Patricia Cerrutti

Inhibitory effects of vanillin on some food spoilage yeasts in laboratory media and fruit purées

The effect of vanillin and essential oil of mint on the growth of different strains of food spoilage yeasts in laboratory media and fruit purées was studied. Growth of Saccharomyces cerevisiae, Zygosaccharomyces rouxii, Debaryomyces hansenii and Z. bailii was inhibited in culture media and apple purée containing 2000 ppm of vanillin for 40 days storage at 27 degrees C and a(w) 0.99 or 0.95. But 3000 ppm of the spice were not effective to inhibit Z. bailii growth in banana purée. Growth of yeasts was not affected by 100 ppm of essential oil of mint.

Combined Effect of Nisin and Pulsed Electric Fields on the Inactivation of Escherichia coli

The Doehlert design was applied in order to investigate the combined effect of nisin and high voltage pulsed electric fields (PEF) on the inactivation of Escherichia coli in simulated milk ultrafiltrate media. Nisin alone was totally inactivated by PEF, but in the presence of bacterial cells a protective effect was observed. However, the effectiveness of nisin was still decreased when bacterial cells were subjected to the combined treatment. In spite of this phenomenon, an almost additive response emerged as a consequence of the combined treatment. A 4-log cycle reduction may be accomplished with around 1,000 IU/ml (7.15 microM) of nisin and three pulses of 11.25 kV/cm or 500 IU/ml for five pulses of the same intensity. The observed efficacy arising from the combination of both treatments suggests the possibility of using PEF for improving the action spectrum of natural antimicrobials.

Commercial baker's yeast stability as affected by intracellular content of trehalose, dehydration procedure and the physical properties of external matrices

Abstract The effects of vacuum-drying and freeze- drying on the cell viability of a commercial bakers yeast, Saccharomyces cerevisiae, strain with different endogenous contents of trehalose were analyzed. An osmotolerant Zygosaccharomyces rouxii strain was used for comparative purposes. Higher viability values were observed in cells after vacuum-drying than after freeze-drying. Internal concentrations of trehalose in the range 10–20% protected cells in both dehydration processes. Endogenous trehalose concentrations did not affect the water sorption isotherm nor the Tg values. The effect of external matrices of trehalose and maltodextrin was also studied. The addition of external trehalose improved the survival of S. cerevisiae cells containing 5% internal trehalose during dehydration. Maltodextrin (1.8 kDa) failed to protect vacuum-dried samples at 40 °C. The major reduction in the viability during the freeze-drying process of the sensitive yeast cells studied was attributed to the freezing step. The suggested protective mechanisms for each particular system are vitrification and the specific interactions of trehalose with membranes and/or proteins. The failure of maltodextrins to protect cells was attributed to the fact that none of the suggested mechanisms of protection could operate in these systems.

Inactivation of Escherichia coli by a combination of nisin, pulsed electric fields, and water activity reduction by sodium chloride.

The effect of nisin combined with pulsed electric fields (PEF) and water activity reduction by sodium chloride (NaCl) on the inactivation of E. coli in simulated milk ultrafiltrate media was studied with a Doehlert design and a response surface method. The reduction of water activity from 0.99 to 0.95 by the addition of NaCl (without any other hurdle) did not affect E. coli viability of approximately 10(8) CFU/ml. A reduction in PEF effectiveness occurred when the NaCl concentration was increased because of an increase in conductance, which reduced the pulse decay time. In cells submitted to PEF nisin activity was decreased, probably as a consequence of the nonspecific binding of nisin to cellular debris or the emergence of new binding sites in or from cells. However, the lethal effect due to nisin was reestablished and further improved when water activity was reduced to 0.95. A synergistic effect was evidenced when low-intensity PEF were applied. Decreasing water activity to 0.95 and applying PEF at 5 kV/cm (a nonlethal intensity when no other hurdle is used) with the further addition of nisin (1,200 IU/ml) resulted in a 5-log cycle reduction of the bacterial population.

Surface esterification of cellulose nanofibers by a simple organocatalytic methodology.

Bacterial cellulose nanofibers were esterified with two short carboxylic acids by means of a simple and novel organic acid-catalyzed route. The methodology proposed relayed on the use of a non-toxic biobased α-hydroxycarboxylic acid as catalyst, and proceeded under moderate reaction conditions in solventless medium. By varying the esterification interval, acetylated and propionized bacterial cellulose nanofibers with degree of substitution (DS) in the 0.02-0.45 range could be obtained. Esterified bacterial cellulose samples were characterized by means of Solid-State CP/MAS (13)C Nuclear Magnetic Resonance spectroscopy (CP/MAS (13)C NMR), Fourier Transform Infrared spectroscopy (FTIR), X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA) and chosen hydrophobicity test assays. TGA results showed that the esterified nanofibers had increased thermal stability, whereas XRD data evidenced that the organocatalytic esterification protocol did not alter their crystallinity. The analysis of the ensuing modified nanofibers by NMR, FTIR, XRD and TGA demonstrated that esterification occurred essentially at the surface of bacterial cellulose microfibrils, something highly desirable for changing their surface hydrophilicity while not affecting their ultrastructure.

Optimization of biomass production of a mutant of Yarrowia lipolytica with an increased lipase activity using raw glycerol

The yeast Yarrowia lipolytica accumulates oils and is able to produce extracellular lipases when growing in different carbon sources including glycerol, the principal by-product of the biodiesel industry. In this study, biomass production of a novel mutant strain of Y. lipolytica was statistically optimized by Response Surface Methodology in media containing biodiesel-derived glycerol as main carbon source. This strain exhibited distinctive morphological and fatty acid profile characteristics, and showed an increased extracellular lipase activity. An organic source of nitrogen and the addition of 1.0 g/l olive oil were necessary for significant lipase production. Plackett-Burman and Central Composite Statistical Designs were employed for screening and optimization of fermentation in shaken flasks cultures, and the maximum values obtained were 16.1 g/l for biomass and 12.2 Units/ml for lipase, respectively. Optimized batch bioprocess was thereafter scaled in aerated bioreactors and the values reached for lipase specific activity after 95 % of the glycerol had been consumed, were three-fold higher than those obtained in shaken flasks cultures. A sustainable bioprocess to obtain biomass and extracellular lipase activity was attained by maximizing the use of the by-products of biodiesel industry.

Facile synthesis of cobalt ferrite nanotubes using bacterial nanocellulose as template

A facile method for the preparation of cobalt ferrite nanotubes by use of bacterial cellulose nanoribbons as a template is described. The proposed method relays on a simple coprecipitation operation, which is a technique extensively used for the synthesis of nanoparticles (either isolated or as aggregates) but not for the synthesis of nanotubes. The precursors employed in the synthesis are chlorides, and the procedure is carried out at low temperature (90 °C). By the method proposed a homogeneous distribution of cobalt ferrite nanotubes with an average diameter of 217 nm in the bacterial nanocellulose (BC) aerogel (3%) was obtained. The obtained nanotubes are formed by 26-102 nm cobalt ferrite clusters of cobalt ferrite nanoparticles with diameters in the 9-13 nm interval. The nanoparticles that form the nanotubes showed to have a certain crystalline disorder, which could be attributed in a greater extent to the small crystallite size, and, in a lesser extent, to microstrains existing in the crystalline lattice. The BC-templated-CoFe2O4 nanotubes exhibited magnetic behavior at room temperature. The magnetic properties showed to be influenced by a fraction of nanoparticles in superparamagnetic state.

Estudios de la viabilidad y la vitalidad frente al congelado de la levadura probiótica Saccharomyces boulardii: efecto del preacondicionamiento fisiológico

El objetivo de este estudio fue evaluar la vitalidad y la viabilidad de la levadura probiotica Saccharomyce boulardii despues de su congelacion y descongelacion, y el efecto del preacondicionamiento fisiologico sobre dichas propiedades. Los resultados indican que la velocidad especifica de crecimiento (0,3 h�1) y la biomasa (2-3 × 108 celulas/ml) de S. boulardii obtenida en frascos agitados tanto a 28 como a 37 °C fueron semejantes. El cultivo de esta levadura en biorreactores en lote, utilizando glucosa o melaza de cana de azucar como fuente de carbono, alcanzo rendimientos de 0,28 g de biomasa/g azucar consumida tras 10 h de cultivo a 28 °C, obteniendose resultados similares en fermentaciones en lote alimentado. Ademas, en los cultivos en lote, la vitalidad de las celulas en fase de crecimiento exponencial fue mayor que la de las celulas en fase estacionaria. En cambio, en el lote alimentado, la vitalidad de las celulas fue semejante a la del lote en fase estacionaria. La supervivencia a la congelacion a �20 °C y posterior descongelacion de las celulas de fermentaciones en lote en fase de crecimiento exponencial fue del 0,31% y la de fase estacionaria del 11,5%. El pretratamiento de esta levadura en medios de actividad de agua (aw) 0,98 aumento 10 veces la supervivencia al congelado de las celulas almacenadas a �20 °C durante 2 meses. La exposicion de las levaduras a medios de aw reducida y/o el proceso de congelacion/descongelacion afectaron negativamente la vitalidad celular. Se concluye que condiciones de estres como las estudiadas en este trabajo disminuyen la vitalidad de S. boulardii. Ademas, la cepa estudiada presento buena tolerancia a las sales biliares aun a bajos valores de pH del cultivo.

Rhodotorula minuta-mediated bioreduction of 1,2-diketones

Abstract The reduction of cyclic and acyclic 1,2-diketones was investigated by employing whole cells of the yeast Rhodotorula minuta as biocatalyst. The reactions showed a variable degree of regio- and enantioselectivity depending on the nature of the substrate. In the case of cyclic diketones, the reduction afforded a mixture of diastereomeric diols only. The reduction of acyclic diketones allowed production of both the hydroxy ketone and the diol, in a two-step reaction. The first step was highly regio- and stereoselective, affording the hydroxy ketone of (S)-configuration with high enantiomeric excess. After longer reaction times the corresponding (S,S)-diols were obtained in high yield and diastereomeric excess.

Acetylation of bacterial cellulose catalyzed by citric acid: Use of reaction conditions for tailoring the esterification extent

Bacterial cellulose (BC) nanoribbons were partially acetylated by a simple direct solvent-free route catalyzed by citric acid. The assay of reaction conditions within chosen intervals (i.e. esterification time (0.5-7h), catalyst content (0.08-1.01mmol/mmol AGU), and temperature (90-140°C)), illustrated the flexibility of the methodology proposed, with reaction variables which can be conveniently manipulated to acetylate BC to the required degree of substitution (DS) within the 0.20-0.73 interval. Within this DS interval, characterization results indicated a surface-only process in which acetylated bacterial cellulose with tunable DS, preserved fibrous structure and increased hydrophobicity could be easily obtained. The feasibility of reusing the catalyst/excess acylant in view of potential scale-up was also illustrated.