Fermented tea production method

Abstract

To provide a drying method capable of keeping fragrance peculiar to fermented tea generated in a fermentation step in dried tea in a production process of the fermented tea.SOLUTION: Tea leaves subjected to at least either a wilting step or a fermenting step are dried under reduced pressure to suppress loss of fragrance components of fermented tea in a production method of the fermented tea, and thus the dried tea leaves keeping fragrance peculiar to the fermented tea can be obtained. Preferably, drying is conducted under a reduced pressure of 610 Pa or lower and the tea leaves are in a frozen state when the pressure is reduced. The loss of the fragrance components is suppressed during drying and storing after drying by subjecting the tea leaves to compression shaping before drying.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing fermented tea, and more particularly to a drying means used in the production process. [Background technology]

[0002] Tea is obtained by processing the leaves and stems of the tea plant (scientific name: Camellia sinensis). Today, there is a wide variety of tea available around the world, which is often classified by the degree of fermentation during tea production, broadly divided into fermented tea and non-fermented tea. Fermentation methods include microbially fermented tea (post-fermented tea), in which microorganisms are involved, and enzymatic fermentation, in which tea leaf components are changed by the oxidative action of enzymes contained in the tea leaves. Generally, microbially fermented tea is classified as non-fermented tea, and enzyme-fermented tea is treated as fermented tea (unless otherwise specified in this specification, fermented tea means enzyme-fermented tea). Furthermore, tea is further classified by the degree of fermentation, such as weakly fermented tea, semi-fermented tea, and fermented tea (fully fermented tea).

[0003] For example, in China, tea is classified into six types depending on the degree of fermentation and the manufacturing method (unfermented tea: green tea, yellow tea, black tea; fermented tea: white tea, green tea, black tea). In a narrow sense, fermentation refers to the process of rolling the tea leaves and then leaving them to stand, allowing the enzymes in the leaves to act and change the chemical components in the tea leaves, but in a broader sense, fermentation also includes the process of withering the freshly picked tea leaves without heat inactivating the enzymes, and the process of stimulating the tea leaves by stirring, in the sense that chemical changes in the components occur within the tea leaves.

[0004] For example, black tea, a typical fermented tea, is produced through the sequential processes of plucking, withering, rolling, fermentation, and drying. Black tea is characterized by its vibrant aroma, refreshing astringency, and vibrant red-orange color, which are the result of enzymes acting during the withering, rolling, and fermentation processes. In the fermented tea production process, the drying process primarily reduces moisture content to enhance shelf life. The drying process typically involves the use of air dryers, fluidized-bed dryers, rotary drum dryers, and water dryers. These dryers all expose tea leaves to hot air, which can result in the loss of aroma compounds in the tea leaves. Furthermore, in black tea production, high-temperature treatment during the initial drying stage after fermentation stops the enzymatic reaction, but this process can also result in the loss of aroma and accelerate fermentation. During the withering and fermentation processes in fermented tea production, enzymatic and oxidative reactions produce the aroma compounds characteristic of fermented tea from aroma precursors in the tea leaves. Such aroma components are highly volatile and are easily lost during the drying process using hot air.

[0005] Therefore, methods for suppressing the loss of aroma that occurs during the production process of fermented tea have been investigated. For example, Patent Document 1 discloses a method for preserving tea leaves in a state where the quality of the aroma is good and the amount of its components is high by flash-freezing the tea leaves that have been rolled during the processing step without putting them into a dryer. Furthermore, Patent Document 2 discloses that fresh tea leaves are heat-treated to inactivate enzymes, and the dried tea leaves obtained by primary drying are then added to an aqueous suspension containing crushed fresh tea leaves, fermented with enzymes contained in the fresh tea leaves, heat-treated again to inactivate the enzymes, and then secondary dried, thereby obtaining semi-fermented tea that is rich in aroma components such as linalool and geraniol and has a strong aroma while maintaining the flavor of green tea.

[0006] Additionally, with regard to green tea, the lush, fresh aroma and color of fresh tea leaves are highly valued, and methods for preserving these characteristics have been investigated. For example, Patent Document 3 proposes a tea production method characterized by vacuum-drying the tea leaves after a steaming process in the green tea production process. It is disclosed that this method prevents oxidation of the tea leaves and maintains the vivid green color of the leaves, thereby improving the marketability of green teas such as sencha and tonocha. Furthermore, Patent Document 4 proposes a simplified method for producing tea leaves with a good flavor and aroma, a vibrant color, and many health-promoting ingredients, which involves a vacuum steaming process and a vacuum drying process for the purpose of preserving the color and components of the fresh leaves. Patent Document 5 also proposes a method for producing processed fresh leaves that can withstand long-term storage without changing the original flavor and active ingredients of the fresh leaves, in which fresh leaves are washed, wet with water, placed in a vacuum chamber, and subjected to a pretreatment process in which the leaves are frozen by the heat of vaporization of the water via vacuum lyophilization, followed by drying by vacuum freeze-drying.

[0007] Furthermore, a method has been proposed in which fresh leaves are withered during crude tea production and then the greening is killed in a short period of time, with the aim of retaining the refreshing green leaf aroma of green tea after retort sterilization during drink production (Patent Document 6). Also proposed is a method for efficiently producing homogeneous fermented tea leaves on an industrial scale, in which fresh leaves are treated with hot air to adjust the moisture content, and then roughly rolled while maintaining the temperature at a level suitable for fermentation (Patent Document 7). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-217803 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-054219 [Patent Document 3] Japanese Patent Application Publication No. 03-127938 [Patent Document 4] Japanese Patent Application Laid-Open No. 2014-036650 [Patent Document 5] Japanese Patent Application Publication No. 11-346702 [Patent Document 6] Japanese Patent Application Publication No. 2019-170356 [Patent Document 7] Japanese Patent Application Publication No. 2017-093469 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, several methods have been proposed for the drying process of tea leaves and for maintaining their aroma. However, these methods either use hot air for the final drying of the tea leaves or are designed to preserve the flavor and color of the raw tea leaves, and do not disclose a means for preserving the aroma unique to fermented tea in the dried tea leaves. Further improvements are therefore strongly desired. Therefore, an object of the present invention is to provide a drying method that can preserve the aroma unique to fermented tea that is generated during the production process of fermented tea in the dried tea leaves. [Means for solving the problem]

[0010] In the course of research aimed at solving the above-mentioned problems, the inventors noticed that the aroma detected during the withering, rolling, and fermentation steps in the fermented tea production process was not very noticeable in hot-air dried tea leaves. They then conducted extensive research into drying methods other than hot-air drying. As a result, they discovered that drying tea leaves that had been withered, rolled, and fermented under reduced pressure allowed a large amount of the aroma characteristic of fermented tea to remain in the dried tea leaves, leading to the completion of the present invention. They also discovered that freezing the compressed and molded tea leaves during vacuum drying reduced the loss of aroma during storage. Furthermore, they confirmed that the extraction efficiency of flavor components such as polyphenols was improved compared to hot-air drying.

[0011] That is, the present invention is as follows. [1] A method for producing fermented tea, characterized in that tea leaves that have undergone at least one of a withering process and a fermentation process are dried under reduced pressure. [2] The method for producing fermented tea according to [1], characterized in that the drying is carried out under reduced pressure of 610 Pa or less. [3] A method for producing fermented tea according to [1] or [2], characterized in that the tea leaves are in a frozen state when dried under reduced pressure. [4] A method for producing fermented tea according to any one of [1] to [3], characterized in that the tea leaves are compressed and molded when dried. [5] A method for suppressing loss of aroma components in fermented tea, characterized by drying tea leaves that have undergone at least a withering step or a fermentation step under reduced pressure in the production of fermented tea. [Effects of the Invention]

[0012] According to the present invention, a production method can be provided in which the aroma characteristic of fermented tea, which is generated during the production process, is maintained even in dried tea leaves. Furthermore, the tea leaves obtained by the production method of the present invention are characterized by improved extraction efficiency of flavor components such as polyphenols. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below. In this specification, unless otherwise specified, "%" indicates % by mass. Furthermore, a numerical range of "lower limit value to upper limit value" means a numerical range of "not less than the lower limit value and not more than the upper limit value," unless otherwise specified.

[0014] The present invention relates to a process for producing fermented tea leaves. Fermented tea leaves, as used herein, refer to dried tea leaves (rough tea or finished tea) produced using harvested leaves, stems, or other materials from the tea plant (scientific name: Camellia sinensis) through a process that includes at least withering or fermentation. The raw tea leaves may be common varieties used for processing into green tea, oolong tea, black tea, etc.; however, varieties improved for fermented tea, such as black tea and oolong tea, are particularly preferred because they exhibit a stronger aroma through withering and fermentation. Suitable tea varieties include domestic varieties such as Benifuuki, Benihikari, Benihomare, and Benifuji. Furthermore, among overseas varieties, varieties used for fermented tea production, such as the large-leaf Assam variety, the small-leaf Chinese variety, and hybrid varieties resulting from crossing these varieties, are suitable. The application of the present technology to the production of fermented tea using these varieties as raw materials is particularly effective.

[0015] In the present invention, fermentation refers to a process in which freshly picked tea leaves are withered without heat-inactivating the enzymes, or a process in which the tea leaves are stimulated by stirring or rolling, followed by a fermentation process in which the leaves are allowed to stand to promote fermentation, thereby developing the characteristic aroma of fermented tea through the action of enzymes in the tea leaves. Therefore, fermented tea leaves in the present invention refer to tea leaves whose aroma has been extracted through a fermentation process that utilizes the action of enzymes in the tea leaves, and more specifically, tea leaves produced through at least a withering or fermentation process. Examples of tea leaf types defined in this way include weakly fermented teas such as white tea, semi-fermented teas such as Baozhong tea and oolong tea, and fermented teas such as black tea. In weakly fermented teas and semi-fermented teas, the withering process triggers a decrease in moisture in the leaves, which causes hydrolases to act and break down aroma precursors present as glycosides, thereby developing the characteristic aroma of fermented tea. In addition to the withering process, the rolled tea leaves are fermented in black tea, which activates the oxidative enzymes in the tea leaves, causing the oxidative decomposition of lipids associated with the oxidation of polyphenols and changes in the aroma components that occur during the withering process, thereby eliciting the aroma characteristic of fermented tea. The present invention provides a means for preserving this unique aroma of fermented tea in dried tea leaves.

[0016] A typical production flow for fermented tea is, for example, as follows: The production method of the present invention is characterized in that the drying step is carried out under reduced pressure, and the other steps may be carried out according to the following general flow. <Black tea> Fresh leaves → withering → rolling → fermentation → drying <Oolong tea / Paoshu tea> Fresh leaves → Wilting (sunlight, indoors) → Stirring (shaking) → Killing (roasting) → Rolling → Drying <White tea> Fresh leaves → Natural wilting (natural fermentation) → Drying

[0017] In the present invention, withering refers to a process of reducing the moisture content of tea leaves to promote the activity of glycoside-degrading enzymes in the tea leaves. Withering methods include sun withering, which exposes tea leaves to sunlight; indoor withering, which involves ventilation; and shaking the leaves during withering. The withering process in the present invention is not limited to these methods; any conventional method can be used as long as it reduces the moisture content of tea leaves to a level that allows enzymes to function effectively. The moisture content is preferably reduced by 5 to 65%, more preferably 10 to 50%, relative to the weight before withering. Furthermore, conditions that result in a weight reduction of 5 to 30% are preferred for weakly fermented teas such as white tea and semi-fermented teas such as Baozhong tea and oolong tea, and a weight reduction of 20 to 60% for fermented black tea, but are not necessarily limited to these. The reduction can be adjusted appropriately depending on the desired flavor and aroma.

[0018] In the present invention, rolling refers to the process of rolling tea leaves. In the production of semi-fermented tea, this process is performed on tea leaves that have been withered and then degreened, while in the production of fermented tea, this process is performed on withered tea leaves. Rolling the tea leaves destroys the tissue of the tea leaves, making it easier to extract the components in the tea leaves during the infusion process when drinking. Furthermore, in fermented tea, the destruction of the tissue brings the oxidizing enzymes in the tea leaves into contact with polyphenols, facilitating the enzymatic oxidation and fermentation. The rolling method is not limited, and includes the traditional hand-rolling method as well as mechanical rolling using a rolling machine. In addition, in the production of black tea, a CTC (Crush, Tear, and Curl) machine, an LTP (Lawrie Tea Processor), or a Rotorvane machine, which are mainly used overseas, can also be used. The rolling conditions can be appropriately set depending on the target aroma and physical properties of the tea leaves and the characteristics of the equipment used.

[0019] When applying the technology of the present invention to the production of black tea, an oxidative fermentation step, which is commonly used in the production of black tea, may be added. In this case, the rolled tea leaves are loosened and allowed to stand at a constant temperature to allow oxidative fermentation to proceed. The conditions for the fermentation step are not particularly limited and may be set appropriately depending on the state of the tea leaves and the desired flavor and aroma. However, fermentation is preferably carried out at a temperature range of 15 to 40°C, more preferably 20 to 30°C. The fermentation time is preferably 30 minutes to 6 hours, more preferably 1 to 3 hours. However, depending on the ambient temperature during fermentation, the time is generally longer at lower temperatures and shorter at higher temperatures. As fermentation progresses, the tea leaves turn reddish-brown, the aroma becomes more complex, and the appearance and flavor unique to black tea are achieved.

[0020] The present invention is characterized by drying under reduced pressure. By adopting these conditions, the loss of aroma formed by withering and fermentation is suppressed, allowing dried tea leaves to retain the state of aroma components immediately before drying. Under reduced pressure, the partial pressure of water vapor in the air decreases, lowering the boiling point of water, allowing drying to proceed at lower temperatures. In other words, reducing the pressure allows drying at temperatures below room temperature. Generally, fermented tea is dried at high temperatures using hot air, but the increase in tea leaf temperature during drying can rapidly promote oxidative fermentation. Therefore, it is necessary to adjust the fermentation state of the tea leaves before the preceding process, taking into account this effect. In contrast, with the reduced-pressure drying method of the present invention, the tea leaf temperature decreases due to the latent heat of evaporation as water evaporates, and the reduced pressure also reduces the amount of oxygen in the atmosphere, preventing the oxidation reaction from proceeding. Therefore, even if drying is started at an optimal fermentation state, the present invention allows tea leaves to be obtained in a stable fermented state with minimal changes during drying. Furthermore, when hot air drying is performed in a wet state, it is inevitable that aroma components will volatilize along with the moisture. However, in the present invention, moisture is allowed to evaporate first, so that more aroma components are retained, which is clearly superior to conventional hot air drying methods.

[0021] The apparatus used for drying under reduced pressure should have a sealed container that can accommodate tea leaves, reduce the pressure inside the container, and remove evaporated water from the sealed container. An oil-sealed rotary vacuum pump or the like can be used to reduce the pressure. Specifically, the drying process involves placing tea leaves in the sealed container and reducing the pressure inside, thereby drying the tea leaves. The pressure (atmospheric pressure) during the drying process is preferably 610 Pa or less, more preferably 110 Pa or less, and even more preferably 10 Pa or less. By maintaining a pressure of 610 Pa or less, the tea leaves are cooled by the heat of vaporization, and the water in the frozen state is sublimated. In other words, freeze-drying can be performed. If the pressure can be lowered, external heating may be used to apply radiant heat to the tea leaves to promote sublimation. The temperature conditions during this process should be set to a level that maintains the frozen state, but are preferably 60°C or less, more preferably 40°C or less. Although the tea leaves can be frozen by the heat of vaporization even if the temperature during vacuum drying is started at room temperature, it is preferable to freeze them beforehand in order to preserve the aroma components. Drying is continued until the moisture content of the tea leaves is 10% or less, but it is not necessary to remove all the bound water in the tea leaves, so the moisture content of the tea leaves after drying is 2 to 10%, preferably 3 to 8%.

[0022] When tea leaves are dried with hot air, they shrink and become thin and firm. On the other hand, the reduced-pressure drying method of the present invention allows the leaves to easily untwist and open up. Therefore, the tightly tangled state created by rolling can be maintained by compressing the tea leaves before drying. If the compression molding frame has ventilation, the leaves can be dried under reduced pressure while still inside the frame. Alternatively, if the leaves are frozen in a compression-molded state, they can be dried while maintaining their compressed state even after being released from the compression molding frame. The compression molding frame may be a metal or resin mesh-like frame or a mortar-like frame with some open sides. Alternatively, the leaves may be compressed and then packed in a nonwoven fabric or mesh bag. The shape of the molded product is not particularly limited, but examples include cubes, cylinders, and spheres. Drying the tea leaves in a compressed state reduces the surface area of the tea leaves, thereby suppressing aroma loss and oxidation over time during storage. The pressure applied during molding should be sufficient to compact the tea leaves to a degree that does not squeeze out the moisture from the tea; for example, a pressure of 0.5 to 5.0 MPa is suggested.

[0023] The fermented tea leaves obtained by the production method of the present invention retain the highly volatile, light aroma components that volatilize during drying in conventional tea leaves, making them suitable for use in forms such as loose leaf tea or tea bags for drinking in a teapot or tea kettle. In the production method of the present invention, if a fixed amount of tea leaves is dried during compression molding and drying, it is possible to produce chunky tea leaves, eliminating the need for measuring. Furthermore, the tea leaves obtained by the production method of the present invention have a physical structure that minimizes loss of flavor components and facilitates infusion of the components, making it possible to quickly obtain an infusion. For this reason, the tea leaves are advantageous for low-temperature extraction, which results in slower infusion than hot-water extraction, and can be used, for example, to produce fermented tea products for cold-brewing. [Example]

[0024] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0025] [Test Example 1] Black tea was produced using benifuuki harvested during the autumn and winter seasons. After harvesting the fresh leaves from the tea garden, the fresh tea leaves (2.4 kg) were left to stand on a mesh in an environment of approximately 28°C, and were withered until the weight reached 1.7 kg (weight loss rate 29.2%) while gently blowing air with a fan. Next, they were kneaded by hand for approximately 40 minutes and fermented in an environment of room temperature 37°C and humidity 50%. When the elapsed fermentation time reached 1, 2, and 3 hours, approximately one-third of the charged amount was taken out each time, and the tea leaves at each fermentation time were further divided in half. One half was vacuum freeze-dried using a vacuum freeze dryer (freeze dryer FDU-2110 type, Tokyo Rika Kikai Co., Ltd.) at a reduced pressure of 4.0 Pa for approximately 20 hours to obtain Example 1 (fermentation for 1 hour), Example 2 (fermentation for 2 hours), and Example 3 (fermentation for 3 hours). The other half was hot air dried at 90°C for 1 hour using a shelf-type hot air roasting machine (fully automatic roasted tea dryer CS-303 type, Kaisei Electric Co., Ltd.) to obtain Comparative Example 1 (fermentation for 1 hour), Comparative Example 2 (fermentation for 2 hours), and Comparative Example 3 (fermentation for 3 hours).

[0026] For these tea leaves and as reference examples, instrumental analysis of the aroma components was performed on commercially available black teas from representative production areas using the SPME-GC / MS method shown below. As reference examples other than the test samples, commercially available black teas from representative production areas (Reference Example 1: Assam, Reference Example 2: Uva, Reference Example 3: Darjeeling) were similarly evaluated, and the results are shown in Table 1.

[0027] 《Analysis and Quantification Method of Aroma Components by SPME-GC / MS》 100 mg of tea leaves crushed with a food processor, 3.0 g of sodium chloride, and 10 mL of ion-exchanged water were placed in a 20 mL SPME vial, and cycloheptanol (manufactured by Tokyo Chemical Industry) was added as an internal standard substance to a final concentration of 500 ppb to prepare an analysis sample. The aroma components were recovered by solid-phase microextraction (SPME: Solid phase Micro Extraction) and subjected to gas chromatography-mass spectrometry (GC / MS analysis). The GC / MS analysis conditions are as follows. The ratio of the peak area of each aroma component to the internal standard substance (IS ratio) was used as the peak area value.

[0028] <SPME-GC / MS Conditions> GC: TRACE GC ULTRA (Thermo Fisher Scientific) MS: TSQ QUANTUM XLS (Thermo Fisher Scientific) SPME fiber: 50 / 30 μm Divinylbenzene / Carboxen / Polydimethylsiloxane Stableflex (Sigma-Aldrich) ·Extraction: 60℃, 30 minutes Column: SUPELCO WAX10 0.25 mm I.D. x 60 m x 0.25 μm (Sigma-Aldrich) Oven program: Hold at 40°C for 2 minutes, then increase temperature to 160°C at 3°C / min, then increase temperature to 280°C at 10°C / min Carrier gas: Helium (100 kPa, constant pressure) Injector temperature: Splitless, 240℃ Ion source temperature: 200℃ Ionization: electron ionization Ionization voltage: 70eV Measurement mode: Scan Evaluated components and their monitoring ions: (E)-2-hexen-1-al: m / z = 69, (Z)-3-hexen-1-ol: m / z = 67, (E)-2-hexen-1-ol: m / z = 57, linalool oxide (cis and trans-franoid): m / z = 93, linalool: m / z = 93, hotrienol: m / z = 71, phenylacetaldehyde: m / z = 91, methyl salicylate: m / z = 120, geraniol: m / z = 69, β-damascenone: m / z = 121, 2-phenylethylalcohol: m / z = 91

[0029] In addition, a sensory evaluation was conducted on the tea leaves of Example 2, Comparative Example 2, and Reference Example 3. The evaluators were five expert panelists who were trained in identifying flavors and off-flavors and their concentrations, and who routinely appraise tea leaves. For the evaluation, 3.0 g of each tea leaf was extracted with 180 g of hot water at 95°C for 3 minutes. Evaluation items were based on the black tea aroma classification described in Reference 1 (Journal of the Society for Odor and Aroma Environment, 2014, Vol. 45, No. 5, pp. 344-350), and the aroma perceived as green, woody, dry, flowery, fruity, sweet, roasted, spicy, and cool was evaluated on a scale of 1 point (low) to 7 points (high), with Comparative Examples 1 and 2 receiving 4 points. The scores of the five panelists were averaged. The results of the sensory evaluation are shown in Table 2.

[0030] Furthermore, the amount of components extracted during extraction was investigated. 3.0 g of tea leaves from Example 1 and Comparative Example 1 were immersed in 180 mL of room-temperature ion-exchanged water and allowed to stand to perform cold extraction. 2 mL of extract was collected after stirring at 30, 60, 90, and 120 minutes after the start of extraction, and then filtered through a membrane filter. The concentrations of catechins, theaflavins, and polyphenols in the filtrate were measured. Catechins were measured under the conditions described in Reference 2 (JP 2018-134052 A). The total content of eight catechins: epicatechin (EC), epicatechin gallate (ECg), epigallocatechin gallate (EGCg), gallocatechin (GC), catechin (C), gallocatechin gallate (GCg), and catechin gallate (Cg) was calculated. Theaflavins were measured under the conditions described in Reference 3 (JP Patent Publication No. 2010-35548). The total content of the four theaflavins (TF1, TF2A, TF2B, and TF3) obtained in the analysis was calculated. Polyphenols were measured under the conditions described in Reference 4 ("Analysis Manual and Commentary for the 2015 Edition (7th Revision) of the Standard Tables of Food Composition in Japan," supervised by the Resources Office, Science and Technology Policy Bureau, Ministry of Education, Culture, Sports, Science and Technology, Kenpakusha, February 2016, pp. 242-243). The results are shown in Table 3.

[0031] [Table 1]

[0032] [Table 2]

[0033] [Table 3]

[0034] The instrumental analysis results shown in Table 1 confirmed that the products of the present invention (Examples 1 to 3) contained approximately twice as many aroma components as their corresponding comparative examples (Comparative Examples 1 to 3). The floral aromas of linalool, geraniol, and linalool oxide, the fresh fruit aroma of (E)-2-hexen-1-al, and the young grass aroma of (Z)-3-hexen-1-ol were particularly notable. These characteristics were also significantly higher in the products than in products from major tea-producing regions. The sensory evaluation results shown in Table 2 support the distinctive component content, confirming the excellent green (young grass) and flowery aromas. This aroma is a vibrant and light aroma unique to fermented tea, and the drying method used in the production method of the present invention demonstrated the characteristic of minimizing loss of these aromas. Furthermore, the results of the component leaching amounts shown in Table 3 confirmed that Example 1 of the present invention had a faster leaching rate and a larger leaching amount than Comparative Example 1 for all analytical items. This suggests that tea leaves prepared by vacuum drying lose less polyphenolic components than tea leaves prepared by hot air drying, and that their structure makes it easier for these components to be extracted.

[0035] [Test Example 2] Black tea was produced using Benifuuki tea picked in the second season. After picking the fresh leaves from the tea plantation, they were withered in an environment of about 30°C until the weight of the tea leaves had decreased by approximately 45%, and then stored in a refrigerator. The tea leaves were thawed, rolled by hand for about 40 minutes, and fermented for 1.5 hours in an environment of 37°C and 50% humidity. The tea leaves were dried with hot air to give Comparative Example 4, dried under reduced pressure to give Example 4, and frozen at -80°C and then dried under reduced pressure to give Example 5. Five grams of fermented tea leaves (dry weight 2.0 g) were placed in a container with a volume of 9 cm. 3 The tea leaves were compressed into a rectangular plastic case (inner dimensions: 20 × 25 × 18 mm) at a pressure of approximately 1.0 MPa and molded. These were then hot-air dried (Comparative Example 5), vacuum-dried (Example 6), and frozen at -80°C and then vacuum-dried (Example 7). Hot-air drying was performed using a shelf-type hot-air roaster (name and model number: Fully Automatic Roasted Tea Dryer Model CS-303, manufactured by Minsei Electric Co., Ltd.). Comparative Example 4 was hot-air dried at 90°C for 1 hour, and Comparative Example 5 was hot-air dried at 90°C for 1 hour followed by 60°C for 6 hours. The vacuum drying conditions for Examples 4 to 7 were a vacuum freeze dryer (Freeze Dryer Model FDU-2110, Tokyo Rikakikai Co., Ltd.) at a vacuum level of 4.0 Pa for approximately 20 hours. Sensory evaluation was performed on these tea leaves in the same manner as in Test Example 1. However, the extract was prepared by extracting 2.0 g of each tea leaf in 120 g of hot water at 95°C for 3 minutes, and the evaluation items were limited to the aroma perceived as "green" and "flowery," which were particularly distinctive in the evaluation of Test Example 1, and these were evaluated using Comparative Example 4 as the standard (4 points for each item). The results are shown in Table 4.

[0036] [Table 4]

[0037] The sensory evaluation results shown in Table 4 indicate that the vacuum-dried product of Example 4 had a stronger aroma than the hot-air-dried product of Comparative Example 4, similar to the results in Test Example 1. In addition, Example 5, which was dried under reduced pressure from a frozen state, had an even stronger aroma intensity. The difference with hot-air drying was further exacerbated when compression molding was performed. Comparative Example 5, in which the compression-molded product was dried under hot air, experienced a greater loss of aroma components than Comparative Example 4, which was dried under hot air as is. However, Examples 6 and 7, which were dried under reduced pressure, maintained a high level of aroma intensity. Compression molding and drying offers advantages in terms of shape (compact, no need to measure, etc.), but hot-air drying requires a long drying time to remove internal moisture, which is thought to be why deterioration occurred during that time. In this regard, vacuum drying, which minimizes component deterioration and loss of aroma components during drying, was effective. Furthermore, Examples 5 and 7, which were dried under reduced pressure in a frozen state, each had a stronger floral aroma than Examples 4 and 6, which were dried under reduced pressure from an unfrozen state, giving a more glamorous impression.

[0038] [Test Example 3] The tea leaves of Examples 5 to 7 prepared in [Test Example 2] were packed into aluminum bags and stored in a refrigerator at 4°C or in an incubator at 37°C for 48 hours. Sensory evaluation was performed on these bags in the same manner as in Test Example 2 to confirm the influence of changes over time. Evaluation was performed using Example 5 under each storage condition as the standard (4 points for each item). The results are shown in Table 5.

[0039] [Table 5]

[0040] The sensory evaluation results shown in Table 5 show that the compression-molded Examples 6 and 7 were evaluated better after heated storage than the non-compression-molded Example 5. This result means that the fragrance of Example 5 was reduced by heating, and it was confirmed that the loss and deterioration of fragrance components during storage was relatively suppressed in the compression-molded product compared to the non-compression-molded product, and that the compression-molded product had excellent storage stability.

Claims

1. This method for producing fermented tea comprises drying tea leaves that have been subjected to at least either a withering step or a fermentation step in a frozen state under a reduced pressure of 610 Pa or less, thereby sublimating the moisture in the tea leaves, suppressing the volatilization of aroma components in the tea leaves, and reducing the moisture content in the tea leaves to 10% or less.

2. 2. The method for producing fermented tea according to claim 1, wherein the tea leaves are compressed and molded when dried.

3. This method for suppressing loss of aroma components in fermented tea, in the production of fermented tea, comprises drying tea leaves that have been subjected to at least a withering step or a fermentation step in a frozen state under a reduced pressure of 610 Pa or less, thereby sublimating the moisture in the tea leaves, suppressing volatilization of aroma components in the tea leaves, and reducing the moisture content in the tea leaves to 10% or less.

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