Method for producing frozen cooked rice

Abstract

The present invention addresses the problem of providing a method which is for producing frozen cooked rice and by which frozen cooked rice having favorable food texture, appearance, and loosening properties after thawing can be produced. The present invention pertains to a method for producing frozen cooked rice, the method comprising: an immersion step for immersing rice serving as a raw material in water at 10-35ºC and causing the rice to absorb the water; a first heating and pressurizing step for steaming the rice after the immersion step with water vapor under a pressure of 130-200 kPa; a liquid contact step for bringing the rice after the first heating and pressurizing step into contact with a liquid having a liquid temperature of at most 60ºC and containing water and edible oil and fat that is liquid at room temperature; a second heating and pressurizing step for steaming the rice after the liquid contact step with water vapor under a pressure of 200-300 kPa; a water absorption step for causing the rice in the second heating and pressurizing step to absorb water; and a freezing step for freezing the rice after the water absorption step.

Description

Frozen rice manufacturing method

[0001] The present invention relates to a method for producing frozen cooked rice.

[0002] In recent years, with the diversification of eating scenes, the trend toward individual meals, and the growing trend toward convenience, there has been an increasing demand for cooked rice products distributed in a frozen and / or chilled state as merchandise for the food service industry, or as merchandise for the ready-meal industry via supermarkets, convenience stores, department store basements, etc., or delivery, etc., and accordingly, the distribution forms of cooked rice products have been diversifying. One of the distribution forms of cooked rice products that has become widespread is one in which cooked rice products are delivered in a frozen state to retail stores or bento businesses, etc., and then thawed and sold at low temperatures such as chilled temperatures. Consumers who purchase cooked rice products sold at low temperatures in this way eat cooked rice products that have been sequentially frozen, thawed, and chilled.

[0003] It is known that cooked rice undergoes significant aging due to retrogradation of the inherent starch when stored at low temperatures, particularly when frozen, thawed, or chilled. When cooked rice ages, its viscosity decreases and it becomes hard, resulting in a significant deterioration in texture. When cooked rice passes through the chilled temperature range (0 to 10°C), where aging is likely to progress, aging progresses, and the rice is more likely to become less sticky and harden. In particular, when cooked rice is naturally thawed, the cooked rice is left at the chilled temperature range or room temperature, and aging is more likely to occur because the time spent in the maximum ice crystal formation temperature range and the chilled temperature range is long.

[0004] Since staled cooked rice can be restored to a certain degree of stickiness and softness by heating, when cooked rice stored at low temperatures is eaten, deterioration in texture can be prevented to some extent by heating the cooked rice immediately before eating. However, there is a need for cooked rice that has a good texture even when eaten without such pre-heating. To meet this need, suppressing the staling of cooked rice caused by low-temperature storage becomes an even more important issue.

[0005] Regarding the prevention of deterioration of cooked rice stored at low temperatures, Patent Document 1 proposes a method for solving the problem of quality deterioration in frozen cooked rice by combining the use of a gelling agent and the amount of water added for cooking. Furthermore, Patent Documents 2 to 4 propose a method for producing cooked rice that is less likely to undergo starch retrogradation even when stored at room temperature or in a low temperature range of about 5°C, by combining water absorption by rice and heating, but make no mention of frozen storage of cooked rice.

[0006] Patent Document 5 discloses a method for producing cooked rice that does not form lumps but tends to break apart, which includes a step of adding edible oil to washed rice and cooking the rice under pressure.

[0007] Japanese Patent Application Laid-Open No. 2009-118760 Japanese Patent Application Laid-Open No. 2008-000086 Japanese Patent Application Laid-Open No. 2006-174780 Japanese Patent Application Laid-Open No. 2005-218372 Japanese Patent Application Laid-Open No. 2-46261

[0008] Generally, staling of cooked rice can be easily prevented by increasing the moisture content of the rice. On the other hand, heating is necessary to increase the moisture content of rice. However, when cooked rice is heated after adding a large amount of moisture, starch is more likely to be eluted from the rice grains, which makes the rice grains more likely to lose their shape, resulting in poor appearance and loosening properties. For these reasons, it is particularly difficult to achieve both grain shape and prevention of staling under low-temperature storage conditions that are prone to staling. Frozen cooked rice that is resistant to quality deterioration after thawing and has a good appearance, loosening properties, and texture in an edible state has not yet been provided.

[0009] An object of the present invention is to provide frozen cooked rice that is less likely to deteriorate in quality after thawing.

[0010] As a result of extensive research, the present inventors have found that frozen cooked rice that is less likely to deteriorate in quality can be obtained by soaking rice in water, then subjecting it to two heating and pressurizing steps using steam, and bringing a specific liquid containing edible oil and water into contact with the rice between the first and second heating and pressurizing steps. The technology described in Patent Document 5 adds edible oil to rice at the rice cooking stage, but does not add edible oil to rice after cooking.

[0011] The present invention is based on this finding and provides a method for producing frozen cooked rice, which includes a soaking step in which raw material rice is soaked in water at 10 to 35°C to absorb water, a first heating and pressurizing step in which the rice after the soaking step is steamed with steam under a pressure of 130 to 200 kPa, a liquid contacting step in which the rice after the first heating and pressurizing step is brought into contact with a liquid containing edible oils and fats that are liquid at room temperature and water, and having a liquid temperature of 60°C or less, a second heating and pressurizing step in which the rice after the liquid contacting step is steamed with steam under a pressure of 200 to 300 kPa, a water absorption step in which water is absorbed by the rice in the second heating and pressurizing step, and a freezing step in which the rice after the water absorption step is frozen.

[0012] The present invention will be described below based on preferred embodiments thereof.

[0013] The present invention relates to a method for producing frozen cooked rice. The method includes a soaking step, a first heating and pressurizing step, a liquid contacting step, a second heating and pressurizing step, a water absorption step, and a freezing step. Each step will be described below in order.

[0014] <Soaking Step> In the soaking step, raw rice (hereinafter referred to as "raw rice") is soaked in water at 10 to 35°C to allow the rice grains of the raw rice to absorb water. The raw rice may be non-glutinous rice, glutinous rice, or a combination of these. In the present invention, as described below, the rice is heated and pressurized twice to promote gelatinization of the rice grains. However, performing pressurized heating twice may cause the rice to become too soft. Therefore, to prevent such problems, the raw rice is preferably non-glutinous rice, and more preferably consists of only non-glutinous rice. In this specification, when the raw rice is non-glutinous rice, this includes cases where the proportion of non-glutinous rice in the raw rice is more than 50% by mass, more preferably 60% by mass or more. When the raw rice is glutinous rice, this includes cases where the proportion of glutinous rice in the raw rice is more than 50% by mass, more preferably 60% by mass or more.

[0015] When the raw rice is non-glutinous rice, it is preferable that the non-glutinous rice be low-amylose rice. This is because cooked rice is less likely to age. Low-amylose rice refers to rice with a low amylose content, i.e., rice with an amylose content of less than 17% by mass. Specific examples of rice that fall within the category of low-amylose rice in the present invention include many varieties such as "Hanabusa," "Irodori," "Takitate," "Silky Pearl," "Snow Pearl," "Milky Queen," "Hokuriku 180," "Soft 158," "Yuwa Komachi," "Asa Tsuyu," "Nebari Kachi," "Iwata 15," "Ou 344," "Nishikai 215," "Milky Summer," and "Yumepirika." In the present invention, these varieties may be used alone or in combination.

[0016] In the present invention, it is preferable that the raw material rice is raw rice. "Raw rice" includes, for example, rice that has not been heated at 60°C or higher for 30 seconds or longer. There are no particular restrictions on the degree of polishing of the raw rice, and it may be brown rice, partially polished rice, or polished rice.

[0017] In the soaking step, it is preferable to soak the raw material rice in water so that the moisture content of the rice after the soaking treatment is 36 to 45% by mass, particularly 37 to 42% by mass. This makes it easier to gelatinize the rice to the center during the first heating and pressurizing step described below, and to obtain frozen cooked rice that is less likely to deteriorate in quality after thawing; specifically, frozen cooked rice that remains edible and has a sticky texture and inhibited hardening even after frozen storage, thawing, and chilled storage. When the raw material rice is non-glutinous rice, from the same viewpoint, it is preferable to soak the raw material rice in water so that the moisture content of the rice after the soaking treatment is 36 to 45% by mass, particularly 37 to 42% by mass. When the raw material rice is glutinous rice, from the same viewpoint, it is preferable to soak the raw material rice in water so that the moisture content of the rice after the soaking treatment is 39 to 45% by mass, particularly 40 to 42% by mass.

[0018] <Method for Measuring the Moisture Content of Rice> In this specification, the "moisture content of rice" is measured by the following method. The term "rice" as used herein includes cooked rice obtained by cooking raw rice. Approximately 3 g of the rice to be measured is weighed into a weighing container, and the weighing container is heated at 135°C for 3 hours to dry the rice in the weighing container. The weighing container can be, for example, a cylindrical metal container (weighing can) with an opening at one end in the depth direction, an inner diameter of the opening of 55 mm, an inner diameter of the bottom of 50 mm, and a depth of 25 mm. The weight of the weighing container after drying (M1) is determined by measuring the weight of the weighing container and subtracting the constant mass of the weighing container from the measured value. The moisture content of the rice is calculated using the thus determined M1 and the previously determined weight of the rice before drying (M0) according to the following formula: Moisture content of rice (unit: mass %) = {(M0 - M1) / M0} × 100

[0019] The temperature of the water in which the raw rice is soaked in the soaking step (hereinafter also referred to as the "soaking temperature") is 10 to 35°C, preferably 15 to 30°C. The soaking temperature is closely related to the rate at which water penetrates the rice, and therefore to the moisture content of the rice after the soaking treatment. By adjusting the soaking temperature within the above range, the moisture content of the rice after the soaking treatment can be easily adjusted to the above preferred range, making it easier to obtain frozen cooked rice that is less likely to lose quality after thawing. If the soaking temperature is less than 15°C, the rate at which water penetrates the rice will decrease, resulting in an insufficient moisture content in the rice after the soaking treatment, and the desired effects of the present invention may not be achieved. If the soaking temperature is more than 35°C, there is a risk of a deterioration in flavor or texture.

[0020] From the viewpoint of ensuring that the desired effects of the present invention are achieved more reliably, the time for which the raw material rice is soaked in water at 10 to 35°C in the soaking step (hereinafter also referred to as "soaking time") is preferably 10 to 300 minutes, more preferably 30 to 180 minutes.

[0021] <First heating and pressurizing step> The first heating and pressurizing step and the second heating and pressurizing step described below are steps adopted to gelatinize not only the surface portions of the rice grains but also the central portions of the rice grains in the frozen cooked rice product. One of the findings behind adopting these steps is that gelatinization of rice grains is more easily promoted by performing the heating and pressurizing step twice as in the present invention, rather than performing the heating and pressurizing step once on rice after the soaking step.

[0022] In the first heating and pressurizing step, the rice after the soaking step is steamed with steam under a pressure of 130 to 200 kPa. The first heating and pressurizing step is a steaming step. "Steaming" here refers to a method of using steam as a heating medium and heating the rice by bringing the steam into direct contact with the rice. In addition to "steaming," a known rice heating method is a method of using water as a heating medium and heating rice while immersed in water (hereinafter referred to as the "water boiling method"). Generally, rice obtained by the water boiling method is characterized by being soft throughout and having a sticky surface, and the surface is particularly sticky when the rice has a relatively high moisture content. On the other hand, the steaming step can soften the interior of the rice while suppressing surface stickiness to some extent. From the viewpoint of making the most of the characteristics of the steaming step, it is preferable to remove water adhering to the surface of the rice after the soaking step by leaving the rice on a sieve or by shaking the sieve (so-called draining) before subjecting the rice to the first heating and pressurizing step.

[0023] The type of steam used in the first heating and pressurizing step is not particularly limited, and examples thereof include saturated steam and superheated steam. From the viewpoint of improving the texture of cooked rice, saturated steam is preferred. The same applies to the steam used in the second heating and pressurizing step described below.

[0024] As mentioned above, in the first heating and pressurizing step, the rice is not only steamed with steam but also pressurized to facilitate gelatinization to the center of the rice grains. In contrast, Patent Documents 2 and 3 state that it is desirable not to gelatinize the rice grains in the heating step after soaking (Claim 1 of Patent Document 2, Paragraph

[0023] of Patent Document 3). These descriptions discourage the combined use of heating and pressurizing of rice, as in the first heating and pressurizing step. It is important that the pressure conditions for the first heating and pressurizing step be 130 to 200 kPa, preferably 140 to 190 kPa. If the pressure is less than 130 kPa, gelatinization will not progress to the center of the rice grains, and the resulting frozen cooked rice may have a hard texture with a core remaining when ready to eat. Furthermore, if the pressure exceeds 200 kPa, the rice grains may lose their distinct texture and become too soft. In this specification, "pressure" refers to absolute pressure unless otherwise specified.

[0025] The temperature of the steam in the first heating and pressurizing step is preferably 101°C or higher but lower than 125°C, more preferably 101°C or higher but lower than 123°C, even more preferably 101°C or higher but lower than 120°C, and particularly preferably 105°C or higher but lower than 115°C. A steam temperature of 101°C or higher facilitates gelatinization to the center of the rice grains, which helps prevent the rice from hardening even after the resulting frozen cooked rice has been frozen, thawed, and chilled. Furthermore, if the steam temperature is too high, there is a risk of damaging the shape of the rice and overheating the rice. However, by setting the steam temperature to less than 125°C, these problems are prevented and the rice tends to have good grain structure. From the same perspective, the first heating and pressurizing time is preferably 1 to 180 minutes, more preferably 5 to 120 minutes, and even more preferably 10 to 60 minutes. Here, the "first heating and pressurizing time" refers to the time during which the first heating and pressurizing step is carried out, and more specifically, refers to the time during which rice is steamed with steam under a pressure of 130 to 200 kPa.

[0026] In the first heating and pressurizing step, from the viewpoints of graininess of the rice, appropriate stickiness, prevention of dryness and stickiness, and hardness control, it is preferable to heat and pressurize the rice so that the moisture content of the rice after the first heating and pressurizing step is 38 to 50% by mass, particularly 43 to 48% by mass. When the rice is non-glutinous rice, it is preferable to carry out the first heating and pressurizing step so that the moisture content of the rice after the first heating and pressurizing step is 38 to 49% by mass, particularly 43 to 48% by mass. When the rice is glutinous rice, it is preferable to carry out the first heating and pressurizing step so that the moisture content of the rice after the first heating and pressurizing step is 40 to 50% by mass, particularly 43 to 48% by mass. The moisture content of the rice after the first heating and pressurizing step can be adjusted to the preferred ranges described above by adjusting the pressurizing conditions, steam temperature, and first heating and pressurizing time in the first heating and pressurizing step, each within the preferred ranges described above.

[0027] <Liquid Contacting Step> In the liquid contacting step, the rice after the first heating and pressurizing step is brought into contact with a liquid having a temperature of 60°C or less (hereinafter also referred to as "specific liquid"). The specific liquid includes edible oils and fats that are liquid at room temperature and water. In this specification, "room temperature" refers to 25°C.

[0028] In the second heating and pressurizing step described below, it is preferable to steam the rice under a higher pressure than in the first heating and pressurizing step in order to further promote gelatinization of the rice grains. However, steaming rice under such high pressure may cause starch to leach out of the rice, resulting in a deterioration in the texture and appearance of the rice. It is presumed that when rice is brought into contact with the specific liquid, the water in the specific liquid is absorbed into the rice, and the oils and fats in the specific liquid adhere to the surface of the rice. By subjecting the rice in this state after the first heating and pressurizing step to the subsequent second heating and pressurizing step, the water absorbed into the rice promotes gelatinization of the rice grains, while the oils and fats adhered to the surface of the rice prevent starch from leaching from the rice, presumably achieving the desired effects of the present invention. In other words, one of the main roles of the edible oils and fats contained in the specific liquid is to protect the rice to be subjected to the second heating and pressurizing step.

[0029] Examples of edible oils and fats that are liquid at room temperature include salad oil, soybean oil, rapeseed oil, corn oil, sesame oil, refined rapeseed oil, refined soybean oil, safflower oil, olive oil, cottonseed oil, rice oil, rice bran oil, palm oil, palm kernel oil, coconut oil, sunflower oil, safflower oil, coconut oil, peanut oil, grapeseed oil, and medium-chain triglycerides (MCTs). In the present invention, these edible oils and fats can be used alone or in combination of two or more. During storage of frozen cooked rice and when eaten at room temperature, the oils and fats may solidify into a white solid, which may impair the appearance of the cooked rice. To prevent such inconveniences, it is preferable to use edible oils and fats that are liquid at room temperature, such as those that are liquid at a product temperature of 10°C or higher.

[0030] The content of the edible oil or fat in the specific liquid is preferably 0.1 to 30.0% by mass, more preferably 0.5 to 20.0% by mass, and even more preferably 1.0 to 10.0% by mass, relative to the total mass of the specific liquid, from the viewpoint of preventing a deterioration in the texture of the rice when eaten and from the viewpoint of preventing the flavor of the edible oil or fat from being too strong.

[0031] The specific liquid may be emulsified or not. When the specific liquid is emulsified, the emulsion form may be, for example, an oil-in-water emulsion or a water-in-oil-in-water emulsion in which the aqueous phase is the continuous phase. In the present invention, it is usually preferable that the specific liquid is not emulsified. If the specific liquid is emulsified, the effect of the water in the specific liquid being absorbed by the rice may be reduced, and the rice may not have a glossy appearance.

[0032] The temperature of the specific liquid (the temperature immediately before contacting the rice) is 60°C or lower. If the temperature of the liquid that is brought into contact with the rice exceeds 60°C, the rice may expand upon contact with the liquid, causing the grain shape to collapse. There are no particular restrictions on the lower limit of the temperature of the specific liquid, but from the viewpoint of making it easier to adjust the moisture content of the rice after the liquid contact step to the range described below, a temperature of 5°C or higher is preferred. Taking the above into consideration, the temperature of the specific liquid is preferably 5 to 60°C, more preferably 10 to 50°C.

[0033] The method of contacting the rice with the specific liquid in the liquid contacting step is not particularly limited, and examples thereof include a method of soaking the rice in a large amount of the specific liquid (hereinafter also referred to as the "soaking method") and a method of applying the specific liquid to the rice by spraying, pouring, coating, or the like (hereinafter also referred to as the "application method"). The soaking method is typical.

[0034] From the viewpoint of improving the texture and appearance of cooked rice, the contact of the rice with the specific liquid in the liquid contacting step is preferably adjusted so that the moisture content of the rice after the liquid contacting step is 45.0 to 65.0% by mass, particularly 50.0 to 64.0% by mass. When the rice is glutinous rice, the moisture content of the rice after the liquid contacting step is preferably adjusted so that it is 45.5 to 65.5% by mass, particularly 50.0 to 65.0% by mass.

[0035] The moisture content of the rice after the liquid contact step can be controlled by the amount of specific liquid that the rice comes into contact with and / or the contact time between the rice and the specific liquid. In the liquid contact step, the amount of specific liquid that the rice comes into contact with is preferably 20 to 100 parts by mass, more preferably 40 to 60 parts by mass, per 100 parts by mass of rice after the first heating and pressurizing step, from the viewpoint of adjusting the moisture content of the rice after the liquid contact step to within the above-mentioned preferred range. Here, the "contact amount of specific liquid" refers to the amount of specific liquid used for soaking in the soaking method (when the soaking method is performed by placing rice and the specific liquid in a container, the amount of specific liquid contained in the container). In the application method, it refers to the amount of specific liquid sprayed, poured, coated, or the like onto the rice. From the same viewpoint, the contact time between the rice and the specific liquid in the liquid contact step is preferably 1 to 600 seconds, more preferably 10 to 300 seconds.

[0036] The specific liquid may contain components other than the edible oils and fats and water. Examples of the other components include salts such as salt; monosaccharides such as glucose, disaccharides such as sucrose and trehalose, trisaccharide or higher oligosaccharides, and sugar alcohols; sweeteners; seasonings such as vinegar, soy sauce, miso, dashi, consommé, sodium glutamate, ketchup, curry powder, and saffron; antioxidants such as vitamin E; emulsifiers, coloring agents, flavorings, preservatives, and shelf-life extenders. These may be used alone or in combination of two or more.

[0037] In the present invention, it is preferable to carry out the first heating and pressurizing step and the liquid contacting step continuously. Here, "continuously carried out" means that the difference in temperature between the rice immediately after the first heating and pressurizing step and the rice immediately before the liquid contacting step is small, specifically, for example, that such temperature difference is within ±5°C. For example, if the temperature of the rice immediately after the first heating and pressurizing step is 60°C, in order to carry out the first heating and pressurizing step and the liquid contacting step continuously, it is preferable to bring the specific liquid into contact with rice at a product temperature of 55 to 65°C in the liquid contacting step.

[0038] <Second heating and pressurizing step> In the second heating and pressurizing step, the rice after the liquid contacting step is steamed with steam under a pressure of 200 to 300 kPa. The second heating and pressurizing step is a steaming step similar to the first heating and pressurizing step. The rice after the second heating and pressurizing step is cooked rice and can be eaten as is. From the viewpoint of making the most of the characteristics of the steaming step described above, it is preferable to remove the specific liquid adhering to the surface of the rice by leaving the rice after the liquid contacting step on a sieve or by shaking the sieve, etc., before subjecting the rice to the second heating and pressurizing step.

[0039] In the second heating and pressurizing step, in order to further promote gelatinization of the rice grains that have undergone the first heating and pressurizing step, the rice is not only steamed with steam but also pressurized, as in the first heating and pressurizing step. It is important that the pressure conditions in the second heating and pressurizing step be 200 to 300 kPa. If the pressure is less than 200 kPa, the rice may become hard and have a strong stale feel. Furthermore, if the pressure exceeds 300 kPa, the rice may become too soft and lose its shape. In either case, this can lead to a deterioration in the texture when eaten and a deterioration in the appearance of the rice. From the perspective of more reliably achieving the desired effects of the present invention, it is preferable that the second heating and pressurizing step be carried out at a higher pressure than the first heating and pressurizing step. Specifically, it is preferable that the second heating and pressurizing step be carried out at a pressure of 200 to 270 kPa.

[0040] The temperature of the steam in the second heating and pressurizing step is preferably 101°C or higher but lower than 133°C, more preferably 101°C or higher but lower than 132°C, even more preferably 101°C or higher but lower than 131°C, and particularly preferably 105°C or higher but lower than 130°C. The reasons for this are the same as those for the temperature of the steam in the first heating and pressurizing step. From the same perspective, the second heating and pressurizing time is preferably 1 to 180 minutes, more preferably 5 to 120 minutes, and even more preferably 10 to 60 minutes. Here, the "second heating and pressurizing time" refers to the time during which the second heating and pressurizing step is carried out, and specifically refers to the time during which rice is steamed with steam under a pressure of 200 to 300 kPa.

[0041] In the second heating and pressurizing step, from the viewpoints of graininess of the rice, appropriate stickiness, prevention of dryness and stickiness, and hardness control, it is preferable to carry out the heating and pressurizing step so that the moisture content of the rice after the second heating and pressurizing step is 45.5 to 70.0% by mass, particularly 50.0 to 69.0% by mass. When the rice is glutinous rice, it is preferable to carry out the second heating and pressurizing step so that the moisture content of the rice after the second heating and pressurizing step is 46.0 to 70.0% by mass, particularly 50.0 to 69.0% by mass. By adjusting the pressurizing conditions, steam temperature, and second heating and pressurizing time in the second heating and pressurizing step within the preferred ranges described above, it is possible to adjust the moisture content of the rice (cooked rice) after the second heating and pressurizing step to within the preferred ranges described above.

[0042] <Water Absorption Process> In the water absorption process, water is absorbed into the rice (cooked rice) after the second heating and pressurizing process. One of its main purposes is to improve the appearance of the rice, as explained below. The frozen cooked rice, which is the product of the present invention, has an ideal moisture content, although this varies depending on the type and consumer preferences. If the water absorption process is not performed on rice after the second heating and pressurizing process, it is necessary to have the rice absorb a large amount of water in a process prior to the second heating and pressurizing process in order to achieve an optimal moisture content. However, if the second heating and pressurizing process is performed on rice with such a high moisture content, the shape of the rice after the second heating and pressurizing process may be distorted, and the rice may appear clumped rather than individual grains, which may impair the appearance of the cooked rice. In the present invention, the water absorption process after the second heating and pressurizing process allows the rice to absorb water, eliminating the need to have the rice absorb a large amount of moisture in a process prior to the second heating and pressurizing process. As a result, the cooked rice produced has an excellent appearance.

[0043] The water absorption step can be carried out by contacting the rice after the second heating and pressurizing step with a liquid containing water. Hereinafter, this liquid will also be referred to as the "water absorption liquid." The water absorption liquid may contain other components as long as they do not deviate from the spirit of the present invention, provided that it contains water.

[0044] The method for bringing rice into contact with the water absorption liquid is not particularly limited, and examples thereof include a method in which rice is soaked in a large amount of water absorption liquid (soaking method) and a method in which the water absorption liquid is applied to rice by spraying, pouring, coating, etc. (application method). The soaking method is typical.

[0045] Below, we will explain the case where water is used as the water absorption liquid. When water is used as the water absorption liquid, it is preferable to have the rice after the second heating and pressurizing step absorb water so that the moisture content of the rice after the water absorption step is 65 to 76% by mass, particularly 69 to 75% by mass. By setting the moisture content of the rice after the water absorption step within this range, it is possible to increase the resistance to aging of the cooked rice when the frozen cooked rice, which is the production result, is thawed after frozen storage, or even when stored in a chilled state, and it is also preferable because it can prevent the cooked rice from becoming sticky and undesirable in texture. When the raw material rice is glutinous rice, it is preferable to have the rice after the second heating and pressurizing step absorb water so that the moisture content of the rice after the water absorption step is 66 to 76% by mass, particularly 70 to 75% by mass.

[0046] The temperature of the water used as the water absorption liquid is preferably 5 to 60°C, and more preferably 10 to 35°C. By setting the temperature of the water to be absorbed by the rice within this range, it is possible to prevent the rice from expanding and losing its grain shape during the water absorption process. From the same perspective, the temperature of the rice that is brought into contact with the water used as the water absorption liquid is preferably 50 to 90°C, and more preferably 60 to 80°C.

[0047] The amount of water used as the water absorption liquid is preferably 10 to 300 parts by mass, more preferably 20 to 200 parts by mass, per 100 parts by mass of rice after the second heating and pressurizing step, from the viewpoint of reliably adjusting the moisture content of the rice after the water absorption step to within the above-mentioned preferred range. The term "amount of water absorption liquid used" as used herein refers to the amount of water absorption liquid used for soaking in the soaking method (when the soaking method is carried out by placing rice and water absorption liquid in a container, the amount of water absorption liquid contained in the container), and in the application method, refers to the amount of water absorption liquid sprayed, poured, coated, etc. on the rice. From the same viewpoint, the contact time between the rice and water as the water absorption liquid in the water absorption step is preferably 1 to 600 seconds, more preferably 30 to 300 seconds.

[0048] In the water absorption process, a seasoning liquid can be used instead of water as the water absorption liquid. This not only achieves the same effects as when water is used as the water absorption liquid (such as improving the appearance of the rice), but also makes it possible to season the frozen cooked rice, which is the resulting product. Furthermore, by making it possible to season the frozen cooked rice, further improvements in the taste, texture, and appearance of the cooked rice can be expected. By using a seasoning liquid as the water absorption liquid, the type of frozen cooked rice can be made into, for example, takikomi gohan (seasoned rice), okowa (sticky rice), chimaki (rice dumplings), ohagi (rice balls), sekihan (red rice), dry curry, and risotto. Regarding the water absorption process using a seasoning liquid as the water absorption liquid, the following mainly describes the configurations that differ from the water absorption process using water as the water absorption liquid. For configurations that are not specifically described for the former, the above-mentioned description of the latter applies as appropriate.

[0049] The seasoning liquid, which is a liquid for water absorption, contains at least water and additives. The additives can be components conventionally used for seasoning purposes, such as salts (e.g., salt); monosaccharides (e.g., glucose); disaccharides (e.g., sucrose and trehalose); trisaccharide or higher oligosaccharides; and sugar alcohols; sweeteners; seasonings (e.g., vinegar, soy sauce, miso, dashi, consommé, sodium glutamate, ketchup, curry powder, saffron); enzymes (e.g., amylase, transglutaminase); oils (e.g., salad oil, soybean oil, rapeseed oil, corn oil, sesame oil, butter); antioxidants (e.g., vitamin E); colorants, flavorings, preservatives, shelf-life enhancers, etc. One of these additives may be used alone, or two or more may be used in combination. The content of the additives in the seasoning liquid may be adjusted as needed to achieve the desired seasoning. However, from the viewpoint of reliably achieving the objective of having rice absorb water, it is preferable that the content be 80% by mass or less of the total mass of the seasoning liquid.

[0050] From the viewpoint of ensuring the effects of using the seasoning liquid, the temperature of the seasoning liquid, which is the water absorption liquid, is preferably 5 to 59°C, and more preferably 10 to 50°C. From the same viewpoint, the temperature of the rice brought into contact with the seasoning liquid, which is the water absorption liquid, is preferably 60 to 95°C, and more preferably 70 to 90°C.

[0051] <Freezing Step> In the freezing step, the rice (cooked rice) after the second heating and pressurizing step is frozen. The method for freezing rice is not particularly limited, and any method known in the art can be used without limitation. Specific examples of freezing methods include a mechanical freezing device and a freezing device using dry ice, etc. The freezing step is preferably rapid freezing. Rapid freezing refers to freezing in which the time it takes for the rice to pass through the maximum ice crystal formation temperature range of -1°C to -5°C (T -1°C to -5°C) is 30 minutes or less. Packaging and freezing the rice after the second heating and pressurizing step is preferable because this avoids thawing and the resulting aging that occurs during the packaging process, compared to packaging after freezing.

[0052] <Cooling step> The method for producing frozen cooked rice of the present invention may include a cooling step of cooling the rice (cooked rice) after the second heating and pressurizing step, after the water absorption step and before the freezing step. The cooling method for the cooked rice in the cooling step is not particularly limited, and any known method can be used without particular limitation. Specific examples of cooling methods include, for example, allowing the cooked rice to cool by loosening it with a rice scoop, differential pressure cooling, vacuum cooling, or a combination thereof.

[0053] The frozen cooked rice produced by the production method of the present invention is preferably for low-temperature storage. "Cooked rice for low-temperature storage" refers to cooked rice that is distributed and sold in a low-temperature stored state, does not require reheating to make it edible, and can be eaten as is in the low-temperature stored state. "Low-temperature storage" refers to storage at a temperature range of 10°C or below, includes chilled storage and frozen storage, and in the case of frozen products, also includes the processes of freezing and thawing. "Chilled storage" refers to storage in a chilled temperature range, and "frozen storage" refers to storage in a freezing temperature range.

[0054] Examples of cooked rice for low-temperature storage include frozen cooked rice stored in a freezing temperature range and chilled cooked rice stored in a chilled temperature range, with frozen cooked rice being preferred. Frozen cooked rice includes frozen chilled cooked rice that is stored in a freezing temperature range and then stored in a chilled temperature range. The freezing temperature range refers to a temperature range below the freezing onset temperature, for example, −18°C or below. The chilled temperature range refers to a temperature range below 10°C up to the onset of freezing. Cooked rice for low-temperature storage can be eaten as is by natural thawing of frozen cooked rice, i.e., thawing at room temperature or a chilled temperature range, and chilled cooked rice that has been chilled from production to distribution and end consumption can be eaten as is. Here, “natural thawing” refers to a method of thawing a frozen product by leaving it in a temperature environment that allows thawing, such as room temperature or a refrigerator, without performing a heating operation such as microwave heating.

[0055] The present invention is not limited to white rice, but can be applied to various cooked rice dishes such as takikomi gohan (seasoned rice), okowa (steamed rice), chimaki (rice dumplings), ohagi (rice balls), zosui (rice porridge), sekihan (red rice), dry curry, and risotto. In these cases, the seasoning ingredients required for seasoning can be added appropriately in the liquid contact step, as described above. In addition to these steps, a seasoning step may be added. Furthermore, a rice washing step may or may not be added as the first step, and pre-washed rice may be used.

[0056] Frozen cooked rice produced by the manufacturing method of the present invention is less likely to deteriorate in quality after thawing. In other words, frozen cooked rice produced by the manufacturing method of the present invention has a higher anti-aging effect than frozen cooked rice produced by other manufacturing methods. A specific example of "less likely to deteriorate in quality after thawing" is when the rice grains have both a good shape and a good texture, and the rice is easy to separate. The "good shape of the rice grains" refers to cooked rice grains that are less crushed, chipped, or distorted, and have a uniform shape, i.e., have good "grain structure." The "good texture" refers to rice grains that have appropriate stickiness and hardness. The "ease of separation" refers to the ease with which rice grains are separated, and "good ease of separation" refers to adjacent rice grains not clumping together to form clumps.

[0057] The present invention encompasses frozen cooked rice produced by the above-described manufacturing method of the present invention. Identifying the frozen cooked rice of the present invention as a product that can be distinguished from cooked rice produced by conventional manufacturing methods would take an extremely long time, and there are circumstances in which this is substantially impossible or impractical under the requirement for early filing under the first-to-file system. For this reason, the present invention defines frozen cooked rice as a product based on the manufacturing method. The frozen cooked rice of the present invention is preferably cooked rice for low-temperature storage as described above.

[0058] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass".

[0059] [Production of Frozen Cooked Rice] Frozen cooked rice of the Examples and Comparative Examples was produced according to the following procedure. The production conditions are shown in Tables 1 and 2.

[0060] (Example 1) (1) Soaking process: Polished rice (variety: Milky Queen, amylose content: 9-12%) was used as raw rice. 100 parts of non-glutinous rice was soaked in 300 parts of water at room temperature (25°C) for 2 hours. The rice was then placed on a sieve to drain. The moisture content of the rice after the soaking process was 39%.

[0061] (2) First heating step: The rice after the soaking step was placed in a heating and pressurizing device and pressurized and heated with saturated steam at 115°C under a pressure of 170 kPa for 15 minutes. The moisture content of the rice after the first heating step was 45%.

[0062] (3) Liquid contacting step After the first heating step, 100 parts of rice at 80°C was brought into contact with 51 parts of liquid at 25°C. The contacting was carried out by placing the rice and liquid in a bowl and gently stirring with a spatula. The first heating and pressurizing step and the liquid contacting step were carried out continuously. The moisture content of the rice after the liquid contacting step was 63%. A water / oil liquid with the following composition was used as the liquid. The water / oil liquid was not emulsified. <Composition of water / oil liquid (total of the following components is 100%)> Edible oil (manufactured by J-Oil Mills Co., Ltd., product name "Super Canola Oil"): 3% Water: 97%

[0063] (4) Second heating step: 100 parts of the rice after the liquid contact step was placed in a heating and pressurizing device, and the rice was pressurized and heated with saturated steam at 127°C under a pressure of 250 kPa for 10 minutes. The moisture content of the rice after the second heating step was 66%.

[0064] (5) Water absorption step: 22 parts of water at 25°C was added to 100 parts of the 80°C rice after the second heating step, and the rice was thoroughly mixed for 1 minute to absorb water. The moisture content of the rice after the water absorption step was 71%.

[0065] (6) Cooling step and freezing step The rice immediately after the water absorption step was cooled to 20°C in a vacuum cooler (Miura Kogyosho), then placed in a container, sealed and packaged, and frozen with cold air at -40°C in a quick freezer (Hoshizaki Co., Ltd.) to obtain the frozen cooked rice of Example 1.

[0066] Example 2 Example 2 is an example in which the following seasoning liquid was used in the water absorption step. Specifically, the soaking step, first heating step, liquid contact step, and second heating step were carried out in the same manner as in Example 1. In the water absorption step, 28 parts of seasoning liquid at 25°C were added to 100 parts of rice at 75°C after the second heating step, and the rice was allowed to absorb water by mixing well for 1 minute. The moisture content of the rice after the water absorption step was 71%. The rice immediately after the water absorption step was subjected to the cooling step and freezing step in the same manner as in Example 1, to obtain frozen cooked rice. The composition of the seasoning liquid was as follows (the total of the following components is 100%): Rice vinegar (manufactured by Mizkan Co., Ltd., product name "Rice Vinegar"): 29% Salt (manufactured by the Salt Business Center, public interest incorporated association, product name "Table Salt"): 4% Sugar (manufactured by DM Mitsui Sugar Co., Ltd., product name "Super White Sugar"): 14% Water: 53%

[0067] Comparative Example 1 Frozen cooked rice was obtained in the same manner as in Example 1, except that the liquid in the liquid contacting step in Example 1 was changed to water.

[0068] (Comparative Example 2) Frozen cooked rice was obtained in the same manner as in Example 1, except that the liquid in the liquid contacting step in Example 1 was changed to water, the pressure in the second heating step was changed to 170 kPa, and the temperature of the saturated steam in the second heating step was changed to 115°C.

[0069] (Comparative Example 3) Frozen cooked rice was obtained in the same manner as in Example 1, except that the pressure in the first heating step in Example 1 was changed to 250 kPa, the temperature of the saturated steam in the first heating step was changed to 127°C, the liquid in the liquid contacting step was changed to water, the pressure in the second heating step was changed to 170 kPa, and the temperature of the saturated steam in the second heating step was changed to 115°C.

[0070] (Comparative Example 4) Frozen cooked rice was obtained in the same manner as in Example 1, except that the pressure in the first heating step in Example 1 was changed to 250 kPa, the temperature of the saturated steam in the first heating step was changed to 127°C, and the liquid in the liquid contact step was changed to water.

[0071] (Comparative Example 5) Frozen cooked rice was obtained in the same manner as in Example 1, except that the pressure in the first heating step in Example 1 was changed to normal pressure, the temperature of the saturated steam in the first heating step was changed to 99°C, the liquid in the liquid contact step was changed to water, the pressure in the second heating step was changed to 170 kPa, and the temperature of the saturated steam in the second heating step was changed to 115°C.

[0072] Comparative Example 6 Frozen cooked rice was obtained in the same manner as in Example 1, except that the water in the soaking step in Example 1 was changed to the water / oil liquid described above, and the liquid in the liquid contacting step was changed to water.

[0073] (Comparative Example 7) Frozen cooked rice was obtained in the same manner as in Example 1, except that the liquid in the liquid contacting step in Example 1 was changed to water, and the water in the water absorption step was changed to the water / oil liquid described above.

[0074] (Comparative Example 8) Frozen cooked rice was obtained in the same manner as in Example 1, except that the liquid in the liquid contacting step in Example 1 was changed to water, the pressure in the second heating step was changed to atmospheric pressure, and the temperature of the saturated steam in the second heating step was changed to 99°C.

[0075] Comparative Example 9 Frozen cooked rice was obtained in the same manner as in Example 1, except that the pressure in the second heating step in Example 1 was changed to 170 kPa and the temperature of the saturated steam in the second heating step was changed to 115°C.

[0076] Comparative Example 10 Frozen cooked rice was obtained in the same manner as in Example 1, except that the pressure in the second heating step in Example 1 was changed to 310 kPa and the temperature of the saturated steam in the second heating step was changed to 134°C.

[0077] (Comparative Example 11) Frozen cooked rice was obtained in the same manner as in Example 1, except that the pressure in the first heating step in Example 1 was changed to normal pressure and the temperature of the saturated steam in the first heating step was changed to 99°C.

[0078] Comparative Example 12 Frozen cooked rice was obtained in the same manner as in Example 1, except that the pressure in the first heating step was changed to 250 kPa and the temperature of the saturated steam in the first heating step was changed to 127°C.

[0079] (Comparative Example 13) This comparative example corresponds to the invention described in Patent Document 4 (JP 2005-218372 A). The (4) second heating step in this comparative example corresponds to the "rice cooking step" described in Patent Document 4. (1) Soaking step: Polished rice (variety: Milky Queen, amylose content: 9-12%) was used as the raw rice. Non-glutinous rice was soaked in water at 25°C for 15 minutes, then placed on a sieve to drain, yielding rice with a moisture content of 33%. (2) First heating step: The rice after the soaking step was subjected to a heat treatment at normal pressure at 100°C for 60 minutes using saturated steam. (3) Liquid contact step: Water (25°C) was added to the rice after the first heating step in an amount that would reduce the moisture content of the rice to 67%, and the rice was allowed to absorb water for 90 minutes, yielding rice with a moisture content of 67%. (4) Second heating step: The rice after the liquid contact step was placed in a container, and the rice was cooked and heat sterilized at the same time by adjusting the execution time, temperature, and pressure of this step so that the F0 value of the cooked rice after this step was 3.1 or more, to obtain packaged cooked rice. The moisture content of the obtained cooked rice was 67%. (5) The cooked rice obtained in the second heating step was cooled and frozen to obtain frozen cooked rice.

[0080] (Comparative Example 14) This comparative example corresponds to the invention described in Patent Document 2 (JP 2008-000086 A). The (4) second heating step in this comparative example corresponds to the "rice cooking step" in Patent Document 2, in which rice is heated in a state where "there is no aqueous phase around the rice." (1) Soaking step: Polished rice (variety: Milky Queen, amylose content: 9-12%) was soaked in water at 5°C for 15 minutes, and then placed on a sieve to drain the water, yielding rice with a moisture content of 25%. (2) First heating step: The rice after the soaking step was subjected to a heat treatment at normal pressure at 100°C for 2 minutes using saturated steam. (3) Liquid contact step: Water (25°C) was added to the rice after the first heating step in an amount that would reduce the moisture content of the rice to 59%, and the rice was allowed to absorb the water for 60 minutes, yielding rice with a moisture content of 59%. (4) Second heating step: The rice after the liquid contact step was placed in a container, and the rice was cooked and heat sterilized at the same time by adjusting the execution time, temperature, and pressure of this step so that the F0 value of the cooked rice after this step was 3.1 or more, thereby obtaining containerized cooked rice. (5) The cooked rice obtained in the second heating step was cooled and frozen to obtain frozen cooked rice.

[0081]

[0082]

[0083] [Evaluation of Frozen Cooked Rice] The frozen cooked rice produced in each Example and Comparative Example was stored frozen (-18°C) for two weeks, then transferred to a refrigerator at 7°C and chilled for 72 hours. It was then left to stand at room temperature (25°C) for 60 minutes. After standing, the samples were eaten by eight expert panelists, who evaluated the appearance (graininess), texture (stickiness and hardness), and ease of disintegration according to the following evaluation criteria. The results are shown in Table 3.

[0084] Appearance (graininess) A: The rice grains are individually shaped like grains, which is good. B: The rice grains look like grains, but some of the outlines are difficult to see, so it's fairly good. C: The rice grains are not individually shaped like grains, and look clumped together, so it's poor.

[0085] Texture (stickiness and hardness) A: Good, with moderate stickiness and hardness. B: Fairly good, with some stickiness and some hardness. C: Hard and not sticky, poor.

[0086] - Breakability A: Moderate breakability, easy to shape and serve, good. B: Somewhat breakable, can be shaped, fairly good. C: Not breakable, rice clumps, poor.

[0087]

[0088] As shown in Table 3, in Examples 1 and 2, 1) the liquid in which the rice was soaked in the soaking step was water at 10 to 35°C, 2) the pressure in the first heating step was 130 to 200 kPa, 3) the liquid in which the rice was contacted in the liquid contacting step was a liquid (water / oil liquid) containing edible oils and fats that are liquid at room temperature and water and having a liquid temperature of 60°C or less, 4) the pressure in the second heating step was 200 to 300 kPa, and 5) a water absorption step was carried out after the second heating step in which the rice absorbed water. Therefore, compared to the comparative examples that did not satisfy 1) to 5), the appearance, texture, and loosening properties of the resulting frozen cooked rice in an edible state after chilled storage were excellent. Comparative Examples 1 to 8 used water in the liquid contacting step and did not satisfy 3), and Comparative Example 6 also used a water / oil liquid in which the rice was soaked in the soaking step and did not satisfy 1). In Comparative Examples 9 and 10, the pressure in the second heating step was outside the range of 200 to 300 kPa, and thus did not satisfy 4). In Comparative Examples 11 and 12, the pressure in the first heating step was outside the range of 130 to 200 kPa, and thus did not satisfy 2). Comparative Example 13 did not satisfy at least 2), 3), and 5), and Comparative Example 14 did not satisfy any of 1) to 5). From the above results, it is clear that the production method of the present invention, which satisfies all of 1) to 5), makes it possible to produce frozen cooked rice that is resistant to quality deterioration after thawing and has good appearance, loosenability, and texture in an edible state.

[0089] According to the present invention, frozen cooked rice is provided which is less likely to deteriorate in quality after thawing and has good appearance, loosenability and texture in an edible state.

Claims

1. A method for producing frozen cooked rice, comprising: a soaking step in which raw material rice is soaked in water at 10 to 35°C to absorb water; a first heating and pressurizing step in which the rice after the soaking step is steamed with steam under a pressure of 130 to 200 kPa; a liquid contacting step in which the rice after the first heating and pressurizing step is brought into contact with a liquid containing edible oils and fats that are liquid at room temperature and water, the liquid having a temperature of 60°C or less; a second heating and pressurizing step in which the rice after the liquid contacting step is steamed with steam under a pressure of 200 to 300 kPa; a water absorption step in which the rice in the second heating and pressurizing step is allowed to absorb water; and a freezing step in which the rice after the water absorption step is frozen.

2. A method for producing frozen cooked rice according to claim 1, wherein the content of the edible oil or fat in the liquid is 0.1 to 30% by mass.

3. A method for producing frozen cooked rice according to claim 1 or 2, wherein the rice is non-glutinous rice.

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