WSU Tree Fruit Research & Extension Center

Postharvest Information Network

Saturday, February 16, 2019

WSU-TFREC/Postharvest Information Network/Flotation Materials For Pear

Flotation Materials For Pear

Specific Gravity

Pears are frequently brought from harvest bins to packing lines in water solutions to minimize fruit bruising during handling. Due to their being of similar density to water, pear fruit do not float adequately 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 briskly and can be handled efficiently. This paper will describe salts currently available for use in pear packinghouses and management strategies to optimize their use.

Measuring Specific Gravity
The density or "strength" of a solution in floating 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 the specific gravity. The specific gravity of a solution may be read from a hydrometer, which is a sealed, weighted glass tube with a scale marked off along its length. The hydrometer is allowed to float freely in the solution, and the specific gravity value is read where the scale intersects the water line.The specific gravity required to float pears varies considerably with the pear variety being floated, the nature of the growing season, and the type of handling system employed by the packinghouse. For example, in a typical year in some Medford district packinghouses, the following values might be sufficient:

  • Bartlett = 1.03
  • Anjou = 1.03
  • Comice = 1.04
  • Bosc = 1.05
  • Seckel = 1.06

However, the actual density of fruit varies from one season, or even from one orchard, to the next. In addition, the speed with which fruit is to move through the dump tank is determined by the design of the packing operation, the flow rate of the tank solution, and the 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 packing lines may allow faster movement than where single lines are fed. Modify the expected specific gravity values for a given pear variety according to the observations of the packinghouse operator.

Materials for Pear Flotation

Increasing the specific gravity of a solution may be accomplished through dissolving a salt in the water. The most common salts used in pear production are sodium carbonate (soda ash), sodium sulfate, and sodium silicate. In the last few years, sodium and calcium lignin sulfonate have been introduced.

Sodium Carbonate

Sodium carbonate (soda ash) has many industrial uses and, consequently, is readily available and usually inexpensive. It is a powder that is commonly loaded into dump tanks by hand from 100-pound sacks. Some packers have noted that much dust is raised during loading, and workers may have difficulty with the heavy sacks. Sodium carbonate solutions are somewhat caustic, tending to dissolve paint and corrode machinery. The solutions also may irritate workers' skin at small cuts. It is difficult to dissolve sodium carbonate unless it is made into a slurry before adding to the tank. Caking at the bottom of the tank is frequently observed. Sodium carbonate is compatible with either chlorine or sodium ortho-phenyl phenate (SOPP) used as a dump tank disinfectant. A 5% solution of sodium carbonate in water with or without SOPP has a pH of approximately 10.

Sodium Sulfate

While it is difficult to dissolve sodium sulfate in cold water, it dissolves adequately in heated water with agitation. It is also a powder, loaded manually into tanks from heavy sacks. 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 to 6.5, varying with the source of material. When SOPP is used, the pH rises to 9.7 to 11.2.

Sodium Silicate

Sodium silicate, sometimes known as "water glass," is formulated as a liquid. Various devices may be used to facilitate delivery of the liquid into dump tanks from barrels or storage tanks, although the material is highly concentrated and 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 use 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 to 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 and Calcium Lignin Sulfonate

Lignin sulfonate salts are produced as byproducts in the sulfite pulping of Western Hemlock wood in paper mills in the Pacific Northwest. Although these salts are available both in powder and liquid formulations, it is difficult to dissolve the powder. The liquid formulation (50% solids) is available in either drum or railroad tank car volumes. Approximately twice the volume of lignin sulfonate as sodium silicate is needed to reach the same specific gravity. Lignin sulfonates are highly soluble and clean up readily in water. "Orzan" is a brand name of lignin sulfonate products of ITT Rayonnier; "Lignosite" is a brand name of lignin sulfonate products of Georgia-Pacific Corporation. Because lignin sulfonates make very dark solutions, analysis of SOPP in the dump tank by color-change titration is not feasible. A pH-based alternative will be discussed below. Lignin sulfonates in water are near neutral in pH; the concentration of SOPP is the primary determinant of solution pH, typically 8 to 9. Lignin sulfonates are not compatible with chlorine in the dump tank, since chlorine ions tend to become inactivated by binding with the lignin sulfonate material.

Flotation Salts and Pear Decay

The effect of flotation salts alone and in combination with disinfectants on spores of decay fungi and incidence of postharvest decay has been the subject of recent research. Two factors appear to be of fundamental importance in the relationship of flotation salts to decay fungi: inherent anti-fungal 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 at 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. Inclusion of 0.35% SOPP accentuated this effect. This clearly indicates an inherent anti-fungal activity in the sodium lignin sulfonate. Similar properties were found in calcium lignin sulfonate, although the performance against spores of Mucor piriformis was not as strong when using calcium as when using sodium 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 is most effective in killing spores in the lowest pH solutions, sodium lignin sulfonate and sodium sulfate. Chlorine, since it is not compatible with lignin sulfonates, is most effective in sodium sulfate solution. The surfactant AG98 (Rohm and Haas Co.) has improved spore kill and decay control by chlorine and may be used in any of the solutions appropriate for chlorine.

Managing SOPP in Lignin Sulfonate Solutions

As an alternative to the color-change titration method for analyzing SOPP in lignin sulfonate solutions, a pH-based method has been developed. The basis for this method is that, in the near-neutral pH lignin sulfonate solutions, SOPP concentration is the main factor affecting pH. However, you must understand two variable conditions: the concentration of lignin sulfonate (as indicated by specific gravity) and the initial pH of the lignin sulfonate solution must be known. Curves have been developed which relate SOPP concentration to solution pH at each specific gravity level and for each likely value of initial lignin sulfonate pH. Producers of lignin sulfonate have indicated that they will try to stabilize the latter factor to allow this procedure to be simplified. These curves may be obtained from Steri-Seal, Inc., of Wenatchee, Michelson Packaging of Yakima, or Washington State University and Oregon State University researchers involved with this material. Hydrometers may be obtained from winemaking suppliers, and relatively inexpensive hand-held pH meters are available from scientific and industrial suppliers.

Flotation Salts and Fruit Injury

Both sodium carbonate and sodium sulfate may stain fruit surfaces if fruit is 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. This effect is enhanced by flotation salts, low pH, and increases as the solution temperature rises and the fruit spends more time in the solution. A 5 minute exposure of Comice pears to 1% SteriSeal in soda ash at specific gravity 1.04 resulted in fruit injury at temperatures higher than 65°F. Under the same conditions, injury was observed in sodium sulfate at temperatures above 70°F, in sodium lignin sulfonate above 80°F, and in SteriSeal alone above 86°F. Despite its relatively low pH, lignin sulfonate appears to protect solutions against SOPP injury. No injury occurred in water alone at the temperatures tested. These temperature conditions are not likely to occur where dump tanks are indoors or shaded, and when fruit has been precooled.

Operating Suggestions

Because of the complexity of factors involved in managing flotation and other dump tank materials, it is important to keep detailed records of activities relating to the clump tank. The amount and time of addition of materials should be noted, as well as any observations on the behavior of the tank solution or the fruit. Note the specific gravity which gives the desired flotation for each pear variety in each season. In predicting volumes of liquid flotation salt needed, it is important to accurately know dump tank volume. Many packers have never measured the water required to fill the tank and have substantially underestimated the volume of flotation salt required.

David Sugar and Robert A. Spotts

Oregon State University, Southern Oregon Experiment Station and Mid-Columbia Agricultural Research and Extension Center

Post Harvest Pomology Newsletter, 7(2): 5-7
September 1989

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