Sources, distribution, and acidity of sulfate–ammonium aerosol in the Arctic in winter–spring

We use GEOS-Chem chemical transport model simulations of sulfate–ammonium aerosol data from the NASA ARCTAS and NOAA ARCPAC aircraft campaigns in the North American Arctic in April 2008, together with longer-term data from surface sites, to better understand aerosol sources in the Arctic in winter–spring and the implications for aerosol acidity. Arctic pollution is dominated by transport from mid-latitudes, and we test the relevant ammonia and sulfur dioxide emission inventories in the model by comparison with wet deposition flux data over the source continents. We find that a complicated mix of natural and anthropogenic sources with different vertical signatures is responsible for sulfate concentrations in the Arctic. East Asian pollution influence is weak in winter but becomes important in spring through transport in the free troposphere. European influence is important at all altitudes but never dominant. West Asia (non-Arctic Russia and Kazakhstan) is the largest contributor to Arctic sulfate in surface air in winter, reflecting a southward extension of the Arctic front over that region. Ammonium in Arctic spring mostly originates from anthropogenic sources in East Asia and Europe, with added contribution from boreal fires, resulting in a more neutralized aerosol in the free troposphere than at the surface. The ARCTAS and ARCPAC data indicate a median aerosol neutralization fraction [NH4+]/(2[SO42−] + [NO3−]) of 0.5 mol mol−1 below 2 km and 0.7 mol mol−1 above. We find that East Asian and European aerosol transported to the Arctic is mostly neutralized, whereas West Asian and North American aerosol is highly acidic. Growth of sulfur emissions in West Asia may be responsible for the observed increase in aerosol acidity at Barrow over the past decade. As global sulfur emissions decline over the next decades, increasing aerosol neutralization in the Arctic is expected, potentially accelerating Arctic warming through indirect radiative forcing and feedbacks.Chemistry and Chemical Biolog

Pea flour as stabilizer of oil-in-water emulsions : Protein purification unnecessary

Plant proteins have recently gained considerable attention as stabilizers of food-grade oil-in-water emulsions. However, the separation of plant proteins from their native matrix can be cumbersome due to the molecular complexity of plants. This issue could be alleviated by avoiding the protein purification step. In this work, we show that native pea flour containing 50 wt% starch and 20 wt% protein has similar interfacial properties compared to concentrated pea protein systems (~55 wt% protein). Interfacial tension profile of pea flour was similar to that of concentrated pea protein, indicating that proteins are the primary stabilizing agents of the interface. The fabricated oil-in-water emulsions (10.0 wt% oil) made with pea flour or pea protein concentrate containing 0.2 and 0.3 wt% protein showed a similar monomodal droplet size distribution. Moreover, both emulsions stabilized by the pea flour and the pea protein concentrate had similar rheological properties, showing that starch granules did not have any impact on the physical properties. This work clearly showed that stable oil-in-water emulsions can be produced with pea flour and further purification of pea proteins is not necessary.</p

On the Emulsifying Properties of Self-Assembled Pea Protein Particles

Pea proteins are promising oil-in-water emulsifying agents at both neutral and acidic conditions. In an acidic environment, pea proteins associate to form submicrometer-sized particles. Previous studies suggested that the emulsions at acidic pH were stabilized due to a Pickering mechanism. However, protein particles can be in equilibrium with protein molecules, which could play a significant role in the stabilization of emulsion droplets. Therefore, we revisited the emulsion stabilization mechanism of pea proteins at pH 3 and investigated whether the protein particles or the protein molecules are the major emulsifying agent. The theoretical and experimental surface load of dispersed oil droplets were compared, and we found that protein particles can cover only 3.2% of the total oil droplet surface, which is not enough to stabilize the droplets, whereas protein molecules can cover 47% of the total oil droplet surface. Moreover, through removing protein particles from the mixture and emulsifying with only protein molecules, the contributions of pea protein molecules to the emulsifying properties of pea proteins at pH 3 were evaluated. The results proved that the protein molecules were the primary stabilizers of the oil droplets at pH 3

Effect of Temperature and Pressure on the Stability of Protein Microbubbles

Protein microbubbles are air bubbles with a network of interacting proteins at the air-water interface. Protein microbubbles are commonly used in medical diagnostic and therapeutic research. They have also recently gained interest in the research area of food as they can be used as structural elements to control texture, allowing for the manufacture of healthier foods with increased consumer perception. For the application of microbubbles in the food industry, it is important to gain insights into their stability under food processing conditions. In this study, we tested the stability of protein microbubbles against heating and pressurization. Microbubbles could be heated to 50 °C for 2 min or pressurized to 100 kPa overpressure for 15 s without significantly affecting their stability. At higher pressures and temperatures, the microbubbles became unstable and buckled. Buckling was observed above a critical pressure and was influenced by the shell modulus. The addition of cross-linkers like glutaraldehyde and tannic acid resulted in microbubbles that were stable against all tested temperatures and overpressures, more specifically, up to 120 °C and 470 kPa, respectively. We found a relation between the storage temperatures of microbubble dispersions (4, 10, 15, and 21 °C) and a decrease in the number of microbubbles with the highest decrease at the highest storage temperature. The average rupture time of microbubbles stored at different storage temperatures followed an Arrhenius relation with an activation energy for rupture of the shell of approximately 27 kT. This strength ensures applicability of microbubbles in food processes only at moderate temperatures and storage for a moderate period of time. After the proteins in the shell are cross-linked, the microbubbles can withstand pressures and temperatures that are representative of food processes.</p

Identification of critical concentrations determining foam ability and stability of β-lactoglobulin

To understand the properties of protein stabilized foam, quantitative parameters, such as the concentration dependence of the foam properties need to be determined. Recently, a concept was proposed that predicts the emulsifying ability (i.e. the droplet size in emulsions) based on different parameters, including the protein concentration. The aim of the present study is to investigate whether a similar concept can be applied to describe the foam ability and stability of protein stabilized foams. To achieve this, the foam, thin film and molecular properties of β-lactoglobulin (BLG) were determined at different concentrations and different pH values (pH 3-7). At each pH, a certain critical concentration for foam ability CFA, could be identified above which the set foam volume was reached, while below that value the set volume was not reached. Furthermore, for all pH another critical concentration (Ccrr32) at C > CFA was identified as the point where the bubble radius (measured at the end of foam formation) reached a minimal value. The foam ability increased with increasing pH (pH 3-7). The difference in foam ability as a function of pH was reflected in the adsorption rate (slope Π/t0.5 curve) of BLG. The foam stability increased with increasing concentration at each pH value but even in the protein rich regime where C > Ccrr32 different foam stabilities were observed, which were highest at pH 7.</p

Starch controls brittleness in emulsion-gels stabilized by pea flour

Pea proteins are widely studied as emulsifying and gelling agents in soft food materials. However, their extraction process consumes energy and often focuses only on protein purity. To reduce extraction-related energy consumption, unpurified pea protein-starch mixtures could be used directly as functional ingredients. Such mixtures provide additional advantages due to their binary role, such as using proteins as emulsifiers and starch as a gelling agent. Therefore, we investigated the heat-induced gelation of emulsions (30 or 50% oil), stabilized by pea flour (PF) containing 20 wt% protein and 50 wt% starch. To understand the effect of starch on gelation, starch was removed from pea flour by filtration. Heat-induced gelation behavior of the pea protein mixture (PPM) stabilized emulsions was investigated and compared with PF emulsions. Both the PF and PPM stabilized emulsions gelled upon temperature treatment, monitored by oscillatory shear rheology. At pH 7, starch (in PF) contributed to a higher G' (2000Pa) compared with the emulsions without starch (PPM:∼1000 Pa). Whereas, at pH 3, the presence of starch did not contribute to a higher G′ in the emulsions. The presence of starch at both pH values affected the microstructure of the emulsion gels. The PF emulsions after heating were more brittle upon applying strain compared to the PPM emulsions. The brittle nature of the emulsions containing starch was most likely due to the starch gel's disruption of the oil droplet network. Our results provide insight into emulsion gelation when using a native pea protein-starch mixture. Our study demonstrates that depending on the pH conditions, native protein blends could have better gelling functionality

Interfacial protein-protein displacement at fluid interfaces

International audienceProtein blends are used to stabilise many traditional and emerging emulsion products, resulting in complex, non equilibrated interfacial structures. The interface composition just after emulsification is dependent on the competitive adsorption between proteins. Over time, non-adsorbed proteins are capable of displacing the initially adsorbed ones. Such rearrangements are important to consider, since the integrity of the interfacial film could be compromised after partial displacement, which may result in the physical destabilisation of emulsions. In the present review, we critically describe various experimental techniques to assess the interfacial composition, properties and mechanisms of protein displacement. The type of information that can be obtained from the different techniques is described, from which we comment on their suitability for displacement studies. Comparative studies between model interfaces and emulsions allow for evaluating the impact of minor components and the different fluid dynamics during interface formation. We extensively discuss available mechanistic physical models that describe interfacial properties and the dynamics of complex mixed systems, with a focus on protein in-plane and bulk-interface interactions. The potential of Brownian dynamic simulations to describe the parameters that govern interfacial displacement is also addressed. This review thus provides ample information for characterising the interfacial properties over time in protein blend-stabilised emulsions, based on both experimental and modelling approaches

The impact of heating and freeze or spray drying on the interface and foam stabilising properties of pea protein extracts : Explained by aggregation and protein composition

The processing of plant protein extracts can affect the protein structure, leading to altered functional properties. In this work, we evaluated the impact of two common processes in pea protein extraction: heating and drying. Non-heated and heated (5 min at 95 °C) samples were compared, which were either freeze- or spray-dried. These processes led to alterations of the proteins, and resulted in changes of their interface and foam-stabilising properties. A mild protein extraction method was used to preserve the native protein structure during aqueous extraction, allowing the extraction of both albumin and globulin proteins. Spray-drying of these fractions led to higher surface hydrophobicity, which resulted in increased surface activity and stiffer interfacial layers at pH 3.8 and 7.0. The heating step induced aggregation of the globulins, while albumins remained soluble. Here, we demonstrated that the albumins had a dominant effect on the interfacial (rheology and ellipsometry) and foaming properties after heating, as the globulin aggregates were too large for effective interface stabilisation. A similar mechanism was also shown at pH 3.8, where the globulins precipitated, as the pH was close to their pI, while albumins remained soluble. Again, the albumins dictated the interfacial properties, leading to highly stable foams after removing the insoluble globulins. We have shown marginal differences in protein functionality after freeze- or spray-drying. More importantly, the changes in soluble protein composition dictate the protein functionality after heating or pH shifts

The dilatable membrane of oleosomes (lipid droplets) allows their in vitro resizing and triggered release of lipids

It has been reported that lipid droplets (LDs), called oleosomes, have an inherent ability to inflate or shrink when absorbing or fueling lipids in the cells, showing that their phospholipid/protein membrane is dilatable. This property is not that common for membranes stabilizing oil droplets and when well understood, it could be exploited for the design of responsive and metastable droplets. To investigate the nature of the dilatable properties of the oleosomes, we extracted them from rapeseeds to obtain an oil-in-water emulsion. Initially, we added an excess of rapeseed oil in the dispersion and applied high-pressure homogenization, resulting in a stable oil-in-water emulsion, showing the ability of the molecules on the oleosome membrane to rearrange and reach a new equilibrium when more surface was available. To confirm the rearrangement of the phospholipids on the droplet surface, we used molecular dynamics simulations and showed that the fatty acids of the phospholipids are solubilized in the oil core and are homogeneously spread on the liquid-like membrane, avoiding clustering with neighbouring phospholipids. The weak lateral interactions on the oleosome membrane were also confirmed experimentally, using interfacial rheology. Finally, to investigate whether the weak lateral interactions on the oleosome membrane can be used to have a triggered change of conformation by an external force, we placed the oleosomes on a solid hydrophobic surface and found that they destabilise, allowing the oil to leak out, probably due to a reorganisation of the membrane phospholipids after their interaction with the hydrophobic surface. The weak lateral interactions on the LD membrane and their triggered destabilisation present a unique property that can be used for a targeted release in foods, pharmaceuticals and cosmetics.Engineering Thermodynamic