Others

Carob (Ceratonia siliqua L.), which has long been regarded as just a nitrogen-fixing tree grown in the Mediterranean region, has recently found its place in the food industry as a biomass substrate due to its very high sugar content [100]. Moreover, it has been established as a viable biomass resource for bioethanol production [101]. Carob extracts have also been used for microbial production of xanthan [52] and pullulan [44] polysaccharides. Roseiro et al. [52] developed a multistep pretreatment process for carob-based feedstocks that involves aqueous extraction of carob pulp followed by pressing. By recycling of press liquor, the final sugar content of the carob extract was improved however, as a result of esterasic activities, the syrup was found to contain increasing concentrations of isobutyric acid with time in a pH-dependent manner. Though accumulation of isobutyric acid could be controlled by an additional heat treatment step, its presence was found to inhibit the growth of X. campestris cells [52]. When carob extracts with 25g/L initial sugar content were used for pullulan production by a pigmented strain of A. pullulans (SU-M18), a pullulan productivity of 2.16 g/L/day could be reached at pH 6.5 and 25 °C [44]. For dextran production, carob pod residues obtained from the galactomannan industry were milled and the sugars were extracted at 70 °C by use of an acetate buffer. A dextran yield of 8.56 g/L could be reached by L. mesenteroides NRRL B512 cultures within 12 h of fermentation period [37].

Condensed corn solubles (CCS) is a by-product of bioethanol industry. While ethanol is separated from the fermentation broth via distillation, the remaining solids are first recovered by centrifugation and then concentrated using evaporators. The final product CCS contains changing levels of carbohydrates, proteins, vitamins, and nutrients [102]. CCS obtained from a dry-mill ethanol plant has been diluted and used for the cost-effective production of scleroglucan by S. glucanicum [51, 103]; however, the yields were lower than those of S. rolfsii cultures grown on sugarcane juice ormolasses [32], coconutwater [32], and waste loquat kernel [50]. In another study, CCS was diluted, neutralized, clarified by centrifugation and filtration, and then used for the poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) production by Rhodospirillumrubrum cultures [102]. CCS from a wet-mill ethanol production plant has been clarified by centrifugation and then used as substrate for pullulan production by Aureobasidium sp. strain NRRL Y-12974 cultures and the yields (4.5 g/L in 9 days) were found to be comparable with those of soluble starch (5.4 g/L in 9 days), however, much lower than glucose containing medium (10.1 g/L in 9 days) [43]. CCS has also been used for curdlan production by Agrobacterium sp. ATCC 31749 shake flask cultures. A maximum curdlan yield of 7.72 g/L was recovered after 120 h of fermentation in media containing 400 g/L CSS [36].

Ram horn hydrolysates were also reported to be a suitable enhancer for xanthan production by X. campestris EBK-4 because of their high amino acid and mineral content. To obtain the hydrolysates, ram horns, which are usually discharged as waste in slaughterhouses, were subjected to acid hydrolysis with sulfuric acid followed by heat treatment at 130 °C. After neutralization, the hydrolysates were clarified by filtration and then added to the fermentation medium [58].

Waste loquat kernel is another potential biomass resource for EPS production due to its high protein and carbohydrate content. Waste kernels are dried and milled and then subjected to acid hydrolysis with 2 M HCl using an autoclave. Then the hydrolysates were detoxified with Ca(OH)2, neutralized, and then used for scleroglu — can production by S. rolfsii MT-6 [50] and EPS production by Morchella esculenta [104].

Other biomass residues used for microbial EPS production include corn-steep, spent grain and spent sulfite liquors, hydrolyzed potato starch, peach pulp, and peat hydrolysate [28, 30].