Normal air contains 21% oxygen and 0.04% carbon dioxide. In a controlled atmosphere (CA; in Turkish KA, kontrollü atmosfer) store the oxygen is lowered to 1–3%, the carbon dioxide is held between 1 and 5% depending on the product, and the temperature is fixed at 0–3 °C. Low oxygen slows the product’s respiration and ethylene production; it preserves the firmness, acidity and colour of the fruit for months. Apples that last 3–5 months in a normal cold store can be kept for 8–10 months in a CA store. The practice of lowering the oxygen below 1.5% is called ULO (ultra low oxygen).
The equipment is added on top of an ordinary cold room: gas-tight panels with joints sealed with special mastic and tape, a gas-tight door whose gasket is compressed, a nitrogen generator that lowers the oxygen, an activated carbon or lime scrubber that captures excess carbon dioxide, a gas analyser that measures continuously, and automation. Because the room pressure moves with temperature, a pressure equalising valve and a flexible breather bag are essential. Tightness is tested before commissioning; a common acceptance criterion is that a pressure of 250 Pa applied to the room takes longer than 20–30 minutes to fall by half.
There are differences on the refrigeration side too. Since the room is not opened for months there is no door load, but the product is alive and moisture loss is direct weight loss; so the evaporator is selected with a large surface and a low temperature difference of around 3–5 K, and the relative humidity is held at 90–95%. Rooms must be filled and closed quickly: in good practice filling takes no more than a week, the product is cooled first, and then the oxygen is lowered. Each variety has a different oxygen and carbon dioxide limit; oxygen below the limit leads the product to fermentation, excess carbon dioxide to internal browning.
The most important warning concerns occupational safety: the air in a CA room makes a person collapse and kills within a few breaths, and it has no smell or warning whatsoever. Nobody enters until the room has been ventilated and the oxygen reading has risen above 20%; the doors are kept locked and carry warning signs. Once a room has been opened the product must be sold within a few weeks, which is why several rooms of 100–300 tonnes are preferred to one large room. The temperature and humidity requirements of products are given in the food storage conditions guide; the cooling load calculator gives a preliminary capacity for each room.