Limonene or 4-isopropenyl-1-methylcyclohexene (C10H16) is a monocyclic monoterpene hydrocarbon naturally synthesized in many plants through the cyclisation of geranyl pyrophosphate by a monoterpene synthase. It constitutes one of the most abundant monocyclic monoterpenes in the plant kingdom, being found in more tan 300 essential oils and principally in Citrus spp. (30-98%). The high availability in nature together with its proved safety, make it widely exploitable in health, as flavoring agent and adjuvant in food and beverage industries, as well as in cosmetic’s for the formulation of perfumes and other personal hygiene products. In particular, this compound has been proven to possess a valuable potential as sustainable replacement to synthetic pesticides and food preservatives. To successfully use limonene in a greener agri-food industry, its preservation has become a top concern for manufacturers. In general, encapsulation conserves and protects limonene from outside aggressions, but also allows its controlled release as well as enchances its low water solubility, which can be critical for the target applications, together with other parameters, such as scalability, low cost and availability.
Limonene occurs as two optical isomers, named d- and l-limonene, as well as a racemic mixture [1][2][3]. The most common, d-limonene ((+)-limonene) is a colorless liquid with characteristic and pleasant lemon-like odor, normally obtained from the cold pressing of Citrus peels and pulps where it can be found at concentrations over 90% [4]. Whereas, l-limonene ((−)-limonene) is more present in other species such as Mentha spp. essential oils [5]. Both are common flavoring additives in cosmetics, food, industrial solvents and pharmaceuticals because of their fragrant and demonstrated harmlessness for humans [6][2][7].
Mechanical process or steam distillation techniques are typically the chosen methods to obtain limonene because they are green and non-organic solvents are involved [8]. However, other less conventional methods have also been tried in order to optimize d-limonene extraction, including high pressure-high temperature extraction (150 °C, 30 min) and supercritical fluid extraction (15 MPa, 40 °C) [9][10]. Not only the methods, but also the extractants have evolved to healthier and greener ones, such as bio-based cyclopentyl methyl ether (CPME) and 2-methyl-tetrahydrofuran (2-MeTHF) that have been confirmed as sustainable alternatives increasing limonene yield by 4 and 2-fold, respectively, in comparison to the conventional solvent hexane [11]. Interestingly, the resulting limonene itself represents a green alternative to replace hazardous petroleum-based chemicals as n-hexane in the extraction or synthesis of other bioactive compounds [12][13][14][15][16].
Renewable applications of limonene are rapidly expanding these days, especially in the agri-food industry. Interesting to highlight is its potential as an antimicrobial, herbicidal and antioxidant agent [17][18], with low risk against non-target arthropods, earthworms, soil microorganisms and terrestrial non-target plants [19][20][21][22][23][24]. In particular, the limited insecticidal activity of limonene was once reported in 1988 [25]. Years after, it was discovered that other aromatic compounds, such as linalool, camphor and isoeugenol, were able to enhance this activity [26]. For instance, the synergic action of R-(−)-limonene/(−)-borneol against the larvae Spodeptera littoralis caused a mortality of 84.6% when only 17.6% was expected. This synergic capacity with multiple natural substances opens divers possibilities of developing new formulations with biological efficacy in the agri-food field.
Nonetheless, the employment of limonene presents certain limitations coming from its instability, fragility and volatile nature. In fact, it can be easily degraded if it is not well protected from external factors like oxygen, light and temperature when applied.
Limonene represents a safer and greener alternative to commercial synthesized antimicrobial products whose environmental and human health safety are disputed. In fact, Ünal et al. demonstrated the broad-spectrum and dose-dependent antifungal effect of limonene, showing higher effectiveness than standard product Fungizone® at even lower doses (10 µL) [27]. Even more, limonene can modulate the antimicrobial effect of commonly used antibiotics against certain strains [28]. As an example, the combination of limonene and gentamicin considerably reduced the minimum inhibitory concentration (MIC) versus both Gram-positive and Gram-negative clinical bacteria, reaching values of 13.7–4 µg/mL against Staphylococcus aureus and 30–20.1 µg/mL, against Escherichia coli, respectively [29].
The antimicrobial efficacy of limonene may vary according to many factors. First, the stereochemistry and the target pathogen. Regarding this, there was a difference of at least three-fold in the antibacterial effect against S. aureus between enantiomeric forms of limonene. Different MIC values were observed when testing d- and l-limonene individually against the foodborne pathogens E. coli (ATCC 11775) and S. aureus (ATCC 12600); and the racemic mixture exhibited lower MIC (8 mg/mL) than both enantiomers separately (27 mg/mL) versus the food contaminant Enterococcus faecalis [30]. Second, temperature and pH. Particularly, bacterial membrane seemed to be more sensitive to limonene at lower temperatures. These observations may be related to the high volatility of the monoterpene hydrocarbon at increasing temperatures [31]. Nevertheless, the bactericidal activity of limonene against E. coli was improved with simultaneous applications of heat and acidic pH (4.0), increasing outer membrane permeability and altering β-sheet proteins [32]. Finally, the presence of other compounds: although limonene has been reported as one of the principal contributors of the antimicrobial activity of Citrus spp. essential oils [33], synergistic effect of minor compounds of finger citron (Citrus medica L. var. sarcodactylis) essential oil enhanced the bactericidal activity against common foodborne bacteria E. coli, S. aureus, Bacillus subtilis and Micrococcus luteus [34]. This additive effect was also detected against three spoilage bacteria (Lactobacillus plantarum, L. brevis and B. coagullans) and fungi (Saccharomyces bayanus, Pichia membranifaciens and Rhodotorula bacarum) of fruit juices [35]. However, limonene showed higher a antimicrobial effect compared to orange extract with mucilages and glycosides against Candida albicans, A. niger, Aspergillus sp. and Penicillium sp. [36]. Similarly, dl-limonene completely inhibited the growth of A. niger and the aflatoxin production at 500 and 250 ppm, respectively, concentrations at which neither C. maxima and C. sinensis essential oils nor their combination reached a total inhibition [37].
The antimicrobial effect of limonene is comparable with other compounds. Limonene has reported as the lowest MIC (0.421 mg/mL) and minimum bactericidal concentration (MBC; 0.673–1.682 mg/mL) when compared with other hydrocarbon (α-pinene, myrcene) and oxygenated (geraniol, linalool, nerol and terpineol) monoterpenes against the Gram-positive food-spoiling bacterium, S. aureus, and the two Gram-negative bacteria, E. coli and Salmonella enterica [38]. However, limonene’s MIC and minimum fungicidal concentration (MFC) were lower (0.75 and 3 µL/mL) than those observed in the oxygenated compounds citral (0.188 and 0.375 µL/mL) and eugenol (0.4 and 0.8 µL/mL) against Zygosaccharomyces rouxi, responsible of the spoilage of apple juices and high sugar foods [39]. In general, limonene has shown stronger antifungal effect (EC50 238 mg/mL) than other hydrocarbons, like 3-carane (EC50 259 mg/mL), myrcene (EC50 288 mg/mL) and β-cymene (EC50 1051 mg/mL) against the aflatoxin-producing fungus Aspergillus flavus. However, this activity is lower compared to the oxygenated monoterpenes, like citral (EC50 212 mg/mL), citronellol (EC50 87 mg/mL) and the aromatic compound thymol (EC50 20 mg/mL) [40].
Limonene has demonstrated a broad-spectrum phytotoxic potential. It has been one of the most active monoterpenes evaluated against the seed germination and primary radicle growth of radish (Raphanus sativus L.) and garden cress (Lepidium sativum L.) achieving a significant inhibition of their germination and root elongation (10−4–10−3 M) [41]. Limonene also showed stronger herbicidal activity facing Arabidopsis plants than other monoterpenes like citral, carvacrol and pulegone [18]. Furthermore, foliar application of d-limonene at concentrations of 100 and 200 kg ai/ha produced the death of certain weed species, being especially sensitive to d-limonene velvetleaf (Abutilon theophrasti Medik.), Indian jointvetch (Aeschynomene indica L.), barnyard grass (Echinochloa crus-galli (L.) Beauv) and southern crabgrass (Digitaria ciliaris (Retz.) Koel) experimenting death after three days of treatment with 50 kg ai/ha of d-limonene [42]. Additionally, interesting results were obtained with limonene against the cosmopolitan weed slender amaranth (Amaranthus viridis L.) whose germination, seedling growth, dry weight as well as chlorophyll content and cellular respiration were significantly affected by this monoterpene hydrocarbon. Limonene totally inhibited the germination of A. viridis at a concentration of 7 µL, and it reduced the radicle length between 70% and 90%, as well as the seedling dry weigh by about 17% and 33% at only 1 and 5 µL, respectively [43].
However, a comparative study of the weedicide activity of key lime (C. aurantiifolia Christm.) essential oil and its main compounds limonene (40.92%) and citral (27.46%) facing three important monocot weeds: Avena fatua L., Phalaris minor Retz. and Echinochloa crus-galli (L.) Beauv., demonstrated the low phytotoxicity of the hydrocarbon monoterpene with respect to the whole essential oil and the oxygenated monoterpene [44], corroborating the fact that monoterpene hydrocarbons usually exhibit less potent allelopathic activity than oxygenated ones [41][44][45][46][47][48][49].
The mechanisms by which limonene and other terpenes affect germination and/or growth of plants are still not well known. It has been observed that solubility may be a key factor implicated in phytotoxicity. The higher lipophilicity of limonene allows a better penetration through the mitocondrial membranes of maize (Zea mays L.) causing more detrimental effects on respiration at increasing doses than more hydrophilic monoterpenes, even reaching abolition of respiratory control between 1.0 and 5.0 mM [50]. Furthermore, lipid peroxidation may also be a possible mechanism of action by which limonene and essential oils that contain it can exert its phytotoxic activity [37]. In fact, limonene increased lipid peroxidation of the root growth of maize, causing high inhibition values of 73.69–90.10% in the radicle elongation from 24 to 96 h of treatment [46]. Finally, limonene also showed a strong antimicrotubule activity at high and low dosages, provoking the breakage and leakage of the plasma membrane and finally causing plant death [18].
The phytotoxic effect of limonene will also depend on the target plant. As illustration, limonene influenced the photochemical processes in carrot cultivar Splendid deriving in a lower shoot and root biomass, while it reduced gas exchange in cultivar Parano resulting in lower stomatal conductance. While for cabbage, cultivar Lennox showed better tolerance and fast recovery to limonene than cultivar Rinda by means of developing photochemical processes of increasing efficiency that provide energy for defense and repair action [51].
The photochemical processes of limonene, have been also studied in the algae Chlorella vulgaris (Chlorophyceae), showing that this monoterpene hydrocarbon caused a drastic degradation of the photosynthetic pigments, among them xanthophyll at 1.6 mM [52].
Unfortunately, limonene such as other monoterpenes and essential oils exerts a non-selective phytotoxicity, affecting not only weeds, but also cultivated plants [53][54]. Both leaves of cabbage (Brassica oleracea L.) and carrot (Daucus carota L.) were directly damaged at limonene’s concentrations higher than 90 and 120 mL/L [51]. Thus, it may be necessary to evaluate previously the threshold concentration of limonene for cultivated plants with the aim to avoid any harm for them. Consequently, limonene has been included as the main active principle in several non-selective herbicidal formulations [55][56].
Limonene is thought to be a possible substitute to these commonly used synthetic antioxidants, particularly in increasing the oxidative stability of vegetable oils in the deep-frying process without affecting the sensory properties of the fried products [57]. Nevertheless, the antioxidant potential of limonene is still significantly lower than other reference antioxidants, such as trolox, concretely at the range of concentrations between 2 and 2000 μM [58].
The natural presence of limonene in certain foods can represent a quality indicator. For instance, the loss of limonene in Citrus during their storage would affect the original flavor and aroma of the product and consequently, a deterioration of the food [59]. In other food products, the ability of limonene to repress the formation of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) and glutathione (GSH) has been confirmed [58][60]. These results demonstrate the disposition of limonene to avoid food degeneration, indicating its usefulness in overcoming storage losses and enhancing the shelf-life of food products. Moreover, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging effect and reducing power of limonene have been confirmed to be even higher than the antioxidant activity exerted by other terpenes like nerol, terpineol, geraniol, linalool and myrcene [38]. This activity can even be enhanced to an IC50 of 116 ppm when encapsulating the monoterpene hydrocarbon in chitosan-NaTPP, as this material protects limonene from degradation and increases the solubility in water [61].
Citrus spp. with remarkable concentrations of limonene enjoys great antioxidant activity, representing potential natural, eco-friendly and safer alternatives to synthetic preservatives in food packaging and preservation [62][63]. In this way, C. sinensis (90.66% limonene) and C. limon var. pompia (pompia; 256.3 mg/mL limonene) have demonstrated a good ability in scavenging radicals [37][64]. The antioxidant activity of C. aurantifolia, C. limon and C. paradisi (40.16%, 57.20% and 73.5% of limonene, respectively) essential oils occurs in a limonene dose dependent manner, reaching C. paradise essential oil values of 84.92% ± 0.5% and 92.45% ± 0.6% in the DPPH and β-carotene-linoleic acid assays, respectively [65].
Limonene’s termination-enhancing antioxidant chemistry, shared with other compounds like linalool and citral, might be relevant in food preservation, too. In this case, the concentration of the terpene compound as well as the characteristics of the substrate would be limiting factors in its antioxidant activity [66].
The efficacy of the encapsulation towards limonene’s oxidation, evaporation or controlled release would generally depend on the chosen encapsulation methodology (atomization, extrusion, fluidized bed, coacervation, etc.) and on the wall materials used.
It is one of the oldest and most widely used techniques of encapsulation. It appears to be an effective technique for encapsulating limonene aroma providing a good barrier against oxidation of sensitive materials: on the one hand, simple coacervation has been used by Souza et al. for the preparation of insect repellent limonene oil microcapsules with chitosan [67]; on the other hand, complex coacervation seems to be more recurrent in the literature for the encapsulation of limonene [68]. One important advantage of this method is the control of the shape, size and release rate of the encapsulated limonene only varying the concentration and ratio of the chitosan and NaOH solution.
The most common composite matrices employed as delivery vehicles for limonene in complex coacervation are made of sustainable polymers such as chitosan, pectin, gelatin, cellulose and gum acacia or Arabic [69][70]. These composite matrices have shown higher limonene encapsulating efficiency (EE) than chitosan crosslinked with sodium tripolyphosphate (46% vs. 51.3%; 89.7% and 98.6% for chitosan-cellulose and gelatin-gum Arabic, respectively) [61]. Release profiles are typically done in two phases, an initial phase of 24 h characterized by a burst release effect, probably due to the release of limonene found on the surface of particles, followed by a decrease release, which can go over 162 h. Tannic acid has been also used as hardening agent in chitosan/gum Arabic microcapsules of limonene [71]. Mono or polinuclear structures were obtained depending on the emulsifier used. In particular, the best EE% (98.6%) was achieved with Span 85 [72].
Viscosity and pH are essential parameters in the preparation of coacervated biopolymers complexes with maximum encapsulation of limonene. Drying methods have also been reported to have an effect on the retention of volatile limonene and in the structure of the wall matrix influencing consequently the storage stability [73]. For instance, significantly lower retention of limonene was observed for freeze dried whey/corn fiber-limonene samples after storage caused by the diffusion of limonene through the wall materials, which are generally more porous and loose than the structures obtained by spray drying [74][75].
Biopolymers such as polyurea, poly(vinyl alcohol) or poly(lactic acid) [76][77][78][79]; along with polysaccharides and inorganic carriers have been spotted as efficient materials for encapsulation of limonene (Table 1).
Table 1. Examples of wall materials used for limonene’s encapsulation.
|
Wall Material |
|
Highlighted results |
Ref. |
|
Polymer |
|
|
|
|
Polymer-blend in a HPMC:PV(OH):EC1 |
|
HPMC:PV(OH):EC w/w/w ratio of 1:1:6. Low limonene’s EE% due to unsaturated hydrocarbon functionality. |
[80] |
|
Acrylic adhesive polymer or natural rubber |
|
Application as pesticide (Solanum melongena). Penetration rate of the active agent, imidacloprid, was enhanced 2.4 times in the presence of D-limonene. Bursting release avoided. |
[81] |
|
Polysaccharide |
|
|
|
|
Amylose |
Amylose-limonene showed less than 5% limonene released at pH acid. At pH 6-7 burst release followed by a controlled and retarded release (6 h with 34 to 79 % release depending on the % of amylose used in the formulation). |
[82] |
|
|
Chitosan |
|
Release tested in five different food simulating liquids (aqueous solutions with 0%, 10%, 50% and 95% of ethanol and isooctane). Kinetic constants augmented with the addition of ethanol, due to the increase of limonene’s solubility. |
[83] |
|
Functionnalized chitosan |
|
Increasing the shelf-life of strawberries during storage. Chitosan functionalized with palmitoyl chloride provided better preservation after 14 days at 4 °C. Chitosan modification increased its hydrophobicity, ensuring limonene controlled release and improved its stability and adhesion to the fruit. |
[84] |
|
Inorganic carriers |
|
|
|
|
Silica |
Limonene oxidation and retention depended on the type of silica (chemical purity, small pore volume/diameter and hydroxylated surface area). |
||
|
Hybrid CaCO3 with lecithin, sodium stearate (NaSt) and acacia gum (AG) |
|
Particles with lecithin and NaSt presenting more hydrophobic surface retained more limonene. CaCO3-lecithin presented minimal loss after 3 months’ storage at r.t.2 Hydrophobicity was more efficient than specific surface area in increasing limonene’s retention and absorption capacity. |
[87] |
|
Protein |
|
|
|
|
Corn’s Zein |
|
Optimal limonene/zein ratio was 2.0 yielding particles with D4.3 of 10 µm and shell thickness of 25 nm. Maximum burst release at 30 min, followed by sustained release of environ 80%. |
[88] |
In general, the release of the limonene is mainly controlled by the initial EOs loading and the ability of the oil molecules to diffuse through the wall barrier into the surrounding environment. Interactions between the limonene molecules and the wall materials, together with the vapor pressure of the volatile substance on each side of the matrix, are the major driving forces influencing diffusion [89][90][91].
In general, due to its lower cost and ability to interact with a wide variety of EOs, β-CD is widely used for the encapsulation of limonene. Although the most popular method to form limonene-CDs inclusion complexes is in solution, the alternative kneading method has also been reported [92]. It is often combined with other techniques such as extrusion, electrospinning and spray drying achieving a more efficient preservation of limonene [93][94][95].
The determination of the stability constant of limonene inclusion complexes with CDs is of critical importance to take advantage of the complexation potential of CDs in the agro-food industry. The limonene-CDs complexation process has been modeled by 1D and 2D ROESY NMR experiments and found to be driven by non-covalent interactions. It was observed that only partial complexation was obtained, with non-complete formation of 1:1 inclusion complexes. Limonene-β-CD complex seems to be slightly more stable than limonene-α-CD with binding energies of -4.54 kcal and -4.05 Kcal, respectively [96]. A similar trend was observed by Astray et al. who determined the binding constant of limonene-( α/ β)-CDs complex formation by UV-Vis technique coupled with molecular mechanics’ calculations [97]. Although β –CD can increase solubility, permeability and adherence of limonene to the bacterial walls, its complexation with d-limonene can result in structural changes, which can prevent the physicochemical interaction with the cellular bacterial system and therefore the complex would be no active [29].
Encapsulation efficiency was found to be monoterpene chemical nature dependent. While oxygenated terpenes such as eucalyptol and thymol were entrapped with an EE > 82%, monoterpene hydrocarbons like limonene presented lower EE (from 15% to 25%). The lower EE% values obtained for monoterpene hydrocarbons have been associated to their very low water solubility.
Spray drying is a physical encapsulation technique used in the protection and release of unstable active materials confined into polymeric matrices. New wall materials for spray drying encapsulation like maltodextrin or whey and soy protein are investigated to substitute the commonly use dones (gum acacia, gum Arabic and modified starches) [98][99][100][101][102]. Although the traditional materials seem to still give the highest flavor retention, soy and whey protein materials have demonstrated to effectively limit the oxidation of limonene [100].
One of the drawbacks of using spray drying for the production of dry flavorings is the high temperatures used during processing, which can lead to the loss of volatile molecules. An interesting solution to reduce volatilization of d-limonene during spray drying process is to use a multilayer emulsion multilayer as the encapsulation system [103]. Emulsions stabilized with two and three-layers of polysaccharides presented greater retention of d-limonene and physico-thermal stability (30–90 ᵒC) than mono-layered membranes and showed the higher aroma retention after 45 days’ storage.
CDs have also been used in combination of spray drying for flavor and aroma encapsulation [104]. While the use of a coating material for the preparation of CD/limonene spray dried powders improved the powder properties at expenses of decreasing the limonene content [105]. Dried forms of different CD/Limonene (α-, β- and γ- and HP-β-CDs) were prepared by spray drying and studied for increasing the flavor and shelf-life of non-alcoholic beverages [106]. Among the CDs tested, β-CD was the more suitable for limonene complexation and retention (66% of encapsulation efficiency and 6.25 w/w of limonene load). Furthermore, accelerated aging analysis showed that limonene content decreased less in the presence of β-CD with 40% of the complexed limonene remaining in the beverage after 9 simulated months of storage.
The influence of the electrospinning process parameters on the encapsulation of (R)-(+)-limonene with poly(vinyl alcohol) (PVA) has been studied by Camerlo et al. [107]. It was found that while temperature increases the evaporation rate of limonene, humidity affects the permeability of the polymer fibers. Polymer concentration can also influence the EE of limonene. Greater EEs were obtained from 9% PVA/limonene emulsions compared to the other PVA lower contents [108]. Higher polymer concentrations caused either an increase in the viscosity of the emulsion or the polymer precipitation, decreasing or even preventing the possibility for the encapsulated limonene to diffuse.
Fuenmayor et al. used a combination of electrospinning and CDs complexation for the encapsulation of R-(+)-limonene in edible nanofibers obtained from pullulan and β-CD emulsions [95]. The critical role that plays the relative humidity on the limonene release was also highlighted by the authors. It was reported that release was taking place at values of water activity higher than 0.9. These results make this system interesting for active packaging applications, in particular for fresh foods, for which the risk of microbial degradation increases at high water activity conditions.
Interesting also to highlight the application of l-limonene as a green and non-toxic solvent alternative for the production of polystyrene (PS) fiber matrix by electrospinning, which is normally limited to organic toxic solvents that cause environmental problems and restrict its use in food-based applications [109].
Nanoemulsions have proven high potential application in the encapsulation of food ingredients and in the enhancement of the antimicrobial activity of EOs. Both, low- and high-energy methods has been reported for the encapsulation of limonene (Table 2).
Table 2. Examples of limonene’s encapsulation using low- and high-energy emulsifying methods.
|
Emulsification Method |
Highlighted results |
Ref. |
|
|
High-Energy |
|
||
|
High pressure homogenizer |
D-limonene/monosterin organogel (4% w/w) presented better antimicrobial activity than free D-limonene due to the higher solubility of encapsulated limonene. Small size nanoemulsion (36 nm) droplets can easily fuse with bacterial cells. |
[110] |
|
|
Sonication |
Nanoliposomes of D-limonene/soy or rapeseed lecithins (150 nm) were added to starch-sodium caseinate (50:50) film forming dispersions. Encapsulation prevented limonene evaporation. Antimicrobial activity against L. monocytogens was inhibited. |
[111] |
|
|
Microfluidization vs. Ultrasound |
Microfluidization produced droplets of 700–800 nm with the highest retention (86.2%) of d-limonene and minimum amounts of non-encapsulated oil at the surface of particles. |
[75] |
|
|
Low-Energy |
|
||
|
CPI |
Water/Tween 80/d-limonene system. Nanoemulsions stored at 28℃ were more stable than those stored at 4℃. |
[112] |
|
|
CPI |
D-limonene/nisin system showing synergistic effects against food-related microorganisms: S. aureus, B. subtilis, E. coli and Saccharomyces cerevisiae. |
[113] |
|
Sustainable emulsifying agents from plants such as mucilage from different seeds, Angum gum, sodium alginate or β-lactoglobulin have started to be used for the encapsulation of food oils and flavors such as limonene [114][115][116]. Despite the vast majority of work reported focuses on the use of conventionally based systems (surfactants and polymers), particle-stabilized emulsions also referred to as Pickering emulsions have also been used for Limonene’s stabilization. These type of emulsions are stabilized by an accumulation of dispersed particles (i.e., silica or cellulose nanocrystals) at the oil–water interface forming a mechanical barrier that protects emulsion droplets against coalescence and yields high EE% (79–100%) [117][118][119].
Nanocapsules of d-limonene were obtained from electrospraying an emulsion of Alyssum homolocarpum seed gum (AHSG) with 0.1% of Tween 20 [120][121]. Due to the less severe experimental conditions used during electrospraying, the EE achieved was greater than those reported for d-limonene encapsulation using other methods such as spray drying (environment 50% to 90%) [99][122].
An interesting approach for d-limonene encapsulation is by using recyclable porous materials (RPMs), which are highly porous and thermally stable 3D framework structures composed of 1D hydrophobic channels [123]. Two types of RPMs materials were compared in performance to encapsulate d-limonene with modified starch (Starch-CAP@DL). Both RPMs were able to absorb a large quantity of d-limonene (200 and 150 mg/g). RPMs demonstrated prolonged release (1.5 h) of d-limonene compared to Starch-CAP@DL (80% of d-limonene released instantly).
Finally, a quite innovative manner to impregnate or encapsulate limonene in modified starches is using supercritical CO2 via particle from gas saturated solutions or suspensions (PGSS) [124]. One of the best advantages of using supercritical fluid technology for encapsulating limonene is the use of relatively low temperatures, which enable the encapsulation of sensitive materials. The encapsulation efficiency of limonene was of 86% compared to conventional spray drying, which showed an efficiency of 91% for the same volatile.
Current studies have proved that limonene also represents a suitable ingredient for the agri-food industry. In particular, it has been demonstrated its strong antimicrobial activity against a broad-spectrum of pests a ecting crops and food-spoilage microorganisms, as well as antioxidant potential to avoid post-harvest decay along processing, storage/packaging processes and extending the shelf-life of food products. In addition, limonene has also shown significant phytotoxicity facing different weeds that represent an alarming hazard for agricultural production and ecology due to their rapid growth, high competitiveness and resistance development.
Its wide range of biological activities, together with its lack of toxicity and diverse mechanisms of action, make limonene a very interesting natural bio-alternative to synthetic pesticides and preservatives for a more sustainable emerging agri-food industry.
Further perspectives in the encapsulation of limonene could include delaying Limonene’s volatility, enhancing its beneficial properties and controlling its release; cost-efficiency analysis of the processes would be demanded to encapsulate limonene at the industrial scale.
This entry is adapted from the peer-reviewed paper 10.3390/molecules25112598