Clarification of olive juice by α-alumina microfiltration membranes with enhanced packing density

The development of innovative designs has enabled the creation of new-generation mineral membranes with enhanced filtration packing densities, improving their competitiveness against polymeric membranes. This study evaluates the performance of two α-Al2O3 membranes with mean pore sizes of 800 and 600 nm and a filtration packing density of 184 m2.m−3, focusing on their operational parameters and the quality of olive juice clarification. Filtration with the 800 nm membrane achieved 2.4 times higher productivity than the 600 nm membrane. However, it left 14–19 NTU of haze in the final product and was more prone to fouling. In contrast, the 600 nm membrane produced fully clarified juice (<2 NTU) and was more resistant to fouling. Clarification with both membranes didn't produce significant retentions of the majority of the 29 examined phenolic compounds. However, retentions of 7.0 to 7.5 % of total dissolved substances, likely polysaccharide colloids, and complete removal of residual oil were achieved. Both membranes were capable of catalyzing spontaneous hydrolysis reactions that produced a significant increase in the contents of tyrosol, hydroxytyrosol and verbascoside, with oleacein concentration rising by up to 893 % in the clarified juices.

[Industrial relevance]: fresh olive juice is an excellent source of phenolic and secoiridoid antioxidants. However, converting it into a value-added product remains a challenge for the olive oil industry due to technical difficulties in its on-line processing. A critical bottleneck of this activity is the removal of suspended solid impurities. This study shows that α-Al2O3 microfiltration membranes with mean pore sizes of up to 600 nm are able to clarify a minimally pre-treated (by centrifugation at 17568.g) olive juice in only one step and with very high quality. The treatment is scalable for industrial applications.

We thank the Consejería de Educación e Investigación from Comunidad de Madrid for the financial support of this study and the PhD contract (IND2022/BIO-23619) of Alba Gutiérrez-Docio. We acknowledge also Prof. Juan Fernández-Bolaños from Instituto de la Grasa (CSIC) (Sevilla, Spain) for the kind supply of 3,4-DHPG reference substance and Luis Fernando Torrijos Fernández de Yepes for improving the understanding of the original English version of the manuscript.

Peer reviewed