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The Chemical and Preservative Properties of Sulfur Dioxide Solution for Brining Fruit

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The Chemical and Preservative Properties of Sulfur Dioxide Solution for Brining Fruit The Chemical and Preservative Properties of Sulfur Dioxide Solution for Brining Fruit Circular of Information 629 June 1969 Agricultural Experiment Station, Oregon State University, Corvallis The Chemical and Preservative Properties of Sulfur Dioxide Solution for Brining Fruit C. H. PAYNE, D. V. BEAVERS, and R. F. CAIN Department of Food Science and Technology Sweet cherries and other fruits are preserved in sul- control are given in papers by Waiters et al. (1963) and fur dioxide solutions for manufacture into maraschino, Yang et al. (1966). cocktail, and glace fruit. The sulfur dioxide solution, Sulfur dioxide and calcium are directly related to commonly called brine, may be prepared from liquid brined cherry quality. Improper use of these chemicals sulfur dioxide, sodium bisulfite, or sodium metabisulfite may result in cherries that are soft, poorly bleached, or using alkali or acid to control pH. Calcium salts such as spoiled due to fermentation. It is the purpose of this calcium hydroxide, calcium carbonate, and calcium chlo- circular to show how the basic chemical and preservative ride are added to the brine to prevent cracking and pro- components of brine solutions are afifected during prep- mote firming of the fruit tissue through interaction with aration, storage, and use. pectic materials. Directions for brine preparation and Chemical Properties of Sulfur Dioxide Solutions When sulfur dioxide or materials containing sulfur range of sulfur dioxide concentrations, the relative dis- dioxide (bisulfite or metabisulfite) are dissolved in tribution of the sulfur dioxide ionic forms is constant. water, three types of chemical substances are formed: Within the initial pH range used for brining cher- sulfurous acid (H2SO3),1 bisulfite (HSO3"), and sulfite ries, there is a predominance of calcium bisulfite (SO~). The amount of each substance formed depends (Ca(HS03)2) and sulfurous acid (HjSOs), both upon the pH (hydrogen ion concentration) of the solu- forms being water soluble. However, calcium sulfite tion, which is regulated by the amount of sulfur dioxide (CaSOs), which is highly insoluble, precipitates as the present and/or by addition of acid or alkali. The effect pH of the brine is adjusted upward, giving the brine a of pH upon the ionic forms of sulfur dioxide is shown milky-white appearance. This usually occurs within a in Figure 1. Brine solutions containing sulfur dioxide range of pH 2.8-3.2, varying with the sulfur dioxide exhibit the same general equilibrium characteristics; concentration and temperature of the sulfur dioxide so- their chemical properties are somewhat dififerent, how- lution. For example, lower sulfur dioxide concentrations ever, since brine solutions form insoluble calcium sulfite. and/or cool brine temperatures shift the precipitation The chemical reaction for sulfur dioxide-calcium brine point of calcium sulfite and permit adjustment of brine is given below. to higher pH values. Formation of white calcium su'fite precipitate indicates excessive use of alkali and should be Ca(OH), Ca(OH) 2 avoided. Cherries placed in brine solutions containing S0 + H 0^±H SO ^± Ca (HSCM^CaSOjJH-H.O 2 2 2 s calcium sulfite or solutions with an initial pH above 3.2 Addition of alkali raises the pH and shifts the reac- may be inadequately bleached and are subject to crack- tion to the right, while lowering the pH with acid forces ing and spoilage. Brines prepared with insufficient alkali a shift to the left. An increase or decrease in tempera- (pH below 2.5) lose excessive amounts of sulfur dioxide ture will bring about a left or right shift respectively. to the atmosphere during brining and this may cause For any given pH and temperature within a limited cracking and softening of the fruit. The alkali/acid ad- justment of brines should be such that the sulfurous 1 Experiments in recent years have tended to disprove the ex- acid portion is approximately half neutralized to give a istence of HvSO:. molecules, or at least show they are present in infinitcsimally small concentrations in aqueous sulfur dioxide theoretically complete conversion of sulfurous acid solutions. The term sulfurous acid (H-SO.-t) as it is used in this (H2SO.,) to calcium bisulfite (Ca(HSO:,)2). In reality, paper descrihes undissociated sulfur dioxide in aqueous solutions. complete conversion to calcium bisulfite is never at- 90-- o(\] CO 80 UJ u QC 70 U. I < 60 H O h- 50 U. o H 40 Z UJ O 30 tr UJ Q. 20 10- Figure 1. Effect of pH on distribution of sulfur dioxide in solution. tained since the three forms overlap (Figure 1) to give condition is upset, calcium sulfite begins to precipitate some sulfurous acid and calcium sulfite. However, half- and continues to do so until equilibrium is restored. neutralization of the brine solution gives an optimal sul- Onset of precipitation may be detected by monitoring furous acid-calcium bisulfite ratio by minimizing cal- levels of sulfur dioxide, calcium, and pH. Appreciable cium sulfite formation and sulfur dioxide volatilization. changes in one or more of these factors will affect Adjusting the pH to give a clear brine does not pre- brined fruit quality. vent precipitation of calcium sulfite indefinitely. Small Variation in initial pH of brine solution within the amounts of calcium sulfite are formed in all brine solu- range of 2.5 to 3.3 and sulfur dioxide concentration tions^ but they remain temporarily soluble in a super- within 0.75 to 1.50% has little effect on retention of cal- saturated condition. Supersaturated solutions are un- cium and sulfur dioxide. There is a gradual decrease in stable and susceptible to temperature changes, seeding, concentration at all levels, with brine containing higher and nucleation of the precipitate by foreign solid mate- initial calcium and sulfur levels maintaining slightly rial introduced into the brine. Once the super.saturated higher levels throughout storage. Elevated temperatures are the primary cause of brine mental brine containing sodium bisulfite and calcium instability during storage, drastically reducing the level chloride was acidified to pH 3.0 with citric acid and held of calcium ions (Figure 2) and sulfur dioxide (Figure eight weeks at 70° F. It retained essentially all of its 3). High temperatures also lower the pH. Low pH calcium and twice as much sulfur dioxide as a control coupled with reduced calcium levels may cause cracking sodium bisulfite brine acidified with hydrochloric acid. and softening of brined fresh fruit. Brines prepared Brines often are prepared in advance and held for and stored at cooler temperature (40° F) are more re- extended periods of time prior to use. This appears to sistant to deterioration. Calcium and sulfur dioxide be an undesirable practice due to brine instability. In losses incurred during storage should be corrected be- view of the changes occurring in brines during storage, fore fresh fruit is brined to assure high quality brined it is recommended that brines be used within 48 hours cherries. following preparation. If this is not convenient and ex- Brines should be prepared and used fresh to provide tended brine storage is necessary, calcium and sulfur maximum firming, bleaching, and preservation. A rapid dioxide losses should be determined and the brine re- method of preparation was given by Weast (1940). It adjusted to desired concentrations before brining fresh provides rapid preparation of brine by introducing fruit. liquid sulfur dioxide into a lime suspension. Using this Recommended levels of calcium ion in fresh brines procedure, brine can be prepared much faster than by range from 3,000 to 5,000 ppm, depending upon the va- the method of adding alkali slurry to sulfur dioxide riety and maturity of fruit. Additional calcium in the solutions. Both methods of preparation have essentially form of calcium chloride may be added to assist in firm- the same storage characteristics in terms of stability. ing the cherry. Brekke et al. (1966) suggest that calcium In laboratory brining trials at Oregon State Univer- also serves to inhibit enzymatic softening. They report sity, improved storage of brines has been achieved that addition of 2% calcium chloride by weight of fruit through use of chemicals which prevent precipitation of above the usual amount of calcium salts prevents enzy- calcium sulfite. Citric acid and citrate salts improve brine matic and non-enzymatic softening in brined cherries. stability by sequestering calcium ions and thus prevent- A rapid method for determination of calcium in brines ing formation of insoluble calcium sulfite. An experi- and brine adjustment is given later in this publication. o H Z LU U CC LJ CL Figure 2. Changes in calcium content with time at different temperatures. 1.5 O I.0-- O 0.5- cr u o.o 20 30 DAYS Figure 3. Changes in sulfur dioxide content with time at different temperatures. Preservative Properties of Sulfur Dioxide Solutions Preservation of cherries and other brined fruit is more effective than bisulfite in controlling yeasts and made possible through the use of sulfur dioxide. The molds (Rehm and Wittmann, 1962). In fact, many in- preservative effect of sulfur dioxide is not permanent vestigators believe that the bisulfite and sulfite forms and is reduced or lost when the sulfur dioxide content is have little or no preservative properties. Rehm and Witt- lowered due to volatilization, oxidation to the sulfate ion, mann (1962) mentioned that the most common orga- precipitation as calcium sulfite, or combination with nisms are inhibited by 200 to 300 ppm sulfite, but with fruit_constituents as orgariic-bisulfite compounds. The notable exceptions. It is the more resistant microbial pH of sulfur dioxide solutions is perhaps the most im- strains that cause difficulty in commercial practice. Cruess portant single factor affecting preservation. (1932) found that at pH 3.5, two to four times as much Sulfur dioxide solutions are effective preservatives sulfur dioxide was required to inhibit microbial growth only at relatively low pH values (high hydrogen ion as at pH 2.5.
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