Malaxation has been recognized as one of the most critical points in the mechanical extraction process for virgin olive oil. It is a low and continuous kneading of olive paste at a carefully monitored temperature. This essential technological operation helps the small droplets of the oil formed during the milling to merge into large drops that can be easily separated through a decanter centrifuge. During this technological phase, complex bioprocess takes place that is very relevant to the quality and composition of the final product. The malaxer is a heat exchanger characterized by a low overall heat transfer coefficient because the surface area to volume ratio is disadvantageous, so it is important to found innovative technology able to improve heat-exchange. In fact, the malaxing step is the only discontinuous phase in a continuous extraction process. In the next future, the essential challenge of olive oil plant manufacturers is to develop more advanced machines able to transform the discontinuous malaxing step in a continuous phase to improve the working capacity of the industrial plants. In order to reduce the malaxing time enhancing the quality of the product, two ultrasound-assisted virgin olive oil extraction processes were tested versus the traditional method. The sonication treatment was applied on olives submerged in a water bath (before crushing) and on olive paste (after crushing). The ultrasound technology provides a reduction of malaxing time improving extractability of oil and its antioxidant content. Better extractability and higher antioxidant contents were obtained by sonication of olives submerged in a water bath than olive paste. After experimental trials the results were employed to evaluate the scale up of the process and new technological solutions for ultrasonic applications in the VOO industry were proposed.
e) Keywords: Virgin Olive Oil extraction process; Virgin Olive Oil quality; Virgin Olive Oil extraction yields; scale up of the ultrasound process.
1) Introduction
A large increase in the demand for high-quality virgin olive oil continuously stimulating the search for new technologies [1]. In this paper, a new technological procedure being developed that includes ultrasound pre-treatment of olives and olive paste has been tested in order to optimizing the extraction process enhancing the overall quality of the resulting oils. Olive oil extraction process can be separated into three phases, fruit crushing, paste malaxing and separation of the oil (Figure 1). In the modern centrifugal plants, both crushing and separation steps are continuous [2; 3; 4]. Nowadays, the malaxing step is the only batch process. In order to guarantee a continuous process a large series of malaxers are used in the olive oil extraction plants with high capital investment costs for plant. One of the latest challenges in virgin olive oil plant manufacture is the conversion of the traditional malaxing batch process into continuous operation [5]. The first step towards the continuous process consists in reducing the length of malaxation time. Malaxation is a low (20-30 rpm) and continuous kneading of olive paste at a carefully monitored temperature. This essential technological operation helps the small droplets of the oil formed during the milling to merge into large drops that can be easily separated through a decanter centrifuge [2; 3; 4]. The malaxer usually is a semi-cylindrical tank equipped by a shaft with rotating arms and stainless steel blades. The walls of the malaxing tank is hollow allowing warm water to flow through the jacket to heat the olive oil paste. The malaxer is a heat exchanger characterized by a low overall heat transfer coefficient because the surface area to volume ratio is disadvantageous, so it is important to found innovative technology able to improve heat-exchange.
The malaxing phase can be divided in two periods. The first period is definable as "pre-heating", i.e. the time required to the olive paste to achieve the process temperature (30°C). The second period is the "effective malaxing phase". The length of the pre-heating is about 50% of the total process time and is influenced also by the room temperature (the oil mill temperature depends from the weather condition and is about 10-20 °C during the olive harvesting season, from September to January). The ultrasound technology can reduce the pre-heating time. In fact, as an acoustic wave propagates through a plant tissue, part of it is absorbed and converted to heat [6].
In order to ascertain if sonication treatments can reduce malaxing time enhancing the overall quality of the resulting VOO , experimental trials were carried out applying the sonication treatment on olives and on olive pastes. The results of this research should lead to meaningful technological advances in virgin olive oil production. After experimental trials the results were employed to evaluate the scale up of the process and new technological solutions for ultrasonic applications in the VOO industry were proposed.
2) Materials and methods
2.1 Olive fruit samples
Olive fruits (Olea europaea L.) of the Coratina variety were harvested in olive groves of the same area near Bari (Apulia-Italy) in the 2011/2012 crop season. Olives were randomly picked at industrial optimum ripening stage, according to their skin colour. Harvesting was done by hand, using rakes. The olives were put into 30 kg boxes and immediately taken to the pilot plant [7]. Sampling was conducted on 3 consecutive weeks in November. The olive maturation index was determined on a representative sample according to the method proposed by the International Olive Oil Council [8], based on the evaluation of the colour of the skin. The maturation index values were between 1.8 and 2.0. Total oil content and moisture content were determined in order to aid the characterization of the fruit. Moisture (% weight/weight) was determined by drying of milled paste at 105°C to constant weight. The total oil content (TOC) of the olive samples was determined by using the Soxhlet extraction and expressed as percentage on fresh matter basis.
2.2 Oil extraction system
The pilot plant is composed by three units: a hammer crusher, a thermo-malaxer and a basket centrifuge. After washing and leaf-removal, each batch was divided in eleven homogeneous portions. The sonication treatment was applied both on olives submerged in a water bath at room temperature before crushing (2,5 kg of olives in 2 L of water at 19±0.5°C) and on olive paste after crushing (2,5 kg). The experiments were conducted according to the experimental plan as shown in Table 1. The flow chart of the experimental plan is shown in Figure 2.
The olive paste obtained during each test was malaxed. The malaxer was a the stainless-steel container (tank volume: 4,5 L) equipped with an helical blade immersed in a water bath at 35°C (rotational speed: 10 rpm). Each sample was pre-heated until 30°C and then malaxed for 30 minutes at 30°C. Then the olive paste was centrifuged for 2 minutes in a basket centrifuge with an inner diameter of 19 cm of the bowl and a rotational speed of 2700 rpm employing NaCl (0,5 kg) as coadjuvant. Then the oil was recovered. Oil must was collected in a 1 L probe allowing 10 min for phase decantation, the volume of oil was measured.
2.3 High-power ultrasound system
The equipment employed in this work is ultrasonic bath Elmasonic S60H: Ultrasonic frequency - 35 kHz; Tank volume - 4,25 l; Tank internal dimensions - WxDxH 240x137x150 mm - ultrasonic power effective 150 W.
2.4 Temperature Measurement
The temperature values of olive paste were measured employing a mercury-in-glass thermometer (temperature ranges from -10 to 50° C in 0.1 graduations) submerged in the olive paste for 1 minute. The temperature values were measured as described in the flow chart of figure 2. The room temperature during the experimental trials was about 19°C (±0.5). In order to ensure the thermal equilibrium with the ambient, the olive samples were kept at room temperature for five hours before extraction. This precaution is useful to guarantee, in each experiment the same initial temperature of olives that were stored in a thin layer inside holed plastic boxes.
Untreated samples: The temperature values of olive paste were measured and recorded after crushing (immediately before the pre-heating) and every 5 minutes.
Ultrasound treatment of olives submerged in a water bath: As a direct measurement of the temperature distribution inside the whole olives is technically difficult, the measure of temperature on olive paste after the crushing appeared to be a reasonable solution for assessing the increment due to sonication treatment of olives submerged in a water bath, taking into account that the hammer crusher in these experimental conditions warmed up to 3°C the olive paste. Then the temperature values of olive paste were measured immediately before the pre-heating and every 5 minutes. Also the temperature of the water in which the olives were submerged was measured at the end of sonication treatment.
Ultrasound treatment of olive paste: The temperature values of olive paste were measured instantly after crushing, after sonication treatment (immediately before the pre-heating) and every 5 minutes.
2.5 Analytical indices
2.5.1 Determination of oil quality parameters
Determination of oil quality indexes: acidity value, peroxide value and UV absorption (K270 and K232) were carried out according to the analytical methods described in Regulation EEC/2568/91 and next amendments and additions [9; 10].
2.5.2 Determination of chlorophyll and carotenoid contents
Chlorophyll and carotenoid contents were determined colorimetrically following the method of Minguez et al. (1991) [11].
2.5.3 Determination of total phenol content
Phenols were recovered from extra-virgin olive oils by liquid-liquid extraction using methanol as solvent and following the procedure reported in [12]. The total phenolic content was expressed as mg of gallic acid equivalents per kg of oil.
2.5.4 Determination of tocopherol compounds
Tocopherol compounds were determined by HPLC according to the method reported by Psomiadou and others (2000) [13].
2.6 Statistical analysis
The experiments were performed in triplicate and the results were expressed as means ± SD (Standard deviation). Statistical analysis was carried out using Microsoft Excel software. Significant differences between treatments were determined using one-way ANOVA followed by ''t- test''.
3) Results and discussion
3.1 Influence of ultrasound treatment on the olive paste temperature and energy balance
When ultrasonics are propagated into a vegetal tissue, a portion of the wave energy is converted into heat [14]. Figure 3 shows the temperature profiles of olive paste during the sonication treatment on olives submerged in a water bath and on the olive paste. In either case the temperature of olive paste linearly increased as ultrasound treatment time was extended.
In the untreated sample (A) the temperature of olive paste measured after crushing was about 22°C (±0.5). This increment of temperature, respect to the room temperature (19±0.5), was due to the effect of the violent mechanical action of the hammer crusher [15]. In fact, after the crushing step the olive paste raised in temperature of about 3°C, because of a part of the mechanical energy of the crusher was converted into heat energy. The temperature of olive paste samples, obtained from sonication treatment of olives submerged in a water bath (temperature of water 19±0.5°C) and subsequently crushed, were about 24°C (±0.5), 26,5 °C (±0.5), 28°C (±0.5), 29°C (±0.5) and 32 °C (±0.5) using 2 (B), 4 (C), 6 (D), 8 (E) and 10 (F) minutes of sonication, respectively. The results obtained analysing the sample F are not confrontable with the others due to the different final temperature reached, in fact the malaxing fixed temperature was 30°C. The temperature of olive paste samples, measured after the sonication of olive paste, were about 23.5°C (±0.5), 25.5 °C (±0.5), 27°C (±0.5), 28°C (±0.5) and 30 °C (±0.5) using 2 (G), 4 (H), 6 (I), 8 (L) and 10 (M) minutes of treatment, respectively. The mean temperature values show that a more rapid heating was attained by sonication of olives submerged in a water bath respect to olive paste. The more rapid heating is probably due to the presence of water that increased its temperature until 23°C after 10 minutes, during the sonication treatment. The thermal effect of ultrasounds is due to attenuation phenomena. In fact, ultrasonic attenuation is the reduction in energy of the beam as it is transmitted through a medium [16]. Usually, the major source of attenuation in a medium is absorption, which is the conversion of acoustic energy into heat (other mechanisms are reflection, refraction and scatter) [17]. During the experimental trials the temperature of the water into the ultrasonic bath increased, but less than the olive temperature. The minor increment of water temperature respect to the olive temperature was probably due to the scientific evidence that solid, viscous liquids, emulsions and liquids with entrained solids generally have a higher ultrasonic attenuation than low viscosity clear liquids such as water [18].
Figure 4 shows the effects of ultrasound treatment on length of pre-heating time. The pre-heating time is the time required to heat up the olive paste until it reaches the malaxation temperature (30°C); it decreases depending of the increase of sonication time. On the contrary, the length of malaxation was equal for all the samples and fixed at 30 minutes. In the untreated olive paste sample (A), the pre-heating time was 30 minutes (±5). In a previous work Jiménez et al. [19] studied the effect of high-power ultrasound on olive paste: all samples were malaxed for 30 minutes and no distinction was made between the pre-heating and the "effective malaxation". The choice of varying the pre-heating time depends on the purpose to give to all samples the same amount of heat, thus reducing the length of extraction time.
Without ultrasound treatment, equation (1) describes the energy balance of the process: it says that the power transferred by the hot water through the jacket corresponds to the power received by the olive paste inside the malaxer (assuming no heat transfer to the environment):
Qtot = Qpre-heating + Qmalaxing = m C ΔTtot
Qmalaxing was constant for all samples.
Qpre-heating = m C ΔTPH
ΔTPH = TfPH - TiPH
Equation (2) says that the amount of heat (Qpre-heating) (J) transferred to the olive paste at the end of pre-heating is computed by multiplying mass (m) (kg) of olives or olive paste by the specific heat (C) (J/(kg°C)) of the olive paste and by the temperature difference ΔTPH between the beginning (initial temperature: TiPH) and the end (final temperature: TfPH) of the process.
With ultrasound treatment, the energy balance is written as in Euation (4):
Qtot = PUS + Qpre-heating + Qmalaxing = m C ΔTtot
where PUS is the ultrasonic power dissipated into heat in the olives or olive paste and, if a basic energy balance is performed onto the system working in steady state, it will be found that the ultrasonic power delivered PUS is thermally recovered by the olives or olive paste as written in Eq.(5) [20]
PUS = m C ΔTUS
Therefore, the total amount of heat transferred to the olives or the olive paste at the end of ultrasound treatment PUS (J) is computed by multiplying mass (m) (kg) of olives or olive paste by the specific heat (C) (J/kg°C) and by the temperature difference ΔTUS between beginning and the end of ultrasonic treatment:
ΔTUS= TfUS - TiUS
where TfUS is the final temperature at the end of ultrasound treatment and TiUS is the initial temperature at the beginning of ultrasound treatment. In all the experiments occurs TfUS ≤ TfPH , with the only exception of sample F (over-heating sample) where TfUS > TfPH .
In the performed tests, mass and specific heat were constant, so the amount of heat transferred depended only on the difference between the initial and final temperature of the process ΔTtot. Since ΔTtot was equal for all the samples (except to the sample F), in all trials the same quantity of heat (Qtot) was transferred.
3.2 Influence of ultrasound treatment on the length of malaxation time
Employing the sonication treatment on the olives submerged in a water bath, the length of pre-heating was about 23, 16, 9, 2, and 0, minutes with 2, 4, 6, 8 and 10 minutes of sonication, respectively. So the length of pre-heating was reduced about of 23%, 47%, 70%, 93% and 100% with 2, 4, 6, 8 and 10 minutes of sonication, respectively. Employing the sonication treatment on the olive paste, the length of pre-heating was about 24.5, 19, 13.5, 8, and 0 minutes with 2, 4, 6, 8 and 10 minutes of sonication, respectively. So the length of pre-heating was reduced about of 18%, 37%, 55%, 73% and 100% with 2, 4, 6, 8 and 10 minutes of sonication, respectively. The analysis of the results shows that the pre-heating time was shorter employing the sonication treatment of the olives submerged in a water bath than of the olive paste.
3.3 Influence of ultrasound treatment on VOO yields
Olive paste malaxation is an important step of process since produces good oil extraction yields. In fact, increasing the malaxing time, in general, improves the oil extraction yield [5]. Considering this economical parameter, the oil millers tend to increase malaxing time. However, the increase in malaxing time results in the decrease of some nutritional characteristics of VOO. The extraction yield (Yield (%)) was calculated as the percentage on weight (g) of the VOO extracted (Woil = volume measured by olive oil density, 0.915 g/ ml) from the weight of olive (Wolives ) (on a fresh matter basis):
Yield (%) = Woil / Woilives ∙ 100
Extractability index (EI) has been defined as the percentage of VOO extracted from the total oil content of the fruit (TOC) (determined by Soxhlet Method on a fresh matter basis) [5]. The "extractability index" (EI) was calculated using the formula:
EI = Yield (%) / TOC (%) ∙ 100
This parameter indirectly takes in account the oil content lost in the by-products (vegetation water and pomace).
In Table 2, the effect of the ultrasound treatments on yield and oil extractability are shown. It can be seen that ultrasound application on olives and on olive paste for 8 minutes can produce a significant improvement (p < 0.1) on oil extractability of "Coratina" variety. While ultrasound treatment on olives and on olive paste for 4 minutes had non-significant extractability differences.
In the pilot plant the medium increase of VOO extracted after 8 minutes of sonication was about 8 g per kilogram of fruits. In an industrial full scale plant with a working capacity of 2000 kg/h, if the plant works 8 h/day, with an average extraction yield of 16%, the quantity of VOO extracted per day is about 2500 kg. Employing the ultrasound technology for 8 minutes (Ultrasonic frequency - 35 kHz; ultrasonic power effective 150 W), assuming at least an increase of about 5 g of VOO extracted per kilogram of fruits, the quantity of VOO extracted per day could increase about of 80 kg with an increment of 3% of the total VOO production. This calculation doesn't take into account that the ultrasound treatment for 8 minutes determines a reduction of the pre-heating time of about 70%; also this aspect could contribute to increase the quantity of VOO extracted per day due to the higher working capacity of the innovative plant in confront of the traditional system.
3.4 Influence of ultrasound on VOO quality indices
According to EU Commission Regulation 1989 (2003), extra virgin olive oil is a liquid fat that conforms to a series of chemical and sensory parameters (free fatty acid content ≤ 0.8 g oleic acid/100 g oil, peroxide value ≤ 20 meqO2/kg, K232 ≤ 2.50, K270 ≤ 0.22, median of defects = 0, median of fruity >0), and is free of defects. Table 1 summarizes data relative to the qualitative parameters of VOO extracted employing the traditional method and after sonication treatment of olives submerged in a water bath and of olive paste for 8 minutes obtained in three different experiments . All the oils examined were defined as belonging to the commercial class of extra virgin olive oil. In fact, all samples showed very low percentages of free fatty acids and peroxide value (index of primary oxidation),and all samples were always below the legal limit. No difference attributable to the innovative technology was highlighted confirming that ultrasounds did not affect oil acidity and peroxide value. Also the values of K232 (another index of primary oxidation) and K270 (an index of hydroperoxides degradation) were not influenced by the ultrasound treatment. All the oils were free from defects. The oils obtained applying the sonication treatment on olives showed a more strong bitter and pungent tastes than the oils obtained applying the sonication treatment on olive paste. This effect was due to the different total phenol content. In fact polyphenols are responsible for the typical bitter taste of VOO [21].
3.5 Influence of ultrasound on VOO antioxidant content
Phenols, tocopherols and carotenes are the most abundant natural antioxidants of VOO. Several important biological properties (antioxidant, anti-inflammatory, chemopreventive and anti-cancer) and the characteristic pungent and bitter tasty properties have been attributed to VOO phenols [22]. Also the study of chlorophyll and carotenoid pigments in virgin olive oil has traditionally been of interest because of their contribution to the VOO stability and to the colour [23]. Colour of VOO has an obvious effect on consumer preference and acceptance. The mechanical oil extraction process affects a number of sensory and health parameters of VOO [24]. The release of antioxidants in the oil, which greatly affect the quality of virgin olive oil, are directly related to the extraction process [25]. Also the ultrasonic process affect these parameters. In particular, two main mechanisms are involved in the ultrasonic treatment on olives and on olive paste: a thermal effect and a mechanical effect. The thermal effect was widely explained in the previous paragraph. The mechanical effect is due to cavitation or particulate streaming which cause violent movement of the particles of the medium. Sound waves, which have frequencies higher than 20 kHz, are mechanical vibrations in a solid, liquid and gas. Unlike electromagnetic waves, sound waves must travel in a matter and they involve expansion and compression cycles during travel in the medium. Expansion pulls molecules apart and compression pushes them together. The expansion can create bubbles in a liquid and produce negative pressure. The bubbles form, grow and finally collapse. Close to a solid boundary, cavity collapse is asymmetric and produces high-speed jets of liquid that have strong impact on the solid surface [26; 27] and can disrupt biological cell walls. The mechanical effect of ultrasounds promotes the release of soluble compounds from the plant body by disrupting cell walls [28] and improves mass transfer also in the olive tissues. The cells of olive mesocarp contain a number of antioxidant compounds. Often the crusher doesn't break all the cells, so many substances remain inside the by-products, such as the olive pomace [29]. Ultrasounds can disrupt a proportion of the oily cells remaining uncrushed during the crushing step allowing the recovery of another oil fraction enriched in antioxidants. Figure 5 shows the changes in total chlorophylls , carotenoids, phenols and tocopherols content in VOO after sonication treatment of olives submerged in a water bath and of olive paste. The results obtained until 6 minutes show that a more rapid extraction of antioxidant compounds was attained by sonication of olives submerged in a water bath respect to olive paste.
In general, the chlorophylls increased as ultrasound treatment time was extended on olives and on olive pastes (Fig. 5a). The rate of this increment seems to differ with the sonication system applied. The oils obtained applying the sonication treatment on olives showed a higher total chlorophyll content than the untreated samples and the oils obtained applying the sonication treatment on olive paste in each experimental conditions. This result was consistent with observations of the bright green colour at the time of processing. As a large proportion of the chlorophylls is contained in the epicarp, or skin, it appears the sonication is an efficient method able to break up the epicarp to release more chlorophyll. Chlorophylls are considered an advantage in providing an aesthetic appearance to the oil and can also works as an antioxidant when the oil are stored in the dark [30].
When the sonication treatment was applied on olives, the curve of total carotenoid content as a function of sonication time had a bell shape distribution (Fig. 5b). These results indicate, effectively, that initially the increased of sonication time improved the total carotenoid content due to the cavitation phenomena able to degrade the wall oil-bearing cells. After achieving the maximum value, the curve of total carotenoid content decreased with the sonication time increasing. When the sonication treatment was applied on olive paste the total carotenoid content increased as ultrasound treatment time was extended.
Also total phenol content increased as ultrasound treatment time was extended if the treatment was applied on olives (Fig. 5c). The high levels of polyphenols in the oils extracted from sonicated samples showed clearly that polyphenols are extracted more efficiently employing the innovative system than the traditional one. On the other hand, when the sonication treatment was applied on olive paste the total phenol content decreased as ultrasound treatment time was extended. These two opposite trends probably were due to the action of oxygen, in fact it serves as a cofactor in many enzymatic reactions and as promoter of non-enzymatic oxidations. When the olives were submerged in a water bath, two factor protected the phenols from oxidation: firstly, in the whole olives the enzymes (i.e. polyphenol-oxidase and peroxidase) and the substrates (phenols) are separated in different compartments of the cell while in the olive paste both enzymes and substrates are released; secondly the presence of water protect the olives against the action of oxygen that promote oxidative reactions.
Similarly the tocopherols, when the sonication treatment was applied on olives, the curve of tocopherols as a function of sonication time had a bell shape distribution. Both classes of compounds showed the highest value after six minutes of sonication (Fig. 5d). When the sonication treatment was applied on olive paste the tocopherols increased as ultrasound treatment time was extended. Tocopherols are associated with the quality of the oils and they are of great nutritional significance in health and disease [31].
It is possible to conclude that in order to reduce oxidation phenomena due to the presence of oxygen, the sonication treatment on olive paste should be conducted under hermetic conditions saturated of inert gas.
3.6 Use of ultrasound technology on an industrial olive mills: From the laboratory to commercial production
Ultrasonic processing on VOO is still in its infancy [32] and requires a great deal of future research in order to develop the technology on an industrial scale, and to more fully elucidate the effect of ultrasound on the properties of olives, olive paste and VOO. The use of ultrasonic industrial processes has two main requirements: a liquid medium (even if the liquid element forms only 5% of the overall medium) and a source of high-energy vibrations (the ultrasound). The vibrational energy source is represented by the transducer which transfers the vibration (after amplification) to the so-called "sonotrode", which is in direct contact with the processing medium.
In order to develop a project idea two types of piezoelectric transducer design were considered: plate transducer and rod-style transducer (figure 6). These transducers are easily installed into an existing facility: the plate transducers are the ideal solution in retrofitting existing devices in areas where space is a premium. They are fitted to the outside of containers with welded or press-on mounting frames, thus saving space. It is possible to collocate this transducer into the olive washing machine in order to apply the sonication treatment the olives submerged in the water; the rod-style transducer has a omnidirectional sonic distribution that creates a homogeneous sound field. It is possible to collocate this transducer into the malaxer in order to apply the sonication treatment on olive paste. A third hypothesis may be represented by a new device placeable between the crusher and the malaxer: an ultrasonic reactor for a continuous process. An ultrasonic reactor consist of the reactor vessel and the ultrasonic sonotrode. After crushing, the olive paste then can fall into a closed ultrasonic reactor which is airtight preventing oxidisation. At the end of sonication treatment, a pump can be utilised to move the sonicated and pre-heated olive paste into the malaxer. In order to reduce the time of sonication treatment it is possible to combine a heat exchanger with the ultrasonic reactor [33]. In fact, in a previous paper Amirante et al. [1] tested the introduction of a heat exchanger in a VOO industrial plant. The heat exchanger gave rise to the faster attainment of the temperature of the malaxation and had a positive effect on the total phenol content. Ultrasonics can enhance the effects of the tested heat exchanger by inducing acoustic cavitation, acoustic streaming, and fluid particles oscillations that are responsible for heat transfer improvement [33]. Obviously, the implementation of the ultrasonic reactor with a heat exchanger requires future research in order to optimize the design and improve the inside fluid dynamic, in analogy to other mechanical systems [2; 34]. Figure 6 shows the schematic representation of the described devices.
Ultrasonic processing is establishing itself as a significant food-processing technology with the capability for large commercial scale-up and good payback on capital investment [35]. Significant improvements in product quality, process enhancement and cost reduction are achievable on a commercial scale. Any ultrasonic process will be scaleable using energy (the energy input per volume treated material (in kWh/L) and intensity (the actual power output per surface area of the sonotrode (in W/cm2) [20] of ultrasound system tested in the pilot plant. In fact, both energy and intensity are independent of scale.
In order to achieve the same technological effects of the pilot experiments in a full-scale plant, it is possible calculate the specific energy required in the sonication treatment. Specific energy (Es) is defined as the energy per unit mass. Common metric units are J/kg. Equation (9) describes the specific energy:
Es= P ∙ t / m
where P is ultrasonic power effective, t is the time of ultrasonic treatment and m is the mass of olives and olive paste. Depending on the application, the amount of energy required per kg material treated is comparable to any other unit operation in the industry.
4) Conclusions and perspectives
Ultrasound is well known to have a significant effect on the rate of various processes in the food industry. Using ultrasound, full reproducible food processes can now be completed in seconds or minutes with high reproducibility and reducing the processing cost. The considerable interest in high-powered ultrasound is due to its promising effects in food processing, such as higher product yields, shorter processing times, reduced operating and maintenance costs, improved taste, texture, flavour and colour. Also VOO industry can take advantage of ultrasound systems in extraction process. In fact, ultrasounds reduce the malaxation time enhancing the quality of the product. Moreover, the ultrasound technology provides a quick-heating of olive paste, improvement in process extractability and an high antioxidant content of the resulting oils. Sonication during VOO extraction process can be easily transferred to an industrial scale. This technology could represent the first step toward a continuous malaxing phase. A continuous process presents potential advantages such as minor operating costs, minor capacity limitations, faster return on investment, lower cost of production, reduced energy demands, reduced work-in-progress, faster and easier cleaning, real-time quality control and significantly reduced facility footprint.
Authors' contributions
ML Clodoveo conceived the study design, wrote the manuscript and with the collaboration of V Durante and D La Notte extracted the oil samples and performed the analytical evaluations, performing the statistical analysis and the data interpretation. All authors read and approved the final manuscript.
Appendix:
ABBREVIATIONS USED
VOO = Virgin Olive Oil
NOMENCLATURE AND UNITS
Qtot = total amount of heat transferred during the thermal process
Qpre-heating = The amount of heat transferred to the olive paste at the end of pre-heating
Qmalaxing = The amount of heat transferred to the olive paste during malaxation
m = mass of olives or olive paste
C = specific heat of the olives or olive paste
ΔTPH = the temperature difference between the beginning and the end of the pre-heating
TiPH = initial temperature of the pre-heating
TfPH = final temperature of the pre-heating
PUS = is the ultrasonic power dissipated into heat in the olives or olive paste
ΔTUS = the temperature difference between the beginning and the end of ultrasonic treatment
TfUS = the final temperature at the end of ultrasound treatment
TiUS = the initial temperature at the beginning of ultrasound treatment
Woil = weight of the VOO extracted
Wolives = weight of olive
TOC = total oil content of the fruit
EI = extractability index
Es = Specific energy (energy per unit mass)
P = ultrasonic power effective
t = time of ultrasonic treatment
