Estudio de recuperación de ácido galacturónico de residuos de piel de cebolla mediante un proceso combinado de extracción con agua subcrítica y diafiltración

Abstract

In this work the feasibility of a diafiltration process using tubular ceramic membranes for the recovery of galacturonic acid, extracted from onion skin wastes, has been studied. In the first stage of the work, galacturonic acid (which is the main component of pectin) was extracted from onion skin wastes (OSW) using subcritical water. In each experiment, 15 g of OSW were mixed with 0,35 L of water in a high-pressure extractor and heated up to 145ºC and 40 bar for 45 min. The liquid extract was characterized in terms of composition by HPLC, in order to quantify the amount of galacturonic acid extracted (either in monomeric or oligomeric form) and the impurities co-extracted: free sugars (monosaccharides), organic acids (mainly formic and acetic) and other degradation products such as furfural and HMF formed from the degradation of the sugars present in the raw material. In the second stage of this work the aim was to study the separation of the oligomeric galacturonic acid from these impurities. For that purpose, multichannel ceramic membranes (Tami Industries) with an active layer of TiO2 and 132 cm2 of effective area were used. The molecular weight cut off (MWCO) of the selected membranes were 1, 5 and 50 kDa. Feed flowrate was set at 15 L/h and transmembrane pressure (TMP, in the range from 0.2 to 1.25 bar) was adjusted to the desired values by using a valve placed after the membrane module. Permeate fluxes (Jp) were determined volumetrically under different operating conditions. The retention of galacturonic acid (either in monomeric or oligomeric form were calculated), as well as that of the impurities was calculated. All the experiments were carried out at 25 ºC and in total recirculation mode. It was found that the higher the MWCO, the higher the permeate flow rate, leading to a final flux of permeate constant due to the high degree of fouling observed on the membrane. It was also observed a dramatic decrease in the permeate flux when using the subcritical water extract (< 90 %) compared to that obtained when distilled water was used. This fact indicated an important membrane fouling due to the presence of colloidal matter. This fouling resulted to be reversible, since after chemical cleaning, the membrane recovered the full filtration capacity, evaluated in terms of permeate flux using water. All in all, a retention of oligomeric galacturonic acid higher than 70% was obtained when the MWCO was1 kDa, which was reduced to 50 % when either the 5 or 50 kDa membranes were used. Then, in order to complete the separation of the oligomeric galacturonic acid and the low molecular weight purities a diafiltration process using the 50 kDa membrane was carried out. After 480 min of experiment the complete removal of the impurities was obtained, leading to a retentate rich in oligomeric galacturonic acid.

En este trabajo se ha estudiado la viabilidad de la ultrafiltración con membranas cerámicas tubulares para el fraccionamiento y purificación de oligómeros de ácido galacturónico obtenidos por extracción con agua subcrítica a partir de residuos industriales de piel de cebolla. En la primera fase del estudio, el extracto de agua subcrítica se caracterizó en términos de composición y ultrafiltró en modo recirculación total a 25 ºC. Se estudió el efecto del tamaño de poro de la membrana (1-50 kDa) y de la presión transmembrana (0,2 -1,25 bar) en la retención y en el flujo de permeado. Los resultados mostraron que la retención del ácido galacturónico oligomérico aumentó al disminuir el tamaño de poro de la membrana, sin embargo, tuvo escasa repercusión en la retención de otros compuestos presentes en el extracto tales como ácido galacturónico libre, monosacáridos, ácidos orgánicos, HMF y furfural. En la segunda parte del trabajo, se llevó a cabo la ultrafiltración del extracto en modo diafiltración utilizando la membrana de mayor tamaño de poro (50 kDa). La eliminación de los compuestos de bajo peso molecular (monosacáridos, ácido galacturónico libre, ácidos carboxílicos, HMF y furfural) fue lenta, alcanzándose valores de purificación de aproximadamente el 80% con tiempos de proceso largos (5 horas). Después de 8 horas de diafiltración, se obtuvo un retenido con un 35% en peso de los oligómeros del extracto inicial y con un grado de purificación superior al 95%.