Revisiting lycopene extraction: Caprylic acid-based emulsion for the highest recovery

Abstract

In this study, we evaluated the caprylic acid-based oil-in-water (O/W) emulsion-assisted extraction of lycopene from tomatoes. Emulsion-assisted extraction was performed using two types of micron-sized O/W emulsions: (a) O/W emulsion with absence or (b) presence of 0.1% (w/w) of Tween 20 emulsifier. This green extraction technique was compared with the conventional method using soybean oil, tributyrin, and caprylic acid. The results show that caprylic acid, a green solvent, is significantly more effective for lycopene recovery than soybean oil and tributyrin. In the absence of an emulsifier, caprylic acid-based O/W emulsion significantly improved the lycopene content by 14.69 mg/g, corresponding to a 98.59% extraction efficiency at 50 ˚C. The capability of the proposed approach to lycopene recovery was explained in terms of lycopene affinity, the ability to swell the tomato cell, and some other standard parameters. In addition, caprylic acid has the significant advantage that once developed with the extracted lycopene can be used directly as a food additive.

Introduction

Lycopene (C₄₀H₅₆) is a natural carotenoid with an acyclic tetra-terpenic hydrocarbon having 13 carbon–carbon double bonds, 11 of which are conjugated in a linear fashion (Fig. 1). Because of this high degree of conjugation, lycopene has an excellent antioxidants effect, making it one of the most potent antioxidants (Zuorro 2020). Lycopene is particularly effective at quenching singlet oxygen (¹O₂), a highly reactive species capable of destroying various biological components such as lipids, proteins, and nucleic acids (Amiri-Rigi and Abbasi 2016). According to previous studies, its ability to quench singlet oxygen is twice and ten times higher than that of β-carotene and α-tocopherol, respectively (Deng et al. 2021). Moreover, epidemiological and clinical studies suggest that lycopene helps to prevent cardiovascular disease, cancer, metabolic syndrome, and neurodegenerative disorders such as Alzheimer’s, Parkinson’s, and Huntington’s diseases (Li et al. 2021; Zuorro 2020). As lycopene is a powerful antioxidant, it aids in the reduction of oxidative stress that is often correlated with the manifestation of aforesaid diseases. Moreover, the mechanisms other than antioxidative protection could also be correlated, such as lycopene can inhibit cell proliferation, induce apoptosis, and enhance intercellular gap-junctional communication. The above observations have increased the interest in producing lycopene-containing supplements, functional foods, and cosmetics. As a result, the lycopene market has grown rapidly in terms of size and value (Deng et al. 2021).

Fig. 1.

Chemical structure of lycopene

Lycopene was first discovered in tomatoes and other red vegetables and fruits (Li et al. 2021). The natural lycopene is mostly produced by extraction from tomatoes, and their by-products or pomace. Tomato is one of the richest agricultural crops with natural antioxidants, and it is often regarded as a functional food with multiple biological properties, which help to reduce the risk of certain types of cancer (prostate, lung, and stomach) (Eh and Teoh 2012). It contains a wide variety of antioxidants including vitamin E, ascorbic, phenolics, flavonoids, and carotenoids. Lycopene is a carotenoid, responsible for the deep red color of ripe tomatoes (Amiri-Rigi and Abbasi 2016). Different extraction methods for lycopene recovery have been already proposed. Economically, nearly all the routes have been determined to be infeasible for large-scale production. Solvent extraction is the most common method of lycopene extraction. However, the use of organic hazardous solvents, such as n-hexane, ethanol, acetone, and ethyl acetate, causes several risks, including toxicity, the presence of solvent traces in the final product, and disposal problems (Eh and Teoh 2012). Lycopene extraction can also be conducted using supercritical carbon dioxide fluid extraction, ultrasound-assisted extraction, microwave-assisted extraction, and enzyme-assisted extraction. However, the existing extraction methods are quite expensive, require a long process time, and need high energy demand (Deng et al. 2021).

Over the last few years, microemulsions have been applied for the extraction of various bioactive compounds. For example, canola oil was extracted from canola plant seed using a lecithin-based microemulsion system (Abbasi and Radi 2016). While, β-carotene and lycopene were extracted from carrot pomace and tomato pomace, respectively, using a microemulsion system (Amiri-Rigi and Abbasi 2019; Roohinejad et al. 2014).

In general, emulsions consist of at least one immiscible liquid, which is dispersed in another in the form of droplets, using an emulsifier. This method is considered a green extraction technique. It can integrate different bioactive compounds due to the presence of both lipophilic and hydrophilic domains in the system (Roohinejad et al. 2014). Moreover, the oil-in-water (O/W) emulsion contains a large amount of water, a protic solvent, which can both donate and accept hydrogen bonds. This behavior may assist to swell the cell wall of the tomato and increase lycopene recovery. The mechanism of the cellular swelling is mainly influenced by three solvent properties: molar volume, hydrogen bonding ability, and basicity (Mantanis et al.1994; Prusov et al. 2014). Moreover, a previous study reported that the protic or aprotic nature of the solvent has an impact on the swelling process of the plant matrix (Fidale et al. 2008).

Although using synthetic emulsifiers in the microemulsion-based extraction is helpful to improve the overall extraction efficiency through increasing the osmotic pressure, causing the cell expansion, many safety concerns are growing regarding their use in high concentrations in both food and cosmetic applications (Roohinejad et al. 2014; Flanagan et al. 2006). On the other hand, no reports have been found on the lycopene O/W emulsion-assisted extraction, in the absence of a synthetic emulsifier. This novel extraction approach may help in reducing the synthetic and hazardous substances in food, pharmaceutical, and cosmetic applications, enhancing their safety, as well as attracting the customers' acceptability.

Caprylic acid is one of the green solvents known for its capability to efficiently extract bioactive compounds. It has a higher swelling ability with lower molar volume, lower viscosity, higher polarity, and higher hydrogen bond capacity. These remarkable characteristics (Table 1) give caprylic acid, the advantage to increase the overall extraction efficiency. Furthermore, caprylic acid is a fatty acid dietary supplement that could have beneficial nutritional value (Nordoy et al. 1991). It is worth mentioning, to the best of our knowledge, no previous studies have been done to investigate the effects of caprylic acid-based oil-in-water emulsion in lycopene extraction.

Table 1.

Molecular properties and Hansen solubility parameters of solvents and lycopene

Compound	Mw (g/mol)	${\mathit{V}}_{\mathit{m}}$ (cm3/mol)	$\mathit{\eta }$ (mPa s)	${\mathit{\delta }}_{\mathit{D}}$ (MPa 0.5)	${\mathit{\delta }}_{\mathit{P}}$ (MPa 0.5)	${\mathit{\delta }}_{\mathit{H}}$ (MPa 0.5)	$\mathit{\delta }$ (MPa 0.5)	D (MPa 0.5)
Soybean oil	920.09	1003.2	39.1 ± 0.14a	16.51	2.01	2.71	16.801	3.47
Tributyrin	302.33	293.5	8.29 ± 0.27b	16.31	3.91	5.71	17.701	6.94
Caprylic acid	144.22	158.4	4.47 ± 0.14c	16.11	5.11	9.51	19.371	10.7
Water	18.015	18.01	-	15.54	16.04	42.34	47.804	45.2
Tween 20	1227.56	1115.9	-	14.97	9.47	13.37	22.077	16.3
Lycopene	536.878	604.2	-	15.68	08	08	15.608	-

Mw molecular weight, $V_m$ molar volume, $\eta$ viscosity, ${\delta }_D$ Hansen dispersion parameter,${\delta }_P$ Hansen polar parameter, ${\delta }_H$ Hansen hydrogen bond parameter $\delta$ total Hansen solubility parameter, D distance between points in Hansen space

Values having the different superscript letters in viscosity ($\eta )$ are significantly different (P < 0.05). The references are mentioned in superscript number for the respective values as reported in (de la Peña-Gil et al. 2016¹; Delgado Naranjo et al. 2021²; Eiteman and Goodrum 1994³; Hansen 2007⁴; Lee et al. 2008⁵; Ortiz-Tafoya and Tecante 2018⁶; Shakeel et al. 2021⁷; Zuorro 2020⁸; Zhang et al. 2014⁹)

Thus, in this work, we are proposing caprylic acid-based oil-in-water (O/W) emulsion-assisted extraction as a green and novel technique for the highest lycopene recovery from tomatoes. In addition, two other green solvents, soybean oil and tributyrin, were used for comparison purposes, which has a high affinity for lycopene (Table 1). Furthermore, the present work was aimed to study the extraction of lycopene from tomatoes using O/W emulsion in the absence or presence of a synthetic emulsifier (Tween 20). The effects of droplet size of O/W emulsion, the interfacial tension between soybean oil, tributyrin, caprylic acid, and water phase as well as different temperature conditions, were evaluated to observe the highest lycopene recovery.

Material and methods

Materials

Red ripe cherry tomatoes were bought from a local grocery store in Tsukuba, Ibaraki, Japan. Standard lycopene (> 90%), acetone, ethanol (99.5%), hexane, refined soybean oil, tributyrin (97%), caprylic acid, and polyoxyethylene 20 sorbitan monolaurate (Tween 20) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka Japan). Deionized water (18 MΩ cm) was produced by a Milli-Q system (Sartorius, Arium® pro, Goettingen, Germany).

Preparation of tomato powder

Tomatoes were washed with Milli-Q water and then freeze-dried (EYELA freeze-drier Shanghai, China) at -80 ˚C and 5 Pa until the moisture content reached approximately 7.6% wt. The dried tomatoes were then ground using mortar and pestle, passed through a 500 µm net sieve, and stored at -20 ˚C until further analysis.

Total lycopene quantification

The total lycopene content in the tomatoes was examined according to Fish et al. (2002) with some modifications. Briefly, 2 g of tomato powder was extracted with a 20 mL fresh mixture of hexane: ethanol: acetone (2:1:1) under continuous stirring (700 rpm) for 15 min at 25 ˚C, then 3 mL of Milli-Q water was added to the suspension and mixed for another 5 min. Thereafter, the suspension was allowed to stand at 25 ˚C for 5 min to spontaneously separate polar and nonpolar phases. The lower phase was re-extracted until the residue became colorless, while the upper phase was filtered through a hydrophobic membrane (0.45 μm) and analyzed for the total lycopene content.

Triglycerides and fatty acid-assisted extraction

Two g of tomato powder was added to soybean oil, tributyrin, or caprylic acid (30 mL) and extracted for 4 h at 25 ˚C. After centrifugation (Tomy MX-307, Tomy Kogyo Co. Ltd) at 9100 × g for 1 h at 25 ˚C and filtration (hydrophobic membrane, 0.45 μm) to remove the solid particles, the supernatants were analyzed for the lycopene content.

O/W emulsion-assisted extraction

O/W emulsion-assisted extraction was determined according to the method of Tsogtoo et al. (2020) with minor modifications. O/W emulsions were prepared by homogenizing 30 mL of the triglycerides or fatty acid phase (soybean oil, tributyrin, or caprylic acid) with 70 mL of the aqueous phase, absence or presence of 0.1% (w/w) Tween 20 emulsifier. The coarse emulsions were prepared using a rotor–stator homogenizer (Polytron, PT-3000 Kinematica-AG, Littace, Switzerland) at 7000 rpm for 5 min. Then, prepared emulsions (100 mL) were added with 2 g of tomato powder and stirred at 750 rpm for 4 h at different temperatures (25, 50, or 70 ˚C). After centrifugation (Tomy MX-307, Tomy Kogyo Co. Ltd) at 9100 × g for 1 h at 25 ˚C and filtration (hydrophobic membrane, 0.45 μm) to remove the solid particles, the supernatants were analyzed for the lycopene content.

Lycopene quantification

The lycopene content in tomatoes was determined using a UV spectrophotometer (V-530, Jasco Corporation, Tokyo, Japan) at 501 nm (Eh and Teoh 2012). A calibration curve (R² = 0.99) was prepared using standard lycopene with a concentration range of 0 ~ 0.05 mg/mL to estimate the lycopene content in the solvent (mg/mL). Then the lycopene content in tomatoes (mg/g) was calculated using Eq. (1):

$$Lycopenecontent\left(m,g,/,g\right)=\frac{C_{\mathit{UV}}}{m}\times V$$ 1

where, $C_{\mathit{UV}}$ is the lycopene content in the extract (mg/mL), $V$ is the volume of total extract (mL), and $m$ is the dry weight (g) of the tomato powder used (Diacon et al. 2021).

Lycopene extraction efficiency

The yield of lycopene extraction $Y\left(\%\right)$ from tomatoes was determined using Eq. (2):

$$Y\left(\%\right)=\frac{C}{C_t}\times 100$$ 2

where $C$ is the lycopene content in tomatoes (mg/g), $C_t$ is the total lycopene content in tomatoes (mg/g) (Amiri-Rigi and Abbasi 2019). In this study, 14.9 mg/g of total lycopene content in tomatoes was found. Previous studies investigated almost two times lower than our findings, such as 5.11 mg/g and 8.14 mg/g of lycopene from tomatoes and tomato pomace, respectively (Eh and Teoh 2012; Vági et al. 2007). However, the production variables such as tomato cultivar, cultivar conditions, and ripening stage have a considerable impact on lycopene content in tomatoes (Taoukis and Assimakopoulos 2010).

Measurement of viscosity

The viscosity of soybean oil, tributyrin, and caprylic acid was measured using a Vibro Viscometer (SV-10, A&D Company Ltd., Tokyo, Japan) at 25 ˚C. Thin sensors including sensor plates (vibrators) and a temperature sensor were absorbed into a sample that was collected in a 10 mL measuring vessel. Viscosity was then determined by detecting the electric current needed to resonate the sensor plates.

Measurement of droplet size

Volume mean diameter ($d_{4,3}$) of the emulsion in the absence of the Tween 20 emulsifier was measured using a static laser diffraction particle size analyzer (LS 13,320, Beckman Coulter, Brea, USA). Refractive indexes of water, soybean oil, tributyrin, and caprylic acid were 1.33, 1.47, 1.43, and 1.42, respectively. The average droplet diameter was defined as volume mean diameter ($d_{4,3}$), according to Eq. (3):

$$d_{4,3}=\frac{\sum n_i{d_i}^4}{\sum n_i{d_i}^2}$$ 3

where, $d_i$= diameter, and $n_i$ = the total number of droplets with a diameter of $d_i$. (Tsogtoo et al. 2020).

Measurement of interfacial tension

Interfacial tension was measured using the pendant drop method on a fully automatic interfacial tensiometer (PD-W, Kyowa Interface Science Co., Ltd., Saitama, Japan). Briefly, the Milli-Q water was loaded into a syringe, then soybean oil, tributyrin, or caprylic acid were placed into a glass cell, and a drop of water was made until it extended into its maximum volume inside the oil sample. A high-resolution camera was used; then the images of the drop and the interfacial tension were calculated using the Young–Laplace equation according to the size and the shape of the drop, and to the density difference between the two phases.

Statistical analysis

At least two duplicates of each experiment are carried out and the mean and standard deviation of the results are reported in this study. The analysis of variance (ANOVA) was used to compare the lycopene content and extraction efficiency under different conditions at a 95% confidence level (p < 0.05) using Statistic 8.1 software (Tallahassee, USA).

Results and Discussion

Effects of triglycerides and fatty acid on lycopene recovery

As shown in Fig. 2a, caprylic acid resulted in the highest lycopene content (8.14 mg/g) compared to soybean oil and tributyrin. The extraction efficiency of lycopene using caprylic acid was approximately 54.60%, compared to 33.05% or 42.01% using soybean oil or tributyrin, respectively. During an extraction process, a low solvent viscosity is usually correlated with an increased solvent migration through the matrix to increase extraction efficiency (Pierre et al. 2002). Therefore, the lowest viscosity of caprylic acid (4.47 mPa s) potentially allows the penetration of caprylic acid into the plant matrix, enhancing the lycopene recovery (Table 1). Moreover, it was found that caprylic acid in ethanol: water mixtures provide the best extraction yields of bioactive compounds from the coffee cherry pulp (Torres-Valenzuela et al. 2020). On the other hand, soybean oil showed the lowest effect on lycopene recovery, which can be ascribed to the high viscosity of soybean oil (39.1 mPa s) compared to tributyrin or caprylic acid (Viscosity: 8.29 or 4.47 mPa s, respectively) (Table 1). Previous studies also reported that oils with high viscosity might obstruct oil diffusion through the solid substrate mass and result in low extraction yields (Goula et al. 2017).

Fig. 2.

Extraction of lycopene from tomatoes using triglycerides and fatty acid under (a) the conventional extraction, (b) O/W emulsion-assisted extraction in the absence, and (c) presence of 0.1% (w/w) of Tween 20 emulsifier

Effects of O/W emulsion on lycopene recovery

Figure 2b shows the lycopene content and extraction efficiency using O/W emulsion-assisted extraction in the absence or presence, respectively, of 0.1% (w/w) of Tween 20 at 25 ˚C. In the absence of an emulsifier, the lycopene content was approximately 13.72 mg/g of tomatoes using caprylic acid-based O/W emulsion, corresponding to 92.08% extraction efficiency. On the other hand, caprylic acid-based O/W emulsion in the presence of emulsifier resulted in a lower recovery of approximately 63.15% (Fig. 2c). This reduced recovery can be explained by the lower interaction between lycopene and triglyceride or fatty acid droplets in the O/W emulsion extraction system. When triglyceride or fatty acid droplets were covered by an emulsifier, it creates a barrier over the solvent, which reduces the interaction between the tomato powder and the solute (Tsogtoo et al. 2020). Overall, it can be concluded that caprylic acid is the best solvent for the extraction of lycopene in both conventional and O/W emulsion-assisted extraction systems and the addition of emulsifier hinders the partition of lycopene from the plant material into the solvent.

Hildebrand solubility, for solubilization in extraction system, is often used to evaluate the performance of solvents in both conventional and O/W emulsion extraction methods. Hildebrand's classic solubility parameter is used to determine the energy needed to generate space in the molecules to fit other molecules (Hansen 2007). Therefore, the comparison of solubility parameters of lycopene and the solvents was used to estimate the contribution of solvent-lycopene affinity for the highest recovery. It is known that closure the solubility parameters of two components, the higher their affinity. On the other hand, the molecular size of the solvent affects the Hildebrand solubility as well.

According to the classic solubility theory of Hildebrand and Scott (1964), the solubility parameter of a molecule (δ) is equal to the square root of the cohesive energy density. Therefore, the cohesive energy (δ) can be written as:

$$\delta =\sqrt{\frac{\mathrm{\Delta }E}{v_m}}$$ 4

where $\mathrm{\Delta }E$ is the cohesive energy of that molecule and $v_m$ is the molar volume.

Hansen proposed that the entire cohesive energy can be divided into three components: dispersion energy, polarity energy, and hydrogen-bonding energy (Hansen 2007). As a result, the total solubility parameter can be expressed as:

$${\delta }^2={\delta }_D^2+{\delta }_P^2+{\delta }_H^2$$ 5

where ${\delta }_D$, ${\delta }_P,$ and ${\delta }_H$ are the dispersion, polar, and hydrogen-bonding solubility parameters, respectively. For O/W emulsion in the absence or presence of an emulsifier, the solubility parameter can be determined using the following equation:

$${\delta }_{O/Wemulsion}=\sum _i{\phi }_i{\delta }_i$$ 6

where ${\phi }_i$ represent the volume fraction of the $i$ th component in the O/W emulsion and ${\delta }_i$ is its Hansen solubility parameter.

Furthermore, the affinity of a solute (A) for a solvent (B) can be expressed as the distance (D) in Hansen space. This distance can be measured by calculating between the points corresponding to the solute ${(\delta }_{D,A}{,\delta }_{P,A},{\delta }_{H,A})$ and the solvent${(\delta }_{D,B}{,\delta }_{P,B}{,\delta }_{H,B})$, as follows:

$$D=\sqrt{{{(\delta }_{D,A}-{\delta }_{D,B})}^2+{{(\delta }_{P,A}-{\delta }_{P,B})}^2{{(\delta }_{H,A}-{\delta }_{H,B})}^2}$$ 7

According to Eq. 7, the molecular interactions between A and B can be shown by distance. With a small D-value, the molecular interactions are similar, which demonstrates high affinity.

Table 1 shows the solubility parameters and D-values for lycopene and the solvents. The most efficient caprylic acid for recovery of lycopene showed relatively high D-values (D = 10.7 MPa^0.5) (Table 1). As a result, the affinity of lycopene for caprylic acid is lower than other solvents. This inconsistent phenomenon was also investigated in previous research, where the most effective solvent mixture for lycopene had a relatively large D value (Zuorro 2020). As a result, other variables than solute–solvent affinity could impact the extraction process. A previous study reported that although a low-distance solvent is present inside Hansen's dissolving sphere, it does not dissolve the solute. This may be due to the solvent's large size, which prevents it to penetrate the solute (Yamamoto et al. 2017). Moreover, Hildebrand’s solubility parameter theory also points to smaller molar volume solvents as being better than those with larger molar volumes, even though they may have identical solubility parameters. Therefore, solvent molecular size could be an essential fourth parameter in Hansen solubility (Hansen 2007). The above considerations suggest a possible explanation of the results in both conventional and O/W emulsion-assisted extraction methods. As shown in Table 1, caprylic acid has a small molecular size compared to tributyrin and soybean oil. The small molecular size of caprylic acid helps to penetrate the plant matrix which enhances lycopene recovery. Moreover, the aforementioned inconsistent phenomenon can be explained using the solvent’s swelling ability. In cellulosic material, cellulose is arranged as microfibrils, which comprise both crystalline and amorphous regions. Microfibrils are formed into larger diameter fibers that are cross-linked by hemicelluloses and immersed in a gel-like pectic matrix (Lavecchia and Zuorro 2016). Intramolecular and intermolecular hydrogen bonding control the degree of cellulose crystallinity and the spatial structure of the cellulose/hemicellulose network. These bonds are produced by hydroxyl groups in the cellulose β-1, 4-linked D-glucopyranose units (Grunin et al. 2015). The small size and high polarity of solvent molecules can penetrate the plant matrix and adsorb to these hydroxyl groups. Some bonds are disrupted as a result of adsorption, increasing the space between cellulose fibers and causing the substance to swell (Prusov et al. 2014). In most cases, swelling is usually limited to the amorphous portions of cellulose, which are more reactive and accessible to solvent. As mentioned previously, the process of swelling is mainly affected by three solvent properties: molar volume, hydrogen bonding ability, and basicity (Mantanis et al. 1994; Prusov et al. 2014). The solvent's protic or aprotic nature also plays an important role in the swelling process. (Fidale et al. 2008). This is because protic solvents, such as water, can both donate and accept hydrogen bonds.

In the case of O/W emulsion extraction, the swelling behavior is increased significantly in the presence of a huge amount of protic solvent water. For the caprylic acid-based emulsion in the absence of an emulsifier, it can be assumed that the presence of protic solvent water allows the tomato cell to swell. This swelling assists caprylic acid to penetrate the plant matrix which increases the extraction efficiency of lycopene (92.08%) (Fig. 2b). According to Table 2, a significant increase in hydrogen bond capacity (32.46 MPa^0.5) was obtained for caprylic acid-based O/W emulsion compared with conventional conditions (9.5 MPa^0.5, Table 1), which contribute to the increased lycopene recovery. In the case of using an emulsifier for preparing the caprylic acid-based O/W emulsion, recovery of lycopene was lower compared to the absence of an emulsifier. It is possible that caprylic acid-based O/W emulsion with 0.1% (w/w) Tween 20 has a lower ability to swell the plant matrix but it was found that the addition of this small amount of emulsifier did not appreciably reduce the hydrogen bond capacity of the emulsion (Table 2). The reduction in the lycopene recovery using Tween 20 emulsifier could be associated, therefore, with the formation of a physical barrier on the droplets’ interface, which reduced the ability of lycopene to penetrate inside of the core of the triglycerides and fatty acid droplets (Amiri-Rigi and Abbasi 2016).

Table 2.

Hansen solubility parameters of triglycerides and fatty acid-based O/W emulsion absence or presence 0.1% (w/w) Tween 20 emulsifier

Types of O/W emulsion	Compound	${\mathit{\delta }}_{\mathit{D}}$ (MPa 0.5)	${\mathit{\delta }}_{\mathit{P}}$ (MPa 0.5)	${\mathit{\delta }}_{\mathit{H}}$ (MPa 0.5)	$\mathit{\delta }$ (MPa 0.5)	D (MPa 0.5)
Absence of emulsifier	Soybean oil	15.80	11.80	30.42	36.25	32.62
	Tributyrin	15.74	12.37	31.32	37.17	33.67
	Caprylic acid	15.68	12.73	32.46	38.23	34.86
Presence of emulsifier	Soybean oil	15.79	11.79	30.39	36.23	32.60
	Tributyrin	15.73	12.36	31.29	37.15	33.65
	Caprylic acid	15.67	12.72	32.43	38.21	34.84

${\delta }_D$ Hansen dispersion parameter,${\delta }_P$ Hansen polar parameter, ${\delta }_H$ Hansen hydrogen bond parameter, $\delta$ total Hansen solubility parameter, D distance between points in Hansen spa

Overall, the effects of caprylic acid-based O/W emulsion on the extraction of lycopene from tomatoes can be summarized as follows: a) the small molecular size of caprylic acid helps to penetrate the plant matrix which enhances lycopene recovery, and (b) the presence of a high volume of protic solvent (water), which enhances the swelling of the plant matrix by forming hydrogen-bond complexes with the molecules of the plant cell wall.

Effects of temperature, droplet size, and interfacial tension on lycopene recovery

We investigated the effect of temperatures, droplet size, and interfacial tension on the lycopene content and the recovery from tomatoes using caprylic acid-based O/W emulsion in the absence of an emulsifier as a model emulsion-assisted extraction system. As shown in Fig. 3, the lycopene content and extraction efficiency increased upon increasing the extraction temperature from 25 to 50 ˚C. However, the lycopene content and extraction efficiency decreased after 50 ˚C. The highest lycopene content of 14.69 mg/g was obtained at 50 ˚C, corresponding to the extraction efficiency of approximately 98.59% (Fig. 3). Dehghan-Shoar et al. (2011) also found that the optimal temperature for lycopene recovery was 50 ˚C. However, any further increase in the extraction temperature reduced the extraction efficiency (53.02%) of lycopene from tomatoes. Previous investigations have shown similar results which reported that temperatures higher than 65 ˚C resulted in increased β-carotene degradation (Baysal et al. 2000).

Fig. 3.

Effects of temperature on the extraction of lycopene from tomatoes using caprylic acid-based O/W emulsion in the absence of emulsifier

Figure 4a shows the effect of droplet size of O/W emulsion in recovering lycopene. Caprylic acid-based O/W emulsion showed the smallest droplet size of 16.11 µm compared to the droplet size of 48.04 µm or 32.73 µm of soybean oil or tributyrin-based O/W emulsion, respectively. This smallest droplet size can be explained by the lower interfacial tension between caprylic acid and water (7.59 mN/m) (Fig. 4b). On the other hand, the interfacial tensions of soybean oil and tributyrin with water are 27.91 mN/m, and 19.75 mN/m, respectively, which are relatively higher than the interfacial tension between caprylic acid and water. Previous studies reported that the small droplet size of O/W emulsion increases the extraction recovery of carotenoids, in accordance with our findings (Tsogtoo et al. 2020), which can be attributed to the increase of the emulsions’ surface area, resulting in higher contact with the plant matrix.

Fig. 4.

Effects of (a) droplet size of soybean oil, tributyrin, and caprylic acid-based O/W emulsion in the absence of emulsifier and (b) interfacial tension between soybean oil, tributyrin, caprylic acid, and water phase on the extraction of lycopene from tomatoes

Conclusion

In this study, lycopene was extracted from tomatoes using conventional and O/W emulsion-assisted extraction. In general, O/W emulsion was more efficient for extracting lycopene compared to crude triglycerides or fatty acid. The swelling ability of the O/W emulsion in the absence of the emulsifier system helps to swell the cell wall resulting in improved recovery. Moreover, it was found that the addition of an emulsifier may prevent lycopene from partitioning to the triglycerides or fatty acid. The lycopene recovery was also influenced by the droplet size of the emulsion, interfacial tension, and temperature. As for the solvent, a non-toxic, environmentally friendly, caprylic acid was found to be the best for lycopene recovery. Therefore, caprylic acid-based O/W emulsion extraction in the absence of an emulsifier can be a promising green technique for lycopene recovery that may consequently be utilized in dietary nutritional supplements and food additives. Further research should be carried by focusing on the molecular behavior of O/W emulsion in the absence or presence of an emulsifier and their effects on the extraction process of lycopene.

Abbreviations

O/W emulsion

Oil-in-water emulsion

w/w

Weight-per-weight basis

$C_{\mathit{UV}}$

Lycopene content in the extract (mg/mL)

$V$

Volume of total extract (mL)

$m$

Dry weight of tomato powder (g)

$Y$

Yield of lycopene extraction (%)

$d_{4,3}$

Volume mean diameter

D

Distance in Hansen space (MPa^0.5)

δ

Solubility of a molecule

$\mathrm{\Delta }E$

Cohesive energy

$v_m$

Molar volume

${\delta }_D$

Dispersion-bonding solubility

${\delta }_P,$

Polar -bonding solubility

${\delta }_H$

Hydrogen-bonding solubility

Authors' contributions

KK experimental design, carried out the experiments, and wrote the manuscript. SE and NT analyzed the data and edited the manuscript. MSRS analyzed the data. IK designed the experiment. MN conceiving the idea, and interpretation of the results. MAN experimental design, supervised the work and edited the manuscript. All authors read and approved the final manuscript.

Funding

The authors have not disclosed any funding.

Availability of data and material

All data generated or analyzed during this study are included in this published article.

Code availability

Not applicable.

Declarations

Conflict of interest

The authors declare that they have no competing interests.

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