Application of ultrasound technology for the effective management of waste from fruit and vegetable

Graphical abstract


Introduction
Globally, the linear economic approach, which emphasizes responsible consumption and production with minimal waste accumulation, is predominant.Certain geographical regions experience higher food production rates due to favorable climatic conditions.However, these increased production rates also result in substantial waste materials.The vast amounts of waste are discarded in landfills, and only 12 % is recycled [1].A circular economic approach is based on consumption and minimum waste production through the recycling and reusing waste components in new products.In this way, waste becomes an important resource for sustainability.There are two major international challenges; the first one is to provide food in surplus as per increased population demands and to overcome food shortage (food security), and the second one is a huge accumulation of food waste materials that has an impact on carbon footprints of the environment (food waste management) [2].These two global issues trigger alterations in production systems to make them more sustainable and environmentally friendly.The circular economy is most appealing due to its major role in sustainable development, improved environment quality, and economic prosperity [3].Fruits and vegetables produce large amounts of waste, which includes stems, leaves, peels, and seeds [4].According to Suri et al., [5] more than 20 % of fruits and vegetables are wasted due to insufficient processing and management.For example, in kiwi production, the percentile is ~ 1 × 106 tons of global waste annually [6].In addition, growing interest in citrus fruit consumption increases waste production, which generates 50 % of the waste of fresh fruit [7].Fruits and vegetables by-products comprise a wide variety of bioactive compounds.As bioactive compounds are biomass's main component, their waste is interesting for the bio-economy model and has a major role in the pharmaceutical, cosmetics, and food sectors.Waste streams such as seeds and peels also contain many essential oils enriched with bioactive compounds.They can sometimes be used directly as value-added products via various techniques [8,9].
The valorization method offers a sustainable way to transform food waste into value-added products.However, improper treatment of this waste can lead to serious environmental issues, such as unpleasant odors and pollution.Valorizing by-products from fruits and vegetables is an effective strategy to not only reduce environmental waste but also to utilize valuable components from the waste, reintegrating them into the economy.This approach addresses both economic and environmental concerns.For example, using waste by-products not only conserves natural resources but also reduces landfill costs.Moreover, it fosters innovation in the development of functional foods and packaging materials [10].Food packaging is a tool utilized to protect food from environmental stress, such as light, heat, oxygen, and preservation of nutrients.
Moreover, consumer demand for natural ingredients in food products has increased due to health-related issues with synthetic products.The overall approach is significant when green technologies are applied to reduce hazardous compound production and decrease the severe environmental and health impact.Due to these reasons, the extraction, characterization, and purification of natural bioactive compounds have increased scientists' interest in using novel technologies to recover bioactive substances from food waste streams to make functional foods [11].In addition, industrial processing water has become a serious concern due to bioactive compounds disposed of in wastewater.It is necessary to recover these bioactive compounds due to their role in developing functional foods and to lessen bad environmental impacts.Various technologies such as pulsed electric field (PEF), high-pressure processing [12], microwave (MW), and US [13] have been utilized for this purpose.In the following sections, the US is explored as a green technology for recovering valuable components from fruit and vegetable waste and the wastewater of these industries.Besides, this review focuses on the latest fruit and vegetable waste applications in functional foods development and for edible packaging to protect food from external issues.

Ultrasound
The US is a green and environment-friendly technique with applications in various food industry processes.This technique is a great alternative to many heat and conventional treatments that can affect the quality of the product.Ultrasound-assisted extraction (UAE) is commonly employed in wave mode for short-term and pulse mode for long-term extraction.Its application in extraction is found to be effective and overcomes the hurdles of conventional techniques with improved yields of constituents [14,15].In this phenomenon, the implosion and cavitation cause rupture in cell walls and improve the mass transfer from solid to liquid state.This creates micro-channels within tissues, thus improving solvent penetration and enhancing mass transfer [16].A US probe system is preferred over a US bath for extraction purposes due to the more powerful intensity delivered through a small surface in the extraction process.Probes are operated at 20 kHz and connected to a transducer, directly delivering US waves to extraction media with less ultrasonic energy loss [17].Fig. 1 illustrates the UAE's setup for the food industry.
This is applied to extract bioactive compounds due to its adaptability, simple operation, less solvent requirement, and biological activity [18,19].The US has been applied to recover the polyphenols and other natural components from various fruits and vegetable by-products by utilizing hydroethanolic mixtures as solvents.For example, it showed more efficiency in extracting bioactive components from lemon waste using water as a solvent [20,21].The UAE has also attracted scientists for protein extraction [22,23].The fruits and vegetable processing industry generates a large amount of wastewater containing various organic compounds, pointing to serious environmental concerns [24].It is an effective method for removing toxic and hazardous organic components from wastewater.Its utilization in wastewater treatment helps remove toxic and resistant pollutants such as aromatics, surfactants, and dyes.This process involves the oxidative breakdown of resistant wastewater compounds [25].
US efficiency is linked to common factors like processing temperature, solvent nature, ultrasonic reactor type, frequency, and power [26].It has advantages over conventional heating treatments, like less energy requirement, short processing time, less solvent consumption, and enhanced yield [27].It has also proved effective for heat-sensitive compound extraction due to less temperature requirement [28].Due to these capabilities, the US' involvement in waste extraction has become a recent trend worldwide.

Recovery of biomolecules from food waste
Fruits and vegetable wastes contain valuable compounds such as vitamins, carotenoids, alkaloids, proteins, phenolics, dietary fiber, and polyphenols.Fruit waste processing contains cellulose (40-50 %), lignin (10-25 %), hemicellulose (30 %), and other polysaccharide [29,30].Due to these reasons, there is a great chance of recovering natural components from these residues within the frame of the circular economy.The recovery of valuable products from fruit and vegetable waste using US technology is described in Fig. 2.
Fruit industries' waste is rich in nutritious ingredients compared to chemical and canning industries, which account for a lower content of flavonoids [31].For instance, 60 % of the vitamin C is reported in acerola waste compared to fruit pulp [32].It produced 40 % of byproducts; seeds and peels contributed 30 % and 10 % of sludge during the juice clarification step that could extract vitamin C to fulfill the increased societal demand [33].Besides, citrus industries produce approximately 50-60 % of waste and over 60 million tons globally [34].For instance, grapefruit peel contributes 25-30 % to the whole grapefruit, while lemon peels give 12-13 % to the whole species.On the other hand, pomelo peel contributes 36-40 % of the fruit [35].Moreover, the citrus peel is rich in pectin, limonene, and molasses [36].Table 1 discusses the important studies of UAE of phytochemicals from fruit waste and byproducts.
Vegetable industrial processing such as carrots, tomatoes, and potatoes generate vast amounts of waste with beneficial bioactive compounds.For example, carrot industry waste is a rich source of pectin, αand β-carotenes, lutein, and tocopherols associated with high antioxidant capacity [91].Waste's main phenolic compounds are flavonoids such as quercetin and kaempferol glycosides [92].Table 2 shows various studies of the valorization of vegetable waste in the US.

Polysaccharides
Polysaccharides contain mainly cellulose, hemicellulose, and lignin; these components are made when three or more sugars are linked together in long chains and thus have complex structures [119].The complicated structure with high lignocellulosic content makes further production complex and less effective.However, the US application transforms the complex polysaccharides into simple monomeric sugars and is then utilized for energy production [120].In addition, it is a promising approach for lignin barrier disarrangement, hemicellulose, and cellulose crystal reduction.The removal of lignin and hemicellulose is related to the bond oxidation and amorphous nature [121], improving cellulose content [122,123].Polysaccharides are available in different plant tissues with the potential for anti-oxidation, anti-cancerous, and hypoglycemic levels.Their health benefits and less toxic nature make them beneficial ingredients for functional food development.Different plant parts not consumed by humans are subjected to the US for their extraction [124].
The Rambutan (Nephelium lappaceum L.) fruit peel was subjected to US for crude polysaccharides extraction.The outcomes demonstrated that US at the following conditions: power (110 W), temperature (53 • C), and processing time (41 min) resulted in the experimental yield of polysaccharides 8.29 % [125].The papaya seed was subjected to alkaline and UAE alkaline extraction for soluble dietary fiber extraction, and the structure and compositions of the extracts obtained were compared.The US-treated samples at optimum conditions NaOH (0.6 %-3.0 %), temperature (30-80 • C), processing time (10-60 min), and power (125-250 W) resulted in the highest yield of dietary fibers (36.99 %).In addition, the US was combined with an alkaline treatment that showed fewer total amino acids but higher essential amino acids (16.18 %) than the simple alkaline treatment.The primary sugars were neutral in both treatments.The results indicated that papaya peel is a rich source of natural dietary fibers [70].In addition another research, the US was used to extract polysaccharides from guava leaves by optimizing Box-Behnken design.The total yield of polysaccharides was 1.00 ± 0.04% at extraction time of 20 min, temperature of 62 ℃ and ultrasound power of 404 W. While the DPPH and ABTS⁺ radical scavenging rate of were 56.38% and 51.73%, respectively.The results indicated that guava leaves are rich source of polysaccharides [126].
Anwar et al. [108] compared the US with conventional and PEFassisted extraction techniques to investigate the effects on yield and properties of water-soluble non-starch polysaccharides of taro peel.The finding showed that the US gave a higher yield (3.65 g/100 g) as compared to PEF-assisted extraction (2.25 g/100 g) and conventional extraction (2.10 g/100 g).The extracted starch sample after UAE treatment showed fewer impurities and lighter color, so it could be concluded that the US exhibits more advantages in extraction compared to other non-thermal and conventional methods [108] The results revealed that triple-frequency US was more efficient in achieving higher polysaccharide yield (10.50 ± 0.20 %) than dual-frequency (9.74 ± 0.30 %).This higher yield was attributed to the higher collapsing of cavitation bubbles.In dual frequency, neutral sugars in samples were 52.14 ± 1.61 % while 58.81 ± 2.09 %, with triple frequency treatment.The uronic acid and protein content for dual frequency were 3.93 ± 0.20 % and 6.02 ± 0.24 %, while for triple frequency, it was 4.82 ± 0.08 % and 6.78 ± 0.11 %, respectively.Overall, these outcomes implied that the US, with triple frequency, improved the extraction of chemical moieties [101].The US and alkaline  solution were used for wampee seed protein extraction.The outcomes demonstrated that the optimum yield was 15.06 % at the LS ratio 1:29 g/ mL, 64 min, and 12 pH [127].Therefore, the final remarks stated that the US is a promising approach for polysaccharides extraction from fruit and vegetable waste at the industrial level.

Pectin
The presence of pectin in various plants' cell walls and middle lamella, including fruits and vegetables, has gained attention to exploring peel waste, pomace, and rind for the UAE.The fig skin was subjected to ultrasound-microwave (US-MW) assisted extraction at various conditions (10-30 min, 300-600 W, irradiation time 5-15 min, and LS ratio 10-30 mL/g) for extraction and obtained the highest pectin yield (13.97 %) [72].Many studies have proven the UAE's efficiency in achieving a high yield of bioactive compounds from peel waste [95].Various researchers evaluated the pectin extraction from different sources, but the citrus peel is a rich source of pectin compared to other fruit peels [128,129].Rhamnogalacturonan-I-enriched pectin was extracted from citrus peel and confirmed as having good thickening attributes compared to commercial pectin.In addition, it is reported as a suitable ingredient in functional food products due to its role in preventing cancer and heart diseases by rhamnogalacturonan-I-enriched pectin [130].The US was applied for pectin extraction from sour orange peel.At the power 150 W, pH 1.5, and time of 10 min, optimum yield was obtained as 28.07 ± 0.67 %.While ash (1.89 ± 0.51), moisture (8.81 ± 0.68), and protein (1.45 ± 0.23 %) were improved, respectively.The maximum total phenolic content (TPC) (39.95 ± 3.13 mg gallic acid equivalents/g pectin), water holding (3.10 ± 0.12), oil holding capacity (1.32 ± 0.21 g), and degree of esterification (6.77 ± 0.43 %) were observed [131].Apple pomace was subjected to the US at amplitude (20 %, 60 %, and 100 %), pH (1.5, 2, and 2.5), and time (10, 20, 30 min) for pectin extraction.The US amplitude greatly impacted the yield and degree of pectin esterification, while pH greatly influenced the yield of galacturonic acid and the degree of esterification.The optimum conditions were amplitude of 100 %, 1.8 pH, and 30 min treatment time, which resulted in a pectin yield of 9.183 %, galacturonic acid content of 98.127 g/100 g, and degree of esterification 83.202 % [74].
The UAE was applied at high intensity on mango peel for pectin extraction.The process was carried out at frequencies 37 kHz and 80 kHz, times 20 min, 25 min, and 30 min, and the ripening stages of the fruit were 0, 2, and 4. The findings showed that pectin yield ranged from 13 % to 30 % without time duration influence.The highest yield was obtained at the lowest frequency (37 kHz) and lowest maturity.Moreover, the lowest frequency (37 kHz) gave high gel strength, purity, and quality.The results demonstrated that the UAE application efficiently extracts pectin from mango waste [132].Banana peel pectin was extracted by UAE application at optimum conditions temperature 33.12   ).Moreover, it showed higher pectin extraction (10.7 %) and a low degree of esterification (68.5 %), but they exhibited lower thermal stability.The outcomes revealed that US steam explosion acid treatment was more effective and provided higher pectin yield [80]. Kazemi et al. [100] applied UAE for pectin extraction from eggplant byproducts.The findings showed a yield of 33.64 % at 50 W, but an increase in power from 50 W to 150 W negatively affected the pectin extraction as it degraded them.So, the US at optimum parameters affects the extraction yield positively.The US was applied at the following conditions: 550 W, 37 kHz, 70 • C /30 min on black carrot pomace for pectin extraction.The results showed the highest yield of pectin (0.08 kg/pomace), anthocyanins (297.9 mg/L), phenolics (1285.3mg/L), and antioxidant activity (37.6 μM/mL).The overall findings indicated the presence of functional groups in pectin, so it can be concluded that US processing causes rapid extraction without adversely affecting its functionality [115].Pectin and polyphenols were recovered from tomato peel waste by application of high hydrostatic pressure extraction (HHPE) and UAE.The HHPE increased by 15 % pectin recovery after 45 min time duration compared to traditional extraction for 180 min.Depectinized residues were treated with US for 15 min with 70 % ethanol, which gave twice lower TPC (1625.7 mg/100 g) than pectin samples (3643.9mg/100 g) [113].In conclusion, UAE is applied for pectin recovery from fruit and vegetable waste.

Essential oils
The UAE has the potential to enhance the recovery and yield of oils by adjusting processing parameters, including time, power, temperature, and frequency.Moreover, it can replace the harmful solvent utilized in chemical treatment [124].It was applied to papaya seeds to evaluate the yield, antioxidant activity, and oil stability.The basic goal of this research was to obtain optimum US conditions for maximum yield of papaya seed oil with high stability and antioxidant activity.The US treatment resulted in 73 % oil recovery from papaya seed.The highest antioxidant capacity was obtained at high temperatures.The overall outcomes suggested optimum conditions: temperature 62.5 • C, power 700 W, processing time 38.5 min with solvent to sample ratio (~7:1 v/w) [134].The watermelon seeds were subjected to UAE for crude oil extraction at 25-75 %, 45-55 • C, and 20-40 min, respectively.The optimum conditions were power 65 %, temperature 52 • C, and processing time 36 min, resulting in the highest crude oil yield (108.62 mg-extract/g-DM).The US-extracted oil yield was lower than that of the Soxhlet method.However, antioxidant activity (35.84 %) and TPC (23.26 mg GAE/g extract) were higher as compared to the Soxhlet method, in which antioxidant activity and TPC were 28.70 %, 11.34 mg GAE/g extract [135].
Mango kernel oil was extracted with MW and in combination with US.The combined treatment gave better oil yields (96.67 ± 1.30) than MW alone (88.42 ± 1.36 %).The US-MW treatment showed a maximum increase in the recovery of mango kernel oil [136].The US pretreatment was applied to citrus waste for essential oil extraction.The outcomes showed that maximum yield (33 %) was obtained at the following conditions: amplitude (52.7 %), processing time (15.7 min), and LS ratio (3.2/1).The increased yield in the case of the US is associated with mass transfer through cell wall breakage and cavities formation improved oil recovery [137].The cranberry seeds were treated in the US for oil extraction.The objective of that research was to determine optimum conditions for oil extraction.The outcomes showed that optimal conditions were amplitude (95 %) and extraction time (11.38 min), which resulted in maximum extraction yields (22.55 ± 0.36 %) [138].Hence, it has been proved that the UAE is an effective technology for recovering and enhancing essential oil from fruits and vegetable seeds.

Polyphenols, flavonoids, and anthocyanins
It has been reported that lime peel waste was subjected to MW and UAE for phenolic compound extraction.The outcomes suggested the optimum condition for MW (ethanol 55 %, power 140 W, times 45 s with eight repetitions) while for US (ethanol 55 %, amplitude 38 %, time min).The results showed that the US was effective, resulted in maximum TPC (54.4 mg GAE/g), and saved 33 % time compared to MW extraction [19].Mango peels were subjected to natural deep eutectic solvents (NADESs) and the US for antioxidant recovery.The optimum conditions were water content 20 %, duty cycle 50 %, density 2 W/cm 3 , LS ratio 30:1, particle size 0.3 mm, 30 min time, and lactic acid glucose 5:1.The NADES gave maximum TPC (69.85 mg GAE/g of MP), TFC (16.5 mg QE/ g of MP), and DDPH (35.37 μg/mL).However, the US, in combination with NADES, provided 1.4 times higher polyphenols, 1.7 times more total flavonoids, and 1.9 times more antioxidant activity.Moreover, % less time was observed, and a 25 % reduction in solvent consumption was observed compared to the batch method with 80 % ethanol [139].The US technology was applied to jabuticaba peels to obtain high-value co-products.The US at 3.7 W/cm 2 and 50 g water/100 g resulted in the best recovery of bioactive compounds.The maximum values detected for bioactive compounds were polyphenol (3391 mg GAE/L), anthocyanin (287 mg/L), flavonoids (667 mg CE/L), and tannins (11265 mg CE/ L).Compared to other conventional techniques, the US approach is promising for phenolic compound recovery at low costs and with no hazards or environmental effects [90].Jujube peel was subjected to US for flavonoid extraction at 200 W, 50 min, K2HPO4 35 % (ethanol %), and LS ratio 30:1 g/mL.Rutin, quercetin 3-β-D-glucoside, and kaempferol-3-O-rutinoside were extracted with an overall yield of 95.55 %.Hence, the US application increased the flavonoid release [82].Pomegranate peel phenolic compounds at 15.12 min, amplitude 30 %, and gave a maximum yield of 42.45 %, while others were recorded as TPC 354.67 mg GAE/g, total ellagitannin 348.0 mg TAE/g, and antioxidant activity 94.78 %.They were enhanced due to cavitation, cell The results showed the bioactive compounds of purple turnip peel, which could be used in various food products [140].Kinnow pomace, which is profuse with polyphenols, antioxidants, and dietary fiber, is discarded by processing industries.These compounds can be obtained from waste and have applications in food products as functional components.The US at the 40 % amplitude, 38:1 LS ratio, 40 • C temperature, and 12 min increased polyphenols (7.89 ± 0.09 mg GAE/g pomace), DPPH (68.63 ± 0.79 %), and FRAP (31.17 ± 0.23 mM Fe 2 + /100 g) was observed.The waste after polyphenol extraction also showed TDF (33.18-45.12%), which included insoluble dietary fiber (68-72 %).This study can be scaled up for highly functional compound extraction from kinnow pomace [41].Tahiti lime pomace was subjected to the US for phenolic extraction.The US was applied at 160 to 792 W power, 2 to 10 min, and ethanol was used as the solvent.These parameters' effects were investigated on yields, hesperidin, polyphenols, narirutin yields, and antioxidant capacity.The optimum conditions were 480 W and 6 min, which resulted in 5 % extraction yield, polyphenols (7 mg GAE/g BS), hesperidin, narirutin (0.85 mg/g DP, 0.15 mg/g DP), and antioxidant capacities (50 mg TE/g DP).This study suggested that the US could have effective applications in pomace utilization [89].Da Rocha & Noreña [141] applied US at 250, 350, and 450 W for 5, 10, and 15 min on grape pomace for bioactive compounds extraction, and results reported 45 % anthocyanin on 10 min exposure time.Phenolic and anthocyanin were extracted from jabuticaba peels by US bath at 25 and 40 kHz.The outcomes showed maximum extraction at 25 kHz with 10 min sample exposure [142].The UAE was applied for polyphenolic extraction from grape pomace.It was subjected to the sample at the following conditions: temperature (20-60 • C), amplitude (20-60 %), LS ratio (8-24 mL/g), and ethanol (0-100 %) was used as a solvent.The maximum extraction (48.76 mg GAE/g dry pomace) was achieved at 56 • C, 8 mL/g LS ratio, and 34 % amplitude within 20 min.The outcomes suggested that UAE applications resulted in purified and higher yields of valuable bioactive compounds [51].Andrade et al. [47] determined the optimum pressure, intensity, solvent concentration, and solvent flow rate conditions in UAE for anthocyanin extraction from black chokeberry (Aronia melanocarpa) pomace.Citric acid was used as an extraction solvent.The best conditions were a temperature of 70 C, 180 bar pressure, 1.5 % wt citric acid, and 200 W power in a US bath.These conditions resulted in 88 % weight extraction of anthocyanin in 45-45 min.It is also stated that the US is more efficient at the lowest temperatures.The effect of temperature was investigated on anthocyanin yield at optimum conditions.In addition, compound stability was evaluated to prove that the system could operate at 80 C. The outcomes reported a 19 % increase in anthocyanin yield.The apple pomace with ethanol (50:50, v/v) in 10:1 LS ratio was subjected to an ultrasonic bath at 45 C for 45 min for bioactive compounds recovery.Fresh pomace yield increased twice, from 7.12 % to 13.61 %, while UAE showed a further yield increase (from 21.64 to 58.09 %) in the case of freeze-dried pomace.The TPC for fresh pomace was under 0.72 mg GAE (gallic acid equivalents)/g, while the freeze-dried sample resulted in up to 10.05 mg GAE/g dry weight (DW) [75].
Moreover, food industrial waste peels (~55 %), seeds (<5%), peduncles (<5%), and the remains of the pulp (~35 %) of oranges, bananas, pears, and apples in variable proportions were pretreated with, air oven and US.The optimized extraction conditions for dried food waste were amplitude 69.7 %, temperature 53.43 C, and time 12 min.The outcomes showed the highest yield (52.6 %), protein content (0.42 mg/g), TPC (116.42 mg GAE/g), and antioxidant activity (44.95 mg Trolox/g) [143].Peach juice waste, either in a frozen or air-dried state, was evaluated by the UAE.The optimum conditions for frozen waste were UAE at 23 % amplitude and 120 s time.They resulted in lower polyphenols, flavonoids, anthocyanin contents, and antioxidant activity (309-317 mg GAE, 94-120 mg QE, 8-9 mg CGE, 2.1-2.2mg TE, respectively) in the frozen waste.While polyphenols in dried waste were high (630-670 mg GAE), the flavonoids showed reduction (75-90 mg QE), while anthocyanin and vitamin C were non-significant in dried waste.This research suggested not only the extraction of bioactive compounds from peach waste but also the economic and environmental role of the US [71].Guava leaf powder was subjected to normal and pulsed US for bioactive compound extraction.The findings revealed that normal US exhibited TPC (15.5-68.8mg GAE/g), antioxidant activity (50.1-80.3%), vitamin C (5.7-25.7 mg/100 g) and TFC (44.7-289.77mg QE/g) with 5-20 min time and 40-70 • C temperature, while in pulsed US following conditions temperature (62.19 • C), extraction time (14.94 min) resulted maximum TPC (72.62 mg GAE/g), TFC (288.13 mg QE/g) and antioxidant activity (86.07 %).The outcomes suggested that the pulsed mode of the US was more efficient than the normal US [85].Tomato, watermelon, and apple wastes were subjected to cascade extraction based on UAE.During the first extraction step, protein and antioxidants were achieved at NaOH 3 wt%, 98.6 W, 100 % amplitude, 6.48 W/cm 2 , and 6 min.The comparison revealed that watermelon peel had higher protein (857 ± 1 mg BSA/g extract) and TPC (107.2 ± 0.2 mg GAE/100 g DW) while the highest antioxidant activity was for apple peel for ABTS (1559 ± 20 μmol TE/100 g DW), FRAP (1767 ± 5 μmol TE/100 g DW), and DDPH (902 ± 16 mol TE/100 g DW).After the first extraction, the remaining residues were subjected to cutin extraction at the following conditions (ethanol 40 wt%, 58 W, 100 % amplitude, 2 W/ cm 2 , 17 min, 1/80 g/mL, pH 2.5).Watermelon showed the highest cutin level (55 %), while tomato and apple had 25 and 40 %, respectively [54].The UAE was applied for 5-, 10-, and 15-min extraction time, amplitude 40, 70, and 100 % with pulse cycle 0.4, 0.7, and 1 s for polyphenolic compounds extraction from seed, pulp, and columella of soursop fruit.The optimal conditions for maximum polyphenol extraction were dependent on the type of raw material, such as peel resulted in 187.32 mg/g DM, columella 164.14 mg/g DM, pulp 33.24 mg/g DM, and seed 36.15 mg/g DM.The variation resulted in contents due to the matrix complexity of the matrix [144].Peel and columella showed a higher yield of polyphenolic compounds (32-37 %) as compared to conventional processing for 2 h (14-16 %) [50].US extracted Anthocyanin and antioxidants from sweet cherries' skin at the following conditions (ethanol 70 %, 40 kHz, 100 W, 40 C, and 30 min).The findings showed that US improved anthocyanin (14.48 ± 1.17 mg cyanidin 3-glucoside/100 g DW) and antioxidant activity (85.37 ± 1.18 B.G. Nabi et al. μMTrolox/100 g DW) [79].Culinary banana dietary fiber was extracted by alkaline and UAE in combination with alkaline extraction.The optimum conditions were 25 mL/g solute, 49.48 % amplitude, temperature 80 • C, and time 10 min.The highest yield and TDF were 71.05 % and 83.38 g/100 g, obtained by the US and alkaline treatment combination.Moreover, regular honeycomb structure, maximum crystallinity (25.86 %), fine particle size, and thermal stability were observed in combined treatment.The overall finding suggested that the US, in combination with alkaline, was promising for extraction [77].Date waste is high in moisture and organic compounds, which lead directly to environmental pollution.On the other hand, the dates processing industry is a rich source of sugars, fibers, proteins, and vitamins, which could be used in various bioprocesses [145].Bioactive compounds were extracted from date seeds by NADESs in combination with UAE.The optimum extraction conditions were US amplitude of 90 %, extraction time of 25 min, NADES content of 70 %, and solid-to-liquid ratio of 1:30 g/mL.TPC and DDPH at optimum conditions were 145.54 ± 1.54 (mg GAE/g powder) and 719.19 ± 2.09 (mmol TE/g powder), respectively.This research demonstrated that UAE, in combination with NADESs, extracted higher amounts of phenolic components, which could be applied in the food, pharmaceutical, and cosmetic industries [46].The UAE was carried out to assess the phenolic compounds of artichoke by-products.The optimum conditions were temperature: 60 • C, time: 60 min, solvent: 50 % ethanol: water.The outcomes showed TPC (22.4 ± 0.2 mg GAE g -1 dw), which mainly included dicaffeoylquinic acid (32.8 ± 0.6 mg CAE g -1 dw) and chlorogenic acid (14.1 ± 0.2 mg CAE g -1 dw) [104].Red beetroot is famous for higher betalains, red pigment, polyphenols, fiber, and nitrate.Beetroot juice is high in demand, leaving much waste behind.Betalains and polyphenols were recovered from whole dried beetroot juice waste.The US was applied at the following conditions: 44 KHz, 30 min, 30 • C, 35 W. Ethanol, and water mixture were proven more effective than single solvents.The UAE was effective in the recovery of betalains and polyphenol compounds from dried pulp.The total betalains and polyphenols were assessed for antioxidant capacities during four weeks of storage.Betalains showed degradation at room temperature compared to − 20 • C, while polyphenols were less affected by temperature.The total betalains extractability with 30 v/v ethanol was 7.7 %, and polyphenols recovery was 6.86 ± 0.23 mg/g at 30 % v/v ethanol.The overall finding concluded that dried pulp waste from the beetroot juice industry demonstrated good betalains and polyphenols content [107].Dietary fiber and phytochemicals were extracted from bottle gourd seeds by alkaline, enzymatic, and US-assisted alkali extraction.The US at the following optimized conditions LS; 1:23.90 mg/mL, time; 27.20 min and amplitude; 47.76 % resulted in 75.81 % dietary fiber.The US-assisted alkaline extraction at amplitude 70 %, temperature 45 • C, and 15 min time duration gave a higher phytochemicals yield of 8.24 %.While TPC (55.96 ± 0.75 mg/g), TFC (14.36 ± 0.45 mg/g), and antioxidant activity (35.08 ± 0.15 %) were improved as well [93].In conclusion, the US is a suitable green technique for extracting bioactive compounds from fruits and vegetable waste for food, nutraceuticals, and medical field applications.

Lycopene, carotenoids, and related antioxidants
The US was employed on orange peel waste for bioactive compound extraction.The findings suggested that the US at optimum conditions (400 W, 30 min, 50 % ethanol in water) resulted in total carotenoids (TCC) (0.63 mg ß-carotene/100 g), vitamin C (53.78 mg ascorbic acid/ 100 g), and polyphenols (105.96mg GAE/100 g).The major TPC in all orange peel samples was hesperidin (113.03 ± 0.08 mg/100 g) [44].Wang and coworkers worked on mandarin orange peel and extracted phytochemicals such as tangerine and nobiletin by UAE at 85 % LS ratio 20:1 (mL/g), 40 min, 50 • C, powder mesh size 100, and 150 W. The outcomes showed that UAE increased the yield 1.5 times compared to simple solvent extraction [38].The US was applied on the citrus peel for extraction efficiency.The UAE decreased extraction time to 20 min compared to the conventional process of 185 min.In UAE, the maximum d-limonene yield (32.9 mg/g, 97 %) was obtained by using hexane as a solvent at the following conditions: biomass: solvent (1:10), agitation speed (300 rpm), temperature (60 • C), ultrasonic power (80 W), 50 % duty cycle and frequency 25 kHz.This could suggest that UAE is an efficient and economical method for d-limonene extraction from fresh sweet lime peel [65].Orange peel β-carotene pigment, in combination with enzyme and ethanol as solvent, was extracted.The optimum conditions were a pectinase concentration of 0.40 % (w/w), time 115.55 min, and pH 5.11.At the following processing conditions, the maximum content of β-carotene (209.14 ± 0.40 ppm), antioxidant activity (91.23 ± 0.40 %), and color parameters (L* 15.22 ± 0.03, a* 3.76 ± 0.21, b* 8.31 ± 0.01) were obtained.The results showed that US processing is effective for pigment extraction [66].Orange peel carotenoids with olive oil as a solvent were extracted.The outcomes showed that carotenoids achieved values of 1.85 and 1.83 mg/100 g DW at optimum conditions of 35 min, 42 • C, and an LS ratio of 15 mL/g [42].They were subjected to US for antioxidant extraction at 30 min, 60 C, and an LS ratio of 15 mL/g.The results showed that gallic acid had the highest value (157.08 mg/100 g DW), whereas the lowest value was found for ferulic acid (20.91 mg/100g DW), and total antioxidant capacity was 2.79 g GAE/ 100g DW [43].The US was applied to determine the bioaccessibility of carotenoids in an in vitro digestion model at the amplitude of 30 %, power 400 W, and frequency 24 kHz from mango peel and pulp.The UAE peel bioactive compound percentage was improved by β-cryptoxanthin (46.93 %), lutein (35.21 %), and β carotene (32.62 %), while in pulp β cryptoxanthin, β carotene, and lutein improved by 44.16, 44.01 and 46.04 % as compared to control-paste.In control-peel, non-bioaccessible β-carotene content was higher (79.48 %) than in UAE samples.The US improved bioaccessibility in vitro digestion in mango byproducts [83].Peach palm peel is a rich source of carotenoids.HPLC-DAD investigated the impact of different extraction methods on the carotenoid profile.Extraction was done by maceration (acetone & ethanol), shaker, magnetic stirring, and US with ethanol.The outcomes demonstrated that UAE gave maximum TCC (67 mg/100g), whereas maceration resulted in acetone and ethanol (63 and 52 mg/100g).The last extraction was shown in magnetic stirring and shake extractions (44 mg/100g).HPLC-DAD data revealed that the carotenoid profile remained unchanged regardless of the extraction method.However, the z isomer β-carotene in shaking was 18 %, US 17 %, and magnetic stirring 15 %, while maceration was 7 and 8 % with acetone and ethanol, respectively.This study suggested that the US, as a safe technology, could be used for carotenoid extraction [86].The US and MW were applied on pumpkin peel for carotenoid extraction using corn and olive oil as solvents.The TCC ranged between 26.91 ± 1.96 and 34.35 ± 0.94 mg/100 g, higher than the conventional technique (22.24 ± 1.03 mg/ 100 g).This yield increase by applying these technologies is either by processing temperature, US, MW penetration depth, or carotenoids' solubility in edible oils.The UAE with olive oil is more effective for major carotenoid extraction (lycopene 87.0; β-carotene 18.7 mg/L).The DDPH inhibition percentage in US and MW-treated samples was 92.49 ± 1.84 and 94.16 ± 1.06 %, respectively, while in the conventional method was 58.57± 2.34 % [146].
The US at 42 kHz, 240 W, 60 • C, 60 min, and LS ratio 0.0004 g/mL was applied on mandarin epicarp for carotenoid extraction.These treatment conditions yielded the highest TCC (140.70 ± 2.66 mg β-carotene/100 g of DW).Moreover, variable variations such as a decrease in LS ratio and an enhancement in temperature caused the increase in TCC extraction [78].This increase in extraction may be attributed to solvent diffusion in the matrix and an increase in carotenoid transfer to the environment [147].The US was applied on tomato waste to recover lycopene content at 50 Hz, 45 min.Lab-prepared tomato waste has higher lycopene content (57.87 ± 5.30 μg/g fresh wt.) as compared to industrial (27.11 ± 0.83 μg/g fresh wt.).(138.82 ± 6.64 μg/g fresh wt).Freeze drying and US improved lycopene content by 2.8 and 0.68 folds, and their combined treatment enhanced industrial tomato waste yield by 4.12 folds.Freeze drying ruptures the cell structure, facilitates US processing, and enhances its extraction ability [114].The US was applied to hot pepper pulp for carotenoid and capsaicinoid extraction.The optimum conditions were amplitude 60 % at 60 • C for 5 min.The US-treated sample outcomes were compared with the conventional method and eight h maceration.The UAE showed 235.46 mg β-carotene/100 g and 1148 μg/g capsaicin content.It was a more effective treatment due to higher extraction and short duration [148].
The US was applied for carotenoid extraction from carrot pomace.The optimum conditions for the US were extraction time ( μg/g carotenoid content was obtained at these processing conditions. The findings concluded that the US is an effective technology for carotenoid recovery from cantaloupe waste [102].The US was assessed for ethanolic antioxidant extract extraction from lettuce waste at 400 power, 24 kHz frequency, and 120 s treatment duration.It gave 81 μg/ mL polyphenol yield and 101 μg TE/mL antioxidant capacity, significantly higher than SLE at 50 • C and 15 min [111].Defatted bitter seeds were subjected to pulsed US for protein extraction.The US at the following conditions was applied on the following power (300, 375, and 450 W) and time (2.50, 5.00, 7.50, 10.00, 12.50, 15.00, 17.50, and 20.0 min) to investigate the effects on protein yield and quality characteristics.The yields were higher at 375 W (31.05 %), followed by 450 W (28.93 %) and 300 W (23.79 %).With the increase in power level, yields decreased gradually due to protein aggregation.The water holding capacity (2.76 g/g), emulsion capacity (68.66 %), and emulsion stability (36.40 %) were higher than in conventional extraction [103].All the above studies proved that the UAE is an effective approach to recovering the bioactive compounds from fruit and vegetable waste.

Upcycling of waste for food product development
Food processing industry waste is a rich source of bioactive compounds.Due to the unique flavor, color, and nutritional value of fruits are incorporated into different items such as confectionery and baking, leading to effective waste utilization [149].Factors such as fruit variety, ripening stage, and post-harvest handling conditions could influence the by-products' nutritional content.In addition, functional ingredient preparation greatly impacted the quality and quantity of functional ingredients.All of these features should be undertaken to maximize the nutraceutical potential of enriched products [150].Table 3 describes the valorization of fruit and vegetable waste in functional food development.
The UAE and maceration used sunflower oil to extract β-carotene from pumpkins to prepare β-carotene-rich sunflower oil-fortified Mayonnaise.At 15/100 g/mL LS ratio, the different values of β-Carotene contents were detected.In the case of UAE (127.93 mg/100 g DM) and microemulsion (149.71mg/100 g DM) with lecithin 0.098 % and polyglycerol 1.902 %, better results appeared than maceration + sunflower oil (99.83 mg/100 g DM) and maceration + n-hexane (125.75 mg/100 g DM).Mayonnaise formed with extracted β-carotene-rich sunflower oil had better sensory quality than control; however, during storage, it was more resistant to oxidation [151].The apple pomace with ethanol (50:50, v/v) in 1:10 LS ratio was subjected to an ultrasonic bath at 45 C for 45 min for bioactive compounds recovery.Fresh pomace yield increased twice from 7.12 % to 13.61 %, while UAE, in the case of freeze-dried pomace, showed a further increase in yield (from 21.64 % to 58.09 %) and TPC up to 10.05 mg GAE/g DW.Apple pomace freezedried powder with 40.19 % dietary fiber was utilized to fortify beef burgers (4 % and 8 %).The sensory evaluation gave better results for fortified burgers than for control.The water activity values were for beef burgers with 0 % apple pomace (0.952), for 4 % fortified (0.948), and for 8 % fortified burgers (0.943) after 8 days.Plate count agar for 4 % and 8 % burgers were 7.25 and 6.90 log CFU/g after 8 days [75].

Upcycling of waste for food packaging and preservation
Food packaging is a part of food processing for its storage, transport, and distribution and ends at the consumer level.It is a tool utilized in food processing to protect the product from environmental stress.The main objective of food packaging is the protection of food commodities from external impacts (light, heat, and oxygen) and the preservation of nutrients.Moreover, it also aids in preventing tempering [152].

Edible coatings and biopolymer films
Edible coating gained attention due to its biodegradable nature and potential to increase the shelf life of products [153].Biopolymers based on fruits and vegetables have been extensively used in biodegradable film preparation, which provides preservation, longer shelf life, and sensory properties [154].Various plant and animal source biopolymers like lipids, proteins, polysaccharides, and bioplastics are synthesized by microbes to develop eco-friendly packaging material that can potentially preserve functional components of food products [155].
The potato processing industry produces a vast amount of waste, which contributed to the circular economy approach.In addition, sweet lime pulp generated pomace after juice extraction, which is considered zero-valued and ineffective.However, biodegradable packaging films could consume this zero-valued waste in food packaging to reduce environmental stress.Composite films were produced by using potato peel powder and sweet lime pulp pomace in different proportions (0:1, 0.5:1, 1:1, 1:0.5, 1:0) with US treatment (45 min) and (0:1, 0.5:1, 1:1, 1:0.5, 1:0) with 60 min.The US applied a film solution at different processing times to break polymers into small pieces to create a film.All the films were investigated for their barrier and mechanical properties.The outcomes showed that increased processing time provided good film and potato peel powder favorable for film formation.So, the films with 0.5:1 with potato peel and sweet lime pulp proportion 0.5:1 were best.The other properties of films with 0.5:1 were water vapor permeability (7.25 × 10-9 g/Pa h m), moisture absorption (12.88 ± 0.348 %), water solubility (38.92 ± 0.702 %), breakage strength (242.01 ± 3.074 g) and elongation capacity (7.61 ± 0.824 mm).The thermal decomposition temperature of films with 0.5:1 was above 200 • C.Moreover, these films were tested for bread as packaging for 5 days with 1.5 % clove essential oil and successfully reduced weight loss, hardness, and surface microbial load [156].
Apple polyphenols and sodium alginate were used as base materials, and glycerol was used as a plasticizer to produce an antibacterial and anti-oxidation composite film by casting.The US-assisted electrospray method was applied for silver nanoparticle deposition in films.Sodium alginate and silver particle ratio in films at different ultrasonic times were assessed on sample films' mechanical, optical, and hydrophilicity.
The outcomes suggested that sodium alginate and silver ration of 7:3 and processing US of 30 min was best for all properties.These films had good strength, softness, and low water vapor permeability (0.75 × 10-11 g/msPa).The composite films demonstrated a wide range of antibacterial activity, and Escherichia coli's (92.01 %) antibacterial activity was higher than Staphylococcus aureus's (91.26 %).Adding apple polyphenols resulted in higher antioxidant activity (98.39 %).In a strawberry model, these composite films enhanced the shelf life for days at refrigeration temperature compared to polyethylene films [157].Upcycling of fruits and vegetable waste by applying ultrasound technique is described in Fig. 3.

Ultrasound technology for the treatment of beverage industrial wastewater
The fruits and vegetables processing industries generate large amounts of wastewater that contains a wide variety of organic compounds and showed 5 days of biochemical oxygen demand (500 to mg/L) and chemical oxygen demand (806 to 7732 mg/L), pointing to serious environmental concerns [24].This wastewater treatment involves high-cost energy; hence, removing bioactive compounds is a serious challenge.This wastewater includes many polysaccharides, pigments, polyphenols, proteins, and fibers [158].The US is an effective method for removing toxic and hazardous organic components from wastewater.Its application causes irreversible changes in the medium involving biological and chemical phenomena such as coagulation, oxidation, and depolymerization [159].Besides these changes, the mechanical effect on the medium was also observed, including structure changes by US vibrational, which rely on the frequency and amplitude of waves.The US utilization in wastewater treatments helps remove toxic and resistant pollutants such as aromatics, surfactants, and dyes.This process involves the oxidative breakdown of resistant wastewater compounds [25].
Increasing coke production in consequent wastewater is a serious environmental issue.This wastewater includes large amounts of toxic compounds; therefore, coke wastewater treatment requires a combined processing of physicochemical and biological methods.The US was applied for coke wastewater treatment in a sequencing batch reactor.The researchers aimed to determine optimum conditions (amplitude and time) and wastewater treatment in a sequencing batch reactor.The  allowed in the US field [160].In conclusion, the US technology is effectively utilized to treat beverage industrial wastewater.

Limitations and future challenges
Apart from identifying bioactive compounds and safety risks, challenges also involve large amounts of solvent, long time for extraction, high temperature, and power of the US, which are some usual drawbacks linked with the extraction of compounds from natural food by-products [129].The UAE is inefficient for heat-liable and oxidation-sensitive bioactive compounds.Limitations associated with by-product extracts are instability, less water solubility, and low bioavailability of components that restrict their application in the food industry [161].For instance, citrus flavonoids have poor bioavailability and high sensitivity towards environmental conditions, which limits their use in industry as a food ingredient.Conversely, with technological advancement, there is a need to explore more approaches toward effectively utilizing waste and its safety assessment [34].Its application in food systems requires special attention for extracted components' stability, including bioactive compounds.Besides US benefits in food processing, the cavitation process and temperature could produce free radicles, which cause degradation issues.It could also cause severe molecule configuration changes and cell wall component degradation, releasing enzymes that oxidize phenolics [162].Cruz et al. [163] demonstrated that grape skin is sensitive to US processing.The TPC ranged between 18.4 and 31.0 %.Bioactive stability should be controlled to increase the positive effects of US processing in the food industry.
Besides the effective utilization of waste, there is also a need for wide research on the safety aspect of these by-products.Recycling generates various value-added products, but its introduction has risks in the food chain.Contamination is a basic challenge of transforming by-products into food products [164].Moreover, by-products are novel food products with specific functions; hence, safe food use must be properly documented.This raises the need for more research on the safe use of byproducts in the food industry [163].In addition, the bioactive compounds in fruits in food require legal assessment and dose recommendations for safety issues.For these reasons, much research was done on the safety assessment of citrus-based compounds.However, there is still a need to explore the risk related to each extracted compound from waste to improve its application in the food industry.It provides advantages in terms of time, temperature, energy, and chemical requirements during the extraction of the bioactive compounds from fruits and vegetables by-products; however, the complete removal of chemical solvents with reasonable yield needs to be explored.The UAE process, in combination with other non-thermal techniques such as MW, PEF, and enzyme-assisted extraction of bioactive compounds from fruit and vegetable waste, requires more research.Moreover, the equipment's development is needed to avoid direct contact with US horns.

Conclusion
Waste from the fruit and vegetable industry is a rich source of bioactive compounds and finds extensive applications across various industrial sectors.Due to their nutrient-dense and diverse composition, these wastes offer numerous health benefits.Consequently, they serve as an excellent source of natural ingredients for functional foods and food packaging.The potential to repurpose fruit and vegetable waste underscores the importance of recycling in boosting productivity and minimizing environmental impact.Ultrasound technology, an ecofriendlier alternative to traditional physical and chemical methods, effectively extracts valuable compounds from food waste and treats wastewater, enjoying high consumer acceptance.Adopting this approach can yield both economic and environmental benefits, transforming this sector into a sustainable, eco-friendly production system and promoting a circular, sustainable economy.It's crucial to evaluate fruit and vegetable waste for the recovery and characterization of bioactive compounds, and to explore their use in chemicals, nutraceuticals, and as food ingredients.Additionally, thorough research into the safety aspects of introducing these waste-derived products into the food chain is essential to enhance their applications in the food industry.

Fig. 2 .
Fig. 2. Recovery of valuable products from fruit and vegetable waste by US.

Fig. 3 .
Fig. 3. Upcycling of fruits and vegetables waste by the application of US.
B.G.Nabi et al.

Table 1
Ultrasound-assisted extraction for fruit waste valorization.

Table 2
Ultrasound for vegetable waste valorization.

Table 2
(continued ) [133]inued on next page) B.G. Nabi et al.• C, time 17.12 min, and pH 3.68.Under the following conditions, the maximum value was achieved (pectin yield 2.62 %, esterification degree 88.26 %, and galacturonic acid content 87 %).The results revealed that banana peel pectin was effectively extracted by US application[63].Mono-sonication (pressure-assisted ultrasound technique) was applied for pectin extraction from various citrus cultivars in combination with US radiation and pressure.Applying optimum pressure during sonication enhances the yield of the target compound[133].The UAE steam explosion with acid extraction and US steam explosion acid extraction were applied to passion fruit peel for pectin extraction.The pectin yields with UAE and acid extraction were 6.5 % and 5.3 %, respectively, but the emulsion stability in the former extraction method was poor.The US steam explosion acid extraction enhanced the emulsion stability in comparison to UAE and had more protein content (0.62 %