Fruit-like and rose-like aroma production via solid-state fermentation of sugarcane bagasseprocess optimization and production strategies

Resumo

This thesis focuses on the use of solid-state fermentation (SSF) as an alternative approach to the bioproduction of value-added aroma compounds through the valorization of selected agro-industrial residues. Specifically, two groups of objective aroma compounds have been studied. First, the fruit-like compounds, among which could be included a set of volatile scented species such as aldehydes, ketones, alcohols and esters. From these, the straight-chain esters could be catalogued as those more appreciated due to their high fruit-like aroma profile. The second group of species studied here are the semi-volatile compounds known as rose-like compounds, which are constituted by 2-phenethyl alcohol (2-PE) and 2-phenethyl acetate (2-PEA), two value-added species widely used as additives due to the rose-like odor they provide. In both cases, the aim was to develop residue-based bioprocesses using as raw materials the agro-industrial residue sugarcane bagasse (SCB) and the industry by-product sugar beet molasses (SBM). With this aim, the Generally Recognized As Safe (GRAS) yeast Kluyveromyces marxianus was used in the study. Developments related to the bioproduction of fruit-like compounds are exposed in the first section of the thesis. They include the initial substrate screening and an initial assessment of the operational variables affecting the process. Then, the optimization of the fruit-like compounds production in a batch SSF at 0.5 L system is developed. In that sense, it was found that the bioproduction is significantly affected by operational variables like the temperature and the air flow rate. While the maximum volatile production (including all the quantified species) was 161 mgVol per gram of dry substrate (g-1TS) at 40°C, 0.14 L h-1 g-1TS and 35% SBM, the ester species were maximized at 30°C, 0.11 L h-1 g-1TS and 25% SBM up to 47 mgEst g-1TS. Finally, based on the optimization of the batch SSF at lab scale, the evaluation of the process at bench-scale was performed (4.5 and 22 L scales). In this case, the analysis has been focused on the effects of some operational strategies named intermittent mixing and fed-batch SSF in the global performance of the bioprocess. Fed-batch has shown interesting characteristics affecting the selectivity of the ester species and also improving some of the evaluated performance indices. Bioproduction of the rose-like compounds via SSF is the core of the second section of this thesis. In this case, the first part is devoted to the evaluation of the feasibility of the process under sterile conditions in a batch SSF 0.5 L system. It was found that extending the K. marxianus activity is a key factor to increase the 2-PE and 2-PEA production. In this sense, by adding supplementary carbon sources to the media or splitting the substrate load (as a manner of a fed-batch mode) resulted in efficient and consistent ways to promote the biotransformation of these compounds. Based on the results at 0.5 L, the process was further evaluated at bench-scale (1.6 and 22 L scales) by releasing some of the constraints found at lab-scale such as the temperature control and the sterilization of the substrates. Again, the analysis was focused on the assessment of the operational strategies fed-batch and sequential-batch as alternative approaches to enhance the global behavior of the process. Here, strategies have succeeded increasing the 2-PE and 2-PEA production from 17 mg2-PE+2-PEA per gram of dry substrate (g-1TS) in a batch scenario, up to 19.2 mg2-PE+2-PEA g-1TS by using a fed-batch approach, and until 21 mg2-PE+2-PEA g-1TS through a sequential-batch. Overall, the results exposed in this thesis represent a step forward in the development of SSF as an alternative approach for producing valuable aroma compounds from agro-industrial wastes.