Salting and Curing.
Two and a half grams of curing salt per kilogram of meat gives 156 parts per million of nitrite: enough to protect against botulism, and a measured limit, not a pinch.
Salt preserves meat and fish by dissolving in their water and lowering its water activity (III.9). What matters is the water-phase salt: the salt as a share of salt plus water. At about 5 % water-phase salt, Listeria and Salmonella slow sharply; at 10 % (water activity about 0.94) most pathogens stop; traditional salt fish and country hams go far higher. Salting also draws water out by osmosis (III.8) and firms the flesh.
Curing adds sodium nitrite, usually as curing salt (Prague Powder #1: 6.25 % sodium nitrite in salt, dyed pink so it is not mistaken for table salt). Nitrite blocks the germination of C. botulinum spores in the airless inside of a ham or sausage, reacts with myoglobin to make the pink of ham and bacon (VI.7), and gives cured flavour. It is toxic in larger amounts, so it is weighed precisely and regulated: in the US at most 156 ppm in minced cured products and 120 ppm in bacon. The workshop salts and cures a kilogram of meat.
Below: the water-phase salt and water activity of meat salted at the percentage you choose (water bench: exact NaCl water activity), with the growth floors of pathogens, and the nitrite in parts per million from the curing salt you add against the US limits.
- Compute water-phase salt and its water activity.
- Explain what nitrite does in a cure.
- Weigh curing salt precisely.
- Know why curing salt is dyed pink.
Weigh the salt; weigh the cure to the tenth of a gram
Meat salted at 3 % has a water-phase salt of about 4 % and a water activity of 0.976: enough to season and to slow spoilage, not to preserve on its own. At 5 % salt the water phase is 6.5 % and the water activity 0.960, near the Listeria floor. That is why fresh sausages and lightly salted fish still need the fridge, while country hams and salt cod, salted far more heavily and dried, keep for months.
Curing salt at 2.5 g per kilogram gives 156 ppm of nitrite, the US limit for minced cured products; bacon is limited to 120 ppm, about 1.9 g of curing salt per kilogram. Weigh it on a scale that reads to 0.1 g, mix it evenly, and never substitute one curing salt for another (Prague Powder #2, for long dry cures, also contains nitrate). Chinese 腊肉 and 腊肠 and Italian salumi use the same chemistry with their own traditional cures. Home dry-curing of whole muscles and sausages has real botulism and Listeria risks: follow a tested recipe exactly.
Weigh 1 kg of pork belly, rub it with 25 g of salt and 10 g of sugar, seal it and refrigerate for a week, turning daily. Weigh it again: the liquid that came out was drawn by the salt. Rinse, dry and cook it as unsmoked bacon.
Terms
- Water-phase salt
- Salt as a percentage of salt plus water in a food.
- Curing salt
- Salt mixed with a small, fixed share of sodium nitrite, dyed pink.
- Nitrite
- A curing agent that blocks botulism, fixes pink colour and adds flavour; toxic in excess.
- 腊肉
- Chinese salt-cured, air-dried and often smoked pork belly.
Sources
- USDA FSIS, Processing Inspectors' Calculations Handbook and 9 CFR 424.21: ingoing sodium nitrite at most 156 ppm in comminuted cured products and 120 ppm in pumped bacon (with ascorbate); curing salts such as Prague Powder #1 contain 6.25 % sodium nitrite. EU Regulation 1333/2008 (as amended 2023): added nitrite limits of the order of 100–150 mg/kg.
- ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens, Blackie, 1996 — growth limits (minimum temperature, pH and water activity) and typical heat resistance; the course rounds them.
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.