JP3653354B2 - How to dry food - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for drying foods for producing dried foods such as vegetables and seafood, and more particularly, to a drying method for preventing food from being altered or discolored and performing drying.
[0002]
[Prior art]
There are various methods for drying fresh food, such as sun drying, hot air drying, infrared drying, freeze drying, and the like. Recently, a vacuum drying method has attracted attention as a method that can be quickly dried at a relatively low temperature.
[0003]
The reduced-pressure drying method increases the evaporation rate of moisture by reducing the pressure in the drying chamber, and by drying at a temperature close to room temperature, for example, 50 ° C. or less, prevents decomposition of food components, alteration of food, and discoloration. It is an excellent method for obtaining a dry food that retains the flavor of the food.
[0004]
However, even with the vacuum drying method, it takes a considerable amount of time to complete the drying of the food, which causes the propagation of various bacteria and the oxidation of the food components, which are not always sufficient to leave the flavor of the original fresh food. I can't say.
[0005]
Furthermore, there is a vacuum freeze-drying method as another method for drying while leaving the flavor of the original food. This is a drying method in which the object to be dried is cooled to 0 to −40 ° C. and frozen, and then the moisture is removed as a gas in the vacuum drying chamber while being frozen. The pressure in the drying chamber is lower than the vapor pressure of the solid moisture. Therefore, it is necessary to create a high vacuum of, for example, about 50 mm (0.5 kPa) of water column with absolute pressure.
[0006]
In recent years, the vacuum freeze-drying method has been widely used as a method for drying ingredients such as instant foods such as instant noodles, tea pickles, sprinkles, soups, and seasonings. This is to dry the material once boiled, and has the advantage that it can prevent the invasion and propagation of germs in the drying process and is suitable for long-term storage. In addition, it has been attempted to produce fresh foods that can be stored for a long period of time by directly vacuum-freezing and drying fresh foods such as vegetables, meat, and seafood.
[0007]
However, the vacuum freeze-drying method has a very low drying speed, and if all the moisture in the food is removed with this, there is a problem that the drying time becomes extremely long and the drying cost is remarkably increased.
[0008]
[Problems to be solved by the invention]
In view of the problems of the conventional food drying methods as described above, the present invention provides a food drying method that better preserves the flavor of the original food, can be stored for a long time, and has a relatively low drying cost. With the goal.
[0009]
In particular, we provide a food drying method that can achieve both of the two objectives of preventing deterioration of food and reducing the cost of drying by properly combining vacuum drying and other conventional drying methods near room temperature and vacuum freeze drying. The purpose is to do.
[0010]
[Means for Solving the Problems]
The inventors of the present invention examined the drying speed of various fresh foods in conventional drying methods, for example, a method of drying at 40 ° C. or lower under reduced pressure (hereinafter referred to as “room temperature vacuum drying method”) and a vacuum freeze-drying method. It was found that there is a relationship as schematically shown in FIG.
[0011]
That is, the normal temperature reduced pressure drying method has a large initial drying rate, but when the moisture in the food becomes about 20 to 30% of the fresh state, the drying rate starts to decrease, especially when it becomes 15 to 20% or less of the fresh state moisture. Drying almost does not proceed, and it is difficult to reach the final dry moisture (about 3 to 10% of the fresh moisture).
[0012]
On the other hand, in the vacuum freeze-drying method, the initial drying rate is very small, but even if the moisture in the food becomes 20 to 30% or less of the fresh state, the drying rate does not decrease so much and reaches the final dry moisture.
[0013]
The reason for the difference in the drying rate in the low moisture region is considered as follows. That is, in the room temperature vacuum drying method, the drying speed is high at the initial stage of drying because mainly free water in the food tissue (water not bonded to the food tissue) evaporates. On the other hand, in the low moisture region, the water adhering / bonding to the multi-molecular layer mainly evaporates in the food tissue, but this moisture has a large affinity with the food tissue, and therefore the evaporation rate is remarkably slow.
[0014]
In contrast, in the vacuum freeze-drying method, the moisture adhering to and bound to the multi-molecular layer on the food tissue as described above freezes and becomes solid moisture together with free water, and the affinity with the food tissue is almost lost. Is called. This is considered to be the reason why the drying rate does not decrease even in a low moisture region (20 to 30% or less of fresh moisture) in the vacuum freeze-drying method.
[0015]
The inventors of the present invention pay attention to the drying behavior of food as described above, and find that it is possible to perform efficient drying with little alteration of food by appropriately combining non-freeze drying and freeze drying. It was.
[0016]
The present invention has been made on the basis of the above knowledge, and the gist thereof is as follows.
(1) When drying a fresh food or a food prepared by boiling it, it was pre-dried at a temperature of 10 to 40 ° C. until the moisture in the dried product became 10 to 30% of the fresh moisture. Then, it is the drying method of the foodstuff characterized by carrying out vacuum freeze-drying.
[0017]
(2) In addition, the pre-drying, the preceding paragraph and characterized in line Ukoto under a reduced pressure of a water column -30mm~-200mm from atmospheric (1) a method of dry food product according.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the food to be dried includes, for example, fresh vegetables such as carrots, potatoes, pumpkins, green onions, spinach, fresh seafood such as salmon, salmon, salmon, squid, shrimp, scallops, fresh meat or these Foods boiled and cooked.
[0019]
In the method for drying a food according to the present invention, firstly, the material to be dried is preliminarily dried at a temperature at which it does not freeze, that is, a temperature of 0 ° C. or higher, and then vacuum freeze-dried. It is characterized by being in the range of 10 to 30% of moisture.
[0020]
Hereinafter, the reason for limiting the moisture after preliminary drying to this range will be described.
When vacuum freeze-drying is performed in a state where the moisture of the material to be dried is higher than 30% of the fresh moisture, ice pieces of several hundred μm to several mm are formed in the tissue of the food. Since the voids remain after the ice pieces have evaporated, even if the dried food with such voids is returned to the water, the original fresh texture and chewy texture cannot be obtained, resulting in a food flavor. It drops significantly.
[0021]
The present inventors have found that in any food, there is almost no decrease in flavor due to the formation of such pores when vacuum freeze-dried from 30% or less of fresh water. Therefore, in this invention, the water | moisture content in to-be-dried material after preliminary drying shall be 30% or less of the water | moisture content of a fresh state.
[0022]
Further, according to the knowledge of the present inventors, in the preliminary drying (drying in a non-frozen state), the drying rate starts to decrease at a moisture content of 20 to 30% or less of the fresh state, and the fresh state When the water content is close to 10%, the drying rate is remarkably reduced.
[0023]
For this reason, predrying until the moisture in the material to be dried is lower than 10% of the fresh state is not preferable because the drying time becomes excessive and the quality of the food may change due to propagation of various bacteria. Therefore, in this invention, the water | moisture content in to-be-dried material after preliminary drying shall be 10% or more of the water | moisture content of a fresh state. In addition, from the viewpoint of more completely preventing the quality of the food from being changed, it is preferable to keep the moisture in the material to be dried after preliminary drying at a higher value, for example, 15% or more of the moisture in the fresh state.
[0024]
The method of decreasing the drying rate in the low moisture region in the preliminary drying (drying in a non-freezing state) differs depending on the type of food. Therefore, by measuring the speed of pre-drying and vacuum freeze-drying for each food, the former grasps in advance the moisture value in the material to be dried, which is slower than the latter, and from pre-drying to vacuum freeze-drying around this moisture value. If it moves, the whole drying time can be shortened most.
[0025]
In the present invention, it is based on the results of the present inventors that the moisture that shifts from pre-drying to vacuum freeze-drying is determined based on the moisture in the fresh state. This is because it has been found that the moisture value at which the drying rate is reduced and the formation of pores in the food tissue due to ice pieces can be displayed based on the fresh moisture.
[0026]
In addition, the fresh state here is a state in which agricultural products, marine products, etc. are collected and can be traded on the market as fresh products. Usually, vegetables are about 80 to 95% by weight, and seafood is about 60 to 80% by weight. Contains moisture.
[0027]
In the present invention according to the first aspect, the pre-drying condition may be that the temperature of the material to be dried is 10 to 40 ° C. , and the drying method is not particularly limited. Sun drying, cold-air drying, can be applied a variety of methods such as infrared heating drying, the pressure of the atmosphere during drying may also be a reduced pressure at atmospheric pressure.
[0028]
Further, the method and conditions for vacuum freeze-drying are not particularly limited, and a normal method, for example, a material to be dried is frozen at 0 to −40 ° C. and then a freezing vacuum chamber (absolute pressure is, for example, 50 mm or less water column). What is necessary is just to use the method of enclosing in and drying for a predetermined time.
[0029]
In the present invention according to claim 2, after pre-drying from atmospheric pressure under a reduced pressure of -30 mm to -200 mm of water, the moisture of the material to be dried becomes 10 to 30% of the moisture in the fresh state, followed by vacuum freezing. It is characterized by drying.
[0030]
The reason for performing preliminary drying under reduced pressure is to increase the drying rate at room temperature or a temperature close to this. In this case, the higher the degree of vacuum, the higher the drying speed, but on the other hand, the drying equipment becomes expensive and is not preferable. According to the drying system that supplies air to the drying chamber and exhausts the air with high humidity at the same time as shown in the following embodiments, the drying chamber is slightly depressurized, for example, at a reduced pressure of about -30 mm to -200 mm. The drying speed can be increased sufficiently.
[0031]
In the preliminary drying according to the second aspect of the present invention, it is preferable to install a heating means such as an infrared heater and a temperature measuring means such as a radiant heat temperature sensor in the drying chamber. By adopting such a configuration, the temperature of the object to be dried can be reliably controlled to 10 to 40 ° C. to perform preliminary drying.
[0032]
In the invention of claim 2, the upper limit of the temperature of the material to be dried during the pre-drying is set to 40 ° C. If the temperature is higher than this, it is difficult to avoid deterioration of the food or decomposition of useful components due to the action of enzymes in the food. Because. The lower limit of the temperature of the object to be dried is set to 10 ° C. because the drying speed is too slow even under a reduced pressure, and efficient drying becomes difficult.
[0033]
It is also possible to kill germs and the like by installing an ultraviolet irradiation device in the drying chamber and irradiating the material to be dried for a predetermined time or periodically. Also in the present invention of claim 2, the vacuum freeze-drying conditions and method are not particularly limited, and a normal method may be used.
[0034]
In the present invention, when the moisture content of the material to be dried reaches 10 to 30% of the fresh state, the preliminary drying is finished and the process is shifted to the vacuum freeze-drying. Measure the relationship between the drying time and moisture in the material to be dried under the specified drying conditions, and (2) measure the weight of the material to be dried continuously or periodically during drying. It is possible to use various methods such as a method of judging and transferring a moisture change from weight loss.
[0035]
【Example】
A method for drying foods of the present invention was carried out, in which fresh vegetables such as carrots, pumpkins and yamato potatoes were pre-dried under reduced pressure in a drying chamber having a supply / exhaust means and an infrared heater and then freeze-dried.
[0036]
In the example, the moisture in the material to be dried is preliminarily dried to about 20% of the fresh state, and then vacuum freeze-dried.
[0037]
In the preliminary drying, the temperature of the material to be dried was 40 ° C., the atmospheric pressure in the drying chamber was reduced to a water column of −50 mm, and preliminary drying was performed for about 10 hours. Thereafter, the material to be dried was frozen at −15 to 20 ° C., and freeze-dried in a vacuum tank below the freezing point under a vacuum of an absolute pressure of 0.1 kPa or less for about 15 hours. The final dry moisture was 3-5% of fresh state.
[0038]
On the other hand, Comparative Example 1 was a case where the final dry moisture was dried at a temperature not frozen under reduced pressure, and was dried for about 40 hours under the same conditions as the above preliminary drying. In Comparative Example 2, vacuum lyophilization was performed immediately from the fresh state, and vacuum lyophilization was performed for about 60 hours under the same conditions as described above. In Comparative Example 2, the final dry moisture was not reached within the above time.
[0039]
In FIG. 1, the relationship between the drying time and the water | moisture content in a foodstuff in the Example at the time of drying a carrot and the comparative examples 1 and 2 is shown. As can be seen from the figure, in this example, it was found that the time required to reach the final dry moisture can be significantly reduced as compared with Comparative Example 1. Moreover, when drying other vegetables, the same tendency as the above was recognized.
[0040]
FIG. 2 shows a schematic diagram of the carrot tissue after drying in Example and Comparative Example 2 (a sketch of a structure photograph of about 8 times). FIG. 2A shows the case of Comparative Example 2 that was immediately freeze-dried from the fresh state, and there are a large number of pores 2 of several hundred μm to several mm in the food tissue 1. On the other hand, FIG. 2 (b) shows the case of this example, where the food structure 1 is uniform and no large voids as described above are observed.
[0041]
Moreover, when the dried vegetables of Examples and Comparative Examples 1 and 2 were returned to water and their flavors were compared, Comparative Example 1 showed slight alteration and discoloration, and Comparative Example 2 showed changes in texture and texture. On the other hand, in the Examples, there was little alteration and discoloration, and the texture and crunch were closest to the original fresh food.
[0042]
【The invention's effect】
According to the present invention, there is provided a method for drying a food that combines non-freeze drying and vacuum freeze drying to leave the original food flavor with little alteration and discoloration of the food, can be stored for a long time, and has a relatively short drying time. It became possible to provide.
[Brief description of the drawings]
FIG. 1 is a view showing a comparison of carrot drying rates in the present example and a comparative example.
FIG. 2 is a schematic view showing an example of carrot tissues after drying in Examples and Comparative Examples.
FIG. 3 is a schematic diagram showing a comparison of the drying speed of fresh food between a room temperature vacuum drying method and a vacuum freeze drying method.
[Explanation of symbols]
1 Food organization 2

Claims (2)

When drying a fresh food or a food prepared by boiling it, the dried product is pre-dried at 10 to 40 ° C. until the moisture in the dried product becomes 10 to 30% of the fresh moisture, and then vacuum freeze-dried. A method for drying a food, characterized by comprising: The preliminary drying, the drying method of a food according to claim 1, wherein the row Ukoto under a reduced pressure of a water column -30mm~-200mm from atmospheric pressure.

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