Harvested fruit and vegetables are still alive: they take in oxygen, give off carbon dioxide and release heat as they do so. That puts one more line in a cold-room load calculation, next to walls, doors, lighting and fans. The warmer the produce, the faster it respires, so the size of that line depends on the commodity and its temperature.
| Commodity | At 0 °C, W/tonne | At 20 °C, W/tonne |
|---|---|---|
| Broccoli | 62 | 887 |
| Sweet corn | 121 | 771 |
| Spinach | 62 | 680 |
| Strawberry | 47 | 443 |
| Head lettuce | 35 | 166 |
| Cabbage | 15 | 124 |
| Apple, fall varieties | 9 | 59 |
| Dry onion | 9 | 24 |
The figures are calculated from the respiration rates in Table 1 of USDA Agriculture Handbook 66. The handbook also gives the conversion: a rate in mg CO2 per kg per hour multiplied by 61 gives kcal per tonne per day, which works out at roughly 3 W per tonne for each unit of rate.
A worked example
Ten tonnes of broccoli arriving from the field at 20 °C produce about 8.9 kW from respiration alone; once the product is down to 0 °C, that load falls to 0.6 kW. Heat from warm produce slows its own cooling, and produce that cools slowly keeps producing a lot of heat for longer. The handbook puts room cooling, where warm produce is simply placed in a cold room, at 24 hours at least.
That is why pre-cooling is planned apart from storage: forced air, hydrocooling or, for leafy vegetables, vacuum cooling brings the produce close to its target temperature before it enters the store. The same handbook states that highway trailers have too little airflow to cool produce and should never be relied on for initial cooling, so a refrigerated body is loaded with produce that has already been pre-cooled.
