Plant innovation in the olive oil extraction process: A comparison of efficiency and energy consumption between microwave treatment and traditional malaxation of olive pastes

Journal of Food Engineering 146 (2015) 44–52

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Plant innovation in the olive oil extraction process: A comparison of efficiency and energy consumption between microwave treatment and traditional malaxation of olive pastes Alessandro Leone a, Antonia Tamborrino b, Riccardo Zagaria a, Erika Sabella c, Roberto Romaniello a,⇑ a b c

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

a r t i c l e

i n f o

Article history: Received 18 June 2014 Received in revised form 21 August 2014 Accepted 25 August 2014 Available online 6 September 2014 Keywords: Microwave Virgin olive oil Malaxation SEM Energy consumption Specific heat Olive paste

a b s t r a c t In this study, we discuss the introduction of microwave radiation replacing the malaxation of olive paste and associated experimentation. A major limitation of olive oil extraction plants is the discontinuity in the extraction process due to the current technology used during the conditioning of the olive paste. In this work, a microwave-assisted system was developed and applied in an industrial-scale olive oil extraction plant to preliminarily analyse the installation and determine any advantages to improving the process continuity. The apparatus, which is specifically designed to be industrially implemented, was evaluated in terms of electrical and thermal energy consumption and the extraction yield of the olive oil. Special attention was given to a microstructural investigation of the olive paste using SEM analyses. The microwave treatment does not significantly influence the extraction yield compared with conventional malaxation, but the SEM results demonstrated that the microwave technique efficiently breaks down cell walls and membranes, thus increasing the release of oil. In addition, some benefits are observed that result from making the olive oil extraction process continuous. The results acquired from this study are promising for microwave implementation in olive oil extraction plants in the future. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The extraction techniques employed to produce olive oil from olives in the last 30 years have not seen the significant innovation necessary to introduce substantial changes in the overall organisation of the mill and the logistics of the process. The most important innovation in olive oil extraction plants was the introduction of the horizontal decanter centrifuge, which allowed the continuous operation of the solid–liquid separation. This innovation has had the effect of rendering obsolete the operations involved in discontinuous pressing (Amirante and Catalano, 1993; Amirante et al., 2010; Daou et al., 2007). The main benefits were the improvement in the olive oil quality by preventing oxidation and the improvement of the management of the mill (Di Giovacchino et al., 2001; Ranalli and Angerosa, 1996; Catalano et al., 2003; Altieri, 2010; Altieri et al., 2013). Time savings, the associated reduction of labour and labour costs, faster and easier cleaning of the equipment and the new layout of the mill that included minor space ⇑ Corresponding author. Tel.: +39 0881 589 120. E-mail address: [email protected] (R. Romaniello). http://dx.doi.org/10.1016/j.jfoodeng.2014.08.017 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved.

dedicated to the machines of the process were the main advantages achieved by the continuous machine. This led to the immediate and widespread dissemination of the decanter over the entire world. Currently, if we look at the olive oil extraction process, we realise that all operations are continuous except for malaxation. This represents the major limitation in olive oil extraction plants associated with the continuity of the olive oil extraction process and leads to connection problems between the continuous operations before washing and crushing and those that follow solid– liquid separation and liquid–liquid separation. To overcome these limitations, the current industrial solutions include a series of tanks with a production capacity equal to the throughput of the horizontal decanter centrifuge. The number of malaxers required is decided on the basis of the plant work capacity, the required malaxing time and the average size of the batches of olives. Examined one way, this processing system ensures continuous processing. However, it has many disadvantages due to operating management for the malaxer machine. The flows of olive paste passing through to the malaxer machine occasionally results in dead spots and stagnation during the loading and unloading phases, which are frequently required

A. Leone et al. / Journal of Food Engineering 146 (2015) 44–52

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Nomenclature MPS EVOO PLC Up Uair m Cp Tp Tf Tair DTML Si Se t

microwave prototype system extra virgin olive oil programmable logic controller service fluid-paste heat transfer coefficient (W m2 °C1) service fluid–air heat transfer coefficient (W m2 °C1) mass of olive paste (kg) specific heat of olive paste (J kg1 °C1) Temperature of olive paste (°C) temperature of service fluid (°C) temperature of atmospheric air (°C) logarithmic mean temperature difference (°C) internal heat exchange area of malaxer (m2) external heat exchange area of malaxer (m2) malaxing time (min)

to ensure the proper operation of the malaxing process (Amirante et al., 2012; Leone et al., 2014a; Tamborrino, 2014). However, one of the great limitations is control over temperature and time of kneading, which have considerable importance for the extraction yield and olive oil quality. Finally, the management of the malaxing is entrusted to the ability and knowledge of the operators, who have a great influence on the quality of the process. Inadequate management of this phase risks quality degradation and a loss of yield (Aguilera et al., 2010; Inarejos-Garcia et al., 2009; Esposto et al., 2013; Leone et al., 2014a; Gambacorta et al., 2010, 2012; Gomez-Rico et al., 2009; Reboredo-Rodríguez et al., 2014; Tamborrino et al., 2014a,b). Therefore, significant innovation is required to develop a continuous malaxing phase. The direction of academic and industrial research is moving toward the establishment of technological solutions and the development of novel innovations to maximise the productivity of this specific sector. With this in mind, in this work, the authors consider employing microwave radiation in olive oil extraction plants during the malaxation phase. In recent years, microwave processing of food has emerged as one of the fastest heating techniques available and is being investigated in various food processes (Leone et al., 2014b; Singh et al., 2014; Mudgett, 1986; Cheng et al., 2006; Datta, 1990; Cocci et al., 2008; Seixas et al., 2014; Catalano et al., 2013). Microwave heating is different from other indirect thermal heating methods. Microwave energy heats the food material at the molecular level, which eventually leads to uniform bulk heating. Because the heat originates in the molecules throughout the bulk, the heating process is faster than other known modes of heating in which depend on conventional modes of heat transfer (Schiffmann, 2010; Salvi et al., 2009; Chandrasekaran et al., 2013). In conventional heating systems attached to the malaxer machine, the olive paste gets heated from the surface of the tank to the interior via a thermal gradient, resulting in excess expenditures of time and thermal energy (Comba et al., 2011). However, microwave heating is characterised as volumetric heating, reducing the thermal gradient and saving thermal energy (Regier, 2014). Thus, the application of microwave energy as a source of heating during the conditioning olive paste may be a cost effective option that can be employed in the olive oil processing industry. Numerous studies have reported on ‘‘no-thermal’’ effects of food materials exposed to microwaves compared to other thermal processes, and changes in microstructure of food materials (Thostenson and Chou, 1999; Uquiche et al., 2008; Starmans and Nijhuis, 1996; Aguilera and Stanley, 1999; Jiao et al., 2013).

q DSC QDSC r w w.m. d.m. EY Woil Wolives SEM V ANOVA R2

thermal energy (J) differential scanning calorimeter heat flow measured by DSC (J s1) DSC scanning rate (°C min1) sample weight subjected to DSC analysis (kg) wet matter dry matter extraction yield (kg [oil]  100 kg [olives]1) mass of the extracted oil (kg) mass of processed olives (kg) scanning electron microscope volume of olive paste (m3) analysis of variance coefficient of determination

A primary goal of this research is to investigate any possible changes in the structure of the olive paste subjected to the microwave treatment to investigate the release of oil drops from the vacuoles of the cells. To analyse the feasibility of the installation of microwave technology for conditioning olive paste to achieve a continuous process, an industrial apparatus specifically designed and was implemented to analyse the electric and thermal energy consumption, as well as the extraction yield in an olive oil extraction plant. The results acquired from the present study are considered very promising for the future of this technology in this specific field. 2. Materials and methods 2.1. Industrial microwave prototype system An MPS system was installed in an industrial processing line for EVOO extraction. The MPS (Fig. 1) was fabricated and assembled by EMITECH s.r.l. (Molfetta, Italy). It consists of a reverberant chamber fabricated from AISI 304 stainless steel. Inserted on the side of the reverberant chamber are four TM060 generator heads packaged in a stainless steel cabinet and housing a water-cooled YJ1600 magnetron (Alter s.r.l., Reggio Emilia – Italy), which is powered by four SM1180T power supplies (Alter s.r.l., Reggio Emilia – Italy). The total output power from the four magnetrons is equal to 24 kW.

Fig. 1. Microwave prototype system: 1. Input; 2. PP tube; 3. Reverberant chamber; 4. Output; 5. Magnetron; 6. Power supply; 7. PLC; 8. Termocopule; 9. Water cooler; 10. Circulator.

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A. Leone et al. / Journal of Food Engineering 146 (2015) 44–52

A polypropylene pipe with a diameter of 65.4 mm passes through the reverberant chamber to permit the exposure the olive paste to microwave radiation when the line is operated in a continuously mode. The length of the pipe was 4000 mm. The four magnetrons were controlled by a PLC that allowed operator control over the power output. This also permitted control over the output olive paste temperature through a thermo-couple inserted in a terminal of the polypropylene pipe. The MPS was equipped with a water refrigeration system to control against overheating the magnetrons. The continuous mass flow rate of the MPS system was 3000 kg h1.

The waste water was sampled from the separator at regular time intervals and stored at 25 °C until analysis. Samples of the pastes were immediately frozen in liquid nitrogen (196 °C) and stored in a freezer at 80 °C until the Scanning Electron Microscope (SEM) analysis could be performed.

2.2. Mechanical extraction plant and experimental program

EY ¼

Experimental testing was performed by processing homogeneous Peranzana (Olea europaea L.) olive batches (750 kg) in an industrial extraction plant located in Incoronata (Foggia, Puglia, Italy). The olives were harvested at a maturity index equal to 2.8 (IOOC, 2001), and immediately transported to the mill for processing. The extraction plant was equipped with a partial de stoner machine, which was used as the crusher machine (Moliden system, Pietro Leone e Figli s.n.c., Foggia, Italy) followed by six parallel malaxer machines with one rotary lobe pump for feeding the olive paste into the decanter. A three-phase solid/liquid decanter (Alfa Laval, mod. NX X32) and a liquid/liquid vertical plate centrifuge completed the extraction plant. The olive flow rate through the plant is about of 3000 kg h1. In this plant, the MPS was implemented and placed between the crusher machine and the decanter. Tests were performed comparing three different olive paste conditioning methods including conventional discontinuous conditioning and continuous microwave treatment conditioning as described below:

where Woil is the mass of the extracted oil (kg) and Wolives is the mass of processed olives (kg).

 M: the olive paste was malaxed for 40 min at up to 28 °C (control test), in the malaxer machine (conventional discontinuous conditioning).  W: the olive paste was sent to the MPS, where microwave radiation exposure caused a temperature increase to about 28 °C before the olive paste was subsequently discharged directly to the decanter (continuous conditioning).  WM: the olive paste was sent to the MPS, where microwave radiation exposure caused a temperature increase to about 28 °C before being subsequently malaxed for 20 min at 28 °C (combined conditioning).

An environmental scanning electron microscope (Zeiss EVOÒ HD15, Carl Zeiss, Jena, Germany) was used to examine structure of the olive pastes. To study the olive paste structure during the olive oil extraction process, a sample of olive paste after being exposed to the partial de-stoner machine (control test) was collected. For the analysis, the olive pastes were kept at room temperature to complete thaw them before being smeared on conductive carbon adhesive tabs stuck to an aluminium stub that is mounted on a multi-sample holder inside the microscope chamber. Samples examination was carried out with an accelerating voltage of 16.0 kV, under a pressure of 80 Pa and at a magnification factor of 695.

The process parameters are described in detail in Table 1. The malaxing time did not include the loading and unloading times of approximately 15 min for each operation. Each conditioning method was repeated four times using the same process parameters. 2.3. Sampling The husks were sampled from the decanter at regular times and stored at 25 °C until analysis.

2.4. Extraction yield The EY is the amount of oil obtained through the milling of 100 kg of olives. The EY was calculated using the following equation:

W oil 100 W oliv es

ð1Þ

2.5. Determination of oil content in husk and waste water The percentage of olive oil in the husks and waste water was determined considering a 25 g sample, which is previously dehydrated until reaching a constant weight. The sample was extracted with hexane in an automatic extractor (Randall 148, Velp Scientifica, Milan, Italy) following the analytical techniques described in our previous study (Cherubini et al., 2009). The sample was processed in three steps: – Immersion in hexane at 139 °C for 60 min. – Washing at 139 °C for 40 min. – Recovery at 139 °C for 30 min; the solvent used was recovered. 2.6. Scanning electron microscope for the structure modifications of olive pastes analysis

2.7. Energy consumption evaluation Here, the olive-processing stages, data collection from the plant electric motors, and the approaches used to calculate the thermal energy values are explained, considering an olive oil extraction plant that is composed of five main sub stages: olive cleaning, olive crushing, olive paste conditioning, solid–liquid separation and liquid–liquid separation. Fig. 2 shows the production path line of

Table 1 Process parameters used in the three conditioning methods. Conditioning methods

Malaxing time (min)

Malaxing temperature (°C)

Output temperature of MPS (°C)

Microwaves exposure time (s)

MWS electric power (kW)

M W WM

40 – 20

28.0 ± 0.4 – 28.0 ± 0.5

– 28.0 ± 0.1 28.0 ± 0.1

– 17 ± 0.2 17 ± 0.2

– 24 24

Data represents mean value ± standard deviation.

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A. Leone et al. / Journal of Food Engineering 146 (2015) 44–52

The overall heat exchange coefficient (Up) was calculated from Eq. (2):

Up ¼

m C p ðT 2  T 1 Þ Si DML 60 t

ð2Þ

where m is the mass of paste in the malaxer, Cp is the specific heat of the olive paste, T1 and T2 are the initial and final temperatures of olive paste, respectively, DTML is the logarithmic mean temperature difference between the olive paste temperature and the service fluid contained in the jacket of the malaxer, Si is the internal heat exchange area of malaxer and t is the malaxing time. The olive paste and the service fluid temperatures were collected in the 70 min during which the four malaxers were operated. The means of the collected data were used in the analysis. Table 3 shows the process parameters and characteristics of malaxer used to calculate the Up value, expressed as (W m2 °C1). The calculation of the exchanged thermal energy (q) is carried out considering the malaxer as an unsteady state energy exchanger. The total exchanged energy (q) was calculated using Eq. (3) and considered six contributions: the energy transferred from the heat exchanger surface to the paste (q1) and to the air (q2) during the malaxer loading, the energy transferred from the heat exchanger surface to the paste during the malaxing operation (q3), the energy transferred from the heat exchanger surface to the paste (q4) and to the air (q5) during the malaxer unloading, and the energy transferred from the external surface of the malaxers to the atmosphere (q6). The contributions q1, q2, q3, q4, q5 and q6 were calculated using Eqs. (4)–(9):

q ¼ q1 þ q2 þ q3 þ q4 þ q5 þ q6

ð3Þ

15 X U p SiðtÞ ðT f  T pðtÞ Þ 60 DT

ð4Þ

Fig. 2. Path lines of olive oil extraction plant.

the mill process for the three different olive paste conditioning methods (M, W and WM). These three different test conditions identify three different configurations of the extraction plan: configuration M, W and WM. The product flow from one configuration to the others is allowed by 3-way valves and/or starts and stops using pumps and conveyors.

q1 ¼

2.7.1. Electric energy consumption evaluation The evaluation of the electrical power consumption was carried out from data on all of the electric motors involved in the oil extraction plant for each of the three different configurations. Production capacity, equipment, and electrical and thermal energy consumed in the olive oil extraction plant are given in Table 2. As seen, the primary type of energy used in an olive oil extraction plant is electrical. Only in the olive paste conditioning operation is any thermal energy lost.

q3 ¼

2.7.2. Thermal energy consumption evaluation To evaluate the thermal energy necessary to enable the heating of the olive paste in six parallel malaxers, we first determined the global heat exchange coefficient (U) necessary to calculate an energy balance.

t¼0

q2 ¼

15 X U air SiðtÞ ðT f  T air Þ 60 DT

ð5Þ

t¼0 55 X

U p Si ðT f  T pðtÞ Þ 60 DT

ð6Þ

U p SiðtÞ ðT f  T pðtÞ Þ 60 DT

ð7Þ

U air SiðtÞ ðT f  T air Þ 60 DT

ð8Þ

t¼15

q4 ¼

70 X t¼55

q5 ¼

70 X t¼55

q6 ¼

70 X U air Se ðT f  T air Þ 60 DT

ð9Þ

t¼0

where Tp(t) is the temperature of olive paste in the malaxer over the considered interval of time (Dt).

Table 2 Machine park and consumed energy types of olive oil extraction plant. Name of production process

Mass flow rate (kg h1)

Machine name (number of units)

Consumed energy type

Olive cleaning

3000

Olive crushing Olive paste conditioning Solid–liquid separation Liquid–liquid separation

3000 3000 3000 3000

Loading hopper (1) Conveyor belt (2) Defoliator (1) Washing machine (1) Screw elevator (1) Partial de-stoner mill (1) Malaxer (6) Decanter (1) Vertical centrifuge (2)

Electrical Electrical Electrical Electrical Electrical Electrical Electrical and thermal Electrical Electrical

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3.1. Quantitative performance of the olive oil extraction plant

Table 3 Process parameters and construction characteristics of the traditional malaxer. Parameters

Value

M.U.

Reel type

Single blades Cradle shape 750 22.6 28.3 31.0 5.02 3.45 3.70 65

–

40

min

Tank shape Mass of paste (m) Initial temperature (T1) Final temperature (T2) Service fluid temperature (T3) DTML Internal heat exchange surface (S1) External heat exchange surface (S2) Service fluid–air overall heat transfer coefficient of the malaxers (Uair) Malaxing time (t)

– kg °C °C °C °C m2 m2 W m2 °C1

During the malaxation operations (from 15th to 55th min), the heat transferred from the service fluid to the air is not considered because the heat exchanger surface was totally in contact with the olive paste. 2.8. Differential scanning calorimetry (DSC) paste analysis The specific heat of olive paste was calculated according to Kaletunc (2007), using a differential scanning calorimeter (Pyris Diamond DSC, Perkin Elmer, Waltham, USA) equipped with a cooling system (ULSP 130, Ultra Low Special Product, Ede, NL). All DSC measurements were conducted using an aluminium capsule filled with the sample, which was sealed so that the sample weight could be determined. An empty capsule with the same properties was used as the reference. An empty capsule versus empty capsule DSC scan was performed to establish an instrumental baseline. All of the DSC measurements were performed over a temperature range from 5 to 45 °C and at a scanning rate of 5 °C min1. The DSC thermograms consisted of the heat flow (J s1) between the sample and the reference capsule as a function of temperature. The empty crucible thermogram (empty sample versus the empty reference capsule) was subtracted from the sample thermogram (sample capsule versus the empty reference capsule), and the specific heat of the sample was calculated using the following equation:

Cp ¼

Q DSC 60 rw

ð10Þ

Each sample was analysed five times. For the calculation of Cp, the average of the measurements of heat flow over a temperature range from 20 to 30 °C was used. 2.9. 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 analysis of variance (ANOVA) and Duncan’s test with p < 0.05 using the MATLABÒ statistics toolbox (MathWorks Inc., Natick, MA, USA).

3. Results and discussion The effects of the inclusion of the MPS in the olive oil extraction process were studied in terms of the extraction yield, structural modification of the olive pastes and the electrical and thermal energy consumption.

Table 4 shows the results of the quantitative performance of the plant, measured as oil content in husks (wet and dry matter) and extraction yield. As reported in Table 4, the samples of the waste water show traces of oil for all tests performed. Data processing with ANOVA did not show any statistically significant difference (p < 0.05) between the considered conditions in terms of oil content in husks and the extraction yield. On the basis of quantitative performances, it is possible to affirm that the microwave treatment can be successfully used for the olive paste conditioning operation. 3.2. Structural changes of the microstructure of olive pastes by SEM observation The different treatments studied produced distinguishable physical changes in the olive paste. According to Fig. 3 and after the crushing phase, the olive pastes exhibit an intact structure with visible oil droplets dispersed and partially leaked on the paste surface. After malaxation (40 min) the olive pastes display well pronounced porosity, with a series of irregular cavities (Fig. 4), which are the result of the partial destruction of the parenchyma cells; this cell walls damage occurred during the paste mixing of the malaxation, in fact, tissues and cells are probably broken with contact to the pieces of stone formed during the crushing phase. This mechanical effect increasing the visibility of the cell outlines in the damaged tissues produces the observed irregular cavities in the samples. However, for the olive pastes subjected to the microwave treatment (Fig. 5) and for those subjected to the microwave treatment and subsequent malaxation (Fig. 6), a more disorganised samples surface, that may be the result of significant destruction of morphological structures, is covered by an aggregation of dense oil droplets. After the MPS treatment, the samples appear completely disrupted, enabling the oil to move through the permeable cell walls, and inducing the formation of the oil phase. In relation to this coalescence phenomenon, the SEM images of the microwave treated samples show oil droplet aggregation that appears as a dense deposited substance on the sample surface. Indeed, the microwave heating vaporises the water in the vegetable’s substrate microstructure, increasing the pressure in its interior. This release causes the disintegration of the material (Starmans and Nijhuis, 1996; Aguilera and Stanley, 1999) and the rupture of the cell membrane, enabling the passage of oil out of the internal structure (Uquiche et al., 2008). In the intact cells, oil droplets are contained inside the vacuole, when the cell wall is completely destroyed, small droplets of oil can leak from vacuole and aggregate between them. Jiao et al. (2014) performed a microwave-assisted aqueous enzymatic extraction of oil from pumpkin seeds. The SEM results demonstrated that the microwave technique efficiently broke down the cell walls and membranes, increasing the release of oil. The microwave treatment can therefore be considered as a valuable alternative to traditional vegetable oil extraction methods (Azadmard-Damirchi et al., 2011). 3.3. Energy consumption evaluation Operations examined in this study were olive cleaning, olive crushing, olive paste conditioning, solid–liquid separation and liquid–liquid separation. The equipment and the consumed energy in the plants are shown in Table 5. The washing machine, crushing machine, three-phase solid/liquid decanter and liquid/liquid vertical plate centrifuge were the same for the three configurations considered. The total electrical consumption for these operations was equal to approximately 86 kW.

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A. Leone et al. / Journal of Food Engineering 146 (2015) 44–52 Table 4 Quantitative performances of the three conditioning methods. Conditioning methods

Moisture content of pomace (%)

Oil content of pomaces (%w.m.)

Oil content of pomaces (%d.m.)

EY (kg [oil]100 kg1 olives])

M W WM

55.5 ± 0.6 a 54.5 ± 0.7 a 55.3 ± 1.0 a

5.4 ± 0.2 a 5.5 ± 0.1 a 5.5 ± 0.1 a

9.7 ± 0.3 a 10.0 ± 0.3 a 10.0 ± 0.4 a

14.4 ± 0.3 a 14.2 ± 0.3 a 14.3 ± 0.3 a

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).

Fig. 6. SEM images of olive paste submitted to condition WM. Fig. 3. SEM images of olive paste after crushing operation.

Fig. 4. SEM images of olive paste submitted to condition M.

Fig. 5. SEM images of olive paste submitted to condition W.

As reported in Table 5, the differences in the total electrical consumption in the three configurations of the plant are due to the different methods used for olive paste conditioning. In particular, the electrical consumption for each conditioning method was approximately 14, 38 and 52 kW for the M, W and WM configurations, respectively. These differences are also due to the absence of a cavity pump for configuration W, where the same cavity pump present on the de-stoner mill is used for both the MPS and the decanter feeding. The overall electrical consumption was approximately 100, 124 and 138 kW for the M, W and WM configurations, respectively. The electrical consumption increase between configuration M and W was 24%, and the increase between configuration M and WM was 38%. Considering the quantitative performances, the configuration WM did not exhibit a significant change in the extraction yield, and thus does not provide any advantage. Thus, the critical analysis was performed between configurations M and W. For the thermal energy consumption, the overall heat exchange coefficient (Up), calculated from Eq. (2), was 343 W m2 °C1. The measured specific heat value of the olive paste was 3327 J kg1 °C1. These parameters permitted the calculation of the energy transferred between the service fluid to the olive paste and the air during an entire malaxation operation. Fig. 7 shows the trends of the thermal energy transferred, the olive paste temperature and the Si V1 ratio inherent to a single malaxer machine, considering the malaxation operation and including the loading and unloading times (15 min for each operation) and the conditioning time (40 min). At the beginning of the malaxer loading, it is possible to notice that there is a significant amount of energy transferred. This is due to the high temperature difference between the service fluid and the olive paste. In addition, the Si V1 ratio is high during the initial phase of malaxer loading and decreases when the malaxer is filled. At the same time, the energy transferred increases until the malaxer is full. During malaxer loading, the exchanged energy varies from 0 to 288 kJ, the olive paste temperature ranges from 20.5 to 23.0 °C and the Si V1 ratio ranges from 33.7 (at the end of 1st min) to 5 m2 m3 (after unloading is completed). Successively, the Si V1

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Table 5 Electric energy consumption data of the olive oil extraction plant. Name of production process

Machine name (number of units)

Motor (number of units)

Olive cleaning

Conveyor belt Rotational defoliator

1 1 (bowl + outlet leaf screw) 1 1 1 (vibrating plane) 2 (blower) 1 (water pump) 1 Total

Electric motor size (kW)

Total motor power (kW)

Total electrical energy consumed (kW) ConfigM

Conveyor belt Centrifugal defoliator Washing machine

Conveyor belt

ConfigWM

1.50 2.00

1.50 2.00

1.50 2.00

1.50 2.00

1.50 1.50 0.33 2.20 4.00 1.50

1.50 1.50 0.33 4.40 4.00 1.50

1.50 1.50 0.33 4.40 4.00 1.50 16.73

1.50 1.50 0.33 4.40 4.00 1.50 16.73

1.50 1.50 0.33 4.40 4.00 1.50 16.73

1.50 15.0 0.35 15.00 1.50 1.50 2.00 36.85

1.50 15.0 0.35 15.00 1.50 1.50 2.00 36.85

14.00

37.80* 37.80

12.00 2.00 37.80 51.80

Olive crushing

Partial de-stoner mill

1 (feed screw) 1 (crusher) 1 (feed screw) 1 (partial destoner) 1 (kneader) 1 (outlet stone screw) 1 (cavity pump) Total

1.50 15.00 0.35 15.00 1.50 1.50 2.00

1.50 15.00 0.35 15.00 1.50 1.50 2.00

1.50 15.0 0.35 15.00 1.50 1.50 2.00 36.85

Olive paste conditioning

Malaxers (6)

1 (horizontal shaft) 1 cavity pump 1 Total

2.00 2.00 37.80*

12.00 2.00

12.00 2.00

1 (bowl-screw) 1 (husk scraper) 2 (liquid pump) 2 (motovibrator) 1 (outlet husk screw) 1 (cavity pump) Total

18.00 0.75 1.00 0.33 1.50 2.00

18.00 0.75 2.00 0.66 1.50 2.00

18.00 0.75 2.00 0.66 1.50 2.00 24.91

18.00 0.75 2.00 0.66 1.50 2.00 24.91

18.00 0.75 2.00 0.66 1.50 2.00 24.91

7.50

7.50

7.50 99.99

7.50 123.79

7.50 137.79

MWS (1) Solid–liquid separation

Decanter

Liquid–liquid separation Vertical centrifuge (1) Overall electrical energy consumption *

ConfigW

1.50 2.00

1

The value was calculated as follow: Pmw/(gm  gs), where Pmw = microwave power required, gm = magnetron efficiency (0.75) and gs = power supply efficiency (0.95).

Fig. 7. Trends of the thermal energy transferred, olive paste temperature and exchange-surface/olive paste volume ratio.

ratio becomes constant for the entire malaxation time and the energy transferred decreases to 92 kJ, whereas the olive paste temperature increases to 28.2 °C. The decrease in the energy transferred is because the olive paste temperatures tend to reach the service fluid temperature quickly and consequently, and the DT reduces its value during the malaxation time.

In the malaxer unloading phase the Si V1 ratio increases until reaching 33.7 m2 m3 (at the 69th min), the olive paste temperature increases until reaching 29.8 °C and the energy transferred decreases until reaching 47 kJ. Fig. 8 shows the trends of the thermal power transferred between the service fluid and the paste and the service fluid and

A. Leone et al. / Journal of Food Engineering 146 (2015) 44–52

Fig. 8. Thermal power transferred on the six malaxers along the processing time.

the air. Is also represents the total thermal power transferred. As shown in Fig. 8, from 0 to 60 min, the thermal power transferred between the service fluid and the air decreases, whereas the thermal power transferred between the service fluid and the olive paste increase. At the 60th min, the system reaches steady state and exhibits a sinusoidal trend as the alternative loading and unloading of the six malaxers is considered. In the steady state, the mean value of the thermal power transferred between the service fluid and the olive paste is equal to 23.5 kW and 13.5 kW between the service fluid and the air. The total thermal power transferred is 37 kW. These results confirm that the time–temperature profile of the olive paste particles is different during the malaxation operation time due to a non-constant filling level, as reported by Comba et al. (2011). 4. Conclusions In this paper, a microwave system was implemented in an olive oil extraction plant to condition the olive paste in a continuous way. This technology was compared with the conventional discontinuous malaxation. The results were evaluated in terms of the quantitative performances and the structural modifications of the olive paste using SEM analysis. Moreover, an estimated calculation of the electrical and thermal energy consumption was presented. The calculation was based on data collected from the electric motors of the industrial plant and the evaluation of the thermal energy consumption for both plant configurations considered. Concerning the quantitative performances, there were no significant differences between the mean values of the different methods in terms of EY and amount of oil loss in the husk. This result was reinforced by the SEM analysis that shows the advantages of the microwave pre-treatment. The heat generated in the olive pastes by the microwaves assist in the microstructural modification of substrate tissue by vaporising the water of the vegetable substrate microstructure. Then, the increasing pressure in the cell interiors increases the mass transfer of oil, which aggregates outside of the membranes. It is this coalescence phenomenon that is responsible for the increase in the extraction oil yield. The energy consumption evaluation shows that the use of the MPS to condition the olive paste requires an additional electric power of approximately 24 kW with respect to the traditional plant extraction involving six malaxers machine. The traditional extraction plant requires also of approximately 37 kW of thermal power, 36% of which is lost to the air due to the absence of insulation in the external walls of the malaxers. Moreover, there are other heat losses to the air due to the different levels of olive paste in each malaxer. Thus, non-direct heating in the malaxer machines are not efficient. For this reason, in an actual scenario, an improvement in the olive paste heating system is required.

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The results of this research confirm that traditional malaxation has some limitations, including the discontinuity of the process and the non-uniformities of the time–temperature profile of the olive paste particles. In particular, the olive paste optimal temperature is reached only after approximately 30 min after the beginning of the malaxation process. Moreover, during malaxer unloading, the olive paste temperature is increased over its optimal value (28 °C). The reasons behind the microwave methodology success may be found in the type of heating generated. In conventional thermal processing, energy is transferred to a material through conduction, convection and radiation. However, microwave energy is delivered directly to materials through molecular interactions with the electromagnetic field. The internal temperature distribution of a material subjected to conventional heating is limited by its thermal conductivity, whereas microwave heating results in all individual elements of the material being heated uniformly. Consequently, heating times using microwaves can often be reduced in comparison with those using conventional heating methods. In fact, by using the MPS (configuration W), which allows a rapid and volumetric heating of the olive paste, the time–temperature profile of the olive paste particles changes in a short time (approximately 17 s), which is all that is necessary to heat the olive paste from 18 to 28 °C. When using traditional heating (configuration M), the time necessary to heat the olive paste to 28 °C was approximately 55 min. Thus, the use of the MPS allows conditioning the olive pastes in a shorter time than traditional system and avoids the problems associated with process discontinuity. In conclusion, based on the results obtained here, the authors believe that industrial application is technically possible. The electrical power consumption using MPS is higher by 24% than traditional malaxation. However, this aspect must be related to the lower of the consumption of thermal energy and with the advantages of the continuity of the process. Finally, to evaluate the significant differences between the two systems compared, a comprehensive cost-benefit analysis of the entire process is required. Acknowledgements This study was financed by Fondazione Cassa di Risparmio di Puglia (CARIPUGLIA). The authors are most grateful to the foundation and to the President, Prof. Antonio Castorani. References Aguilera, J.M., Stanley, D.W., 1999. Microstructural Principles of Food Processing and Engineering, second ed. Aspen Publishers Inc., Gaithersburg, MD, pp. 325372. Aguilera, M.P., Beltran, G., Sanchez-villasclaras, S., Uceda, M., Jimenez, A., 2010. Kneading olive paste from unripe ‘‘Picual’’ fruits: I. Effect on oil process yield. J. Food Eng. 97, 533–538. Altieri, G., 2010. Comparative trials and an empirical model to assess throughput indices in olive oil extraction by decanter centrifuge. J. Food Eng. 97, 46–56. Altieri, G., Di Renzo, G.C., Genovese, F., 2013. Horizontal centrifuge with screw conveyor (decanter): optimization of oil/water levels and differential speed during olive oil extraction. J. Food Eng. 119, 561–572. Amirante, P., Catalano, P., 1993. Analisi teorica e sperimentale dell’estrazione dell’olio d’oliva per centrifugazione [theoretical and experimental analysis of olive oil extraction by centrifugation] La rivista italiana delle sostanze grasse. LXX 1993, 329–335 (in Italian). Amirante, P., Clodoveo, M.L., Leone, A., Tamborrino, A., Patel, V.B., 2010. Influence of different centrifugal extraction systems on antioxidant content and stability of virgin olive oil. In: Preedy, V.R. (King’s College London; University of London, UK), Watson, R.R. (University of Arizona, U.S.A.) (Eds.), Olives and Olive Oil in Health and Disease Prevention. Academic Press, Elsevier Book. pp. 85–93. ISBN 978 0 12 374420 3. Amirante, P., Clodoveo, M.L., Tamborrino, A., Leone, A., 2012. A new designer malaxer to improve thermal exchange enhancing virgin olive oil quality. Acta Horticolture (ISHS) 949, 455–462.

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