1. Introduction
The extraction process affects significantly the virgin olive oil (VOO) composition and its health and sensory characteristics [1-3]. Oil extraction begins with fruit crushing to liberate the oil drops contained in the mesocarp cells. Then olive paste malaxation was carried out to group the oil drops and produce a continuous oily phase. Solid-liquid separation is carried out in a horizontal screw solid bowl (HSSB), to separate olive oil from the water and solid organic matter. Finally to eliminate the water and suspended particles which still remain in the oily must, oil clarification is carried out by «vertical centrifugal separators» (VCS), or/and natural settling.
Vertical centrifugation is the most used clarification method since is faster, requires little labor and lets to separate impurities effectively. A variable amount of lukewarm water, from 1:1 to 1:2 v/v oil/water, is added to the centrifuge generating a liquid effluent with high contaminant activity (from 20 to 40L of aqueous-by-product per 100 kg of olives), but also affects to VOO characteristics decreasing its phenol content, volatile compounds, induction time, and bitterness [3-6].
The natural decantation is carried out by conical bottom settling tanks (CBSTs), this clarification system does not require water addition although it is not instantaneous since solid particles and water contained in the oil are progressively settled out by gravity action. These tanks, usually, are built in stainless steel, have a cone angle between 45 and 60°, a capacity between 400 and 10000 L, and can be used for both static and dynamic conditions. The time for VOO clarification before its storage ranges between 24 and 48 h. To preserve VOO quality, impurities should be periodically purged from the bottom of the settling tanks. Therefore, this clarification method needs more time and personal attention.
Despite the importance of oil clarification on VOO quality a limited number of studies have been reported. For vertical centrifugation the experiments have been carried out always adding a substantial volume of water [1,5-7]. Although natural settling is used for oil clarification, works describing the clarification of VOO from 2-phases decanter are not available. Furthermore, data about comparison between natural settling and vertical centrifugation were not carried out yet.
In the present study, «Picual» virgin olive oils obtained from two phase continuous process were subjected to either natural settling in static conditions and vertical centrifugation working with a considerable reduction of the added water amount (only 5% of water regarding to oil), with the aim to evaluate how both clarification methods affect the VOO quality parameters, composition, and sensory characteristics.
2. Material and methods
2.1 Plant material.
This work was carried out during three non-consecutive crop years, 2010/2011 (first crop year), 2011/2012 (second crop year), and 2013/2014 (third crop year). For each year, around 5000 kg of «Picual» olive fruits were picked from the experimental orchard of the center «IFAPA Venta del Llano» (Mengíbar, Jaén). Then, olive fruits were immediately transported to the oil mill for processing.
2.2 Olive oil extraction system
The experiments were performed in the experimental oil mill (Pieralisi, Spain), of IFAPA Center «Venta del Llano» in Mengíbar (Jaén, Spain). It consists of a metallic hammer mill with a hole grid diameter of 6mm, a thermobeater formed by three containers of 600 kg each, a two-three phase horizontal centrifuge Pieralisi SC-90 (working at two phases way).General processing conditions were: 45min malaxation time, 28°C malaxation temperature, 3.500 rpm centrifugation speed for horizontal decanter and an olive paste flow rate of 800-900 kg/h.
2.2 Clarification systems
The experimental oil mill was also equipped with two different clarification systems working as follows:
- A vertical centrifugal separator (P1500, Pieralisi, Spain), with a nominal working capacity of 1500-2000 L/h, which operated during the experiment at 6500 rpm, fed with an oil:water ratio of 1:0.05 and a water temperature similar to the oil. Before starting each experiment, a discharge of VCS was carried out.
- A battery of conical bottom settling tanks (Secovisa, Spain) with a capacity of 400 liters each were used. The tank dimensions are: 67×122 cm (0 x h) for the top cylinder and an angle of the bottom cone of 35°. The time needed to fill the tank was 205 minutes approx., the oil temperature during settling was 22 ± 2°C. During the oil settling, purges were carried out at 24 hours for the first and second crop year, and at 5 h and 24 h for the third crop year. These purges were performed at the tank bottom until clear oil was observed.
2.4 Experimental design and sampling.
For each year, around 800 L of VOO were clarified. The VOO from HSSB was passed through a 1mm vibrational sieve. Then, the oil was pumped to the clarification systems simultaneously; half of this oily must (400 L) was clarified by VCS whereas the rest was used to fill a CBST, in order to compare both clarification systems using the same oil (Fig. 1). In the second crop year (2011/2012), some variables parameters of the horizontal centrifuge «decanter» were regulated to obtain a VOO with higher impurities content than the other two crop years studied.
During the CBST filling, oil samples were taken at different time of the experiment. Three oil samples were taken from horizontal centrifuge as reference after passing the vibrational sieve («HSSB oil»). Then three oil samples were taken from the VCS outlet (VCS oil). Both VCS and HSSB oil samples were picked at the beginning, the half-fill and the end of the CBST filling.
When CBST was full, oil was allowed to settle for 48 h. Oil samples were taken by triplicate, at 24 h («CBST oil 24 h») and 48h («CBST oil 48 h») of settling, from a depth of 18 cm from the surface.
During the CBST filling, oil samples were taken at different time of the experiment. Three oil samples were taken as references after passing the vibrational sieve (‘HSSB oil’). Then three oil samples were taken from the VCS outlet (VCS oil). Both VCS and HSSB oil samples were picked coinciding with the beginning, the half-fill and the end of the CBST filling.
When CBST was full, oil was let settling for 48h. Oil samples were taken at 24h (‘CBST oil 24h’) and 48h (‘CBST oil 48h’) of settling, from a depth of 18 cm from the surface. Samples were taken by triplícate.
Figure 1. Diagram of the assay for the three crop years with sampling points of decanter oil (HSSB oil), vertical centrifuge separator oil (VCS oil) and conical bottom settling tank oil (CBST oil).
2.5 Analytical determinations.
Moisture and impurities content were quantified in the oils before filtering. Then, oil samples were paper-filtered and stored at -24°C until analysis.
2.5.1 Moisture and solid organic impurities.
To determine water content, samples of olive oil (approximately 10 – 20 g in a capsule with filter paper) were dried in an oven at 105 °C until weight stabilizing. The loss of weight gave the amount (%) of water and volatile matter in the sample [8]. Solid organic impurities content (%) was determined by introducing the dried sample in a SOXHLET system to eliminate the oil [9]. From these data, the clarification efficiency for the different clarification systems was calculated by Equation 1:
where Ce is the clarification efficiency (%) for each clarification system, c¡o is the moisture and solid organic impurities (MSOI) content of the initial oil from ‘HSSB’ (%) and cco is the MSOI content of the oil after clarification (%) by VCS, CBST at 24h or CBST at 48h.
2.5.2 Quality indexes.
Free acidity (FA), peroxide value (PV), and ultra-violet absorption at 232 and 270 nm (K232 and K270) were measured following the analytical methods described in European Regulation EEC 2568/91 [10]. FA was expressed as percent of oleic acid, PV were expressed as milli-equivalents of active oxygen per kilogram of oil (mEq O2/kg); K232 and K270 extinction coefficients were calculated from absorption at 232 and 270 nm, respectively. For the last year, fatty acid ethyl esters (FAEEs) content was analysed [11], results were expressed as mg/kg.
2.5.3 Total polar phenol content.
Phenolic compounds were extracted from an oil-in-hexane solution with methanol:water (60:40) and their concentration was measured using Folin-Ciocalteau reagent and colorimetric measurement at 725 nm [12]. Results are expressed as mg/kg of caffeic acid.
2.5.4 Index K225.
The bitterness index K225 was determined applying the method described by Gutierrez et al. [13].
2.5.5 Tocopherol content.
Tocopherol composition was analyzed by HPLC, applying the IUPAC method 2432 [14]. Results are expressed as mg/kg of VOO.
2.5.6 Pigments content.
Carotenoid and chlorophyllic pigments were determined measuring the absorbance of olive oil weighed and dissolved in cyclohexane at 470 and 670 nm as described by Mínguez-Mosquera et al. [15]. The results were expressed as mg/kg.
2.5.7 HPLC analysis of phenolic compounds.
The individual phenolic compounds were determined for the third crop year. The Phenolic Compounds were extracted with methanol/water and the extracts were analyzed by RP-HPLC [16]. Phenolic compounds were quantified at 280 nm using syringic acid as internal standard and the response factors determined by Mateos et al. The results were expressed as mg/kg.
2.5.8 Sensory analysis.
For the second and third year, sensory characterization was performed by a virgin olive oil analytical sensory panel of Fundation Citoliva formed by trained tasters as described by EUC Regulation [10]. The results were expressed as the median of the intensity of the sensory attributes.
2.6 Statistical analysis
ANOVA analysis was performed in order to compare the effect of vertical centrifuge and natural decantation at two different times (24h and 48h) on ‘Picual’ virgin oils quality and composition. When a significant F value was found, Tukey’s HSD was used to test differences between means (p < 0.05). These statistical analysis were carried out using software Statistix, version 8.0.
3. Results and discussion.
3.1 Characteristics of the initial VOO from HSSB.
HSSB before clarification for the three crop years are shown in Table 1. Because of the variability in the agroclimatic conditions for the crop years studied, the fruit characteristics changed and then, the oils from HSSB used for the clarification experiments showed important differences in composition.
Oil moisture and solid organic impurities ranged between 1.56 and 13.82% for moisture and from 0.32 to 1.09% for organic impurities. In general, oil composition showed high variability although it agreed with the values reported for «Picual» oils FA, PV, K232, K270, and FAEEs values let to classify all «HSSB oils» into the category extra-virgin as established by the EU regulation [10].
Table 1. Quality indexes, minor compounds and bitterness index of the oils from HSSB for the three crop years.
