Fresh produce is alive: it burns the sugar it has stored with oxygen and produces carbon dioxide, water vapour and heat. This heat arises continuously in the room and has to be removed by the refrigeration system. The amount varies greatly with the product. Approximate values at 0 °C: onions 7–10 W/tonne, grapes 5–8, apples 10–12, lettuce 27–50, strawberries 36–52, carrots 45, broccoli 55–65, mushrooms 80–130, sweetcorn 125 W/tonne. As a rule, products that are leaves, flowers or shoots respire a great deal, and tubers and mature fruits little.
The effect of temperature is very strong: the respiration rate roughly doubles or triples for every 10 K. While strawberries give 36–52 W/tonne at 0 °C they give 300–580 W/tonne at 20 °C, and broccoli goes from 55–65 at 0 °C to 800–1,000 W/tonne at 20 °C. This is why the heat of respiration matters most in the first hours, when the product enters the store warm; as the product cools the load falls rapidly. This is also where the value of pre-cooling lies: product that is cooled quickly both produces less heat and does not use up its sugar and water.
The calculation is direct: respiration load = product mass × unit heat of respiration. 100 tonnes of apples produce about 1.1 kW of continuous heat at 0 °C; 20 tonnes of broccoli 1.2 kW at 0 °C, but 18 kW at the moment it enters the store at 20 °C. In a long-term apple or potato store respiration is a small but constant part of the total load; in banana ripening and ripening rooms, by contrast, it is the main item. Meat, dairy, fish and frozen products do not respire, and this item is taken as zero.
The by-products of respiration also have to be managed. The middle of a tightly stacked, unventilated carton warms up with its own heat; a stack that leaves no air path leads to spoilage of the product even if the room thermometer reads correctly. In a closed room carbon dioxide builds up and oxygen falls; this must be borne in mind when entering fruit stores that have not been opened for a long time, and fresh air exchange must be planned. The cooling load calculator shows the heat of respiration of the fresh product you select as a separate item; the storage values of products are given in the food storage conditions guide.