Preservation by Chemicals, Preservatives and Fermentation
What this domain tests
You must classify preservatives, state their permitted limits and effective pH range, calculate preservative doses, and explain the microbiology of pickling and fermentation.
Preservative classes
- Class I: salt, sugar, vinegar, spices, honey and edible oil; used as normal ingredients without a stated maximum.
- Class II: benzoic acid and sodium benzoate, sorbic acid and sorbates, sulphur dioxide and potassium metabisulphite (KMS), propionates and citric acid. These are permitted only within legal limits.
Benzoate works best below pH 4.5 and is weak in neutral foods. Sorbate is effective over a wider pH range and is useful against yeasts and moulds. Sulphur dioxide inhibits moulds and yeasts, bleaches, prevents browning and acts as an antioxidant. KMS contains about 57% available SO2, so the weight of KMS is always greater than the weight of SO2.
Fermentation
Lactic acid bacteria such as Leuconostoc and Lactobacillus convert sugars to lactic acid in sauerkraut and pickles. Salt brine of about 2.5-10% selects these bacteria, and anaerobic conditions plus a fall in pH below 4.0 suppress spoilage. Yeasts produce alcohol in wine and cider, and Acetobacter converts alcohol to acetic acid in vinegar, which is about 5% acetic acid.
Worked example
10 kg of squash must contain 350 ppm SO2. SO2 needed = 350 mg/kg × 10 kg = 3500 mg = 3.5 g. KMS required = 3.5 / 0.57 = 6.14 g. Weighing 3.5 g of KMS would under-dose the product by nearly half.
Traps that cost marks
Benzoate is not effective in a high-pH food. Fermentation is controlled microbial action, not spoilage. Preservative limits are legal maximums, and KMS weight is not SO2 weight.