et al. [103] investigated the cell surface characteristics, including its relation to the development of biosurfactants for diverse uses. This research group [103] cultivated six strains of Candida in fermentation medium supplemented with glucose, corn‐steep liquor, soybean refinery oil residues, peanut oil refinery residues, and n‐hexadecane. Their studies indicated that the biosurfactants produced with yeast could be used to extract hydrophobic substances with 90% capability to eliminate hydrophobic pollutants from soil.
3.6.2 Development of Biosurfactants Using Waste Frying Oil
Several crops are cultivated primarily for the production of foodstuffs for industries, which are then used in industrial processes that generate byproducts [76, 104, 105]. Go et al. [17] reported that increasing population and growing living standards have contributed to increased demand for edible oils, as they supply the necessary nutritional components and energy for physical activities. Edible vegetable oils are usually composed of triacylglycerols (more than 95%) and various fatty acids [106]. The waste cooking oil/frying oil contains several hazardous chemicals that cause health risks when consumers use it or process it. The composition of the used frying oil depends on the food fried in it and also the number of times it is reused; usually, though, recycled oil has 30% higher polar‐hydrocarbons compared to fresh frying oil [107]. Waste frying oil represents a renewable energy source for the production of new industrial products and alternative feedstock in place of pure and expensive chemicals. Haba et al. [108] studied sunflower oil and olive oil waste for rhamnolipid production by using P. aeruginosa 47T2 (2.7 g/l). They reported 0.34 g/g rhamnolipid production with NaNO3 (5 g/l) and waste frying oil (40 g/l).
Researchers [109] developed a modified approach of biosurfactant production under submerged culture conditions through Bacillus subtilis MTCC2423 strain using sunflower bran, paddy bran, waste frying oil (50 g/l), yeast extract (5 g/l) and rock salt (1.7 g/l). They observed that the surface tension decreased in glucose + sunflower bran, paddy bran, and frying oils waste by approximately 29.0, 32.0, and 34.5 mN/m, respectively, while glucose produced the best output (2.1 g/l) of surfactin. It was concluded from their results that the surfactin production process was safe for waste disposal and low‐cost biosurfactant development. Pan et al. [110] had also reported rhamnolipids production using P. aeruginosa strain DG30 in fermentation medium enriched with 5% w/v discarded vegetable oil (w/v) and reported 15.6 g/l of biosurfactant during the fermentation process.
Moreover, the coconut fried oil waste (2%) was used by George and Jayachandran [99] for the synthesis of rhamnolipids using P. aeruginosa strain D, with a recorded emulsification index (EI) of 71% and a yield of 3.55 g/l. It was reported that Mucor circinelloides, grown in culture media with 5% waste frying oil, produced ~12.4 g/l of glycolipids [111]. The obtained glycolipids lowered the surface tension by up to 26 mN/m, produced a consistent 129 mm diameter hollow area in the oil dispersal assay, and proved the emulsification capability of up to 65% of crude oil in marine water. All the results reported in this section provide scientific evidence that waste oil has been a good source of carbon to support microbial growth and production of biosurfactants. The waste substrates used for the production of biosurfactants themselves can reduce the cost of production and can be considered environmentally safe by reducing the pollution problem.
3.6.3 Fruit and Vegetable Industry Byproducts for Biosurfactant Processing
The commercial manufacturing units use vegetable and fruit items (cassava, apple, banana juice and peels, pineapple, mango, carrot, and lime) for the production of various consumer products, but also in this process enormous quantities of residual waste is produced that is rich in carbon content and can be used to produce biosurfactants [112, 113]. The production of cashew nut generates a massive amount of cashew apples as waste, with only 12% being used as a fruit or for commercial processing, whereas more than 70% of cashew apples remain as waste in the soil and cause pollution [114]. Rocha et al. [115] reported that the cashew apple is an invaluable raw material for varied practical applications due to its abundant carbohydrate, vitamin, and mineral content. They evaluated the A. calcoaceticus strain RAG‐1's ability to produce emulsions by utilizing cashew apple juice, which lowered the kerosene surface tension by ~17 and 59% of EI value. They also assessed the potential of ATCC‐10145 strain of P. aeruginosa in the nutrient media enriched with cashew apple juice as having 90–97 g/l of carbohydrate for rhamnolipid production. The maximum surface tension reduction was 29.5 mN/m, whereas the maximum rhamnolipid synthesis was 3.8 g/l, which was obtained by adding peptone (5 g/l) to cashew apple juice. During the research, they analyzed the surfactin synthesis using B. subtilis LAMI008 in nutrient media supplemented with 86.1 g/l carbon content with cashew apple juice.
Subsequently, a related study was conducted by Giro et al. [116] using B. subtilis LAMI005, where they reported that after 48 hours of fermentation, the highest amount of surfactin was 123 mg/l with clarified cashew apple juice. The production levels were, however, two times smaller than those of mineral media enriched with glucose 10 g/l and fructose 8.7 g/l. The CMC value of biosurfactant from cashew apple juice was 2.5 times lower than that of the CMC value of biosurfactant derived from glucose and fructose media, indicating an increase in efficiency of biosurfactant. These results suggest that cashew apple juice can be used as an appropriate substrate for the production of biosurfactants using B. subtilis LAMI005 and may be used as a rich source of carbon for large‐scale industrial production.
3.6.4 Starch‐Rich Byproduct from the Industry for Biosurfactant Production
High volumes of effluents, extremely rich in starch and cellulose, are generated during the commercial extraction of starch using various staple crops like maize, rice, cassava, wheat, and potato, which could be utilized as a growth medium to produce different products like surfactins [76, 117, 118]. Potato waste, for example, comprises 16–20% of starchy material, 2–2.5% of proteins, 1–1.8% of fibers, and 0.15% of fatty acids. It was earlier reported [119] that potato with surface skin contains elevated potassium, B complex vitamins and vitamin C, and minerals like P, Mg, and Fe.
Thompson et al. [120] examined potato waste as a possible source of carbon in shake flask culture to produce biosurfactants using B. subtilis ATCC‐21332. They evaluated different potato‐based fermentation media for biosurfactant production, which includes defined potato media, liquefied and solid potato waste media, synthetically made starchy medium by the addition of pure starch in mineral media. In a solid medium, the surface tension decreased from 71.3 to 28.3 mN/m, and CMC 100 mg/l was reported when only 60 g/l of potato substrate was used for microbial cultivation, without adding any other nutrient in the fermentation medium. They also examined the surfactin synthesis by using B. subtilis 21332 strain in a medium containing potato industrial effluent with 16.2 and 6.5 g/l of potato solid components. The potato effluent was diluted at 1 : 10 by adding minerals and corn steep liquor to the modified and unmodified media. Surfactin produced using small potato solids showed a better production of biosurfactants with a production concentration of 0.44 g/l than that of large potato solids. Thompson et al. [120] and Noah et al. [76] demonstrated the usage of corn steep liquor for surfactin production. Noah et al. [76] subsequently produced surfactin with a low‐solid potato effluent with the same microbial strain in batch‐mode operated chemostat and recorded ~0.8–0.9 g/l production after 52 h of fermentation. Another study conducted by Das and Mukherjee [121] documented the production of lipopeptides using B. subtilis DM03 and DM04 strain with 5 g potato peel waste under solid‐state fermentation and 2% w/v substratum in submerged fermentation. During fermentation, the production of lipopeptide by B. subtilis DM‐03 was reported with 80 and 67 mg/g in submerged and solid‐state fermentation, respectively.
Wang et al. [122] used B. subtilis B6–1 for fengycin and poly‐β‐glutamic acid(α‐PGA) production by incorporating 5 g/l of soy curd and 5 g/l of sweet potato residue in solid‐state fermentation. The quantity of lipopeptide was reached at the maximum level after 54 hours of incubation; however, the highest amount of γ‐PGA (3.63%) was achieved after 42 hours of incubation. The researchers also emphasized the potential use of these lipopeptides as a biocontrol agent and fertilizer synergists.