Milk must fall below 40°F within 1-2 hours of milking, and without a refrigerator the work is done by cold running water, ice or evaporation. Chilling 20 liters from body heat to 4°C removes about 2,500 kJ, which equals the melting of roughly 7.5 kg of ice or the warming of about 150 kg of 10°C spring water by 4°C.
Part of Dairy Production at Scale
Cooling without refrigeration means moving the heat of freshly drawn milk into a cold-water bath, ice or evaporating water quickly enough to slow bacterial growth.
A Cooling Method Is Judged By The Heat It Can Carry Away, Not By How Cold It Feels
Milk leaves the udder warm, close to body temperature. The parent article gives the target as below 40°F within 1-2 hours, and lists the methods: a spring house, a cold stream, a root cellar at 50-60°F as a stopgap, evaporative cooling that drops temperature 15-20°F below ambient, and nature’s free refrigeration in winter. This article puts a number on the job so you can tell which method is actually enough.
Worked Example: The Heat In A 20-Liter Can
The constants are standard physical values, not figures from this wiki: the specific heat of milk is about 3.93 kJ per kg per degree, its density about 1.03 kg per liter, and the latent heat of melting ice 334 kJ per kg. The 35°C starting temperature is an assumption for freshly drawn milk.
- Mass of 20 liters of milk: 20 × 1.03 = 20.6 kg.
- Temperature drop: 35°C to 4°C = 31°C.
- Heat to remove: 20.6 kg × 3.93 kJ/kg·°C × 31°C = 2,510 kJ, about 2,500 kJ.
- Ice: 2,510 kJ ÷ 334 kJ/kg = 7.5 kg if the ice only melts. Because the meltwater then warms a little, treat 7.5 kg as the minimum.
- Spring water: if 10°C spring water is allowed to warm by 4°C (to 14°C) and water absorbs 4.19 kJ/kg per degree, the flow must carry 2,510 ÷ (4.19 × 4) = about 150 kg, or 150 liters, past the can during the hour or two. In a bath this is why the water must flow, not sit: a still bath warms up and stops cooling.
Milk can only get as cold as the coolant. In a 10°C bath the milk will approach 10°C, not 4°C, so a spring house alone does not meet 40°F (4.4°C) when the spring is typically 45-55°F as the parent article says; it meets it only as a first stage followed by ice or a colder cellar.
Method: Choose And Combine
Spring house or cold-stream bath. Stand sealed cans in flowing water up to the level of the milk, stirring the milk with a clean paddle every few minutes. Small cans cool faster than one big can because a small can has more surface for its volume. The build is in Springhouse.
Ice. Where ice is stored, a bath of crushed ice and water around the can reaches near 0°C and meets the target. See Ice Houses and Cold Storage.
Evaporative cooling. Wet burlap on a breezy shaded surface can drop the container 15-20°F below ambient, which works only when the air is dry enough to evaporate water; in humid weather it fails. See Cold Storage: Passive Cooling for Perishables.
Root cellar. At 50-60°F it only buys time. Use it to hold milk you are about to process, not to store it.
Summer rule. When none of these reaches the target, the parent article’s rule stands: process the milk into cheese or butter within hours, since a product that is acidified or salted is no longer the same food.
Common Mistakes
- Assuming a cold-feeling bath is cold enough. A 12°C bath cannot bring milk below 12°C.
- Not stirring. Warm milk in the core of a can stays warm because milk is a poor heat conductor in the middle of a still can.
- Pouring warm milk onto yesterday’s cold milk. It warms the whole can. Cool each milking separately before combining.
- Lids left off in a stream. Water entering the can is a contamination.
Safety
Cooling slows bacteria, it does not remove them
Milk chilled to 4°C is not pasteurized. Some bacteria, listeria among them, continue to grow slowly in the cold, so chilled raw milk still carries the diseases described in Milk Handling & Safety. Cool first, then pasteurize or boil milk meant for the community. Never draw drinking milk from a stream bath that livestock or people use upstream.
What’s Next
- Learn next: Pasteurization Methods — the heat treatment that cooling cannot replace
- Combine with: Cream Separation Techniques — separating warm milk versus cooled milk