Categories
Production of marmalade-Pastila products

Fundamentals of drying tin of apple jelly

Marmalade, selected from the following forms of cages, contains up to 29 31% of water.

Its humidity almost coincides with the humidity of the marmalade mass before bottling, since the loss of water due to free evaporation during sinking is rather insignificant and is no more than 0,5 — 1%. Marmalade comes out of the form of a wet, with a soft and friable texture and with a sticky surface. To obtain marmalade in the form of a stable, transportable and fully decorated product, it is necessary to dry the selected semi-finished product selected from the forms.

The final moisture content of tin apple marmalade is brought to 22 — 25%.

Instead of a sticky surface, which has a crude marmalade, you must get it fine crystalline crust, which gives it the appropriate type and forms a protective coating on it nenamokayuschee.

Thus, drying the marmalade achieves the removal of excess water from the marmalade and the formation of a crust on its surface.

Despite the small size of the pieces of tin marmalade, it is a difficult drying material. This is primarily due to the nature of water binding in it.

There are two main forms of bound water in jellies:

  •  and capillary
  • colloid, or adsorption-related.

Capillary water can be removed relatively easily.

It can be assumed that the bulk of this is the water that extends from the open capillaries and fills the micropores on the surface of raw marmalade.

Colloidal amenable water removal much more difficult since its vapor pressure is lower than the capillary water.

The first type of moisture is characterized by the fact that for the most part it obeys the laws of evaporation from the free surface of water. The rate of evaporation of this moisture depends on the air velocity and is directly proportional to the difference between the vapor pressure at the temperature of the evaporating water and the partial pressure of vapor in the air.

The process of evaporation of more tightly bound colloidal water from marmalade is governed mainly by the laws of diffusion and depends on the movement of water inside the marmalade (from the migration of moisture and the deepening of the surface of evaporation). The course of this process is characterized by drying curves and drying speed curves. The first show changes in material humidity over time. The drying speed curves express the graphic relationship between the drying speed (in g / cm2hour) and the humidity of the material (in%). .

In the presence of capillary water on the surface of marmalade, the drying rate remains at the beginning of the process at a constant level. When the transition from the evaporation of this water to the evaporation of colloidal water occurs, the drying rate drops. Depending on this, there are periods of constant and falling drying rate or periods of external and internal diffusion of moisture, respectively.

The internal diffusion of moisture in marmalade depends on the gradient of humidity and on the state of the temperature field of the material during the drying process, that is, on the difference in water content and on the temperature difference in the adjacent layers of marmalade.

The higher the gradient of moisture, the more intense the drying takes. The movement of moisture goes with the direction from the center to the periphery.

If there is a significant temperature gradient inside the material, drying is delayed, since moisture migrates at the same time due to thermal and thermal diffusivity from the outer to the inner layers, i.e. in the direction opposite to the first one, which delays drying.

Very important for the rate of migration of moisture inside the marmalade is the viscosity of the medium. The viscosity of the marmalade, in turn, is determined by its drying temperature: the higher the temperature of the marmalade, the lower its viscosity and the easier the movement of water in it. Along with the temperature, the viscosity of the marmalade is greatly influenced by the ingredients of the marmalade recipes and the cooking mode.

Of the prescription components of marmalade in this respect are of particular importance applesauce and syrup. - Due to its hydrophilic properties, pectic substances mashed potatoes and dextrins molasses prevent the removal of water from marmalade. This explains the fact that with an increase in the proportion of molasses in the recipe for marmalade or when working with a highly gelatinous puree, the removal of water from the marmalade is delayed. On the contrary, with an increase in the proportion of sugar in the recipe, drying is greatly facilitated. In the first case, the so-called “long-drawn” marmalade is obtained, in the second case - “sugary”. A puffy marmalade takes much longer to dry than a sugary.

The rate of removal of water from marmalade is limited to some extent by the allowed temperature of its drying, which is usually maintained within 55 — 65 °. At higher temperatures, the destruction of pectin and the weakening of the marmalade consistency, as well as some darkening of it, are possible. An increase in drying temperature can also lead to excessive inversion of sucrose, which has an adverse effect on crust formation and marmalade strength. In particular, this is observed with increased acidity of marmalade.

The quality of the finished marmalade also depends on the correct formation of the crust during the drying process. As water is removed from the marmalade, supersaturation of the sugar solution occurs, which is accompanied by crystallization of sugar on the surface of the marmalade. Drying of marmalade should be carried out in such a way that the crystallized layer forms a thinner crust as possible, since the presence of a thick crust leads to the coarsening of the marmalade and to the deterioration of its marketable appearance.

The research process of drying tin of marmalade

The rationalization of the drying process should be based on the study of specific drying properties of tin marmalade, kinetics and dynamics of its drying.

In a study carried out in VKNII [18], it was established that there is a close relationship between the rate of formation of a crust on the surface of the marmalade and the rate of drying.

Proper coordination of the formation of the crust with the process water yielding marmalade is crucial to control the drying of marmalade.

In this connection, the mechanism of the formation of a crust was studied in detail. This process is determined by the rate of sucrose crystallization. As is known, this value is characterized by the amount of sugar crystallized in 1 min. on xnumx mxnumx crystal surface. According to Silin, the rate of sucrose crystallization from thick viscous media is expressed by the following relationship:7.1

where: K is the rate of sucrose crystallization;

T - the absolute temperature;

C is the concentration of the supersaturated solution at a distance r from the crystal face;

c is the concentration of a saturated solution at the crystal face;

ƞ — medium viscosity; r is the length of the diffusion path;

Ʀ is a constant coefficient.

It was found the effect of individual factors on the formation of a crust.

The decisive role played by the causative capacity of the prescription mixture. Changes in the viscosity of the latter have a dramatic effect on the formation of a crust. The properties of apple puree, which goes to the recipe (its ability of gelatinization and mineral composition), as well as the ratio of apple puree, sugar and molasses in the recipe are important. In general, any changes in the prescription mixture that increase the caustic capacity of the latter, slow down the formation of a crust.

The value (C – s) expresses the degree of supersaturation of the marmalade mass with sucrose. This value depends on the number of sugars contained in marmalade, and on their ratio. Crystallization of sucrose on the surface of marmalade should occur gradually, within certain limits. The content from 20 to 23% of reducing sugars in ready-made marmalade with the constancy of all other conditions is most favorable in this regard.

The presence of molasses dextrins, which increase the viscosity of the marmalade, also inhibits crystallization.

The same effect as dextrin, pectin and renders.

On the other hand, pectin, like dextrins, plays a certain positive role in the formation of the crust structure. The sugar that crystallizes from the viscous medium forms small crystals, cemented together by these adhesives; the crust is obtained with a fine crystalline structure. The presence of pectin and dextrins gives the crust strength, elasticity and shine.

For successful drying of marmalade, it is necessary that the full development of the crust on the marmalade does not occur before the bulk of the water to be evaporated does not diffuse out. Otherwise, a rough crust is formed on the marmalade, which can significantly delay the evaporation of water from the inner layers of marmalade. With a significant content of water inside the marmalade and at an elevated temperature of drying, the finished crust / is eroded. With the re-formation of the crust, it turns out to be cloudy and weak and the marmalade has an unattractive appearance.

Obviously, the regulation of both processes should be carried out by selecting the appropriate parameters of air (temperature, relative humidity and speed) for different periods of drying. With increasing temperature, the formation of a crust is accelerated. As for the relative humidity of the air, it depends on the speed of free evaporation of water from the surface of marmalade, the latter determines the degree of supersaturation of the surface layer (C – s), i.e. the ability to crystallize sucrose in it. Changes in air velocity within 0,5 — 2,0 m / s do not have a noticeable effect on the rate of formation of a crust,

It is necessary to focus on the maximum allowable temperature of the marmalade, since under this condition it is possible to achieve a reduction in the process duration.

The maximum permissible drying temperature of marmalade depends on its pH.Drying curves and speeds of marmalade drying at 65 °.

Fig. 14. Curves of drying and drying rate of marmalade at 65 °.

It was found that fruit jelly with a pH equal to 3,3 — 3,5 (acidity around 0,6%) can be exposed to 65 ° for 9 — 12 an hour.

For drying fruit jelly with pH, ​​equal to 3,0-3,2 (acidity ~ 0,7-0,9%), the maximum permissible temperature 60 °. Drying of the marmalade over the 12 hours. at 70 ° causes some depolymerization of pectin and increased accumulation of invert sugar.

The drying curves and drying speeds of marmalade at 60, 65 or 70 ° obtained in laboratory and semi-production experiments showed that the drying process proceeds from the first hours in the period of decreasing drying speed, that is, in the period of internal diffusion. The period of constant drying speed in this case is practically not detected.

The process of drying the marmalade is illustrated by the curves of drying and its drying speed at 65 ° (Fig. 14).

When drying marmalade, prone to the rapid formation of a crust, as well as at a high drying temperature, there is a sharp drop in the drying rate due to the resistance created by the crust to evaporation of moisture from the marmalade surface.

After reaching the standard humidity of marmalade, the process of moisture return continues. Approximation to the equilibrium state of marmalade in the dryer is observed at absolute humidity around its 2C%.Changing the moisture gradient of marmalade during the drying process.

Distance from the central axis in mm

Fig. 15. Changing the moisture gradient of marmalade during the drying process.

The moisture gradient diagrams for two samples of “long” and “sugary” marmalade (Fig. 15) illustrate the distribution of moisture in the outer (2 zone), intermediate (2 zone) and average (1 zone) marmalade layers at different drying points.

These data show that marmalade already before “drying has a significant moisture gradient. The initial drying period is in 2 hours and in 6 hours. characterized by a sharp increase in the moisture gradient of marmalade from the inner to the outer layers. Alignment of moisture in marmalade occurs slowly throughout the drying process, and at the end of drying there is still a significant difference between the humidity of the inner and outer layers.

It was noted above that the movement of moisture in marmalade is largely determined by the state of its temperature field. In the center of the marmalade (fig. 16), the temperature quickly rises during the first drying period (about 2 hours), moving away from the temperature curve of the wet thermometer and approaching some constant distance from the temperature curve of the dry thermometer. The observed known constancy of the temperature of the marmalade (below the temperature of the dry thermometer) after the first 2 — 4-hour drying period until the end of the process is characteristic of the internal diffusion of moisture. Only after 10 — 12 hours, i.e., at the end of “drying” (when the marmalade has a moisture content equal to 26 — 31% of dry matter), there is a tendency for the marmalade temperature to approach the dry thermometer temperature, i.e., a transition to the period equilibrium.

The equilibrium humidity of marmalade after drying fluctuates depending on the humidity of the ambient air in the range from 22,86 to 33,86% by weight of dry matter (the corresponding limits of relative air humidity are 65 and 80%).

The optimal drying mode and the cooling pan of apple jelly

Based on the analysis of drying curves, speed, marmalade drying, humidity gradient and marmalade temperature, it was proposed to distinguish two periods in the process of drying.7.16

The first drying period is characterized by a high gradient of moisture at the surface of marmalade, i.e. there is a basic condition for the intensive development of internal diffusion of moisture. In the same period, there is a rapid drop in the drying rate due to the crust formed, which creates (ever-increasing resistance to evaporation of moisture through the marmalade surface. During this period, it is advisable to maintain the softest possible drying mode (i.e. low temperature and moderate degree of air saturation), aimed at slowing down the formation of a crust.

In the second period, the conditions for internal diffusion of moisture deteriorate due to the partial leveling of the humidity of the inner and outer marmalade layers and with an increase in the viscosity of the medium. In addition, the presence of a formed crust causes a decrease in the moisture conductivity of the outer layer of marmalade. Under these conditions, there is a need to raise the drying temperature to a permissible maximum.

In experiments conducted in a special semi-industrial type drying unit based on the above provisions, indicators were developed for the optimal drying mode for sugar marmalade in relation to the conditions of the factory process:5.2

In the cross-flow between the tiers, on which sieves with marmalade are installed, the air moves with a change of direction in 1 hours.

For puffy marmalade, a somewhat lower drying temperature is required, respectively, for the first period of 55 ° and for the second 60 °.

The total duration of drying with the initial moisture content of the product (relative) 34 — 37% (52 — 61% absolute humidity) is 8 — 10 hour.

In connection with the transition in recent years to a deeper boiling of the marmalade mass with a decrease in the relative initial moisture of the marmalade to 29 — 31%, the drying time can be taken equal to 6 — 7 an hour.

Upon leaving the marmalade from the dryer, it must be cooled to a temperature of 20 — 30 °, at which it can be laid in boxes.

A study of the marmalade cooling process showed that when using air for this purpose at room temperature and circulating it in the interfloor space at a speed of about 1 m / s, the duration of cooling of the marmalade is 45 — 60 minutes. at a temperature of cooling air 15 — 20 ° (in winter) and about 1,5 hours - at a temperature of air 20 — 30 ° (in summer).

Described (the mode of drying and cooling of marmalade can be carried out in continuously operating dryers of various types. After drying, the product must be transferred to the cooling chamber, which is a continuation of the same dryer.

For medium and small enterprises, the constructive solution of the marmalade dryers can be provided in the form of a tunnel in which the rack trolleys with sieves of marmalade are continuously moving. The forward movement of the trolleys through the tunnel is provided by a special mechanism. In the absence of mechanized dryers, use chamber or cabinet dryers of periodic action. In all cases, the operation of the drying device must be designed for air characteristics that meet the requirements of the optimal drying mode and with the obligatory condition of uniform distribution of the coolant - air in the working space of the dryer.

Cooling marmalade chambers must be performed in a circulating air stream in a horizontal passing through mezhyarusnye space, under the conditions specified above. For small businesses, "wait in line" marmalade to cool indoor plant. At the same time good ventilation and relative humidity should be provided not more than 70%. Vystoyka marmalade extends from the 4 8 to an hour. depending on the ambient temperature.

Add a comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to combat spam. Find out how your comment data is processed.