WSU Tree Fruit Research & Extension Center

Postharvest Information Network

Sunday, March 24, 2019

WSU-TFREC/Postharvest Information Network/Chemical Safety: What You Can Mix and What You Can't

Chemical Safety: What You Can Mix and What You Can't


This talk is not about a list of specific chemicals and their restrictions in combination. The problem with a list such as that is that it becomes exceedingly complex because of the many physical variables involved and because of the changing availability of the chemicals being used, compounded by the changing rules and regulations under which we operate. I will, however, mention some of that type of information, using it to show the results of actual instances of chemical misuse. I hope that this will illustrate the complexity of the problems of some of the possible chemical combinations in today's warehouse operations.

I will attempt to increase your understanding of some of the complexities in combining chemicals and increase your awareness of their use in your particular situation. Every warehouse is traveling down a different path because of their equipment, fruit surface residues, decay problems, equipment fruit waxing capabilities and fruit varieties. Hopefully, it is within this context that this information will cause you to question how you might use chemicals in your warehouse before you actually begin.

When we contemplate the use of new chemicals or changing combinations of old chemicals or any of the factors affecting the combinations of chemicals certain concerns should cause the alarm bells to begin ringing. The first of these concerns should be the safety of the warehouse personnel. Secondly, we should be concerned about the potential damage to the fruit being exposed to the chemicals and, third, we should be concerned about the effect of the chemical mix on the equipment. Finally, we must be concerned with the time lost and excessive additional labor utilized to correct a chemical problem.


What must we be aware of as we attempt to control the risks inherent with the use of chemicals in our operations?

  1. The first factor to watch will be the pH of the solutions that we are working with. For example, two chemicals in a solution at a certain pH might be quite compatible with no adverse activity occurring. Change the pH to a level higher or lower and something quite unexpected might occur. An example of this would be the acidification of a sodium silicate pear float solution. Such a solution at a pH of 9.5 or above would cause no problems. If it were to be lowered to pH 9.0 or below a solidification to a jelly-like mass would occur. In this example, a change in the pH of a solution changed the physical characteristics of the solution.

    This occurred in a pear packing facility when the operator decided to change float material. They added lignosulfonate to a solution of sodium silicate. This lowered the pH to a level below 9.0 and resulted in a solidification in the dump tank.

    A change in the pH of a solution can change the phytotoxicity level. For example, when lowering the pH of a solution of sodium silicate and SOPP, the phytotoxicity of the solution dramatically increases as the pH of the solution drops. Interestingly, phytotoxicity does not increase when the pH in a solution of calcium lignosite and SOPP is lowered.

    A problem of a different sort can be created when the pH of a chlorine solution is lowered, as chlorine gas is formed at low pH. A warehouse was using a chlorine treatment in their pre-sizing equipment and disposing of the overflow through a floor drain system. This drainage system also picked up the overflow from their packing line, where an acid solution was being used to clean the fruit for packing. The overflow from this operation mixed with the pre-size overflow in the drainage system, and the mixture generated chlorine gas.

    Here are some examples of how the pH of a solution, depending oh the constituents of that solution, can affect the safety of personnel, damage to fruit or equipment.

  2. Temperature changes in certain chemical solutions can have dramatic effects on the solution itself and upon the effectiveness of the chemicals. Temperatures can be raised or lowered in a number of often unexpected ways. While both of the following examples occurred with the use of SOPP, it could happen with any chemical.

    A packinghouse in California was seeing an increase in phytotoxicity as the day progressed. It was finally determined that the cause of the temperature change was due to the fact that the dump tank was outside with only a roof covering. The air temperature at the start of packing was about 55 °F Late in the afternoon it would approach 100 °F. Because of the tank location the water temperature would rise causing the phytotoxicity to increase as the day progressed.

    In another warehouse, field run fruit was run in the last hour of the packing day. This fruit had traveled a long way in August temperatures and had a fruit surface temperature of about 100 °F. When placed in a SOPP mixture, it had the same result as raising the mixture temperature to 100 °F, and phytotoxicity resulted.

    Temperature can also affect the solubility of certain chemicals. When the solution in the tank is cold, new chemicals may not go completely into solution. Thus the concentration may be below that which is recommended and time may be lost while chemicals are heated before being added to the treatment tank.

  3. Time has its own unique way of affecting the chemical solutions we use. In one instance, a chlorine solution was being used in a drench tank and additional chlorine was being added daily. The tank was used for over a week without the solution being changed and as a result the fruit began to show damage due to the buildup of excessive amounts of sodium salts.

    In another warehouse lignosite was being used by itself as a pear float material. Following a packing period of about 4 days the tank stood idle for a week. Without the pumping action to add oxygen to the solution, anaerobic conditions developed which resulted in an odor very similar to a septic tank. Many chemical reactions are not stable over time and this must be understood by the operator. With the use of biological controls, time will become an even more critical factor.

  4. The concentration or strength of the chemicals or agents being used is of extreme importance. The fact that we are dealing with a solution in constant change adds to the difficulty of doing an effective job while steering clear of the dangers or hazards to people, fruit and equipment.

    To maintain a constant concentration of chemicals in a tank to which a dilutant is constantly being added takes vigilant monitoring. This may be done by humans or by some type of mechanical or computerized system.

    If chemicals are being added manually, caution must be exercised so double dosages do not occur. This is every common cause of high concentration of chemicals. On the other hand, when low concentrations develop (which results in a lack of effectiveness) often the reason is that additional chemicals have not been added to take care of the effect of dilution.


    In summary, when working with chemical mixtures on the packing line be aware that any changes in the chemistry of that solution must be carefully judged before changes are made. If after reviewing all of the information available you are still not comfortable with the mix, contact the supplier of the chemicals being considered. If there are no answers to the specific mix which you are contemplating then perform a test with a small sample of the fruit to see firsthand the results which occur.

    Always remember the four factors which can have an effect on the solution. Those four factors are the pH of the solution, the temperature of the solution, the age of the solution or time, and the concentration of the solution. A change in any one of these four factors will almost always have an effect on the solution you are using and usually with negative results.

Maurice Smith

SteriSeal of Washington, Inc

13th Annual Postharvest Conference
March 1997

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