GB2185379A

GB2185379A - Packaging and sterilising food

Info

Publication number: GB2185379A
Application number: GB08629512A
Authority: GB
Inventors: Ko Sugisawa; Yasushi Matsumura; Hidefumi Okamoto; Makoto Hirayama; Yukari Inada; Takashi Nishi; Ryusuke Nakanaga
Original assignee: House Food Industrial Co Ltd
Current assignee: House Foods Corp
Priority date: 1985-12-11
Filing date: 1986-12-10
Publication date: 1987-07-22
Also published as: KR900006865B1; KR870005601A; IT8622646A0; DE3642194C3; DE3642194C2; DE3642194A1; CN86108458A; CN1021285C; GB8629512D0; IT1199773B; GB2185379B

Abstract

A method of producing a packaged food in which non-dry noodles such as spaghetti, Japanese noodles, Chinese noodles or uncooked noodles; boiled rice and boiled beans are placed in a heat-resistant container so that an air volume in the container is in the range of 40 to 85 volume percent, sealing the container, and subjecting the sealed container to sterilising treatment under pressure. It is possible to effectively prevent individual noodle strings, rice grains or beans from sticking to each other in the container by this method.

Description

SPECIFICATION Method of producing packed food The present invention relates to a method of producing a packed food in which non-dry noodles such as spaghetti, Japanese noodles, Chinese noodles or other uncooked noodles, boiled rice or boiled beans are packed in a sealed container. More particularly, the invention relates to a method of producing a packed food in which it is possible to effectively prevent individual noodle strings, rice grains or beans from sticking to each other in the container.

From the viewpoint of enabling saving of the time normally required for soaking dry noodles in water to prepare them for cooking, and also of enjoying the very taste of cooked noodles, various types of packed food have heretofore been placed on the market. In general, a typical product includes a plastic container, such as a pouch or a cup, in which uncooked noodles, such as Japanese noodles, Chinese noodles or spaghetti, are packed followed by heat treatment under pressure. It is true that such packed uncooked noodles possess the above-mentioned advantage when they are to be cooked. However, the noodle strings packed in the container such as a pouch stick to each other during the noodle-packing process, the ensuing pressure or heat treatment, the product distribution process, or during the time they are displayed in a shopwindow or other display.In consequence, when consumers obtain such types of packed food, the appearance of the packed noodles is already damaged. In addition, the noodles stuck together in the container are hard to separate, and the quality inherent to their texture cannot be enjoyed when they are eaten. There is a particular problem in that, when the consumer attempts to separate noodle strings which are stuck to one another, the noodle strings are often damaged or broken around places where they are adhered to each other.

Various proposals have heretofore been made with respect to the above-described problems.

For example, the Japanese Patent Laid-open KOKAI No. 170155/1982 states that, for the production of "retort-packed" noodles (retort: a sealed bag made of thin metal film), if uncooked noodles coated with aqueous organic acis containing dispersed fat is subjected to heat treatment, it is possible to prevent individual noodle strings sticking to each other. In addition, this patent states then when in particular, noodles are to be packed into a sealed container, it is preferable for the air content in the container to be maintained at a level of 30 to 50 volume percent of the volume of the packed noodles (such values are substantially equivalent to 23 to 33 volume percent of the total volume of the container.) According to this method, it is possible to effectively prevent individual noodle strings from sticking to each other unlike other prior-art methods.However, it is still impossible to achieve a completely satisfactory result.

Various types of "retort-packed" rice (packed in a sealed metal bag) have also been placed on the market. When the rice is to be eaten, the entire pouch is heated in boiling water, before being opened and the contents then transferred to, for example, a dish.

It is true that such packed rice is simple to prepare; however, the packed rice grains become stuck to one another during the rice-packing process, the ensuing pressure or heat treatment, the product distribution process, or during the time they are displayed in a shop window or other display. In consequence, when consumers obtain such types of rice, the appearance of the packed rice is already damaged. In addition, the rice grains stuck together in this way are hard to separate, and, since the rice grains are partially crushed in the container, the quality inherent to their texture cannot be enjoyed when they are eaten.

The present invention has been devised on the basis of the knowledge that, if non-dry noodles such as uncooked noodles, boiled rice grains or beans are packed into a sealed container so that the air content in the container may be maintained within a specific range, followed by heat treatment under pressure, it is possible to effectively prevent the noodle strings, rice grains and beans from sticking to one another within the container.

It is therefore a primary object of the present to provide a method of producing packed food in which individual noodle strings, rice grains and beans are less likely to become stuck to each other than in the prior art.

This and other objects of the present invention will be clear from the following description.

In accordance with the present invention, there is provided a method of producing packed food in which non-dry noodles such as uncooked noodles, boiled noodles, semi-boiled noodles, steamed noodles or semi-boiled noodles, or boiled rice or boiled beans or other similar food are hermetically packed in a heat-resistant container in such a manner that the air content in the container may be maintained between 40 and 85 volume percent of its total volume after completion of such packing, and the container is then subjected to heat treatment under pressure.

Brief Description of the Drawings Figure 1 is a graph showing the relationship between temperature changes and pressure changes in a heat sterilization step performed in a method of producing packed noodles in accordance with the present invention; and Figure 2 is a graph showing the relationship between temperature changes and pressure changes in a heat sterilization step performed in a method of producing packed rice in accordance with the present invention; and Figure 3 shows a process in which the lid of the container is deformed by the heat treatment under pressure; and Figure 4 shows a process in which the bottom of the container is deformed by the heat treatment under pressure; and Figure 5 shows a process in which the side wall of the container is deformed by the heat treatment under pressure; and Figure 6 is a graph showing the relationship between temperature changes and pressure changes in case where the lid of the container is deformed by the heat treatment under pressure.

In Figs. 1 and 2, a curve A represents temperature changes within a sterilizer, a curve B represents temperature changes in packed noodles, a curve C represents pressure changes in a container and a curve D represents pressure changes in the sterilizer.

The present invention handles: non-dry noodles, such as Japanese noodles, buckwheat noodles, spaghetti, Chinese noodles or chow mein which are uncooked, boiled, steamed or semi-steamed; rice grains, such as steamed rice, semi-steamed rice, boiled rice or semi-boiled rice; and beans such as boiled soybean. As a matter of course, the aforementioned food is not limitative, and the present invention can also be applied to boiled noodles or semi-boiled noodles which are obtained by boiling dry noodles by a known method. In order to further improve the effect of preventing these noodle strings or rice grains from sticking to each other, an emulsifier agent, such as lecitin or saccharose fattey acid ester, is added to the noodles or rice grains before they are packed in a heat-resistance container.Preferably, 0.01 to 0.1 percent of the emulsifier by weight is applied over the noodle strings or rice grains. Also, for the same purpose, various kinds of fat could be added to them at a level of 0.5 to 2 percent by weight.

Substantially, the above-mentioned food, such as non-dry noodles, is packed in the heatresistant container by a known method until the air content within the container reaches a level of 40 to 85 percent by volume. In this case, when non-dry noodles are to be packed, the air content is maintained at a level of 40 to 70 percent by volume, preferably 50 to 70 percent by volume. When rice grains are to be packed, the air content is maintained at a level of 40 to 85 percent by volume, preferably 45 to 80 percent by volume. It is preferable to use a heatresistant container made of material which resists heat of about 135"C, such container being suitably formed, for example, in the shape of a pouch, a "retort-pouch" (or sealed pouch made of a metal film), a polygon, such as a column, a cube or a rectangular pallelepiped, a cup, or a bowl.Also, the heat-resistant container is preferably made of unilaminate or laminate material such as polyethylene, polystyrene, polypropylene, or polyester which are water-resistant, flexible material. Moreover, in the present invention, it is particularly suitable to use a tray-like container having rigidity or semi-rigidity. Such rigid or semi-rigid container can be directly heated by a microwave oven. For example, after a seasoned sauce is added to non-dry noodles or curry sauce is added to rice grains, the container including such food can be easily heated in the microwave oven. Also, of course, the thus-heated food can be eaten in the container immediately after heating without the need to use a plate.

In the present invention, it is particularly critical when non-dry noodles or rice grains are to be packed in a container of the aforementioned type to maintain the air content in the container in the previously-described specific range. The term "air content in the container" means the value obtained by subtracting the volume percent of the packed non-dry noodles or the rice grains from the total volume (100 percent) of the container, that is, it refers to the total volume of inert gases such as nitrogen or air present in the container. In order to provide the aforementioned air content, when the non-dry noodles or rice grains are to be packed in the container, it is preferable to arrange them uniformly within the container. After the non-dry noodles have been packed in the container in this manner, the container is sealed by known means such as heat sealing.

In accordance with the present invention, noodle strings, rice grains or beans are packed in a rigid or semi-rigid container and the air content in the container is maintained within the specific range. This prevents noodle strings, rice grains and beans from being squeezed between the inner walls of the container during a pressure-heat treatment or product distribution process, and thus, the rice grains do not get stuck to each other.

In accordance with the present invention, the above-mentioned container is then subjected to the heat treatment under pressure. This treatment is preferably effected so that the differential pressure obtained from the external pressure of the container minus the internal pressure of the container may be varied in the range of - 1.0 to +2.0 kg/cm2, preferably 0.1 to 1.0 kg/cm2, more preferably 0.1 to 0.5 kg/cm2. It is to be noted here that the method disclosed in Japanese Patent Laid-open (KOKAI) No. 1371/1986 filed by this applicant can be effectively applied to this method to a remarkable extent, and thus is remarkably suitable for use as the pressure-heat treatment in the production method of this invention.

The method described in Japanese Patent KOKAI No. 1371/1986 will be specifically described below. This method is a pressure-heat sterilization method which is effected under a pressure control process. In this method, the rate of rise of the external pressure acting on the container is varied in accordance with the rate of rise of the internal pressure acting on the same and at the same time the point for dropping the external pressure of the container is controlled such as to lag behind the point for starting an ensuing cooling step.

In this pressure control method, a first step is to detect the pattern of changes in the pressure in the container under a predetermined temperature condition. A package containing air and contents to be sterilized is used as an example of such container. In this detection step, a thermocouple, for example, is arranged within the package in order to measure the temperature of the contents and the space within the same, and the thus-arranged package is placed in a sterilizer. In this state, while the sterilization of the package is being effecte,d under a predetermined sterilizing temperature, measurement is made with respect to the pattern of changes in the temperature of the contents and that of the space in the package. The pattern of changes in the internal pressure of the package is calculated on the basis of the results of this measurement.

More specifically, the approximate equation for the pressure in the container is as follows: PRESSURE WITHIN CONTAINER (kg/cm2) =ATMOSPHERIC PRESSURE (kg/cm2 abs)x TEMPERATURE OF SPACE AFTER PREDETERMINED PASSAGE OF TIME ("K) INITIAL TEMPERATURE OF SPACE ("K) +SATURATED VAPOR PRESSURE (kg/cm2 abs) WITH RESPECT TO TEMPERATURE OF CONTENTS AFTER PREDETERMINED PASSAGE OF TIME (kg/cm2 abs) -ATMOSPHERIC PRESSURE (kg/cm2 abs) where the pressure within the container before effecting the sterilization step is assumed to be atmospheric pressure, and the initial temperature of the space is assumed to be substantially the same and the intial temperature of the contents.

When the aforementioned sterilization step is to be effected for a flexible container, it is preferable to adjust the pressure in the sterilizer in order to minimize measurement errors which might be caused by the volume change of the container accompanying the pressure change in the package.

In addition, it is possible to adopt another method of detecting a pressure change pattern by directly measuring the internal pressure of the container.

On the basis of the change pattern of the pressure in the container by the above-described method, detection is made with respect to a point at which a change is effected during the rise of this pattern and the peak pressure within the container. In addition to these results, differential pressures provided when sterilization is started and when the internal pressure of the container reaches its peak value are taken into consideration.Thus, the rate of rise of the internal pressure of the container is calculated so that this rate of pressure rise may be varied near the above-mentioned point at which a change is effected. (In order to assure a suitable level of differential pressure, the manner of conducting the pressure change in the sterilizer preferably matches that in the container.) On the basis of the results, a decision is made as to each of the control conditions for the rate of rise in the pressure in the sterilizer. In addition, a decision is made as to each of the control conditions for the pressure drop in the sterilizer so that the point at which the pressure starts to drop in the bath lags behind the point at which the ensuing cooling step is started. The control conditions relating the pressure control method are thus finalized in this manner.

There is no limitative condition relating to the differential pressure provided when the aforementioned sterilization step is started or the differential pressure provided when the internal pressure of the container reaches its peak value. However, the differential pressure obtained from the external pressure of the container minus the internal pressure of the container is preferably maintained within the range of - 1.0 to +2.0 kg/cm2, preferably +0.1 to + 1.0 kg/cm2, more preferably +0.1 to +0.5kg/cm2.

In addition, the point for changing the rate of rise of the pressure in the sterilizer is set close to the point for changing the pattern of the pressure rise in the container, i.e., within a range equivalent to (the required period between the starting of sterilization and the point for changing the pressure in the container) x 30 percent. It is thereby possible to maintain a suitable differential pressure at a constant level while the internal pressure of the container rises.

Incidentally, the above-described pressure control method can be performed automatically and stably by the use of a computer.

In addition to the above method, it is preferable that the lid or the bottom of the container be made of a material capable of being momently deformed by the heat treatment under pressure, and that the lid or the bottom be momently deformed by controlling the pressure at heat treatment so as to make the lid contact the upper surface of the food in the container during the temperature in the container is not less than 115 C, more preferably not less than 100"C. It is preferable that the pressure be controlled, so that area of the lid contacting the upper surface of the food in the container be not less than 60%, preferably between 70 and 100%.

That is, this effectively prevents browning and scorch that occur on the upper surface of the food faced to the head space at the heat treatment in which the temperature in the container becomes not less than 100 C, more particular not less than 115"C. In this case, the air content of the container does not change, since the container is sealed, so that the air in the container moves into the food even if the lid is momently deformed as described above.

Examples of the deformation described above include a method of deforming the lid referred to Fig. 3, a method of deforming the bottom of the container referred to Fig. 4 and a method of deforming a part of the side wall of the container referred to Fig. 5.

In accordance with the present invention, the phenomenon by which noodle strings usually become stuck to each other can be successfully prevented, thereby enabling the production of packed noodles which are superior in appearance, quality of texture, and separability of noodle strings. In particular, if the method of this invention incorporates the pressure control method which is disclosed as a pressure-heat sterilization method in Japanese Patent KOKAI No.

1317/1986, it is possible to more effectively prevent the noodle strings from sticking to each other.

Accordingly, the packed noodles produced by the method of this invention are suitably utilized as a form of instant food with sauce or soup. In particular, in accordance with the present invention, the air content in the container is large and few noodle strings adhere to each other.

When sauce or soup is poured over the same after opening the lid of the container, the sauce or soup can easily penetrate between the individual noodle strings through the clearances between them, and, since the container has sufficient space for accommodating sauce or soup, no sauce or soup overflows out of the container. Moreover, the sauce or soup can be uniformly applied over the noodle strings, and it is possible to eat them immediately after the entire container is heated in a microwave oven. It is to be noted, however, that the packed noodles of this invention could, as a matter of course, be marketed with no sauce or soup attached thereto.

The packed rice produced in accordance with the present invention is suitably used as a form of instant food with curry sauce, and the present invention provide favorit merits in this case as well.

Although the microwave oven is illustratively used as means for heating non-dry noodles and rice in the above description, the use of such microwave oven is not limitative. For example, noodles may be heated in boiling water or by other suitable heating means.

The present invention will be further illustrated by the following non-limitative examples.

Description of the Examples In the following examples, the air content in the container was measured in the following manner. A container including noodle strings was placed in water, and the volume of the air in the same was measured. On the basis of the result, the air content was calculated by the following equation. Regarding the rice as well, the same procedures were carried out.

AIR CONTENT (%) VOLUME OF AIR WITH CONTAINER INCLUDING NOODLES x 100 VOLUME OF CONTAINER Example 1 Dry spaghetti, each string having a diameter of 1.9mm, was boiled.for 6 minutes and 30 seconds, and a sample of semi-boiled spaghetti was obtained. The thus-obtained sample was transferred onto a colander, and, after the water was drained off the sample, refined rapessed oil containing 1.2 weight percent of refined soybean leticin was applied over the sample at 1.25 weight percent. Subsequently, 200 grams of the sample was packed in a polypropylene tray having a thickness of 0.7 mm (130 mm in width, 180 mm in length and 20 mm in depth), and the tray was heat-sealed with a film composed of a polypropylene inner layer and a nylon outer layer (130 mm in width and 180 mm in length).In this case, the value of the obtained air content was about 57 volume percent.

Subsequently, the tray was placed in a sterilizer. It should be noted that a thermocuple was attached to the tray in order to measure the temperature of the contents.

Next, after the sterilizer was sealed, the interior of the sterilizer was initially pressurized at 0.25kg/cm2, 90"C of water was transferred from a hot-water tank into the sterilizer. Subsequently, the temperature in the sterilizer was increased as indicated by a line A shown in Fig. 1, and, after 123"C was reached, the tray was subjected to sterilization treatment for 12 minutes.

(During this time, the pressure within the sterilizer was regulated by a manual valve so that the tray was not deformed.) Subsequently, measurement was made with respect to the pattern of temperature change of the contents. The result was plotted by a line B of Fig. 1.

On the basis of the result obtained by the measurement of this temperature change pattern, the pressure in the tray was calculated by the following equation. A line C in Fig. 1 shows the pattern of the pressure change in the tray obtained from this calculation. In this calculation, the temperature of the space in the container sample was assumed to be equal to the temperature of the contents.

PRESSURE WITHIN TRAY (GAUGE PRESSURE) =ATMOSPHERIC PRESSURE (kg/cm2 abs)x TEMPERATURE OF SPACE AFTER PREDETERMINED PASSAGE OF TIME ("K) INITIAL TEMPERATURE OF SPACE ("K) +SATURATED VAPOR PRESSURE WITH RESPECT TO TEMPERATURE OF CONTENT AFTER PREDETERMINED PASSAGE OF TIME (kg/cm2 abs) -ATMOSPHERIC PRESSURE (kg/cm2 abs) In this equation, the internal pressure of the tray was assumed to be atmospheric pressure.

On the basis of the thus-obtained pattern of the pressure change in the tray, the internal pressure in the tray was changed 21 minutes after the sterilization was started. At this time, the pressure in the sterilizer was set to 2.2 kg/cm2. (The differential pressure, namely, the internal pressure of the sterilizer minus the internal pressure of the tray was +0.2 kg/cm2).The application of a constant level of pressure was started 31 minutes after the heat sterilization was started, and the pressure in the sterilizer at this time was set to 2.4 kg/cm2.l(The differential pressure was +0.2 kg/cm2.) The pressure applied to the tray was maintained at a constant level until 33 minutes passed after the sterilization was started (until about 1 minute after the commencement of cooling), and the pressure in the sterilizer was reduced to 1.3 kg/cm2 (a differential pressure:=0-2 kg/cm2) 37 minutes after the commencement of the sterilization (5 minutes after the commencement of cooling). In the above-described manner, the pattern of the pressure change in the sterilizer was determined on the basis of the pattern of the pressure change in the tray. A line D shows the pattern of pressure change in the sterilizer.In this case, although the temperature of the space in the container was assumed to be the same as the temperature of the content in order to calculate the pattern of the pressure change in the tray, the temperature of the space was actually higher than the temperature of the content at least until cooling was started. The actual pressure in the tray was higher than the values shown by the pattern of pressure change in the same. For this reason, the pattern of pressure change in the sterilizer was maintained at a level higher than the values shown by the pattern of pressure change in the tray. Moreover, in order to prevent a sealing material from being removed from the tray, it is preferably to maintain the pressure in the tray at a higher level than the pressure in the sterilizer.

On the basis of the above-described pattern of pressure change in the sterilizer, the tray was subjected to sterilization under the aforementioned conditions, and thus a tray-packed spaghetti was obtained. The appearance of the spaghetti was good. In this example, the contacting area of upper surface of the spaghetti and the lid is about 1% relative whole area of the upper surface of the spaghetti in the container during the temperature in the container is more than 1 1 5 C.

After the tray containing the spaghetti had been heated in 95"C of boiling water for five minutes, the tray was opened, and 150 grams of meet sauce was poured into the tray. When the spaghetti was eaten in this state, the spaghetti strings were not stuck to each other, the separability was good and the quality of texture of the strings was good.

Example 2 Pressure-heat sterilization was carried out at a constant level of pressure. The level of pressure was 1.5 kg/cm2, sterilization temperature was 120"C and sterilization time was 25 minutes.

Apart from these conditions, tray-packed spaghetti was obtained in the same manner as that of Example 1. In this example, the upper surface of the spaghetti does not contact the lid during the temperature in the container is more than 115 C.

Example 3 355 grams of spaghetti was packed in the tray (the air content was about 40 volume percent.) Pressure-heat sterilization was carried out at a constant level of pressure. The level of pressure was 1.5 kg/cm2, sterilization temperature was 120"C and sterilization time was 25 minutes. Apart from these conditions, tray-packed spaghetti was obtained in the same manner as that of Example 1. However, in this example, the upper surface of the spaghetti does not contact the lid during the temperature in the container is more than 115"C.

Comparative Example 1 384 grams of spaghetti was packed in the tray (the air content was about 32 volume percent.) Pressure-heat sterilization was carried out at a constant level of pressure. The level of pressure was 1.5 kg/cm2, sterilization temperature was 120"C and sterilization time was 25 minutes. Apart from these conditions, tray-packed spaghetti was obtained in the same manner as that of Example 1. However, in this example, the upper surface of the spaghetti does not contact the lid during the temperature in the container is more than 115"C.

Comparative Example 2 413 grams of spaghetti was packed in the tray (the air content was about 27 volume percent.) Pressure-heat sterilization was carried out at a constant level of pressure. The level of pressure was 1.5 kg/cm2, sterilization temperature was 120"C and sterilization time was 25 minutes. Apart from these conditions, tray-packed spaghetti was obtained in the same manner as that of Example 1. However, in this example, the upper surface of the spaghetti does not contact the lid during the temperature in the container is more than 115"C.

A panel of ten members tried the tray-packed spaghetti prepared in the above-mentioned Examples 1 to 3, Comparative Examples 1, 2, and checked each of the items, "separability", appearance and "texture quality" relating to the spaghetti. Table 1 shows the results in summarized form.

TABLE - l Separa- Appear- Texture bility ance Quality Comment Example 1 8.2 8.0 8.3 The tray-packed spaghetti strings were not at all stuck to each other. The separability, the appearance, and the quality texture were excellent.

Example 2 7.8 7.6 7.5 Few of the tray-packed spaghetti strings were stuck to each other. The separability, the appearance, and the quality texture were good.

Separa- Appear- Texture Comment bility ance Quality Example -3 6.8 7.0 7.1 The tray-packed spaghetti strings were effectively prevented from being stuck to each other. The separability, the appearance, and the quality texture were good.

Compara- 4.7 5.1 5.1 The tray-packed spaghetti tive strings were partially Example 1 stuck to each other. The separability, the appearance, and the quality texture were bad.

Compara- 4.4 4.7 5.0 The same as Comparative tive Example 1 Example 2 Note: The numeric values shown in Table 1 are based on a rating of spaghetti's quality on ten levels, with respect to each of the items, "Separability", "Appearance" and "Texture Quality".

Each of the members marked the spaghetti on a maximum scale of 10 points representative of the spaghetti idealized by each of them. The above-noted values were respectively obtained by averaging the thus-obtained values (rounded off to the first decimal point).

Example 4 Chinese noodles, each string having a diameter of 1.5 mm, was boiled for two minutes, and a sample of semi-boiled Chinese noodles was obtained. The semi-boiled noodle sample was transferred onto a colander, and, after the water was drained off the sample, refined rapeseed oil containing 1.2 weight percent of refined soybean leticin was applied over the sample at 1.25 weight percent. Subsequently, 200 grams of the sample was packed in a polypropylene tray having a thickness of 0.7 mm (130 mm in width, 180 mm in length and 20 mm in depth), and the tray was heat-sealed with a film composed of a polypropylene inner layer and a nylon outer layer (130 mm in width and 180 mm in length). In this case, the value of the air content was about 59 volume percent.

Subsequently, the Chinese noodles contained in the tray were sterilized under a constant level of pressure in the conditions of a pressure of 1.5 kg/cm2, a sterilization temperature of 120"C and a sterilization time of 25 minutes, and thus the tray-packed Chinese noodles was obtained.

The appearance of the Chinese noodle strings was good. In this example, the upper surface of the noodle does not contact the lid during the temperature in the container is more than 115"C.

After the tray containing the noodles had been heated in 95"C of boiling water, the tray was opened, and 250 grams of Chinese soup at 80"C was poured into the tray. When the Chinese noodles were eaten in this state, the noodle strings were not substantially stuck to each other, and also the separability and the quality of texture of the strings were good.

Example 5 180 grams of boiled rice which was obtained in the same manner was packed in a polypropylene tray having a thickness of 0.7 mm (150 mm in upper diameter, 80 mm in lower diameter and 30 mm in depth), and the tray was heat-sealed with a film composed of a polypropylene inner layer and a nylon outer layer (150 mm in diameter). In this case, the value of the obtained air content was about 47 volume percent.

Subsequently, the tray was placed in a sterilizer. It should be noted that a thermocouple was attached to the tray in order to measure the temperature of the content.

Next, after the sterilizer was sealed, the interior of the sterilizer was intitially pressurized at 0.25 kg/cm2, 90"C of water was transferred from the hot-water tank into the sterilizer. Subsequently, the temperature in the sterilizer was increased as indicated by a line A shown in Fig. 2, and, after 123"C was reached, the tray was subjected to sterilization for fifteen minutes. (During this time, the pressure within the sterilizer was regulated by a manual valve so that the tray was not deformed.) Subsequently, measurement was made with respect to the pattern of temperature change of the content. The result was plotted by a line B of Fig. 2.

On the basis of the result obtained by the measurement of this temperature change pattern, the pressure in the tray was calculated by the same method as that of Example 1.

On the basis of the thus-obtained pattern of the pressure change in the tray, the internal pressure in the tray was changed 20 minutes after the sterilization was started. At this time, the pressure in the sterilizer was set to 1.6 kg/cm2. (The differential pressure, namely, the internal pressure of the sterilizer minus the internal pressure of the tray was +0.2 kg/cm2). The application of a constant level of pressure was started when 30 minutes had passed after the heat sterilization was started, and the pressure in the sterilizer at this time was set to 2.5 kg/cm2.

(The differential pressure was+0.2 kg/cm2.) The pressure applied to the tray was maintained at a constant level until 36 minutes passed after the sterilization was started (for about 1 minute after the commencement of cooling), and the pressure in the sterilizer was reduced to 1.6 kg/cm2 (differential pressure:+0.2 kg/cm2) 40 minutes after the commencement of the sterilization (5 minutes after the commencement of cooling). In the above-described manner, the pattern of the pressure change in the sterilizer was determined on the basis of the pattern of the pressure change in the tray. A line D shows the pattern of pressure change in the sterilizer.In this case, although the temperature of the space in the container was assumed to the be the same as the temperature of the content in order to calculate the pattern of the pressure change in the tray, the temperature of the space was actually higher then the temperature of the content at least until cooling was started. The actual pressure in the tray was higher than the values shown by the pattern of pressure change in the same. For this reason, the pattern of pressure change in the sterilizer was maintained at a level higher than the pattern of pressure change in the tray. Moreover, in order to prevent a sealing material from being removed from the tray, it is preferable to maintain the pressure in the tray at a lower level than the pressure in the sterilizer.

On the basis of the above-described pattern of pressure change in the sterilizer, the tray was subjected to sterilization under the aforementioned conditions, and thus a tray-packed rice was obtained. The appearance of the rice was good. In this example, the upper surface of the content (rice) does not contact the lid during the temperature in the tray is more than 115"C.

After the tray containing the rice had been heated in 95"C of boiling water for five minutes, the tray was opened, and 150 grams of curry was poured into the tray. When the rice was eaten in this state, the rice grains were not stuck to each other, and also the separability and the texture quality of the rice grains were good.

Example 6 Pressure-heat sterilization was carried out at a constant level of pressure. The level of pressure was 2.2 kg/cm2, sterilization temperature was 120"C and sterilization time was 25 minutes.

Apart from these conditions, tray-packed rice was obtained in the same manner as that of Example 5. In this example, the upper surface of the content (rice) does not contact the lid during the temperature in the tray is more than 115"C.

Example 7 The rice was packed in the tray in such a manner that the air content was about 40 volume percent. Pressure-heat sterilization was carried out at a constant level of pressure. The level of pressure was 2.2 kg/cm2, sterilization temperature was 1200C and sterilization time was 25 minutes. Apart from these conditions, tray-packed rice was obtained in the same manner as that of Example 5. In this example, the upper surface of the content (rice) does not contact the lid during the temperature in the tray is more than 115"C.

Comparative Example 3 The rice was packed in the tray in such a manner that the air content was about 32 volume percent. Pressure-heat sterilization was carried out at a constant level of pressure.

The level of pressure was 2.2. kg/cm2, sterilization temperature was 120"C and sterilization time was 25 minutes. Apart from these conditions, tray-packed rice was obtained in the same manner as that of Example 5. In this example, the upper surface of the content (rice) does not contact the lid during the temperature in the tray is more than 1 15"C.

Comparative Example 4 The rice was packed in the tray in such a manner that the air content was about 27 volume percent. Pressure-heat sterilization was carried out at a constant level of pressure. The level of pressure was 2.2 kg/cm2, sterilization temperature was 1200C and sterilization time was 25 minutes. Apart from these conditions, tray-packed rice was obtained in the same manner as that of Example 5. In this example, the upper surface of the content (rice) does not contact the lid during the temperature in the tray is more than 115"C.

A panel of ten members tried the tray-packed rice prepared in the above-mentioned Example 1 to 3, Comparative Example 1, 2, and checked each of the items, "separability", "appearance" and "texture quality" relating to the rice. Table 2 shows the results in summarized form.

TABLE 2 Separa- Appear- Texture bility ance Quality Example 5 8.4 8.2 8.1 The unsuitably stucked rice grains were not found at all. The separability, the appearance, and the quality texture were excellent.

Example 6 7.9 7.4 7.6 Few of rice grains were unsuitably stuck to each other. The separability, the appearance, and the quality texture were good.

Example 7 6.6 7.1 7.0 Rice grains were effectively prevented from being unsuitably stuck to each other. The separability, the appearance, and the quality texture were good.

Separa- Appear- Texture Comment Separa- Appear- Texture bility ance Quality Compara- 4.5 4.7 4.9 Rice grains were tive unsuitably stuck to each Example 3 other. The separability, the appearance, and the quality texture were bad.

Compara- 4.3 4.7 4.8 The same as the comment tive of Comparative Example 3.

Example 4 Note: The numeric values shown in Table 2 are based on a rating of rice's quality on ten levels, with respect to each of the items, "Separability", "Appearance" and "Texture Quality".

Each of the members marked the rice on a maximum scale of 10 points representative of the rice idealized by each of them. The above-noted values were respectively obtained by averaging the thus-obtained values (rounded off to the first decimal point).

Example 8 1300 g of rice which has been immersed in water is steamed at a temperature of 100"C for 5 minutes and then it is immersed in hot water (98"C) for 2 minutes. On the other hand, there is prepared a seasoning liquid (80 cps) which contains 18 parts by weight of soysauce, 1.0 part by weight of tuna extract, 0.7 by weight of sodium glutamate, 5.3 parts by weight of salt, 6 parts by weight of sugar, 0.2 part by weight of xanthane gum and 68.8 parts by weight of water. The rice was treated with 300 g of the seasoning liquid, and thereafter the rice was steamed at a temperature of 100"C for 20 minutes. The water content of the resulting rice was 64% by weight.The rice (i.e., pregelatinized rice) was subjected to air-drying treatment for about 3 minutes in condition of 40"C and RH 47%.

200 g of the resulting rice (water content: 60.2% by weight) was packed in a polypropylene tray having a thickness of 0.7 mm (134 mm in upper diameter, 90 in lower diameter, 35 mm in depth and 350 ml in volume), and the tray was heat-sealed with a film composed of a polypropylene inner layer and a nylon outer layer (135 mm in diameter). In this case, the value of the obtained air content was about 52 volume percent.

Subsequently, the tray was placed in a sterilizing sterilizer. The pressure in the bath was changed as shown in Fig. 6 so as to carry out heat treatment under pressure. In this connection, the pressure was controlled by hand in such a manner that the tray and lid would not be deformed at the initial stage, and that about 75% of the whole area of the upper surface of the content (rice) would contact the lid during the temperature in the container was more than 100"C, and that the tray and the lid would not be deformed again when the temperature in the container lowered than 100"C.

Regarding the resulting rice packed in the tray, browning and scorching were not observed on the upper surface of the rice in the tray.

The resulting rice was eaten after the tray containing the rice was heated in a microwave oven. The rice grains were not stuck to each other, and also the separability and the texture quality of the rice grains were good.

Comparative Example 5 The rice packed in the tray was prepared by the same method is set forth in Example 8, except that the pressure was controlled by hand in such a manner that the lid was not deformed.

Regarding the resulting rice packed in the tray, browning and scorching were observed on the upper surface of the rice in the tray.

Claims

1. A method of producing packed food, which comprises the steps of: packing non-dry noodles, rice grains or beans in a heat-resistant container so that an air content in the heatresistant container is in the range of 40 to 85 volume percent of the total volume of the container, and subjecting the container to pressure-heat treatment.

2. A method according to Claim 1, wherein the non-dry noodles are packed in the container so that the air content is in the range of 40 to 70 percent.

3. A method according to Claim 1 or 2, wherein the non-dry noodles are selected from the group consisting of uncooked noodles, boiled noodles, semi-boiled noodles, steamed noodles and semi-steamed noodles.

4. A method according to Claim 1, wherein the rice grains are selected from the group consisting of steamed rice, semi-steamed rice, boiled rice and semi-boiled rice.

5. A method according to any of Claims 1 to 4, wherein the pressure-heat treatment is effected such that the differential pressure obtained from the external pressure of the container minus the internal pressure of the container may be varied in the range of - 1.0 to 2.0 kg/cm2.

6. A method according to any of Claims 1 to 5, wherein the pressure-heat treatment is effected by a pressure control system in which the rate of rise of the external pressure of the container is varied to match that of the internal pressure of the container, and in which the timing for dropping the external pressure of the container lags behind a time at which cooling is started.

7. A method according to any of Claims 1 to 6, wherein the heat-resistant container is made of a flexible material.

8. A method according to any of Claims 1 to 7, wherein the heat-resistant container is a rigid or semi-rigid tray.

9. A method according to any of Claims 1 to 8, wherein the pressure is controlled during the heat treatment under pressure so as to contact not less than 60% of an upper surface of the food in the container to the lid.

10. A method according to claim 1, substantially as described in any of the foregoing Examples.

11. Packed food produced by a method according to any of claims 1 to 10.