3.1. Plants

Carotenoids, considered as secondary metabolites, are necessary for many biological activity and growth of plants. They give the bright red, yellow, and orange color to plants, leaves, flours, and fruits. Many flowers contain chromoplasts that contain carotenoids giving them their bright colors. Similar thing occurs in case of fruits. Phytoene, phytofluene, lycopene, and β-carotene are found in red-, yellow-, and orange-colored fruits and xanthophylls are found in green vegetables. Dark green leaves such as spinach and kale have higher concentration of carotenoids than light green vegetables such as lettuce and cabbages. Bright colored fruits such as tomato, watermelon, apricot peaches, mango, guava, and orange have high concentrations of carotenoids. Other vegetables such as beet roots, carrot, sweet potatoes, and bell papers also contain carotenoids [27,29].

3.2. Micro-organisms

Microalgae are a great source of carotenoids. They can be classified according to their photosynthetic pigments such as red algae (rhodophyta), green algae (chlorophyta), and brown algae (phaeophyta). Bioactive compounds such as lutein, β-carotene, and lycopene are widely available in microscopic algae. They are some significant carotenoids with high economic merit [26]. Many bacteria are able to produce carotenoids by chromatophores. They act as secondary metabolites to play key roles in cell adaptability. Bacteria, namely, Flavobacterium spp., Agrobacterium spp., Chromobacterium spp., Arthrobacter spp., Pseudomonas aeruginosa, and Rheinheimera spp. are some prominent producer of carotenoid pigments [27,30]. In comparison with other micro-organisms, fungi have the ability for production and accumulation of appreciably greater levels and concentrations of carotenoids intracellularly. Several classes of fungi including zygomycetes, ascomycetes, basidiomycetes, and the deuteromycetes are capable of producing greater level of carotenoids. Some example include Basidiomycete yeast, Blakeslea trispora, Neurospora crassa, and Xanthophyllomyces dendrorhous [27,31].

3.3. Animals

In case of bright colored birds, their red, orange, yellow, pink, and various color are the result of carotenoids colorization. These colors come from the various carotenoid rich diets of the birds, such as fruits and seeds high in a and b-carotene. Some examples of these birds are song birds and flamingo [32]. In case of fishes, most of the red fish and fishes with more fat concentration have high carotenoids stored in them, such as salmon, trout, and tuna. These carotenoids mainly come from their diet of microalgae, plankton, small insect, and shrimps. Red shrimps and lobsters also contain high level of dietary carotenoids such as astaxanthin which got released while cooking [33]. In case of eggs, the yellow-orange color comes from carotenoids also. β-carotene can also be detected in cow milk.

3.4. Human Diet

Human body can acquire carotenoids from various fruits and vegetables, animal foods, and other nutritional supplements. Microalgae are widely used to produce nutritional supplements containing carotenoids. Some of the examples are brown sea-weed (Undaria pinnatifida) and red algae (Chondrus crispus) [34]. When it comes to foods derived from animals, the yolk of eggs is particularly high in carotenoids. Milk, flesh, liver, and fat tissues of poultry and cattle also contain significant level of carotenoids. Red fishes, such as salmon and trout, have carotenoids in their tissue [35,36]. In case of plant and plant-based food products, carotenoids are available in bright red, yellow, and orange colored fruits (mango, orange, guava, papaya, watermelon, apricot, and peach) and vegetables (tomato, bell paper, carrot, beet roots, spinach, and kale), herbs (basil and coriander), legumes [37], cereals (wheat, maize, rice, and corn) [38], and seeds and seed oil (palm oil). In Table 1, various carotenoids found in a human’s daily dietary intake are shown.

Table 1: The different sources of carotenoids that can be found in human dietary intake.

3.5. Food Wastes

Although carotenoids are present in several human dietary sources, a huge amount of them get wasted as a form of food waste such as, some of fruits peels contain more carotenoids than the pulp. Such fruits and vegetables of which the peels and pulps (after extracting juice out of it) get wasted as most of the time those things were used in landfill or animal feed or in biofertilizer/biogas production. Food waste such as mango peel, peach peels and pulp, apricot waste, tomato peel and pulp, carrot peels, and passion fruit peels are high sources of carotenoids. Some other food waste except fruits and vegetables include coffee grounds, having rich number of carotenoids present in them. Sea food waste like shrimp shells also contain collectable amount of carotenoid.

4.1. Lutein-Zeaxanthin

Zeaxanthin is the stereoisomer of lutein. Both of them have similar function in human health, first as an antioxidant [12]. They act as a screen to safeguard one’s skin from ultraviolet (UV)-light; in addition, lutein has been suggested as having the potential to diminish cardiovascular system risks. Zeaxanthin and lutein are found in the macular pigment of human retina. Supplementation of lutein and zeaxanthin can increase macular pigment optical density (MPOD) values and reduce the risk of age-related macular degeneration in individuals with low MPOD (0.2 or lower) [39,40]. They also protects retina from photochemical injury and UV-induced peroxidation [41]. Several studies showed lutein having anti-cancerous properties against certain types of cancer. It improves the immune response also, both humoral and cell mediated [42].

4.2. Astaxanthin

It has similar antioxidant properties such as lutein as well as anti-inflammatory properties. Human clinical trials on eye health showed positive effects of dietary astaxanthin on diabetic retinopathy, eye strain and fatigue, and macular degeneration [27]. It increases the phagocytic and fungicide capacity of neutrophils. It increases lymphocyte proliferation and the cytotoxic activity of natural killer cells [43]. Increased consumption of astaxanthin decreases the risks of certain types of cancers [44]. Moreover, it has been shown to reduce cardiovascular health disease since it reduces low-density lipoprotein peroxidation and improves the blood flow capacity and blood lipid profiles [45]. It is one of the strongest antioxidant of carotenoids family, exhibiting Vitamin E and β-carotene. Application of astaxanthin with aspirin shows sun-proofing and anti-aging properties as well as anti-inflammatory activities [26].

4.3. Canthaxanthin

Canthaxanthin shows positive effect on neurological disorders, as a neuroprotective agent due to its antioxidant and anti-inflammation properties [13]. It enhances the communication between cells through gap junctions either directly or by forming 4-oxo-retinic acid, stimulating the retinoic acid stimulator. These mechanisms provide it with antitumor activities. Canthaxanthin can prevent some blood disorder disease; however, consumption of large quantity is required which might be unsafe [27,46].

4.4. Lycopene

The carotenoid is reported to have anticancer properties especially prostate cancer. A study conducted by Zu et al. [47] on 49,898 male health professionals showed lower risk of fatal prostate cancer for the candidates having dietary intake of lycopene as compared to those who consumed less lycopene. The sources of lycopene in diet were pink grapefruit, tomato, tomato sauce, tomato juice, watermelon, and pizza [47]. Lycopene enhances the integrity of vascular barrier, inhibits the barrier permeability, cells adhesion molecule, and prevents leucocyte adhesion and trans-endothelial migration. Doing so lycopene suppresses pro-inflammatory response. It also shows antioxidant activities and has potential roles in cardiovascular health [48].

4.5. β-carotene

Beta-carotene shows provitamin A functions in human body. It is used as a supplement for Vitamin A deficiency. Regular intake of β-carotene can affect eye health and reduce macular diseases. It also shows anti-cancerous and antioxidant activities. In various studies, β-carotene has shown protection against sunburns [49].

5.1. Soxhlet SE

SE is one of the traditional methods. It utilizes a solid-liquid extraction method that employs continuous reflux of natural solvents to enable the transfer of non-harmful biological molecules in large quantities. Some of the solvents used for carotenoids extraction include hexane, ethanol, acetone and chloroform. Although this technique extracts the largest amount of carotenoids from a given sample; nowadays, it is not preferred due to more time consumption, lengthy processing time, requirement of high amount of organic solvents, and increased extraction cost [51,52].

5.2. MAE

This method employs microwave radiation to heat up the solvent that further enhances transfer of solutes present in the matrix to the solvent. Microwave radiation is capable of rupture plant cell walls, resulting in an easy release of bioactive compounds present in cells into solvents. Both polar and non-polar solvent are used in microwave extraction such as aglycones, flavonoid glycosides, acetone, chloroform methanol, and ethanol with different concentration. This method is more appreciated presently, for being simple, time convenient and cost effective. Although this method requires less time and less amount of solvent, the yield of this technique is very low as compared to Soxhlet extraction method. Increase of extraction time can cause thermal degradation of bioactive compound, so microwave time and power optimization is critical issue in this technique [51,53].

5.3. UAE

This technique utilizes the mechanical energy produced by ultrasound waves. The sonication generates small bubbles or gaps in the liquid, resulting in the falling apart of the internals, due to reaching high temperature (4500°C) and pressure (50 MPa) [54]. In this method, ultrasonic probe is submerged in the solution and generates ultrasonic power at least 100 times greater than that of the bath, with a typical sonication time of 5 min or less. In the study of Goula et al., [55] a Ti-Al-V sonoprobe (13 mm) equipped 130 W maximum nominal output power, 20 kHz frequency VCX-130 Sonics and Materials (Danbury, CT, USA) sonicator was employed for pulsed mode ultrasound-assisted extraction for extracting carotenoids from pomegranate waste. The sample was combined with 200 mL solvent to obtain different peels/solvent ratios. The sample container was kept in a thermostat-controlled water bath during the extraction procedure. The extracts were collected at 10, 20, 30, 40, 50, and 60 min and filtered through glass microfiber paper to eliminate particle residues before to carotenoids analysis. As this technique is performed in low temperature, thermal damages can be avoided [55].

5.4. Super Critical Fluid Extraction of Carotenoids

Supercritical fluids are either liquid or gas used above critical pressure and temperature where gas and liquid can coexist. For this nature, gas and liquid phase become indissociable form each other. In this method, mostly pressurized fluid is used as solvent and carbon dioxide gas is being used. It becomes advantageous for its greater range of extraction ability [56]. Total carotenoids were extracted by super fluid extraction (SFE) utilizing S-CO₂ at 15 g/min and optimized processing settings were 30 min at 59°C, 350 bar, and 15.5% (v/v) ethanol [57].

5.5. Enzyme-Aided Extraction

Enzymes act as catalysts in the process of extracting bioactive compounds. The raw material is treated with enzymes, allows it to hydrolyze the cell wall and the phytochemicals will come out along with carbohydrate chains and lipid molecules. Drawback of this process is factors affecting the extractions such as type, dosage, and condition of enzyme particle size of sample materials and its water content among other factors. Some of its advantages compared to previous methods are high yield and low-energy consumption. Some of the enzymes used in this method are proteases, tannases, pectinases, and cellulose [50].

5.6. High-Shear Dispersant Extraction

In this technique, cell wall and membrane of source is disrupted by mechanical pressure, using high-shear dispersant, releasing the compound present within the cells. To effectively and selectively extract water-soluble proteins, carbohydrates, and C-phycocyanin, the use of a mechanical pre-treatment like high-shear homogenization (HSH - 20,000 rpm, 96 kJ/kgSUSP) in combination with (PEFs - 20 kV/cm, 100 kJ/kgSUSP) has been advocated [58]. This process is less time consuming than many other traditional techniques. Vegetable oils are used as an organic solvent in this process, which reduces the energy consumption as well as environment friendly. It also avails the possibility to obtain uncontaminated carotenoids, directly usable without any need of purification. A disadvantage of using vegetable oil is its viscosity, elevated viscosity reduces solvent diffusivity [55,59].

5.7. PEF-Assisted Extraction

PEF-assisted techniques are based on the electroporation, or electropermeabilization, phenomenon, which depicts the development of pores in cell membranes as a result of electric pulses with short durations (from several nanoseconds to several milliseconds) and high electric field strengths (from 100–300 V/cm up to 300 kV/cm) [60]. Using electric fields for extraction processes can decrease the necessary temperature and increase the overall yield. This is extremely important for thermolabile compounds like carotenoids. The cellular components must be exposed to a strong electric field (external) in range of 1–50 kV/cm for a brief period during the extraction process. The cell membrane barrier allows for the electrical potential to divide molecules depending on its charge. The formation of pores is a result of this electro-permeabilization technique, which may or may not be reversible, in the cell membrane or wall, allowing the components of the cells to come out. Low-energy consumption and processing cost along with high efficiency and low temperature are some of the advantages of this process. The absence of any petrochemical solvent makes this technique environment friendly. Without damaging the matrix, the technique enhances the selective separation of biologically active components. However, the technique must be adjusted differently for different types of samples, as the parameters for a process depend on the texture and electrical conductivity of the sample [61-64].

6.1. Extraction from Tomato Waste

Tomato (Lycopersicon esculentum) one of the most important components in Mediterranean diet making it the second most important vegetable worldwide. Approximately 80 million tons of waste is generated from over 130 million tons of production worldwide. After industrial processing, the remaining tomato pomace including peels and seeds are mostly used as animal feed. However, these wastes contain a plethora of bioactive compounds, including phenolic components, vitamins, and carotenoids. Of all the pigments, lycopene [Table 1] makes up about 80–90%. Consumption of lycopene shows various health benefits including cardiovascular and anticancer activities, supported by various epidemiological studies [66,67].

The rudimentary carotenoids present in tomato are lycopene and β-carotene, where lycopene is present mostly in the peels and the seeds contain β-carotene. For being beneficial to human health, the demand of low cost available and high purity carotenoids is increasing. The extraction of lycopene from tomatoes is commonly done using traditional methods that involve the application of a range of organic solvents and their combinations in established protocols. One of the most efficient ways to obtain lycopene is by utilizing the peels of tomato generated by juice production industries.

Using enzymes to aid in extraction is often viewed as a pre-treatment step rather than a standalone method of extraction. This technique breaks down the polysaccharide chains of the tomato cell wall in which the majority of pigments are located. This helps the solvents to penetrate and improve the release of carotenoids and reduce extraction time and temperature [68]. Using cellulolytic and hemicellulolytic enzymes as a pre-treatment method for tomato peels is an effective method for recovering carotenoids [69]. In an experiment of Catalkaya and Kahveci, the study demonstrated that the polar nature of the solvent used is a significant factor in the process of extraction (e.g., 1:2 acetone-chloroform, 50:25:25 hexane-acetone-ethanol [70].

It is one of the safest techniques where carbon dioxide is used as super critical solvent among many, for being environmentally safe, non-toxic, and easily available. In an experiment performed by Kehili et al., the extraction yield for lycopene and β-carotene was up from 32.02% to 60.85%, and 28.38% to 58.8%, respectively. The raw material used here was Tunisian industrial tomato peels [71]. Latest research has shown that usage of additional cosolvent can increase the carotenoids solubility. de Andrade Lima and the team used ethanol as a cosolvent in SFE method, that increases the polarity of CO₂ which ultimately increases the recovery rate of more than 90% of carotenoid. Lycopene recovery was also efficient (almost 95%) due to the high content of moisture found in peels of tomato [57].

The electrical and/or heat treatment’s induction of cell disintegration at the cuticular level enhances the solvent’s entry into the cytoplasm, amplifying the carotenoids’ ability to be extracted. Degradation is made possible by PEF extraction technology, which also cuts down on time, energy costs, and solvent usage. An investigation was conducted to determine the relationship between the breakdown index of cells present in tomato peel tissues and different electric field strengths as well as temperatures used in steam blanching (SB). The result of steam blanch treatment 60°C applied with PEF treatment with an intensity more than 0.75 kV/cm shows a much higher yield of extraction of total carotenoids (PEF-188% and steam blanching-189%) and power of antioxidant (PEF-372% and steam blanching- 305%) when compared to untreated peels of tomato (Pataro et al., 2018). In case of solid liquid extraction, pretreatment with PEF (5 kV/cm, 5 kJ/kg) increases the extraction yield of lycopene to 12-18% and total extraction rate to 27–38%, in acetone or ethyl lactate solvent [72].

Application of UAE method on tomato pomace for lycopene extraction using ethyl lactate and ethyl acetate mixture as solvent along with optimized extraction conditions (30% v/v ethyl acetate in solvent mixture, 63.4°C, 100 mL/g solvent to sample ratio, 20 min) yielded higher (1334.8 ± 83.9 µg/g) at shorter time and lower temperature. Compared to this, the yield without UAE was 9% lower [73]. Combined treatment of UAE and freeze drying increases the yield by 4.12 folds from industrial tomato waste of lycopene extraction [74]. Hydrophobic eutectic mixtures of DL-methanol (hydrogen-bond acceptor) and lactic acid (hydrogen-bond donor) in the UAE yielded as high as 1.45 g of lycopene/Kg dry weight of tomato pomace. The yield is reportedly higher than the traditional organic solvent mixture of ethyl acetate and ethyl lactate for the same raw material [75,76].

6.2. Extraction from Carrot Waste

Carrots are a kind of tuber or root vegetables, present in various colors, including, purple, red, orange, yellow, and white [77]. Carrots are considered the most abundant source of β-carotene [Table 1] [78]. Carrot waste is abundant in carotenoids, particularly β-carotene. Researchers have shown that the peel of carrots contains a higher concentration of β-carotene (204.5 mg/g) as compared to carrot pomace waste (19.81 mg/g) and carrot pulp waste (39.2 mg/g) [79,80]. β-carotene is the rudimentary carotenoid present in carrot. Selective SE techniques are used to extract both polar and non-polar carotenoids using various mixtures of organic solvents [81]. Other green extraction processes such as MAE, ultrasound extraction, and super critical CO₂ assisted extraction methods are also can be used. Water-induced hydrocolloidal complexation method was used to co-extract β-carotene and pectin from carrot peels where complexation of these two bioactive components under different operating stirring conditions yielded in 1.17 mg of β-carotene in 100 g wet sample [82].

Carrot juice processing waste was frozen at –40°C and then freeze dried to reduce moisture content to <1%. A definite amount of waste powder was mixed in flaxseed oil, used as solvent. The mixture was placed in an extraction vessel and it was put in a microwave extraction device. The temperature and time can be set in different variation to obtain maximum yield but should not exceed 110°C. A closed microwave system is used to carry out the process. After extraction process, the flaxseed oil, enriched with carotenoids, was stored at –20°C in a dark or black glass bottle. The highest recovery yield was achieved at 170 W microwave power, 9.46 min of extraction time with oil to waste ratio as 8:1. The recovery percentage was 77.91 ± 0.33 [83]. The quantification of extracted β- carotene was done by high-performance liquid chromatography (HPLC) using methanol/acetonitrile (9:1) with 9 µM triethylamine mixture as mobile phase at 0.9 mL/min flowrate, 450 nm wavelength [83]. Changes in carrot tissue caused by sonication impacted total carotenoid content and color change [84]. Carrot pomace was used as a raw material, while the UAE was carried out for 16 min of sonication at 29°C under 59% v/v concentration of ethanol yielded 14.37 μg/g of β-carotene, 5.35 μg/g of lutein, and 2.5 μg/g of lycopene [23].

The aim of this process was to assess and improve the method for extracting carotenoids from peels of carrots using supercritical CO₂ (SCO₂) in combination with a cosolvent like ethanol. The factors that were assessed included temperature and pressure and the concentration of a cosolvent [23]. Carrot peels were used as sample, which were dried and mixed 5 g of it with 95 g inert glass beads for homogenization. The mixture was put in an extraction vessel and CO₂ flow was applied at a rate of 15 g/min with ethanol as a cosolvent. The extraction time was 80 min. After every run, the collected carotenoids, dissolved into ethanol, were evaporated using a rotavapor to remove the ethanol and the collection was weighed and redissolved in methanol for total carotenoid concentration analysis [85]. According to a validated model, the best conditions for achieving the highest mass yield (5.31%, d.b.) were determined to be 58.5°C, 306 bar, and 14.3% ethanol. In addition, the most efficient conditions for carotenoid recovery (86.1%) were identified as 59.0°C, 349 bar, and 15.5% ethanol [85].

The results of this study indicate that using PEF technology can effectively enhance the carotenoid resistance of natural food ingredients, such as carrot pomace. The use of electroporation in PEF treatment has the potential to enhance the ability to extract carotenoids from carrot pomace. Roohinejad et al. [79] have optimized β-carotene extraction through microemulsion (made of medium chain triglycerides, phosphate buffer, and tween 80) using Box–Behnken experimental design. The optimal conditions at 49.4 min of extraction time under temperature of 52.2°C having carrot to microemulsion ratio as 1:70 (w/w) yielded 19.6 µg/g of β-carotene in the microemulsion. Based on the given conditions, it was predicted that the β-carotene content would have a response value of 19.6 µg/g [79,86].

6.3. Extraction from Mango Waste

Mango (Mangifera indica L.) is cultivated in sub-tropical and tropical regions, which belongs to Anacardiaceae family. It is produced in more than ninety countries [87-89]. Mangoes contain a diverse array of bioactive compounds, such as phenolic compounds and carotenoids [Table 1], that can be found in ripe mangoes [90,91]. Depending on cultivars and weight of pulp can vary from 33 to 85%, skin 7-24%, and seeds 9–40% (w/w). Therefore, the processing of mangoes generates 35-60% (w/w) agro-industrial waste [92,93]. Peeling and recovery of skins and seeds, respectively, as rich source of bioactive compounds can not only aid in managing waste but also yield substances that can be utilized in various industries, such as food and beverages, pharmaceutical, and cosmetic products [94]. Peel and seed are the main by-product of mango fruit. Ripen mango skin contains various kind of carotenoids including α and β-carotene. Green extraction process such as MAE and supercritical CO₂ (SrCO₂) extraction is used to extract the bioactive compounds present in the mango waste. Mango peels were freeze dried at –35°C for 48 h under 4.0 Pa pressure or air dried (50°C, 24 h), size reduced and extracted using SrCO₂ (operated at 40°C temperature, 30 MPa pressure, and 1.1 L/min CO₂ flow rate). The process yielded total carotenoid of 5.6 ± 0.51 mg β-carotene/g dry basis of mango peel [95,96]. High pressurized ethanol can be used as a co-solvent [97] or the residue of SrCO₂ extraction can be treated with it under the same conditions [96]. The output of extracted carotenoid is affected by the duration of extraction, the temperature, the pressure, and the concentration of the cosolvent. The amount of carotenoids can be determined through methods such as UV-visible spectrophotometry and ultra-performance liquid chromatography having diode array detector [97]. Application of polar solvent such as ethanol in SrCO₂ extraction increases the extraction yield due to the presence of polar components in sample [98]. Segatto et al. [99] extracted secondary metabolites, namely, hyperoside (flavonol) and mangiferin (xanthone) under MAE system and matrix solid-phase dispersion (MSPD) technique from mango processing waste (MPW). They revealed higher extraction yield of hyperoside (398.52 mg/Kg of dried MPW) and mangiferin (352.90 mg/Kg of dried MPW) using MSPD technique as compared to MAE system [100].

6.4. Extraction from Orange Waste

Orange has various biochemical compounds loaded with secondary metabolites including amino acids, proteins, carbohydrates, phenolic compounds, flavonoids, and carotenoids [101]. Carotenoid is one of the main natural pigment presents in orange give it its orange color. A variety of carotenoids are present in orange including mutatoxanthin, β-cryptoxanthin, violaxanthin, zeaxanthin, and antheraxanthin. The main compound in orange juice is α- and β-carotene measuring almost 1.5 µg/mL [102]. Of the oranges grown worldwide, four out of ten are used to manufacture concentrated juice. While extracting the juice, it produces by-products including peels core and frit. The pulp is also produced as by-product during filtration of extracted juice. These by-products contain a variety of carotenoids which can be extracted using green extraction methods [101]. After extraction of juice, the by-products can be treated for carotenoid extraction. Orange peel is the most favorite sample for these extraction process probably due to less moisture content in it. Traditional green SE techniques can be used to extract carotenoids from orange as well as green extraction techniques such as ultrasound-assisted extraction can be used.

Carotenoids are extracted from homogenized orange peels by SE technique. In this process, various organic solvents, such as acetone, hexane, petroleum ether, and KOH, is used to extract the carotenoids from the peels. Final separation of carotenoids from solvent is done using separating funnel and diethyl ether and distilled water for washing, where the upper lipophilic phase contains carotenoids. Rota vapor and nitrogen was used to concentrate the final extract [103]. Orange peels were dried to reduce moisture content and then grounded using electric mill. An ultrasound-assisted extraction method was utilized by combining powdered orange peel with various solvents, including olive oil and different types of ionic liquids including olive oil [104] and different types of ionic liquid such as 1-n-butyl-3-methylimidazolium tetrafluoroborate, 1-hexyl-3-methylimidazolium chloride, and 1-butyl-3- methylimidazolium chloride [105] and 1-butyl-3- methylimidazolium hexafluorophosphate [106]. In case of ionic liquids, ethanol is used as a cosolvent. The utilization of frequency and power of ultrasonication can vary depending on the specific experiment, as well as factors such as time required for extraction, temperature, and the ratio of liquid to solid. Utilizing a 4000 rpm speed on the centrifuge, the plant material was divided through spinning for a quarter of an hour [104,105]. The analysis of extracted carotenoid can be done using HPLC with DAD detector [104,107].

6.5. Extraction of from Shrimp Waste

Shrimp, a highly sought-after seafood item, is prepared for consumption by removing the shell, head, and tail, which are typically considered as unusable parts. These were part measures almost 60% of shrimp’s body weight [108,109]. The waste parts are good source of proteins, lipids, chitins, and carotenoids, having potential industrial uses [110,111]. In terms of carotenoid representation within raw shrimp, astaxanthin and β-carotene dominated with astaxanthin contributing 75.18–88.86% of the total carotenoid content [112]. Astaxanthin having high antioxidant activity and anti-diabetic, neuroprotective, and anti-inflammatory activity [113-116] and the current pollution issues faced by the sea food industries for dumping the waste generate a demand for utilization of shrimps wastes to produce astaxanthin [117]. The main carotenoid content of shrimp waste is astaxanthin and there is very little information available on its extraction process. Powdered sample of shrimp waste is mixed with different solvents (acetone and methanol) with different ratio and put in a high-pressure container and different pressure (210 MPa) and holding time (10 min) is applied. The research team then used a centrifuge to separate the sample and collected the liquid layer on top. Mixture of distilled water and hexene (1:1 v/v) was added to it and the resulting upper layer was collected and dried using oxygen free nitrogen [118].

Total carotenoid content was determined using spectrophotometer and the yield of astaxanthin was measured using HPLC [118,119]. Utilizing commercial, crude and unconstructed enzymes for the purpose of enzymatic hydrolysis are a technique that has been investigated for the extraction of astaxanthin from shrimp. A combination of proteolytic enzymes (barbel and bovine trypsin) and ethylenediamine tetraacetic acid EDTA (0.5 M) is used on Dried Shrimp shell powder to obtain the dry matter “carotenoprotein.” This carotenoprotein underwent treatment with lipase and protease for hydrolysis, resulting in the production of 900 mg/g of soluble protein and 66 mg/g of total carotenoids through the use of 10 lipolytic and 15 proteolytic units [120,121].

The enzyme alcalase can also be used for recovering carotenoids although suitable range of pH is needed for proper activity of this enzyme. The range of recovery was 57.5–64.6% measuring 4.7–5.7 mg/100 g of dry waste [113]. Cabanillas-Bojórquez et al. [122] used super critical CO₂ and ethanol to extract astaxanthin from dried exoskeleton of shrimp. After extraction, the supernatant was collected form the extract through centrifugation, dried, and resuspended into ethyl acetate for astaxanthin concentration analysis using HPLC.

6.6. Extraction from Pumpkin Waste

Pumpkin is a good source of bioactive compound which includes more than 800 varieties of its species [123]. Pumpkin pulp is considered a good source of carotenoids. Major carotenoids present in pumpkin is β-carotene, as well as little amount of lutein, lycopene, and α-carotenes are present also [124]. In spite of having the potential to use pumpkin waste as an alternative for carotenoid production, most of the waste of pumpkin is used as landfill material or converted into biofertilizer. It is also used for animal feeding [125]. To extract bioactive compounds from vegetables, green extraction technologies using vegetable oil as a solvent or cosolvent were studied before. These techniques are environment friendly and do not require any use of harmful chemicals. To harvest carotenoids from pumpkin waste, it is suggested to use freeze dried sample. Using methods such as ultrasound-aided extraction and microwave-aided extraction can be implemented to implement green extraction techniques.

In their experiment, M. Sharma and Bhat extracted carotenoids from pumpkin peels and pulp, using corn oil as a green solvent and ultrasonication technique. The final extract was separated from the oil through centrifugation. The total carotenoid content was determined by spectrophotometer at 470 nm [126]. In the same experiment, they used MAE technique to extract carotenoids from pumpkin waste using same solvent and MAE extractor (130W, 30 min, 45°C). After extraction process, the cooled extract were separated from the oil through centrifugation [126]. The total carotenoid content was determined by and the antioxidant capacity of extracted carotenoid was estimated by 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) assay. Ghosh, Biswas [127] used pumpkin peels and pumpkin industrial waste in their experiment to extract carotenoids, such as lycopene, using enzymes such as cellulase and pectinase. The hand blended pumpkin wastes are treated with these two enzymes separately to extract carotenoids and the extracts are filtrated and undergone SE using acetone and petroleum ether. Effects of pectinase were shown better than cellulase in case where the raw material was fruit pulp waste. Same conclusion can be drawn in case of whole carotenoid extraction and lycopene extraction from pumpkin pulp. However, pectinase showed better yield in case where industrial pumpkin waste, such as peels, were used as raw material [127].

6.7. Extraction from Passion Fruit Waste

Passion fruit waste refers to the parts of the passion fruit that is not typically consumed, such as the peel and seeds. The waste created by juice processing industries weigh almost 60% of the whole fruit gets discarded [128]. These waste parts may contain valuable nutrients such as carotenoids, vitamins, and minerals. Passion fruit waste parts contain carotenoids such as beta-carotene and lutein. These carotenoids have antioxidant properties and may have health benefits such as promoting eye health and reducing the risk of certain diseases [12]. Passion fruit waste can be used for various purposes such as animal feed, composting, and extraction of bioactive compounds. The extraction of biologically active compounds from these types of waste materials has the potential to provide significant industrial benefits.

Conventional extraction process using petrochemical solvents is useful for extraction of carotenoid from plant based materials but they comes with issues such as generates harmful chemical compounds, having low extraction rates and long extraction time [129]. Therefore, methods of green extraction such as ultrasound-assisted and microwave-assisted extraction using bio-based solvents like vegetable oils are gaining popularity due to its low to zero chances of environmental harming by products production, low extraction time, and high efficiency and low-energy costs [130,131].

In their experiment, Chutia and Mahanta used UAE technique and MAE technique to extract carotenoids from freeze dried passion fruit peels. In both techniques, olive oil and sunflower oil were taken as solvent and in case of UAE method; the mixture of solvent and passion fruit peels were ultrasonicated using a probe and in case of MAE method microwave system was used for extraction process. Further, filtration was done using glass microfiber paper [18]. The total antioxidant activity of extracted carotenoid was measured using DPPH free radical scavenging activity.

Passion fruit peel powder was mixed with a solvent containing mixture of n-hexane, ethanol, ethyl acetate, and acetone (2:1:1:1) in volumetric ratio. The mixture was put into extraction thimble and extraction was done inside Soxhlet apparatus. After the end of extraction process, the solvents were evaporated using vacuum evaporator to separate the extracted carotenoids [83]. To quantify the amount of carotenoids and βcarotene, a dilution of carotenoids extract in n-hexane was performed for the investigation [18].

6.8. Extraction from Guava Waste

Guava fruit contains several carotenoids, including lycopene, beta-carotene, and lutein. These compounds are responsible for the red or yellow color of the fruit and have been found to have antioxidant properties [132]. Guava is used in food industry for production of juice, syrup, jelly, and flavored food. It is a popular tropical fruit. However, during various stages of industrial productions, it produces wastes containing seeds, peel, and leftover pulp, making almost 30% of the fruit weight [133]. This waste can create environmental problems. However, the amount of carotenoids left in the waste parts of guava holds industrial value if extracted properly [132]. The extraction of carotenoids from guava waste products can be done through maceration but is not accepted well due to its labor intense process, high extraction time, and use of excessive solvents. Hence, alternative non-conventional green extraction techniques, such as UAE, are now being interested due to their high extraction yield, low extraction time, and processing labor and most importantly by production of less to none toxic chemicals [134,135]. da Silva Lima et al. [136] used different UAE techniques to extract carotenoids, mostly lycopene and β-carotene from dried guava pulp, peel, and left-over waste. Three different methods, maceration extraction, bath type and probe type UAE, were used for comparison purpose to understand the best possible method for extraction while ethyl acetate was being used as a solvent. Lycopene was found to be the major carotenoid present in the extract. Ultrasonic bath type extraction method was found to be most effective with high yield, where probe type extraction method showed the most degradation of carotenoids. Maceration extraction process was also found to be efficient for extraction but did not get entertained due to its high usage of organic solvent making it impractical for large scale commercial use [136].

6.9. Extraction from Pomegranate Waste

Pomegranate fruit contains various carotenoids, including lycopene, β-carotene, and lutein. Research has demonstrated that certain carotenoids possess antioxidant properties and may offer potential health advantages, such as a decreased likelihood of heart disease, cancer, and diabetes. In addition, pomegranate fruit also contain high amounts of punicalagins and punicic acid, which are unique antioxidants found in pomegranate and are responsible for many of its health benefits. As the yield of extracted juice from pomegranate is less than half of its weight, it produces a lot of waste by-products. Pomegranate waste, such as the peels and seeds, also contain various carotenoids, including lycopene, beta-carotene, and lutein. Although previously pomegranate peels are mostly applied as animal feed with recent advance in studies, the industrial value of it came to known for carotenoid production [137,138]. There has not much studies been published regarding the extraction of carotenoids from pomegranate. Instead of using traditional method, green SEs such as use of vegetable oils as solvents are now being considered as alternatives.

Goula et al. [55] used vegetable oil as an alternative solvent to extract carotenoids from pomegranate peels by applying ultrasound-assisted techniques. They used both bath type and probe type ultrasonication device and sunflower and soy oil for extraction and the extracts were filtered using glass microfiber. In this process, they managed to extract almost 85.7 and 93.8% of total carotenoid present in the peel waste. Moreover, with increase in temperature (20–40°C), the extraction yield was also increased [55].

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