Ana M.R. Pilosof

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.

Application of Doehlert designs for water activity, pH, and fermentation time optimization for Aspergillus niger pectinolytic activities production in solid-state and submerged fermentation

Abstract The Doehlert design was applied to optimize water activity, pH, and fermentation time conditions for Aspergillus niger 148 pectinolytic activities production in solid-state (SSF) and submerged (SmF) fermentation. The fermentation technique had a great influence on the composition of pectinases produced by the fungus. Production of polygalacturonase was 5-fold higher in SmF than in SSF. However, pectin lyase production was 3 times higher in SSF than in SmF. Pectinesterase was only a 30% higher in SSF than in SmF. Optimization of the process was based on minimum pectinesterase production and maximal pectin lyase production. The optimal conditions to obtain the aforecited composition by a SSF process were initial pH 6.5–7, initial a w 0.93–0.94, and 2 days of fermentation. For the SmF process they were initial pH 5.5–6.2, initial a w above 0.99, and 3 days of fermentation. The results suggest that A. niger 148 overproduces pectin lyase under the optimized SSF culture conditions, as the concentration in the fermentation extract was around 0.25 g/l, representing ≈65% of total extracellular proteins produced by the fungus.

Modification of foaming properties of soy protein isolate by high ultrasound intensity: Particle size effect

The effect of high intensity ultrasound (HIUS) may produce structural modifications on proteins through a friendly environmental process. Thus, it can be possible to obtain aggregates with a determined particle size, and altering a defined functional property at the same time. The objective of this work was to explore the impact of HIUS on the functionality of a denatured soy protein isolate (SPI) on foaming and interfacial properties. SPI solutions at pH 6.9 were treated with HIUS for 20 min, in an ultrasonic processor at room temperature, at 75, 80 and 85°C. The operating conditions were: 20 kHz, 4.27 ± 0.71 W and 20% of amplitude. It was determined the size of the protein particles, before and after the HIUS treatment, by dynamic light scattering. It was also analyzed the interfacial behavior of the different systems as well as their foaming properties, by applying the whipping method. The HIUS treatment and HIUS with temperature improved the foaming capacity by alteration of particle size whereas stability was not modified significantly. The temperature of HIUS treatment (80 and 85°C) showed a synergistic effect on foaming capacity. It was found that the reduction of particle size was related to the increase of foaming capacity of SPI. On the other hand, the invariable elasticity of the interfacial films could explain the stability of foams over time.

Rheological properties of food gums as related to their water binding capacity and to soy protein interaction

Abstract Water binding capacity and hydration time of several food gums were measured by the Baumann method. Xanthan gum was by far the best water binder (232 mL/g) followed by guar gum (40 mL/g) and sodium alginate (25 mL/g). Propylene glycol alginate and locust bean gum (LBG) were the poorest water binders (15 and 11.6 mUg). Hydration time ranged from 50 min for sodium alginate to 3500 min for guar gum. A good correlation was obtained between the consistency coefficient of gum dispersion and the imbibed water fraction calculated from Baumann measurements. Soy protein (20 g/kg) showed synergism with xanthan and guar gum (1.5 or 3 g/kg) as shown by the increase of the consistency coefficients of mixed systems over the expected values from 81 to 139%. For the xanthan gum and soy protein systems, yield stress was greatly enhanced too. The interaction between locust bean gum and soy protein changed from antagonism at 20 g/kg soy protein and 1 g/kg LBG to synergism for 1.5 and 3 g/kg gum concentration. When soy protein interacted with a blend (1:1) of xanthan and guar, the synergistic increase in the consistency coefficient and yield stress was also apparent. However, soy protein showed an antagonistic interaction with the blend xanthan/LBG (2 g/kg) as it decreased gel properties of the system.

Effects of denaturation on soy protein–xanthan interactions: comparison of a whipping–rheological and a bubbling method

The effect of xanthan on foam formation and on physical mechanisms of destabilization involved in the breakdown of foams made from native and denatured soy protein at neutral pH was studied by a bubbling and a whipping-rheological method. Parameters describing foam formation and destabilization by liquid drainage and disproportionation obtained by the two methods showed that the addition of xanthan was accompanied by delayed rates of drainage and disproportionation and reduced foam height decay (collapse). Drainage showed the largest reduction, mainly because of the increased bulk viscosity. In the absence of xanthan, protein denaturation enhanced foam formation and stability against drainage and disproportionation, but increased the collapse of foams. In the presence of xanthan, differences in foam formation and drainage/disproportionation stability between native and denatured soy protein were greatly reduced. However, differences in foam collapse were greatly enhanced. The increased stability of foams in the presence of xanthan could not be explained purely in terms of increased aqueous phase viscosity. More specific interactions of xanthan and soy proteins at the air-water interface influencing the surface rheology, and the protein composition and aggregation, are involved.

Combined effect of water activity depression and glucose addition on pectinases and protease production by Aspergillus niger

Abstracthe combined effect of depressed water activity (a) and glucose addition on the production of extracellular pectinases and protease by Aspergillus niger 148 was studied. A differential response in the production of enzyme activities was observed. Glucose (0.7%) acted as catabolite repressor for all the enzymes at a=1 and at a=0.975 for pectinesterase, pectinlyase and protease. For polygalacturonase at a=0.975 a synergistic stimulating effect was observed. By depressing a a pectinase complex containing mainly pectinlyase and polyglalacturonase could be obtained.

Interactions in the aqueous phase and adsorption at the air–water interface of caseinoglycomacropeptide (GMP) and β-lactoglobulin mixed systems

The aim of this work was to study the interactions and adsorption of caseinoglycomacropeptide (GMP) and GMP:beta-lactoglobulin (beta-lg) mixed system in the aqueous phase and at the air-water interface. The existence of associative interactions between GMP and beta-lg in the aqueous phase was investigated by dynamic light scattering, differential scanning calorimetry (DSC), fluorometry and native PAGE-electrophoresis. The surface pressure isotherm and the static and dynamic surface pressure were determined by tensiometry and surface dilatational properties. The results showed that GMP presented higher surface activity than beta-lg at a concentration of 4%wt but beta-lg showed higher film forming ability. In the mixed systems beta-lg dominated the static and dynamic surface pressure and the rheological properties of interfacial films suggesting that beta-lg hinders GMP adsorption because, in simple competition, GMP should dominate because of its higher surface activity. The surface predominance of beta-lg can be attributed to binding of GMP to beta-lg in the aqueous phase that prevents GMP adsorption on its own.

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.

Electrophoretic studies for determining soy proteins–xanthan gum interactions in foams

Abstract The aim of this work was to elucidate soy proteins–xanthan gum interactions at a molecular level by studying protein composition at the air–water interface of foams and in the solutions used to make them and to see if the different properties of heat denatured protein were reflected in the proportions in which they were present at the interface or in the ability to interact with xanthan. To this end SDS-PAGE and densitometry was employed. Initial protein concentration and xanthan influenced the composition of proteins in the solutions used to make the foams. The increase in NSP concentration of solutions (0.5–6 wt.%) in the absence of xanthan promoted the formation of aggregates of low molecular weight (160 kDa), the association of A an B polypeptides and a decrease in α and α ′ subunits. As DSP concentration of solutions increased, an increase in the proportion of aggregates of high molecular weight (above 200 kDa) and B-polypeptide was observed. On addition of xanthan (0.025 and 0.05%) to protein solutions (0.5 and 2%), the formation of aggregates of high molecular weight was favoured for both NSP and DSP. In the absence of xanthan, no preferential adsorption of soy polypeptides was observed at the air–water interface of NSP foams. However in DSP foams, there was a preferential adsorption of B-polypeptides. Xanthan present in NSP foams (0.5 or 2%), caused an increase in the proportion of aggregates of high molecular weight at the interface as compared with the composition of solutions used to make the foams. An increase in proportion of AB-polypeptides (for 0.5% NSP and 0.025% xanthan) and B-polypeptides together with polypeptides of molecular weight lower than 14 kDa (for 0.5% NSP and 0.05% xanthan) was also observed at the interface in NSP foams. On the contrary, the presence of xanthan in DSP foams caused a decrease in the proportion of aggregates of high molecular weight and a concomitant increase in B-polypeptide. The B-11S polypeptide predominated the interface of DSP foams probably for its hydrophobicity and basic characteristics.

A model for the dependence of the electrical conductance with the applied stress in insulating-conducting composites

A model for the dependence of the electrical conductance, G, with the strain induced by external mechanical stress in conducting particles-polymer composites is presented. The model assumes that the percolation probability between neighboring particles must depart from a scale-invariant behavior but saturate at moderated-high strains, reaching percolation path’s saturation, with sigmoid dependence. This dependence is obtained by proposing a dynamic picture where contacts or bonds between neighboring particles are created but also destructed when a stress is applied and relatively moderated or high strains, e, are produced in the composite. The electrical conductance of prepared graphite-polydimethylsiloxane composites were measured as function of the applied pressure and fitted by the presented model. The elastic response to the uniaxial compression was studied using a texture analyzer. The possibility of nonuniversal effects in the conduction critical exponent, t, was taken into account. It is concluded ...