Flotation Materials For Pears
Pears are frequently transferred from harvest bins to packing lines 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.
Measuring 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 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 , 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 packing lines may require 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
The most common salts used for pear flotation 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 (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. Caking at the bottom of the tank frequently happens. 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.
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 bags. Sodium sulfate is compatible with either chlorine or SOPP in the dump tank. Tile 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 10.7 and to 7.8 with 100 ppm chlorine.
Sodium silicate, sometimes known as "water glass," is a liquid. 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 well at the end of each day all machinery involved with sodium silicate. 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 by-products of sulfite pulping in Pacific Northwest paper mills. Although these salts are available as powder or liquid, it is difficult to dissolve the powder. 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. "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 is discussed below. Lignin sulfonates in water are 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.
A pH-based method has been developed as an alternative to the color-change titration method for analyzing SOPP. The basis for this method is that, in the near-neutral pH lignin sulfonate solutions, SOPP concentration is the main factor affecting pH. However, there are two variable conditions: the concentration of lignin sulfonate (as indicated by specific gravity) and initial pH of the lignin sulfonate solution. Curves have been developed that 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 simplify this procedure. These curves may be obtained from Steri-Seal, Inc. of Wenatchee, AG98 Packaging of Yakima, or Washington State University and Oregon State University Cooperative Extension offices in major fruit producing areas. Hydrometers may be obtained from winemaking suppliers, and relatively inexpensive hand-held pH meters are available from scientific and industrial suppliers.
Flotation Salts and Pear Decay
The effect of flotation salts and disinfectants on fungi 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 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 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 is most effective in killing spores in the lowest pH solutions, sodium lignin sulfonate and sodium sulfate. Since chlorine is not compatible with lignin sulfonates, it 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.
Flotation Salts and Fruit Injury
Both sodium carbonate and sodium sulfate 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. This effect is enhanced by flotation salts and 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 0.3% SOPP 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, with 0.3% SOPP in sodium lignin sulfonate above 80°F and in 0.3% SOPP 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 damaging temperature conditions are not likely to occur where dump tanks are indoors or shaded and fruit has been precooled.
Mike Willett(1), Gene Kupferman(2), Rodney Roberts(3), Robert Spotts(4), Dave Sugar(5), Gary Apel(6), Hugh W. Ewart and Bill Bryant (7)
(1)WSU Cooperative Extension Agent, Yakima, WA; (2)Postharvest Specialist, WSU TFREC, Wenatchee, WA; (3)USDA Pland Pathologist, Tree Fruit Research Lab, Wenatchee, WA; (4)OSU Plant Pathologist, Mid-Columbia Research and Extension Center, Hood River, OR; (5)OSU Plant Pathologist, Southern Oregon Experiment Station, Medford, OR; (6)Michelsen Packaging Company, Yakima, WA; (7)Northwest Horticultural Council, Yakima, WA
Post Harvest Pomology Newsletter, 7(3): 8-9