The Chemistry of Sulfur Dioxide in Boerewors
By Eben van Tonder
15 November 2018
When making boerewors for commercial sale, we add sulfur dioxide as preservative and giving longevity to the product colour.
Spoilage of meat by microorganisms leads to the development of off-flavors, oxidative rancidity, discoloration, gas production and, often, slime formation (LLOYD- PURYEAR et al., 1991; COCOLIN et al., 2004). The reason for fresh sausage being highly perishable are their characteristic pH and aw values. In boerewors the pH is kept low through the addition of vinegar which itself is a string anti-microbial which should aid greatly in preserving the sausage.
It may however not be sufficient and sulfur dioxide is added as an additional hurdle. Along with factory cleaning and maintaining processing temperatures of the meat between 2 and 3 deg C, boerewors should have a good shelf life under chilled conditions and vacuum packed or packed in a foamo tray.
Here we review sulfur dioxide.
Mechanism as antimicrobial
Sulfur dioxide (SO2) is a broad spectrum antimicrobial agent and antioxidant. It has been known since the early 1900 that only the free form of sulfur dioxide (i.e. unbound to another molecule) have any antimicrobial efficacy. In the 1960’s it was shown that molecular SO2 is several hundred times more effective than bisulfite. (Henderson, Pat. 2009)
The antimicrobial mechanism of SO2 is that it enters the microbe and disrupts the activity of the enzymes and proteins of the cell. (Henderson, Pat. 2009)
Only the SO2 molecule can enter through the cell membrane, it is the concentration of SO2 that controls the antimicrobial efficacy. (Henderson, Pat. 2009)
The percentage of SO2 is again in turn dependent on pH. The lower the pH, the greater the percentage of SO2. SO2 readily dissolves in water. The reaction is
H2O + SO2 ↔ H+ + HSO3– ↔ 2H+ + SO3=
SO2 is what we refer to as molecular SO2. The products to the right of the balanced reaction are called the sulfites. HSO3 is called bisulfite and SO3= is called sulfite. “The negative signs (– and =) denote the negative charge of the bisulfite and sulfite ions (molecules with a charge are called ions). The double arrows (↔) of the equation denote that the reaction is at equilibrium.” (Henderson, Pat. 2009)
At equilibrium, the rate at which bisulfite ions become sulfite is the same as the rate at which sulfite ions become bisulfite. The reaction between the different types of sulfite is going both ways at a steady state so the concentration of the sulfite compounds remains steady.” (Henderson, Pat. 2009)
“While the concentration of the different forms of sulfites may be steady, it does not mean there are equal amounts of the compounds in solution; the acidity or pH of the water has a huge effect on their concentration. The more acidic or the lower the pH of the water, the more heavily the reaction is weighted to the molecular SO2 side. The more basic or higher the pH is, the more sulfite is present.” (Henderson, Pat. 2009)
“Sulfites will also react with other chemical constituents found in a meat cure such as sugars, acetaldehyde, and phenolic compounds, added to the meat as liquid smoke or during the smoking process. When a sulfite reacts with another molecule and becomes part of its structure it no longer takes part in the equilibrium reaction and it is called bound. Sulfites that still are part of the equilibrium reaction are called free. The combined amounts of free and bound sulfites are called “total SO2.”” (Henderson, Pat. 2009)
The more compounds that are available in meat and in the meat cure for sulfites to bind to, the higher the ratio of bound to total sulfites there will be. Therefore, smoked sausages that are produced with a sweet cure and where SO2 is relied on for preservation will have a lower ratio of free to total SO2. (Henderson, Pat. 2009)
“Knowing both the amount of free and total sulfites is very important because only the free forms of sulfites are available for providing a preservative role. This is often expressed as ppm free SO2/ppm total SO2 to denote which number is free and which is total; these numbers can readily be determined by chemical analysis.” (Henderson, Pat. 2009)
Effect in Meat Spoilage Organisms
“Some common spoilage organisms are Acetobacter, Lactobacillus, Pediococcus, and Brettanomyces. All of these are sensitive to some degree to sulfur dioxide but the best results come from a combination of sulfur dioxide and good factory hygiene practices.” (Henderson, Pat. 2009)
“Acetobacter is also known as acetic acid or vinegar bacteria. As the name implies, it can grow in meat and produce vinegar (acetic acid). It can be controlled using sulfur dioxide.” (Henderson, Pat. 2009)
If Lactobacillus has already become established, lysozyme (an antimicrobial enzyme that is effective at high pH), can be added to control growth. Pediococcus produces an off-aroma that is described as “vegetal” or “dirty socks” and often comes from equipment and meat working surfaces that have not been kept clean. (Henderson, Pat. 2009)
Mechanism as antioxidant
The role of an antioxidant in the boerewors will be to provide cour stability. Fading of the cour will happen due to oxidation or the action of light.
Although the sulfite ion (SO3=) can bind with oxygen, there is almost no sulfite ion present in solution at the pH range found in boerewors. Rather sulfur dioxide prevents oxidation by binding with the precursors involved in oxidative reactions preventing them from reacting with oxygen or by binding with compounds already oxidized to reverse oxygen’s effect. (Henderson, Pat. 2009)
In fruit juices, sulfur dioxide acts by reducing the activity of the degenerative enzyme tyrosinase (polyphenol oxidase). (Henderson, Pat. 2009)
If colour is the only reason for adding sodium or potassium metabisulfite, I would seriously consider rather using ascorbic acid or erythorbic acid. The latter is a stereoisomer of ascorbic acid and a lot less expensive even though one sacrifice a considerable amount of functionality. It may be easier to work with erythorbic acid. In combination with isocitric acid, these have been proven to be highly effective. Isocitric acid is a structural isomer of citric acid.
I refer to the use of the acids, but of course, the salts may be used with the same results, depending on price and availability. Care must, however, be taken that the pH of the sausage does not drop below 5, to prevent denaturing of the meat proteins.
Mix the ingredients in a solution and determine its pH. Adjust to around 5.7 by using sodium hydroxide or potassium carbonate or sodium hydrogen carbonate or something similar.
The amount of isocitric acid to be added is 0.2 to 20 times the ascorbic or erythorbic acids. Ascorbic or erythorbic acid is normally added at 0.05% of FP.
Such a blend was proposed in 1969 by Nakao, Seishi Takagi, and Hiromi Nakatani on behalf of Takeda Pharmaceutical Co Ltd.
Organic acids, which vinegar is a part off (acetic acid) is of course the “oldest” emerging preservative. Of these, benzoic acid and parabens have the greatest overall effect against a broad range of bacteria, yeast, and moulds. Acetic acid, like lactic, and fumaric acids are effective, but only against bacteria. The limitations of acetic acid must therefore also be understood. (Søltoft-Jensen, J., Hansen,F., 2007)
How to add SO2?
“Sulfur dioxide is available in its pure form as a compressed gas that can be made into an aqueous solution. Most processors use a stable, powdered form of sulfur dioxide called potassium metabisulfite or sodium metabisulfite. Potassium metabisulfite has the molecular formula of K2S2O5 and is 57.6% available SO2 by weight. Potassium metabisulfite is usually abbreviated as PMBS or sometimes KMB or KMBS (K is the chemical symbol for potassium).
The molecular formula for sodium metabisulfite is Na2S2O5 and is 66.5% available SO2 by weight. we abbreviate it SMBS.
The formula and calculations for determining how many grams of PMBS you need to add for a given rise in ppm of SO2 are:
final product weight x ppm required /1000 x 0.576 = grams of SMBS to add
The formula for SMBS is:
final product weight x ppm required /1000 x 0.665 = grams of SMBS to add
1000 converts mg/L to g/L.
0.576 and 0.665 are the g’s of SO2 in PMBS and SMBS respectively.
There is a certain amount of guesswork in how much SO2 will be available. Always add a bit more. Many countries around the world, including South Africa, allows 500ppm in the final product. At least 30% of the SO2 from PMBS or SMBS added to meat will be lost immediately. Therefore, aim for 600 ppm inclusion which should get you to the 500 ppm. I suggest 1g to 1.2g per kg FP in sausages.
The differences between PMBS and SMBS are sodium metabisulfite has a molecular weight of 190.1 g/mole. A maximum of 650 g of this chemical can be dissolved in 1 liter of water at 20 degrees Celsius. Potassium metabisulfite has a molecular weight of 222.32 g/mole and is less soluble in water. Only 450 g can be dissolved in 1 liter of water. (Morgan, S. 2017)
Some processors prefer to use a premixed aqueous solution of sulfur dioxide rather than PMBS. The liquid is typically 5% to 10% SO2 by weight and it can be purchased or made up at the processing plant by dissolving SO2 gas or PMBS into distilled water. The liquid can be directly added to meat without mixing and the proper amount is measured volumetrically instead of weighed on a scale.
Measuring Sulfur Dioxide
The exact amount of both free and total sulfur dioxide in meat can only be determined by chemical analysis. Two primary methods that are used are known as the Ripper method and the Aeration-Oxidation method. Both methods have limitations and require an investment in laboratory equipment and chemicals and a degree of expertise in laboratory practices.
Vinegar or acetic acid provides powerful antimicrobial action in boerewors. Adding Sulfur Dioxide will contribute to the overall antimicrobial action and enhance and prolong the fresh meat colour. It will also increase the molecular Sulfur Dioxide in the sausage matrix. Smoking boerewors will reduce this by depleting the molecular Sulfur Dioxide.
Producers must consider adding sulfur dioxide carefully since it is a known allergen. Despite this, it is a very popular chemical to add to fresh sausages. Greater care should be taken with hygiene during production, keeping meat temp < 3 deg C and it should not be necessary to add this. A suggestion that will horrify meat scientists around the world and masters butchers alike is that if there is any doubt about the micro on the meat such as will be the case if mince is used, I would wash the trim first with a 2% acetic acid solution in cold water. Dip it using a clean crate. I would then use sodium ascorbate or erythorbate in conjunction with isocitric acid to address the matter of colour fading.
Overall Boerewors should have a good shelf life.
COCOLIN, L. et al. Study of the ecology of fresh sausages and characterization of populations of lactic acid bacteria by molecular methods. Applied and Environmental Microbiology, v. 70, p. 1883-1894, 2004. PMid:15066777 PMCid:PMC383013. http:// dx.doi.org/10.1128/AEM.70.4.1883-1894.2004
Søltoft-Jensen, J., Hansen,F., 2007, New Chemical and Biochemical Hurdles; https://doi.org/10.1016/B978-012676757-5/50017-7
Henderson, Pat. 2009. Science behind this anti-microbial, anti-oxidant, wine additive. Practical Winery & Vineyard Journal. January/ February 2009.
LLOYD-PURYEAR, M. et al. Meningitis caused by Psychrobacter immobilis in an infant. Journal of Clinical and Microbiology, v. 29, p. 2041-2042, 1991. PMid:1774332 PMCid:PMC270256.
Morgan, S. 2017. Sodium Metabisulfite Vs. Potassium Metabisulfite.