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Saturday, February 16, 2019

WSU-TFREC/Postharvest Information Network/Flotation Materials for Pears

Flotation Materials for Pears

Materials for Pear Flotation

In the packinghouse, pears are frequently transferred from harvest bins to sorting tables in water solutions to minimize fruit bruising. Because pears have almost the same density as water, they do not float well enough to move quickly out of bins and through the dump tank. Addition of a salt raises the density of the water solution so that pears will float better for easier handling. This section describes salts currently available for use in pear packinghouses and explains how best to use them.

The most common salts used for pear flotation are sodium lignin sulfonate, sodium sulfate, sodium silicate and sodium carbonate (soda ash).

Sodium and calcium lignin sulfonate. This is the most widely used (73% of pear packers surveyed) of the flotation salts in Washington State. Lignin sulfonate salts are by-products of sulfite pulping in Pacific Northwest paper mills. The liquid formulation (50% solids) is available in drums or railroad tank car volumes. Approximately twice as much lignin sulfonate as sodium silicate is needed to reach the same specific gravity. Lignin sulfonates are highly soluble and clean up readily with water. "Lignosite" is a product of Georgia-Pacific Co. Lignin sulfonates in water are neutral in pH. Lignin sulfonates are not compatible with chlorine in the dump tank. They react irreversibly with chlorine, reducing its disinfecting properties. Using lignin sulfonate without sodium orthophenylphenate (SOPP) will allow the lignin sulfonate to ferment rapidly and create unpleasant odors. Heating lignin sulfonate reduces pH.

Determining the concentration of SOPP in lignin sulfonate can be complicated since the solution pH, typically 8-9, must be taken into account. Both pH-based and a colorimetric-based system have recently become available for checking the concentration of SOPP in a flotation tank charged with lignin sulfonate. Each has its limitations in field use. See your supplier for test kits and instructions.

Sodium silicate. Sodium silicate, sometimes known as "water glass," is a liquid. It is used by 20% of the pear packinghouses surveyed in Washington State. Various devices are used to move it into dump tanks from barrels or storage tanks. When the material is highly concentrated, it may be difficult to pump or pour when cold. Sodium silicate is slippery, and spills are both dangerous and difficult to remove unless cleaned up promptly. Rinse all machinery involved with sodium silicate well at the end of each day. Sodium silicate is compatible with either chlorine or SOPP in the dump tank. The approximate pH value of a 7.5% solution of sodium silicate with or without SOPP is 11.2-11.4. Sodium silicate solutions may gel at low pH values so they should never be mixed with acids or lignin sulfonates in the tank.

Sodium sulfate. Sodium sulfate is a powder, loaded manually into tanks from bags. It is used by 6% of the pear packers surveyed in Washington. Sodium sulfate is compatible with either chlorine or SOPP in the dump tank. The approximate pH of a 6% sodium sulfate solution in water is 4-6.5, varying with the source of material. When SOPP is used, the pH rises to 9.7-10.7 and to 7.8 with 100 ppm chlorine. While it is difficult to dissolve sodium sulfate in cold water, it dissolves adequately in heated, agitated water.

Sodium carbonate. Sodium carbonate (soda ash) has many industrial uses and consequently is readily available and usually inexpensive. It is a powder commonly loaded into dump tanks by hand from 100-pound sacks. Sodium carbonate solutions are somewhat caustic, tending to dissolve paint and corrode machinery. The solutions also may irritate workers' skin. It is difficult to dissolve sodium carbonate unless it is made into a slurry before adding it to the tank. It frequently cakes at the bottom of the tank. Sodium carbonate is compatible with either chlorine or SOPP. A 5% solution of sodium carbonate in water with or without SOPP has a pH of approximately 10.

Measurement of Specific Gravity

The density or strength of a solution used to float pears is measured along a scale known as specific gravity. The specific gravity of pure water is arbitrarily given the value of 1.00. Dissolving materials such as salts in water increases specific gravity. The specific gravity of a solution is measured using a hydrometer, a sealed, weighted glass tube with a scale marked off along its length. The hydrometer is floated in the solution, and the specific gravity value is read where the scale intersects the waterline.

The specific gravity required to float pears varies considerably with the pear variety being floated, nature of the growing season and type of handling system employed by the packinghouse. In addition, the speed that fruit moves through the dump tank is determined by design of the packing operation, flow rate of the tank solution and goals of the operator. For example, vertical dumpers may require a higher specific gravity solution than horizontal pass-through systems, and tanks feeding multiple sorting and packinglines may require faster movement than those feeding single lines. Modify the expected specific gravity values for a given pear variety according to the observations of the packinghouse operator.

Flotation Salts and Pear Decay

The effect of flotation salts and disinfectants on fungal spores and incidence of postharvest decay has been the subject of recent research. Two factors appear to be important in the relationship of flotation salts to decay fungi: inherent antifungal properties of the flotation salt and the effect of the flotation salt on the activity of SOPP or chlorine. The latter effect appears to be primarily a consequence of the pH environment in which the disinfectant functions.

In a comparison of flotation salts alone in specific gravity 1.05, sodium lignin sulfonate inhibited germination of spores of Botrytis cinerea (gray mold), Mucor piriformis (mucor rot), Penicillium expansum (blue mold) and Phialophora malorum (side rot) to a much greater extent than did either sodium carbonate, sodium sulfate or sodium silicate. Including 0.35% SOPP accentuated this effect. This clearly indicates an inherent antifungal activity in the sodium lignin sulfonate. Similar properties were found with calcium lignin sulfonate.

As noted in the descriptions of each salt, there is a considerable range among the pH values of flotation salt solutions. In general, SOPP most effectively kills spores when used with sodium lignin sulfonate and sodium sulfate. Since chlorine is not compatible with lignin sulfonates, a sodium sulfate solution is the obvious choice. The surfactant AG98 (Rohm and Haas) has improved spore kill and decay control by chlorine and may be used in any of the solutions appropriate for chlorine.

Flotation Salts and Fruit Injury

Sodium carbonate (soda ash), sodium sulfonate and sodium lignin sulfonate may stain fruit surfaces if fruits are left in flotation solutions overnight. In normal use of these salts, there is not a high risk of fruit injury. However, SOPP can also injure fruit surfaces. The amount of injury is determined by the flotation salts and low pH. Injury increases as the solution temperature rises and the fruit spends more time in the solution.

For example, a 5-minute exposure of Comice pears to 0.3% SOPP in soda ash at specific gravity of 1.04 resulted in fruit injury at temperatures higher than 65°F. Injury was observed in sodium sulfate containing 0.3% SOPP at temperatures above 70°F and in sodium lignin sulfonate above 80°F. Injury was also observed in SOPP alone at temperatures above 86°F. Despite its relatively low pH, lignin sulfonate appears to enhance protection against SOPP injury when used as a flotation agent. No injury occurred in water alone at the temperatures tested.

These damaging temperature conditions are not likely to occur where dump tanks are indoors or shaded and the fruit has been precooled. In certain years, however, due to growing conditions, Bartlett or Comice pears can be more susceptible to SOPP injury. In these years, surface injury may occur especially during initial packing start-up when dump water is warm and fruit temperatures are high. Do not use SOPP on Bartletts or Comice until the dump tank is below 55°F and the fruit is cooled to below 50°F. Some injury to Bartletts has occurred after two dips in SOPP in sodium lignin sulfonate at 46°F for 15 minutes. Single exposures to SOPP, shorter exposure time, and moving fruit to cold storage immediately after treatment all helped to reduce the severity of the injury.

Additional Cautions When Using Flotation Materials

Do not acidify the dump tank when using flotation materials. Using chlorine in an acidified tank will cause the release of chlorine gas. Using SOPP in an acidified tank can cause fruit damage. Acidifying sodium silicate will cause it to form a gel.

When using SOPP, do not use warm fruit or warm water or fruit damage may result. Do not use SOPP on Asian pears when using lignin sulfonate to avoid fruit damage. Do not allow any pears to remain in flotation tanks with SOPP for any length of time.

Gary Apel

Michelsen Packaging, Yakima WA

Tree Fruit Postharvest Journal 4(1):19-21
June 1993

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