Accounting for 61% of the world’s citrus fruit production [1
], the global production of sweet orange (Citrus sinensis
(L.) Osbeck) in 2017–2018 exceeded 47 million tons, 36% of which (17 million tons) was used in orange juice production [2
]. Production for 2018–2019 was predicted to grow by another 4.2 million metric tons. Large amounts of by-products, estimated at a level between 50% and 60% of the harvest, consist of discarded fruits, peels, and seeds. Effective technologies to upgrade the value of these said by-products, which have been so far mostly dealt with as waste, are of direct and significant relevance to all orange-growing countries and regions, including Brazil, Florida, India, South Africa, Spain, Turkey, and Italy [3
]. Waste orange peel (WOP), in particular, contains highly valuable bioproducts, such as carbohydrate polymers (cellulose, hemicellulose, and pectin), polyphenols (including naringin and hesperidin), and essential oils (mostly d
The affordable, large-scale extraction and valorization of these compounds would also result in the size reduction of the relevant waste stream, thus relieving the environmental burden related to the still frequent disposal of the WOP in landfills or saving valuable biocompounds before the energy conversion of the residues. Anaerobic co-digestion, carried out after the extraction and removal of d
-limonene, an inhibitory compound, was assessed as the most environmentally performing technique for the energetic valorization of WOP by means of biogas generation [3
]. Indeed, the latter practice has been increasingly applied in some orange intensive production areas, such as Sicily.
Extracted from the orange peel prior to squeezing via a mechanical process (a jet of water breaking the oil-containing glands), orange essential oil (EO) mostly contains d
], a monoterpene whose average content in Citrus sinensis
fruit peels is 3.8 wt % on a dry weight basis [5
]. This molecule was first used in the 1950s as a bio-solvent and today is the main ingredient of numerous bio-based functional products whose demand is rapidly growing [6
]. In the early 1990s, its plant anti-fungal and antibacterial properties were first identified [7
], leading to the development and utilization of biopesticide formulations, in which orange oil, and thus d
-limonene, was the active ingredient [8
]. After the discovery of its natural ozone scavenging properties, in 2005 d
-limonene was proposed as an effective adjuvant in preventive therapies against asthma [9
]. Due to its broad spectrum of antimicrobial, antioxidant, and anti-inflammatory properties, d
-limonene is now used in many cosmetic and nutraceutical applications, as well as an anti-spoilage additive in food [10
Pectin is currently mostly produced from citrus peels (56% from lemons, 30% from limes, and 13% from oranges) and, to a lesser extent (14%), from apple pomace [11
], and it is the most valued natural hydrocolloid [12
]. Since the early 2000s, it was established that pectin has various beneficial effects on health and nutrition as a dietary and prebiotic fiber, with numerous applications in the food, feed, cosmetic, medical, and pharmaceutical industries [12
]. Effectively reducing the interfacial surface tension between the oil and water phases, pectin is also an excellent emulsifier and emulsion stabilizer [14
]. Orange-extracted pectin powder added to an oil-in-water sub-micron size emulsion (20% w/w
of orange oil), prepared with a standard homogenizer, exhibited substantial stability up to at least 30 days from preparation [14
In the last fifteen years, numerous green chemistry processes were applied to extract the valued components of WOP resulting from the orange juice industry. WOP is a potential source of fat (oleic, linoleic, linolenic, palmitic, and stearic acids, and phytosterols), mono- and disaccharides (glucose, fructose, and sucrose), organic acids (mainly citric, malic, and tartaric, but also benzoic, oxalic, and succinic acids), polysaccharides (cellulose, hemicellulose, and pectin), enzymes (pectinesterase, phosphatase, and peroxidase), flavonoids (hesperidin, naringin, and narirutin), terpenes (d-limonene, linalool, and myrcene), and pigments (carotenoids and xanthophylls).
A few years ago, solvent-free extraction processes using microwave and ultrasound techniques were successfully applied to obtain essential oils, polyphenols, and pectin through microwave hydrothermal processing [16
]. Promising results were achieved using solar-driven vapor steam distillation, to obtain valued pectin, terpenes, and biophenols [17
], as well as employing a solvent-free process based on microwave distillation, hydrodiffusion, and gravity [13
Hydrodynamic cavitation (HC) is generally achieved via pumping a liquid through one or more constrictions of suitable geometry, such as Venturi tubes and orifice plates. Controlled HC results in the generation, growth, and collapse of microbubbles due to pressure variations in the liquid flow [18
]. The increase in kinetic energy at the constriction occurs at the expense of pressure, leading to the generation of microbubbles and nanobubbles, which subsequently collapse under pressure recovery downstream of the constriction [19
]. The violent collapse of the cavitation bubbles results in the generation of localized hot spots endowed with extremely high-energy density [20
], highly reactive free radicals, and turbulence, which can result in the intensification of various physical/chemical phenomena. These include wastewater remediation and enhancement of biogas generation [22
], preparation of nanoemulsions, biodiesel synthesis, water disinfection, and nanoparticle synthesis [25
], among others.
In the recent past, cavitation has emerged as a green extraction technology for natural products, reducing process time and energy consumption, while achieving higher extraction yields, as well as a useful tool for the intensification of food and pharmaceutical processes [25
]. The growing variety of applications has also stimulated the development of other promising arrangements, such as those based on rotating parts [27
] and variants of fixed constrictions based, for example, on vortex dynamics [28
], which are in the process of proving the respective affordability and straightforward scalability.
Real-scale applications of cavitation are quickly spreading in the food and beverage industries, including the processing of food waste [29
]. Again, the HC processing of vegetable raw materials, such as grains and hops for beer-brewing [30
] or plant leaves [32
], and its application to the extraction of bioactive compounds [27
], offer distinct advantages, such as shorter process times, higher energy efficiency and yields, and enhanced extraction rates. When compared with both conventional techniques and newer ones, including acoustic cavitation sustained by ultrasound irradiation, the HC-based processes showed superior performance, due to enhanced process yields and straightforward scalability [18
HC-based techniques appear as natural candidates for applications to the valorization of WOP. Nevertheless, to the best of our knowledge, no studies have been reported so far on the application of hydrodynamic cavitation processes to extract the valued components of waste orange peel. This study reports the first results concerning a novel route to valorize WOP based on criteria of effectiveness, reliability, efficiency, and affordability. The starting idea was that waste orange peel contains EOs, water-soluble pectin, and polyphenols, which can be transferred to the aqueous phase. In particular, the EOs could form oil-in-water emulsions, stabilized by the presence of pectin. All this, carried out by means of HC processes and without additives, as elucidated in Section 2.2
After the HC-based extraction process, the liquid phase could be used as such to functionalize foods and beverages, affecting both the nutraceutical properties and the shelf life. The residual WOP solid fraction, mostly composed of cellulose and hemicellulose and deprived of inhibitory compounds, such as the EOs, could be effectively used to produce biogas in an anaerobic digester, and the resulting digestate used as a soil amendant or easily converted into biochar or hydrochar [34
The device used to process the orange peel waste, employing no proprietary components, is easy to construct and maintain, and its operation, at the pre-industrial scale, was verified by experiments carried out on real scale (more than 100 L of water, quantity of WOP raw material of about 6.4 and 42 kg). The scalability of the proposed device, up to the industrial scale (1700 L), was recently demonstrated in the brewing sector [66
]. Additionally, the reliability of the device was proven by the absence of any wear of flow components after thousands of hours of operation, as was already noted in a previous study using the device [30
The hydrodynamic cavitation processes, sustained by a circular Venturi-shaped reactor, allowed us to effectively and completely separate and extract the most valued components of the waste orange peel. It is remarkable that no solvents or any additives, other than tap water, were used in the extraction processes.
As shown in Section 3.1
, the biomethane generation potential was boosted in terms of both total cumulated production and generation rate. Within only 3 min at 14.5 °C, corresponding to less than 10 passes of the entire processed mixture through the cavitation reactor, the BMP was already at 61% of its theoretical value. Additionally, the specific energy content of the generated methane (chemical energy) was about 30 times higher than the specific consumed energy (electricity). Since then, the BMP increased up to the Th-BMP at the end of the process WOP1 (273 min, temperature of 96 °C), but the energy balance became negative.
From the energy balance point of view, it would be imperative to limit the processing time as much as possible, i.e., to a few min. However, the processing time should be optimized based on the assessment of the overall value of the extractable materials, such as pectin, polyphenols, and terpenes, as well as on the processing of the substrate resulting from the anaerobic digestion (e.g., disposal, composting). Such topics will require further research.
Due to the apparent suboptimal cavitation regime during most of the WOP1 process, especially during the first 60–90 min, it is likely that simple technical adjustments, such as a different centrifugal pump, could produce even better results. However, with a lower concentration of WOP in the aqueous mixture, as in the WOP2 test, the HC process was carried out in the optimal regime, as proven by the low levels of the cavitation number. Thus, an optimized HC process is expected to lead to higher methane generation in a shorter process time, even for higher WOP concentrations, thereby further improving the energy balance.
According to the results presented in Section 3.2
, the pectin isolated in the sample collected at the end of the WOP1 process showed a very low degree of esterification, namely 17.05 ± 0.60%, meaning that it would be particularly appropriate for food and beverage, pharmaceutical, and nutraceutical applications, because it does not require sugar or acidic conditions to form stabilized gels. It should be noted that this result nicely agrees with previous studies, in which pectin from WOP originating from red oranges from the same area of Sicily, extracted via microwave hydrodistillation and gravity, was shown to have a DE of 25%, suggesting that the pectin from the red orange pulp is likely to have a very low DE [67
]. However, a distinct beneficial role of the HC-based extraction method on the pectin DE cannot be ruled out, which deserves further comparative research.
We remind that WOP (exo-, meso-, and endocarp) contains not only the outer skin (exocarp) and the peel (exo- and mesocarp), but also endocarp residues. It is remarkable that, as mentioned in Section 2.3.2
., pectin, analyzed 18 months after extraction and lyophilization, remained stable during prolonged storage at room temperature in direct contact with oxygen. In fact, after another three months in the same plastic vessel, pectin continued to show no sign of degradation. This evidence pointed to the stabilization effect of powerful antioxidant orange biophenols, including the flavanones (Section 3.3
) found in the WOP2 aqueous solutions, and is likely available in an even higher concentration in the sample T14 from the WOP1 test.
Overall, the WOP1 test proved that the HC process allowed the effective extraction of high-quality pectin from the waste orange peel and a very efficient exploitation of the biomethane generation potential from the solid residues of the process. Additionally, there was no evidence of microbiological degradation or spoilage in the T14 liquid sample, even though it was unlikely that any relevant concentration of antimicrobial d
-limonene remained in the aqueous solution, due to the very high working temperature (as shown for sample T214 from the WOP2 test). We hypothesize that the reason for the apparent microbiological stability lies in the well-known effective disinfection carried out by the HC-thermal process [40
]. The stabilization effects produced by the extracted flavanones and the process-driven disinfection could be distinctive features of the HC-based extraction method.
As shown in Section 3.3
, water-soluble flavanones, naringin and hesperidin, constituted the majority of polyphenols in the WOP. Both compounds were extracted in the aqueous solution quite effectively and efficiently through the HC process and were partially transformed into other compounds, mostly other flavanones and possibly hydroxycinnamic acid derivatives. Overall, the extraction process yield was nearly 60%, regarding the sum of the detected compounds. Such a level was achieved within 10 min of processing, while after just 2 min it was at about 53%, thus proving the effectiveness of the extraction.
The HC-based polyphenols extraction rate was remarkably greater than achieved by means of a state-of-the-art hydro-distillation extraction method [17
], where the total polyphenol content in the aqueous phase was only about 17% of the original content, as well as the HC-based extraction was much faster.
We hypothesize that the other flavanones (peaks F1 to F4 in Figure 7
) might have derived from hesperidin and/or naringin, following the loss of at least one hexose unit. In their turn, since these peaks were practically undetectable in the chromatogram of the process residues, this decomposition could have been due to cavitation processes occurring in the liquid phase. In addition, the peaks shown just on the left of the peak F1 region in the chromatogram for the aqueous phase (Figure 7
, unlabeled peaks), attributed to HAD, were not observed in dry WOP or process residues and could be considered as a distinct effect of the cavitation process.
From the decrease of d
-limonene concentration in the solid residues (Section 3.4
), a lower limit of 45% for the respective extraction yield in the aqueous phase was inferred, such a compound being by far the most abundant monoterpene in the WOP. However, the actual extraction yield is expected to be much higher, as suggested by two pieces of evidence. First, the abrupt drop of its concentration in the aqueous phase shortly after its highest value (6 min of process time) is achieved, pointing to its fast volatilization. Second, the mass loss from the solid residues due to the continuous extraction leads to the overestimation of the respective total content of d
-limonene, based on its concentration. In forthcoming practical applications, airtight HC extractors will be used in order to retain liquid limonene, both floating and emulsified in the aqueous solution due to the emulsifying action of pectin [15
While postponing the comparison of EO extraction rates to future experiments, based on the available data it can be safely stated that the HC-based EO extraction was remarkably faster than achieved by means of a state-of-the-art hydro-distillation extraction method [17
], which took about 120 min to complete. The same holds with regard to an innovative solvent-free process based on microwave distillation, hydrodiffusion, and gravity [13
], where the semi-industrial process took about 60 min.
The high volatility of orange peel EOs under environmental conditions (in particular d
-limonene, which is chemically unstable) hinders their effectivity as flavorings in the food industry (affecting the shelf-life) and as biopesticides in agronomic applications [68
]. Moreover, the antimicrobial action of d
-limonene was found to markedly increase when applied as an oil-in-water nanoemulsion, for example reducing the thermal resistance of Listeria monocytogenes
by 100 times, against only two to five times when added directly [69
Therefore, methods have been proposed to reduce the volatility, increase the stability, and control the release of such compounds. Two recent studies suggested the nanoencapsulation of orange peel EOs [70
] and d
], respectively, in oil-in-water nanoemulsions prepared by ultrasonic irradiation (acoustic cavitation) and stabilized with a mixture of pectin and whey proteins. Thus, the combination of cavitation processes and pectin appears very promising for the retention and effectivity of d
-limonene, provided its early volatilization is prevented.
Indeed, the residual retention of d
-limonene in the aqueous solution, up to sample T27 (30 min, 35 °C) in the WOP2 test (Figure 9
a), could have been favored by two factors. First, the likely micronization and partial emulsification of the terpenes in water, based on the well-established effectivity of HC processes in the creation of stable sub-micron oil-in-water emulsions [41
]. Second, the effectivity of pectin as an emulsifying compound, as well as a stabilizer for emulsions [15
]. Due to the effective extraction of high-quality pectin in the aqueous phase (Section 3.2
), the micronized limonene drops could have been partly emulsified and stabilized, concurring to the limitation of its volatilization. Future research will investigate these relevant emulsion chemistry aspects.
Further research using optimized devices and processes will allow more comprehensive and rigorous comparison of the presented process with either conventional or newer extraction techniques. As an example, the effective retaining and recovery of orange peel oil during the HC process will allow the determination of comprehensive performance indices, such as those recently advanced, based on the extraction yield, energy efficiency, and quality of the product [73
Finally, hydrodynamic cavitation techniques were compared many times with competing techniques, very often resulting in higher process yields [33
]. Nevertheless, separating and quantifying the contribution of cavitation to the achieved results, in comparison with other processes, such as pumping, heating, and turbulence, would be desirable. Although prevented in this study due to technical limitations, this issue could be solved by means of the installation of a bypass excluding the cavitation reactor, all else being equal, which is recommended for further research. Additionally, the direct visualization of cavitation could be useful in order to assess its features (extent, intensity), which could be done by means of reactors made of transparent material.