatechin, and ferulic acid were found to be the main polyphenolics in pineapple peels. Results of polyphenolics' interactions indicated no synergistic effects. In the combinations of ferulic acid-epicatechin and ferulic acidgallic acid, additive effects were found using both antioxidant activity assays [55]. However, some researchers have reported that pineapple peel is a potential source of bioactive compounds, such as vitamin C, carotenoid, phenolic compounds, flavonoids [38] [39] [40] [41], and these compounds have been reported to have antioxidant activity and various biological activities [21] [56] [57]. However, these bioactive compounds are varied depending on many factors, including cultivar, pre-and postharvest treatment and particularly method for raw material and extract preparation [ [65]. Chlorogenic acid and ferulic acid in aqueous pineapple extract is considered having an important role as reducing agent or antioxidant [44].

Anticancer Activity
Ferulic acid is a hydroxycinnamic acid, a type of organic compound. It is an abundant phenolic photochemical found in extracted from pineapple peel. It is related to trans-cinnamic acid and contributes to the antibacterial, anticancer activity and antioxidant properties of the plant [66]. The antioxidant potential for extracted and standard ferulic acid was calculated by DPPH assay of 87.3% and 81.92% respectively and the total antioxidant capacity was calculated by FRAP assay which is expressed as 2315 μM and 3985 μM. The anticancer activity of extracted ferulic acid was evaluated based on viability of cells using MTT assay. The optimum cell viability was 48.04% at a concentration 125 μg/ml and the maximum cell viability was 91.63% obtained at the concentration of 7.8 μg/ml using MTT assay. So, pineapple is a potent source of ferulic acid that can be extracted and used as anticancer and antioxidant [66]. Bromelain also showed significant anticancer activity against breast cancer cells, melanoma cells, human epidermoid carcinoma cells, malignant peritoneal mesothelioma cells and mammary carcinoma cells. The major anticancer mechanism of bromelain is by inhibiting cancer cell proliferation and inducing differentiation of leukemic cells [31].

Antibacterial Activity
Recycling pineapple wastes by using pineapple waste as potential lead to discover new drugs to control some food poisoning and other infectious bacteria is a promising aim. Lubaina et al. [67] showed that the antibacterial potential exhibited by oxyoxan-2-yl]oxy-4-hydroxy-6 (hydroxymethyl)oxan-3-yl]acetamide. The chemical structure has been drawn by ChemDraw Professional software ( Figure 5). Pineapple peel extract demonstrates antibacterial properties because it contains chemical compounds. In addition to flavonoids as the highest concentration of chemical compounds in pineapple peel extract, there are other active compounds, namely: saponins, tannins, and enzymes bromelain. Each of these chemical compounds has a specific role in inhibiting the growth of Enterococcus faecalis [42] [75]. Flavonoid compounds have the ability to form complex bonds with extracellular protein through hydrogen bonding. Hydrogen bonds with extracellular proteins cause bacterial cell membrane structures containing proteins to become unstable. Consequently, the permeability of bacterial cell wall will be disrupted, resulting in bacterial cell death [76]. Tannin compounds can damage bacterial cell membranes and result in reduced permeability, so the growth and activities at bacterial cells is inhibited [77]. In other chemical compounds, saponin, as an antibacterial agent, can reduce the surface tension leading to the increasing release of intracellular compounds from the cell that leads to bacterial lysis [77].
The action mechanism of the bromelain enzyme as an antimicrobial lies in altering or damaging the wall structure of bacteria-containing protein. The bromelain enzyme will break down and denature the protein constituent of bacterial cell walls. As the cell weakens it can allow small molecule to leak out [70]. Okoh et al. [44] performed the antibacterial activity of the peel extract and showed a greater inhibition for Staphylococcus aureus, Pseudomonas aeruginosa and Salmonella typhi. The data obtained show that the pineapple peel contains potential antimicrobial component that may be of great use for the development of pharmaceutical based therapies against various diseases. Drinking the juice or its peel eating can help to hydrate the body and restore the immune system.

Other Uses
Along the pineapple processing chain, waste products such as peels, crowns, stems W. M. Hikal et al. and cores result, and these parts are usually containing significant amounts of the enzyme bromelain [84] [85]. Bromelain, a protease found in pineapples, is of high demand in the pharmaceutical, cosmetic and in the food industries, which use as a meat tenderiser, anti-browning agent and in the production of infant formulas [86]. As a protease, it hydrolyses proteins in these formulas, thus making amino acids more readily available to infants. Cosmetic and pharmaceutical usage of bromelain, as well as usage in the textile industry, has also been reported [87] [88] [89] [90].
Pineapple peel has the potential to being used as a nonpharmacological treatment in the form of processed food and instant drinks. One of the causes of hypertension is reduction of intracellular potassium concentrations in the body.
Increased potassium intake could be done by consuming pineapple, because it is one of the superior fruits which have a high content of potassium [91]. Safriani et al. [91] showed that potassium levels in pineapple peel were 938.48 mg/kg.
The potassium content is greater than flesh and core which are 485.28 mg/kg and 12.98 mg/kg, respectively. From the result, it was appeared that, the peel contains more potassium than flesh and core. Also, producing vinegar from fermented pineapple by-products (peels). The vinegar was produced after fermenting the peels of pineapple using three selected strains of acetic acid bacteria. such as propionic bacterium acidipropionici, panteo agglomerans, and pantea dispersa. The results of Chalchisa et al. [92] work have revealed that it is possible to produce vinegar from pineapple peels. This was done via the use of yeast (Saccharomyces cerevisae) as an aerobic degradation of sugar to ethanol and Acetobacter aceti oxidizers ethanol to acetic acid (vinegar) by using different strain of acetic acid bacterias. The study also revealed that while cleaning the environment, an added value can be achieved through recycle or conversion of supposed wastes into useful products [93]. In addition to pineapples consumption as fresh, cooked, juiced, and preserved and are found in a wide array of cuisines., the pineapple leaves are used to produce the textile fibre pina employed as a component of wallpaper and furnishings, among other uses. Pineapple peel, is normally used as animal feed [94], development of new products such as cereal bars [95] [96], cakes [97] and jellies [31] [98], citric acid extraction [31], phenolic compounds production [31], bromelain from stem and rind [31], hydrogel from peel cellulose [31], nanocellulose from leaf fibres [32].
Generally, Pineapple peel waste (PPW) has been used for animal feed or land fertilizer [99], and substrate for production of bio-ethanol [41]. Antioxidant or reducing agent plays an important role both in health sector and chemical industry. Therefore, explored PPW as a potential of natural antioxidant is one of interesting pineapple applications as reducing agent is in biosynthesis of metal nanoparticles. It is found that biosynthesis of metal nanoparticles can be carried out by aqueous pineapple extract [45], and had successfully produced silver and cooper nanoparticles. Although fruit is the most used part of plant, other parts (root, rhizome, leaf, cortex, and stem) also have various nutritional and medici- Pineapple has been used as a medicinal plant in numerous cultures, both Lawal [105] and Dabesor et al. [106] reported that the solvent extracts of the various parts of A. comosus exhibit antibacterial, antiviral, antifungal, antiparasitic and anti-inflammatory properties. Moreover, pineapple juice, can be taken to alleviate sore throat and seasickness. The functional bioactivity of a plant extract, in general, depends upon the presence of compounds such as polyphenols, carotenoids and chlorophyll [14] [107], due to the antioxidant activity of polyphenolic compounds [108]. Flavonoids, mainly present as colouring pigments in plants also function as potent antioxidants at various levels. Some studies showed that flavonoids could protect membrane lipids from oxidation [109]. The anti-inflammatory properties in the fruit help reduce the symptoms of arthritis, and help reduce pain after surgery and sport injuries. A. comosus juice is taken as a diuretic and to expediate labour, also as a gargle in cases of sore throat and as an antidote for sea sickness, also to expel intestinal worms; and as a drastic treatment for venereal diseases. In Africa the dried, powdered root is a remedy for edema. The crushed rind is applied on fractures and the rind decoction with rosemary is applied on hemorrhoids. Indians in Panama use the leaf juice as a purgative, emmenagogue and vermifuge [2] [3]. Figure 6 shows the health benefits of pineapple waste (Ali et al., 2020) [110].

Conclusion
Waste generated from pineapple fruit is an important concern in the context of effective waste management. Therefore, it is useful to exploit of such waste with a view to the optimal exploitation of the waste. By appropriate extraction technologies to extract important nutritional and bioactive components is an important concern in the waste. This study reviews pineapple peel as becomes a profitable by-product, given its unique aroma, abundant volatile compounds, and nutritional values and the processing of waste into food and other application. In pineapple waste, many reusable substances with a high final value are found. There are significant amounts of high-value bioactive compounds in various waste generated from pineapple, such as a crown, peel or skin, core, etc. The results obtained in this study showed that pineapple peel has varying degrees of antimicrobial activity, anticancer and antioxidant activities. The results also revealed the presence of medicinally important constituents in peel. Many evidences gathered in earlier studies have confirmed the identified phytochemicals to be bioactive. Therefore, pineapple peel could be used as a good source of nonpharmacological treatment in the form of processed food and instant drinks and a promising source of metabolites for therapeutics, functional foods, and cosmeceutical applications.