Method for reducing sodium content and simultaneously increasing potassium content of a food

The sodium content of a food is reduced and the potassium content simultaneously increased by a process wherein an aqueous solution of a sodium-containing food and an aqueous solution containing potassium ions are circulated on opposite sides of a cation exchange membrane for a time and at a rate sufficient to exchange at least a portion of the sodium and potassium ions.

BACKGROUND OF THE INVENTION 
This invention relates to and has among its objects the provision of a 
novel method for reducing the sodium content of a food and simultaneously 
increasing the potassium content by use of a cation exchange membrane. 
It is estimated that in the United States alone over 25 million people 
suffer from hypertension and its related problems. Although the role of 
salt (sodium chloride) in hypertension remains contraversial, a number of 
scientific studies have shown a positive correlation between salt intake, 
systolic blood pressure and the incidence of hypertension. In its 1980 
report, the National Research Council (Toward Healthful Diets, Food and 
Nutrition Board, National Academy of Sciences, Washington, D.C., pp 12-13 
(1980)) recommended that the American population reduce its average daily 
salt intake from 10 to 3 grams per day. 
It has been suggested that a diet high in potassium may be helpful 
particularly in high salt diets. In animal studies in which animals were 
fed extra potassium chloride in a diet containing salt at levels which 
ordinarily produced high levels of hypertension, the extra potassium had 
the effect of ameliorating the hypertension developed. When extra 
potassium chloride was added to a diet containing 5.6 per cent sodium 
chloride, the level of hypertension was not changed, but improvement of 
the animals' life span resulted. Presently, human studies regarding the 
effect of potassium are inconclusive, although some studies suggest that 
increased potassium has therapeutic effects. The National Research Council 
(Recommended Dietary Allowances, Ninth Edition, National Academy of 
Sciences, Washington, D.C. pp 169-177 (1980)) recommended a potassium 
intake for adults of between 1.9-5.6 grams/day with a sodium /potassium 
ratio of 1/1.7. 
Consumer concern over salt in foods has caused some food processors to 
voluntarily decrease the amount of sodium directly added to foods or to 
decrease the amount which may be incorporated indirectly during 
processing. In some foods, however, such as fermentation sauces (e.g., soy 
sauce, tamari sauce, oriental fish sauce, pickling brine), a high salt 
concentration is required to effectively exclude undersirable organisms 
during production and thus a product having a high salt content is 
inherently produced. Several methods have been tried to reduce the sodium 
content of these foods. For example, Japanese Pat. No. 77,148,699 
discloses dialysis of soy sauce across a semipermeable membrane which 
separates substances based on different molecular size to reduce the 
sodium chloride content from 18 to 9 percent. This method has the 
disadvantage that some protein, carbohydrates and flavor components are 
removed by the process. Japanese No. 72 46,360 discloses the production of 
a reduced-sodium soy sauce by electrodialysis wherein cation- and 
anion-exchange resin membranes are placed alternately in a bath, and soy 
sauce is introduced in alternating compartments. Disadvantages of this 
method are the requirement of electric power to drive the process and the 
production of undesirable flavor or other substances at the electrodes. 
Some efforts have been made to reduce the salt content of high salt foods 
such as soy sauce using cation exchange resins. The disadvantages of this 
method include losses of solids due to sorption of non-polar substances 
onto the resin (inbibition losses) and losses due to liquid remaining 
between the resin beads (interstitial losses). 
No method has been disclosed for both reducing the content of sodium while 
at the same time increasing potassium content of sodium-containing foods. 
SUMMARY OF THE INVENTION 
I have discovered a process wherein the sodium content of a food is reduced 
and the potassium content is simultaneously increased. In my process, an 
aqueous solution of a sodium-containing food wherein at least a portion of 
the sodium is ionized and an aqueous solution containing potassium ions 
are circulated on opposite sides of a membrane which is selectively 
permeable to cations (cation exchange membrane) for a time and at a rate 
sufficient to exchange at least a portion of the sodium ions in the food 
solution with potassium ions in the potassium solution. The process is 
continued until the desired replacement of sodium is achieved or 
equilibrium between the two solutions is reached. 
The primary object of the invention is to provide an economical and 
efficient means for reducing the salt content of a food and simultaneously 
increasing its content of potassium, to provide a more healthful product 
which is lower in sodium and higher in potassium than the initial food. 
Another object of the invention is to minimize the loss of protein, 
carbohydrates and flavor components or production of undesirable 
substances which occur in the previous methods to reduce salt content. 
A further object of the invention is the elimination of the problem of 
imbibition and interstitial losses which occur when ion exchange resins 
are used to reduce salt content of a food. 
Further objects and advantages of the invention will become readily 
apparent from the following description. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the method of the invention, an aqueous solution of a sodium-containing 
food wherein at least a portion of the sodium is ionized and an aqueous 
solution containing potassium ions are simultaneously circulated on 
opposite sides of a membrane which allows cations to pass through but 
which substantially excludes the passage of anions or other substances 
through the membrane (cation exchange membrane). The circulation of the 
solutions is carried out at concentrations and for a time and at a rate 
sufficient to allow the passage of at least a portion of the sodium ions 
from the food solution through the membrane to the side containing the 
potassium ion solution and to allow passage of potassium ions in the 
potassium solution to the other side of the membrane thereby resulting in 
the reduction of the sodium content and increase in the potassium content 
of the food solution. The process is continued until the desired 
replacement of sodium in the food solution with potassium is achieved 
(i.e., sodium is decreased and potassium is increased the desired amounts) 
or until equilibrium between the solutions is reached. 
In this process, called Donnan dialysis, the cations will redistribute 
between the solutions until the following condition of equilibrium is 
reached 
##EQU1## 
wherein Na.sup.+.sub.FS and K.sup.+.sub.FS concentrations of sodium ions 
and potassium ions, respectively, in the food solution and Na.sup.+.sub.KS 
and K.sup.+.sub.KS are the concentrations of sodium ions and potassium 
ions in the potassium solution. As can be seen, the driving force of the 
process is the inequality of the two ratios. Thus to have the exchange 
occur, the ratio of sodium ion concentration in the food solution to the 
sodium ion concentration in the potassium solution must not equal the 
ratio of potassium ion in the two solutions. (For more details on Donnan 
dialysis and Donnan membrane equilibrium, see "Concentration and 
Separation of Ions by Donnan Membrane Equilibrium," by R. M. Wallace, I & 
EC Process Design and Development, Volume 6, No. 4, pp 423-431 (1967). 
Examples of membranes which are selectively permeable to cations include 
cation exchange membranes comprising sulfonated polymerizates of polyvinyl 
aromatic compounds as described in U.S. Pat. No. 2,731,411, which is 
hereby incorporated by reference and perfluorinated ionomer membranes 
described in Perfluorinated Ionomer Membranes, Eds. A. Eisenberg and H. L. 
Yeager, American Chemical Society, Washington D.C. (1982), which is hereby 
incorporated by reference. Such membranes include perfluorosulfate cation 
exchange membranes such as those sold under the tradename Nafion by E. I. 
duPont de Nemours and Company, perfluorocarbon ion exchange membranes 
composed of sulfonic acid and carboxylic acid groups such as those sold 
under the tradename Neosepta-F by Tokuyama Soda Company, and 
perfluorocarboxylate membranes such as Flemion made by Ashahi Glass 
Company, Limited. Examples of other cation exchange membranes will be 
readily apparent to those in the art. 
The process may be carried out over a broad temperature range so long as 
the solutions remain in the liquid phase and the temperature does not 
adversely effect the food product quality. In general, room temperature is 
preferred so that energy input required to vary the temperature above or 
below room temperature is avoided. 
The invention may be practiced over a broad range of flow rate. Optimum 
flow rate in each case will depend on such process parameters as the 
geometry of the flow path, mechanical design, flow path length, whether 
the process is continuous or batch, and the like. In general, the flow 
rate should be slow enough to allow transfer of cations across the 
membrane and fast enough to minimize concentration polarization of the 
membrane surface. 
The preferred pH of the solutions is the natural pH of the food solution. 
In this way, hydrogen ions do not pass through the membrane and the pH of 
the solutions is not changed during processing. 
The process may be carried out continuously or batchwise. In either case, 
it is preferred that the food and potassium ion solutions flow 
countercurrent to one another to achieve the most efficient exchange of 
ions. 
After the completion of the desired replacement of sodium with potassium in 
the food solution, the solution is used as is, concentrated or dried 
depending on the type of food product desired. For example, as shown in 
the following examples, soy sauce which has been treated in accordance 
with my method may be used without further concentration. 
The potassium ion "waste solution" which is obtained after treatment of a 
food and which contains sodium chloride, potassium chloride and possibly 
some volatiles from the food solution may be used in the production of a 
sodium-potassium containing food. For example, the "waste solution" 
resulting after soy sauce has been treated by my method can be used in the 
production of a reduced sodium, increased potassium, chemically hydrolyzed 
soy sauce. 
The following examples are given to further illustrate the invention and 
are not intended to limit the scope of the invention which is defined by 
the claims.

EXAMPLE 1 
A countercurrent flow membrane device was set up with 10 lucite plates, 
each with flow channels that exposed 60 cm.sup.2 of active membrane, for a 
total of 600 cm.sup.2 of active membrane. The plates and membranes were 
17.8 cm by 17.8 cm, and the flow channel was 1 cm wide and cut in a 
sinuous path. The membrane used was a Modacrylic (copolymer of 
vinylchloride and acrylonitrile) fiber reinforced cation-transfer membrane 
61CZL386 made by Ionics, Inc of Waterton, Mass. as described in U.S. Pat. 
No. 2,731,411. It has a thickness of 0.6 mm and has been approved by the 
FDA for food use. 
A potassium chloride solution (1600 grams of KCl in 6 liters of water) was 
circulated through the device countercurrent to 400 ml of soy sauce 
(initial NaCl content, 16.90 percent (wt/vol)) both at a rate of 
approximately 300 ml per minute. Samples were taken for determination of 
sodium and potassium at various intervals over an 85-hour circulation 
period. The sodium chloride content of the sauce was reduced to 6.4 
percent after 21/4 hours, to 4.8 percent after 7 hours and to 2.1 percent 
after 85 hours. The results are tabulated in the following table: 
______________________________________ 
Soy Sauce 
Running 
Sodium Potassium Potassium Chloride Soln 
Time Chloride Chloride Sodium Chloride 
(hours) 
(%, wt/vol) 
(%, wt/vol) 
(%, wt/vol) 
______________________________________ 
.sup. 0.sup.a 
16.90 2.00 -- 
0.50 16.74 3.42 -- 
2.25 6.42 15.44 0.85 
7.00 4.82 17.50 1.01 
31.10 3.70 19.78 1.21 
61.50 2.56 20.66 1.36 
62.83 2.36 21.00 1.40 
84.50 2.13 21.84 1.48 
______________________________________ 
.sup.a Input soy sauce 
EXAMPLE 2 
A countercurrent flow membrane was set up with a single piece of a 
perfluorosulfonic acid polymer cation exchange membrane sold under the 
tradename Nafion N-125 by E. I. du Pont de Nemours and Company. The 
membrane thickness was 0.13 mm; 316 cm.sup.2 of active membrane was 
exposed. One liter of soy sauce (initial NaCl content, 18.28 percent 
(wt/vol)) was run continuously and countercurrent against 4 liters of a 
potassium chloride solution (1067 grams of KCl per 4 liters of water). The 
flow rate on both sides of the membrane was approximately 550 ml/min. The 
content of sodium chloride in the soy sauce was reduced to 11 percent 
(wt/vol) after 114 hours. The results are tabulated below: 
______________________________________ 
Soy Sauce 
Running 
Sodium Potassium Potassium Chloride Soln 
Time Chloride Chloride Sodium Chloride 
(hours) 
(%, wt/vol) 
(%, wt/vol) 
(%, wt/vol) 
______________________________________ 
0.0.sup.a 
18.28 1.09 0.24 
1.0 16.52 1.20 0.39 
2.25 16.32 1.39 0.40 
3.25 16.27 1.54 0.41 
18.00 15.96 2.48 0.69 
26.00 14.62 3.16 0.74 
42.00 13.62 3.54 0.88 
79.25 12.45 6.29 1.33 
98.00 11.84 6.58 1.51 
114.00 11.13 7.02 1.64 
______________________________________ 
.sup.a Input soy sauce