Influence of malaxation time of olive paste on oil extraction yields and chemical and organoleptic characteristics of virgin olive oil obtained by a centrifugal decanter at water saving

Se realizaron pruebas experimentales para verificar la influencia del tiempo de batido sobre los rendimientos en aceite y sobre las caracteristicas de la calidad de los aceites obtenidos con un decanter centrifugo con ahorro de agua. Los resultados conseguidos indicaron que el tiempo de batido no debe ser inferior a 45 minutos para poder obtener rendimientos en aceite satisfactorios. Ademàs, se pudo verificar que la operacion de batido, aùn siendo prolongada a 90 minutos, no influencia significativamente en las caracterìsticas cualitativas y organolépticas de los aceites. Sòlo el contenido de fenòles totales en los aceites disminuyò cuando el tiempo de batido fue incrementado de 15 a 90 minutos. Sin embargo, se demonstrò que en algunos casos el contenido de fenòles totales en los aceites aumentò durante los primeros 15-45 minutos de batido mientras, succesivamente, disminuyò de acuerdo con los resultados de la literatura. Esto se debe a la variaciòn del contenido de fenòles totales en las aguas de vegetaciòn que anteriormente aumentaron y luego disminuyeron. Debido a la ley del equilibrio de particiòn, también el contenido de los fenòles totales en sus correspondientes aceites variò de la misma manera. Finalmente, se hace notar que la composiciòn de las sustancias volàtiles del espacio de cabeza de los aceites no varia significativamente al variar el tiempo de batido de las pastas de aceitunas de buena calidad.


INTRODUCTION
Olive processing by mechanical systems (pressure, percolation, centrifugation) needs the malaxation of olive paste to increase the extraction yields.The malaxation is an useful technological operation that entails stirring the olive paste slowly and continuously to favour the droplets of oil to merge into large drops (Martinez Moreno et al., 1957), that can be separated into a continuous liquid phase ("free oil") extractable by pressure (Di  Giovacchino et al., 1984-87), by percolation (Di  Giovacchino et al., 1988), and centrifugation systems (Vitagliano et al., 1972; Di Giovacchino, 1991).
The use of metallic crusher to crush olives, because of its violent action, generally causes an emulsion of olive paste that reduces oil yields.To avoid or cut down the emulsion state, it needs to increase the malaxation time and, in some cases, the malaxation temperature also (Solinas et al., 1978).This may cause a negative influence on the qualitative characteristics of virgin olive oil because of the activity of the enzymes, naturally present in the olive paste, which may develop affecting the percentage of free fatty acids content, peroxide value and phenol content of oil.
At begins of '90s, new centrifugal decanters were launched on the market with the end to reduce the quantity of water added to olive paste (Di  Giovacchino et al., 1994 b).Actually the centrifugal decanters utilized in olive oil mills, in Italy and in other countries, are the following : i) centrifugal decanter at 3 phases, type conventional : it operates by adding 50-70 liters of lukeworm water per 100 kg of olive paste, and separates oil, pomace (45-55% of moisture) and vegetable water (70-90 L/100 kg of olives) by 3 flow exits; ii) centrifugal decanter at 2 phases, type integral: it operates without adding lukewarm water to olive paste and separates oil and pomace (65-72% of moisture) by 2 flow exits only; iii) centrifugal decanter at 3 phases, type "water saving": it operates by adding 0-20 liters of lukewarm water per 100 Kg of olive paste, and separates oil, pomace (55-60% of moisture) and vegetable water (5-30 L/100 kg olives) by 3 flow exits.
All types of centrifugal decanters have the possibility to give a good oil yields if the olive paste is fairly mixed and for this the malaxation operation is very important for the quality of oil.
In this paper, the influence of malaxation time on extraction yields, qualitative characteristics, total natural antioxidants content and composition of volatile compounds of oils obtained by the new centrifugal decanter at water saving is studied.In addition, the coefficient of partition of total phenols between oil and vegetable water is calculated.

EXPERIMENTAL PROCEDURES
Olive oil extraction tests by centrifugation system were performed during olive harvest season, using a new centrifugal decanter, manufactured by Rapanelli S.p.A., Foligno, Italy.
This decanter, type 4500-S and called at water saving, has the possibility to separate oil from the pomace and vegetable water using a small quantity of lukewarm water added to olive paste.
Tests were carried out using different cultivars of olives (Dritta 1; Mixed varieties 1; Leccino; Dritta 2; Mixed varieties 2), picked on November and processed following the working diagram reported in the Figure 1.After leaf-removal and washing, an homogeneous lot of kg 1,600 of olives of each cultivar was utilized.This quantity was divided in four batches, each one of 400 kg, and utilized in this way: the first batch was used to make ready the machinery and other three batches for the tests at different times of malaxation.Each batch of olives was crushed by a mobil hammer metal crusher and the olive paste obtained was mixed at 24 o C for 0, 30 and 75 minutes in the first sep of malaxation.Then, olive paste was poured in the homogenizer (2nd step of malaxation) and immediately centrifuged in the above mentioned centrifugal decanter at water saving loaded with 800 kg/h of olive paste and with addition of 0-25 L of water at 24 o C per 100 kg of olive paste.
Oil was separated from the oily must (olive oil + vegetable water) by an automatic discharge vertical centrifuge, without addition of clean water.
To estimate the oil yields of the olive processing, the weight of olive fruits and pomace and the volume of vegetable water were determined.Oil extraction yields have been also calculated as the percentage of oil extracted by industrial centrifugal decanter (kg/100 kg olives) and oil content (kg/100 kg olives) of olive fruits measured by Soxhlet apparatus.
On olive fruits and pomace samples were determined: -moisture content, in oven at 105 o C; oil content, by Soxhlet apparatus, using petroleum ether 40-60 o C. On vegetable water samples were determined: -dry matter, in oven at 105 o C; oil content, by Soxhlet apparatus (Di Giovacchino, 1986); total phenol content (Singleton et al., 1965),expressed as gallic acid.On olive oil samples were determined: 180 Grasas y Aceites -free fatty acids (%), peroxide value (m.eq.O 2 /kg), specific spectrophotometric absorptions in the UV region and organoleptic assessment (score) (EEC Rule, 1991); -total phenol content (Ragazzi et al., 1973), expressed as gallic acid; -induction time (hour), measured by Rancimat 679 apparatus (Laubli et al., 1986), at 120 o C and with an air flow of 20 L/h; -chlorophill pigments (Wolff, 1968); -the composition of volatile substances after stripping the oil with nitrogen at 37 o C for 2 hours (Camera et al., 1990).The volatile substances were adsorbed on active carbon and then transferred into test-tubes containing 1 ml of diethyl ether.The solution thus obtained was injected on column into gas-chromatograph (type HRGC 5160 Mega series, Carlo Erba Instruments S.p.A., Rodano, Italy) fitted with a fused silica capillary GC column (NB 20 M, 50 m x 0.32 mm, film tickness 0.50 m, HNU-Nordion Ltd, Helsinki, Finland) using hydrogen as the carrier gas.Temperature program varying from 25 to 155 o C, with a FID detector at 180 o C, was used.The identification of components was carried out by comparison of RRT of standard components following the same conditions reported in other paper (Camera et al., 1990).The concentration of each component was determined by using 1-nonanol as internal standard.The results obtained were tested by t or ANOVA tests.

RESULTS AND DISCUSSION
Table I shows the influence of malaxation time of olive paste on oil yields and by-products characteristics obtained processing 5 different olive varieties by centrifugal decanter at water saving.Table II shows the average values of the same results statistically treated (test t).
Oil yields significantly increased when malaxation time changed from 15 to 45 and 90 minutes, in agreement with results reported in other papers (Vitagliano et al., 1972; Di Giovacchino et al.,  1984-87; Di Giovacchino, 1991).The quantity of oil mechanically extractable by the centrifugation system depends from the quantity of free oil present in the olive paste after crushing and malaxation operations.
Olive crushing by metallic hammer crusher, generally, determines an emulsion that it is possible to reduce only by the malaxation, carried out for 45 minutes at least, in relation to the olive characteristics.When the malaxation time was 15 minutes, the oil yield was very low, especially for some olive cultivars, difficult to process, as shown in the Figure 2.
The different oil yields are due to the different quantity of oil lost in the by-products (pomace and vegetable water), obtained processing olive paste by the centrifugal decanter.The average quantity of oil lost in the pomace changed from 3.1 to 2.2 kg/100 kg olives, when the malaxation time increased from 15 to 90 minutes, as shown in Table II, in agreement with the results reported in other paper (Hermoso et , 1996).The average quantity of oil lost in vegetable water changed from 0.7 to 0.3 kg/100 kg olives, when the malaxation time increased from 15 to 90 minutes, as shown in the same Table II.Finally, the total oil lost in the by-products was, on average, 3.8 and 2.5 kg/100 kg olives (statistically significant difference), when the malaxation time changed from 15 to 90 minutes.This confirms that the malaxation time of olive paste is important to have good yields and for this it has to be no lower than 45 minutes, when olives are processed by centrifugal decanter, as reported in other paper (Di Giovacchino, 1991).
Results obtained in the analysis of virgin olive oil samples are shown in Table III and in Table IV, where the average results, statistically treated, are also reported.The values of free fatty acids percentage, peroxide number, specific spectrophotometric absorptions in the UV region and organoleptic assessment of oils, obtained changing the malaxation time, are not statistically different.This means that oils, obtained from good quality olives by centrifugal decanter after a 15-90 minutes of   ).This is due to the protective action of the great mass of olive paste utilized in the industrial tests.In these conditions, during the malaxation, only a little part of olive paste is exposed to the air and may be oxidized, but other large part is protected from the oxidation and oil cannot change its chemical characteristics in the short time of malaxation operation.
The total phenol content only significantly diminished (ANOVA test) when the malaxation time increased from 15 to 90 minutes, in agreement with the results reported in other papers, before named.
As reported in Table IV, the total phenol content of oils, obtained from olive paste mixed for 15 and 90 minutes, was, on average, 269 and 225 mg/L, with a 16% of reduction.This small difference is normal when tests are carried out with industrial plants and large quantity of olive paste is mixed for different times.In this case, only the superficial layer of olive paste (a very small percentage), and relative phenolic compounds, is exposed to air and to oxidation, whereas a large part is covered and effectively protected by the reducing conditions due to the antioxidant properties of phenolic substances present in large quantity.
When the same tests are carried out using small quantity of olive paste (1-10 Kg) and small laboratory plants, it is possible to have oils with a higher difference of total phenol content.This happens because the superficial layer of olive paste exposed to air and oxidized represents a high percentage of total paste.This causes a remarkable reduction of total phenol content of olive paste, vegetable water and oil, as reported in other papers (Servili et al.,  1992; Servili et al., 1994; Montedoro et al., 1994;  Lercker et al., 1999).
In particular, in this research, it was observed that, in some industrial tests, the total phenol content rose when the malaxation time changed from 15 to 45 minutes, as shown in the Figure 3.This phenomenon was unexpected, and to have a confirmation, the total phenol content of vegetable waters was determined.
Results show that, at beginning of malaxation, the phenol content of vegetable water rose, and after it diminished, as reported in the Figure 4.This is  probably due to the crushing operation carried out by the mobile hammer metallic crusher, that operates quickly and not very violently determining a partial breakage of the cells of the olive flesh.The observed phenomenon, in fact, does not happen when olives are crushed slowly by the granite millestone, that takes 20-30 minutes to grind fairly olives, as reported in other paper (Solinas et al., 1978).The variation of phenol content of oils and corresponding vegetable waters, during the malaxation step, can be explained in this way: (i) at beginning of the operation, the slow movement of the olive paste favours the breakage of the cellular structure of the flesh, by enzymatic and mechanical actions, and a part of phenolic substances, bound with other chemical compounds to form complex structures (Wallace et al., 1994), liberates phenolic glycosides soluble in the aqueous phase.This causes the increment of total phenol content of vegetable water; (ii) a part of phenolic glycosides, containing tyrosol and hydroxytyrosol, hydrolyses liberating the corresponding aglycones, more soluble in the oily phase.This causes, in some case, the increment of total phenol content of oils, during the first time of malaxation.
Increasing the malaxation time, the total phenol content of vegetable water diminishes because of the oxidative activity of phenoloxydase enzymes (Ragazzi et al., 1967).This causes the reduction of phenol content of oils also, in agreement with the results reported in other before named papers, because of the law of partition equilibrium of phenolic compounds between oily and aqueous phases.
The partition equilibrium is expressed from the ratio of the concentrations of a chemical compound in two immiscible liquids in contact, as the following equation: K = [ A ] aqueous phase / [ A ] oily phase where: K is the constant of partition equilibrium; [ A ] is the concentration of the compound A, expressed as mol/L.
The value of K, in conditions of chemical-physical equilibrium, depends from the temperature only and, therefore, it is constant at constant temperature.
The results obtained in this research, changing the malaxation time of olive paste, indicated that the ratio values of total phenol content in vegetable water and oil of the same olive varieties were sensibly constant, as shown in the Figure 5.The differences of the K values are not statistically significant and indicate that a satisfactory equilibrium is reached between the immiscible liquid phases in contact and quickly rotating into the centrifugal decanter (speed = 3,600 r/min).
Because of the invariance of the K value, at constant temperature, when the total phenol content of vegetable water diminishes, the total phenol content of oil is also decreasing (Welsh et al. ,1989;Di Giovacchino et al., 1994 b, c;Bianchi, 1999).
Table V shows the results obtained in the determination of volatile substances present in the headspace of virgin olive oils.The average values, reported in Table VI, indicate that the content of each volatile compound did not vary significantly when the malaxation time of olive paste increased from 15 to 90 minutes, while a significant difference (P < 0.05 with ANOVA test) was ascertained for total volatile substances content only.This means that several volatile compounds, especially those naturally present in virgin olive oil and derived from the enzymatic decomposition of 13-L-hydroperoxide of the linolenic and linoleic acids (Olias et al., 1993), are formed in a short time, soon after olive crushing and the cells of the pulp are broken.Then, protracting the malaxation time up to 90 minutes at the temperature of 24 o C, the content of each compound generally increases, but in not significant way, in partial agreement with the results obtained in other tests carried out by laboratory little plants (Angerosa et al., 1998;Lercker et al., 1999).
It is important, however, that the content of some volatile compounds, as n-octane, iso-amyl alcohol, iso-butyl alcohol, acetic acid and ethyl acetate, responsible of specific off-flavors of oils (fusty and winey), did not vary significantly when the malaxation time of olive paste increased up to 90 minutes.This happens because the enzymatic activity of microrganismes (Angerosa et al., 1996), present in the olive paste, cannot develop during the normal time of malaxation (45-90 minutes) so much to make possible the formation of the above mentioned substances.For this, all oils obtained in this present research had good organoleptic properties, as ascertained from the tasters.
Figure 1 Diagram of olive processing by new centrifugal decanter called at water saving.
Figure 2 Influence of malaxation time of olive paste with different rheological characteristics, on oil extraction yields obtained by the centrifugal decanter at water saving.

Figure 3
Figure 4Influence of malaxation time on total natural phenols content of vegetable waters obtained by centrifugal decanter from different olive varieties.

Figure 5
Figure 5Experimental values of the ratio (K) of total phenol content of vegetable water and oil obtained by centrifugal decanter from olive pastes mixed for different times.