The present invention relates to the field of preservative treatments of grains during storage, and more particularly, to treatments applied to grains containing more than about 15% moisture, to prevent microbial spoilage which might otherwise result, as for example due to the growth of various fungi, molds, etc.
It is often advantageous to store grains such as corn, wheat, sorghum and the like after harvest and prior to their eventual conversion into products such as flour, molasses, etc. Unfortunately, without some form of treatment after harvest, most grain will spoil, particularly when stored in semi-closed conditions, such as grain storage bins, elevators, etc. After harvest, most grain contains approximately 20% to 25% moisture. This moisture content is sufficient to promote microbial growth, such as fungi and mold growth, which may very rapidly cause spoilage of the stored grain.
One treatment for preserving grain during storage which has achieved considerable success is the rapid drying of grain through the application of heat, such as hot air, which reduces the moisture content of that grain to below about 15%. At this moisture content, grain may be stored for extended periods of time while retaining suitable odor, flavor, and color characteristics. Unfortunately, this technique normally requires the consumption of substantial amounts of energy which is used to provide the heat necessary for quick drying of grain to be preserved in this manner. Accordingly, a substantial need has arisen for alternative preservative treatments which may be used either alone or in combination with ambient air drying techniques, to thereby provide an energy efficient method of treating grain for storage.
Another treatment for preserving grain which has achieved some commercial success is the use of propionic acid. This treatment normally entails the provision of less than 2%, usually about 1%, by weight of propionic acid to the grain to be treated. While this method is effective for inhibiting microbial growth, grain treated with these amounts of propionic acid has an objectionable taste and color, and therefore may not be sold for human consumption. Nonetheless, under E.P.A. regulations, grain containing not more than 2% propionic acid can be sold for animal fodder. It is estimated that propionic acid treatment is not used on more than about 1% of the grains stored in the United States.
It has also been suggested to treat grains during storage with various gases. According to one process, ammonia gas is slowly "trickled" through grain to be treated. Unfortunately, this treatment discolors the treated grain, producing a darkened product. Accordingly, grain treated with an ammonia gas trickle process is permitted only for animal consumption.
Another gas which has been suggested for use in preserving grains is sulfur dioxide. Not surprisingly, sulfur dioxide gas, while exhibiting preservative effects, may produce some odor and flavor problems in the treated grain. Additionally, treatments with sulfur dioxide gas are extremely corrosive on storage equipment, presumably as the result of sulfurous and sulfuric acids which are formed within the moist treatment environment.
The above described treatments, and other chemical materials, have been described in various publications as exhibiting preservative or pesticidal effects:
For various patents referring to the use of gases, such as ammonia gas or sulfur dioxide gas, please see U.S. Pat. No. 1,777,044 (Legendre) (Grain preserved through treatment with ammonia gas, sodium carbonate or alkaline-reacting gas by adjusting the hydrogen ion concentration of the adherent moisture to a point within the range pH 7-10.); U.S. Pat. No. 3,928,577 (Kochurova et al.) (vegetable food products, flower bulbs, plants, etc. preserved with tablets containing potassium metabisulfite, gelatin or starch, stearic acid and salts thereof which are described as replacing use of sulfur dioxide, potassium metabisulfite, sulfurous acid and sodium bisulfite prior art techniques); U.S. Pat. No. 1,524,494 (Warth) (sulfur dioxide or other gases such as hydrocyanic acid gas, carbonic acid gas, and chlorine disclosed as treatments for composition cork to destroy or prevent the development of mold spores, bacteria, fungus growths, yeasts, insect larvae, etc.); U.S. Pat. No. 2,132,786 (Hockenyos) (sulfur dioxide intermixed with carbon dioxide for use as fumigant for carpet beetles, moths, bedbugs, etc.); U.S. Pat. No. 2,912,793 (Stone et al.) (soil treatment to reduce nematode populations comprising injection of sulfur dioxide substantially below the surface of the soil followed by a soil pH adjustment by application of an alkaline fertilizer, such as anhydrous ammonia, calcium oxide, etc.).
Organic acids, such as decanoic, undecylenic, and acetic acids, have also been suggested in the literature as treatments for storage grains:
In U.S. Pat. No. 3,962,475 a method is disclosed for preserving high moisture content agricultural grains comprising treating grains with, (1) a composition consisting essentially of an organic food-grade acid or phosphoric acid, (2) a water soluble, noncorrosive, nontoxic, alkali or alkaline earth metal sulfate salt, and (3) a synthetic organic cationic or anionic surfactant for enhancing the penetration of said acid into said grains. In accordance with this disclosure, exemplary organic acids include propionic and acetic acids, while exemplary alkali or alkaline earth metal sulfate salts particularly include such sulfate salts as sodium and potassium sulfates.
In U.S. Pat. No. 3,404,987 (Kooistra et al.), a preserving agent, such as propionic acid, sorbic acid, benzoic acid (and its methyl- and ethyl esters) and a potentiating agent, typically an edible mineral salt, such as the phosphates, carbonates, chlorides, nitrates, sulfites, pyrophosphates and hydroxides of iron, manganese, zinc, tin and silver, are disclosed as being effective in inhibiting microbial growth in food systems. In this patent, the cooperative effect between the preserving agent and potentiating agent is described as exhibiting outstanding activity against microorganisms.
Another chemical which has been reported in the literature as exhibiting a preservative effect is ammonium bisulfite. In Chemical Abstracts, Vol. 87, No. 19, p. 451 (1977) preservation with 0.4% ammonium bisulfite (NH.sub.4 HSO.sub.3) of oat-pea, corn, clover-timothy, or alfalfa silage containing 72-84.6% water was described as improving feed quality. Preservation was described as increasing the soluble sugar, nitrogen and lactic acid content of the feeds and of preventing butyric acid formation. Good results were also reported by mixing corn with straw (5:1, 67.3% moisture) and treating the mixture with 0.4% ammonium bisulfite. Similarly, in Chemical Abstracts, Vol. 66, No. 7, p. 2616 (1967) (Abstract 27779K) selected preservatives were tested to determine the pH of a 1% solution, acidity or buffer value, and the preserving properties of selected preservatives. The preserving capacity was determined by the degree of suppression of germination of moist grain, the growth of mold, the activities of oxidative-reductive, proteolytic, and amylolytic enzymes. Thiourea, ammonium bisulfite (NH.sub.4 HSO.sub.3 ), a mixture of urea and NH.sub.4 pyrosulfate, and ammonium bisulfate (NH.sub.4 HSO.sub.4) were described as good preservatives which enriched green matter with nitrogen and sulfur. Other ammonia containing compounds were described as weak preservatives. Of these compounds, ammonium bisulfite, ammonium bisulfate, and ammonium chloride (NH.sub.4 Cl) were recommended for further study and industrial tests. Finally, in Chemical Abstracts, Vol. 72, No. 1 (1970), p. 203 (Abstract No. 2303m) various sulfur preparations were described as preserving green fodder. Among these, liquid sulfur dioxide, NaHS.sub.2 O.sub.3 and an 80% solution of NH.sub.4 SO.sub.3 were tested at given doses. Upon comparison with "conventional methods", strong decomposition of all soluble carbohydrates was inhibited, the latent phase of fermentation was prolonged, total traceable acidity was decreased, and the formation of volatile and non-volatile organic acids was suppressed. The resultant silage was considered to be superior to conventional silages in organoleptic properties, as well as in maintenance of its structure. Preservation with sulfur preparations was described as reducing the loss of nutritive substances by 50%.
It is known to commercially prepare ammonium bisulfite by bubbling ammonia and sulfur dioxide into water, forming an essentially 100% yield of pure ammonium bisulfite solution having a pH of about 5.5. Aqueous solutions of 47-50 weight percent of ammonium bisulfite are thus readily available. Upon extended storage, particularly when exposed to air, ammonium bisulfite is known to spontaneously undergo various "disproportionation" reactions. For various discussions of these disproportionation reactions, their kinetics, and the products which result therefrom, please refer to the following articles, each of which are hereby incorporated by reference:
Landrooth et al, Thermodynamics, Vol. 88, 1976, p. 427, "Thermodynamics of the reaction of ammonia and sulfur dioxide in the presence of water vapor";
Zelionkaite et al, Chemical Abstracts, Vol. 76, 1972, p. 329, "Decomposition of Ammonium hydrosulfite solutions under the action of thiosulfate";
Scargill, Air Pollution and Industrial Hygiene, Vol. 75, 1971, p. 167, "Dissociation constants of anhydrous ammonium sulfite and ammonium pyrosulfite prepared by gas phase reactions";
Chertkov, Chemical Abstracts, Vol. 53, 1959, Cols. 22770-22771, (citing Zhur. Priklad. Khim. 32, 1695-1707, 1959), "Kinetics of the autodecomposition of ammonium bisulfite-sulfite solutions";
Chertkov, Zhur. Priklad. Khim. 32, 1732-1742, 1959, "Kinetics of Spontaneous Decomposition of Ammonium Sulfite-Bi-sulfite Solutions";
Najbar et al, Catalyst, Kinetics, Vol. 78, 1973, p. 309, "Kinetics and stoichiometry of the heterophase reaction sulfur dioxide with ammonia";
Hisatsune, Chemical Abstracts, Vol. 83, 1976, p. 600, "Infrared spectroscopic study of the ammonia-sulfur dioxide-water solid state system";
Mizoguchi et al, Bulletin of the Chemical Society of Japan, Vol. 49(1), 1976, pp. 70-75, "The Chemical Behavior of Low Valence Sulfur Compounds. X.sup.1) Disproportionation of Thiosulfate, Trithionate, Tetrathionate and Sulfite under Acidic Conditions";
Chertkov et al, Soviet Chemical Industry (English Translation), Vol. 49(6), 1973, pp. 383-387, "Spontaneous Decomposition of Concentrated Ammonium Sulfite-Bisulfite Solutions";
Encyclopedia of Chemical Technology, Vol. 14, pp. 90-91, edited by Raymond E. Kirk and Donald F. Othmer, published by The Interscience Encyclopedia, Inc., New York, "Thionic Acids";
Goehring et al, Zeitschrift fuer anorganische und allgemeine Chemie, Vol. 263, 1950, pp. 138-144, "Ueber die Einwirkung von Schwefeldioxyd auf Ammoniak";
M. Goehring, Ergebnisse and Probleme der Chemie der Schwefelstickstoffverbindungen, Akademie Verlag, Berlin, 1957.
These "disproportionation" reactions apparently cause ammonium bisulfite to be oxidized and disproportionated into several other compounds, which contribute to a solution exhibiting a lowered pH. Analysis of ammonium bisulfite which had aged for several years indicated that up to 20-30% may have been converted to ammonium bisulfate, and minor amounts of thiosulfate, metabisulfite, dithionate, imidodisulfonate, and various polythionates. In addition to a lowered pH, a disproportionated solution of ammonium bisulfite exhibits a yellow-greenish color which is not exhibited by a "fresh" (undisproportionated) aqueous solution of ammonium bisulfite.
As seen from the above, while many efforts have been made to provide preservatives which are suitable for use in preserving grains, no chemical preservative has yet been found which provides an efficient, low cost method of preserving grains, while maintaining or enhancing the color, odor and flavor characteristics of those grains so that the treated grains remain suitable for human consumption.