Influence of the 3-Hydroxyvalerate Content on the Processability, Nucleating and Blending Ability of Poly(3-Hydroxybutyrate-co-3-hydroxyvalerate)-Based Materials.

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate (P(3HB-co-3HV) copolymers are an attractive class of biopolymers whose properties can be tailored by changing the 3-hydroxyvalerate monomer (3HV) concentration, offering the possibility of counteracting problems related to high crystallinity, brittleness, and processability. However, there are few studies about the effects of 3HV content on the processability of copolymers. The present study aims to provide new insights into the effect of 3HV content on the processing step including common practices like compounding, addition of nucleation agents and/or amorphous polymers as plasticizers. P(3HB-co-3HV)-based films containing 3, 18, and 28 mol % 3HV were processed into films by extrusion and subsequent molding. The characterization results confirmed that increasing the 3HV content from 3 to 28 mol % resulted in a decrease in the melting point (from 175 to 100 °C) and an improvement in mechanical properties (i.e., elongation at break from 7 ± 1% to 120 ± 3%). The behavior of P(3HB-co-3HV) in the presence of additives was also investigated. It was shown that an increase in the 3HV content leads to better miscibility with amorphous polymers.

Impact of nitrogen deficiency on succinic acid production by engineered strains of Yarrowia lipolytica.

Yarrowia lipolytica strains PGC01003 and PGC202 engineered for succinic acid production were studied and compared to the wild type strain W29. For the first time, these two strains were characterized in a chemically defined medium. Strain growth and organic acid production were investigated in fed-batch mode with glycerol as carbon and energy source. This study evaluated the impact of nitrogen deficiency strategy to redirect carbon flux toward succinic acid synthesis. Strain PGC01003 produced 19 g L-1 succinic acid with an overall yield of 0.23 g g-1 and an overall productivity of 0.23 g L-1 h-1, while strain PGC202 produced 33 g L-1 succinic acid with an overall yield of 0.12 g g-1 and a productivity of 0.57 g L-1 h-1. Nitrogen limitation effectively stopped biomass growth and increased succinic acid yield of PGC01003 and PGC202 by 18 % and 62 %, respectively. However, the specific succinic acid production rate was reduced by 77 % and 66 %, respectively.

Determination of yeast viability during a stress-model alcoholic fermentation using reagent-free microscopy image analysis.

A dedicated microscopy imaging system including automated positioning, focusing, image acquisition, and image analysis was developed to characterize a yeast population with regard to cell morphology. This method was used to monitor a stress-model alcoholic fermentation with Saccharomyces cerevisiae. Combination of dark field and epifluorescence microscopy after propidium iodide staining for membrane integrity showed that cell death went along with important changes in cell morphology, with a cell shrinking, the onset of inhomogeneities in the cytoplasm, and a detachment of the plasma membrane from the cell wall. These modifications were significant enough to enable a trained human operator to make the difference between dead and viable cells. Accordingly, a multivariate data analysis using an artificial neural network was achieved to build a predictive model to infer viability at single-cell level automatically from microscopy images without any staining. Applying this method to in situ microscope images could help to detect abnormal situations during a fermentation course and to prevent cell death by applying adapted corrective actions.

Single-cell analysis of S. cerevisiae growth recovery after a sublethal heat-stress applied during an alcoholic fermentation.

Interest in bioethanol production has experienced a resurgence in the last few years. Poor temperature control in industrial fermentation tanks exposes the yeast cells used for this production to intermittent heat stress which impairs fermentation efficiency. Therefore, there is a need for yeast strains with improved tolerance, able to recover from such temperature variations. Accordingly, this paper reports the development of methods for the characterization of Saccharomyces cerevisiae growth recovery after a sublethal heat stress. Single-cell measurements were carried out in order to detect cell-to-cell variability. Alcoholic batch fermentations were performed on a defined medium in a 2 l instrumented bioreactor. A rapid temperature shift from 33 to 43 °C was applied when ethanol concentration reached 50 g l⁻¹. Samples were collected at different times after the temperature shift. Single cell growth capability, lag-time and initial growth rate were determined by monitoring the growth of a statistically significant number of cells after agar medium plating. The rapid temperature shift resulted in an immediate arrest of growth and triggered a progressive loss of cultivability from 100 to 0.0001% within 8 h. Heat-injured cells were able to recover their growth capability on agar medium after a lag phase. Lag-time was longer and more widely distributed as the time of heat exposure increased. Thus, lag-time distribution gives an insight into strain sensitivity to heat-stress, and could be helpful for the selection of yeast strains of technological interest.

Assessing yeast viability from cell size measurements?

During microbial cell cultures, environmental conditions affect cell physiology and subsequently process efficiency. Physiological changes result in changing cell morphology, such as cell size variations. The aim of this work was to study cell size evolution of a Saccharomyces cerevisiae population exposed to various stresses during alcoholic batch fermentations, and to evaluate the potential use of cell size measurements to infer cell viability. During a reference culture, without perturbation, viability as assessed by propidium iodide staining (PI) remained 100% and mean cell diameter was found to be above 5microm. A rapid temperature shift from 33 to 43 degrees C at 50gl(-1) of ethanol resulted in an immediate arrest of growth and triggered a progressive loss of viability from 100% to 0% and a decrease of mean cell diameter from 5.2 to 3.7microm. Cell size distribution curves obtained with a cell counter showed an increasing subpopulation of significantly smaller cells. At single-cell level, combined microscopy size measurements and PI staining showed that this subpopulation was exclusively composed of dead cells. Similar results were obtained after acetic acid or furfural additions. Accordingly, a multivariate data analysis was achieved to estimate the ratio of dead cells from cell size distributions obtained using the cell counter.

Antimicrobial paper based on a soy protein isolate or modified starch coating including carvacrol and cinnamaldehyde.

Soy protein isolates (SPI) and octenyl-succinate (OSA) modified starch were used as paper coating and inclusion matrices of two antimicrobial compounds: cinnamaldehyde and carvacrol. Antimicrobial compound losses from the coated papers were evaluated after the coating and drying process, and the two matrices demonstrated retention ability that depended on the compound nature and concentration. Whereas carvacrol losses ranged between 12 and 45%, cinnamaldehyde losses varied from 43 to 76%. The losses were always higher from OSA-starch-coated papers than from SPI-coated papers. During storage in accelerated conditions, at 30 degrees C and 60% relative humidity, carvacrol retention from coated papers was found to be similar whatever the coating matrices and the carvacrol rate. In contrast, the retention from SPI-coated papers was particularly high for the cinnamaldehyde concentration of 30% (w/w) compared to the lowest (10% w/w) or highest concentration (60% w/w). Compared to carvacrol, faster release was observed, particurlarly when OSA-starch was used. The antimicrobial properties of the coated papers were shown against Escherichia coli and Botrytis cinerea and explained by favorable conditions of total release of the antimicrobial agents.

Enzymatic synthesis of aroma compound xylosides using transfer reaction by Trichoderma longibrachiatum xylanase.

Enzymatic synthesis of aroma compound xylosides was performed by Trichoderma longibrachiatum xylanase. Information concerning the nature of xylosides present in the reaction medium was obtained by GC-EI-MS, by GC-NCI-MS of TFA derivatives, and by positive FAB-MS of the reaction mixtures. Moreover, the structures of isolated benzyl beta-D-xylopyranoside and 4-O-beta-xylopyranosyl-beta-D-xylopyranoside were established by (1)H and (13)C NMR and heteronuclear two-dimensional ((1)H-(13)C) chemical shift correlation. The results obtained for hexyl and benzyl alcohol xylosides indicated that a reaction implying a transfer of one to two or three xylose units from xylan was involved. The enzyme was able to recognize xylobiose, xylotriose, and xylan as xylose donors. Benzyl xyloside, produced independently of xylobioside and xylotrioside, was found as the major kinetic product of the reaction. Benzyl xyloside was produced in higher quantities and at a higher rate than that obtained for the di- and trixyloside derivatives. The maximum production for benzyl xyloside, 1.29 g/L, was obtained in the presence of hexane (50%) used as cosolvent. Xylosides and xylobiosides of several aroma compounds, (Z)-hex-3-en-1-ol, heptan-2-ol, geraniol, nerol, and citronellol, were synthesized in different amounts, from 850 mg/L for (Z)-hex-3-en-1-yl xylosides to 1.5 mg/L for citronellyl xylosides. No synthesis occurred when menthol, linalool, and eugenol were used as acceptors.