Ory Ramon

Self-Assembly of Bovine β-Casein below the Isoelectric pH

Beta-casein is an intrinsically unstructured amphiphilic protein that self-assembles into micelles at neutral pH. This paper reports that beta-casein self-organizes into micelles also under acidic conditions. The protein association behavior and micelle characteristics at pH 2.6, well below the p I, are presented. The pH was found to strongly affect the micelle shape and dimensions. Cryogenic transmission electron microscopy (cryo-TEM) experiments revealed disk-like micelles of 20-25 nm in length and approximately 3.5 nm in height in acidic conditions. An aggregation number of 6 was determined by sedimentation equilibrium under these conditions. Isothermal titration calorimetry experiments verified the association below the p I and allowed determination of the micellization enthalpy, the critical micellar concentration, and the micellization relative cooperativity (MR). Small-angle X-ray scattering results at concentrations below the critical micellization concentration (CMC) suggest that the monomeric protein is likely in a premolten globule state at low pH. Calculations of the protein charge at acidic and neutral pH reveal a similar high net charge but considerable differences in the charge distribution along the protein backbone. Overall the results show that beta-casein is amphiphilic at low pH, but the distribution of charge along the protein chain creates packing constraints that affect the micelle organization, leading at concentrations above the CMC to the formation of disk micelles.

Osmotic dehydration phenomena in gel systems

Abstract Swelling or contraction plays an important role in affecting the mass transfer during osmotic dehydration (OD) of hydrophilic and hydrophobic gels. The initial stage of the OD process is characterized by mass loss until the gel volumes reach a minimal value. Two types of behaviours are observed after this point. The first one (Type I) shows a turning point and re-swelling of the gel. The main reason for such behaviour is the relatively high swelling pressure of the gel in a sugar or salt solution. In certain solutions, such as of sugars and sodium chloride, hydrophilic polyacrylamide gels swell even to a larger extent than in water. The second type of behaviour (Type II) is characterized by total lack of re-swelling capability, which is most likely the result of a phase separation process. In hydrophobic gels, phase transition may take place under critical conditions that are determined by the combined effect of temperature, type and concentration of low molecular solutes. In hydrophilic gels, on the other hand, “salting-out” is the most likely reason for phase separation. The driving forces of all these phenomena are attributed to the effects of the preferential interaction between the low molecular solutes and the gel polymeric matrix.

Collapse processes in shrinkage of hydrophilic gels during dehydration

The driving force and the end point of the shrinkage process, during dehydration, were evaluated using poly(acryl amide) and poly(acrylic acid) gels, as a model of food systems. The maximal shrinkage of the gels is practically the extent of their full collapse when water is removed and replaced by air. The drastically diminishing polymer mobility due to transition into its glassy state is one of the two mechanisms that determine the end point of the contraction process. Addition of low molecular weight solutes, such as sugars, lowers the moisture content required for the onset of the glass transition and thus extends the rubbery region at which shrinkage may take place. However, their own volume of the sugars sets a bottom limit for contraction, thus resulting in less shrinkage the higher the sugar content. The second mechanism that stops contraction may come into effect when a rigid filler is present in the system. The percolation of its particulates, while the polymer is in its rubbery state, forms a rigid solid matrix that resists further bulk shrinkage. However, until it reaches its glassy state, the polymeric network may continue its contraction around the filler particulates, thus forming a porous structure.

Shrinkage behaviour of hydrophobic hydrogel during dehydration

Abstract A gel of poly-n-isopropylacrylamide (PNIPA) spontaneously shrinks when dehydrated at a temperature above 35 °C. The latter is the critical volume phase transition point where the system separates into a rich-polymer phase in equilibrium with pure water thus the water activity of the system at that point is practically one. This phenomenon may be attributed to hydrophobic attraction that is strong enough to push water molecules away from the vicinity of the polymer. This impacts also on the sorption isotherms of the hydrophobic gels. Compared with the hydrophilic ones, these gels have a relatively low water holding capacity. Even under very mild dehydration conditions, such as a temperature of 40 °C and water activity of 0.85, the moisture content of the gel is readily reaching a level (0.1 g water /g solids ) where the system becomes glassy. Since dehydration processes normally take place in an environment of relatively low water activity, the surface water is readily removed to a very low level resulting in the formation of a glassy rigid skin. When strong enough, this rigid skin resists further macroscopic shrinkage.

Effects of microwave reheating on surimi gel texture

Microwave re-heating of a surimi gel may change its mechanical properties by introducing defects into the protein gel matrix. These defects result from the localized hot spots that are generated due to the non-uniform nature of microwave heating, especially at high output power. When the temperature at these hot spots is above the boiling point, the resulting high water vapor pressure may overcome the gel cohesion forces and create structural defects. This phenomenon will not take place in the absence of loci having a temperature above boiling point. The latter is the case when re-heating is done by hot water, steam, low power microwave, and even by high-power microwave, as long as the temperature is well below the boiling point. The structural changes due to these defects seem to weaken the gel structure, which is opposite to the effect caused by moisture lost.

Air-Suspension Fluidized-Bed Microencapsulation of Probiotics

In the present research, an air-suspension fluidized-bed technique for generation of core and shell microcapsules containing probiotic Lactobacillusparacasei cells was evaluated. The air-suspension process was performed in a Würster coater system with a bottom-spraying atomizer. In the first stage, a solution containing trehalose, maltodextrin, and probiotic cells was spray-coated onto and absorbed by the inert carrier microcrystalline cellulose to produce nonagglomerating dry coated particles with high probiotic cell viability (109 colony-forming units [cfu]/g particles). The effect of inlet air temperature, spray flow rate, solids concentration, cell concentration, and encapsulation formulation on survival was investigated. The inlet air temperature had the most pronounced effect; a 15°C increase in inlet air temperature led to a 250-fold decrease in survival percentage. There was no agglomeration of the coated adsorbed particles at spray flow rates of 1 to 3.5 mL/min. Spraycoating was performed with both laboratory- and pilot-scale Würster systems. Scaling up the equipment from the lab scale to the pilot scale did not affect cell survival percentage. Low solids concentration (<20%) and low probiotic cell concentration (6%) resulted in a fair survival percentage (35–40). Spraying a solution formulation of trehalose–maltodextrin with dextrose equivalent (DE) 3 at a 1:1 ratio provided the best protection in terms of cell viability during the spray-coating process as well during storage. Layering with ethylcellulose (ETHOCEL) provided some protection in a low acidic environment (12%); however, the nonuniform coating and cracks detected by scanning electron microscopy (SEM) imaging were the main reasons for the layerings relatively poor protection of the cells against humidity, oxygen, and acid environment.

The effect of topological constraints on polymer network pressure

Recent data on comparative osmotic deswelling of poly(acrylic acid) gels and solutions shows that the difference between osmotic pressure of the gel and of the solution, P gel (c, f) - P solution (c,f ), taken at equal polymer concentration (c) and fraction of the ionized acrylic acid monomers (f), changes its sign from negative to positive as the concentration c increases; this effect is enhanced with increasing f whereby it is shifted toward lower concentrations. In order to explain this effect, a model is suggested for the elastic pressure of the gel network which takes into account the effect of topological constraints combined with the effect of network ionization. According to the model, the sign-change of P gel - P solution originates from the topological constraints on conformations of the network; the ionization of the network chains enhances this effect and shifts it to the concentration range of the experiment.