RU2727671C1 - Green tea processing method - Google Patents

Abstract

FIELD: food industry.SUBSTANCE: invention relates to the tea industry. Redox enzymes are inactivated by thermal action on green tea leaves with subsequent treatment of green tea leaves to residual moisture content of 60–75 %. Tea leaves are twisted. Drying of tea leaf is performed in two stages and tea leaf is sorted. At the first stage for 8–12 minutes the formation of tea leaves with height of not more than 10 cm, with moisture content of 60–75 % of electromagnetic microwave field, at constant pressure in drying chamber with values selected from range of 40–80 kPa, with uniform heating temperature growth rate 3–4 °C/s. Speed is selected proceeding from the necessity to reach the heating temperature values from range of 70–85 °C by the end of the first stage and taking into account duration of the first stage. At the second drying stage pulse heating is used in range of 35–45 °C by means of electromagnetic microwave field at pressure reduction in drying chamber to values of 15–4 kPa until residual moisture content is less than 10 %. Then the vacuum pump is switched off and atmospheric pressure is restored in the chamber.EFFECT: invention ensures simplification of green tea drying technology, provision of uniform heating of each batch of tea loaded into drying chamber, prevention of caramelisation of sugars at the initial stages of drying mode, increase of green tea infusion extractiveness and improvement of its organoleptic properties.1 cl

Description

Technology area

The invention relates to methods for processing tea and can be used in the tea industry.

State of the art

Methods for processing green tea are known from the prior art, including the following processing steps for raw materials:

- withering (pre-drying)

- fixation

- twisting

- final drying.

The withering operation makes the tea leaf more suitable for both manual and mechanical processing (the leaf becomes more “pliable”).

Fixation means the processing of small portions of withered tea leaves during manual processing for several minutes (mainly from 7 to 10) at a temperature of 180 ° C and during mechanical processing at a temperature of about 250 ° C inside a rotating drum for 2-3 minutes. With such a short-term heating, enough moisture remains in the leaf to further subject it to the rolling operation, and at the same time, the temperature effect is sufficient to fold the vegetable protein contained in the tea leaf, which prevents the process of destruction of vitamins and trace elements in the leaf (http: // tea.finepokupka.ru/).

The rolling operation is aimed at shaping the sheet into the desired shape. Final drying prevents oxidation of polyphenols, thereby preserving the antioxidant properties of tea, in addition, drying determines the ability of tea for long-term storage and affects the most important organoleptic properties of tea - aroma and color of the infusion, removing the herbal odor, developing tea aroma.

The disadvantages of traditional methods of processing raw materials are unstable quality characteristics of the resulting tea and low (mainly due to the technologies used at the stages of fixation and final drying) productivity.

Known technical solutions related to drying green tea, aimed at overcoming the noted disadvantages.

So there is a known method of drying green tea (USSR Inventor's Certificate No. 1375223, published 02/23/1988) D1, according to which the drying of green brick tea is carried out in a dry nitrogen gas at a stepwise increasing temperature. In the description of the method it is said that an improvement in the opening of the stomata of coarse tea leaves is achieved due to gaseous nitrogen, depending on the temperature of which the intensity of the opening of the stomata changes. At the same time, the temperature at the initial stage of drying is low in order to ensure uniform heating of the tea brick. Since at the initial stage of drying, moisture intensively evaporates from the surface layers of the brick, this can lead to rapid drying, the appearance of cracks on the surface of the brick, to its deformation, while the middle of the brick remains wet.

If the first stage of drying is carried out while maintaining the temperature of nitrogen gas below 29 ° C, then drying will take a long time and the proportion of catechins in the total tannin content will decrease, which will negatively affect the quality of the tea. It is the presence of catechins with P-vitamin activity that provides the antioxidant properties of green tea, its ability to affect the permeability of capillaries and the elasticity of their walls and other useful properties of green tea.

Therefore, the first stage of drying is carried out at a temperature of 30 to 32 ° C. When tea humidity reaches 12.5-13.5%, tea briquettes are heated evenly throughout the entire volume, moisture transpiration increases in them, and then the temperature is raised to 50-52 ° C, which speeds up the drying process.

In fact, the known method protects the temperature regime of convective drying, i.e. type of drying, in which the transfer of heat by the raw material occurs with the help of a drying agent / neutral gas, which simultaneously performs the function of a heat carrier and a moisture carrier / - a transport medium into which the removed moisture passes.

The disadvantages of convective drying are significant energy consumption of 1.8-3.0 kWh / kg, a decrease in the thermal conductivity of the product at the end of drying, which significantly lengthens the process, which, in turn, as already mentioned, negatively affects the quality of the product.

Convective drying, in addition to the noted drawbacks, does not provide the possibility of long-term storage of products, being significantly inferior in this to infrared drying and freeze drying, as well as microwave drying. At the same time, drying using microwave radiation surpasses other known types of drying in a number of important indicators, as shown in the table given in the source (Ministry of Agriculture of the Russian Federation. Federal State Educational Institution of Higher Professional Education "Irkutsk State Agricultural Academy" Scientific and practical journal "Bulletin of the Irkutsk Agricultural Academy", issue 36 September: Irkutsk, 2009) D2.

It is known to use microwave radiation for drying green tea leaves in a vacuum (US Patent No. 8617633, published 12/13/2013) D3, while the description of the patent does not disclose the parameters of the drying mode, except for the indication that the leaves are dried in a microwave oven under conditions vacuum until the leaves begin to burn, which occurs when the leaf temperature reaches values in the range of 150-170 ° C. By vacuum is meant a pressure below atmospheric pressure, preferably in the range from 3 to 7 kPa. In the example given, pre-prepared whole tea leaves (drying is preceded by keeping them in water at a temperature not exceeding room temperature, to prevent leaf damage, it is advisable to keep them at a temperature of 10 to 6 ° C) are placed in an oven with a capacity of 48 kilowatts and then the pressure in the oven is reduced up to 3 kPa. When the temperature in the upper layer of leaves reaches a temperature of about 160 ° C (after about 32.5 minutes), the state of the leaves is monitored, if a small part of the leaves is burnt, the stove is turned off, the leaves are unloaded from the stove and cooled. This method results in a tea that combines the flavor of roasted tea with the tangible herbal flavor of green tea. To reduce the herbal flavor of green tea, the leaves may be further roasted by methods such as induction heating, conductive heating, and convection heating.

As a result, in order to obtain green tea with stable organoleptic properties and with the required shelf life without deteriorating its properties, a long process is required with a change in a number of technological modes and significant energy consumption. In addition, there are high risks of spoilage of the batch of tea loaded into the oven due to the fact that the moment of stopping heating is determined by the beginning of burning of the leaves, which by this moment are practically dried. Accordingly, after the dried and starting to burn leaves are unloaded from the oven and begin to cool, they, at best, will continue to smolder, saturated with a fried smell, and at worst, they will burn.

Also known in the art is a method for processing green tea (RF patent No. 2689694, published on 05/28/2019) D4, including inactivation of redox enzymes by thermal exposure to green tea leaves, followed by processing green tea leaves to a residual moisture content of 60-75%, rolling tea leaf, drying of tea leaves in vacuum using an electromagnetic microwave field and sorting of tea leaves, characterized in that the drying of green tea leaves is carried out in three stages with a total duration of 85-157 minutes, at the first stage a layer of tea leaves with a moisture content of 60 -75% electromagnetic microwave field at constant pressure in the drying chamber with values selected from the range of values of 90-200 mm Hg, with a uniform rate of growth of the heating temperature, which is selected based on the need to reach the heating temperature values from the range of 40 by the end of the first stage -60 ° С and taking into account the duration of the first stage, at the second stage of drying, the d pressure in the drying chamber to values of 200-500 mm Hg, and the heating temperature - to values of 60-90 ° C, at the third stage of drying, the heating of the tea leaf layer is stopped and the pressure in the drying chamber is reduced to 80-30 mm Hg by the time the temperature in the drying chamber reaches the temperature values in the production room in which the drying chamber is installed, the rate of pressure change is controlled in accordance with the rate of change in the temperature of the tea leaf bed, and the ratio of the durations of the first, second and third stages is selected from the ranges of values 1 , 5-1 h: 0.05-0.15 h: 0.35-0.95 h.

The method presented in D4 is rather complicated and contains a multi-stage drying stage, which leads to a deterioration in the quality of tea due to excessive heating of the raw material.

Information source D4 is selected as the closest analogue to the claimed invention.

Disclosure of the invention.

The problem to be solved by the claimed invention is to develop a method for processing green tea, which will eliminate the disadvantages of the prior art.

The technical result to be achieved by the present invention is to simplify the technology of drying green tea, ensure uniform heating of each batch of tea loaded into the drying chamber, prevent caramelization of sugars at the initial stages of the drying regime, increase the extract of green tea infusion and improve its organoleptic properties.

The specified technical result, the achievement of which the claimed technical solution is aimed at, is achieved due to the fact that the method of processing green tea, including inactivation of redox enzymes by thermal action on green tea leaves, followed by processing green tea leaves to a residual moisture content of 60-75%, rolling the tea leaf, drying the tea leaf in vacuum using an electromagnetic microwave field to a residual moisture content of less than 10%, and sorting the tea leaf, characterized in that the drying of green tea leaves is carried out in two stages, in the first of which the layer is heated for 8-12 minutes from tea leaves, no more than 10 cm in height, with a humidity of 60-75% by an electromagnetic microwave field, at a constant pressure in the drying chamber with values selected from the range of 40-80 kPa, with a uniform heating temperature rise rate of 3-4 ° C / s, which is selected based on the need to reach the heating temperature values from the range 70 -85 ° C and taking into account the duration of the first stage, at the second stage of drying, pulsed heating is applied in the range of 35-45 ° C by means of a microwave field, with a decrease in the pressure in the drying chamber to 15-4 kPa until residual moisture is reached, after which vacuum the pump is turned off and atmospheric pressure is restored in the chamber

Implementation of the invention

"Tea" for the purposes of the present invention means the leaf material Camellia sinensis var. sinensis or Camellia sinensis var. assamica

"Leaf tea" means plant material obtained from a tea plant that has not been infused.

Microwave radiation or high-intensity microwave radiation is used for non-contact heating of bodies. The product is heated by the presence of dipole molecules in it, at one end of which there is a positive electric charge, and at the other - a negative one. Under the influence of an electric field, dipole molecules line up along its lines of force. Changing the direction of the field entails a change in the direction of the molecules. Water molecules are dipoles, respectively, at an oscillation frequency of 1000 MHz, the electric field 2,000,000,000 times per second changes its direction and with the same frequency the direction of water molecules changes in it. Due to the resulting friction of the molecules, the product is heated and the liquid evaporates. Microwaves work only in a relatively small surface layer of the product, penetrating inside to a depth of several centimeters, heating occurs due to the heating of the surface layer by microwaves and the subsequent penetration of heat into the depth of the heated product due to its thermal conductivity.

High-voltage vacuum devices - magnetrons are used as sources of the electromagnetic microwave field. Magnetrons can operate at various frequencies from 0.5 to 100 GHz, with powers from several W to tens of kW in continuous mode, and from 10 W to 5 MW in pulsed mode with pulse durations mainly from fractions to tens of microseconds.

Microwaves enter the oven chamber through a waveguide - a channel with metal walls that reflect microwave radiation. Microwaves entering through the waveguide into the oven chamber are randomly reflected from its walls and come to the processed product from various directions. The interference of waves amplifies the waves that arrived in phase, such waves will warm up the area they hit, and those that arrived in antiphase will extinguish each other. There are various ways to provide a uniform wave field - mechanical (using various types of dissectors), constructive, in which the negative effect of interference on the uniform heating of the product in the oven is reduced by creating optimal conditions for the reflection of waves from its walls and due to a certain geometry of the oven.

In household microwave ovens, uniform heating is ensured by placing the processed product on a rotating table. But this way of solving the problem is unacceptable (for reasons of energy consumption and dimensions) for drying the product in an industrial environment.

One of the ways to solve the problem of uniform heating of a batch of a product in a drying oven is to ensure the arrival of microwave radiation to a selected area of the processed product from different sources by switching on sources of an electromagnetic microwave field in a pulsed mode, and the radiation of each of these sources affects the selected plot from different directions.

Switching off the sources of the electromagnetic microwave field is due to the fact that the radiation power of the magnetrons is unchanged and to weaken the intensity of the effect of microwave radiation on the processed product, it is necessary to switch off the magnetrons.

There are inverter power systems, widely used in air conditioners, that allow smooth power changes. In microwave ovens, inverter power systems also make it possible to smoothly change the power of the radiation source (magnetron), instead of turning it off at specified time intervals.

When green tea leaves are treated with an electromagnetic microwave field, it undergoes profound changes, including a significant change in its dielectric properties, which affects the kinetics of microwave heating. This creates certain difficulties in maintaining optimal processing conditions that provide an adequate response to changes in product properties throughout the technological cycle.

Especially the influence of the effect of changing the dielectric properties is manifested during drying, when the humidity and the dielectric loss factor decrease.

Another problem of using microwave ovens for heating dielectrics is the unevenness of heating associated with a drop in specific power, heterogeneity of the composition and moisture content, as a rule, the irregular shape of the product.

It is recommended to choose the shape of the workpiece so that its linear dimensions at least in one dimension do not exceed twice the penetration depth, which is the distance at which the absorbed power of the electromagnetic microwave field decreases exponentially. If this condition is not met, high rates of temperature and overpressure change arise, which for moisture-containing products can lead to the formation of cracks and voids in the product, thereby affecting the quality of the product.

Accordingly, when determining the rational technological modes of heating using microwave radiation, it is necessary to take into account the factors of changing the dielectric and physical-mechanical characteristics of products in the technological cycle.

And although dependences are known that allow calculating the power values of the electromagnetic microwave field, providing a given drying time for raw materials, reliable mathematical modeling of technological heating modes using microwave radiation is difficult due to the influence, along with the above factors, of influence on the quality of the processed product of such an indicator as cohesive strength and other physical and mechanical properties of the product being processed.

Taking into account the difficulties noted, the most effective technology for drying green tea in an oven with microwave radiation is to create a technology based on experimental data obtained during drying a batch of green tea in an oven with microwave radiation.

In this case, the geometry of the oven and the location of microwave sources (magnetrons) in it are designed to ensure the uniformity of the effect of microwave radiation on the product being dried.

The batches of green tea loaded into the drying oven were layers of green tea leaves that had passed the withering stage, forming rectangular horizontally elongated stacks up to 10 cm high. The material objects in the experiments were a green tea leaf Camellia sinensis var. Sinensis or Camellia sinensis var. Assamica.

In the course of the experiments, the influence of microwave dielectric heating modes on the kinetics of the properties of products was investigated, as well as methods related to the study of the composition, structure, mechanical, thermophysical, chemical and organoleptic properties of the objects under study.

Comparative experimental studies made it possible to select the modes of temperature treatment of green tea leaves that are optimal in terms of the combination of the obtained physicochemical and organoleptic properties of the products and the energy consumption of the equipment used for the values of the moisture content of the original product varying from 60 to 75%.

The claimed method is carried out as follows:

freshly harvested tea leaves were wilted in the traditional way - by storage for several hours, possibly up to 24 hours. At the same time, tea leaves lost moisture at a rate depending on the temperature and humidity of the environment and the amount of surface moisture. In the withering process, about 15 percent of moisture was lost: from 70 to 85% in a fresh leaf and up to 55-70% in tea leaves that had passed the withering stage.

Fermentation in the cut tea leaves begins immediately after the cut. So green tea, which, unlike black tea, is considered not fermented, actually goes through, albeit for a very short period of time.

Fixation (inactivation of redox enzymes) is carried out by heat exposure of green tea leaves for several minutes, followed by drying the leaves to a residual moisture content of 60-75%.

During the rolling stage, essential oils begin to form in the tea leaves, giving the tea its characteristic aroma. Green tea leaves were rolled using a rotary vane machine (RLM) to shape them.

Then, a layer was formed from the withered twisted tea leaves on a pallet, the dimensions of which in the horizontal plane were determined by the dimensions of the pallet, in turn, depending on the dimensions of the drying chamber in which the tea goes through the drying stage. The seam height should not exceed 10 cm.

Drying of tea leaves in vacuum is carried out using an electromagnetic microwave field, so that drying of green tea leaves is carried out in two stages.

At the first stage, a layer of tea leaves, no more than 10 cm in height, with a moisture content of 60-75%, is heated for 8-12 minutes by an electromagnetic microwave field at constant pressure in a drying chamber with values selected from the range of 40-80 kPa, with a uniform the heating temperature rise rate of 3-4 ° C / s, which is selected based on the need to reach the heating temperature values from the range of 70-85 ° C by the end of the first stage and taking into account the duration of the first stage.

At the second stage of drying, pulsed heating is applied in the range of 35-45 ° C by means of a microwave field, when the pressure in the drying chamber is reduced to 15-4 kPa until the residual moisture is reached, after which the vacuum pump is turned off and atmospheric pressure is restored in the chamber. lowering the temperature, which helps to stabilize the entire processing of green tea. Drying by pulse heating is carried out until the residual moisture is less than 10%.

The duration and intensity of the processing steps depend both on the properties of the green tea leaves and the parameters of the equipment, and on maintaining the possibility of vaporization in the tea leaves, which is necessary for drying them.

The impact on the leaves of green tea with microwave radiation is carried out under vacuum conditions (the pressure is changed using a pump) at a pressure value in the range of 40-80 kPa. This eliminates mold contamination, reduces the intensity of the oxidation process of tea, tea raw materials and reduces the rate of division of bacteria.

The choice of specific pressure values is determined by the moisture content of the green tea leaves before drying.

There are two widespread methods of food disinfection: pasteurization and sterilization. Pasteurization includes a single heating to a temperature of 60 ° C with further exposure at this temperature for 60 minutes or heating to 70-80 ° C with exposure for 30 minutes or heating to 98 ° C and above with exposure for several seconds. Sterilization is carried out by various chemical and physical methods, the latter include exposure to ultraviolet radiation, high-frequency currents, ultrasonic vibrations, radioactive radiation, infrared rays, etc., as well as exposure to high temperatures on sterilized objects (heat sterilization). As a result of pasteurization, vegetative forms of microorganisms die, but the spores survive and, when favorable conditions occur, intensive reproduction begins.

The use of high temperature during sterilization for a long time helps to completely destroy not only the microorganisms themselves, but also their spores. But prolonged exposure to high temperatures destroys the substances that give the taste and aroma of tea, as well as the vitamins it contains.

Accordingly, the temperature exposure regime should ensure the destruction of microbes that form mold on tea in the range of their growth temperatures and, in addition, ensure the rapid passage of temperatures at which the growth rate constant is maximum. The curve of the dependence of the growth rate on temperature for each organism is different, while the Q10 law is in effect: with an increase in temperature by 10 ° C, the reaction rate increases by 2-4 times. When passing through the optimum temperature, growth slows down significantly.

Therefore, the task of disinfection due to heating is the rapid passage of temperature values at which the growth rate of microorganisms (number of generations per hour), and, accordingly, the intensity of reproduction, is maximum and the transition to those temperatures at which the rate of death of microorganisms is maximum.

The temperature ranges at which green tea undergoes thermal treatment in an evacuated drying chamber equipped with microwave radiation sources (usually magnetrons) should be selected based on the above considerations. In turn, the value of the pressure drop is selected based on the synchronization of the temperature values as it decreases after switching off the sources of the electromagnetic microwave field with the pressure values at which the current values of the temperature of green tea leaves provide the process of vaporization.

If in the mode of active temperature change the problem of reducing the microbial load (molds and bacteria) to the required values is solved, then in the mode of "passive" temperature change, accompanied by a decrease in pressure, the process of steam formation from the water molecules remaining in the green tea leaves is intensified.

The steam draws out aromatic essential oils from the deep layers of the tea leaf. Essential oils settle on the surface of the leaf and firmly "stick" to it and retain their properties for quite a long time, being extracted only under the influence of boiling water at the time of brewing.

At the same time, due to the short-term exposure to high temperature and the main time of exposure to temperature with decreasing values, the destruction of vitamins and organic acids does not occur in the tea leaf.

As the results of experiments have shown, under the influence of high temperature, together with the removal of moisture in the tea leaf, thermochemical processes occur, as a result of which the specific aroma, color and taste of the finished tea are finally formed. As a result of drying with microwave radiation in a vacuum, the taste softens, the infusion intensifies, its color, the destruction of substances (chlorophyll and others) that negatively affect the taste characteristics of tea.

The improvement in aroma is due to the increased interaction of phenolic compounds with amino acids and other substances that affect the formation of volatile aldehydes, which have a pleasant odor.

Moreover, as experiments show, not only the aroma of the infusion increases, but also its taste improves - the bitterness decreases, the taste becomes thinner, and a light sweetness appears in the aftertaste, which indicates that the infusion is rich in the amino acid L-theanine. Depending on the variety, the tea leaf has a concentration of L-theanine from 0.2 to 2.25%, the amount of L-theanine is directly proportional to the amount of catechins in the tea. This amino acid, which acts like a neurotransmitter, owes many of its health benefits to tea, including its effects on heart rate and blood pressure. A high content of L-theanine is characteristic of young leaves; during the processing of tea leaves, the amount of this amino acid decreases during fermentation and a greater amount is released into the solution depending on the duration of drying at a low temperature - the maximum content is reached after drying at a temperature of 40-55 ° С within 7.0-8.5 hours.

As shown by chromatographic analysis, the concentration of L-theanine after processing green tea leaves with an electromagnetic microwave field in a low vacuum for 3-10 minutes at temperatures close to 90 ° C and followed by a decrease in pressure and temperature-pressure for 8-12 minutes to values close to the boundary values of the average vacuum, and temperatures to room values (temperature in the workshop, etc.), is comparable to that obtained as a result of drying at a temperature of 40-55 ° C for 7.0-8.5 hours.

Carrying out the initial stage of drying with the operation of microwave sources in the maximum power mode allows not to carry out the fixation stage, thereby reducing the total processing time of green tea leaves from the moment they are cut to packaging of finished products intended for delivery to the market.

The claimed method makes it possible to obtain tea with a microbial load several times lower than that allowed by the standards in force in the tea industry and with improved organoleptic properties.

The claimed method can be carried out using known devices for drying ovens with microwave radiation, which confirms its industrial applicability

During drying, the magnetrons were switched off and on in a pulsed mode, proceeding from the provision of the possibility of uniform heating of all sections of the processed tea layer.

Claims (2)

A method for processing green tea, including inactivation of redox enzymes by thermal action on green tea leaves, followed by processing green tea leaves to a residual moisture content of 60-75%, rolling a tea leaf, drying a tea leaf to a residual moisture content of less than 10% in vacuum using an electromagnetic microwave - fields and sorting of tea leaves, characterized in that the drying of green tea leaves is carried out in two stages, in the first of which a layer of tea leaves, no more than 10 cm high, with a moisture content of 60-75% by electromagnetic microwave frequency is heated for 8-12 minutes -field, at constant pressure in the drying chamber with values selected from the range of 40-80 kPa, with a uniform heating temperature rise rate of 3-4 ° C / s, which is selected based on the need to reach the heating temperature values from the range 70 by the end of the first stage -85 ° С and taking into account the duration of the first stage, at the second stage of drying, pulsed heating is applied in the range of 35-45 ° C by means of an electromagnetic microwave field, when the pressure in the drying chamber is reduced to 15-4 kPa until the residual humidity is reached, after which the vacuum pump is turned off and atmospheric pressure is restored in the chamber.