Sodium Bisulfite (Food) - China

Brief Overview 

Sodium bisulfite (sodium hydrogen sulfite) is a compound with the chemical formula NaHSO₃. Sodium bisulfite is a food additive with E number E222. It is a white crystalline solid that turns yellow in solution. It is acidic and, therefore, corrosive. Sodium bisulfite in contact with chlorine bleach (aqueous solution of sodium hypochlorite) generates heat and forms sodium bisulfate and sodium chloride.

Manufacturing Process

This salt of bisulfite can be prepared by bubbling sulfur dioxide in a solution of sodium carbonate in water. Aqueous sodium hydroxide, sodium bicarbonate, sodium carbonate, or sodium sulfite solution are treated with sulfur dioxide to produce sodium metabisulfite solution. In one operation, the mother liquor from the previous batch is reinforced with additional sodium carbonate, which need not be totally in solution, and then is treated with sulfur dioxide. In some plants, the reaction is conducted in a series of two or more stainless steel vessels or columns in which the sulfur dioxide is passed countercurrent to the alkali. The solution is cooled, and the sodium metabisulfite is removed by centrifuging or filtration. Rapid drying, e.g., in a stream-heated shelf dryer or a flash dryer, avoids excessive decomposition or oxidation to which moist sodium metabisulfite is susceptible.

Food Industry

In fruit canning, sodium bisulfite prevents browning (caused by oxidation) and kills microbes. In the case of winemaking, sodium bisulfite releases sulfur dioxide gas when added to water or products containing water. The sulfur dioxide kills yeasts, fungi, and bacteria in the grape juice before fermentation. When the sulfur dioxide levels have subsided (about 24 hours), fresh yeast is added for fermentation. It is later added to bottled wine to prevent the formation of vinegar if bacteria are present and to protect the wine's colour, aroma and flavour from oxidation, which causes browning and other chemical changes. The sulfur dioxide quickly reacts with oxidation by-products and prevents them from causing further deterioration. Sodium bisulfite is added to leafy green vegetables in salad bars and elsewhere to preserve apparent freshness under names like LeafGreen. The concentration is sometimes high enough to cause severe allergic reactions. 

Chemical Industry

Sodium bisulfite is used as a reducing agent to manufacture many organic compounds. It is used as a decolourization agent in many purification processes. It can efficiently remove trace amounts of chlorine, bromine, iodine, hypochlorite salts, chromium trioxide and potassium permanganate.

Other Applications

Sodium bisulfite is used in large piping systems to prevent oxidative corrosion. It helps to maintain anaerobic conditions inside biochemical reactors. It is used in water treatment to remove residual chlorine after super chlorination. It is a strong reducing agent in wastewater treatment, textile dye preparation and film development. It is also used to bleach pulp in the paper industry.