Not every product obeys the rule "the colder, the better". While apples, grapes and carrots keep for months at 0 °C, the cell membranes of products of warm-climate origin lose their function below a certain temperature. The product does not freeze, but its tissue breaks down. Typical lower limits: bananas 13 °C, mature green tomatoes 12–13 °C, lemons and grapefruit 10–13 °C, cucumbers, aubergines and okra 10 °C, mangoes 10–13 °C, peppers and green beans 7 °C, oranges 3–8 °C, pomegranates 5 °C. In potatoes starch turns to sugar below 4 °C; they darken when fried.
Symptoms: pitting of the skin and water-soaked patches, darkening or greying, browning of the internal tissue, failure to ripen, loss of flavour and rapid decay. The insidious part is this: the symptoms mostly do not show in the cold room; they appear a day or two after the product has reached the supermarket shelf or the kitchen and warmed up. The store operator believes the product was delivered sound, and the complaint arrives days later. The damage is the product of temperature and time; a few hours of low temperature leave no trace on most products, several days do.
In cold room design this translates into the correct set point and a narrow temperature band. If a banana room is set to 14 °C, the air blown from the evaporator must not fall below 13 °C; this means a large-surface evaporator, a low temperature difference (TD, the difference between the room air and the evaporating temperature) and stepped or inverter capacity control. The top-row cartons that the discharge air strikes directly are the first to be damaged. The same limit applies in pre-cooling: cucumbers and tomatoes are not cooled with water or air at 0 °C.
Mixed storage is the most frequent source of error: in a greengrocer’s store a single room is set to 2 °C, the apples and carrots are content, and the tomatoes, cucumbers, aubergines and bananas are damaged. The solution is at least two temperature zones: 0–2 °C and 8–12 °C. Chilling injury must not be confused with freezing injury; freezing is the formation of ice in the tissue and begins between −0.5 and −2 °C in most fresh produce. You can find which product is to be stored at which temperature, product by product, in the food storage conditions guide.