Process for producing pregelatinized potato starch for use as a binder in feeds for fish
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for producing pregelatinized potato starch (hereinafter referred to as alpha-potato starch) for use as a binder in feeds for fish, particularly, feeds for eel. 
     In cultivation of fish, for example, eel, alpha-potato starch is commonly incorporated in feeds as a binder. Because of its transparency and higher viscosity, alpha-potato starch is best suited for use as a binder in feeds for eel. However, the potato-starch which is to be thermal gelatinized is a natural product and its quality differs greatly from one lot to another. Furthermore, the quality of the potato-starch has direct effects on the final quality of the alpha-potato starch. As a matter of fact, manufacturers of alpha-potato starch have long found it necessary to maintain the quality of their products by using select and therefore costly starch. 
     The conventional feed for cultivating eel is based on white fishmeal which is supplemented with vitamins or nutrients plus 20-25% of alpha-potato starch as a binder. The eel grower adds 5-10% feed oil and 110-130% water to the feed, kneads them with a mixer, and feeds the eel with the resulting mixture. The alpha-potato starch is required to satisfy the following conditions: (1) it keeps the integrity of the feed in water until the eel has finished its eating; (2) it is easily digestable and retains the viscoelasticity of the feed that assits in the eating by the eel; (3) it inhibits the dissolution of water-soluble additives; (4) it ensures easy removal of the feed from the mixer. The quality of alpha-potato starch largely depends on the quality of white fishmeal, the temperature of water in which the eel is grown, and the temperature of atmosphere. Therefore, actual alpha-starch products have various viscosity characteristics so adjusted as to provide the proper viscoelasticity according to the quality of the fishmeal and the seasonal change. As already mentioned, alpha-starch producers attain this object by using select potato starch. In addition, they control the operating conditions of a conventionally used drum dryer, particularly, the heating temperature and the rotational speed of the drum. However, the change in the operating conditions of the drum could cause a significant drop in the efficiency of the dryer and is not desired. 
     As is well known, the physical properties of gelatinized potato starch vary greatly according to the content of phosphoric acid that forms an ester with the glucose of which the starch is made, as well as to the composition of metallic ions bonded to that phosphoric acid. Phosphoric acid is present in the potato tissue in the form of a potassium salt, but during the starch production, the potassium ion is replaced by divalent calcium and/or magnesium ion in the process water, which reduces the gelatinizing properties of the starch and adversely affects the quality of alpha-starch. The starch slurry also contains free ions that are not bound to the phosphoric acid. Extensive studies have been made on the effects of various free ions on the viscosity characteristics of the starch, but nobody has studied their effects on the quality of alpha-starch. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a process that ensures easy production of alpha-potato starch with the desired quality by adjusting the electrical conductivity of a potato starch slurry. 
     Another object of the present invention is to provide a process that ensures the production of alpha-potato starch on an industrial scale without requiring technical skill and the change in the operating conditions that lead to lower productivity. 
     Other objects and advantages of the invention will become apparent by reading the following part of this specification. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As a result of our studies made on the relation between the free ions present in the starch slurry and the quality of the resulting alpha-starch, we have found that the free ions caused greater effects on the quality of alpha-starch than the composition of metallic ions bound to the phosphoric acid in the starch. We have also found that the determination of the content of free ions can be replaced by an industrially far simpler determination of the electrical conductivity of the starch slurry. The present invention is based on these findings, and provides a process for producing alpha-potato starch for use as a binder in feeds for fish, which comprises adjusting the electrical conductivity of a potato starch slurry to 300μ cm or less, then thermal gelatinizing and dehydrating the same. 
     The potato starch from which the final alpha-starch is produced may be an aqueous suspension of dehydrated potato starch or a starch slurry directly produced from a starch mill or potato processing mill. The electrical conductivity of the starch usually depends on the electrical conductivity of the process water, the time-dependent change of the starch slurry, and the pH of the slurry during storage. A potato starch slurry having an electrical conductivity in excess of 300μ /cm is washed with pure water to reduce its electrical conductivity below 300μ /cm, preferably to a certain value between 10 and 200μ /cm, and the so adjusted slurry is thermal gelatinized and dehydrated with a drum dryer, and subsequently ground into particles. The determination of the electrical conductivity of the starch slurry may be made at a starch concentration in the range of 10 to 45%. The quality of the final alpha-potato starch reflects the electrical conductivity of the starch slurry. With a conductivity of 10-40μ /cm, a soft and easily spreading product is obtained by kneading the alpha-potato starch with white fishmeal. Therefore, the alpha-potato starch is suitable for mixing with a hard meal. If the electrical conductivity of the starch slurry is 100-200μ /cm, the resulting alpha-starch provides hard and highly elastic properties and hence is suitable for mixing with a soft meal. If the electrical conductivity is more than 40μ /cm and less than 100μ /cm, an alpha-starch product which is high both in elasticity and in spreadability is obtained and this is suitable for use with every type of meal. If the starch slurry has an electrical conductivity in excess of 300μ /cm, the resulting alpha-starch is not practical because it is low both in integrity and in viscoelasticity. A starch slurry having an electrical conductivity less than 10μ /cm cannot be adjusted to a suitable value even if it is washed with pure water. 
     The potato starch may be washed with pure water in the presence of an ion-exchange resin or fiber: first, the starch is rinsed in the presence of a cation-exchange resin or fiber, and a starch slurry is separated; then, the so separated slurry is rinsed in the presence of an anion-exchange resin or fiber and the desired starch slurry is obtained. Alternatively, the starch may be rinsed with pure water in the presence of a metallic ion sequestering agent or chelating agent such as citric acid, polyphosphoric acid or ethylenediaminetetraacetic acid, and this technique is also effective in adjusting the electrical conductivity to the desired range. If the potato starch slurry has an undesirably low electrical conductivity, the desired electrical conductivity may be obtained by adding a suitable inorganic or organic salt such as sodium chloride which is neutral and non-toxic and imparts the starch slurry with an adequate electrical conductivity. There is no limitation on the type of the electrolyte except that it is preferably selected by considering the safety features of the final feed. 
     In the prior art technique, the production of alpha-starch of best quality has required adjustments in two aspects: selecting a potato starch with a maximum phosphoric acid content which is combined more with potassium ions than with divalent metallic ions such as calcium and magnesium ions; and properly controlling the operating conditions such as the heating temperature of the drum dryer and the rotational speed of the drum. However, according to the process of the present invention, once the operating conditions are adjusted to an optimum, alpha-starch of the desired quality can be easily obtained by simply adjusting the electrical conductivity of the potato starch slurry. This is certainly a great advantage for commercial operation because, for one thing, adjustment of the operating conditions of the drum dryer which requires a high degree of technical skill is eliminated, and for another, the decrease in productivity that accompanies the change in the operating conditions can be avoided. Needless to say, the present invention does not preclude the production of the desired alpha-starch by means of optimal adjustment of the operating conditions as effected in the prior art technique. 
     The present invention is hereunder described in greater detail by reference to working examples, which are given here for illustrative purposes only and are by no means intended to limit the scope of the invention. 
    
    
     EXAMPLE 1 
     A potato starch slurry having a starch concentration of 35% and an electrical conductivity of 360μ /cm was recovered from process water in the slicing step of the process of producing French fried potatoes. A 10 liter-portion of this slurry was filtered in vacuo, and the resulting starch cake was rinsed with pure water having an electrical conductivity of 0.5μ /cm. The cake was dispersed in pure water of the same conductivity so as to provide a starch concentration of 30% and an electrical conductivity of 110μ /cm. Two more portions of the same starch slurry each measuring 10 liters were washed with an even larger volume of pure water in order to prepare two potato starch slurry sample seach having a starch concentration of 30%. One of them had an electrical conductivity of 50μ /cm, and the other had a conductivity of 20μ /cm. Each of the three samples was fed to the top of a double drum dryer (vapor pressure: 4kg/cm 2  G), and according to the conventional manner, the slurry was thermal gelatinized and dehydrated on the drum. 
     Subsequently, the dehydrated alpha-starch was ground into particles. As a control, a potato starch slurry having an electrical conductivity of 360μ /cm was processed into alpha-starch without rinsing with pure water. 
     The four alpha-starch samples were checked for their viscoelasticity, integrity and organoleptic properties. The viscoelasticity was determined by a conventional method with an emylograph-viscograph of Bravender AG., West Germany. The integrity was evaluated with a mixograph of National Corporation, U.S.A., using a mixture of white fishmeal (24 g), alpha-starch (6 g) and tap water (37.5 ml). The organoleptic evaluation was performed on a sample prepared by kneading with hand a mixture of white fishmeal (80 parts) and alpha-starch (20 parts) in the presence of tap water (120 parts, 20° C.). The results of these evaluations are shown in Table 1. 
     
                       TABLE 1
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Test results       Organoleptic results
Electical
         maximum  results  knead-
conductivity
         B.U. on  with     ing           de-
of starch
         amylo-   mixo-    time          teri-
slurry   graph    graph    (sec.)
                                 remarks oration
______________________________________
110 μ /cm
         1,400    rather   45    hard and
                                         absent
                  slow to        highly
                  attain         elastic,
                  integrity,     suitable
                  but high       for mixing
                  peak           with soft
                  value    meal
 50 μ /cm
         930      quick to 35    high both
                                         absent
                  attain         in elas-
                  integrity      ticity and
                                 in spread-
                                 ability,
                                 suitable
                                 for mixing
                                 with every
                                 type of
                                 meal
 20 μ /cm
         700      low peak 25    soft and
                                         absent
                  value but      easily
                  quick to       spreading,
                  attain         suitable
                  integrity      for mixing
                                 with hard
                                 meal
360 μ /cm
         430      slow to  60    unsuitable
                                         present
                  attain         for use
                  integrity,     as binder
                  poor vis-
                  coelasti-
                  city
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     EXAMPLE 2 
     Dehydrated potato starch powder (3 kg) was mixed with 7,000 ml of pure water having an electrical conductivity of 0.5μ /cm to provide 10 kg of a starch slurry having an electrical conductivity of 75μ /cm. Sodium chloride was added to the slurry in an amount sufficient to provide an electrical conductivity of 120μ /cm. The so adjusted slurry was sprayed onto the top of a double drum dryer (vapor pressure: 6 kg/cm 2  G) in the conventional manner. Without retaining the gelatinized starch on the drum, the starch was momentarily thermal gelatinized and dehydrated. Subsequently, the dehydrated alpha-starch was ground into particles. Another sample was prepared in the same manner except that it was prepared from a starch slurry that was not incorporated with sodium chloride and had an electrical conductivity of 75μ /cm. Still another sample was prepared as in Example 1 from a starch slurry that was rinsed with pure water to have its electrical conductivity adjusted to 25μ /cm. The three samples were subjected to the same tests as in Example 1 and the results are shown in Table 2. 
     
                       TABLE 2
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Test results       Organoleptic results
Elecrical
         maximum  results  knead-
conductivity
         B.U. on  with     ing           de-
of starch
         amylo-   mixo-    time          teri-
slurry   graph    graph    (sec.)
                                 remarks oration
______________________________________
120 μ /cm
         1,050    slow to  50    hard and
                                         absent
                  attain         highly
                  integrity,     elastic,
                  but high       suitable
                  peak           for mixing
                  value    with soft
                                 meal
 75 μ /cm
         1,100    quick to 35    high both
                                         absent
                  attain         in elas-
                  integrity      ticity
                                 and in
                                 spread-
                                 ability,
                                 suitable
                                 for mixing
                                 with every
                                 type of
                                 meal
 25 μ /cm
           820    low peak 25    soft and
                                         absent
                  value but      easily
                  quick to       spreading
                  attain         suitable
                  integrity      for mixing
                                 with hard
                                 meal
______________________________________
 
    
     EXAMPLE 3 
     A potato starch slurry having a starch concentration of 35% and an electrical conductivity of 360μ /cm was recovered from process water in the slicing step of the process of producing French fried potatoes. A 10-liter portion of this slurry was mixed with 50 g of H-form strong acidic cation exchange fiber (2-3 cm long) having an ion exchange capacity of 1 mg eq. per gram (ab. dry basis). The mixture was stirred for 30 minutes and passed through a sieve (100 mesh) to remove the ion-exchange fiber while washing with pure water. The slurry was subsequently mixed with 50 g of OH-form strong basic anion exchange fiber (2-3 cm long) having an ion exchange capacity of 1 mg eq. per gram (ab. dry basis). The mixture was stirred for 30 minutes and passed through a sieve (100 mesh) to remove the ion-exchange fiber while washing with pure water. By this procedure, a potato starch slurry having a starch concentration of 20% and an electrical conductivity of 22μ /cm was obtained. One more portion of the same starch slurry measuring 10 liters was washed with pure water in the presence of cation and anion exchange fibers each weighing 20 g, so as to prepare a potato starch slurry having a starch concentration of 20% and an electrical conductivity of 105 μ /cm. The two samples were fed to the top of a double drum dryer (vapor pressure: 4 kg/cm 2  G), and by the conventional procedure, the slurry was thermal gelatinized and dehydrated on the drum. 
     Subsequently, the dehydrated alpha-starch was ground into particles. As a control, a potato starch slurry having a starch concentration of 20% and an electrical conductivity of 340 μ /cm was processed into alpha-starch by rinsing with tap water in the absence of ion exchange fibers. The three samples were subjected to the same tests as in Example 1 and the results are shown in Table 3. 
     
                       TABLE 3
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Test results       Organoleptic results
Electical
         maximum  results  knead
conductivity
         B.U. on  with     ing           de-
of starch
         amylo-   mixo-    time          teri-
slurry   graph    graph    (sec.)
                                 remarks oration
______________________________________
105 μ /cm
         1,200    rather   40    hard and
                                         absent
                  slow to        highly
                  attain         elastic,
                  integrity,     suitable
                  but high       for mixing
                  peak           with soft
                  value    meal
 22 μ /cm
         720      low peak 25    soft and
                                         absent
                  value,         easily
                  but quick      spreading,
                  to attain      suitable
                  integrity      for mixing
                                 with hard
                                 meal
340 μ /cm
         450      slow to  60    unsuitable
                  attain         for use
                  integrity      as binder
                  poor vis-
                  coelasti-
                  city
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