885 Journal of Food Protection, Vol. 61, No.7, 1998, Pages 885-890 Copyright ©, International Association of Milk, Food and Environmental Sanitarians Use and Removal of Sul'fite by Conversion to Sulfate in the Preservation of Salt-Free Cucumberst R. F. MCFEETERS* u. S. Department of Agriculture, Agricultural Research Service, and North Carolina Agricultural Research Service, Department of Food Science, North Carolina State University, Raleigh, North Carolina 27695-7624, USA MS 97-159: Received 30 July 1997/Accepted 10 December 1997 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/61/7/885/1670094/0362-028x-61_7_885.pdf by guest on 27 September 2021 ABSTRACT Cucumbers (Cucumis sativus) were microbiologically stable in cover liquid containing 300 ppm of added sodium metabisulfite (calculated as SOz), 20 mM calcium chloride, and HCl to give an equilibrated pH of 3.5. The sulfur(IV) oxoanions could be easily removed to nondetectable levels «3 ppm) by addition of an equimolar amount of hydrogen. peroxide, which rapidly converted S(IV) primarily to sulfate ions. When sulfur(IV) oxoanions were removed from stored fresh cucumbers, 85% of the added metabisulfite could be accounted for by formation of sulfate ions. If cucumbers were heated before addition and removal of sulfur(IV) oxoanions, 96% of that added was converted to sulfate by hydrogen peroxide. Preservation of cucumbers in this way does not require fermentation, so addition of salt is not needed to select for lactic acid bacteria. Over one-third of the pickling cucumber (Cucumis at low pH sulfites are very effective at inhibiting microbial sativus L. var. sativus) crop of the United States is fermented growth, although the pH and S(IV) concentrations required in bulk tanks and stored prior to final processing into pickle for microbiological stability have been defined for only a products. Cucumbers are typically fermented in open fiber- few products. (In this paper, free sulfite species [SOz-HzO, glass tanks in 5 to 6% NaCl brine. After the active HS03 - and SO/- ] as a group will be called "sulfur(IV) fermentation, additional NaCl may be added to raise the oxoanions" or "S(N)," following the suggestion of Wedzi- concentration to as much as 12% to protect against dilution cha (27). Reversibly boundforms of sulfite will be referred to of the brine by rainwater and subsequent growth of spoilage as "bound S(IV). "When concentrations of S(N) are given in organisms, or, in colder climates, to protect against freezing parts per million, the amounts are calculated in terms of (9). As restrictions on discharge of chloride in wastewater sulfur dioxide [molecular mass, 64 DaJ). At pH 3.5, 600 streams have increased, processors have gradually reduced ppm of added S(IV) prevented microbial growth in palm the salt levels used in fermentation and bulk storage and wine for at least 30 days (7). Splittstoesser and Mattick (21) increased the amount of fermentation brine that is recycled. considered 200 ppm of S(IV) to be optimal for preventing However, with current fermentation procedures the potential growth of yeasts in refrigerated grape juice during an exists for spoilage of fermented cucumbers by clostridia if 8-month storage period. Mango, guava, and pawpaw pulps the salt level is too low and the pH is above 3.6 (8). Despite (pH 3.4 to 4.3) were stored 18 months at 30°C with 2,000 the efforts to reduce salt waste, there remains a large amount ppm of added S(IV) (5). of dilute brine generated from desalting operations for which This work was undertaken to explore the use of sulfites no practical reuse method is available. to prevent growth of microorganisms in cucumbers so they Salt-free storage of olives (23) is in general commercial could be stored in bulk without the need to add salt. Since use. The procedure uses anaerobic storage conditions, a the cucumber is a low-acid fruit, the idea was to lower the mixture of lactic and acetic acids, and sodium benzoate to pH by direct acidification and then to add sufficient S(IV) to prevent microbial growth. The only published work on ensure microbiological stability of stored cucumbers. Then, salt-free storage of cucumbers was that of Shoup et al. (20) before the stored cucumbers were processed into final in which an approach was used similar to that used for olive products, S(IV) was removed (i.e., converted to sulfate) by storage to preserve the cucumbers. Sulfites have been used addition of an equimolar amount of hydrogen peroxide to preserve a variety of acid fruit products. However, the (HzOz). S(IV) causes an off flavor in foods (3, 16) and primary objective for use of sulfites has often been to asthmatic reactions in some people (2, 17). S(IV) has been preserve color rather than microbiological stability (25). But shown to be nearly quantitatively oxidized to sulfate by a 1:1 reaction with HzOz (Fig. 1) (11). At pH 4 or below this reaction is complete in less than a millisecond (12). HzOz is t Mention of a trademark or proprietary product does not constitute a currently used in several food applications (15). guarantee or warranty of the product by the U.S. Department of Specific objectives were to define the pH and added Agriculture or North Carolina Agricultural Research Service, nor does it S(IV) concentrations that would ensure the microbiological imply approval to the exclusion of other products that may be suitable. * Author for correspondence. Tel: 919-515-2979; Fax: 919-856-4361; stability of stored cucumbers, to find conditions which E-mail: [email protected]. would allow removal of S(IV) to nondetectable levels by 886 MCFEETERS J. Food Prot., Vol. 61, No.7 Removal of S(IV) from stored cucumbers. Whole cucum- bers (680 ± 5 g) were packed in 1,360-ml (46-oz) jars. Jars were filled with 680 ml of cover liquid that contained 40 or 80 mM FIGURE 1. Reaction of bisulfite ion with hydrogen peroxide at pH calcium chloride and HCl to lower the equilibrated pH to 3.5 as 3.5. determined by titration of cucumber slurry (see above). A rubber septum was placed in the jar lids so that after closure a solution of 200 mg of sodium metabisulfite per ml could be injected with a using H202' and to measure the efficiency of conversion of syringe to give 300 ppm of S(IV) (4.68 mM) equilibrated. To assess sulfur(IV) oxoanions to sulfate in stored cucumbers by the effect that heating cucumbers might have on removal of S(IV) H202· by H202, some cucumbers were blanched for 3 min in boiling water and then cooled in tap water before being packed in jars and MATERIALS AND METHODS covered with the solution. In addition, some jars were pasteurized Size 2B cucumbers (32 to 38 mm diameter) obtained from after being filled with cucumbers and cover liquid by heating in a local processors were used for all experiments. Sodium metabisul- steam-jacketed kettle to a center temperature of 74°C for 15 min fite (Sigma Chemical Co., St. Louis, Mo.) was the source of S(IV) and then cooling with cold tap water to 40°C or less. Sodium Downloaded from http://meridian.allenpress.com/jfp/article-pdf/61/7/885/1670094/0362-028x-61_7_885.pdf by guest on 27 September 2021 for all experiments. CaCh . 2 H20 and HCl were purchased from metabisulfite was added to these jars after cooling. Aldrich Chemical Company (Milwaukee, Wis.). Hydrogen perox- Sequential removal of S(IV) by addition of H202 was done by ide (30%) was obtained from J. T. Baker Chemical Company determination of the concentration of sulfur(IV) oxoanions by (Phillipsburg, N.J.). using HPLC and then addition of an equimolar amount of 30% H202 through the septum sufficient to react with the SeN) present Measurement of pH. The pH was measured with a Fisher in the cover liquid, but not the cucumbers in the jar. After Accumet digital pH meter (model 825MP) and a Coming combina- re-equilibration of the S(IV) inside the cucumbers with the cover tion electrode (#476570). The pH meter was calibrated with pH 7.0 liquid, H202 sufficient to remove it from the cover liquid was added and pH 4.01 buffers. Samples and standard buffers were at the same again. This was repeated five or six times until the S(IV) in the temperature. cover liquid was reduced below the detectable limit. In an Determination of pH and S(IV) concentration required for experiment to determine if sufficient H202 to remove S(IV) in both preservation. Fresh cucumbers were cut into 6-mm-thick slices. the cucumbers and cover liquid could be added in a single addition, Slices (180 ± 2 g) were packed into 360-ml (l2-oz) jars and 30% H202 was added to each jar sufficient to react with 50,75,90, covered with 180 ml of cover liquid to give a 50:50 pack-out ratio 100, and 120% of all the S(IV) determined to be in the jar by HPLC on a volume basis. Jars were hermetically sealed by heating caps in analysis. boiling water and then closing by hand. Asingle rubber septum was Conversion of added S(IV) to sulfate in nonpasteurized put in each cap to allow removal of liquid samples during and pasteurized cucumbers. Cucumber slices (680 ± 5 g) were incubation. Jars were stored at 2YC. Calcium chloride (20 mM packed in 36 1,360-ml (46-oz) jars. An equal weight of cover equilibrated) was added to all treatments in all preservation solution containing 40 mM CaCl2 and sufficient HCl to give an experiments.