The impact of fruit softening on avocado cell microstructure changes monitored by electrical impedance and conductivity for cold‐pressed oil extraction

Abstract

The impact of avocado fruit firmness on cold‐pressed oil recovery was investigated using a laboratory oil extraction unit. Intact, ground flesh and the malaxed (mixed) fruit pulp samples were collected during the extraction process to evaluate cell structure by electrical impedance spectroscopy, electrical conductivity, and light microscopy. The intact flesh of soft fruit at 105 firmometer value (Fv; over ripe) had the lowest electrical resistance compared to fruit with a firmness of 55 Fv (minimally ripe). The grinding step resulted in the greatest reduction in electrical resistance, suggesting the greatest cellular disruption in this step. Additionally, intact fruit and fruit pulp from fruit at 105 Fv had a higher conductivity and a lower electrical resistance value, which indicated more cell rupture occurred when softer, riper avocado fruit were processed. A greater number of unbroken parenchyma cells remained in samples from firmer fruit at various sampling points during the extraction process. Practical applications Parenchyma cells in the mesocarp of soft ripe “Hass” avocado were easier to disrupt during cold‐pressed extraction. Extraction yield of oil, using the cold‐press method, increased with riper avocados. However, the cold‐pressed oil quality decreased with avocado fruit ripeness, based on percentage of free fatty acids and peroxide value. Electrical impedance spectroscopy (EIS) was found to be a useful technique for following mesocarp cellular changes during the extraction of oil.

Fatty acids and quality criteria of avocado oil obtained by three solvent-free extraction systems

“…El rendimiento obtenido por prensado hidráulico resultó superior a lo reportado por Restrepo et al (2012), quienes obtuvieron un valor de 16,52 % para pulpa de palta Hass prensada en frío, y a la vez fue cercano a lo reportado por Dos Santos et al (2014), quienes obtuvieron un rendimiento de 24,83 % en pulpas de palta de la variedad Fortuna, secadas a 40 °C. El rendimiento obtenido por el método de termobatido (16,39 ± 1,38) resultó ser superior a lo reportado por Yang et al (2019) quienes al evaluar la firmeza de la palta lograron clasificarlas como mínimamente maduras, completamente maduras y demasiado maduras, obteniéndose rendimientos de 7,38%, 8,92% y 9,95%, respectivamente. Los valores de índice de acidez y peróxido son parámetros comúnmente usados en la especifica-ción de la calidad de los aceites.…”

Effect of malaxing time and temperature on rheological properties of 'Hass' avocado pulp and oil yield during oil extraction

“…The fruit ripeness was determined by measuring fruit firmness using a firmometer (Anderson Manufacturing, Tauranga, NZ) as described in Yang et al (2019b), dry matter content was determined after drying pulp samples at 65 C until constant weight, and total oil content was determined using accelerated solvent extraction, with hexane, on the freeze-dried flesh or pulp, using an ASE 300 (Dionex Corp, Sunnyvale, CA) as described in Yang et al (2019b).…”

“…Avocados are predominately grown in NZ for fresh fruit export, after second grade fruit is sold locally, any reject or unsaleable fruit is generally processed to extract the avocado oil to produce a highquality culinary oil. To achieve maximum oil yield, a number of preharvest and postharvest factors for avocados need to be considered because they influence the release of oil from the parenchyma cells as reported in Yang et al (2019bYang et al ( , 2020. The recovery of oil from the avocado flesh is dependent on the fruit maturity, the time of harvest during the fruit season and the ripeness of the fruit (Woolf et al, 2009;Yang et al, 2019bYang et al, , 2020.…”

“…To achieve maximum oil yield, a number of preharvest and postharvest factors for avocados need to be considered because they influence the release of oil from the parenchyma cells as reported in Yang et al (2019bYang et al ( , 2020. The recovery of oil from the avocado flesh is dependent on the fruit maturity, the time of harvest during the fruit season and the ripeness of the fruit (Woolf et al, 2009;Yang et al, 2019bYang et al, , 2020.…”

“…For extraction of olive oil it is recommended that the temperature of the pulp does not go above 27 C to ensure the required oil quality (European Commission, 2002). Cold extraction to obtain an avocado oil of good quality is carried out at temperatures below 50 C, at this temperature avocado oils with low percentages of free fatty acids (FFA) and low peroxide values (PV) have been reported (Wong et al, 2013;Yang et al, 2019b).…”