extracts of oak, birch, St. John's wort; infusion of green tea; solutions of sugar and
ascorbic acid; ascorbic acid and sodium chloride.
The pre-treatment methods aimed at reducing the activity of the water include
dehydration, osmotic dehydration, freezing in concentrated solutions. Pre-drying at
temperatures of 100°C and above ensures the inactivation of the oxidative-reducing
enzymes, promotes the preservation of the biologically valuable components,
accelerates the freezing process, prevents lacing of the products during storage.
The aerial dehydration is sometimes replaced by osmotic. This changes the ratio
of the free and osmotically absorbed moisture which contributes to the preservation of
the microstructure of the tissues and stabilization of the consistency. However, despite
the benefits of osmotic dehydration, namely, increasing the content of the dry matter,
preserving the vitamin value, colour and structure of the tissues of the fruits, from a
technological point of view, the osmotic dehydration is a rather costly process.
In order to reduce the activity of water, the fruits are frozen in sugars or in its
solutions of different concentrations. Adding to a solution of starch sugar helps to
soften the sweet taste of the fruits at high concentrations of the first; the addition of
ascorbic acid, citric acid and their mixture guarantees the preservation of the colour;
and the addition of gelatin, pectin, agar - ensures the stability of the consistency.
A significant factor in the formation and preservation of the consumer properties
of frozen foods is the detection of an optimal method of freezing and its method -
contact or non-contact. The choice of freezing methods is determined by the economic
and technological aspects. The most used is air freezing. Along with its significant
advantages, it has significant disadvantages: low thermal conductivity of air which
slows down the heat transfer intensity and activates oxidation processes under the
condition of free access of oxygen to air; massive loss of mass.
When freezing fruits and vegetables in solutions (calcium and sodium chlorides,
propylene glycol, ethyl alcohol, mixtures of salts and alcohols), the process of freezing
is intensified. This is due to higher thermal conductivity of the solutions compared with
air, reducing mass losses during freezing. In addition, rapid immersion freezing,
compared with air, reduces the dehydration of the product. A significant drawback is
the negative influence of the solution on the taste properties of some types of plant
material (except eggplant, pepper, potato) and the need for special waterproof film and
rapid corrosion of equipment.
The use of low-boiling cryogenic liquids, in particular, liquefied nitrogen, carbon
dioxide, reduces the product temperature from plus 21°C to minus 18°C, depending on
the size of the product for 1 to 5 minutes. However, the high freezing rate, the intense
increase in the internal volume of the product can cause cracking of the fruits. The
advantages of freezing in vapors of liquefied nitrogen include the lack of drying. The
passive nitrogen atmosphere formed on the surface of the product helps to slow down
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