Introduction

Cotton is grown as annual crop in many countries for commercial purpose. Different commercially valuable Gossypium hirsutum, Gossypium arboreum, Gossypium herbaceum, Gossypium barbadense, and other cotton varieties developed conventionally to produce improved agronomic varieties of cotton for their application in textile, fiber, oil, and animal feed. The worldwide cottonseed production in 2019–2020 was 43 million metric ton (Kumar et al. 2022). Cottonseed oil (CSO) is the by-product of cotton manufacturer; extracted from the decorticated and delinted cottonseed for their used as edible oil and industrial applications (Orhevba and Efomah 2012; Shah 2017). CSO production industries are found in India, China, Turkey, Pakistan, the USA, Uzbekistan, and Brazil (Ghazani and Marangoni 2016), and worldwide consumption of CSO is nearly 5.2 million metric tons (List 2016).

Clean and dry cottonseeds are used in oil extraction, and it contains 15–20% CSO depending on the quality and varieties. Oil percent is also dependent on weather, growth, and maturity of cottonseed, and oil yield also varies from the season to season and place to place of cottonseed varieties. Oil percentage of 20 wild varieties of Gossypium are ranging from 11.22 to 24.82% (Sharif et al. 2019). CSO is extracted by different conventional and non-conventional methods. Conventional methods include organic solvent extraction; other biochemical and mechanical methods included in the non-conventional methods to extract oil from oilseeds in large extent (Nde and Anuanwen 2020). After extraction, CSO is evaluated for fatty acid content and physiochemical properties including free fatty acid content, acid value, iodine value, saponification value, peroxide value, specific gravity, refractive index, color, quality, and stability of oil. CSO is also evaluated for bioactive compounds including total antioxidants, flavonoids, sterols, tocopherols, and gossypol for their use in human consumption and industrial applications (Pliego-Arreaga et al. 2013). CSO is regarded as nutritionally balanced oil due to higher content of unsaturated fatty acid (USFA) such as oleic (17%) acid and linolenic acid (56%) with 23% of saturated fatty acid (SFA) (palmitic acid).

Genetic approaches like gene silencing and transgenic development used to enhance quality and quantity of CSO depend on the industrial and consumers’ demand. Level of oleic acid was enhanced by Agrobacterium transformation. Oleic acid converts to linoleic acid by enzyme fatty acid desaturase 2 (FAD2), and suppression of FAD2 enzymes leads to increase in oleic acid level, while overexpression of FAD2 with co-expression of Δ12-epoxygenase gene (Capl2) leads to increase in vernolic acid content in transgenic CSO. Antisense, RNAi, ribozyme, and co-suppression techniques were used in gene silencing of stearoyl-acyl-carrier protein Δ9-desaturase (ghSAD-1) to enhance the steric acid level, and downregulation of oleoyl-phosphatidyl choline ω6-desaturase (ghFAD2-1) uplifted the level of oleic acid (Sharif et al. 2019). Silencing of microsomal oleate desaturase (GhFAD2-1) and palmitoyl-acyl carrier protein thioesterase (GhFATB) enhances the quality of CSO (Liu et al. 2017). Overexpression of seed-specific Diacylglycerol acyltransferase (GhDGAT1) gene was observed to increase the total CSO content from 4.7 to 13.9% (Wu et al. 2021).

CSO is commonly used as cooking oil for commercial and home cooking, for example, use in the production of crackers, biscuits, mayonnaise, pastries, potato chips, salad, margarine, shortening, dressing, doughnuts, ice cream substitutes, baking, frying, oriental dishes and for industrial applications, which are special lubricants, inks, fabric dispersants, protective coatings, soft soaps, detergents, rubber and leather manufacturing, alkyl resins for interior paints, and biofuel to improve their quality and properties (Pliego-Arreaga et al. 2013; Orhevba and Efomah 2012; Sharif et al. 2019). For use of CSO as edible oil, it is necessary to check their safety profile; use of toxic organic solvents for extraction and presence of gossypol in higher concentration limit their use for human consumption. Different methods of extraction of CSO with the use of less toxic organic solvents are developed to produce healthy CSO with high oil yield and low free and total gossypol content (Bhattacharjee et al. 2007; Gadelha et al. 2014; Taghvaei et al. 2015). CSO has many applications in processed food as edible oil and other technical applications such as production of paints, cosmetics, detergents, biodiesel, agricultural, and pharmaceuticals. This review aims to study the extraction, characterization, and safety profile of CSO for their food and non-food applications.

Extraction of Cottonseed Oil

The process of triglyceride separation from various seeds is known as oil seed extraction. Extraction methods have been evolved from conventional to non-conventional methods to improve oil yield, quality, and their physiochemical properties. Chemical method includes solvent extraction (SE); biochemical method includes enzyme-assisted extraction (EAE); and mechanical method includes supercritical fluid extraction (SFE), high pressure extraction (HPE), microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), and mechanical extraction (ME) (Al Khawli et al. 2019; Putnik et al. 2018, 2017). Before extraction, clean and dry cottonseeds need to be grinded and sieved to obtain uniform particle size of cottonseed meal/cake. Grinding is an essential process in oil extraction to control flow of extraction and to achieve higher oil yield, and same particle size of oilseeds is positively influenced in the downstream process of oil extraction. In ME extraction method, clean and dry cottonseeds are directly used for extraction by screw or hydraulic pressing. Screw and hydraulic pressing techniques are physical pressing of oil extraction and it gives high quality of oil with natural color, aroma, taste, and other natural ingredients as it does not contain any additives and toxic chemicals than other methods which required chemicals which results in colorless, odorless, and less nutrient content oil after many steps of extraction and purification of crude oil (Kristoferson and Bokalders 1986; Nde and Anuanwen 2020; Pliego-Arreaga et al. 2013).

CSO is conventionally extracted by Soxhlet method using different solvents at laboratory scale. SE is based on the ability of solvent used to dissolve and extract oil from oilseeds. Chloroform, methanol, acetone, ethanol, water, n-hexane, petroleum ether, etc., solvents are involve in the SE method (Nde and Anuanwen 2020). Gossypium hirsutum seeds were grinded, vacuum dried, and segregated to optimize different parameters: 0.6-, 1.2-, and 1.6-mm particle size, 5:1 to15:1 solvent to solid ratio, 288 to 318 °C for 1- to 180-min time interval to study the kinetics and thermodynamics of CSO extraction by using hexane and ethanol as a solvent. Small particle size of seeds was easily dissolved in the solvent than large particles, which is highly resistant to the oil seed extraction; solvent to solid ratio depended on the ability of solvent to solubilize large amount of oil in the solvent during process, and solubility of CSO in n-hexane is slightly higher than ethanol. Higher temperature for CSO extraction process also helps to increase the solubility of oil and decrease viscosity. Surface oil easily dissolves in the solvent, and diffusion of remaining CSO in the solvent increases with the increase in the extraction time interval. Higher oil yield is obtained at 0.6-mm particle size and at 318 °C temperature after 3-h extraction in both hexane and ethanol extraction process; and this study concluded that ethanol was good alternative for hexane (Saxena et al. 2011). Twenty grams of Lankart-57, RH-112, F-20, K-25, and D-9 cottonseed varieties were used to extract CSO by Soxhlet method using 300 mL n-hexane as solvent at 70 °C temperature for 5 h, and result shows oil content in the range of 12 to 14.55% (Shah 2017).

Different enzymes are used in the EAE method to degrade cell wall and hydrolyze cellular material to extract oil from oilseeds, and the choice of enzyme depends on the structure of oilseed and experimental conditions. Phytezyme, Natuzyme, Kemzyme, Feedzyme, and Allzyme were used in the aqueous-enzyme-assisted extraction of CSO at optimum conditions such as 2% (w/w) enzyme/substrate ratio with 45% moisture at 40 °C temperature for 6 h, and enzyme inactivated at 100 °C for 20 min. Oil content obtained in the range of 9.7 to 12.89% oil after drying at 60 °C in vacuum oven and pressing for 20 min at 49 MPa. Aqueous-enzyme-assisted extraction with cold pressing increased the oil content than control; it also improved the quality of oil for consumption purpose (Latif et al. 2007; Nde and Anuanwen 2020). Use of enzymes and mixture of enzymes in equal ratio for hydrolysis of cottonseed flakes increases the quality and extractability of in comparison to control (Taha and Hassanein 2007).

SFE has gained lots of importance due to its selective, high efficiency, short interval of oil extraction, lower requirement of refining and does not leave harmful residue in extracted oil and safe for environment. Carbon dioxide with non-flammable, non-toxic, high diffusivity, low viscosity, power of good solvent and easily removable properties, and use of low temperature makes them most common solvent to be used in SFE method. Hence, SFE method is also known as supercritical carbon dioxide extraction method. Supercritical carbon dioxide extraction parameters of CSO were optimized for three variables: 250–550 bar pressure, 60–80 °C, and 1–3-h time by RSM and central composite rotate design (CCRD) to obtain higher oil yield with low gossypol content. The obtained optimum condition improves the 43.16% yield of CSO with 0.023% low gossypol content at 550 bar pressure, 70 to 80 °C temperature for 2–3 h in comparison with Soxhlet extraction method after 16-h extraction at 60 to 80 °C using n-hexane obtained 40% oil yield with 0.24% free gossypol content. SPE reduces the time of oil extraction without using any harmful organic solvents (Bhattacharjee et al. 2007; Nde and Anuanwen 2020).

MAE is another efficient and attractive method of oil extraction. Non-ionizing radiations of MAE cause dielectric heating and destruction of cells which ensures extraction of high-quality oil with small amount of solvent and time requirement. Ionic conduction is the migration of ions in applied electromagnetic field, and dipole rotation of ions results in the increase in temperature which destroys the cell structure, denatured protein, and extracted oil from oilseeds. Oil extraction from MAE is dependent on the temperature, solvent, mass diffusivity, partition coefficient, dielectric properties, sample to solvent ratio, nature of the sample, solubility of sample into the solvent, and time. In an independent study, CSO was extracted from Pak variety of cottonseed by using MAE at 1:4 sample to solvent ratio with 14% moisture content after 3.57-min irradiation treatment which resulted in 32.6% oil yield in comparison to 34.7% yield obtained in Soxhlet extraction of CSO after 16 h; MAE method gives higher yield within a short interval of time and it enhances with the increase in moisture content (Nde and Anuanwen 2020; Taghvaei et al. 2014, 2015).

UAE is another high efficiency method of oil extraction with 20- to 100-MHz frequencies and short extraction time. Cavitation produced during the UAE leads to formation and collapse of bubbles which disrupt the cell wall structure and release the extracted compound from cell matrix to the solvent. Oil extraction increases with the increase in ultrasound power but when ultrasonic power exceeds 90 W, then it disrupts the cell wall and molecular structure of oilseed and decreases oil yield (Nde and Anuanwen 2020). Pliego-Arreaga et al. (2013) studied effect of Soxhlet, ME, and UAE process of CSO by measuring the extraction efficiency on the basis of effect of extraction time, temperature, and use of organic solvents and solvent seed to ratio and found higher 38.2% in UAE at 10:1 ratio, 45 °C after 1 h of extraction using Folch mixture, and 30% yield in UAE at 10:0.5 ratio, 45 °C after 40 min of extraction using chloroform:methanol (1:2). UAE extraction of CSO using different organic solvents is higher than the Soxhlet and ME method; optimization of process shows that efficiency of extraction process is a variable-dependent, particular variable, and group of variables.

ME is commonly used and oldest method of oil extraction in rural areas due to their low-cost initial investment and requirement of trained people to operate machines. Oilseeds are mechanically pressed either by screw press or hydraulic press techniques driven by a motor to release quality oil in volume of solvent used. ME is efficient process of oil extraction but results in low oil yield than other methods of oil extraction (Nde and Anuanwen 2020). Solvent extraction, hydraulic pressing, and screw pressing process are used in the mills for CSO extraction either by batch or continuous process. After extraction, crude CSO goes through the further processing, refining bleaching, and streaming to obtain refined grade CSO for their use in food. Refining process removes the impurities and dark color materials present in the oil, leaving with clear yellow oil, activated clay bleaching, and finally streaming to remove traces of odor under vacuum to obtain refined grade of CSO (Latif et al. 2007; Orhevba and Efomah 2012). CSO extraction is highly influenced by cottonseed varieties, particle size of cottonseed meal/cake, and types of extraction method used. Extraction of CSO is dependent on the interest and their applications in food products and other industries. The studies on the extraction of CSO are presented in Table 1.

Table 1 An overview of extraction method and conditions of cottonseed oil
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Physiochemical Characterization of Cottonseed Oil

The quality of oil obtained by different methods of oil extraction is most often measured by composition of fatty acid and physiochemical characteristics of oil which include value, iodine value, phosphorus content, moisture content, refractive index, specific gravity, and saponification value. CSO contains bioactive compounds such as antioxidants, including tocopherols, sterols, and flavonoids as well as yellow color pigment known as gossypol.

CSO has 2:1 PUFA to SFA ratio, and unique fatty acid composition of CSO makes them different from other vegetable oils. CSO contains 73% total fatty acid (TFA) content due to the presence of a distinctive group of 17% oleic acid (MUFA), 56% linoleic acid (PUFA), 23% of palmitic acid (SFA), 1% myristic acid (SFA), 2.33% stearic acid (SFA), 0.6% palmitoleic acid (SFA), 0.17% linolenic acid (SFA), and other little amount of SFA, which are lignoceric acid, arachidic acid, sterculic acid, behenic acid, malvalic acid, and palmitic acid. High content of palmitic acid in CSO enhances the oxidative stability in products like shortening, margarine, and confectionery products. SFA raises the low density cholesterol while stearic acid lowers the cholesterol level and increases the melting point of CSO and provides plasticity and solidity in margarine and shortening (Sharif et al. 2019). Malvalic acid and sterculic acid are unique types of FA present in the CSO and are also known as cyclopropenoid fatty acids due to the presence of one-double bond in the propene ring and have some properties of toxicity (Ghazani and Marangoni 2016; Shah 2017; Yang et al. 2019). Cooked, uncooked, and conventional extraction of CSO was significantly affected on the FA composition and physiochemical properties (Taghvaei et al. 2014). Most abundant sterol present in CSO is β-sitosterol followed by campesterol, stigmasterol, and D5-avenesterol. Phillips et al. (2002) reported 292 mg/100 g total sterol in CSO. Studies on the fatty acid profile of CSO from different CSO varieties and extraction process are shown in Table 2.

Table 2 Fatty acid profile of cottonseed oil
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CSO is studied for their proximate analysis and physiochemical properties to study quality of oil for their use as cooking oil and industrial applications. Gas chromatography (GC), high-performance liquid chromatography (HPLC), Fourier transform infrared (FTIR) spectroscopy, and ultraviolet–visible (UV) spectrometry methods are used in the evaluation of physiochemical properties of oil that include acid value which indicates the amount of fatty acid present in oil; saponification value is an index of average molecular mass of fatty acid; free fatty acid content indicates that oil is edible; iodine value gives the degree of unsaturation; peroxide value is used to measure rancidity reaction and an indicator of quality of oil during storage; refractive index used to indicate the purity and quality of oil, viscosity of oil depends on the nature of triglycerides present in oil; and low moisture content of oil makes them suitable as edible oil and increases storability for long duration. Physiochemical properties of oil vary with variety of oil, extraction methods, use of solvents and temperature, fatty acid content, and degree of unsaturation (Orhevba and Efomah 2012). Lankart-57, RH-112, F-20, K-25, and D-9 varieties of cottonseed were evaluated for their physiochemical characteristics and result observed in the range of 12.01–14.55% oil content, 17.3–38.8% free fatty acid content, 181.83–190.55 mg KOH/g saponification value, 93.90–105.76 g I2/100 g iodine value, 1–6 m Eq/Kg peroxide value, and 1.95–2.65-h induction period. Also, similar correlation of physiochemical properties of CSO varieties was found in the FTIR analysis (Shah 2017). Orhevba & Efomah extracted CSO by Soxhlet method and studied for proximate analysis and physiochemical characterization and obtained 15.05% reddish brown color CSO with mild taste, 5.75 mg KOH/g free fatty acid, 11.5 mg KOH/g acid value, 94.7 g I2/100 g iodine value, 189 mg KOH/g saponification value, 9.25 m Eq/kg peroxide value, 1.464 refractive index, 0.92 specific gravity, 4.82 pH, and 74 viscosity. Daho et al. (2012) studied vaporization of different vegetable oils; vaporization of CSO droplets was evaluated by fiber-suspended droplets vaporization techniques at 578–917 K under atmospheric pressure. Results concluded that vegetable oil vaporized at equal to or higher than 773 K temperature, and bubbles were observed in 40% CSO at 684 K at the end of the process. A physiochemical property of CSO shows it is suitable for their use as edible oil, storability for long duration, and industrial applications.

Total phenolic content stabilizes the vegetable oil against oil oxidation during storage and plays an important role as antioxidants against various diseases. Vitamin E terms used for bioactive compounds are tocotrienols and tocopherols; while tocopherols found in oil and fats are known as primary antioxidants. α, β, γ, and δ are four derivatives of tocopherol, in which α-tocopherol is biologically most active compound in nature. These compounds are most effective on the fat-soluble antioxidants; also protect PUFA from peroxidation of lipids, cell membrane against mutagenic nitrogen oxide species, and pyroxylated radicals. Higher tocopherol content decreases the risk of coronary heart and cardiovascular diseases (Matthäus and Özcan 2015; Zio et al. 2021). Crude CSO contains total tocopherol content about 1000 ppm with 41% α-tocopherol and 58% β-tocopherol content (Ghazani and Marangoni 2016). Matthäus and Özcan (2015) reported 1.1 mg/100 g plastochromanol-8, 36.2 mg/100 g α-tocopherol, 0.2 mg/100 g β-tocopherol, 48.7 mg/100 g γ-tocopherol, 0.3 mg/100 g δ-tocopherol, and tocotrienols content. Total phenolic content (TPC) of MAE extracted whole CSO was increased up to 70 ppm as gallic acid equivalents at high moisture content and irradiation time (Taghvaei et al. 2014). HPLC and DPPH methods were used in the evaluation of TPC, tocopherol content, and antioxidant activity of 16 samples of CSO, and results showed average 0.64 mg/100 g of gallic acid equivalents, 56.4 mg/100 g α-tocopherol content, and 0.27 mg trolox/100 g antioxidant activity (Zio et al. 2021). Wen et al. (2020) reported highest total tocopherol content (845–1118.1 mg/kg) in CSO than peanut, rapeseed, rice bran, and soybean oils; CSO has antioxidant activity due to the presence of high tocopherol content.

CSO is considered as edible oil due to its distinctive fatty acids profile, vitamins (E, A, and K), tocopherol, phosphorus, phytosterols, and neuroactive N-acylethanolamines content which maintains the quality and stability of oil during long storage ultimately enhances the nutritional importance (Sharif et al. 2019; Zia et al. 2022); and studies dealing with the physiochemical characteristics of CSO from different sources are discussed in Table 3.

Table 3 Physiochemical properties of cottonseed oil
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Safety Profile of Cottonseed Oil

CSO is used as edible oil but it limits due to harsh solvents used during extraction, gossypol, and cyclopropenoid acid content of CSO. CSO contains cyclopropenoid fatty acids which are sterculic acid, malvalic acid, and dihydrosterculic acid. Cyclopropenoid acids present in CSO are responsible to alter the lipid metabolism and also cause deleterious health effects in many animals such as discoloration and defects in eggs and egg production depression (Obert et al. 2007; Ghazani and Marangoni 2016; Yang et al. 2019). In some cases, CSO was extracted at higher temperature using harsh chemicals which alter the chemical nature of oil. Hexane is commonly used as solvent for oilseeds to extract oil but because of health risks, environmental and safety issues, the edible oil industry needs to replace hexane. The strict guidelines were issued for use of hexane as solvent in edible oil extraction from different sources by U.S. Environmental Protection agency (2001), and new incentives were highlighted to discover other methods for extraction to produce better quality of edible oils containing valuable nutrients, oleochemicals by continuous efforts to develop efficient process.

CSO naturally contains one toxic polyphenolic compound known as “gossypol”, and presence of gossypol in crude CSO gives them dark brown-red color and other contaminants. A high concentration of gossypol causes gossypol poisoning which includes anorexia, apathy, weakness, respiratory distress, impairment in body weight, and death. It also showed adverse effects on the male and female reproductive system (Gadelha et al. 2014). Gossypol is removed during refining and neutralization, and CSO is considered safe when gossypol levels are 1–5 ppm (specified limits of FDA) (Ghazani and Marangoni 2016). Different concentrations 0, 25, 50, 75, and 100% of commercial refined CSO and gossypol-free CSO used in replacement of soybean oil in broiler diet and found that 50% CSO was replaceable to soybean oil as it was decreased the total protein content, cholesterol level, albumin, and alkaline phosphatase in blood serum with increasing concentration of CSO. It also increased the antioxidant activity and fatty acid profile of breast muscle without affecting liver function and growth performance (Yang et al. 2019).

CSO acts as anti-inflammatory, anticancer, anti-allergic, and antioxidants; and cardio-protective properties help to reduce risk of various diseases (Sharif et al. 2019). Gossypol has anticancer activity against different types of cancers including breast, colon, ovary, and prostate cell lines; hence, gossypol-enriched CSO acts as anti-proliferative activity which causes toxicity towards proliferation of cancerous cells (Zia et al. 2022). Risk of hypercholesterolemia enhances with increasing the level of LDL cholesterol which causes cardiovascular diseases. Gossypol-enriched CSO (95 g/week) was used in a diet which results in decrease in the LDL and total cholesterol level in adults (Davis et al. 2012).

Applications

Food Applications

CSO with no gossypol and higher vitamin E content are used as edible oil in commercial and home cooking and formulation of many food products. Pigments and other impurities were removed by refining and bleaching during CSO extraction. About 56% of CSO is used for human consumption as cooking oil, other 36% used for frying and baking; remaining oil used in margarines and other uses (Zio et al 2021).

CSO is rich in unsaturated fatty acids than other vegetable oils, and it is suitable and safe for human consumption. CSO is an excellent example of deep fat frying oil due to the presence of high SFA content, omega-6, free from linolenic acid, and high tocopherol content which improves the shelf life of food products (List 2016). CSO is used to enhance natural flavor in many food products like oriental food, snacks, and frying seafood to maintain their taste, while the refined CSO has golden color with soft flavor (Latif et al. 2007; Orhevba and Efoma 2012). In addition, CSO also provides buttery or nutty taste, texture, mouthfeel, and storage stability to fried snack products. For example, CSO-fried potato chips have nutty taste (List 2016). CSO is rich in USFA content, non-oily consistency, light, high smoke point (232.2 °C), and more stable at higher temperature than other oil with no further processing or formation of trans-fatty acids. CSO is also rich in natural antioxidant, and these antioxidants make them as a natural preservative which increases the shelf life of food. CSO prevents the hardening of coronary arteries and considers as a “HEART OIL” according to list of “American Heart Association” products (Desrochers and Szurmak 2017).

Hydrogenation of oil refers to the adding pairs of hydrogen atoms to unsaturated compounds to produce large amount of monounsaturated and saturated groups. Hydrogenated oil contributes in the 75% of trans-fatty acids are major health concern, and CSO is good alternative for hydrogenation; while non-hydrogenated CSO acts as deep-frying medium and used to reduce trans fatty acid content in fried food. When CSO was partially hydrogenated, then it converts into MUFA and SFA content and increases the stability of CSO. SFA of CSO does not cause any health issues but production of trans-fatty acids during processing and hydrogenation of CSO reduces the high density cholesterol and raises low density cholesterol in blood serum which effectively causes health problems (Sharif et al. 2019). Canola oil and partially hydrogenated CSO were used in the production of margarine but it causes health issues. Combination of inter esterified canola oil (50%) and fully hydrogenated CSO (50%) used to produce healthy margarine with desirable triacylglycerol profile and physiochemical and sensory characteristics (Imran and Nadeem 2015). Blending of CSO with palm oil in different (0:1, 2:3, 3:2, 1:0) ratios at 170 °C for 10 h enhances the quality and stability of CSO during deep-frying of frozen French fries. SFA content was increased with the increase in concentration of palm oil which enhances the stability of CSO in comparison to pure oil (Arslan et al. 2017). Inter esterified CSO and palm oil blending used in the preparation of favorable cookies without any adverse effect on sensory properties and also found close to the standard cookies (Waheed et al. 2010).

Non-Food Applications

Alkyd resin (polyester) is used as raw material for the coating of household finishes and industrial applications like paint industry. After production of alkyd resin by polycondensation (polybasic acid + polyhydric alcohol) and it is modified by triglyceride vegetable/seed oils, CSO is used in the modifications of alkyd resin (Isaac and Ekpa 2013). Long and short alkyd resins were produced by 40 and 60% concentration of CSO using monoglyceride method to study their physiochemical properties such as free fatty acids, acid value, saponification value, iodine value, color, specific gravity for their use in modification of alkyd resin, and it was observed that triglycerides from CSO was successfully applicable in the preparation of biopolymers. This modified alkyd resin also evaluated for their use in the formulation of white gloss paint to study their adhesion, color, hardness, flexibility, durability, specific gravity, solid content properties with chemical resistance and drying time. Result shows short alkyd chains prepared by CSO possessed best drying time and hardness; also found highly resistant to water, sulfuric acid, and brine solution but lower resistance to alkali (Waheed et al. 2010; Isaac and Ekpa 2013).

Transesterification lowers the viscosity of oil; cottonseed oil methyl ester (CSOME) biodiesel derived from cottonseeds provides a better efficiency compared to commercial diesel and produces a much cleaner environment (Kittur et al. 2021). Aspergillus sp. was observed as chemical alternative in CSO biodiesel production as it was used as whole-cell biocatalyst to enhance the properties of long chain fatty acid alkyl (ethyl, propyl, and butyl) esters (Nain et al. 2020). Similarly, 27.9% yield of CSOME optimized for production of biodiesel by using Rizopus oryzae as whole-cell biocatalyst in CSO and glucose-supplemented basal medium (Athalye et al. 2013). CSOME extraction experiments were conducted at different input variables 5–9 methanol/oil molar ratio, 0.25–1.25 w/w concentration of catalyst, 42–72 °C temperature, and 500–900 rpm speed by using RSM and obtained 98.3% yield at optimum value at 6:1 methanol/oil molar ratio, 63.8 °C temperature, 0.97% (w/w) concentration of catalyst, and 797 rpm speed (Jamshaid et al. 2019). Diesel fuel and 5, 20, 50, 75, and 100% of CSOME with diesel fuel were tested at fully loaded air cooled, single cylinder and direct injection diesel engine, and variable speeds to investigate the engine torque, engine power, brake specific fuel consumption, and temperature of exhaust gases by performance tests and blend influences on NOx, SO2, CO, and smoke capacity by emission test of CSOME in diesel fuel. Result of these tests shows that engine torque decreases with the increase in CSOME in diesel due to the lower heat and high viscosity of CSOME. Brake-specific fuel consumption was observed lower in 20% CSOME than others while temperature of exhaust gases was found higher in diesel fuel. CO emission maintained by the presence of CSOME in diesel fuel, NOx decreases in all blends, and SO2 observed lower in biodiesel than diesel fuel. These investigation concluded that CSOME substituted in diesel fuel without any diesel engine modifications (Aydin and Bayindir 2010).

Maleinized CSO is used as plasticizer in films of poly(lactic acid) to increase their flexibility than commercially available maleinized vegetable oils (Carbonell-Verdu et al. 2017). After Soxhlet extraction of Nigerian CSO by using n-hexane as solvent which gives 48% yield of oil and studying the characterization of extracted CSO, it was concluded CSO with good saponification, iodine, and acid value could be utilized in the soap production. The soap produced by using CSO was white in color with good foam properties and slightly soluble in water (Warra et al. 2011). Various food and non-food applications of CSO are discussed in Table 4.

Table 4 Applications of cottonseed oil
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Conclusion and Future Perspectives

CSO is extracted conventionally by screw press and hydraulic extraction method as it is an efficient method of extraction without using any toxic solvent. These ME methods give higher quality of oil with natural color and flavor but require long time of interval for processing and refining of oil. SE is commonly used method of oil extraction at laboratory scale for many studies. Other non-conventional methods of oil extraction such as MAE, UAE, SPE, and EAE at optimum conditions also give better oil yield than conventional method within a short period of time. The oil quality, yield, and physiochemical properties are depending on the varieties, places, and season of cotton production. Physiochemical properties of CSO state that it has good fatty acid profile including acid value, iodine value, saponification value, peroxide value, specific gravity, and refractive index which increases their importance for their used as for human consumption but still due to the presence of polypropenoid content, gossypol content, and used of toxic chemicals during extraction of CSO limits their use as edible oil due to their adverse effect on human health. Hence, this review concluded that, extraction of CSO with less gossypol content and without used of toxic chemicals is an important task for researches. Many studies work on the extraction of CSO low or free gossypol content for their applications. Raw CSO, refined CSO, and blending of CSO with other oils have many applications in food processing as they enhance the stability, color, flavor, taste, and other sensory properties of food products without affecting the other properties. Other than these, CSO has antioxidant, anti-allergic, anticancer, anti-inflammatory, and cardio protective activity which has many applications in agricultural, pharmaceutical, cosmetics, paint, detergent, and other industrial applications. This review concluded that CSO with well develop extraction procedure, good composition of fatty acid profile, physiochemical characteristics, and low gossypol content is healthier and reduces the toxicity of oil.