Dump Tank Chemicals
Chemical Combinations to Avoid
A large variety of chemicals can be used to perform different functions in warehouse dump tanks. Not all of the available materials are compatible with each other. It is often difficult to know which chemicals are compatible. The trick is to select and use those needed for specific purposes without creating a witch's brew. A list of substances that go into dump tanks would include water, fruit, heat, disinfectants, flotation agents, acids, buffers, and surfactants. Each of these can potentially impact the others; certain chemical combinations are incompatible.
Here are some chemical combinations to avoid:
Sodium Silicate + chlorine + an acid
If sodium silicate is used with chlorine and acid is added to reduce the pH of the dump tank solution, the sodium silicate will turn into a stiff gel. Dave Sugar (OSU) told me that the same results can be achieved by combining silicate and lignin sulfonate flotation agents. The lignin sulfonate reduces the pH enough to cause the silicate to gel.
Sodium ortho phenyl phenate (SOPP) + acid
This is a combination you might try if you used SOPP for an apple disinfectant and then came across a grower lot with heavy mineral residues. You might be tempted to add acid to reduce the mineral residue. Ortho phenyl phenate (OPP) is a weak acid, which exists in equilibrium between the undissociated free phenol and the dissociated phenyl phenate ion. The free phenol form causes fruit damage. The two forms exist in equal concentrations at a pH of about 10. Diluting commercial formulations of SOPP results in pHs greater than 11. Reducing the pH results in higher levels of free phenol and greater potential for fruit damage.
There is an exception to the statement that SOPP solutions below pH 10 will cause fruit damage. I refer to this exception as the "Orzan paradox." A solution of 0.3% SOPP tetrahydrate in lignin sulfonate at a specific gravity of 1.03 has a pH below 9.5, yet does not cause damage. No one knows just why this is so.
SOPP + heat
Heat increases the activity of SOPP and reduces the safe treatment time. The damage that results from SOPP at high temperatures is typical of SOPP damage. Morrie Smith at Steri-Seal has exposed apples to 0.32% SOPP at various temperatures to determine a safe exposure time (Figure 1). At 62°F, and even at 80°F, no damage occurred after a 60-minute exposure! If the temperature is increased to 90°F, however, the safe treatment time drops to 5 minutes. Based on his experience, Morrie recommends reducing the concentration of SOPP to 0.2 to 0.25% above 70°F. He recommends against using Steri-Seal at all above 80°F.
Lignin sulfonate without SOPP
This use will not cause any damage that I know of. It has been used by some packers on Bartletts that are going to be shipped immediately and where the packer is not concerned with decay. However, most lignin sulfonate materials contain a large amount of sugar. After running a few hundred bins through the dump tank, the tank water is well inoculated with fungal spores and bacteria. The dump tank actually starts to ferment. If the surface is smooth, you can see gas bubbles rising to the surface. Fermentation can reduce the specific gravity because it replaces sugars with alcohol. A disagreeable septic tank odor also is produced.
Lignin sulfonate + SOPP + Asian pears
Someone may be using this combination successfully. However, the only attempt I know of nearly resulted in a disaster. Fortunately, the packer involved tried a few pears to see their response and found that they damaged the skin. Apparently Asian pears have a very fragile skin.
Lignin sulfonate + chlorine
Chlorine reacts with, or is "bound up" by, organic materials. Lignin sulfonates are organic in nature. Chlorine added to the dump tank will react with the lignin and be unavailable to act as a disinfectant. The reactions may, in fact, produce some undesirable chlorinated products.
Chlorine + acid
Acids are no more compatible with chlorine than they are with SOPP. In fact, adding acid to a dump tank containing chlorine can cause a dangerous situation. The forms of chlorine change with changing pH. Chlorine is usually added to dump tanks as a hypochlorite salt. The hypochlorite salt exists in equilibrium with hypochlorous acid:
HOCl <--> OCl- = H+ pKa 7.46.
At a pH of 7.46 the two species exist in equal concentrations. Lowering the pH with acid causes the hypochlorous acid to form chlorine gas, which will escape the dump tank and make the air around the dump tank unpleasant or poisonous:
Cl2 (gas) + H2O <--> Cl- + H+ = HOCl pKa 3.33.
Acidification of the dump tank has certain disadvantages. Use this procedure only when an acid cleaner alone is not removing the mineral residues sufficiently to achieve an adequate surface for waxing. Dump tank acidification is not compatible with either of the two popular disinfectants, chlorine or SOPP. Also, lowering the pH of your dump tank water can promote corrosion.
Acidification of the Dump Tank
Acidification of the dump tank is done to help remove mineral deposits from fruit. The four materials commonly used for acidification are acetic acid, hydrochloric or muriatic acid, and the two commercial formulations, Mineral-X and F-800. Buffering capacity is the ability of a chemical to hold the solution at a particular pH. Figure 2 compares the buffering capacity of 1 mL Mineral-X, 1 mL F-800, 1 mL acetic acid, 1 g citric acid monohydrate, and 1 g 86% phosphoric acid. Citric acid was included because it is an ingredient in Mineral-X. If you selected an acidification agent strictly according to cost of maintaining a pH within a certain range, you could use these values for buffering capacity along with current prices to determine which material has the greatest buffering capacity per dollar.
There are, however, other factors to consider. Acetic acid has the disadvantage of volatility. If you heat the dump tank, acetic acid in the air can make the packing shed smell like a pickle factory. Also, the formulated products may do a better job of cleaning than an acid alone.
Neither muriatic nor hydrochloric acid was included in this figure. Hydrochloric acid differs from these others in that it is a strong acid which completely dissociates in weak solutions. In dump tank conditions it has no buffering capacity. It takes relatively less hydrochloric acid to reach a pH of 3.0, but the pH is less stable than if a weak acid were used. If you want to use hydrochloric acid and maintain a stable pH, use an electronic pH controller. As the pH rises this device senses the change and opens a valve or energizes a pump to add more acid. An adequate pH controller, including the remote sensor, is available for under $600. You could use a system like this with any of the acidification materials.
I compared corrosiveness of the various acidification agents by adding enough acid to a 0.01 N KOH solution to reduce the pH to 3.0. Then I soaked five one-inch pieces of steel wire in the solutions for 5 days. I rinsed the wires in a mild KOH solution, then in water. I weighed the wires before and after the 5-day exposure to determine weight loss. Each bar on the figure is the average of three measurements. In Figure 3, all the acids were corrosive. Do not read too much into this graph, as the experiment did not necessarily duplicate dump tank conditions. If the acids had been diluted to the same concentration or if the wires had not been rinsed with potassium hydroxide, the results probably would be different. What this does show is that any acidification agent is potentially corrosive.
In summary, take care to ensure that chemicals used in dump tanks are not only used in accordance with the label, but are actually physically compatible. If you plan to use acids to remove mineral deposit buffering capacity, consider both cost and corrosiveness.
Gary W. Apel
Michelsen Packaging, Yakima WA
Post Harvest Pomology Newsletter, 7(2): 10-12