**5. Production of Organic Acids**

Organic acids are multipurpose compounds that have been applied in animal production [89], food processing [90], cosmetic preservation [91] and battery recycling [92], for instance. Due to their importance and the current trends to improve sustainability within the organic acid production sector, several studies have been carried out to explore the use of pomaces in the production of high-added value compounds (Table 5). One relevant example of this strategy is the study performed by Vashisht et al. [93] who evaluated the production of acetic acid using *Acetobacter pasteurianus* SKYAA25 from apple pomace. These authors indicated that the production of acetic acid was affected by the temperature (37 ◦C), concentration of bioethanol (8%, produced from the same strain), and apple pomace (2%) in fermentation media. Similarly, the production of acetic acid from the fermentation of apple pomace was reported in another study using *Acetobacter aceti* [94].


**Table 5.** Organic acids produced from pomace fermentation.

Piwowarek et al. [95] studied the optimization of the production of propionic acid from apple pomace fermentation with *Propionibacterium freudenreichii* T82. According to these authors, the accumulation of propionic acid was increased due to a better control of the fermentation process, i.e., adding biotin to fermentation media, carrying out the pH control at 24 and 48 h of fermentation, and increasing the nitrogen level (supplementing the apple pomace with peptone). However, no significant effects were obtained for the variations in temperatures from 30 to 37 ◦C. In another study from the same research

group with apple pomace, the effect of supplementation (potato wastewater, yeast extract, and peptone) to increase the production yield of propionic acid was evaluated [96]. The use of yeast extract and peptone in apple pomace in the fermentation medium improved the propionic acid yield to a maximum of 14.54 g/L after 120 h of fermentation with *Propionibacterium freudenreichii*. Additionally, the production of acetic acid was also evaluated in this study. A continuous increase in the accumulation of this acid was reported until the end of the fermentation period (120 h) and the most efficient supplement for apple pomace was potato wastewater (maximum yield of 5.01 g/L).

Apple pomace can also be fermented to produce citric acid [97]. In this case, a recent experiment explored the effect of temperature, pH, and substrate amount in the fermentation batch with the combination of *Aspergillus ornatus* and *Alternaria alternate*. The pH and temperature had optimum values of 5 and 30 ◦C, respectively. Increasing the substrate caused a significant increase in the production of citric acid, which led to choosing the maximum substrate amount tested in this study (25 g). Additionally, supplementing the apple pomace with arginine favored the production of citric acid (maximum yield of 2.7 g/L).

Another relevant acid produced from pomaces is the fumaric acid. The production of this acid with *Rhizopus oryzae* was dependent on the fermentation time [98]. The maximum yield was reported after 14 days (52 g/kg) and no additional increase was observed at up to 21 days of fermentation. The production of fumaric acid using the same microorganism and pomace was also explored in a bench scale fermenter [99]. The system comprised by a rotary drum increased the production of fumaric acid to 138 g/kg within the same fermentation period (14 days).
