Specification and implementation of a continuous microwave-assisted system for paste malaxation in an olive oil extraction plant

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ScienceDirect journal homepage: www.elsevier.com/locate/issn/15375110

Research Paper

Specification and implementation of a continuous microwave-assisted system for paste malaxation in an olive oil extraction plant Alessandro Leone a, Antonia Tamborrino b, Roberto Romaniello a,*, Riccardo Zagaria a, Erika Sabella c a

Department of the Science of Agriculture, Food and Environment, University of Foggia, Via Napoli, 25, 71100 Foggia, Italy b Department of Agricultural and Environmental Science, University of Bari Aldo Moro, Via Amendola 165/A, 70126 Bari, Italy c Department of Biological and Environmental Sciences and Technologies, University of Salento, Via Prov.le Lecce e Monteroni, 73100 Lecce, Italy

article info

An industrial prototype continuous microwave-assisted system (MWS) to condition olive

Article history:

paste was specified, built and implemented as an industrial process. The developed system

Received 2 May 2014

was tested to assess its performance during implementation in an industrial olive oil

Received in revised form

extraction plant. The extraction efficiency of the olive oil plant was investigated for

12 June 2014

different operating conditions of the MWS and compared with conventional methods to

Accepted 18 June 2014

condition the olive paste. The results indicate that exposing the olive paste to microwaves

Published online

determines the thermal and non-thermal effects that influence the coalescence phenomena and the extraction efficiency.

Keywords:

The experiments showed the feasibility of the continuous microwave-assisted proto-

Microwave prototype

type and great the potential to become an alternative technique to effectively condition

Extra virgin olive oil extraction plant

olive paste. The MWS removes the limitations of the batch malaxation process and pro-

Malaxer machine

duces an olive oil extraction process that is truly continuous.

Specific heat

© 2014 IAgrE. Published by Elsevier Ltd. All rights reserved.

Image analysis Textural property

1.

Introduction

The only batch process in olive oil extraction plants is malaxation; all of the other processes, from olive washing to liquideliquid separation, are continuous. During malaxation,

* Corresponding author. Tel.: þ39 0881 589 120. E-mail address: [email protected] (R. Romaniello). http://dx.doi.org/10.1016/j.biosystemseng.2014.06.017 1537-5110/© 2014 IAgrE. Published by Elsevier Ltd. All rights reserved.

the loading and unloading of the olive paste occurs at different times. After the olives are washed, the structural components (stone, pulp, and skin) of the olives are reduced by continuous mechanical crushers such as hammers and disc mills (Amirante, Clodoveo, Tamborrino, Leone, & Paice, 2010; Leone, 2014). The olive paste is then transferred into a malaxer in

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Abbreviations MWS

Continuous microwave-assisted system for conditioning the olive paste EVOO Extra virgin olive oil PLC Programmable logic controller DSC Differential scanning calorimeter Cp Specific heat of olive paste (J kg1 K1) Heat flow measured by DSC (J s1) QDSC r DSC scanning rate (K min1) m Sample weight subjected to DSC analysis (kg) Thermal power required (W) Pt Microwave power required (W) PMW Mass flow entering in the MWS (kg s1) QMW DT Temperature difference (K) Cavity efficiency hc w.m. Wet matter d.m. Dry matter EE Extraction efficiency (%) Mass of the extracted oil (kg) Woil Wtotal oil Mass of oil in the processed olives (kg) ANOVA Analysis of variance Coefficient of determination R2

which the material is kneaded and thermally conditioned. Following this process, the conditioned olive paste is discharged into a decanter (a horizontal centrifuge); this decanter performs the solideliquid separation. In this process, the husk (solid phase) is separated from the olive oil and water (liquid phases). The obtained liquid phases are then transferred into two different vertical centrifuges. These centrifuges perform high-speed centrifugation to efficiently separate the oil and water (Altieri, Di Renzo, & Genovese, 2013; Amirante, Clodoveo, Leone, Tamborrino, & Patel, 2010; Boncinelli, Catalano, & Cini, 2013; Boncinelli, Daou, Cini, & Catalano, 2009). The malaxation process is performed in a machine that consists of a stainless steel chamber fitted with a horizontal shaft (fitted with stainless steel blades). The heating of the olive paste is achieved through energy exchanges with a service fluid (hot water) that flows through an external coil (Amirante, Clodoveo, Tamborrino, & Leone, 2012; Tamborrino, Clodoveo, Leone, Amirante, & Paice, 2010). Malaxation is performed to separate the emulsions formed during the crushing process and to promote the coalescence of the oil drops that determine the reduction of the viscosity. The slow mixing improves the extraction yield and is an important process for maintaining the quality of the olive oil. During malaxation, several modifications to the chemical and organoleptic properties of the olive oil occur. These modifications are a result of enzyme kinetics that are typically dependent on the time, temperature, and time of exposure of olive pastes to air during the mechanical extraction process (Amirante, Clodoveo, Tamborrino, Leone, & Dugo, 2012; Esposto et al., 2009;
nGambacorta et al., 2004; Garcıa-Mesa, Pereira-Caro, Ferna
ndez, Garcı´a-Ortiz Civantos and Mateos, 2008; dez-Herna
nchez-Ortiz, & Garcı´a-Rodrı´guez, Romero-Segura, Sanz, Sa
rez, 2011; Gomez-Rico, Inarejos, Salvador and Fregapane, Pe

25

2009; Inarejos-Garcı´a, Santacatterina, Salvador, Fregapane, &
mez-Alonso, 2010; Kalua et al., 2007; Leone, Romaniello, Go & Tamborrino, 2013; Ranalli, Pollastri, Contento, Iannucci, & Lucera, 2003). During the conventional operation of the malaxer, which is batch process, several negative aspects occur that significantly affect the functionality of the plant. These aspects are described below: - The mass flow is not constant for the entire processing period. This involves manual operations such as the on-off switching the conveyor belt, progressive cavity pumps, and/or opening and closing the valves to regulate the flow of products from the washing machine to the centrifuge. - The time and temperature profiles of the olive paste depend on the filling levels of the malaxer. As explained by Comba, Belforte, and Gay (2011) heat exchangers transfer thermal energy through the exchange surface shared between the processed product and the service fluid. Therefore, the thermal energy that flows from the service fluid to the processed product at a given time is a function of the shared surface and temperature difference between the two compartments. The active heat exchange surface therefore depends on the amount of processed product loaded and the time and temperature profiles depend on the ratio between the heat exchange surface and the volume of the processed product. During regular operation, different filling levels and, consequently, different timeetemperature profiles are possible in malaxers from one processing cycle to another. This condition particularly occurs when level sensors are not present on the malaxers, and when the batch of olives has a mass less than that necessary to allow the complete filling of the machines. - The timeetemperature profile is not uniform for all olive paste particles in the malaxer. At the end of the malaxation process, the unloading does not necessarily completely remove all of the paste from the malaxer; a layer of paste often remains attached to the internal wall of the machine. Therefore, a small portion of olive paste can remain attached to the walls of the malaxer through many subsequent cycles. Therefore a non-uniform timeetemperature profile exists in the malaxer and this varies from particle to particle. This portion of paste undergoes degrading reactions that produce chemical and sensory defects in the oil (Tamborrino, 2014). - The discontinuous malaxation process can cause discontinuity in the feeding of the decanter, thus unbalancing the flow inside the bowl. This imbalance may adversely affect the efficiency operation. - The operational time increases. This is particularly significant since the malaxation process consumes approximately two-thirds of the total time required for extra-virgin olive oil processing (Tamborrino, 2014). Industrial and academic investigations have focused on improving the malaxing process by altering specific components of the malaxer to improve the kneading and heating processes of the olive paste and to reduce the process time. The most significant changes have included improving the heat exchange surface-to-volume ratio that is typical of old cradle malaxer models by designing malaxer machines with

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circular sections, more efficient reels, and diffusing the mixer blades. The improved performance of these malaxers was demonstrated by Amirante, Clodoveo, Tamborrino and Leone (2012). Recent studies have focused on flash heating the olive paste by introducing a heat exchanger in an industrial olive oil extraction plant (Amirante, Clodoveo, Dugo, Leone, & Tamborrino, 2006; Esposto et al., 2013). These investigations have reported that using a heat exchanger before malaxing results in optimal processing temperatures and thereby improves the yield and aromatic fraction of the olive oil, whereas decreasing the malaxation time. Other studies have been performed to design a circular malaxer machine to control the oxygenation and the rheological properties of the olive paste with an on-line monitoring system (Catania et al., 2013; Leone, Romaniello, Zagaria, & Tamborrino, 2014; Selvaggini et al. 2014; Tamborrino, Catalano, & Leone, 2014; Tamborrino, Catalano, et al., 2014; Tamborrino, Pati, Romaniello, Quinto, Zagaria, Leone, 2014). Improving the control of malaxation parameters allows the selection of the optimal ending time of malaxation, thus reducing the process time. However, despite research and innovation, the malaxation process is currently performed in a batch manner. Additionally, the malaxation phase represents an ever-present problem in extra virgin olive oil (EVOO) extraction plants. In large-capacity industrial olive mills, the malaxers are connected in series or in parallel to simulate a continuous process. However, employing multiple malaxers involves significant plant costs and large footprints. This scenario prompted the authors to evaluate the possibility of thermal conditioning of the olive paste to provide a more continuous olive oil extraction process. To this end, microwave heating technologies were investigated.

1.1. Microwave applications in olive oil extraction equipment In food processing, microwave heating is often used as an alternative system to replace indirect heating through service fluids (Chandrasekaran, Ramanathan, & Basak, 2013; Cocci, Sacchetti, Vallicelli, Angioloni, & Dalla Rosa, 2008; Datta, 1990; Karaaslan & Tunc¸er, 2008; Rosenberg & Bogl, 1987; € Soysal, Ayhan, Es‚tu¨rk and Arikan, 2009; Soysal, Oztekin, & Eren, 2006). The widespread use of microwaves in food processing is a result of the ability of food materials to adsorb microwave energy and convert the energy into heat. In microwave processing, energy is supplied directly to the material through an electromagnetic field. This direct application results in rapid heating throughout the material thickness with reduced thermal gradients. Microwaves are electromagnetic waves with a frequency that varies from 300 MHz to 300 GHz (Datta & Anantheswaran, 2000; Lauf, Bible, Johnson, & Everliegh, 1993). Microwaves interact with materials through dipolar and ionic mechanisms. The presence of moisture or water in the materials permits microwave heating. When an oscillating electric field interacts with water molecules, the permanently polarised dipolar molecules try to realign in the direction of the electric field. Because of the high frequency of the electric field, this realignment occurs millions of times per second and causes internal friction between the molecules. This friction results in the volumetric heating of the material. Volumetric heating as been shown to reduce processing times

(Aciemo, Barba, & d'Amore, 2004; Barringer, Davis, Gordon, & Ayappa, 1995; Basak, 2003; Basak, 2004; Khraisheh, McMinn, & Magee, 2004; Mudgett, 1989; Singh & Heldman, 2014, pp. 265e419; Thostenson & Chou, 1999). In addition to volumetric heating, energy transfer at a molecular level can have several additional advantages. Numerous studies describe the nonthermal phenomena that have been broadly termed “microwave effects” (Thostenson & Chou, 1999). One study indicated that the application of microwaves can affect the molecular structure of the food materials; thus, the mechanical property of the food may be affected. Several studies have focused on using microwaves to induce reactions that are impossible with traditional methods (Gong, Liu, & Huang, 2010; Liu, Wang, Wang, Zou, & Sonomoto, 2012; Seixas et al., 2014;
z, and Ortı´z (2008) studied Yuan et al., 2012). Uquiche, Jere the effect of pre-treatment with microwaves on mechanical extraction yield and the quality of vegetable oil from hazelnuts. They found that exposing the hazelnut paste to highpower microwaves with a frequency of 2.45 GHz caused microstructural modifications in the treated substrates. These microstructures enabled the oil to move through the permeable cell walls and therefore, increased the extraction yield. Other authors have established that microwave heating improved the mechanical extraction efficiency of oil from the vegetal matrix. This phenomenon vaporises the water in the vegetable substrate and increases the inner pressure of the vegetal tissues. This increased pressure disintegrates the material and ruptures the cell membranes (Aguilera & Stanley, 1999, pp. 325e372; Jiao et al. 2013). By subjecting the olive paste to microwaves after crushing phase, a release of oil from the vacuoles can occur in addition to increased heating rates, thus accelerating the coalescence phenomenon. The heating ability of microwaves also allows for a rapid and uniform heating decreasing the processing time (Regier, 2014;
n, Zu´n ~ iga, & Moyano, 2007; Schiffmann, 2010; Reyes, Cero Venkatesh & Raghavan, 2004). Although research on the design of microwave equipment, the interaction of microwave/materials, and materials processing continues to increase the interest in microwave techniques, there is a lack of information about the application of microwaves as a pretreatment for olive paste and its effect on the microstructure of the substrate and extraction yield in an industrial olive oil extraction plant. The aim of this study is to develop a continuous microwave-assisted system (MWS) to condition the olive paste, to implement the process in an industrial olive oil extraction plant, and to investigate the effect of microwave pre-treatment on the microstructure of the olive paste and on the extraction efficiency of the plant.

2.

Materials and methods

2.1. Design of a continuous microwave-assisted prototype system to condition the olive paste (MWS) This phase focused on identifying and establishing the necessary equipment for the MWS. After an iterative procedure of testing and adjusting different parameters, the performance of the MWS has been tested in an industrial olive oil extraction line.

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2.1.1. Specification of the continuous microwave-assisted system An MWS was designed to be included in an industrial processing line for EVOO extraction. The MWS was required to work continuously. Therefore, a polypropylene tube was placed in a reverberant chamber. One end of the tube was used for the input of the olive paste, and the opposite end was used for the output. To size the MWS, two constants were fixed: the maximum flow rate, which is equal to 3000 kg h1, and the maximum temperature difference (DT) between the input and output, which is equal to 7.5  C. The maximum flow rate was identical to that of the industrial extraction plant chosen for the experimental tests, allowing easy implementation of the MWS in the process. To calculate the thermal power of the microwave generators required in the MWS, the specific heat (Cp) of the olive paste was determined experimentally using a differential scanning calorimeter (DSC) (Mettler Toledo DSC822e, Im Hackacker 15, CH-8902 Urdorf, Switzerland). The Cp of the olive paste was calculated for two types of olive paste: traditional olive paste (with pits) and partial de-stoned olive paste (with a proportion of pits). The partial de-stoned olive paste was used for the experimental tests. Table 1 shows the composition of traditional and partially de-stoned olive pastes used in the specific heat measurements. The Cp of the olive paste measured experimentally by a DSC analysis (described below) was 3250 J kg1 K1 and 3310 J kg1 K1 for traditional and de-stoned olive paste, respectively. The thermal power required (Pt) to heat the olive paste in the MWS with a flow rate of 3000 kg h1 and a DT of 7.5  C has been calculated using the following equations: Pt ¼ QMW Cp DT

(1)

Pc hc

(2)

PMW ¼

27

sample and sealed, and the sample weight was determined. An empty capsule with the same properties was used as a reference. An empty capsule versus empty capsule DSC scan was performed to establish the baseline of the instrument. The DSC measurements were performed over the temperature range of 5e45  C at a scanning rate of 5  C min1. The DSC thermograms are expressed as heat flow (J s1) between the sample and the reference capsule as a function of temperature. The empty crucible thermogram (empty sample versus empty reference capsule) was subtracted from the sample thermogram (sample capsule versus empty reference capsule), and the specific heat of the sample was calculated using the following equation: Cp ¼

QDSC 60 rm

(3)

where QDSC is the heat flow measured by DSC (J s1), r represents the DSC scanning rate (K min1), and m is the sample weight that is subjected to DSC analysis (kg). Each sample was analysed five times, obtaining five mean values of QDSC over the temperature range of 20e30  C. For each mean value of QDSC was used to calculate the Cp value, by using the Eq. (3). Finally, the five Cp values were used to calculate the mean.

2.1.3. Features of continuous microwave-assisted system and its placement in an industrial-scale olive oil extraction plant The MWS was built and implemented in an industrial-scale olive oil extraction plant. A reverberation chamber was constructed from AISI 304 stainless steel. The geometry of the machine allows the stationing of the electromagnetic waves in the volume of the room and impedes their release to the external environment (Fig. 1). Considering that the required microwave power was calculated to be 24.3 kW, the MWS has been equipped with four TM060 generators heads (Alter srl, Reggio Emilia e Italy) equipped with a water-cooled YJ1600C magnetron (Alter srl,

Considering that a partial de-stoner was used for the experimental tests, the Cp value in Eq. (1) was for the destoned paste. The value of PMW was calculated using the theoretical value of cavity efficiency (hc), which is equal to 0.85, and equalled 24.3 kW.

2.1.2.

Differential scanning calorimetry (DSC) paste analysis

The specific heat of the olive paste was calculated according to the method of Kaletunc, 2007. All DSC measurements were conducted using an aluminium capsule that was filled with

Table 1 e Composition of traditional and partial destoned olive paste used for the specific heat measurements. Olive paste components Water Oil Pit Soft solids from pulp

Traditional paste (%)

Partial de-stoned paste (%)

56.4 22.1 12.3 9.2

59.3 24.1 6.5 10.1

Fig. 1 e Schematic diagram of the MWS and its elements: 1. reverberation chamber; 2. power supplies; 3. magnetrons; 4. input of olive paste; 5. output of olive paste.

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Reggio Emilia e Italy) with a maximum power output of 6.0 kW and operating at 2.45 GHz. The magnetron was water-cooled and equipped with flow-control to modulate the flow of water depending on the temperature. Moreover, an aircooling system is installed to control the temperature of the electrical circuit. Each generator head is connected to a SM1180T power supply (Alter srl, Reggio Emilia e Italy). The output power can be adjusted in a continuous manner through a programmable logic controller (PLC). The microwave generator was mounted on an aluminium plate to support the structure and it employed a security system that prevented the system starting when the cover protection was open. A poly-propylene pipe was placed inside volume of the machine, through which the olive paste passed and was exposed to microwaves (Fig. 2). The pipe section was 65.4 mm diameter, and its length was 4000 mm. Several electromagnetic shields were placed at the ends of the pipe. Moreover, a temperature probe was placed inside the output pipe to monitor the temperature of the treated paste. The instantaneous temperature value was displayed on the touchscreen of the PLC. The PLC controlled and set the parameters of the machine and monitored the real-time functionality of the microwave generators. The MWS was fabricated and assembled by EMITECH s.r.l. (Molfetta, Italy). The microwave machine was installed in an industrial olive oil mill by connecting it in parallel with the malaxers using three 3-way valves (Fig. 3). With this layout, the malaxers could be by-passed depending on the test conditions.

2.2. Experiments on an industrial olive oil extraction line with a continuous microwave-assisted system 2.2.1. Experimental procedure: Extra virgin olive oil mechanical extraction plant and process extraction conditions The industrial oil mill used in the experimental tests was located near Foggia, Puglia, Italy and was characterised by a

Fig. 2 e Schematic diagram of the MWS showing the interior: the pipe for olive paste is shown in green.

semi-continuous line that can be visualised as a series of interconnected machines: a defoliator, a washing machine, a partial de-stoner (Pietro Leone e Figli s.n.c., Foggia - Italy), six malaxer machines, a three-phase solid/liquid horizontal centrifugal decanter (mod. NX X32, Alfa Laval Corporate AB, LundeSweden), and two liquid/liquid vertical plate centrifuges which were used to separate the phases of the liquids. This industrial plant was able to process a maximum throughput of 3000 kg h1 of olives. During the tests, the water mass flow rate for olive paste dilution was identical in all of the experimental tests. The partial de-stoner machine used during the experimental tests contained two sections: one that crushed the stone and pulp with a hammer mill, and the other which partially separated the stone fragments. This machine can remove stone fragments in a variable proportions from 0 to 100%. During the test, approximately 50% of the stone fragments were extracted (approximately 6 kg [pit] 100 kg-1 [olives]), whereas the remaining 50% of the fragments of stone and the pulp continued in the process. The composition of the olive paste from the partially destoned machine is shown in Table 1. To complete the data, the composition of the traditional olive paste is shown; these data were not used in the tests. Olive fruits of the cultivar Ogliarola Garganica (Olea europaea L.) at a maturity index of 2.9 were harvested and transported on the same day to the industrial mill. The ripeness of the fruits was determined according to the method proposed by the International Olive Council (IOOC, 2001). The olives were processed 6 h after harvesting. Tests were performed in three different trials during two different cropping seasons: in the 1st and the 2nd trial were carried out in the 2012-2013 crop season and the 3rd trial was carried out in the 2013-2014 crop season. In each trial, three different conditioning systems for the olive paste were compared:  C-MWS: After crushing, the olive paste was continuously fed into the MWS, where the paste was microwaved for a variable amount of time according to the flow rates set. At the output, the olive paste is discharged directly to the decanter. The process temperature was 24.5  C (C-MWSLT) and 28  C (C-MWSHT).  C-MWS-MX: after crushing, the olive paste was continuously fed into the MWS, where the paste was microwaved for a variable amount of time according to the flow rates set. At the exit, the olive paste was discharged to the malaxing section in which the paste was malaxed for 15 min and subsequently discharged to the decanter. The process temperature was 24.5  C (C-MWS-MX LT) and 28  C (C-MWS-MX HT).  C-MX: after crushing, the olive paste was fed to the malaxing section, where the paste was malaxed for 40 min and then discharged to the decanter. The process temperature was 24.5  C (C-MX LT) and 28  C (C-MX HT). The processing conditions are described in detail in Table 2. The malaxing time did not include the times required to load and unload the malaxer. Figure 3 shows a flow-chart of the EVOO mechanical extraction process. During the trials, the flow of the product from one machine to the other was controlled by a human operator. The operator opened and

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Fig. 3 e Lay-out of industrial olive oil extraction plant and process: A loading hopper; B defoliator; C washing machine; D partial de-stoner mill; E discharge of pit fragments; F malaxer machines; G MWS; H PLC panel; I solid/liquid horizontal centrifugal decanter; L liquid/liquid vertical centrifuges; M 3-way valve.

closed three-way valves and started and stopped pumps and conveyors. Each test was performed four times by processing homogenous olive batches (700 kg per batch). The first and second trial (cropping season 2012-2013) studied the effect of microwave treatment on the plant performance using identical mass flow rates of approximately 2150 kg h1 with two different MWS powers and two different output temperatures. In the first trial, the MWS was set to low power (9.0 kW) and an output temperature of 24.5  C, whereas in the second trial, the MWS was set to a medium power (18.0 kW) and with an output temperature of 28.0  C. In the third trial the focus was to assess the performance of the extraction plant at the same temperature used for the second trial (28.0  C) but to increase the mass flow rate from 2150 kg h1 to 3000 kg h1. This mass flow increase set the working capacity of the MWS to the nominal mass flow rate of the extraction plant. Thus, the power of the MWS was increased from 18 kW to 24 kW. To achieve 9.0 kW in the first trial, three magnetrons were set at 2.0 kW, and one was set at 3.0 kW. To achieve 18.0 kW in the second trial, the four magnetrons were set at 4.5 kW. To achieve 24.0 kW in the third trial, the four magnetrons were set at 6.0 kW.

2.2.2.

Sampling

The olives used for the Wtotal oil determination were sampled and stored at 25  C until analysis. Husk was sampled from the decanter at regular time intervals and stored at 25  C until analysis. The waste water was sampled from the separator of the water phase at regular time intervals and stored at 25  C until analysis.

2.3.

Extraction efficiency

The extraction efficiency (EE) is the percentage of oil extracted respect to total oil content in the processed olives determined using Soxhlet extraction process. The extraction efficiency was calculated using the following equation: EE ¼

Woil 100 Wtotal oil

(4)

where Woil is the mass of the extracted oil (kg), and Wtotal oil is the oil mass in the processed olives (kg).

2.4.

Oil content in olives, husk and waste water

The total oil content was determined on 25 g of sample, previously dehydrated until reaching constant weight. The sample was extracted with hexane in an automatic extractor (Randall 148, Velp Scientifica, Milan, Italy) following the analytical technique described by Cherubini et al. (2009). The sample was initially subjected to an immersion phase at 139  C for 60 min; the sample porous container was immersed directly in the boiling solvent. The sample was then subjected to washing at 139  C for 40 min; the sample container was removed from the solvent and reflux washed. The final part of the process which was conducted at 139  C for 30 min was solvent recovery. Results were expressed as percentage of oil on wet and dry matter.

2.5. Oil drop aggregation analysis on a laser scanning microscope The oil drop coalescence analysis used three samples of olive paste for each of the four test conditions reported below:

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Table 2 e Experimental plan and process parameters used during all test conditions. Test conditions 1st trial

2nd trial

3rd trial

C-MWSLT C-MWS-MXLT C-MXLT C-MXHT C-MWSHT C-MWS-MXHT C-MXHT C-MWSHT C-MWS-MXHT C-MXHT

Mass flow (kg h1) 2150 2150 2150 2150 2150 2150 2150 3000 3000 3000

± 30 ± 36 ± 29 ± 24 ± 31 ± 27 ± 28 ± 26 ± 25 ± 31

Out temperature of MWS ( C)

Malaxing temperature ( C)

24.5 ± 0.1 24.5 ± 0.1 e e 28.0 ± 0.3 28.0 ± 0.3 e 28.0 ± 0.3 28.0 ± 0.4 e

e 24.4 ± 24.5 ± 28.0 ± e 28.2 ± 28.1 ± e 28.1 ± 28.0 ±

0.4 0.6 0.5 0.4 0.5 0.3 0.5

Residence time* (s)

Microwave power (kW)

Malaxing time (min)

23.7 ± 0.3 23.7 ± 0.3 e e 23.7 ± 0.3 23.7 ± 0.3 e 17.0 ± 0.2 17.0 ± 0.2 e

9 9 e e 18 18 e 24 24 e

0 15 40 40 0 15 40 0 15 40

Data represents mean value ± standard deviation. * Residence time: the time of exposition of the olive paste to the microwaves.

1. Olive paste sampled after the crushing phase (control test); 2. Olive paste sampled after microwave treatment (C-MWSHT); 3. Olive paste sampled after microwave treatment and 15 min of malaxation (C-MWS-MXHT); 4. Olive paste sampled after malaxation for 40 min (C-MXHT). Samples of pastes were immediately frozen in liquid nitrogen (196  C), the samples were frozen until the laser scanning microscope (LSM) analysis. Sudan black B staining was used to assess structural modifications involving the aggregation of oil drops, which is critical in the efficient oil separation during centrifugation. Samples of pastes were first smeared on a slide and directly emulsified with the staining solution (0.3 g of Sudan black B in 100 ml of 70% ethanol); the slides were then left undisturbed at room temperature for 15e20 min. The excess stain was drained, and the smear was washed with water by pipetting several times. A cover glass was then adhered with a drop of Canada Balsam (SigmaeAldrich, St. Louis, MI, USA). Oil drops were observed on an LSM 700 laser scanning microscope (LSM 700 Inverted, Carl Zeiss, Jena, Germany). Sudan black B was detected using the tetramethylrhodamine isothiocyanate (TRITC) filter set (>650 nm). A digital image of each sample was acquired. For each of the four process conditions considered, 84 digital images were collected: four replicates of each condition with three samples for each replicate and seven images for each sample. For all four different process conditions, the total number of digital images acquired was 336. The digital images were acquired using a colour CCD camera with a spatial resolution of 1024  1024 pixels and a colour depth of 24 bits pixel1. The olive paste images acquired on the LSM were processed by and analysed using an algorithm developed in MATLAB® (The MathWorks Inc., Natick, MA, USA). The images were thresholded using the Otsu's method (Otsu, 1979), and the binary images of the oil drops were obtained. For each image, the number and area of the oil drops were calculated. A spatial reference in each image was used to calculate the area in mm2 (Baiano, Romaniello, La Macchia and La Notte, 2009). The image processing toolbox of MATLAB® was used. The area data collected from the digital images of the olive paste were processed using the statistical toolbox of MATLAB®. A cumulative distribution fit has been performed on each data series relative to each digital image analysed. For

each distribution, the mean, mode, sigma, kurtosis and skewness were calculated. The kurtosis (g2) and skewness (g1) indexes have been calculated as follows: h i E ðx  mÞ4 m4 g2 ¼ n h io2  3 ¼ 4  3 s 2 E ðx  mÞ " 3 # xm m g1 ¼ E ¼ 33 s s

(5)

(6)

for both formulae, m is the mean of the distribution, s is the standard deviation, and E represents the expected value of the quantity y.

2.6.

Statistical analysis

Each industrial test was performed four times and all laboratory analyses were performed in triplicate. All of the experimental data were analysed with an analysis of variance (ANOVA) and Duncan's test with p < 0.05 using the MATLAB® statistics toolbox.

3.

Results and discussion

3.1. Influence of the continuous microwave-assisted system on the performance indices of the olive oil extraction plant The performance indices for the mechanical olive oil extraction plant were analysed when a continuous microwaveassisted system for olive paste conditioning was implemented and compared with the conventional batch method. The performance indices analysed were the following: the oil content of the husk measured both as dry matter % and wet matter %, the oil content of the waste water, and the extraction efficiency (EE). The EE is one of the most important parameters to consider when the quantitative performance of the olive oil extraction process is assessed. The EE together to the EVOO quality directly affects the overall economy of the extraction process. Table 3 shows the results of the quantitative performance of the plant, measured as the oil content in

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the husk (wet and dry matter) and extraction efficiency. As reported in the notes in Table 3, the samples of waste water displayed trace oil content levels in all tests. As shown in Table 3, the oil content in the husk and the EE in the first trial are significantly different under different conditions. The C-MXHT condition resulted in the lowest oil content in the husk and the highest EE (77.3%). The C-MXLT condition resulted in the highest oil content in the husk and the lowest EE (71.5%). The C-MWS-MXLT and C-MWSLT conditions resulted in intermediate EEs compared to the conditions involving traditional malaxation. The results of this trial confirmed that mechanical kneading and the thermal effect obtained by the traditional malaxation (conducted at the optimal temperature of 28  C (C-MXHT) are fundamental to obtaining better EEs. This was previously found by several authors (Amirante et al., 2006; Inarejos-Garcia, Gomez-Rico, Desamparados, & Fregapane, 2009; Tamborrino et al., 2010). Microwave treatment displays a positive effect on the EE of the process. In the comparison tests between the C-MWSLT and C-MXLT conditions performed at identical temperatures (24.5  C), the EE of the process using the MWS displayed significantly higher EEs than those obtained with C-MXLT. The total increase was 2.8%. This result shows that microwaves in addition to the thermal effect, have a non-thermal effect that is significant for the extraction of oil from the olive cells. When the treatment temperature was increased to 28  C (second trial), the results showed that the pre-treatment of olive paste with the MWS did not show significant differences in the oil content in the husk for all tests. These data were confirmed by the absence of significant differences in EEs. By comparing the results of the first and second trial, the increase in the electromagnetic energy transmitted by the MWS (from 9.0 kW to 18.0 kW) and, consequently, the increase of the final temperatures of the olive paste (from 24.5  C to 28.0  C) at the same flow rate (2150 kg h1), improved the EE and reduced the oil content in the husk until the values became equal to the C-MXHT values. The results of the first and second trial

31

confirmed that the microwave treatment has a positive effect on the EE because of thermal and non-thermal effects. Additionally, as shown in the second trial, increasing the electromagnetic energy transmitted from the MWS to the olive paste increases the EE until no significant differences are present among the different test conditions. Finally, kneading in combination with the microwave (C-MWS-MXHT) did not significantly improve the EE. When the MWS used the nominal mass flow rate of the extraction plant (third trial), the results did not display significant differences in both the oil content in the husk and the EE for the three conditions. The results of the third trial confirmed the results of the second. Also, the third trial demonstrated that increasing the mass flow rate while maintaining the thermal differential constant is possible by increasing the power of the MWS. Figure 4 shows the relationship (as described in Eq. (7)) between the specific supply energy (PMW/QMW) and the measured and theoretical DT obtained in all experimental tests. The theoretical DT was calculated using the following equation: DT ¼

hc PMW Cp QMW

The data plotted showed a good linear correlation (coefficient of determination R2 ¼ 0.977). The points are placed in a range of efficiency (hc) between 0.80 and 0.90 (dashed lines in Fig. 4). The overall average efficiency equalled 0.85. This efficiency value coincides with the one used to calculate PMW (Eq. (2)), confirming the correct dimensioning of the MWS. Figure 5 presents the amount of the oil content in the husk versus the EE of the plant for all experimental tests. The plotted data are linearly correlated with a good correlation (R2 ¼ 0.964). This good correlation between the oil content in the husk and the EE potentially allows the oil content in the husk to be a predictor of the EE (as long as the oil is present in trace amounts in the waste water). Thus, the oil content in the husk can be considered a fundamental parameter in the evaluation of the performance of the olive oil extraction process when operating conditions change.

Table 3 e Oil content of husk and extraction efficiency for all test conditions. Test conditions 1 trial

2 trial

3 trial

C-MWSLT C-MWS-MXLT C-MXLT C-MXHT C-MWSHT C-MWS-MXHT C-MXHT C-MWSHT C-MWS-MXHT C-MXHT

Oil content Oil content of pomaces of pomaces (% w.m.) (% d.m.) 7.5 6.9 8.1 6.5 6.5 6.4 6.6 5.6 5.5 5.6

± 0.2b ± 0.3c ± 0.3a ± 0.2c ± 0.3a ± 0.2a ± 0.2a ± 0.2a ± 0.2a ± 0.2a

16.4 14.4 17.6 14.3 14.5 13.5 15.0 12.5 12.3 12.1

± 0.3b ± 0.4c ± 0.4a ± 0.4c ± 0.3a ± 0.4a ± 0.4a ± 0.3a ± 0.3a ± 0.3a

EE (%)

73.5 75.4 71.5 77.3 77.9 78.2 77.5 80.9 81.2 80.3

± 0.4c ± 0.4b ± 0.4d ± 0.5a ± 0.4a ± 0.5a ± 0.5a ± 0.5a ± 0.4a ± 0.5a

In all the tests performed the samples of waste water have shown a content of oil in trace. Data represents mean value ± standard deviation. Different letters in the same column denotes statistical significant differences (p < 0.05).

(7)

Fig. 4 e Relationship between the specific supply energy (Pmw/Qmw) versus the measured and theoretical DT.

32

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Fig. 5 e Linear correlation of pomace oil content and the EE of the plant.

3.2. Influence of the microwave-assisted process on the textural and surface property of the olive paste To investigate the effect of the olive paste conditioning using the microwave-assisted system on the substrate, a characterisation of the textural and surface was performed using image analysis.

Figure 6 displays the processed images of the digital images acquired on the LSM. Figure 7 shows the mean cumulative distribution of the areas of the oil drops relative to olive paste treated with the three processing conditions considered and the olive paste sampled after the crushing stage (control test). The olive pastes analysed were from the third trial. The trend of the distributions described the progressive formation of the oil phase, i.e., the coalescence phenomenon. Table 4 shows the mean parameters of the distributions. The areas of oil drops are normally distributed, as confirmed by the skewness values (Table 4). The mean area value of oil drops increase from the control test to the conditioned by malaxing olive paste (C-MXHT) condition, with an increase of 55%. The mean, mode and sigma show significant differences between the control test and the other three conditions analysed. The kurtosis values showed that all three curves relative to the different conditioning process were platykurtic. However, the distribution relative to the paste sampled after the crushing stage (control test) was leptokurtic. Moreover, the curves show a significantly different value of kurtosis (except for C-MWSHT and C-MWS-MXHT). The results of the textural and surface characterisation showed that the coalescence phenomenon is promoted by the conditioning process of the olive paste. By evaluating the textural data (Table 4) in combination with the quantitative performance data (Table 3), the

Fig. 6 e Laser scanning microscope images of olive pastes coloured with Sudan black B: A control test; B C-MWS; C CMWSMX; D C-MX.

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b i o s y s t e m s e n g i n e e r i n g 1 2 5 ( 2 0 1 4 ) 2 4 e3 5

paste and replace the malaxers. The results obtained demonstrated that the microwave produces a non-thermal effect in addition to the thermal effect to release oil from the vacuoles and increase the coalescence phenomenon. The microwave process resulted in a high extraction efficiency that was comparable to the extraction efficiency obtained with traditional malaxation. This EE was confirmed in the image analysis of the olive pastes that highlighted the absence of significant differences among the means of the oil drop areas relative to the olive paste conditioned with traditional malaxation and those conditioned with a MWS. Microwaves can potentially be applied in olive oil extraction plants to improve the olive oil extraction process and overcome the batch nature of the malaxation process by producing a continuous process. The following conclusions can be drawn:

Fig. 7 e Fitted frequency distributions of the oil drop areas for the various tests.

coalescence level that corresponds to a kurtosis value equal to 1.75 (relative to the C-MWSHT treatment) is sufficient to obtain high values of EEs. Kurtosis values less than 1.75, such as those relative to C-MXHT treatment, corresponded to a higher level of coalescence and did not result in a significant increase in EE. Moreover, malaxation for 15 min after the microwave treatment did not significantly affect oil drop aggregation, as confirmed by the absence of significant differences in EE values.

4.

Conclusion

The introduction of microwave treatment in an industrial olive oil extraction plant was carried out. An industrial prototype continuous MWS was designed, built and implemented in an industrial setting. This should provide useful information to producers worldwide. The efficiency of the developed system was analysed by performing comparative tests with a conventional method that used kneading and heating of the olive paste. The experiments showed the feasibility of the continuous process assisted by a microwave. The results of this work demonstrated that it is possible to insert a MWS in an industrial line when the MWS is correctly dimensioned to guarantee process continuity. The microwave pre-treatment system could be applied to condition the olive

- As demonstrated in this study, the effective conditioning time of the olive paste was reduced from 40 min (traditional malaxation) to a few seconds (MWS treatment) and removed the unloading and loading stages typical of the malaxer machine. - The direct heating system of the microwave application, compared to the indirect system of the malaxer machine, had a more uniform timeetemperature profile for all particles in the olive paste during its conditioning avoiding the olive paste overheating that could occur in the traditional malaxer. - The use of a continuous MWS to condition the olive oil paste produced a constant mass flow to all machines connected in the line. The system allows a constant timeetemperature profile during the process avoiding the negative effects due to the different filling levels of the malaxer that occur when level sensors are not installed and when the batch of olives has a mass lower than that necessary to allow for the complete filling of the malaxers. - Another important advantage is that the conditioning room of the MWS required a minimal volume compared to the high volume of the malaxer machine. Thus, MWS reduced the amount of water used in cleaning the conditioning room between batches and reduces the volume of waste water to dispose. In conclusion, microwave-assisted systems could represent a new frontier for olive paste conditioning in olive oil extraction plants and promote the revisiting of the layout of the extraction olive oil line, also reducing the plant complexity. Further investigations are necessary to assess the

Table 4 e Mean parameters of the distribution curves of the oil drops area. Mean Crusher* C-MWSHT C-MWS-MXHT C-MXHT

7.73 ± 16.64 ± 15.51 ± 17.14 ±

1.12b 1.36a 2.01a 1.87a

Mode

Sigma

15.54 ± 0.76b 19.20 ± 1.11a 18.53 ± 1.43a 19.32 ± 0.89a

9.75 ± 5.33b 24.78 ± 12.54a 30.78 ± 14.54a 42.42 ± 18.41a

Kurtosis** 1.98 1.75 1.05 2.66

± 0.07a ± 0.54b ± 0.43b ± 0.32c

Skewness 1.24 0.89 1.11 1.24

± 0.91a ± 0.86a ± 0.87a ± 0.78a

Data represents mean value ± standard deviation. Different letters in the same column denotes statistical significant differences (p < 0.05). *Olive paste sampled after crushing phase (control test). ** A kurtosis value <0 indicates a leptokurtic distribution; a kurtosis value >0 indicates a platykurtic distribution.

34

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MWS setting considering different olive oil cultivars harvested at different maturation indexes, in order to evaluate the olive oil quality. Finally, studies on the energy required and the investment and management costs of the microwaves application in a mill are important for an overall assessment.

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