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This work provides a review about the biotechnological production of citric acid starting from the physicochemical properties and industrial applications, mainly in the food and pharmaceutical sectors. Several factors affecting citric acid fermentation are discussed, including carbon source, nitrogen and phosphate limitations, pH of culture medium, aeration, trace elements and morphology of the fungus. Special attention is paid to the fundamentals of biochemistry and accumulation of citric acid. Technologies employed at industrial scale such as surface or submerged cultures, mainly employing Aspergillus niger, and processes carried out with Yarrowia lipolytica, as well as the technology for recovering the product are also described. Finally, this review summarizes the use of orange peels and other by-products as feedstocks for the bioproduction of citric acid.

Olive mills generate a large amount of waste that can be revaluated. This work aim to improve the production lignocellulolytic enzymes by solid-state fermentation using ultrasounds pretreated olive mill wastes. The composition of olive mill wastes (crude and exhausted olive pomace) was compared and several physicochemical characteristics were significantly different. The use of both wastes in SSF was evaluated and a screening of fungi for xylanase and cellulase production was carried out. After screening, the use of exhausted olive pomace and Aspergillus niger led to the highest enzyme activities, so that they were used in the study of ultrasounds pre-treatment. The results showed that the sonication led to a 3-fold increase of xylanase activity and a decrease of cellulase activity. Moreover, the liquid fraction obtained from ultrasounds treatment was used to adjust the moisture of solid and a positive effect on xylanase (3.6-fold increase) and cellulase (1.2-fold increase) production was obtained.

In recent years, the interest to find alternative extraction methods has increased. Extraction of natural phenolics by enzymes produced during solid-state fermentation (SSF) is a useful and novel technique environmentally friendly. On the other hand, agro-industrial wastes are an excellent source of natural antioxidants as phenolics. The aim of the work was to evaluate the increase of antioxidant compounds extraction from agro-industrial wastes after SSF and to relate it to the production of lignocellulolytic enzymes. Several filamentous fungi were evaluated as candidates to increase the extraction of antioxidant compounds by solid-state fermentation of wineries, olive mill and brewery wastes. Fermented and unfermented agro-industrial wastes were extracted with water and lignocellulolytic enzymes, total phenolic compounds and antioxidant activity were determined in the extract. The maximum xylanase and cellulase activities were achieved by A. ibericus strains using brewer´s spent grain (BSG) as substrate and ranged from 300 to 313 U xylanase/g and 51-62 U cellulase/g. The best producer of β-glucosidase was A. niger CECT2088 using BSG as substrate (94 ± 4 U/g). The results of extraction of phenolic compounds revealed a higher extraction by SSF in olive mill wastes followed by winery wastes. In the same form, the maximum increase of antioxidant activity was achieved by SSF of exhausted olive pomace by Rhizopus oryzae MUM 10.260, increasing 12.9-fold in relation to unfermented waste. In addition, the production of enzymes, the extraction of phenolic compounds and the increase of antioxidant activity were related by principal component analysis. The first component grouped the BSG along with maximum enzymes production, the second component related positively the production of enzymes and the extraction of phenolic compounds and the increase of antioxidant activity. SSF proved to be an innovative environmentally friendly process that can improve the extraction of antioxidant compounds and simultaneously to produce lignocellulolytic enzymes from different agro-industrial wastes.

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Valencia orange (Citrus sinensis) peel was employed in this work as raw material for the production of h starting from 62 h. The addition of methanol was detrimental for the CA production. Under these conditions, the SSF ensured an effective specific production of CA (193 mg CA/g dry orange peel), corresponding to yields of product on total initial and consumed sugars (glucose, fructose and sucrose) of 376 and 383 mg CA/g, respectively. These results, which demonstrate the viability of the CA production by SSF from orange peel without addition of other nutrients, could be of interest to possible, future industrial applications.

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