Leobardo Serrano-Carreón

The influence of impeller type in pilot scale Xanthan fermentations

The rheological complexity of Xanthan fermentations presents an interesting problem from a mixing viewpoint, because the phenomena of poor bulk blending and low oxygen mass transfer rates inherent in highly viscous fermentations (and their consequences) can be systematically investigated, even at the pilot plant scale. This study in a 150 L fermentor compares the physical and biological performance of four pairs of impellers: a standard Rushton turbine, a large diameter Rushton turbine, a Prochem Maxflo T, and a Scaba 6SRGT. Accurate in-fermentor power measurements, essential for the comparison of impellers in relation to operating costs are also reported. It is demonstrated that the agitator performance in Xanthan fermentations is very specific and the choice of which impeller to use in bioreactors to obtain enhanced performance is dependant on the applied criterion. None of the criterion favored the use of the standard Rushton turbine, therefore suggesting that there are strong grounds for retrofitting these impellers with either large diameter impellers of similar design or with novel agitators. In addition, fluid dynamic modeling of cavern formation has clearly highlighted the importance of a well mixed and oxygenated region for providing the capacity for high microbial oxygen uptake rates which govern Xanthan productivity and quality. Copyright 1998 John Wiley & Sons, Inc.

Increasing Pleurotus ostreatus laccase production by culture medium optimization and copper/lignin synergistic induction

Laccases have great biotechnological potential in diverse industries as they catalyze the oxidation of a broad variety of chemical compounds. Production of laccases by basidiomycetes has been broadly studied as they secrete the enzymes, grow on cheap substrates, and they generally produce more than one isoenzyme (constitutive and/or inducible). Laccase production and isoenzyme profile can be modified through medium composition and the use of inducers. The objective of this work was to increase laccase production by Pleurotus ostreatus CP-50 through culture medium optimization and the simultaneous use of copper and lignin as inducers. Increased fungal growth was obtained through the use of a factorial fractional experimental design 26–2 where the influence of the nature and concentration of carbon and nitrogen sources was assessed. Although specific laccase production (U/mg biomass) decreased when malt extract medium was supplemented with carbon and nitrogen sources, fungal growth and laccase volumetric activity increased four and sixfold, respectively. The effect of media supplementation with copper and/or lignin on laccase production by P. ostreatus CP-50 was studied. A positive synergistic effect between copper and lignin was observed on laccase production. Overall, the use of an optimized medium and the simultaneous addition of copper and lignin improved growth, laccase volumetric activity, and process productivity by 4-, 60-, and 10-fold, respectively.

γ-Decalactone Production by Trichoderma harzianum in Stirred Bioreactors

Production of γ‐decalactone, a compound having peach‐like aroma characteristics, by a strain of Trichoderma harzianumis reported here for the first time. Cultures were carried out in a 14 L fermentor using castor oil as the sole carbon source. The filamentous‐like growth of the microorganism resulted in highly viscous and non‐Newtonian broths, which caused the formation of stagnant zones in the fermentor. T. harzianum was able to produce 260 mg/L of γ‐decalactone after 7 days of culture in a stirred fermenter operated at 3.3 s−1 and 1 vvm of air flow rate. The production of γ‐decalactone was mainly non‐growth associated. The use of high agitation rates—to improve mixing—resulted in celullar lysis. Cell damage occurred at volumetric power drawn as low as 0.2 W/L. Mycelial morphology (mainly hyphal thickness) changed drastically along the culture and was an important influence on the rheology of the broths. All these characteristics make this system an interesting model of study of hydrodynamically complex fermentations involving up to four phases.

The potential application of aqueous two-phase systems for in situ recovery of 6-pentyl--pyrone produced by Trichoderma harzianum

Commercial production of aroma compounds by de novo microbial biosynthesis has been principally limited by the low productivity so far achieved. Production of 6-pentyl-alpha-pyrone (6PP), a coconut-like aroma compound, by Trichoderma harzianum has been limited by the toxic effect that occurs even at low concentration (<100 ppm). This work evaluated the feasibility of the use of aqueous-two phase systems (ATPS), as in situ extraction systems, in order to overcome the toxic effects of 6PP and to improve culture productivity. The partition behaviour of 6-pentyl-alpha-pyrone and Trichoderma harzianum mycelium in polyethylene glycol (PEG)-salt and PEG-dextran two-phase systems was investigated and it is reported for the first time. The evaluation of system parameters such as PEG molecular mass, concentration of PEG as well as salt, volume ratio (Vr) and dextran molecular mass, was carried out to determine under which conditions the 6PP partitions to the opposite phase that mycelium does. PEG-dextran systems proved to be unsuitable for the in situ recovery of 6PP because either 6PP and biomass partitioned to the same phase or a large extraction phase was required for the process. ATPS extraction comprising Vr = 0.26, PEG 1450 (7.2% w/w) and sulphate (16.6% w/w) provided the best conditions for the maximum accumulation of the biomass into the bottom phase and concentrated the 6PP in the opposite phase (i.e. 86% of biomass and 56% of 6PP of the total amount loaded from the fermentation extract into the ATPS) for ex situ bioseparation. However, this system caused complete inhibition of the growth of the microorganism during the in situ bioseparation, probably as a consequence of the high ionic strength resulting from the salt concentration. Consequently, two ATPS PEG 8000-sulphate (12%/7% and 6%/14%) were evaluated and proved to be more suitable in the potential application for the in situ recovery of 6PP.

Prediction of xanthan fermentation development by a model linking kinetics, power drawn and mixing

A model for xanthan fermentation which considers the interaction between kinetic and mixing phenomena is described. Fermentation kinetics are represented by expressions reported in the literature. The volume of well-mixed regions or ‘caverns’ can be calculated as a function of the power number and apparent yield stress according to expressions proposed in the literature. A comprehensive data base containing aerated power data was used to predict the power drawn. The model assumes that kinetic changes occur only in well-mixed caverns or ‘actives zones’. The model was able to estimate an agitation rate profile which allowed complete broth mixing during fermentation at a minimum power drawn. The model predictions were able to indicate that the evolution of gum concentration is a function of impeller speed (if insufficient agitation power is drawn) and this is due to a mixing problem. In model-controlled fermentations, the bacterial growth and the xanthan productivity (g/litre h−1) of the process were lower than those obtained in cultures in which the dissolved oxygen tension (DOT) was kept above 10% saturation (DOT-controlled), however, the specific xanthan productivity (g xanthan/g cells h−1) remained unchanged. DOT-controlled culture consumed at least 46% more power than the model-controlled fermentation. When compared at the same xanthan gum concentration, the broths obtained with model-controlled fermentations were more viscous than those obtained with DOT-controlled cultures.

Rhizoctonia solani, an elicitor of 6-pentyl-α-pyrone production by Trichoderma harzianum in a two liquid phases, extractive fermentation system

Abstract6-Pentyl-α-pyrone (6PP) production by Trichoderma harzianum, in an extractive fermentation system, was elicitated by Rhizoctonia solani. The extent of 6PP elicitation was related to the state of Rhizoctonia and to the Trichoderma inoculum type. The use of non-viable Rhizoctonia solani mycelium in mycelium-inoculated Trichoderma harzianum culture, yielded the maximal 6PP production (474 mg l−1) compared to control cultures (147 mg l−1) and decreased the process time from 192 to 96 h.

Production of 6-pentyl-α-pyrone byTrichoderma harzianum from 18∶n fatty acid methyl esters

Biosynthesis of 6-pentyl-α-pyrone byTrichoderma harzianum in two different media was evaluated. Best yields were found in nitrogen deficient medium (C/N=60). Limited growth seems to favour the production of this lactone. When fungal cells, precultured in low nitrogen medium, were incubated on methyl ricinoleate (10 g/l, C/N=60) an increase in 6-pentyl-α-pyrone production was observed in comparison with the media containing methyl oleate or methyl linoleate.

Aroma compounds recovery from mycelial cultures in aqueous two-phase processes

This paper presents the evaluation of the potential use of aqueous two-phase systems (ATPS) for the recovery of 6-pentyl-alpha-pyrone (6PP) produced by Trichoderma harzianum. The partition behaviour of 6PP and Trichoderma harzianum mycelium (biomass) in polyethylene glycol (PEG)-salt (phosphate and sulphate) and PEG-dextran ATPS was investigated. The influence of defined system parameters (e.g. molecular mass of PEG and dextran, volume ratio, etc.) on the partition behaviour of 6PP and Trichoderma harzianum mycelium was evaluated to select under which conditions 6PP and mycelium partition to opposite phases. In PEG-dextran systems either large extraction phases were required or mycelium and 6PP partitioned to the same phase. ATPS comprising V(r)=0.23, PEG 8000 6.6% w/w and sulphate 14.0% w/w provided the best conditions to satisfy the process requirement of biomass accumulation into the bottom phase and 6PP concentration in the top phase.

Production of laccases by Pleurotus ostreatus in submerged fermentation in co‐culture with Trichoderma viride

Aims:  To evaluate the production and stability of laccases by Pleurotus ostreatus in liquid co‐cultures with Trichoderma viride as a function of infection time and agitation rate.

Microbial biosynthesis for the production of food flavours

Abstract The production of flavouring materials by microorganisms has recently attracted a great deal of research interest and a considerable number of patent applications. There are several possible reasons for such developments: the need to overcome variations in the availability and cost of natural flavourings attempts to diversify by producing new flavouring materials, consumer preferences for natural food additives, and the potential of microbial cells to produce particular stereo-isomers by de novo synthesis.