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ABSTRACT. The purpose of this study was to compare two extraction processes for the production of lemon liqueur (limoncello)—the traditional maceration of ...
Journal of Food Process Engineering ISSN 1745–4530

COMPARISON BETWEEN THE KINETICS OF CONVENTIONAL MACERATION AND A CYCLIC PRESSURIZATION EXTRACTION PROCESS FOR THE PRODUCTION OF LEMON LIQUEUR USING A NUMERICAL MODEL DANIELE NAVIGLIO1,6, ANDREA FORMATO2, MANUELA VITULANO1, IMMA COZZOLINO1, LYDIA FERRARA3, EVERTON FERNANDO ZANOELO4 and MONICA GALLO5 1

Department of Chemical Sciences, University of Naples Federico II, 80126 Naples, Italy Department of Agriculture, University of Naples Federico II, 80055 Portici, Naples, Italy 3 Department of Pharmaceutical and Toxicological Chemistry, Faculty of Pharmacy, University of Naples, Via Domenico Montesano 49, 80131 Naples, Italy 4 Department of Chemical Engineering, Polytechnic Center (DTQ/ST/UFPR), Federal University of Paran a, Jardim das Am ericas, 81530-990, Curitiba, Parana, Brazil 5 Department of Molecular Medicine and Medical Biotechnology, University of Naples Federico II, 80131 Naples, Italy 2

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Corresponding author. TEL: 139-81-674-063; FAX: 139-81-674-063; EMAIL: [email protected] Received for Publication August 12, 2015 Accepted for Publication December 21, 2015 doi:10.1111/jfpe.12350

ABSTRACT The purpose of this study was to compare two extraction processes for the production of lemon liqueur (limoncello)—the traditional maceration of lemon peels and a cyclically pressurized extraction process also known as rapid solid-liquid dynamic extraction (RSLDE) that uses a Naviglio extractor. To evaluate the extraction efficiency of the two processes, dry matter obtained by the evaporation of the extracts was used to construct kinetics curves. The dry matter was directly proportional to the amount of the active principle (terpenes) extracted and consequently to the total concentration. Alcoholic extracts were analysed by gas chromatography (GC) to monitor the extraction kinetics for major components of the terpene fraction and for minor components of essential oils. Moreover, alcoholic extracts were analysed by UV spectrophotometry to identify the more abundant chemical species, while the organoleptic tests (i.e., a consumer test) performed on the final product (limoncello) provided an indication of the taste of the final product and revealed features undetectable with instrumental analytical techniques. To better understand the phenomenon considered, a numerical simulation was performed to evaluate and compare the matter flow of extractable compounds during the process.

PRACTICAL APPLICATIONS The mathematical model shown in this work is a scientific novelty because it is not reported in the previous literature. Indeed, the curves obtained from the kinetic analysis of the two solid-liquid extraction procedures were interpolated to obtain a numerical model, which regulated the two extraction processes. Comparison between the kinetics of the extraction procedures showed that the rapid solidliquid dynamic extraction (RSLDE) process was 120 times faster than maceration and had a greater efficiency in a short time. Finally, a method proposed to follow the kinetics of the dilution of alcohol by water contained in the peels using the Karl Fisher titration method was important for establishing the alcoholic content of the beverage.

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INTRODUCTION Lemon liqueur, commonly called “limoncello,” is a typical lemon-flavored alcoholic beverage from the Campania region of Italy. It has become increasingly popular both in Italy and abroad and is appreciated for its aroma and taste as well as for its digestive properties (Di Giacomo et al. 1991; Kimball, 1991; Di Giacomo and Mincione, 1994; Versari et al. 2003). The cultivation of citrus, especially lemons, is greatly important in Southern Italy (Di Giacomo et al. 1991; Di Giacomo and Mincione, 1994). The production of “limoncello” from lemon peels represents a considerable source of revenue, especially in tourist areas such as Capri, the Sorrento peninsula and the Amalfi coast. Currently, almost the entire production of lemons in these areas is used to make lemon liqueur (Di Giacomo et al. 1991; Di Giacomo and Mincione, 1994). Because the process is technologically simple, production is largely carried out by small firms that are often family-run. The production process for “limoncello” includes the following phases: (1) washing the fresh lemons; (2) peeling the fruit by hand; (3) maceration of the lemon peel in ethyl alcohol for 7–10 days at room temperature; (4) filtration of the alcoholic extract; (5) dilution of the extract with an appropriate amount of sugar solution; and (6) bottling. The procedure for production of lemon liqueur is the same for both small scale (homemade limoncello) and industrial scale production. The most timeconsuming phase of the process is the maceration of the lemon peels in ethanol. In this phase, the essential oils (terpenes, aldehydes, hydrocarbons etc.), are released from the peels into the alcohol together with dyes and water, and the alcohol simultaneously migrates into the interior of the peel (Dugo et al. 2000; Versari et al. 2003; Crupi et al. 2007). The process stops when the chemical potential of each component in the liquid phase is equals to the chemical potential of the same component in the solid phase, applied to all components in analysis. According to tradition, a time shorter than 7 days (168 h) cannot guarantee the complete extraction of the essential oils from the peel and cannot ensure the production of a satisfyingly aromatic liqueur. Generally, as happens in laboratory scale production, no controls are run to verify when the extraction of essential oils is complete during the industrial maceration process, and the termination of the process is governed by experience and conventional application. It is evident that once equilibrium is reached, extending the extraction is a waste of time and increases the cost. Moreover, the risk of transferring bad flavours increases because of the prolonged contact of the lemon peel with the alcohol. In this study, conventional maceration (CM) was compared with an innovative rapid solidliquid dynamic extraction (RSLDE) process that relies on cyclic pressure application in a solid-liquid system (Naviglio, 2003). Therefore, the aim of this study was to follow the 2

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kinetics of the alcoholic extraction of essential oils from lemon peel performed by applying the two solid-liquid extraction procedures. Dry matter was used as a parameter for kinetics, together the absorbance at a fixed wavelength of the extract and organic compounds that were analysed by GC. A comparison between the results of maceration and the RSLDE process was obtained in this way. Moreover, we also proposed a method to follow the kinetics of the dilution of alcohol by the water contained in the lemon peel, which was important to establish the alcoholic content. An evaluation of the alcohol exchange between the lemon peel solid matrix and the alcohol during the RSLDE process was obtained using a numerical model. The analytical procedures performed were very simple to apply, so they could be used by companies producing lemon liqueur that have modestly equipped chemical laboratories. The maceration is a classical solid-liquid extraction technique that is widely used. It consists in covering the solid to be extracted with the liquid and allow to stand the system in this state for a prolonged time. The extraction is done at ambient temperature and therefore there is no alteration of thermolabile compounds; however, the extraction times are long on average for the fact that the extraction takes place primarily by diffusion effects, so that it requires to stir the system from time to time to promote the diffusion of the compounds extracts and avoid a localized supersaturation in the immediate vicinity of the surface of the solid to be extracted, which leads to a slowing down of the extraction process overall. A valid alternative to the method of maceration is represented by a technology of solid-liquid extraction innovative, that uses the Extractor Naviglio. This instrument uses a modern solidliquid extraction system that by means of alternation of cycles of pressure and depression, allows the extraction of the test compounds without altering the chemical characteristics.

MATERIALS AND METHODS Instrumentation and Chemicals A UV-VIS 1601 mod. Spectrophotometer (Shimadzu, Tokyo, Japan) equipped with a 1 cm optical path couvette; an Auto System XL gas chromatograph equipped with a programmable split-splitless injector and a flame ionization detector (Perkin Elmer, Norwalk, CT); a capillary column Rtx-5, stationary phase 5% diphenyl and 95% dimethyl silicone, with l 5 30 m, i.d. 5 0.25 mm, and f.t. 5 0.25 micron (Restek, Bellefonte, PA); a gas chromatograph mod. 17A equipped with a split-splitless injector and interfaced with a mass spectrometer mod. QP-5000 (Shimadzu, Tokyo, Japan); a capillary column SPB-5, with l 5 60 m, i.d. 5 0.25 mm, and f.t. 5 0.25 micron (Supelco, Bellefonte, PA); and a Karl Fisher water titrator mod. KF 2026 (Crison Instruments, Baar, Switzerland) were used. Ethyl alcohol 95% (v/v) C 2016 Wiley Periodicals, Inc Journal of Food Process Engineering 00 (2016) 00–00 V

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(Fluka, Dublin, Ireland), Hydranal Titrant 5 and Hydranal Solvent (Riedelde Haen, Seelze, Germany), n-dodecane, limonene, 3-pinene, y-terpinene, a-pinene, sabinene, and geranial were all analytical grade (Fluka, Dublin, Ireland).

From this initial total volume (Vpt), the volume of the accessory masses (Va), which was previously determined, was subtracted to give the real volume (Vp) of the sample, which was determined as follows: Vp 5 (Vpt2Va).

Gas Chromatographic Conditions

Procedure of Extraction Process

The conditions for the Perkin Elmer Auto System XL gas chromatograph were as follows: the carrier gas (helium) flow-rate was 2.0 mL/min; the split ratio was 1:60; the oven was programmed at 75C for 8 min, raised in 8C/min increment to 240C and held for 5 min at 240C; the injector was programmed at 60C for 12 s, raised in 900C/min increment to 250C and held for 3 min; the detector temperature was 260C; and a Rtx-5 capillary column was used. The conditions for the Shimadzu GC-17A gas chromatograph were as follows: the carrier gas (helium) flow rate was 1.5 mL/min; the split ratio was 1:80; and the temperature program was the same as above. A SPB-5 capillary column was used. The MS operating conditions were a scan mode (TIC) from 20 to 250 amu at 2.9 scan/s, ionization by El at 70 eV by autotuning and an ion source temperature of 250C.

After the determination of the initial weight and volume, the lemon peel samples were subjected to a conventional extraction process (CM) (T) or to an extraction process by means of a rapid solid-liquid dynamic extractor (RSLDE) (I). The weight and volume parameters were determined again at the end of the extraction process. The samples treated with process (T) were macerated in a 1 L volume closable container with 500 mL of ethyl alcohol at room temperature (20 6 1C). The samples treated with process (I) were subjected to programmed cycles of pressure applied to the ethyl alcohol in contact with the lemon peels. A total of 30 cycles (with a maximum pressure of 10 bar) over a period of 2 h was used. The number of hits in the dynamic phase (nd) was 12; the duration of the dynamic phase (td) was 2 min; and the duration of the static phase (ts) was also 2 min. Three replicates of all the experiments were performed, and the averages were used for numerical modelling and analyses.

Samples The lemons were washed with water then peeled with a specific tool. Ovale of Sorrento lemon peels were weighed (300 g) with a Gibertini Europe 1700 balance to an accuracy of 0.1 g and stored at 10C in plastic containers that were hermetically sealed to prevent changes in the moisture content. The lemon peels were also aerated and mixed several times for 2 min. Prior to extraction, the moisture content Mi of the lemon peel was determined by an oven method at 105C for 36 h (AOAC, 1984).

Determination of the Size of the Lemon Peels The size of the solid matrix of lemon peel pieces, including the length, width, and thickness, were measured with a digital calliper before and during the process of solid-liquid extraction. From these values, the sphericity and geometric mean diameter (GMD) of the solid matrix of the lemon peels were determined (Jain and Bal, 1997). The values obtained were used in numerical simulations for the construction of a mathematical model for the process.

Determination of Total Volume To evaluate the volume without changes in the moisture content, the solid matrix of lemon peels was sealed with plastic material (food-grade polyethylene) that adhered perfectly to the shape of the solid matrix, and the volume was measured as the initial total volume (Vpt) by immersion for a few seconds in 500 mL of water in a 1000 mL graduated cylinder. These measurements were carried out at temperature of 20 6 1C. C 2016 Wiley Periodicals, Inc Journal of Food Process Engineering 00 (2016) 00–00 V

Lemon Peel Sample Preparation Experiments were performed to evaluate the solute extraction from the solid matrix for the two extraction processes. A solid matrix of lemon peels was chosen. The samples consisted of the raw material used by a famous “limoncello” liqueur farm in Sorrento, Italy. The samples had been stored at 220C in tightly sealed plastic containers to prevent changes in the moisture content and were mixed several times for 2 min. Before the extraction process, the concentration content (Mi5Ci) of the solid matrix of the lemon peel types was determined (ASAE, 1997; Pereira, 1999).

Determination of the Dry Residue of the extract Alcohol at 105C A 10 mL aliquot of an alcoholic extract previously filtered on paper was transferred to a calibrated porcelain dish, and the alcoholic extract was evaporated to dryness in an oven at 50C. When the volume of the liquid had become negligible, the temperature was raised to 105C. The capsule was transferred to a desiccator, cooled to room temperature, and weighed. This operation was repeated until a constant weight was attained. The weight of the dry matter was calculated by the difference with the tare. Each determination was repeated three times, and the average was reported. After the determination of the initial weight and volume, the samples were subjected to the extraction processes (I) and (T). Determinations of the weight and volume were performed as described above every 20 min for an hour for 3

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samples treated with process (I), while the determinations were performed every two hours for six times and then at 24, 48, and 72 h for samples treated by the traditional extraction process (T). The samples processed by method (T) were soaked in 500 mL of alcohol in a 1 L closable container at room temperature, simulating the industrial process while maintaining a solid/liquid ratio of 1:3. The samples processed by method (I) were subjected to programmed cycles of pressure applied to the alcohol in contact with the product, as described above, while maintaining the same solid–liquid alcohol ratio. All experiments were repeated three times, and the maximum difference between the results obtained was never more than 5%, so the averages were used for numerical modelling and analyses.

Sensory Test A sensory analysis using a consumer test was also conducted on the final beverage to determine whether differences in the perception and pleasantness of the dried product that had been processed by methods (I) and (T) existed. An analytical and numerical analysis was also conducted to gain a better understanding of the phenomenon of alcohol diffusion into a solid matrix under the operating conditions considered. The results of the experiments were compared with results obtained using analytical and numerical approaches described in the current literature that had been successfully applied to other foods.

Traditional Maceration Procedure (T) Extraction of essential oils from lemon peels with ethyl alcohol 1. Wash and dry the lemons. 2. Peel the lemons, separating the albedo from the flavedo as much as possible. 3. Place the appropriate amount of lemon peel in a glass container (300 g in 1000 mL of ethanol 95% v/v). 4. Cover the solution and keep it at a constant temperature of 20C. 5. Macerate, stirring from time to time. Transfer the raw material into two sterile bags with a label indicating the procedure (traditional), the starting and ending time of the extraction process, the product, quality, batch, and date of sampling.

Determination of the Kinetics of the Extraction of Essential Oils from the Peel by Spectrophotometry At predetermined intervals, shake the container, withdraw a sample of the extract, and measure the absorbance at 400 nm without diluting; draw another part, and measure the absorbance at 275 nm diluting sample 1:10 with ethanol 4

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(95% v/v). Plot the absorbance against the extraction time for each wavelength.

Determination of the Kinetics of the Extraction of Essential Oils from the Peel by GC At pre-determined intervals, shake the container, withdraw 1.00 mL of the extract, and add 1.00 mL of an ethanol solution containing 500 mg/L of n-dodecane as an internal standard (IS). Analyse the solution by GC. Report the ratio between the peak areas of the major compounds such as limonene, beta-pinene, and gammaterpinene and the peak area of the IS. Report the ratio between the peak areas of the minor compounds such as alpha-pinene, sabinene, and geranial and the peak area of the IS. This analysis allows to follow kinetics of extraction on the basis of essential oil extracted from lemon peels.

Determination of the Kinetics of the Dilution of Ethanol by Water Contained in the Peel During the Essential Oil Extraction Step Shake the container, then transfer 100 lL of the alcoholic extract to the Karl Fisher titrator, and determine the water content. Measure the water content of the extract every 15 min for the first 2 h, then every hour thereafter. Plot the water content against the extraction time.

Procedure for Using the Naviglio Extractor (I) First Phase. The first phase of the procedure with the Naviglio extractor was identical to that of the traditional procedure because the raw material (300 g) initially sampled was split into two batches. Second Phase. 1. Remove a sample (300 g) of the raw material from the Naviglio extractor at the end of the extraction process. 2. Transfer the raw material into a sterile bag with a label indicating the Naviglio procedure, ending time of the extraction, initial time, final time, product, quality, batch, and date of sampling. Subsequently, the results obtained were compared with methods (T) and (I). For a better understanding of the extraction phenomenon, analytical and numerical approaches already available in the current literature were considered (Carillo et al. 2011; Formato et al. 2012, 2014; Naviglio et al. 2013).

Analytical Approach It is assumed that a diffusion process at a constant temperature follows Fick’s second law of diffusion. For an C 2016 Wiley Periodicals, Inc Journal of Food Process Engineering 00 (2016) 00–00 V

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axisymmetric diffusion, Fick’s three-dimensional (3D) equation is given as follows:  2  @M @ M @2M @2M 5D 1 2 1 2 @t @x2 @y @z

(1)

where M is the instantaneous concentration of the solute extract in the solid matrix at a specified time t and D is the diffusion coefficient. A solution using the above equation for an object having a spherical shape with radius r was presented by Crank (1975) as follows:  X     M2Mi 6 1 1 2 2 t MR5 512 2 p exp 2Di Me 2Mi p i51 i2 r2

MR5

M2Mi Me 2Mi

(2)

(3)

where M is the instantaneous concentration content at a specified time t     1   6 2 t 2 t MR512 2 exp 2Dp 2 1 exp 24Dp 2 p r 4 r (4) where MR is the concentration ratio and Mi and Me are the initial and equilibrium concentrations of the object during the extraction process, respectively. In most cases, only a finite number of terms of Eq. (2) are used to estimate the MR values. In fact, most researchers use only the first two terms of this equation. In this study, the experimental MR values were calculated at specific time intervals and used as an input to the curve fitting tool box of MATLAB (R2006a) software. The diffusion coefficient of the solid matrix, D, was estimated at time t. Because the general shape of a typical lemon peel solid matrix is closer to an oblate ellipsoid than a sphere, the diffusion coefficient in Eq. (1) should be adjusted. Gaston et al. (2004) presented a procedure to estimate the diffusion coefficient for an ellipsoid (De) using the following equation: De 5fe 2 3D

(5)

where fe is the sphericity factor of the ellipsoid (Gaston et al. 2002). Then, the diffusion coefficient D in Eq. (2) should be replaced by the calculated De. For the best understanding of the cyclically pressurized extraction process (I), a numerical simulation was obtained using the ANSYS 15 software, which allowed an evaluation of the alcohol exchange between the lemon peel solid matrix and the alcohol during the process. The C 2016 Wiley Periodicals, Inc Journal of Food Process Engineering 00 (2016) 00–00 V

finite element model was used to numerically determine a time-dependent concentration distribution at specific times. The grain shape was considered as an ellipsoid. Due the symmetry of the problem, a two-dimensional domain considering only a quarter of the projected area as a spheroid with the same volume as the lemon peel piece was assumed. Moreover, the chemical and physical behavior was defined through the specification of the following parameters:  effective diffusion coefficient for a spheroidal geometry;  initial essential oil content of the lemon peel piece;  equilibrium essential oil content of the lemon peel piece;  total time. The domain was discretized using 1400 2D 8-node PLANE238 diffusion elements, with axial symmetry and one degree of freedom (i.e., concentration) at each node. Six static linear transient analyses were performed, one for each of the six species involved. For a certain value of the initial concentration content and for the proper diffusion coefficient of the material, the analysis evaluated the time-dependent concentration distribution at the nodes. A similar approach has previously been used to determine the moisture and temperature fields during the cooling process in the cryomaceration of grapes (Carillo et al. 2011). In a finite element approach, an estimated value for D is supplied in Eq. (1), and the MR values are numerically determined for specified locations at given time intervals. In applying this approach, the following assumptions were made for the individual solid matrix: 1. The diffusion coefficient is independent of the concentration. 2. The solid matrixes are isothermal, allowing heat transfer during the extraction process to be neglected. 3. The solid matrixes are homogeneous and isotropic. 4. Throughout the extraction process, the surface of a solid matrix maintains a saturation concentration (the boundary condition). 5. The solute initial content of the solid matrix is constant and uniformly distributed within the solid matrixes (the initial condition). The commercial finite element analysis software ANSYS (Rel.15, 2014) was used to evaluate instantaneous solute distribution at the nodes. The diffusion coefficient estimated by fitting Eq. (2) to the experimental data was provided to the software, and the concentration content of each node was calculated at one-second time steps. The overall concentration content 5

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RESULTS AND DISCUSSION The Extraction Kinetics of the Essential Oils From Lemon Peel by Spectrophotometry

FIG. 1. ABSORBANCE VALUES OF ALCOHOLIC EXTRACT AGAINST EXTRACTION TIME AT TWO WAVELENGTHS (k 5 275 AND k 5 400 NM)

for a solid matrix was also calculated at every range of time considered by averaging the moisture contents of the nodes. All these steps were performed for the extraction process (I).

The extraction kinetics of the essential oils in ethanol can be determined by instrumental measurements. The absorption spectrum of the alcoholic extract between 200 and 500 nm shows that the solution significantly absorbs over the entire range of UV wavelengths. The absorption spectra between 200 and 500 nm, obtained from the extracts withdrawn at different maceration times, are similar. This absorbance behaviour shows that the principal compounds in the essential oil are extracted in the same ratio during the entire extraction time. Figure 1 shows the absorbance at 275 and 400 nm with extraction time. The absorbance at 400 nm follows the kinetics of the extraction of natural colorants and can be obtained with an undiluted extract up to 30 min after the beginning of extraction. For the absorbance at 275 nm, it is necessary to dilute the extract with ethanol (1:10). At both wavelengths, the absorbance initially increases during maceration and then becomes constant. Both curves indicate that the absorbance becomes constant at the same extraction time. Overlapping the two curves indicates that the absorbance reaches a constant value at the same time, so the extraction of essential oils from the lemon peel can be followed either at 400 nm (yellow) or at 275 nm. Figure 2A, B show the kinetics of extraction at 275 and 400 nm for three alcoholic extracts prepared by macerating 30 g of lemon peels in 100 mL of ethyl alcohol. The peels originated from the same lemon tree. The curves cannot be superimposed, indicating that the maceration process is not reproducible. Absorbance values taken from samples extracted for the same amount of time can vary by 20% of their average value at equilibrium (plateau). This variation can be explained even by the irregular distribution of essential oils and dyes in the peels of different fruits, even if the fruits originate from the same plant. Moreover, the three curves show a similar trend, and the absorbance values become constant after the same extraction time.

The Kinetics of the Extraction of the Essential Oils from Lemon Peel by GC

FIG. 2. A: ABSORBANCE VALUES (k 5 275 NM) OF THREE ALCOHOLIC LEMON PEEL EXTRACTS AGAINST EXTRACTION TIME Lemons were picked from the same plant. B: Absorbance values (k 5 400 nm) of three alcoholic lemon peel extracts against extraction time. Lemons were picked from the same plant.

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A typical gas chromatogram of an alcoholic extract after one day of lemon peel maceration was obtained with GC/MS. Peak identification was done using a GC/MS library search and by a comparison with the retention time of standards. Table 1 shows the identities of the peaks and their percentage composition. Samples were taken at different times during the maceration process and analysed by GC. Figure 3 shows the ratios between the areas of limonene, beta-pinene, and gamma-terpinene, which represent the major components of the essential oils of lemon, and the IS n-dodecane. Figure 4 shows the ratios between the areas of alpha-pinene, sabinene, C 2016 Wiley Periodicals, Inc Journal of Food Process Engineering 00 (2016) 00–00 V

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TABLE 1. COMPOUNDS OF ALCOHOLIC EXTRACT IDENTIFIED BY GC/MS AND THEIR PERCENTAGE COMPOSITION.

FIG. 3. THE KINETICS OF EXTRACTION OF THE COMPOUNDS SHOWING A GREATER PRESENCE IN THE LEMON PEEL The ratios between the peak areas of the substances and the peak area of the IS are reported over extraction time.

geranial, which represent the minor components of the essential oils of lemon, and the IS n-dodecane. The concentrations of the major components in the alcoholic extract reach a constant value at the same time - after 24 h, these compounds have been completely extracted. The curves for the minor components show a different trend, indicating that they are extracted more slowly. This behavior was expected because these compounds are extracted more slowly because of a lesser concentration gradient. An extraction time of approximately three days is necessary for the complete extraction of the minor components. Comparison of the spectrophotometric and gas chromatographic curves for the major components indicates that these curves always reach the same value at the same time. Therefore, the progress of the extraction of the major components can be followed by UV-Vis spectrophotometry or by GC. Conversely, the progress of the extraction of the minor components can only be followed by GC.

FIG. 4. THE KINETICS OF EXTRACTION OF THE COMPOUNDS SHOWING A LESSER PRESENCE IN THE LEMON PEEL The ratios between the peak areas of the substances and the peak area of the IS are reported over extraction time.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Compound

Rt (min)

%

a-Thuyene a-Pinene Sabinene b-Pinene Myrcen a-Phellandrene a-Terpinene Limonene Transocimene c-Terpinene Trans-sabinene hydrate Cis-sabinene hydrate Linalol Citronellale 4-Nonanol Terpinen-4-olo a-Terpineol Neral Geranial Neryl acetate Geranyl acetate b-Caryophyllene a-Bergamotene b-Bisabolene

13.493 13.883 15.597 15.883 16.323 16.798 18.038 18.351 18.401 19.612 19.994 21.280 21.413 22.333 23.225 23.555 25.326 27.170 28.262 31.587 32.329 34.118 34.390 36.687

0.40 1.81 1.85 13.25 1.43 0.35 0.19 65.29 0.06 9.81 0.24 0.31 0.33 0.15 0.18 0.14 0.35 0.87 1.47 0.44 0.46 0.28 0.43 0.61

Effect of Diluting the Alcohol During the Maceration of the Peels Fresh lemon peel contains a significant amount of water (about 70% w/w). We assumed that during maceration, the alcohol diffused into the peel and the water diffused out of the peel into the alcohol. According to this hypothesis, the alcohol becomes diluted during extraction and diffuses into

FIG. 5. THE KINETICS OF EXTRACTION OF WATER CONTAINED IN THE LEMON PEEL The quantity of water (G) contained in 100 mL of alcoholic extract is reported against extraction time.

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FIG. 6. KINETICS OF ALCHOLIC EXTRACTION FOR MAJOR COMPONENTS OBTAINED BY CONVENTIONAL PROCESS OF MACERATION (T)

the peel, becoming unrecoverable. Because the water content of the peel is 70% w/w and 30 g of peel per 100 mL of 95% v/v ethyl alcohol is usually macerated, when equilibrium is reached, the concentration of the alcohol should decrease to 80% v/v, and 20 mL of the solution should remain in the pores of the peel. Analysis of the alcoholic extract at equilibrium indicated that the concentration of alcohol changed from 95 to 78% v/v, which is in accordance with the hypothesized values. Figure 5 shows the experimental curve for the dilution kinetics of the alcohol during maceration and indicates that 4 h is necessary to reach a wateralcohol equilibrium. Therefore, it seems advisable to limit the maceration time to one day. Because a 24 h maceration time is necessary to completely extract the major components of the essential oils of lemon peel, it is not possible to avoid the dilution of the alcohol in the final extract.

Kinetics of Extraction of Maceration (T) Figures 6 and 7 show the extraction kinetics for the major and minor components of lemon peel subjected to tradi-

FIG. 7. KINETICS OF ALCHOLIC EXTRACTION FOR MINOR COMPONENTS OBTAINED BY CONVENTIONAL PROCESS OF MACERATION (T)

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tional maceration, and the experimental results are shown in Table 2. The process is slow, and the system reaches equilibrium after several days of extraction. Equilibrium is indicated by the portion of the curve with zero slope and occurs at 150 h of extraction, that is, after about 6 days. The experimental data diagrams indicate that with stirring, the system reaches saturation after 150 h at room temperature (20 6 1C). Earlier work by Locatelli et al. (2012) showed that some of the major components of the essential oil of lemon peel, including limonene, beta-pinene, and gamma-terpinene, were extracted more quickly in ethyl alcohol than other minor components such as sabinene, alphapinene, and geranial; however, the minor components are no less important for the flavor of the beverage than the major components. This difference in the extraction times is due to the different solubilities of the compounds in ethyl alcohol and to the different concentrations of the compounds in the essential oil of lemon.

Kinetics of Extraction by Means of the Cyclically Pressurized Extraction Process (I) Figures 8 and 9 show the extraction kinetics of the cyclically pressurized process with the Naviglio extractor for the major and minor components. The trend of the rapid solid-liquid dynamic extraction kinetics is the same as that for maceration and essentially changes the kinetic constant k. The initial extraction is very rapid, and the system reaches final equilibrium for the minor components at 120 min, indicating that the extraction with the Naviglio extractor is faster by at least 20 times. This enormous acceleration of the extraction occurs because the Naviglio process is based on an innovative phenomenon compared to the traditional maceration process. The Naviglio extractor generates a negative pressure gradient between the interior and the exterior of the solid matrix. The latter is the driving element for the solid-liquid extraction, and the gradient actively forces the molecules inside the solid matrix to the outside. Diffusion and osmosis are negligible in comparison to the effect of the Naviglio principle, which has been described as follows: “The generation, with an appropriate solvent, of a negative pressure gradient between the outside and the inside of a solid matrix containing extractable material, followed by a sudden restoration of the initial equilibrium conditions, induces the forced extraction of the compounds not chemically bonded to the main structure of which is made the solid” (Naviglio, 2003). This difference occurs because prolonged contact between the liquid and the solid extractant restricts the freedom of the colloidal particles that are dispersed in the system and increases the value of the dry residue. These substances are generally undesirable, as they are the result of the disintegration of the C 2016 Wiley Periodicals, Inc Journal of Food Process Engineering 00 (2016) 00–00 V

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TABLE 2. SOME PHYSICAL CHARACTERISTICS OF THE EXPERIMENTAL LEMON PEEL PARTICLES Time (min)

L1 Length (mm)

L2 Width (mm)

L3 Thickness (mm)

GMD 5 D (mm)

Sfericity /

Sphericity factor-fe

0 5 15 20 25 30 40 60 80 100 120

12.06 6 1.92 12.01 6 1.87 11.82 6 1.78 11.63 6 1.57 11.45 6 1.71 11.27 6 1.73 11.18 6 1.72 11.11 6 1.69 11.07 6 1.71 11.05 6 1.75 11.03 6 1.71

9.22 6 1.62 9.12 6 1.43 8.93 6 1.35 8.90 6 1.63 8.85 6 1.59 8.81 6 1.34 8.52 6 1.29 8.37 6 1.63 8.21 6 1.27 8.16 6 1.43 8.11 6 1.25

8.27 6 1.31 8.06 6 1.23 7.80 6 1.08 7.76 6 1.13 7.69 6 1.05 7.65 6 1.06 7.46 6 1.04 7.35 6 1.12 7.25 6 1.03 7.10 6 1.21 7.05 6 1.03

8.73 8.57 8.34 8.29 8.24 8.20 7.98 7.89 7.80 7.71 7.56

0.973 0.971 0.970 0.970 0.969 0.969 0.968 0.968 0.967 0.967 0.965

0.956 0.952 0.940 0.940 0.938 0.938 0.937 0.936 0.935 0.934 0.932

Data are reported as the mean 6 SD. GMD: geometric mean diameter.

solids that form the solid matrices (cellulose, cellular components, polymeric substances, etc.). A rapid initial alcohol uptake occurs during the cyclically pressurized extraction of the solid matrix, most likely because the capillaries on the surface of the seed coats and hiluim become full (Hsu et al. 1983). As alcohol absorption proceeds, the rate begins to decline because of the effect of the increased extraction of soluble material from the solid matrix and the filling of capillaries and intermicellar spaces with water (Plhak et al. 1989). The principal factor controlling the rate of solute absorption in alcohol during the solid matrix soaking is the seed coat. In this study, the cyclically pressurized extraction process (I) was found to have a significant effect on increasing the extraction rates of lemon peel, and the equilibrium conditions were thus attained in much shorter times compared with the process (T). A cyclically pressurised extraction process has the advantage of enhancing the plasticity of the seed coat and eliminating the presence of hard-shelled cells that fail to imbibe alcohol during the process by alternating the pressure values (0–10 bar). The rapid changes of pressure (0–10 bar) produce an impulsive force that allows the opening of new channels within the solid matrix, through which the alcohol

flows, increasing its overall solute concentration. That does not occur when the pressure value is constant during the process because it does not induce the formation of new channels within the solid matrix. The higher De calculated for the solid matrix of the lemon peel indicates that the solvent alcohol in the cyclically pressurised extraction process conduction (I) dissolved the solute compounds at a higher rate than in maceration process.

FIG. 8. KINETICS OF ALCHOLIC EXTRACTION FOR MAJOR COMPONENTS OBTAINED BY INNOVATIVE PROCESS (I)

FIG. 9. KINETICS OF ALCHOLIC EXTRACTION FOR MINOR COMPONENTS OBTAINED BY INNOVATIVE PROCESS (I)

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Sensory Analysis Results A “consumer test” showed that a group of regular consumers of limoncello liqueur (18 individuals: 9 men and 9 women) to whom anonymous samples of product were offered did not successfully identify which samples were prepared using the Naviglio extractor using a triangular test method. For this number of tasters, the number of correct answers (i.e., the number of individuals correctly identifying which of the three samples was different) must be P < 0.10 for the results to be significant at the 95% confidence level (i.e., an error factor equal to 0.05).

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FIG. 12. NODAL FLUX-TIME DIAGRAMS OF THE EXTRACTION PROCESS: LEMON PEEL C2-BETA PINENE RELATIVE NODAL CONCENTRATION VS. TIME

FIG. 10. FINITE ELEMENT MODEL (FEM) MODEL FOR LEMON PEEL

The tasters answered correctly only three times. Based on these results, we can conclude that the products obtained with the two systems have similar textural characteristics. However, the data obtained in the preference tests suggest that the samples prepared with the Naviglio extractor, although there were fewer of them, had an average preference value higher than that obtained with the traditional method (5.13 vs. 7). These preliminary results showed that the two systems of extraction yielded analogous results and could therefore be considered equivalent. Furthermore, as the tests performed have demonstrated, the Naviglio extractor can also be used for the simultaneous extraction and aromatisation of the final product. The samples were taken from the extraction chamber and tasted and the passage of the aroma into the alcohol was detected. This is explained by the extractor having two functions: the low pressure phase of the extractor drives many active ingredients, including aromatics such as the spices, into the alcohol. The high pressure

FIG. 11. NODAL FLUX-TIME DIAGRAMS OF THE EXTRACTION PROCESS: LEMON PEEL C1-LIMONENE RELATIVE NODAL CONCENTRATION VS. TIME

10

phase allows the simultaneous extraction of aromatic compounds into the alcohol.

Determination of the Characteristic Parameters for Numerical Model Table 2 shows characteristic values of the parameters for the pieces of the solid matrix of lemon peels and the diffusivity De calculated by Fick’s law:  L1 length along the x axis of the considered particle (mm) largest principal dimension, mm  L2 length along the y axis of the considered particle (mm) second largest principal dimension, mm  L3 length along the z axis of the considered particle (mm) smallest principal dimension, mm Moreover, the value of the sphericity / has also been calculated following the method described by Jain and Bal (1997). The sphericity of the grain is an index of its roundness. For nonspherical particles, it is calculated as the ratio of the surface area of an equivalent sphere to the surface area

FIG. 13. NODAL FLUX-TIME DIAGRAMS OF THE EXTRACTION PROCESS: LEMON PEEL C3-GAMMA TERPENE RELATIVE NODAL CONCENTRATION VS. TIME

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FIG. 14. NODAL FLUX-TIME DIAGRAM OF THE EXTRACTION PROCESS: LEMON PEEL C4-SABINENE RELATIVE NODAL CONCENTRATION VS. TIME

of the grain (McCabe and Smith, 1984). For this case, the results are reported in Table 2. However, due to a relative decrease in the thickness and width of the solid matrix, the sphericity of the solid matrix decreased from 0.973 to 0.965. The sphericity factor (fe) for an average solid matrix particle was calculated to be [0.901–0.989]. This decrease in dimensions was due to the migration of solute content in the solid matrix within the lemon peel cells, which causes an overall deflation of the solid matrix. Ahromrit et al. (2006) provides a detailed discussion on the changes in the dimensions during the extraction process. Considering the calculated values of the diffusivity De at the instants considered and applying the expression of Fick using Eqs. (2) and (3), the coefficient of diffusion De was precisely calculated:  For 008  For 008  For 008

the C1 component, De values ranged from 1.16 em2/s. to 5.55 e-011 m2/s the C2 component, De values ranged from 1.16 em2/s. to 4.79 e-011 m2/s the C3 component, De values ranged from 1.16 em2/s. to 4.11 e-011 m2/s

FIG. 15. NODAL FLUX-TIME DIAGRAM OF THE EXTRACTION PROCESS: LEMON PEEL C5-ALPHA PINENE RELATIVE NODAL CONCENTRATION VS. TIME

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FIG. 16. NODAL FLUX-TIME DIAGRAM OF THE EXTRACTION PROCESS: LEMON PEEL C6-GERANIAL RELATIVE NODAL CONCENTRATION VS. TIME

 For 009  For 009  For 009

the C4 component, De values ranged from 1.13 em2/s. to 0.914 e-013 m2/s the C5 component, De values ranged from 1.02 em2/s. to 0.913 e-013 m2/s the C6 component, De values ranged from 1.03 em2/s. to 0.915 e-013 m2/s

The values of De obtained in this case have been compared with those obtained with other types of extracts for traditional maceration, for example, as follows: Cisse` et al. (2012) calculated the diffusion coefficient for the extraction of anthocyanins from Hibiscus sabdariffa within a range of 3.9 3 10211 to 1.35 3 10210 m2 s21 between 25 and 90C, and Linares et al. (2010) estimated this coefficient for yerba mate aqueous extraction kinetics, which ranged between 6.092 3 10211 and 9.469 3 10211. A higher De value for an extraction process indicates that the solute diffuses into the solid matrix at a higher rate.

Numerical Analysis Results To perform a numerical simulation of the process (I), a FEM model for lemon peel was used (Fig. 10). The calculated diffusion coefficient was substituted into Eq. (1), and the equation was solved by a finite element approach to estimate the Nodal Flux-time diagrams for the principal and secondary components (Figs. 11–16) and the distribution of the concentrations in the solid matrix at various times. These Figures show the nodal flux vs. time at locations indicated by 1, 2, 3, and 4 for the major and minor components: limonene (C1), beta pinene (C2), gamma terpinene (C3), sabinene (C4), alpha pinene (C5), and geraniol (C6). The visualization of finite element predictions for the concentration content are presented in Fig. 17 that reports the minutes after the extraction process began. These Figures show the concentration at various points along a cross 11

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FIG. 17. TYPICAL DISTRIBUTION OF SOLUTE CONCENTRATION (DIFFUSION) IN THE SOLID MATRIX OF LEMON PEEL DURING THE EXTRACTION PROCESS AS DETERMINED BY A FINITE ELEMENT ANALYSIS AFTER 36, 900, AND 1800 S FOR THE MAJOR COMPONENTS AND AFTER 144, 7200, AND 14,400 S FOR THE MINOR COMPONENTS The bar below each shape represents the level of concentration content (%) within a particle region at a given time.

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TABLE 3. COMPARISON BETWEEN MACERATION PROCESS OF LEMON PEELS AND EXTRACTION PROCESS BY CYCLIC PRESSURIZATION FOR THE LEMON LIQUOR PREPARATION Extraction technique

Granulometry

Solvent

Yield

Time

Quality extract

Stability extract

Taste test

MACERATION NAVIGLIO EXTRACTOR

Important Not important

Essential Irrilevant

Exhaustive Exhaustive

Long (days) Minimum (h)

Good Excellent

Excellent Excellent

good Very good

section of the particle from the surface to the center of the solid matrix of lemon peel particles. Figure 17 shows the typical distribution of solute concentration (diffusion) in the solid matrix of lemon peel during extraction as determined by a finite element analysis after 36, 900, and 1800 s for the major components limonene (C1), beta pinene (C2), and gamma terpinene (C3) and after 144, 7200, and 14400 s for the minor components sabinene (C4), alpha pinene (C5), and geraniol (C6). The bar below each shape represents the level of concentration content (%) within a particle region at a given time. Moreover, Fig. 17 shows a gradual migration of the solute extract from the centre of a solid matrix to the surface. Theoretically, the finite element model, as shown in Fig. 17, indicates that even at the end of the extraction process, the concentration content throughout the solid matrix is not quite uniform. This conclusion is supported by sensory taste of lemon liquor that highlights a better test for liqueur obtained by using innovative extraction compared with traditional one (See Table 3). Figure 17 shows that the solute extract gradient within a solid matrix is large during the initial stages of extraction and that the gradient decreases with the progress of the extraction. At the end of the process, the concentration is practically uniform and reaches a final value at all the nodes.

CONCLUSION This report compared the extraction of lemon peels by a maceration process (T) with that of a RSLDE process (I). In summary, the alcoholic extraction of the major components of the essential oils from lemon peel using ethyl alcohol was characterized using visible and/or UV spectrophotometric measurements. GC analysis allowed the kinetics of the major and minor components to be followed. The major components were extracted completely after one day, whereas the minor components required approximately three days of infusion of lemon peel in ethyl alcohol. When equilibrium was reached during the maceration phase, the concentration of alcohol decreased from 95 to 80% v/v because of the dilution of alcohol by the water naturally present in the lemon peel. The dilution kinetics of the alcohol were faster than the kinetics of the extraction of the major components of the essential oils, so it was not possible to extract the latter compounds without diluting the ethanol used for extraction. The diagrams obtained for the CM indicated that the process (T) is complete in 10 days (240 h). The (I) process is C 2016 Wiley Periodicals, Inc Journal of Food Process Engineering 00 (2016) 00–00 V

complete in 2 hours. Therefore, the effectiveness of the Naviglio extractor is evident. The difference in the extraction kinetics was characterized by a ratio of 120 between maceration and the process implemented by the Naviglio extractor. A numerical simulation indicated that even at the end of the extraction process, the concentration throughout the solid matrix is not quite uniform and the solute extract gradient within the solid matrix is large during the initial stages of the extraction process. The gradient decreases with the progress of the extraction, and at the end of the process, the concentration is practically uniform and reaches a final value at all the nodes. This procedure has been applied to cases previously studied in the literature and has led to some good preliminary results for the use of the waterway kinetic mining extractor in the prediction of the extraction kinetics of maceration. The results show that the kinetics is faster in the Naviglio extractor, and all the other parameters of recovery and efficiency are unchanged. Therefore, the replacement of this new technology of solid-liquid extraction for the now obsolete and outdated maceration technique is a natural consequence of obtaining more favorable results. Furthermore, this model can be applied to the alcohol extraction of herb mixtures, but it is clearly necessary to study these systems in relation to the characteristics of the solid matrix such as the entirety, the freshness, and the part of the plant extracted. The temperature of the extraction system must be controlled because diffusion and osmosis are strongly dependent on temperature. Finally, this work has allowed us to lay the scientific and quantitative bases for RSLDE with the support of a mathematical model, for the final purpose of extending its application to other types of plant matrices.

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