High-power ultrasound in olive paste pretreatment. Effect on process yield and virgin olive oil characteristics

The effect of high-power ultrasound on olive paste, on laboratory thermo-mixing operations for virgin olive oil extraction, has been studied. Direct sonication by an ultrasound probe horn (105W cm2 and 24 kHz) and indirect sonication with an ultrasound-cleaning bath (150 W and 25 kHz) were applied and their effects compared with the conventional thermal treatment.
A quick-heating of olive paste, from ambient (12–20 C) to optimal temperature conditions (28–30 C), and an oil extractability
improvement were observed when applying sonication. Better extractability was obtained by direct sonication for high moisture olives(>50%) whereas indirect sonication gave greater extractability for low moisture olive fruits (<50%).
Optimal application of ultrasound was achieved with direct sonication for 4 min at the beginning of paste malaxation and with indirect sonication during the malaxation time.
Effect of high-power ultrasound on oil quality parameters and nutritional and sensory characteristics were studied. Changes in quality parameters (free acidity value, peroxide value, K270 and K232) were not found, however significant effects on the levels of bitterness, polyphenols, tocopherols (vitamin E), chlorophyll and carotenoids were observed. Oils from sonicated pastes showed lower bitterness and higher content of tocopherols, chlorophylls and carotenoids. Related to sensory characteristics, off-flavour volatiles were not detected in oils from sonication treatments. Total peak areas of volatiles and the ratio hexanal/E-2-hexenal, as determined by SPME analysis, were lower than non-sonicated reference oils; sensory evaluation by panel test showed higher intensity of positive attributes and lesser of negative characteristics than those untreated.

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Effect of ultrasound treatment on olive oil

When high-power ultrasound is applied to olive pastes,their effect on chemical, nutritional and sensorial characteristics of oil obtained should be considered. In Table 2 the chemical quality parameters of oils are presented, according to UE Regulation, from the different paste treatments. All the oils obtained were classified into ‘extra virgin’ category. For the first harvesting date nonsignificant differences were found between treatments for acidity value, peroxide value and UV absorbance at 270 nm; only values for K232 in IUSO treatment were found significatively different to DUSO and TEST, with values of 1.465 ± 0.064, 1.570 ± 0.057 and 1.590 ± 0.014, respectively. In general, for quality parameters IUSO oils showed the lowest values.
In second harvesting date, IUSO oils showed again the lowest values for all the quality parameters, with significant differences in peroxide value and K232. In general, no oxidation or hydrolysis effects were observed in oils when ultrasounds were applied.
In Table 2, the effect of olive paste treatments on some important nutritional parameters of the olive oil areshown. Significant differences between mean values for TEST and ultrasound treatments were found for both harvesting dates.

tabla 2

For total polyphenol content, the lower values were obtained from olive paste treated with ultrasound: DUSO oils from first harvesting date and IUSO oils from second harvesting date, with 349 ± 30 and 273 ± 2 ppm, respectively.
These compounds are important components in olive oil due both to their antioxidant effect and organoleptic characteristics since they have been related to pungency and bitterness. High polyphenol content give oils with high stability, but very pungent and bitter which may be rejected by some consumers.

In total tocopherols, only significant differences were found at first harvesting date between DUSO and TEST, showing DUSO the lowest value (225 ± 9 ppm). At second harvesting date no differences were noticed between paste treatments, with higher value for DUSO treatment (245 ± 5 ppm) and lower for IUSO (226 ± 13 ppm). Ultrasound treatments gave oils with significant higher
contents in carotenoids and chlorophylls that those untreated. A similar behaviour was observed in both harvesting dates, since no differences between DUSO and IUSO were found. Therefore oils from ultrasound treatments were more green and had greater provitamin A content than those obtained without sonication. With regard to the sensorial characteristics, these were analysed first chemically for both harvesting dates measuring bitterness by K225 and volatiles by SPME-GC-FID; second by panel test on oils obtained at second harvesting date. Results are presented in Table 3. At the first harvesting date oils were more bitter than those from the second one. Bitterness index (K225) is related with total polyphenol content Thus, oils obtained from sonicated olive paste were significatively less bitter than those untreated; the lower values are shown by DUSO oils (0.24 ± 0.01) at first harvesting date, and IUSO (0.19 ± 0.00) at the last one. The hexanal/E-2-hexenal ratio was found to be lower in oils from olive paste ultrasound treated at both harvesting dates; these volatiles are related to oxidation of linolenic and linoleic fatty acids, E-2-hexenal from lipoxygenase pathway and hexanal from direct autoxidation, respectively. They are present at different concentrations in virgin olive oils. High quality oils show higher E-2-hexenal levels than hexanal. When oil oxidation is induced a fast increase of hexanal and decrease of E-2-hexenal levels takes place, and then off-flavour ‘rancid’ appears. From experimental
samples, the hexanal/E-2-hexenal ratio became smaller in oils from sonicated paste, this indicates that oil oxidation did not occur being confirmed because no increase in total area of volatile and another compounds were observed, compared with TEST oils (Fig. 3).With regard to the panel test evaluation at second harvesting date, the overripe fruits gave a slightly ‘winey’ off-flavour in TEST oils, appearing at lower intensity in oils from DUSO treatment and was not detected in IUSO oils.

tabla3

Oils from ultrasound treatments were more fruity, green, pungent and less bitter than those from TEST. IUSO oils showed the lower bitterness, as observed for K225 value; other positive attributes were higher than those obtained in DUSO oils.
Briefly, for the experimental conditions tested, highpower ultrasound application on olive paste has shown a positive effect on malaxation step. It provides a quick-heating of olive paste, improvement in process extractability and modulation of olive oil composition without alteration. Instrumentation and conditions on ultrasound application are needed to be optimized according to olive fruit characteristics and the sensory and nutritional properties of oil required. All suggest the viability of application of this technology in malaxation step of olive oil process; translation to industrial plant will be the following step to be carried out.

Autores del articulo: Antonio Jimenez, Gabriel Beltrán  y Marino Uceda

Estación de Olivicultura y Elaiotecnia, ‘Venta del Llano’, Instituto de Investigación y Formación Agraria y Pesquera, Consejería de Innovación,
Ciencias y Empresas, Junta de Andalucía, Ctra. Bailen-Motril, Km. 18.4, 23620 Mengíbar, Jaén, Spain

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