title: Submerged citric acid fermentation on orange peel autohydrolysate.
abstract: The citrus-processing industry generates in the Mediterranean area huge amounts of orange peel as a byproduct from the industrial extraction of citrus juices. To reduce its environmental impact as well as to provide an extra profit, this residue was investigated in this study as an alternative substrate for the fermentative production of citric acid. Orange peel contained 16.9% soluble sugars, 9.21% cellulose, 10.5% hemicellulose, and 42.5% pectin as the most important components. To get solutions rich in soluble and starchy sugars to be used as a carbon source for citric acid fermentation, this raw material was submitted to autohydrolysis, a process that does not make use of any acidic catalyst. Liquors obtained by this process under optimum conditions (temperature of 130 degrees C and a liquid/solid ratio of 8.0 g/g) contained 38.2 g/L free sugars (8.3 g/L sucrose, 13.7 g/L glucose, and 16.2 g/L fructose) and significant amounts of metals, particularly Mg, Ca, Zn, and K. Without additional nutrients, these liquors were employed for citric acid production by Aspergillus niger CECT 2090 (ATCC 9142, NRRL 599). Addition of calcium carbonate enhanced citric acid production because it prevented progressive acidification of the medium. Moreover, the influence of methanol addition on citric acid formation was investigated. Under the best conditions (40 mL of methanol/kg of medium), an effective conversion of sugars into citric acid was ensured (maximum citric acid concentration of 9.2 g/L, volumetric productivity of 0.128 g/(L.h), and yield of product on consumed sugars of 0.53 g/g), hence demonstrating the potential of orange peel wastes as an alternative raw material for citric acid fermentation.
authors.affiliation: Departmento de Ingeniería Química, Universidad de Vigo (Campus Ourense), Edificio Politécnico, As Lagoas, 32004 Ourense, Spain.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Rivas
authors.forename: Ana
authors.initials: A
authors.lastname: Torrado
authors.forename: Paolo
authors.initials: P
authors.lastname: Torre
authors.forename: Attilio
authors.initials: A
authors.lastname: Converti
authors.forename: José Manuel
authors.initials: JM
authors.lastname: Domínguez
mesh_terms: D001628:Beverages; D002119:Calcium Carbonate / Q000494:pharmacology; D019343:Citric Acid / Q000378:metabolism*; D032084:Citrus sinensis / Q000378:metabolism*; D005285:Fermentation; D005638:Fruit / Q000737:chemistry / Q000378:metabolism*; D006868:Hydrolysis; D007220:Industrial Waste; D000432:Methanol / Q000494:pharmacology
chemical_list: D007220:Industrial Waste; D019343:Citric Acid; D002119:Calcium Carbonate; D000432:Methanol