Agriculture 4. Technologies of storage and processing of agricultural products

Assoc. Prof. V. Yu. Ovsyannikov, postgraduate student Ya.I. Kondrateva,

undergraduate student N.I. Bostynets

Voronezh State University of Engineering Technologies, Russia

Influence of process of freezing on allocation pectin

Substances from fruit juice

The majority of fruit juice very much мутны and hardly filtrations give in. It is caused by a peptization of pectin and other substances which are in vegetable fabrics, and passed into solution, at preliminary crushing of raw materials. As a result it turns out high-disperse sedimentation - steady system with characteristic for pectinaceous Zola, as lyophilic colloid properties.

Stability of colloidal parts is caused mainly by the electrokinetic’s potential interfering them at Brownian motion to face one with another and thanks to forces of a mutual attraction to merge in larger units (flakes). That there was a coagulation, it is necessary either to remove electrokinetic’s potential or to lower it to the critical.

On a surface of a solid body at its contact with liquid spontaneously there is an excess electric charge which is compensated by counter ions. As a result on limit of the section of phases a solid body - liquid is formed a double electric layer.

The big specific surface of particles of a disperse phase promotes accumulation on limit of the section of phases of considerable electric charge which becomes the reason of the electrokinetic’s phenomena — movements of a phase or Wednesday under the influence of external electric field, and also emergence of electric charges at movement of particles of a disperse phase or the liquid dispersive environment.

Emergence of charges on a solid body leads to that on limit of the section of phases there is an electric potential.

Emergence of a layer forming potential influences the ions which are in liquid. To a firm surface from the liquid environment the ions which sign is opposite to a sign of a layer forming potential are attracted.

According to Stern's theory, counter ions are in two provisions: one their part forms the adsorptive layer of counter ions which is forming as a result of electrostatic interaction with ions of a layer forming potential and specific adsorption and other part of counter ions is in a diffusion layer. Unlike the adsorptive layer counter ions of a diffusion layer aren't fixed; as a result of thermal motion of the molecules of the environment and mutual pushing away they are capable to move within a diffusion layer to depth of liquid, i.e. from area bigger to area of smaller concentration.

At movement of particles of a disperse phase the part of the liquid which is on a surface of a solid body to border of sliding, is fond of particles and moves together with them. In case of movement of the dispersive environment this part of liquid remains on a surface of a solid body and is motionless.

Potential on border of sliding represents electrokinetic’s potential.

Potential of a double electric layer decreases in process of removal from a surface of a solid body. And in the adsorptive, most dense bed of a double electric layer linear decrease in potential is observed. Violation of linear dependence happens on sliding border, and in a diffusion layer decrease in potential happens under the exponential law and isn't so sharp, as in the adsorptive layer.

The inflection point on crossing of the line characterizing change of electric potential in process of removal from a firm surface, with border of sliding determines the size of electrokinetic’s potential.

Now stability of colloidal system break introduction of various coagulants containing in the structure inorganic salts dissociating with formation of negatively loaded ions, using properties of a double electric layer.

Formation of crystals of ice when freezing provides decrease in electric potential as on its surface in solution spontaneously there is an excess electric charge which is compensated by counter ions. There is a layer forming potential which influences the ions which are in liquid. To a firm surface from the liquid environment the ions which sign is opposite to a sign of a layer forming potential are attracted.

Coagulation of hydrosols and hydro gels under the influence of freezing is explained by a gradual freeze concentration of water, increase thereof concentration of the dissolved substances in blankets of particles, compression of a double layer and decrease in stability. Freeze concentration of water can proceed at the same time on two mechanisms: due to film diffusion of molecules from a coagulant layer to a surface of earlier formed ice and owing to emergence of the centers of crystallization of water in the layer. The particles which have stuck together at freezing form the coagulate structures remaining at thawing.

On coagulating freezing actions have strong impact structure of the dissolved substances and a temperature mode of freezing.

In case temperature of freezing is lower than temperature of education by the dissolved substances of the cryohydrate, the dissolved substance hardens in layers between ice crystals with formation of an eutectic. Thus irrespective of initial concentration of the dissolved substance there is a deep frost penetration in gel and its volume after thawing considerably decreases.

If temperature of freezing is higher than temperature of education by the dissolved substance of cryohydrate, in layers between ice crystals the dissolved substance reaches such concentration at which at this temperature the freeze concentration doesn't occur any more, compression of gel stops, and deposit volume after thawing of subjects is higher, than more initial concentration of the dissolved substances.

Fast freezing is much less effective, than slow as in this case water stiffens in the form of continuous weight and concoction of the dissolved substances near a surface of particles doesn't happen.

Solution freezing to the eutectic point leads not only to coagulation of pectin’s substances, but also to an exception of the return process of a peptization and transition of a deposit to a suspension owing to violation of spatial structure of molecules of pectin’s substances and creates favorable conditions for further concoction of fruit juice by a method of a freeze concentration of moisture.