Refrigeration and the cold chain

Every decision below follows from these numbers:

• Vaccines: 2–8 °C, and never frozen. Heat destroys potency slowly and invisibly. Freezing destroys aluminium-adjuvanted vaccines — those with tetanus, diphtheria or pertussis components, hepatitis B and others — and they look unchanged afterwards.
• Chilled food: 5 °C or below. Bacteria multiply fastest between 5 and 60 °C; below 5 °C most almost stop, though Listeria still grows slowly.
• Frozen food: −18 °C or below. Nothing grows, but freezing does not kill: thawed food spoils at the normal rate.
• Cooling cooked food: from 57 °C to 21 °C within 2 hours, and to 5 °C within 6 hours in total, in containers under 5 cm deep. A deep pot left overnight spends hours at the temperatures Clostridium perfringens grows fastest at.

Prove every thermometer first: in a stirred slurry of crushed ice and a little water it must read 0 °C. Any error is in every reading after.

Where winters freeze, stored ice needs no machine and no fuel; natural ice supplied the food trade of cities into the early 1900s.

• Walk only on at least 10 cm of clear, hard ice. White or slushy ice is weaker. Cut from still, clean water: freezing does not purify water, so ice that touches food or drink must come from a source safe to drink.
• Build into the ground or a bank on the shaded side — north-facing in the northern hemisphere — as a brick- or stone-lined pit with a drain at the bottom, because meltwater standing against the stack melts it fast. Keep the entrance small, with two doors.
• Pack the blocks tight, with no gaps, and bed the stack in at least 30 cm of dry straw or sawdust on every side and on top. Wet insulation conducts heat: keep it dry.
• Build big. Heat enters through the surface. Doubling every dimension of the stack gives eight times the ice for four times the surface — half the loss per tonne. Small ice houses fail.

Evaporating water takes heat with it. A pot-in-pot cooler — an unglazed clay pot inside a larger one, the gap filled with wet sand, a wet cloth over the top, standing in shade and moving air — keeps vegetables and fruit fresh days to weeks longer than open storage.

It works only in dry air. No evaporative cooler gets below the wet-bulb temperature, which humidity sets:

• 40 °C air at 20% humidity — best possible inside, about 23 °C
• 35 °C at 30% — about 22 °C
• 40 °C at 50% — about 31 °C
• 30 °C at 70% — about 26 °C

So it is never cold enough for vaccines or meat: use it for vegetables, fruit and drinking water. Keep the sand wet with safe water — daily in hot weather — and keep the outer pot unglazed, or it cannot sweat.

Every compression refrigerator runs one loop:

1. Compress the refrigerant vapour, which heats it.
2. Condense it in a coil outside the cold space — the heat leaves here, so air or water must flow over the condenser.
3. Expand it through a narrow valve, dropping its pressure.
4. Evaporate it in a coil inside the cold space. Boiling at low pressure, it takes heat from everything around it.

Ammonia is the refrigerant this tier can make: it is recovered from gasworks liquor when coal is carbonised. It boils at −33 °C at atmospheric pressure and carries more heat per kilogram than almost any alternative.

• Build ammonia circuits in steel. Ammonia attacks copper and brass.
• Design for the pressure. Ammonia condensing at 35 °C is at about 13.5 bar. Test every vessel and joint on the high-pressure side with water at 1.5 times its working pressure before any ammonia goes in — the same rule as the boiler.
• Find leaks with a burning sulphur stick, which makes white smoke at an ammonia leak, or wetted red litmus paper, which turns blue.

An absorption refrigerator replaces the compressor with heat. A kerosene or gas flame, or hot water from wood or the sun, boils ammonia out of an ammonia–water solution; the ammonia condenses and evaporates in the cold space as in a compression machine, then is re-absorbed into water and returned to the heater. Ferdinand Carré built the first in 1859. In the three-fluid design of 1922 — ammonia, water and hydrogen — a bubble pump driven by the same flame replaces the pump, and nothing moves at all. Kerosene absorption refrigerators carried vaccines to clinics without electricity for most of the 20th century.

It uses several times the energy of a compression machine for the same cold, but the energy can come from anything that burns.

• Stand it level. The fluids circulate by gravity; tilted more than a few degrees it stops cooling.
• Keep the flame blue and the wick trimmed. A yellow, smoky flame soots the flue and the cabinet warms. Clean the flue weekly.
• Hold a week of fuel, and log the temperature twice a day as for any other store.

A vaccine is only as good as the warmest and the coldest hours it has spent.

• Store: a 2–8 °C cabinet, with the thermometer read and written down twice a day, every day. Nothing in the door, nothing against the cooling plate, space between boxes for air.
• Hold through outages: an ice-lined cabinet, or bottles of water filling empty space, holds the temperature through hours without power or fuel.
• Transport: insulated cold boxes and carriers with ice packs conditioned first — leave frozen packs out until water sloshes when shaken and beads form on the surface. A pack straight from the freezer is below 0 °C and freezes the vaccine against it.
• Read the vaccine vial monitor before every use: usable while the inner square is lighter than the ring; discard once it matches or is darker.
• Record every break in the chain, and discard exposed stock on the record, not on judgement.

A freeze-damaged adjuvanted vaccine looks normal. The WHO shake test shows the damage:

1. Take a vial of the same vaccine from the same manufacturer — the same batch if possible — and freeze it solid deliberately, overnight at −10 °C or colder. Let it thaw and mark it FROZEN. This is the control.
2. Hold the control and the suspect vial together in one hand and shake them hard for 10–15 seconds.
3. Stand them side by side and watch them settle.
4. If the suspect vial settles as fast as the control — clear liquid above a layer of sediment — it has been frozen: discard it and everything stored with it. If it settles clearly more slowly, it was not frozen and can be used.

Never give the control to anyone. Test each suspect lot separately.