Removal of dark compounds from fruit juices by membrane separation

CARRIN, MARIA ELENA; BUGLIONE, MARÍA BELÉN; LOZANO, JORGE E.

Copenhagen, Dinamarca

Congreso; European Congress of Chemical Engineering ECCE-6; 2007

      Concentration of clarified fruit juices by evaporation reduces costs and increases shelf‑life by removing water without changing the solid composition. Multiple‑effect evaporators were designed to concentrate apple juice at reduced temperatures but in practice, temperatures become very high in the first effects. Fruit juice concentrates (> 65% total solids) are stable from the standpoint of fermentation at any temperature but, when stored at relatively high temperatures, nonenzymatic browning (NEB) reactions occur. NEB between reducing sugars and amino-acids is the major route of color formation in fruit juices concentrate during processing and storage. These reactions involved the formation of brown melanoidins of high molecular weight; increasing color until levels higher than those commercially acceptable. Various pre- and post-treatment are available to avoid post-turbidity and discoloration of apple juices. The objective of this work is to carry out de-coloration of browned apple and grape juice by physical separation of melanoidins with combined use of ultrafiltration (UF) and nanofiltration (NF) membranes. Proposed actions are focused to reduce de-coloration agents added to juice in according with current trends in food processing.  Apple fruit (Granny Smith) and grape fruit (Merlot) were sorted, washed and crushed. Juice was produced by pressing in a hydraulic press, followed by screening, steam heating to 95°C, clarification by ultrafiltration (50 kDa hollow fiber) and concentration under vacuum at 60 °C to 72 °Brix. These concentrated juices were stored at relatively high temperatures to obtain a considerable browning level. Ultra and nanofiltration de-coloration of browned juices were performed with a cross-flow membrane filtration unit Sepaâ CF Membrane Cell (Osmonics; Minnetonka; Mn; USA). The cell body accommodates any 19 cm x 14 cm flat sheet membrane for a full 155 cm2 of effective membrane area, with cut-off in the range 150-450 Da (thin-film composite based in polyamide) and 2-30 kDa (cellulose acetate or polyvinylidene fluoride). Restored browned juice was treated at 30 °C in the UF/NF equipment, in batch mode. UF experiences were done consecutively in diminishing order of membrane molecular weight cut-off (MWCO), using permeate obtained in previous experience like feed to the next separation. On the other hand, NF experiences were done using as feed the same ultrafiltered juice batch. All experiences were done at 0.05 L/min of retentate flow with a transmembrane pressure in the range 41 to 90 kPa. The following parameters were analyzed in the obtained permeates and compared with the non-browned clarified juice sample: Soluble sugars were quantified using a VARIAN VISTA 5500 (Varian, Assoc. Inc., Palo Alto, CA, USA) liquid chromatograph equipped with a differential refractometer VARIAN SERIES RI-3 and a Aminexâ HPX-87C (Bio Rad; USA) column; pH (DigipHase Cole-Parmer pHmeter); absorbance at 420 nm (Perkin-Elmer Lambda3 UV/VIS spectrophotometer); soluble solids concentration as °Brix (Reichert Abbe Mark II digital refractometer); and tristimulus color parameters, L, a, b and DE in a HunterLab UltraScan XE colorimeter (Hunter Assoc. Laboratory; VA, USA). Browned apple juice color was reduced to a level close to a fresh clarified juice after filtration with NF membranes (Cut-off < 2,000 Da). Although changes in pH of permeates were not observed, reduction in °Brix occurred when ultrafiltration changed to nanofiltration. After membrane treatment of highly browned apple juice with nano-filtration membranes (< 2,000 Da) permeates with color characteristics closer to a no-browned juice were obtained. Contrarily, color recovering by NF resulted unviable for highly colored grape juices and traditional active carbon discoloration should be used. Moreover, from a practical point of view, soluble solids concentration reduction after NF must be carefully taking into account, since additional concentration step may be required.