Patent Publication Number: US-2012045540-A1

Title: Fermented food refrigerator and control method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 2010-0079581, filed on Aug. 18, 2010 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments relate to a fermented food refrigerator, which measures a state of fermented food and stores the fermented food according to the measured state, and a control method thereof. 
     2. Description of the Related Art 
     Recently, a fermented food refrigerator specifically adapted to fermented food storage has been developed and placed on the market. Such a fermented food refrigerator applies an appropriate temperature to fermented food introduced into storage chambers to ferment the food, and then preserves the fermented food at an appropriate storage temperature so as to store the fermented food for a long time. 
     Heaters generating heat to maintain the appropriate temperature during ripening of the fermented food, and components of a refrigerating system to maintain a regular low temperature state of the fermented food during storage of the fermented food are installed in the fermented food refrigerator. When fresh fermented food is introduced into the fermented food refrigerator, a ripening process in which an appropriate temperature is supplied to the fermented food to ripen the fermented food is carried out. Such a ripening process is a process of maintaining an appropriate temperature to ripen the fermented food using heat generated from the heater. When the ripening of the fermented food through the above process is completed, a process of storing the fermented food is carried out by operating the components of the refrigerating system. Such a storage process is a process of circulating a refrigerant using the refrigerating system to maintain a proper low temperature state so as to prevent fermented food from spoiling. 
     The rate at which fermentation proceeds varies according to temperature. That is, if a temperature is high, the rate at which fermentation proceeds is high and thus food is rapidly ripened, and if a temperature is low, the rate at which food fermentation proceeds is low. In general, long-term fermented food storage is carried out at a low temperature. 
     Further, the freezing point of fermented food varies according to a fermentation degree and salinity of the fermented food. That is, if fermentation proceeds and the fermented food has high salinity, the freezing point of the fermented food is lowered. Therefore, it may be necessary to properly adjust a storage temperature of the fermented food so as to achieve ripening of the fermented food at a low temperature without freezing the fermented food. 
     However, the conventional fermented food refrigerator sets the same storage temperature in all cases, and then changes the storage temperature after a designated time elapses. Therefore, the conventional fermented food refrigerator is unsatisfactory in measuring an actual state of fermented food and varying the storage temperature according to the measured state of the fermented food. 
     SUMMARY 
     Therefore, it is an aspect to provide a fermented food refrigerator, which sets a storage temperature of fermented food according to a measured ripening degree or salinity of the fermented food, and a control method thereof. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
     In accordance with one aspect, a control method of a fermented food refrigerator includes measuring acidity of the fermented food and judging a ripening degree of the fermented food according to the measured acidity, determining whether or not measurement of salinity of the fermented food is performed through the judged ripening degree, and measuring salinity of the fermented food, and adjusting a storage temperature of the fermented food by lowering the storage temperature, if the measured salinity is higher than reference salinity, and raising the storage temperature, if the measured salinity is lower than the reference salinity. 
     The ripening degree may be divided into an initial stage, a middle stage, and a late stage according to the acidity, and it may be determined that measurement of salinity of the fermented food is performed, upon judging that the ripening is in the middle stage. 
     The storage temperature may be set to a reference temperature corresponding to the ripening degree in the initial stage, upon judging that the ripening degree is in the initial stage, and be set to be uniformly lower than the current storage temperature by a designated temperature, upon judging that the ripening degree is in the late stage. 
     In accordance with another aspect, a fermented food refrigerator includes an acidity measurement unit to measure acidity of the fermented food, a salinity measurement unit to measure salinity of the fermented food, and a control unit to judge a ripening degree of the fermented food according to the measured acidity, to determine whether or not measurement of salinity of the fermented food is performed through the judged ripening degree, to measure salinity of the fermented food through the salinity measurement unit, and to adjust a storage temperature of the fermented food by lowering the storage temperature, if the measured salinity is higher than reference salinity, and raising the storage temperature, if the measured salinity is lower than the reference salinity. 
     The ripening degree may be divided into an initial stage, a middle stage, and a late stage according to the acidity, and the control unit may determine that measurement of salinity of the fermented food is performed through the salinity measurement unit, upon judging that the ripening degree is in the middle stage. 
     The fermented food refrigerator may further include a memory unit to store a reference temperature corresponding to the ripening degree in the initial state and a temperature measurement unit to measure the current storage temperature of the fermented food, and the control unit may set the storage temperature to the reference temperature corresponding to the ripening degree in the initial stage stored in the memory unit, upon judging that the ripening degree is in the initial stage, and set the storage temperature to be uniformly lower than the current storage temperature measured by the temperature measurement unit by a designated temperature, upon judging that the ripening degree is in the late stage. 
     In accordance with another aspect, a control method of a fermented food refrigerator includes measuring acidity of the fermented food and judging whether or not a ripening degree of the fermented food according to the measured acidity corresponds to an initial stage, a middle stage, or a late stage, measuring salinity of the fermented food and judging whether or not the measured salinity is high or low according to the judged ripening degree, and adjusting a storage temperature according to the judged salinity. 
     The storage temperature may be adjusted to be higher than a reference temperature corresponding to the judged ripening degree, upon judging that the salinity is low, and be adjusted to be lower than the reference temperature, upon judging that the salinity is high. 
     The storage temperature may be set to be uniformly lower than the current storage temperature by a designated temperature, upon judging that the ripening degree is in the late stage. 
     Upon judging that the ripening degree is in the late stage, the storage temperature may be set to be uniformly lower than the current storage temperature by a first temperature, upon judging that the salinity is reference salinity corresponding to the ripening degree in the late stage, be set to be uniformly lower than the current storage temperature by a second temperature, upon judging that the salinity is lower than the reference salinity, and be set to be uniformly lower than the current storage temperature by a third temperature, upon judging that the salinity is higher than the reference salinity. 
     The second temperature may be lower than the first temperature, and the third temperature may be higher than the first temperature. 
     The salinity of the fermented food may be measured upon judging that the ripening degree in the middle stage. 
     Upon judging that the ripening degree is in the middle stage, the storage temperature may be adjusted to be higher than a reference temperature corresponding to the ripening degree in the middle stage, upon judging that the salinity is low, and be adjusted to be lower than the reference temperature, upon judging that the salinity is high. 
     The storage temperature may be set to a reference temperature corresponding to the ripening degree in the initial stage, upon judging that the ripening degree is in the initial state, and be set to be uniformly lower than the current temperature by a designated temperature, upon judging that the ripening degree is in the late stage. 
     Upon judging that the ripening degree is in the late stage, the storage temperature may be set to be uniformly lower than the current storage temperature by a first temperature, if the salinity measured in the middle stage is reference salinity corresponding to the ripening degree in the middle stage, be set to be uniformly lower than the current storage temperature by a second temperature, if the salinity measured in the middle stage is lower than the reference salinity, and be set to be uniformly lower than the current storage temperature by a third temperature, if the salinity measured in the middle stage is higher than the reference salinity. 
     The second temperature may be lower than the first temperature, and the third temperature may be higher than the first temperature. 
     In accordance with another aspect, a fermented food refrigerator includes an acidity measurement unit to measure acidity of the fermented food, a salinity measurement unit to measure salinity of the fermented food, and a control unit to judge whether or not a ripening degree of the fermented food according to the acidity measured by the acidity measurement unit corresponds to an initial stage, a middle stage, or a late stage, to judge whether or not the salinity measured by the salinity measurement unit is high or low according to the judged ripening degree, and to adjust a storage temperature according to the judged salinity. 
     The fermented food refrigerator may further include a memory unit to store a reference temperature and reference salinity corresponding to the ripening degree in each stage, and the control unit may set the storage temperature to be higher than the reference temperature stored in the memory unit, upon judging that the salinity is low, and set the storage temperature to be lower than the reference temperature, upon judging that the salinity is high. 
     The fermented food refrigerator may further include a temperature measurement unit to measure the current storage temperature of the fermented food, and the control unit may set the storage temperature to be uniformly lower than the current storage temperature measured by the temperature measurement unit by a designated temperature, upon judging that the ripening degree is in the late stage. 
     Upon judging that the ripening degree is in the late stage, the control unit may set the storage temperature to be uniformly lower than the current storage temperature measured by the temperature measurement unit by a first temperature, upon judging that the salinity is reference salinity corresponding to the ripening degree in the late stage, and the control unit may set the storage temperature to be uniformly lower than the current storage temperature by a second temperature, if the salinity is lower than the reference salinity, and set the storage temperature to be uniformly lower than the current storage temperature by a third temperature, if the salinity is higher than the reference salinity. 
     The second temperature may be lower than the first temperature, and the third temperature may be higher than the first temperature. 
     The control unit may control the salinity measure unit to measure the salinity of the fermented food upon judging that the ripening degree in the middle stage. 
     The fermented food refrigerator may further include a memory unit to store a reference temperature and reference salinity corresponding to the ripening degree in each stage, and the control unit may set the storage temperature to be higher than the reference temperature corresponding to the ripening degree in the middle stage stored in the memory unit, if the salinity measured in the middle stage is judged to be low, and set the storage temperature to be lower than the reference temperature, if the measured salinity is judged to be high. 
     The fermented food refrigerator may further include a temperature measurement unit to measure the current storage temperature of the fermented food, and the control unit may adjust the storage temperature to the reference temperature corresponding to the ripening degree in the initial stage stored in the memory unit, upon judging that the ripening degree is in the initial stage, and set the storage temperature to be uniformly lower than the current storage temperature measured by the temperature measurement unit by a designated temperature, upon judging that the ripening degree is in the late stage. 
     Upon judging that the ripening degree is in the late stage, the control unit may set the storage temperature to be uniformly lower than the current storage temperature by a first temperature, if the measured salinity is judged to be reference salinity corresponding to the ripening degree in the middle stage, set the storage temperature to be uniformly lower than the current storage temperature by a second temperature, if the salinity is lower than the reference salinity, and set the storage temperature to be uniformly lower than the current storage temperature by a third temperature, if the salinity is higher than the reference salinity. 
     The second temperature may be lower than the first temperature, and the third temperature may be higher than the first temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a perspective view of a fermented food refrigerator in accordance with one embodiment; 
         FIG. 2  is a control block diagram of a fermented food refrigerator in accordance with one embodiment; 
         FIG. 3  is a flow chart illustrating a control method of a fermented food refrigerator in accordance with one embodiment; 
         FIG. 4  is a graph illustrating variation in storage temperature according to the control method of the fermented food refrigerator of  FIG. 3 ; 
         FIG. 5  is a flow chart illustrating a control method of a fermented food refrigerator in accordance with another embodiment; 
         FIG. 6A  is a graph illustrating variation in storage temperature according to the control method of the fermented food refrigerator of  FIG. 5 ; and 
         FIG. 6B  is a graph illustrating variation in storage temperature according to the control method of the fermented food refrigerator of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  is a perspective view of a fermented food refrigerator in accordance with one embodiment. 
     A fermented food refrigerator  10  in accordance with this embodiment includes a plurality of storage chambers  13  provided in a cabinet  11  forming a box-shaped external appearance of the fermented food refrigerator  10  so as to ripen fermented food and store the fermented food at a low temperature, heaters (not shown) to supply heat to ripen the fermented food, and a refrigerating system (not shown) to lower the temperature of the storage chambers  13  so as to store the fermented food at the low temperature. The refrigerating system includes a compressor, a condenser, and an evaporator. 
     The plurality of storage chambers  13  is prepared in a pair such that the storage chambers  13  are arranged in parallel at both sides of the cabinet  11  and are provided with opened upper surfaces. Doors  12  are hinged to the opened upper surfaces of the storage chambers  13  so as to be vertically rotated, thereby enabling the storage chambers  13  to receive fermented food. Further, a plurality of fermented food storage containers  15  is provided in each storage chamber  13 . 
     The heaters are provided to supply heat of an appropriate temperature to fermented food introduced into the fermented food storage containers  15  so as to ripen the fermented food. The heaters, which are installed to supply heat to the fermented food storage containers  15 , may be provided as a sheet manufactured by covering electric wires integrally formed with a rectangular sheath made of aluminum. 
     An operation unit  14  and a display unit  16  are provided on the front surface of the cabinet  11 . The display unit  16  displays storage temperature and storage time, and the operation unit  14  receives user input to operate the fermented food refrigerator  10 . The operation unit  14  and the display unit  16  may be integrated into a touch screen type, or be provided separately. The display unit  16  may include LCDs, LEDs, or PDPs, but is not limited thereto. The operation unit  14  may include switches, buttons, slide bars, and dials, but is not limited thereto. 
       FIG. 2  is a control block diagram of a fermented food refrigerator in accordance with one embodiment. 
     With reference to  FIG. 2 , a fermented food refrigerator  10  in accordance with this embodiment includes an acidity measurement unit  111 , a salinity measurement unit  112 , a temperature measurement unit  113 , a control unit  120 , a memory unit  130 , a display unit  141 , a heater  142 , and a refrigerating system  143 . 
     The acidity measurement unit  111  includes a sensor to measure acidity of the fermented food. Here, acidity means an acidity level of a solution or an intensity of an acid. In more detail, an acidity level of a solution is generally expressed by pH (hydrogen exponent), and the pH means the concentration of hydrogen ions in  11  of the solution. Further, an intensity of an acid means of an intensity of protons emitted from the acid, when the acid is dissolved in a solvent. 
     That is, the acidity measurement unit  111  measures a pH value or acidity of gas or a fermented food liquid generated from fermented food. 
     Measurement of acidity of fermented food using the acidity measurement unit  111  is carried out through one of methods described below. 
     First, there is a non-contact method in which acidity of fermented food within a storage chamber is judged by sensing gas generated from the fermented food. In the non-contact method, the acidity measurement unit  111  may be installed at the upper portion of the storage chamber proper to sense the gas generated from the fermented food, and may include a semiconductor thin film sensor or a solid electrolyte sensor. 
     Second, there is a contact method in which acidity of fermented food is judged by sensing a concentration of hydrogen ions in a fermented food liquid. In the contact method, since the acidity measurement unit  111  needs to contact the fermented food liquid, the acidity measurement unit  111  may be installed within a storage chamber, and may include a pH sensor. 
     The salinity measurement unit  112  includes a salinity sensor to measure salinity of the fermented food. That is, the salinity measurement unit  112  measures salinity of the fermented food through contact between the salinity sensor and a fermented food liquid or the fermented food. Measurement of salinity of the fermented food using the salinity measurement unit  112  is carried out through one of methods described below. 
     First, there is a refractive index measurement method using a principle in which, when light generated from a laser module is irradiated onto a light-transmitting target article, the light is refracted by the article and a refractive index of the target article varies according to the salinity of the target article. 
     Second, there is a chloride ion (C − ) concentration measurement method in which salinity of a target article is calculated by injecting a reagent containing silver ions (Ag + ) into the target article and then calculating a concentration of chloride ions (Cl − ) through an amount of precipitated silver chloride (AgCl). 
     Third, there is an electric conductivity measurement method in which salinity of a target article is calculated by applying voltage to a pair of electrodes contacting the target article and then measuring current. This method uses a principle in which electric conductivity of the target article is varied according to an ion concentration of the target article. Here, the applied voltage may be generated using a DC power supply or a high-frequency AC power supply. 
     The temperature measurement unit  113  is installed on a storage chamber or a fermented food storage container, thus measuring storage temperature of the storage chamber or the fermented food storage container. Further, the temperature measurement unit  113  transmits the measured temperature as an electrical signal to the control unit  120 . 
     The control unit  120  receives signals from the acidity measurement unit  111 , the salinity measurement unit  112 , and the temperature measurement unit  113 , and then controls the display unit  141 , the heater  142 , and the refrigerating system  143  based on the received signals and a reference temperature stored in the memory unit  130 . 
     Particularly, in this embodiment, the control unit  120  judges a ripening degree of the fermented food based on the acidity measured by the acidity measurement unit  111 . Further, the control unit  120  sets a storage temperature of the fermented food based on the judged ripening degree or the salinity measured by the salinity measurement unit  112 . 
     Hereinafter, operation of the control unit  120  will be described in detail. 
     The control unit  120  judges the ripening degree of the fermented food. The ripening degree of the fermented food may be divided into three stages, i.e., an initial stage, a middle stage, and a late stage, according to the acidity of the fermented food measured by the acidity measurement unit  111 . 
     As the ripening degree of the fermented food transitions from the initial stage to the late stage, fermentation of the fermented food proceeds. Here, when fermentation of the fermented food proceeds, a sour taste of the fermented food becomes strong and thus the acidity of the fermented food is lowered. That is, as the ripening degree of the fermented food transitions from the initial stage to the late stage, the acidity of the fermented food is lowered. 
     In this embodiment, Table 1 below indicates the criteria by which the ripening degree is divided according to acidity. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Acidity (pH) 
                 Ripening degree 
               
               
                   
                 More than 5.2 
                 Initial stage 
               
               
                   
                 5.2~4.8 
                 Middle stage 
               
               
                   
                 Less than 4.8 
                 Late stage 
               
               
                   
                   
               
            
           
         
       
     
     With reference to Table 1, this embodiment defines the ripening degree of the fermented food as the initial stage if the acidity of the fermented food is more than 5.2, defines the ripening degree of the fermented food as the middle stage if the acidity of the fermented food is 5.2˜4.8, and defines the ripening degree of the fermented food as the late stage if the acidity of the fermented food is less than 4.8. For example, if the acidity measured by the acidity measurement unit  111  is 6.0, the ripening degree of stored fermented food is judged as the initial stage. 
     The definition of the ripening degree according to the acidity, as stated in Table  1 , may be obtained experimentally in consideration of storage temperature and storage time of the fermented food, or may be varied by a designer. That is, the definition of the ripening degree may be modified by varying the above-described values of the acidity. Further, the ripening degree of fermented food may be divided into two stages or larger number of stages than three stages. However, the stages of the ripening degree of the fermented food may be set to be in the ranges which a user at least feels a difference between the stages of the ripening degree of the fermented food. 
     The control unit  120  sets the storage temperature based on the ripening degree or the salinity. 
     As the salinity of the fermented food is raised, the freezing point of the fermented food is lowered. Further, the freezing point of the fermented food in the late stage, in which an osmotic action is finished, is lower than that of the fermented food in the initial stage. 
     In this embodiment, the control unit  120 , using the above principle, controls the storage temperature based on the salinity and the ripening degree so as to enable low-temperature ripening of the fermented food so as to achieve long-term fermented food storage without freezing the fermented food. 
     A method of setting the storage temperature through the control unit  120  based on the ripening degree and the salinity will be described later in detail with reference to  FIGS. 3 to 66 . 
     The memory unit  130  stores the criteria to judge the ripening degree of the fermented food according to the acidity of the fermented food and the criteria to judge whether or not the salinity of the fermented food is low or high according to the ripening degree of the fermented food. Further, the memory unit  130  stores reference temperatures corresponding to the ripening degree in the initial stage and the middle stage and values by which the storage temperature is raised or lowered from the reference temperature according to salinity. The reference temperatures, stored in the memory unit  130 , corresponding to the ripening degree in the initial stage and the middle stage may be equal to or different from each other. 
     The display unit  141  displays an operating state of the fermented food refrigerator, various set values, and temperatures. 
     The heater  142  serves to raise the temperature in the fermented food storage container to ripen fermented food, and may be attached to the fermented food storage container. 
     The refrigerating system  143  includes a condenser, a compressor, and an evaporator, and lowers the temperature in the fermented food storage container to store fermented food at a low temperature. 
       FIG. 3  is a flow chart illustrating a control method of a fermented food refrigerator in accordance with one embodiment. 
     First, the acidity measurement unit measures acidity of the fermented food (operation  210 ), and the control unit judges the ripening degree of the fermented food based on the measured acidity (operation  220 ). In more detail, the control unit defines the ripening degree of the fermented food as the initial stage if the measured acidity (pH) of the fermented food is more than 5.2, defines the ripening degree of the fermented food as the middle stage if the measured acidity (pH) of the fermented food is 5.2˜4.8, and defines the ripening degree of the fermented food as the late stage if the measured acidity (pH) of the fermented food is less than 4.8. 
     Further, the control unit judges whether or not the judged ripening degree satisfies a storage temperature adjustment condition, and controls the salinity measurement unit to measure salinity of the fermented food upon judging that the judged ripening degree satisfies the storage temperature adjustment condition. 
     For example, in a case where the fermented food is napa cabbage kimchi in the initial stage, osmotic pressure equilibrium between the kimchi and seasoning is not achieved and thus a salinity difference between the kimchi and the seasoning occurs. Thereby, if the storage temperature is set based on the salinity of the kimchi in the initial stage, an error may be generated. 
     Therefore, the control unit defines the storage temperature adjustment condition as the middle stage, and controls the salinity measurement unit to measure salinity of the kimchi only in the middle stage. 
     That is, the control unit judges whether or not the ripening degree of the fermented food is in the middle stage (operation  231 ). If it is judged that the ripening degree of the fermented food is in the middle stage as a result of the judgment of the ripening degree, the control unit measures salinity of the fermented food (operation  240 ), and judges whether or not the measured salinity is equal to or lower or higher than reference salinity corresponding to the ripening degree in the middle stage (operation  251  and operation  252 ). Table 2 below indicates the criteria to judge whether or not the measured salinity is low or high when the ripening degree of the fermented food is in the middle stage. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 0~2% 
                 Low salinity 
               
               
                 2~3% 
                 Reference salinity 
               
               
                 3~5% 
                 High salinity 
               
               
                   
               
            
           
         
       
     
     With reference to  FIG. 2 , in this embodiment, the reference salinity of fermented food in the middle stage is defined as 2˜3%. In this embodiment, if the measured salinity of the fermented food is 0˜2%, the salinity of the fermented food is defined to be low, and if the measured salinity of the fermented food is 3˜5%, the salinity of the fermented food is defined to be high. 
     Although the values, as stated in Table 2, are obtained through experimentation, the values may be varied by a designer. That is, the judgment as to whether or the measured salinity is high or low may be modified by varying the above values or dividing salinity into a larger number of stages. 
     The control unit adjusts the storage temperature based on the salinity judged through Table 2. 
     As the salinity of the fermented food is raised, the freezing point of the fermented food is lowered. The embodiment discloses a method of adjusting the storage temperature using such a principle so as to enable low-temperature ripening of the fermented food to achieve long-term fermented food storage while preventing freezing of the fermented food. 
     Concretely, upon judging that the salinity of the stored fermented food is equal to the reference salinity in the middle stage (operation  251 ), the control unit sets the storage temperature to the reference temperature in the middle stage (operation  261 ). 
     Further, upon judging that the salinity of the stored fermented food is lower than the reference salinity in the middle stage (operation  252 ), the control unit sets the storage temperature to be higher than the reference temperature in the middle stage (operation  262 ). Here, a value by which the storage temperature is raised from the reference temperature may be in the range of 0.3˜0.5° C. Since the freezing point of fermented food having low salinity is relatively high, the storage temperature of the fermented food having low salinity is set to be higher than the reference temperature corresponding to the ripening degree in the middle stage so as to prevent freezing of the fermented food. 
     Further, upon judging that the salinity of the stored fermented food is higher than the reference salinity in the middle stage (operation  252 ), the control unit sets the storage temperature to be lower than the reference temperature in the middle stage (operation  263 ). Here, a value by which the storage temperature is lowered from the reference temperature may be in the range of 0.5˜1° C. Since the freezing point of fermented food having high salinity is relatively low, the storage temperature of the fermented food having high salinity is set to be lower than the reference temperature corresponding to the ripening degree in the middle stage so as to enable low-temperature ripening of the fermented food to achieve long-term fermented food storage. 
     If the judged ripening degree does not satisfy the storage temperature adjustment condition, the control unit sets the storage temperature based on the ripening degree. That is, the control unit sets the storage temperature without consideration of the salinity, if the ripening degree is in the initial stage or the late stage. 
     Concretely, upon judging that the ripening degree is in the initial stage (operation  232 ), the control unit sets the storage temperature to be reference temperature in the initial stage (operation  264 ). 
     Further, upon judging that the ripening degree is in the late stage (operation  232 ), the control unit sets the storage temperature to be lower than the current storage temperature by a designated temperature and then maintains the set storage temperature (operation  265 ). Further, if fermented food in the late stage is stored directly, the storage temperature may be set to be lower than the reference temperature in the middle stage by a designated temperature and the set storage temperature may be maintained. Here, a value by which the storage temperature is lowered may be in the range of 0.5˜1° C. Using the principle that the freezing point of ripened fermented food in which osmotic action is finished is lower than fermented food in which a ripening degree is low, the ripened fermented food may be ripened at a low temperature. 
     Therefore, the storage temperature of fermented food is controlled based on the ripening degree or the salinity of the fermented food through the control method of  FIG. 3 , thereby achieving long-term fermented food storage through low-temperature ripening and preventing freezing of the fermented food. Further, differing from the conventional control method in which the same storage temperature is determined in all cases and is then changed after a designated time elapses, the control method of the fermented food refrigerator of  FIG. 3  provides a system which senses an actual state of fermented food and changes the storage temperature of the fermented food according to the state of the fermented food. 
     In this embodiment, definition of the ripening degree according to acidity, reference temperatures in the initial and middle stages, and values by which the storage temperature is raised or lowered from the reference temperatures may be modified by a designer, and be varied according to respective systems. 
       FIG. 4  is a graph illustrating variation in the storage temperature according to the control method of the fermented food refrigerator of  FIG. 3 . 
     In  FIG. 4 , a point of time ‘a’ means a point of time when acidity (pH) of fermented food is 5.2 and is located at the boundary between the initial stage and the middle stage, a point of time ‘b’ means a point of time when acidity (pH) of the fermented food is 4.8 and is located at the boundary between the middle stage and the late stage. That is, a section up to the point of time ‘a’ where acidity (pH) of the fermented food exceeds 5.2 corresponds to the initial stage, and a section between the point of time ‘a’ and the point of time ‘b’ where acidity (pH) of the fermented food is in the range of 5.2˜4.8 corresponds to the middle stage. Further, a section after the point of time ‘b’ where acidity (pH) of the fermented food is less than 4.8 corresponds to the late stage. 
     With reference to  FIG. 4 , when the ripening degree of fermented food is in the initial stage, the storage temperature is set to the reference temperature in the initial stage and is maintained during the initial stage. 
     Further, when the ripening degree of fermented food is in the middle stage, the graph is divided into curves  1 ,  2  and  3  according to salinity of the fermented food. That is, in the middle stage, salinity of the fermented food is measured, and the variation aspect of the storage temperature varied according to the measured salinity. 
     The curve  1  represents variation in the storage temperature if fermented food having low salinity of 0˜2% is stored. In more detail, in case of fermented food having low salinity, the storage temperature is raised from the reference temperature in the middle stage by about 0.3˜0.5° C. For example, when the reference temperature in the middle stage is −1° C., the storage temperature of the fermented food is raised to −0.7˜−0.5° C. 
     The curve  2  represents variation in the storage temperature if fermented food having salinity of 2˜3% is stored. In this case, the storage temperature is set to the reference temperature in the middle stage. Although in this embodiment, the reference temperatures in the initial stage and the middle stage are set to be equal to each other, the reference temperatures in the initial stage and the middle stage may be different. 
     The curve  3  represents variation in the storage temperature if fermented food having high salinity of 3˜5% is stored. In more detail, in case of fermented food having high salinity, the storage temperature is lowered from the reference temperature in the middle stage by about 0.5˜1° C. 
     Finally, when the ripening degree of fermented food is in the late stage, the current storage temperatures are respectively lowered by a designated temperature, and the lowered storage temperatures are maintained. That is, the current storage temperatures in the curves  1 ,  2  and  3  are lowered by about 0.5˜1° C. 
     In  FIG. 4 , definition of the ripening degree according to acidity and values by which the storage temperature is raised or lowered from the reference temperatures may be modified by a designer, and be varied according to respective systems. 
       FIG. 5  is a flow chart illustrating a control method of a fermented food refrigerator in accordance with another embodiment. 
     First, the salinity measurement unit measures salinity of a fermented food liquid (operation  310 ), and the acidity measurement unit measures acidity of the fermented food (operation  320 ). Further, the control unit judges the ripening degree of the fermented food based on the measured acidity (operation  330 ), and judges whether or not the measured salinity is low or high (operations  351 ,  352 ,  353 , and  354 ). Further, the control unit controls the storage temperature of the fermented food according to the judged level of the salinity (operations  361 ,  362 ,  363 ,  364 ,  365 , and  366 ). 
     Hereinafter, a process of judging whether or not the salinity of the fermented food is low or high based on reference salinity according to the ripening degree will be described in detail. 
     Tables 3 and 4 below indicate the criteria to judge whether or not the salinity is low or high according to the ripening degree. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                 1~3% 
                 Low salinity 
               
               
                 3~4% 
                 Reference salinity in initial stage 
               
               
                 4~7% 
                 High salinity 
               
               
                   
               
            
           
         
       
     
     With reference to Table 3, in this embodiment, the reference salinity of fermented food in the initial stage is defined as 3˜4%. If the measured salinity of the fermented food is 1˜3%, the salinity of the fermented food is defined to be low, and if the measured salinity of the fermented food is 4˜7%, the salinity of the fermented food is defined to be high. For example, the control unit judges that fermented food is low-salinity fermented food, if the measured salinity of a fermented food liquid in the initial stage is 2%. 
     
       
         
           
               
               
             
               
                 TABLE 4 
               
               
                   
               
             
            
               
                   0~1.5% 
                 Low salinity 
               
               
                 1.5~2.5% 
                 Reference salinity in late stage 
               
               
                 2.5~4.5% 
                 High salinity 
               
               
                   
               
            
           
         
       
     
     With reference to Table 4, in this embodiment, the reference salinity of the fermented food in the late stage is defined as 1.5˜2.5%. If the measured salinity of the fermented food is 0˜1.5%, the salinity of the fermented food is defined to be low, and if the measured salinity of the fermented food is 2.5˜4.5%, the salinity of the fermented food is defined to be high. For example, the control unit judges that the fermented food is low-salinity fermented food, if the measured salinity of a fermented food liquid in the late stage is 1%. 
     The criteria to judge whether or not salinity of fermented food in the middle stage is low or high are the same as in Table 2, and a description thereof will thus be omitted. 
     The values of salinity of the fermented food in the initial stage of Table 3 are higher than the values of salinity of the fermented food in the middle stage of Table 2 by about 1˜2%. These values are obtained in consideration of the fact that salinity of the liquid of the fermented food in the initial stage is higher than that of fermented food, ripening of which has proceeded. 
     Further, the values of salinity of the fermented food in the late stage of Table 4 are lower than the values of salinity of the fermented food in the middle stage of Table 2 by about 0˜0.5%. These values are obtained in consideration of the fact that salinity of the liquid of the fermented food is lowered as ripening of the fermented food proceeds. 
     These values of salinity are obtained through experimentation, and may be varied by a designer. That is, the judgment as to whether or the salinity is high or low may be modified by varying the above values or dividing salinity into a larger number of stages. 
     Hereinafter, a method of controlling the storage temperature of fermented food according to a result of the judgment as to whether or not salinity is low or high will be described. 
     First, the method of controlling the storage temperature of fermented food if the ripening degree of the fermented food is in the initial or middle stage (operation  340 ) will be described. In this case, upon judging that the salinity of the fermented food is equal to the reference salinity in the initial or middle stage (operation  353 ), the control unit sets the storage temperature to the reference temperature corresponding to the ripening degree in the initial or middle stage (operation  364 ). 
     Further, upon judging that the salinity of the stored fermented food is lower than the reference salinity in the initial or middle stage (operation  354 ), the control unit sets the storage temperature to be higher than the reference temperature corresponding to the ripening degree in the initial or middle stage (operation  365 ). Here, a value by which the storage temperature is raised from the reference temperature may be in the range of 0.3˜0.5° C. Since the freezing point of fermented food having low salinity is relatively high, the storage temperature of the fermented food having low salinity is set to be higher than the reference temperature so as to prevent freezing of the fermented food. 
     Further, upon judging that the salinity of the stored fermented food is higher than the reference salinity in the initial or middle stage (operation  354 ), the control unit sets the storage temperature to be lower than the reference temperature corresponding to the ripening degree in the initial or middle stage (operation  366 ). Here, a value by which the storage temperature is lowered from the reference temperature may be in the range of 0.5˜1° C. Since the freezing point of fermented food having high salinity is relatively low, the storage temperature of the fermented food having high salinity is set to be lower than the reference temperature so as to enable low-temperature ripening of the fermented food to achieve long-term fermented food storage. 
     Next, the method of controlling the storage temperature of fermented food if the ripening degree of the fermented food is in the late stage (operation  340 ) will be described. In this case, the control unit varies a temperature by which the storage temperature is lowered from the current storage temperature according to the salinity. Further, if fermented food in the late stage is stored directly, the storage temperature may be lowered from the reference temperature in the middle stage by a designated temperature, which varies according to the salinity. That is, the freezing point of the fermented food is lowered as the salinity of the fermented food is raised, and thus the storage temperature of the fermented food having high salinity in the late stage is lower than that of the fermented food having love salinity in the late stage. 
     When it is judged that the salinity of the stored fermented food is in the reference salinity corresponding to the ripening degree in the late stage (operation  350 ), the control unit sets the storage temperature to be lower than the current storage temperature by a first temperature and maintains the set storage temperature (operation  361 ). Here, the first temperature may be 0.6˜0.8° C. 
     Further, upon judging that the salinity of the stored fermented food is lower than the reference salinity corresponding to the ripening degree in the late stage (operation  352 ), the control unit sets the storage temperature to be lower than the current storage temperature by a second temperature and maintains the set storage temperature (operation  362 ). Here, the second temperature may be 0.5˜0.6° C. 
     Further, upon judging that the salinity of the stored fermented food is higher than the reference salinity corresponding to the ripening degree in the late stage (operation  352 ), the control unit sets the storage temperature to be lower than the current storage temperature by a third temperature and maintains the set storage temperature (operation  363 ). Here, the third temperature may be 0.8˜1.0° C. 
     In this embodiment, the values of the storage temperatures raised or lowered from the reference temperature in the initial or middle stage and the value of the storage temperature lowered from the reference temperature in the late stage are not important. This embodiment provides a process for preventing freezing of fermented food and achieving low-temperature ripening of the fermented food by setting the storage temperature of the fermented food in consideration of the ripening degree and salinity of the fermented food. 
       FIGS. 6A and 6B  are graphs illustrating variations of storage temperature according to the control method of the fermented food refrigerator of  FIG. 5 . 
     In  FIGS. 6A and 6B , definitions of a point of time ‘a’ and a point of time ‘b’ are the same as those in  FIG. 4 , and thus a detailed description thereof will be omitted. 
       FIG. 6A  is the graph illustrating variation in the storage temperature of fermented food, if salinity of the fermented food is uniform while ripening of the fermented food proceeds. For example, the fermented food maintains low salinity or high salinity from the initial stage to the late stage. 
     In more detail, a curve  1  represents a case in which the salinity of fermented food measured while ripening of the fermented food proceeds from the initial stage to the late stage is lower than the reference salinity. 
     In the curve  1 , in the initial stage, the storage temperature of the fermented food is set to be higher than the reference temperature in the initial stage, and the set storage temperature is maintained until the middle stage. Then, when the ripening degree of the fermented food enters the late stage, the storage temperature of the fermented food is lowered from the storage temperature in the middle stage by about 0.5˜0.6° C. 
     A curve  2  represents a case in which the salinity of fermented food measured from the initial stage to the late stage is equal to the reference salinity. 
     In the curve  2 , in the initial stage, the storage temperature of the fermented food is set to the reference temperature in the initial stage, and the set storage temperature is maintained until the middle stage. Then, when the ripening degree of the fermented food enters the late stage, the storage temperature of the fermented food is lowered from the storage temperature in the middle stage by about 0.6—0.8° C. 
     A curve  3  represents a case in which the salinity of fermented food measured from the initial stage to the late stage is higher than the reference salinity. 
     In the curve  3 , in the initial stage, the storage temperature of the fermented food is set to be lower than the reference temperature in the initial stage, and the set storage temperature is maintained until the middle stage. Then, when the ripening degree of the fermented food enters the late stage, the storage temperature of the fermented food is lowered from the storage temperature in the middle stage by about 0.8˜1° C. 
       FIG. 6B  is the graph illustrating variation in the storage temperature of fermented food, if salinity of the fermented food is varied while ripening of the fermented food proceeds. The graph of  FIG. 6B  illustrates a case in which it is judged that salinity of the fermented food measured in the initial stage is equal to the reference salinity in the initial stage but salinity of the fermented food measured in the middle stage is lower or higher than the reference salinity in the middle stage. 
     In more detail, a curve  1  represents a case in which the salinity of fermented food measured in the initial stage is equal to the reference salinity in the initial stage but the salinity of the fermented food measured in the middle stage is lower than the reference salinity in the middle stage. That is, the curve  1  represents a case in which the fermented food is changed into a low-salinity state when the ripening degree of the fermented food is in the middle stage. 
     In the curve  1 , when the ripening degree of the fermented food is in the initial stage, the storage temperature of the fermented food is set to be equal to the reference temperature in the initial stage. Then, when the ripening degree of the fermented food transitions from the initial stage to the middle stage, the fermented food is changed into the low-salinity state and the storage temperature of the fermented food is set to be higher than the reference temperature in the middle stage, and when the ripening degree of the fermented food enters the late stage, the storage temperature of the fermented food is lowered from the storage temperature in the middle stage by about 0.5˜0.6° C. 
     In more detail, a curve  2  represents a case in which the salinity of fermented food measured in the initial stage is equal to the reference salinity in the initial stage but the salinity of the fermented food measured in the middle stage is higher than the reference salinity in the middle stage. That is, the curve  2  represents a case in which the fermented food is changed into a high-salinity state when the ripening degree of the fermented food is in the middle stage. 
     In the curve  2 , when the ripening degree of the fermented food is in the initial stage, the storage temperature of the fermented food is set to be equal to the reference temperature in the initial stage. Then, when the ripening degree of the fermented food transitions from the initial stage to the middle stage, the fermented food is changed into the high-salinity state and the storage temperature of the fermented food is set to be lower than the reference temperature in the middle stage, and when the ripening degree of the fermented food enters the late stage, the storage temperature of the fermented food is lowered from the storage temperature in the middle stage by about 0.8˜1.0° C. 
     Variations of the storage temperature of fermented food according to the control method of the fermented food refrigerator of  FIG. 5  have thus far been described in detail with reference to  FIGS. 6A and 6B . The ripening degree according to acidity and the values by which the storage temperature raised or lowered from the reference temperatures in the initial and middle stages, as shown in  FIGS. 6A and 6B , are not limiting. That is, the ripening degree according to acidity and the values of the storage temperature raised or lowered from the reference temperatures in the initial and middle stages may be modified as long as the variations of the storage temperature are set based on the ripening degree and salinity of the fermented food. 
     For example, although  FIGS. 6A and 6B  illustrate that the reference temperature in the initial stage and the reference temperature in the middle stage are equal, the reference temperature in the initial stage and the reference temperature in the middle stage may be different. 
     Further, although  FIG. 6B  illustrates that only variation in salinity from the initial stage to the middle stage is considered, variation in salinity from the middle stage to the late stage may be also considered. 
     As is apparent from the above description, in a fermented food refrigerator and a control method thereof in accordance with one embodiment, a storage temperature of the fermented food is set according to the measured ripening degree or salinity of the fermented food, and thus the storage temperature is set to the optimum temperature proper for the current state of the fermented food. 
     Further, the storage temperature of the fermented food is lowered or raised from a reference temperature according to the ripening degree or salinity of the fermented food, thereby achieve long-term fermented food storage and preventing freezing of the fermented food. 
     Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.