Shimon Mizrahi

Leaching of soluble solids during blanching of vegetables by ohmic heating

Ohmic heating may provide an effective method for blanching, especially of whole large vegetables where the process may be accomplished in a relatively short time regardless of the shape and the size of the product. Such a process eliminates any need for dicing of these large vegetables as commonly done prior to water blanching. The extent of solute leaching during blanching by both ohmic heating and by hot water follows the same pattern. It is proportional to the surface to volume ratio of the product and to the square root of the process time. Therefore, by eliminating the need for dicing, blanching by ohmic heating may considerably reduce the extent of solute leaching, as compared to a hot water process, by a favorable combination of a low surface to volume ratio and a short blanching time. For example, in the case of whole beets, the extent of soluble pigment loss during blanching by ohmic heating was about one order of magnitude lower than that of 1 cm cubes of the same product during the equivalent process in hot water.

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.

FLOW BEHAVIOUR OF CONCENTRATED ORANGE JUICE

Orange concentrate, at the 60-65° Brix concentration level, is a non-Newtonian fluid with yield stress and time dependent behaviour. While recovery from low-rate shear is reversible, shear at high rate causes irreversible destruction of the viscous structure. Part of this effect is due to disintegration of pulp particles. Pulpless concentrate (serum) is also non-Newtonian, but yield stress and time dependent behaviour are present only when pectin concentration is high. Depectinized serum is Newtonian. The effect of temperature on flow properties of all three types of material was studied.

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.

Mechanisms of objectionable textural changes by microwave reheating of foods: a review.

Microwave reheating, compared to a conventional method, is notorious for lack of crust formation and severe toughening of flour and starch-based products. This review discusses how the typical thermal characteristics of microwave heating are involved in affecting the texture as well as the possible role of non-thermal effects. While low surface temperature is the well known mechanism why microwave heating is incapable of crust formation, the most severe toughening problems are caused by internal boiling. Beside moisture loss, the internally generated steam causes 2 main textural effects when it is vented out. The first is the replacing of non-condensable gases (air) in the product voids with a condensable one (steam). When the latter is condensed by cooling, a vacuum may be created in the voids causing their collapse and a formation of a more compact and tougher structure. The second textural effect involves amylose extraction from starch granules and its redistribution to eventually form a rich layer on the walls of the structural foam cells of the baked goods. Relatively fast crystallization of the amylose seems to be the main cause of toughening a short while after microwave heating. This mechanism is relevant mainly to products where starch is an important structural element. Structural disruptions by localize excessive steam pressure at hot-spots are also discussed in this review as well as methods of preventing or alleviating the most objectionable textural changes. The most effective ways of preventing these undesirable changes are by avoiding internal boiling and/or by manipulating the starch content and properties.

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.

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.