Method for sublimation drying of food products

FIELD: food-processing industry, in particular, processes for preservation of animal food products.

SUBSTANCE: method involves removing about 60-70% of moisture from product to be dried by stepped reduction of pressure accompanied by freezing of product at each stage to minimum temperature corresponding to pressure at the given stage, and heating product between steps by means of working fluid to -5, -4 C; providing holding of product in frozen state at minimum temperature at each stage. Heating of product between stages is carried out with the use of inert gas, such as carbon dioxide or nitrogen. Frozen product holding time at minimum temperatures is increased with decrease of pressure at stages.

EFFECT: improved quality of end product.

2 cl

 

The present invention relates to methods of preserving foods of animal and plant origin, intended for long-term storage, and can be used for dehydration of mainly fish, seafood, meat.

The method of freeze drying of food products was proposed and patented in 1921, the Soviet engineer GEEPA-Strzelecki (great hotel AI and other Freeze-drying in the food industry. - M.: Food. prom the terrain, 1972, p.5).

The main disadvantage of this method of dehydration is high energy consumption for the implementation process. This is because freeze drying is a complex synthetic process in which three interrelated energy-intensive process; vacuum, freezing, heating product, process characterized by long duration of the working cycle (from 8 to 14 hours or more), depending on the degree of grinding of the dried product, and other factors.

There is also known a method of freeze drying of food products, according to which prior to the removal of the dried material 60-70% moisture drying is carried out at step pressure decrease with freezing the material in each stage to a minimum temperature corresponding to the pressure in this step and between steps heat the material about wojennym air -5, -4°With (USSR author's certificate No. 283034, publ. in BI, 1970. No. 30 - prototype).

The disadvantage of this method of freeze-drying is abundant contact dryable material with oxygen dehydrated air during heating of the material in each stage from the minimum temperature corresponding to this degree, -5, -4°C. the Use of dehydrated air eliminates the receiving material additional moisture, but dry the material interacts with an increased amount of oxygen. The latter contributes to the development of the material undesirable oxidative processes are actively occurring during the heat treatment of products of animal origin.

Quantitative evaluation of components of the gas stream is an important characteristic of the technological mode of processing products in a vacuum, so when the freeze dehydration of meat, fish (biologically active materials) required to maintain acceptable oxygen concentration in the residual gases. When implementing the freeze-drying method, adopted as a prototype, the oxygen in sublimator is due to two sources: from the leakage of air from the outside through leaks and filing in sublimator dehydrated air when heated product from the minimum temperature to -5, -4°C. Kolichestvennogo, coming into sublimator two mentioned sources, the corresponding valid, when the oxygen concentration of hinin the flow, can be calculated by the formula

where a is the allowable air flow when the concentration of oxygen, hin, l·PA/s;

In actual flow of air entering the vacuum system, l·PA/s;

h - permissible concentration of oxygen in the stream, %;

hinthe oxygen concentration in notcause air, hin=20,95%.

It follows that to reduce the amount of oxygen entering the system with air, it is possible in two ways: by reducing the oxygen concentration in the entering air, or a reduction in the incoming air stream. The first path is difficult to implement practically, since the concentration of oxygen in the air hin=20,95%=const. To reduce the amount of oxygen entering into the vacuum system with air, acceptable the second way, i.e. by reducing the incoming air stream.

When implementing the method of drying, taken as a prototype, entering the vacuum system, air flow is formed, again, the leakage of air from the outside through leaks and supply of dehydrated air to heat the dried product between steps. Airflow Andnnotcause is Zune through leaks is an important parameter in any vacuum system, depending on many factors. The numerical value of this parameter is kept constant Andn=const, it is difficult to change especially in the direction of reducing its magnitude, i.e. in the direction of decreasing entering through leaks air flow, l·PA/C. In such conditions, to reduce the amount of oxygen coming from the air in the vacuum sublimation system installation, practically acceptable remains the possibility of reducing or complete exclusion of oxygen from the gaseous working agent (gaseous environment)that is used to heat a drying product between stages of the dehydration process from the minimum temperature to -5, -4°C.

When implementing the freeze-drying method, adopted as a prototype, the main supplier of oxygen in the vacuum system is dehydrated air used for blowing a drying product. The improvement of this method and the efficiency of its implementation can be achieved by a significant reduction in the amount of oxygen that interacts with the dried product.

The present invention is to improve the process of freeze-drying and improving the quality of dehydrated food products.

The solution of this problem is achieved by the fact that, in accordance with which the procedure of freeze drying fish or meat, including the removal of the dried product is about 60-70% water by stepwise pressure reduction with freezing the product in each stage to a minimum temperature corresponding to the pressure in this step, by heating the product between the levels of the gaseous working agent to -5, -4°To provide exposure of the product in a frozen state at a minimum temperature of each stage, starting at a temperature of -7,15°for the first stage, when -9,8°for the second stage and ending at a temperature of -21, -26,6°for the last step, and heating of the product between the two stages to a temperature of -5, -4°With lead dioxide or nitrogen.

The duration of exposure of the frozen product with minimum temperatures increase with decreasing pressure in stages, with the shutter speed at a minimum temperature of the first stage is 5-10 min, and in each subsequent stage, the exposure time increases by 5-7 min in comparison with exposure in the previous stage.

The main purpose of the process of freeze-drying as a method of preserving fish, meat and other objects is to preserve the properties of fresh products. In such circumstances, the active oxidation of the dried products is undesirable process. Although completely eliminate irreversible oxidation processes in foods with limacina drying practically it is not possible, dehydrated by sublimation fish and meat retains the color, shape, smell and digestibility of fresh food for long term storage. This indicates the feasibility of improving the freeze-drying method, ensuring in the process of dehydration minimum oxidation of the dried products

The extract dried product at a minimum temperature of each stage has its advantage, which follows from the next. With decreasing residual pressure in sublimator the temperature of the product decreases as the system pressure of the gas environment is the temperature of the product tends to equilibrium. However, the decrease in temperature lags (offset) at the time of lowering the residual pressure. Lags temperature of the external and internal layers of product on the progress of the pressure is different. Extract the product with a minimum temperature provides the necessary temperature in the volume of the product to conform it to the lower pressure stage. The temperature equalization in the amount of the product to conform it to the pressure level is favorable, since the main driving force for evaporation of the pair is the difference of partial pressure of vapor at the surface of the product and in the surrounding gas environment. In addition, you extract shivamoga product at the lower temperature of each stage contributes to the destruction of unfrozen moisture (about 10%), which in practical terms samosopryazhennye products contain even at -20°C.

When implementing the proposed freeze-drying method for heating a product, instead of dehydrated air, it is recommended to use an inert gas such as carbon dioxide (CO2) or nitrogen (N2). This ensures the creation of malotilate gas environment in the process of dehydration of fish, meat and other, improving the quality of the final product.

The increased duration of exposure of the frozen product as reduce the lower the residual pressure in stages contributes to a more complete temperature equalization in the amount of the product and increase the time steps is reached the difference of partial pressures of steam, which determines the intensity of the evaporation process.

This method of drying can be carried out with different number of stages of reduction of residual pressure. Successive stages differ from one another by the values of the lower residual pressure. The step between the lower pressure adjacent stages is determined empirically and is expressed by a corresponding difference in minimum temperatures. Practical is the difference in 3, 4, and 5°C. the Method allows for different values of steps between the steps of lowering the residual pressures involved in the process both the varovanie products.

The proposed method of freeze drying of food products is carried out as follows.

Subject to freeze-dehydration product with a room temperature, is fed into the vacuum chamber (sublimator), after which the chamber is sealed and connected to a running vacuum pump. Upon reaching sublimator a residual pressure of less than 4.5 mm Hg process begins samozatachivanie product. When samozatachivanie is the use of heat accumulated in the product, on evaporation of the product of the water, so when a certain residual pressure of water in the product freezes (samosobirayutsya). When samozatachivanie (depending on the final temperature, i.e. the depth of samozatachivanie) of the product is removed 8-12% moisture.

After lowering the residual pressure in sublimator to the bottom of the first stage pressure, for example 2.5 mm Hg, the achieved pressure stand for 5-10 minutes to equalize the temperature throughout the volume of the product accordingly to -7,15°C. After that, the system is isolated from the vacuum pump and sublimator serves inert gas, for example carbon dioxide at room temperature, to heat the product. After increase of the residual pressure up to 3.0 mm Hg, i.e. when the product temperature -5,1°With heat of the product gas stop, and sublimator connect the to a running vacuum pump.

If the step between the first and second steps of lowering the residual pressure is assumed equal to, for example, 3°C, the lower the residual pressure of the second stage is 2.0 mm Hg, which corresponds to the temperature of the product -9,8°C. the Second and subsequent steps of lowering the residual pressure freezing of the product is carried out in sequence, similar to the one described. Exposure of the product at the lowest temperature in each subsequent stage increase by 5-7 min in comparison with exposure in the previous stage.

Stepwise reduction of residual pressure freezing of the product lead to 0.7-0.4 mm Hg, respectively, to -21, -26,6°C. Further dehydration of the product are typical when the contact heat input to the product.

Step freezing of products to 21, - 26,6°accepted considering the fact that the eutectic point of the solution of salts in foods of animal origin (fish or meat) is in these temperatures, and they allow complete vigorazhivat from the product free of moisture.

1. The method of freeze drying fish or meat, including the removal of the dried product is about 60-70% water by stepwise pressure reduction with freezing the product in each stage to a minimum temperature corresponding to the pressure in this step, heating the product of the and between the levels of the gaseous working agent to -5, -4°C, characterized in that provide exposure of the product in a frozen state at a minimum temperature of each stage, starting at a temperature of -7,15°for the first stage, when -9,8°for the second stage and ending at a temperature of -21, -26,6°for the last step, and heating of the product between stages to a temperature of -5, -4°With lead dioxide or nitrogen.

2. The method according to claim 1, characterized in that the duration of exposure of the frozen product with minimum temperatures increase with decreasing pressure in stages, with the shutter speed at a minimum temperature of the first stage is 5-10 min, and in each subsequent stage, the exposure time increases by 5-7 min in comparison with exposure in the previous stage.



 

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