Method for chemically peeling fruits and vegetables
The present invention relates to a method for chemically peeling fruits or vegetables. 
     Fruits or vegetables are eaten in the fresh state, and further, they are often preserved after processing in the form of bottled, canned and frozen foods. Many fruits or vegetables should be subjected to a peeling treatment when these processed foods are manufactured. 
     As the conventional methods for the peeling treatment for processing of fruits or vegetables, there can be mentioned (1) so-called hot water peeling methods in which the rind or peel is damaged by hot water or steam and is then removed, (2) alkali peeling methods in which a fruit or vegetable is dipped in or showered with an alkali solution, (3) acid peeling methods in which a fruit or vegetable is dipped in an acid solution, (4) a method using an acid and an alkali in combination, (5) enzymatic peeling methods using enzymes, and (6) mechanical peeling methods. The methods (1) are applied to peeling of white peaches, methods (2) are applied to peeling of peaches and apples, the method (4) is applied to peeling of oranges, and the methods (6) are applied to peeling of apples and pears. The methods (3) and (5) have not been put into practical application. 
     The method (1) is defective in that a troublesome operation is necessary for making the ripening degree uniform, and therefore, this method is not suitable for mass production. The methods (2) and (3) are advantageous over the methods (1) in that the treatment can be accomplished in a short time and mass production can be performed conveniently, but they are defective in that because of the strong action of the chemicals used, such disadvantages as surface roughening, collapse of flesh, reduction of the yield by excessive peeling and discoloration of flesh are caused. In the methods (1), the degree of damaging of flesh is lower than in the methods (2) and (3), but in the method (3), since hydrochloric acid is used, corrosion of the equipment cannot be avoided and a large quantity of a neutralizing agent should be used for neutralizing hydrochloric acid at the waste water treatment. Moreover, since waste water which is poor in the chemical coagulating or precipitating property is produced, a chemical pretreatment is necessary, and the BOD (biochemical oxygen demand) load necessary for the biological treatment is still very high and large expenses are required for the waste water treatment. The methods (5) involves a practical problem of a long treatment time, and in some cases, not only the rind but also the flesh is damaged to cause surface roughening and collapse of flesh. The method (6) is defective in that the application field is limited, large expenses are required and a large loss is caused. 
     Furthermore, there has been proposed a method in which a specific surface active agent, that is, sodium alkyl benzene-sulfonate, is used and incorporated at a high concentration (at least 10%) into an alkali aqueous solution, and a method in which a polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether is used for the above-mentioned treatment using an acid and an alkali in combination. In these methods, however, degradation of the quality, reduction of the operation efficiency and decrease of the yield are observed, and problems involved in the conventional methods such as mentioned above are not solved even according to these methods. 
     The present invention has been completed based on the results of our researches made with a view to developing a method in which the foregoing defects involved in the conventional methods can be solved, the yield, quality and operation efficiency can be improved and the processing and waste water treatment can be performed economically and advantageously. 
     More specifically, in order to attain the above object, we investigated oils, fats, surface active agents and the like in the peeling treatment of fruits and vegetables, and we found that if a fruit or vegetable is dipped in an alkali aqueous solution containing specific fatty acids, the peeling treatment can be performed very effectively and the above object can be attained. 
     In accordance with the present invention, there is provided a method for peeling fruits or vegetables which comprises dipping a fruit or vegetable in an alkali aqueous solution containing fatty acids having 10 to 18 carbon atoms and subjecting the fruit or vegetable to the peeling treatment. 
     According to the present invention, an excellent peeling effect can be attained at a low alkali concentration (lower than 3%) at which no substantial peeling effect can be attained when sodium alkyl benzene-sulfonate is used for the alkali peeling treatment or when a polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether is used for the acid-alkali peeling treatment, and furthermore, the treatment time can be remarkably shortened. 
     The fatty acids that are used in the present invention have 10 to 18 carbon atoms. In the present invention, there can be used saturated and unsaturated fatty acids having 10 to 18 carbon atoms, such as capric acid, undecylenic acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid, oleic acid, linoleic acid, linolenic acid and stearic acid. 
     Fatty acids having more than 18 carbon atoms are hardly available and the practical utility thereof is low. Fatty acids having less than 10 carbon atoms, such as valeric acid, caproic acid, caprylic acid and pelargonic acid, have no substantial peeling effect and have peculiar undesirable smells, and they have no practical utility and cannot be used in the present invention. 
     The fatty acid is used in an amount of 0.005 to 1.0% by weight, preferably 0.1 to 1.0% by weight, based on the alkali aqueous solution. Even if the amount is increased beyond this range, no further improvement of the peeling effect can be attained, but bubbling becomes conspicuous and the operation efficiency is reduced, resulting in economical disadvantages. 
     As the fruit that is peeled according to the present invention, there can be mentioned, for example, orange, peach, apple, apricot, loquat, grape, pear, fig and persimmon, and as the vegetable that is peeled according to the present invention, there can be mentioned, for example, tomato, potato, sweet potato, cucumber and radish. 
     In the conventional alkali peeling method using an alkali aqueous solution alone, when peach, apricot or apple is peeled, an aqueous solution having such a high caustic soda concentration as 5 to 30% by weight is employed. In contrast, in the present invention, a sufficient effect can be attained when the alkali concentration is as low as 0.1 to 3% by weight, ordinarily about 0.2 to about 0.5% by weight. 
     According to the present invention, there can be provided a high-quality processed food excellent in the properties of flesh hardness, flavor and luster. Moreover, the ratio of generation of the broken fruit or vegetable can be remarkably reduced, the surface roughening can be prominently moderated, the yield can be increased and the browning speed of the flesh can be remarkably reduced. Still further, the peeling treatment can be performed at a relatively low temperature smoothly and completed in a short time. The waste water from the peeling step can be coagulated and precipitated by calcium hydroxide more easily than the waste water from the conventional acid-alkali peeling step. Accordingly, the disadvantage of the conventional method that because of a high BOD load, the waste water treatment is not promptly performed by the activated sludge treatment method can be eliminated in the present invention. 
     According to the invention, there is also provided a method for peeling fruits or vegetables which comprises dipping a fruit or vegetable in an alkali aqueous solution containing (1) fatty acids having 10 to 18 carbon atoms and (2) at least one non-ionic surface active agent having an HLB value of 6 to 18, which is selected from the group consisting of polyoxyethylene sorbitan fatty acid esters, sucrose fatty acid esters, sorbitan fatty acid esters, glycerin fatty acid esters and propylene glycol fatty acid esters, and subjecting the fruit or vegetable to the peeling treatment. 
     The polyoxyethylene sorbitan fatty acid ester having an HLB value of 6 to 18, that is used in the present invention, is preferably formed by adding 3 to 30 moles of ethylene oxide to a sorbitan fatty acid ester derived from a fatty acid having 8 to 18 carbon atoms. 
     In the sucrose fatty acid ester, sorbitan fatty acid ester, glycerin fatty acid ester and propylene glycol fatty acid ester, that are used in the present invention, the carbon number in the fatty acid portion is 6 to 18, preferably 12 to 18. In short, the carbon number in the fatty acid portion constituting the non-ionic surface active agent having an HLB value of 6 to 18, that is used in the present invention, is preferably 6 to 18. 
     Such polyhydroxyl compound fatty acid ester that is used in the present invention may be a monoester, a diester or a mixture of a monoester and a diester or of a monoester, a diester and a triester. For example, in case of the sucrose fatty acid ester, there may be employed a mixture comprising 35% of a monolauryl ester and 65% of di- and tri-lauryl esters (HLB value=9.0), and a mixture comprising 40% of a monooleyl ester and 60% of di- and tri-oleyl esters (HLB value=9.0). In the case of the sorbitan fatty acid, there may be employed, for example, a mixture comprising 30% of a monolauryl ester and 70% of di- and tri-lauryl esters (HLB value=8.5). 
     In the present invention, a mixture of a non-ionic surface active agent having a low HLB value and a non-ionic surface active agent having a high HLB value can be used, so far as the HLB value of the mixture is in the range of from 6 to 18. For example, there can be used a mixture (HLB value=9.0) comprising equal amounts of sucrose monooleate (HLB value=13.0) and glycerin mono-oleate (HLB value=5.0). 
     As is well-known, the term &#34;HLB&#34; used herein is an abbreviation to the hydrophilic-lipophilic balance. The HLB value is used for selection of an appropriate surface active agent and estimation of the effect thereof. Namely, the surface active agent is an amphiphatic compound having both the hydrophilic and lipophilic groups in the structure thereof, and because of this specific characteristic, the surface active agent exerts a surface-activating capacity. 
     The method proposed by Griffin [J. Soc. Cosmetic Chemists, 5, 249 (1054)] is now ordinarily adopted for calculation of HLB values. For example, in case of polyoxyethylene sorbitan fatty acid esters, sucrose fatty acid esters and the like, an HLB value is calculated according to the following formula: 
     
         HLB=20(1-S/A) 
    
     wherein S stands for the saponification value of the fatty acid ester and A stands for the neutralization value of the fatty acid. 
     As the HLB value is small, the lipophilic characteristic is increased, and as the HLB value is large, the hydrophilic characteristic is increased. In the art, it is known that the following relation is established between the HLB value of a surface active agent and its water solubility. 
     
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HLB Value      Water Solubility                                           
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1-4            not dispersed                                              
3-6            hardly dispersed                                           
6-8            emulsified dispersion                                      
 8-13          semi-transparent or transparent                            
               dispersion                                                 
above 13       transparent solution                                       
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     In the present invention, the non-ionic surface active agent having an HLB value of 6 to 18 is used in an amount of 0.005 to 1.0% by weight based on the alkali aqueous solution. Even if the non-ionic surface active agent is used in an amount exceeding the above range, no further improvement of the peeling effect can be attained, but bubbling becomes conspicuous and the operation efficiency is degraded, resulting in economical disadvantages. 
    
    
     The characters and effects of the method of the present invention will now be described in detail by reference to the following Examples that by no means limit the scope of the invention. In these Examples, all references to &#34;%&#34; are by weight. 
     EXAMPLE 1 
     In this Example, peeling of white peaches is illustrated. 
     White peaches were dipped in hot water for 30 seconds and each fruit was cut into 4 pieces. The cut pieces were subjected to the peeling treatment in a peeling solution maintained at a predetermined temperature for a certain period. After the treatment, the treated fruit pieces were dipped in a 0.05 N aqueous solution of citric acid for 30 seconds and washed with water, and the peeling ratio was examined. The peeling ratio means the ratio of the number of completely peeled fruit pieces to the total number of the treated fruit pieces. The obtained results are shown in Table 1. 
     White peaches treated with the peeling solutions of Sample Nos. 3, 4, 7 and 8 shown in Table 1 were examined with respect to the browning speed, hardness and flavor to obtain the results shown in Table 2. 
     
                                           Table 1                                 
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Sample                     Treatment                                      
                                 Treatment Tem-                           
                                          Peeling                         
No.    Peeling Solution    Time (min)                                     
                                 perature (°C.)                    
                                          Ratio (%)                       
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Present Invention                                                         
1      water containing 0.5% of caustic soda                              
                           2     80       95                              
       and 0.2% of capric acid                                            
2      water containing 0.5% of caustic soda                              
                           2     80       98                              
       and 0.2% of undecylenic acid                                       
3      water containing 0.5% of caustic soda                              
                                 80       100                             
       and 0.2% of lauric acid                                            
4      water containing 0.5% of caustic soda                              
                           2     80       100                             
       and 0.2% of palmitic acid                                          
5      water containing 0.5% of caustic soda                              
                           2     80       98                              
       and 0.2% of oleic acid                                             
Comparison                                                                
6      5% caustic soda aqueous solution                                   
                           3     80       100                             
7      water containing 5% of caustic soda and                            
                           2     80       85                              
       0.2% of sodium dodecyl benzenesulfonate                            
8      0.5% caustic soda aqueous solution                                 
                           2     80       21                              
9      water containing 0.5% of caustic soda and                          
                           3     80       46                              
       0.2% of sucrose monopalmitate                                      
10     water containing 0.5% of caustic soda and                          
                           2     80       28                              
       0.2% of caproic acid                                               
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                       Table 2                                                     
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Sample   Browning                                                         
No.      Speed      Yield (%)  Hardness                                   
                                      Flavor                              
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Present Invention                                                         
3        low        93         good   good                                
4        low        94         good   good                                
Comparison                                                                
7        extremely  79         ordinary                                   
                                      ordinary                            
         high                                                             
8        high       84         good   ordinary                            
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     EXAMPLE 2 
     This Example illustrates peeling of apricots. 
     Apricots (variety: Heiwa) were dipped in a peeling solution maintained at a predetermined temperature for a certain period. 
     After the dipping treatment, the treated apricots were washed with water and the peeling state was examined to obtain the results shown in Table 3. 
     
                                           Table 3                                 
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Sample                  Treatment                                         
                              Treatment Tem-                              
                                       Peeling                            
No.    Peeling Solution Time (min)                                        
                              perature (°C.)                       
                                       Ratio (%)                          
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Present Invention                                                         
11     water containing 3% of caustic soda                                
                        1.5   85       100                                
       and 0.2% of lauric acid                                            
12     water containing 3% of caustic soda                                
                        1.5   85       100                                
       and 0.2% of myristic acid                                          
13     water containing 3% of caustic soda                                
                        1.5   85       95                                 
       and 0.2% of oleic acid                                             
14     water containing 3% of caustic soda                                
                        1.5   85       95                                 
       and 0.2% of capric acid                                            
Comparison                                                                
15     30% caustic soda aqueous solution                                  
                        1.5   85       95                                 
16     3% caustic soda aqueous solution                                   
                        1.5   85       0                                  
17     water containing 3% of caustic soda                                
                        1.5   85       45                                 
       and 0.2% of caprylic acid                                          
18     water containing 3% of caustic soda                                
                        1.5   85       28                                 
       and 0.2% of sucrose oleate                                         
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     Each of the samples treated with the peeling solutions of the present invention and then washed with water had a smooth surface, but each of the samples treated with the comparative peeling solutions and then washed with water had convexities and concavities on the surface. 
     EXAMPLE 3 
     Peach fruits were divided into two pieces, and subjected to the dipping treatment in a peeling solution maintained at 80° C. for a predetermined period. 
     After the treatment, the treated fruit pieces were washed with water and the peeling state was examined to obtain the results shown in Table 4. 
     
                       Table 4                                                     
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Sam-                                                                      
ple                      Treatment Peeling                                
No.  Peeling Solution    Time (min)                                       
                                   Ratio (%)                              
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Present Invention                                                         
5    water containing 0.5% of                                             
     caustic soda and 0.2% of                                             
                         2         100                                    
     oleic acid                                                           
3    water containing 0.5% of                                             
     caustic soda and 0.2% of                                             
                         2         100                                    
     lauric acid                                                          
Comparison                                                                
6    5% caustic soda aqueous                                              
                         2         90                                     
     solution                                                             
7    water containing 5% of caustic                                       
     soda and 0.2% of sodium dodecyl                                      
                         2         98                                     
     benzene-sulfonate                                                    
8    0.5% caustic soda aqueous                                            
                         2         10                                     
     solution                                                             
10   water containing 0.5% of caustic                                     
                         2         22                                     
     soda and 0.2% of caproic acid                                        
19   water containing 0.5% of                                             
     caustic soda and 0.2% of                                             
                         2         41                                     
     polyoxyethylene sorbitan                                             
     oleate                                                               
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     Each of the samples treated by the peeling solutions of the present invention and then washed with water had a smooth surface, but each of the samples treated by the comparative peeling solutions and then washed with water had convexities and concavities on the surface thereof. 
     EXAMPLE 4 
     This Example illustrates the adaptability of the waste water from the peeling step to the waste water treatment. More specifically, the peeling solutions used for the peeling treatment in Examples 1 and 3 were tested with respect to the adaptability to the waste water treatment in the following manner. 
     The peeling solution from the peeling step was mixed with calcium hydroxide so that the concentration of calcium hydroxide was 1%. Then, the solution was agitated and allowed to stand still for 30 minutes to form flocs. Then, the solution was filtered through two piled gauze sheets. 
     The total organic carbon (TOC) was measured by a TOC measurement apparatus manufactured by Toshiba-Beckmann before and after addition of calcium hydroxide and filtration treatment, and the TOC removal ratio was calculated to obtain the results shown in Table 5. 
     
                       Table 5                                                     
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                   TOC                                                    
                             ppm   ppm   re-                              
Sam- Fruit                   before                                       
                                   after moval                            
ple  Treat-                  treat-                                       
                                   treat-                                 
                                         ratio                            
No.  ed      Peeling Solution                                             
                             ment  ment  (%)                              
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3    white   water containing                                             
     peach   0.5% of caustic soda                                         
                             1385  300   78.3                             
             and 0.2% of lauric                                           
             acid                                                         
6    white   5% caustic soda 1515  817   46.1                             
     peach   aqueous solution                                             
5    peach   water containing                                             
                             1425  240   83.2                             
             0.5% of caustic soda                                         
             and 0.2 of oleic acid                                        
7    peach   water containing 5%                                          
             of caustic soda and                                          
                             1759  1122  36.2                             
             0.2% of sodium dodecyl                                       
             benzene-sulfonate                                            
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     EXAMPLE 5 
     In this Example, peeling of white peaches is illustrated. 
     White peaches were dipped in warm water for 30 seconds and each fruit was cut into 4 pieces. The cut pieces were subjected to the peeling treatment in a peeling solution maintained at a predetermined temperature for a certain period. After the treatment, the treated fruit pieces were dipped in a 0.05 N aqueous solution of citric acid for 30 seconds and washed with water, and the peeling ratio was examined. The peeling ratio means the ratio of the number of completely peeled fruit pieces to the total number of the treated fruit pieces. The obtained results are shown in Table 6. 
     
                                           Table 6                                 
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Sample                    Treatment                                       
                                Treatment Tem-                            
                                         Peeling                          
No.    Peeling Solution   Time (min)                                      
                                perature (°C.)                     
                                         Ratio (%)                        
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Present Invention                                                         
20     water containing 0.5% of caustic soda,                             
       0.1% of lauric acid and 0.1% of                                    
                          3     75       100                              
       sucrose mono-oleate (HLB = 13)                                     
21     water containing 0.5% of caustic soda,                             
       0.1% of myristic acid and 0.1% of                                  
                          3     75       100                              
       polyoxyethylene(20) sorbitan oleate                                
       (HLB = 15)                                                         
22     water containing 0.5% of caustic soda,                             
       0.1% of oleic acid and 0.1% of sorbitan                            
                          3     75       100                              
       monolaurate (HLB = 8)                                              
Comparison                                                                
23     5% caustic soda aqueous solution                                   
                          3     85       72                               
24     0.5% caustic soda aqueous solution                                 
                          3     85       11                               
25     water containing 0.5% of caustic soda                              
       and 0.2% of sucrose mono-oleate                                    
                          3     75       38                               
       (HLB = 13)                                                         
26     water containing 0.5% of caustic soda                              
                          3     75       88                               
       and 0.2% of capric acid                                            
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     White peaches treated with the peeling solutions of Sample Nos. 20, 21, 22, 23 and 25 shown in Table 6 were examined with respect to the browning speed, hardness and flavor to obtain the results shown in Table 7. 
     
                                           Table 7                                 
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       Treatment                                                          
             Treatment Tem-                                               
                      Browning                                            
Sample No.                                                                
       Time (min)                                                         
             perature (°C.)                                        
                      Speed Yield (%)                                     
                                  Hardness                                
                                       Flavor                             
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Present Invention                                                         
20     3     75       low   95    good good                               
21     3     75       low   94    good good                               
22     3     75       low   95    good good                               
Comparison                                                                
23     3     85       extremely                                           
                            77    soft bad                                
                      high                                                
25     3     75       high  89    good ordinary                           
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     EXAMPLE 6 
     This Example illustrates peeling of apples. 
     Apples of a variety shown below were dipped in a peeling solution maintained at a predetermined temperature for a certain period. 
     After the dipping treatment, the treated apples were washed with water and the peeling state was examined to obtain the results shown in Table 8. 
     
                                           Table 8                                 
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                                       Treatment                          
Sample                     Variety                                        
                                 Treatment                                
                                       Temperature                        
                                              Peeling                     
No.    Peeling Solution    of Apple                                       
                                 Time (min)                               
                                       (°C.)                       
                                              Ratio (%)                   
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Present Invention                                                         
27     water containing 3% of caustic soda,                               
                           Kokko 3     85     100                         
       0.1% of lauric acid and 0.1% of                                    
                           Star King                                      
                                 3     85     100                         
       sucrose oleate (HLB = 13)                                          
                           Kogyoku                                        
                                 2.5   85     100                         
                           Fuji  2     85     100                         
28     water containing 3% of caustic soda,                               
                           Kokko 3     85     100                         
       0.1% of stearic acid, 0.05% of sorbitan                            
                           Star King                                      
                                 3     85     100                         
       oleate (HLB = 4.3) and 0.05% of poly-                              
                           Kogyoku                                        
                                 2.5   85     100                         
       oxyethylene(20) sorbitan laurate (HLB =                            
                           Fuji  2     85     100                         
       18) (HLB of mixture of two non-ionic                               
       surface active agents being about 11.2)                            
Comparison                                                                
29     30% caustic soda aqueous solution                                  
                           Kokko 3     85     100                         
                           Star King                                      
                                 3     85     99                          
                           Kogyoku                                        
                                 2.5   85     100                         
                           Fuji  2     85     100                         
30     3% caustic soda aqueous solution                                   
                           Kokko 3     85     0                           
                           Star King                                      
                                 3     85     0                           
                           Kogyoku                                        
                                 2.5   85     0                           
                           Fuji  2     85     0                           
31     water containing 3% of caustic soda                                
                           Kokko 3     85     15                          
       and 0.2% of sucrose oleate                                         
                           Star King                                      
                                 3     85     20                          
       (HLB = 13)          Kogyoku                                        
                                 2.5   85     35                          
                           Fuji  2     85     30                          
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     The apples treated with the peeling solutions of Sample Nos. 27, 28, 29, 30 and 31 shown in Table 8 were tested with respect to the browning speed and flavor to obtain results shown in Table 9. 
     
                       Table 9                                                     
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Sample No.    Browning Speed  Flavor                                      
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Present Invention                                                         
27            slightly low    good                                        
28            slightly low    good                                        
Comparison                                                                
29            high            lost                                        
30            slightly high   ordinary                                    
31            slightly high   good                                        
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     EXAMPLE 7 
     This Example illustrates the adaptability of the waste water from the peeling step to the waste water treatment. More specifically, the peeling solutions used for the peeling treatment in Example 5 were tested with respect to the adaptability to the waste water treatment in the following manner. 
     The peeling solution from the peeling step was mixed with calcium hydroxide so that the concentration of calcium hydroxide was 1%. Then, the solution was agitated and allowed to stand still for 30 minutes to form flocs. Then, the solution was filtered through two piled gauze sheets. 
     The total organic carbon (TOC) was measured by a TOC measurement apparatus manufactured by Toshiba-Beckmann before and after addition of calcium hydroxide and filtration treatment, and the TOC removal ratio was calculated to obtain the results shown in Table 10. 
     
                       Table 10                                                    
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                            ppm   ppm                                     
Sam- Fruit                  after after removal                           
ple  Treat-                 treat-                                        
                                  treat-                                  
                                        ratio                             
No.  ed      Peeling Solution                                             
                            ment  ment  (%)                               
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20   peach   water containing 0.5%                                        
             caustic soda, 0.1%                                           
                            1305  300   77                                
             of lauric acid and                                           
             0.1% of sucrose mono-                                        
             oleate (HLB = 13)                                            
23   peach   5% caustic soda                                              
                            1620  956   41                                
             aqueous solution                                             
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