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BACKGROUND: The final prices of fermentative culture broths must be reduced in order to extend biotechnological processes to an industrial scale. In this way, residual lees from the beer, cider and wine industries were evaluated as a source of nitrogen and micronutrients for xylitol production by Debaryomyces hansenii. RESULTS:The liquid fraction of wine or beer lees was employed as suitable culture media for xylitol production (Q P = 0.30-0.24 g L −1 h −1 , respectively). Complete beer lees diluted to 25% produced 27.2 g L −1 after 168 h (Q P = 0.16 g L −1 h −1 ). Autolysis of beer lees produced 15.6 g L −1 of xylitol using whole lees (Q P = 0.11 g L −1 h −1 ), or 34.8 g L −1 using the solid fraction (Q P = 0.36 g L −1 h −1 ). The specific xylitol productivity (0.12 g g −1 h −1 ) achieved with the solid fraction was even higher than that obtained with the synthetic control (q P = 0.074 g g −1 h −1 ). CONCLUSIONS:Residual lees can be used as economical nutrients. In some cases, autolysis treatments are necessary to increase the amount of available nitrogen thus improving the fermentative parameters.

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were grown on corncob to produce cell wall polysaccharide-degrading enzymes, mainly xylanases, by solid-state fermentation (SSF). A. niger CECT 2700 produced the highest amount of xylanases of 504±7 U/g dry corncob (dcc) after 3 days of fermentation. The optimization of the culture broth (5.0 g/L NaNO 3 , 1.3 g/L (NH 4) 2 SO 4 , 4.5 g/L KH 2 PO 4 , and 3 g/L yeast extract) and operational conditions (5 g of bed loading, using an initial substrate to moistening medium of 1:3.6 (w/v)) allowed increasing the predicted maximal xylanase activity up to 2,452.7 U/g dcc. However, different pretreatments of materials, including destarching, autoclaving, microwave, and alkaline treatments, were detrimental. Finally, the process was successfully established in a laboratory-scale horizontal tube bioreactor, achieving the highest xylanase activity (2,926 U/g dcc) at a flow rate of 0.2 L/min. The result showed an overall 5.8-fold increase in xylanase activity after optimization of culture media, operational conditions, and scale-up.

Xylitol is a pentahydroxy sugar alcohol coming from xylose with many applications in the food and pharmaceutical industries as a low caloric sweetener suitable for diabetics and as an active ingredient in several biomedical applications. The microbial bioproduction of xylitol from natural xylose coming from lignocellulosic materials appears a sustainable and a promising alternative to chemical synthesis, which works at stronger reaction conditions and generates undesirable co-products which must be removed. There are several reviews that study the metabolic pathways in wild and transformed xylitol producing yeasts and the culture conditions that enhance xylitol accumulation, which are mainly related to the need of microaerobiose for the best producing wild yeasts. Nevertheless, there are relatively few studies focusing on the engineering aspects related to scalable systems and bioreactors that could result in a final industrial stage. This review explores recent advances on xylitol production using immobilized systems, which have been proposed to facilitate the reuse of the biocatalyst for extended periods and the main types of bioreactors available assayed for this purpose.