Extracción con agua subcrítica y ultrafiltración para la obtención de pecto-oligosacáridos a partir de residuos de piel cebolla

Abstract

In this work the purification by ultrafiltration using tubular ceramic membranes of pectic oligosaccharides (POS) extracted from onion skin wastes (OSW) was studied. In the first stage of the work, subcritical water was used to promote the hydrolysis of OSW and obtain a liquid hydrolysate rich in POS. For this purpose, three different combinations of temperature and extraction time (115 ºC – 180 min; 125 ºC – 150 min and 135 ºC – 90 min) were used in a batch extractor at 25 bar. In order to consider the combined effect of the temperature and the extraction time, the severity factor (log Ro) was calculated, resulting to be in the range 2.88 to 3.29. The best results in terms of POS concentration (5643 ± 84 mg/L) in the hydrolysate was obtained in the experiment carried out at 125 ºC (log Ro = 3.16). It was also possible to demonstrate that the composition of the liquid hydrolysate was strongly influenced by the extraction conditions: the higher the severity factor the higher the concentration of impurities (organic acids, HMF and furfural) and higher the POS hydrolysis rate. This was demonstrated by the increased concentration of free monosaccharides (glucose, xylose, galactose and arabinose) and the decrease in the POS molecular weight. In the second stage of the work, the aim was to study the separation of POS from the impurities formed during the subcritical water hydrolysis stage, using an ultrafiltration process in diafiltration mode. For this purpose, a 50 kDa tubular membrane was firstly used (Tami Industries, active layer of TiO2 and 132 cm2 of effective area). The fouling mechanism, hydraulic resistance and the purification extent were studied. Important differences in the permeate flux (J) were observed as a function of the subcritical water hydrolysate composition. The permeate flux obtained when filtering the 115 ºC hydrolysate decreased dramatically at the beginning of the experiment to achieve a constant value, whereas the hydrolysates obtained at 125 and 135 ºC exhibited a different behaviour: an initial fast decline of the permeate flux followed by a slower continuous decrease. This result for 125 and 135 ºC hydrolysates can be justified due to the effects of the membrane fouling (fouling mechanism was cake layer formation, according to Hermia’s model). Using this 50 kDa membrane, the POS recovery was 76 % (115 ºC hydrolysate), 58 % for the 125 ºC hydrolysate and 56% for the 135 ºC hydrolysate. In all the cases, a complete removal of the impurities was achieved after the dilution factor of 3.5. To complete the purification experiments, the effect of the removal of the colloidal matter (by centrifugation) present in the 125 ºC hydrolysate was studied using two different membranes (50 and 5 kDa). In the hydrolysate, it was observed the formation of a black precipitate that was responsible for the membrane fouling, hindering the filtration process. The removal of this colloidal matter, which did not affect the POS fraction, was responsible for the reduction of the colour of the hydrolysate by 60 % and the promotion of the filtration process (a faster removal of the impurities was achieved) without loss of the POS fraction. Finally, when a 5 kDa membrane was used, the retention of the POS fraction was higher than 80 % in comparison to the 60 % obtained for the 50 kDa membrane.