How Does a Cold Store Work? Temperature Bands and Storage

From the outside, a cold store looks like an oversized fridge; from the inside, it is a finely balanced system in which every product group demands its own temperature, humidity and airflow, and whose balance is upset each time a door opens. The field heat still carried by fruit and vegetables, the surface moisture on meat, and frozen product's intolerance of thawing and re-freezing are all managed under one roof. What follows, from temperature bands through to stacking rules, alarm logic and FEFO discipline, reflects how this is applied on site. The aim is not merely to keep the store running, but to operate it in a way that carries the product through to the end of its shelf life.

Temperature bands: chill room, blast freezing and frozen storage

A cold store does not run at a single temperature; it is divided into three main bands according to product physiology. The chill room band generally spans 0 to +8 °C and holds fresh fruit and vegetables, dairy products, fresh meat and delicatessen items. Even within this band there are sub-ranges: fresh meat and fish require 0 to +2 °C, milk and yoghurt +2 to +4 °C, while chill-sensitive products such as bananas and tomatoes must be kept above +10 °C. Placing a chill-sensitive product at too low a temperature causes irreversible damage such as pitting and internal browning.

The blast freezing room is not a storage area but a processing area. Its purpose is to carry the product through the critical temperature range in the shortest possible time; where freezing is involved, it operates at around -30 to -40 °C with strong airflow until the product's core reaches -18 °C. The faster a product freezes, the smaller the ice crystals remain, the less damage the cell walls sustain, and the less liquid is lost on thawing. Leaving product sitting in the blast freezer for hours is both a waste of energy and disrupts the sequencing of the next batch.

The frozen storage band targets -18 °C and below. -18 °C is regarded as the industry threshold; many operators run at -20 to -22 °C to allow a margin for fluctuation. Here the real enemy is not low temperature but temperature fluctuation: a room cycling between -18 and -12 °C causes surface recrystallisation, frost inside the packaging, and dry, discoloured patches known as freezer burn.

The transitions between the three bands are also part of the design. The door of the frozen room must not open directly onto a warm corridor; there should be an anteroom or chilled corridor in between. Otherwise, every door opening lets in humid air that turns to ice on the evaporator, increasing defrost frequency and making it harder for the room to hold its set point.

Relative humidity and storage by product group

A parameter as important as temperature, yet discussed far less, is relative humidity. The gap between a product's water activity and the humidity of the room air causes the product either to lose moisture or to develop condensation on its surface. Both are a loss of quality: leafy greens that lose water wilt, while a meat surface that gathers condensation becomes an ideal environment for microbial load.

The fruit and vegetable group generally requires 85-95% relative humidity. High-surface-area products such as leafy greens, broccoli and carrots need the upper end of this range, while dry-skinned products such as onions, garlic and squash prefer much lower humidity, around 65-75%. When placed in the same room, one dries out while the other goes mouldy.

Milk and dairy products are kept in the +2 to +4 °C range at moderate-to-high humidity; the critical point here is that the product packaging remains sealed and the room is free of strong odour sources. For fresh meat, the target is 0 to +2 °C and 85-90% humidity; if humidity drops, the surface dries out and weight loss occurs, while if it rises, a sticky surface layer and spoilage begin. Carcass meat is hung, while cut meat is packaged and transported on pallets, in order to manage air contact.

For frozen products, measuring relative humidity loses practical meaning; at -18 °C, air already holds very little moisture. Here, the focus is on packaging integrity and temperature stability, in order to limit the moisture the evaporator draws from the product. A frozen product with torn packaging loses moisture and quality within weeks in storage.

The practical rule is this: identify the product group, note the temperature and humidity pair, and set up the room according to this pair. In single-room facilities, settings are made according to the needs of the most sensitive product, and incompatible products are not accepted.

Ethylene gas and co-storage incompatibilities

Ethylene is a natural plant hormone released by fruit as it ripens; it is colourless and odourless, so it goes unnoticed in storage, but its effects appear within a few days. Products such as apples, pears, bananas, tomatoes, melons, avocados and apricots produce ethylene in high quantities. Leafy greens, cucumbers, broccoli, carrots, lettuce, cauliflower and cut flowers, on the other hand, are highly sensitive to ethylene: yellowing, softening, bitterness and premature spoilage are all caused by this gas.

A classic mistake is placing apples and lettuce, or bananas and broccoli, in the same room. Even if both products are at the correct temperature, the sensitive one loses its shelf life within a few days. Bitter taste in carrots, yellowing in cucumbers and sprouting in potatoes are typical complaints caused by ethylene.

Management operates on three levels. The first is separation: ethylene producers and sensitive products are placed in separate rooms; if only one room is available, they are not accepted in the same batch. The second is ventilation: exchanging outside air dilutes ethylene build-up, although this increases energy costs during hot summer months. The third is active control: potassium permanganate cartridges, catalytic ethylene scrubbers or controlled-atmosphere rooms remove ethylene from the environment.

Ethylene is not the only source of incompatibility. Odour transfer must be treated with equal seriousness: onions and garlic taint apples, fish taints butter, and citrus peel taints meat. Dairy products, butter and eggs absorb odours quickly due to their fatty structure. Sealed packaging reduces the risk but does not eliminate it; a separate room or separate dispatch day is planned for strong-smelling products.

In a well-run warehouse operation, the goods receipt form includes markers for ethylene producers, ethylene-sensitive products and odour sources; shelf assignment is made accordingly. This simple form prevents a significant share of quality complaints reaching the customer right at the door.

Air circulation and stacking rules

The evaporator doesn't cool the room; it cools the air. What keeps the product cold is that cold air being able to pass around it. This is why stacking is just as decisive as cooling capacity. In the same room, at the same set point, a correctly stacked pallet sits at the target temperature while a tightly packed pallet stays several degrees higher.

The basic rules applied on site are as follows. The space in front of the evaporator's discharge and intake surfaces is kept clear; a pallet placed there blocks airflow and leaves the room's back corners warm. Pallets are kept at least a hand's width from the wall, since the wall surface is where heat ingress is heaviest. Product is never placed directly on the floor; a pallet or grate is used. The upper limit stays below the ceiling air duct; if product is stacked up to evaporator level, cold air only circulates in the front half of the room and never reaches the back half.

Air corridors are left between pallets. When row gaps close up in block stacking, the pallets in the middle behave as if insulated; cold on the outside, warm on the inside. In racking systems, the space above and below each shelf must stay clear, and product must not rest against the back of the rack. Perforated cartons and perforated shrink wrap make airflow easier; on pallets fully wrapped in solid stretch film, product temperature reaches its target far too late.

There is also the reality of hot spots: the area by the door, the corner furthest from the evaporator, and the space under the ceiling beams are the warmest zones in every room. Sensitive product is never placed there; hardier products or batches waiting only briefly are kept there instead. Mapping the room and marking its hot spots is a small task, yet it delivers years of order.

Finally, there is load balance: once a room is loaded beyond its capacity, air cannot circulate even with the evaporator running. Occupancy is calculated not by the room's volume but by preserving the air corridors; the forklift operator's reflex to squeeze in one last pallet is the most common cause of upsetting that calculation.

Temperature logging, sensor placement and alarm logic

Temperature logging serves three purposes: proving the product has been kept in the right condition, catching a fault before the product spoils, and providing retrospective data whenever a customer or an audit asks for it. A single sensor cannot fulfil all three purposes.

Sensor placement is the first decision. A sensor right in front of the evaporator shows the room as colder than it is; a sensor by the door triggers a false alarm every time it opens. The right approach is to place one sensor at the room's expected warmest point (the corner furthest from both the door and the evaporator), one at product level, and one in the return air. In large rooms, using a dummy product (a probe in a glycol-filled bottle) to represent product temperature is a practical way to show that momentary swings in air temperature aren't reflected in product temperature.

Recording frequency is determined by product risk; for a cold room, a reading every few minutes can be considered standard. Records are stored permanently, not just displayed on screen; they must remain accessible retrospectively for a period exceeding the product's shelf life. Sensors are compared against a reference thermometer at least once a year; if there is a deviation, they are corrected or replaced. A record from a sensor without calibration documentation carries no evidentiary value in the event of a dispute.

Two thresholds are set in the alarm logic: a warning threshold (e.g. 2°C above the set point) and a critical threshold (the dangerous limit for the product). Each threshold has a defined delay period; this delay is needed so that short-term rises caused by a door opening or a defrost cycle do not trigger an alarm. Defrost times are entered into the system, so that planned rises do not appear on the alarm list. A critical alarm is sent to the on-duty person by SMS or phone call; email alone is not sufficient, because no one reads email at three in the morning.

The response given to an alarm is recorded just as much as the alarm itself: who did what, and at what time. Without this record, the same fault recurs three months later. For the transport side of this topic, you can take a look at our article explaining why the cold chain must not be broken.

Door discipline and pre-cooling

In cold storage, the biggest source of energy and quality loss is the door. Hot, humid air entering through every door left open does three things: it raises the room temperature, forms a layer of ice on the evaporator, and creates a safety hazard by leaving ice or standing water on the floor. For this reason, door discipline is not a courtesy rule but an operating procedure.

Physical measures follow a sequence. A strip (PVC) curtain is the cheapest and most effective first layer; it significantly reduces air passage even while a forklift is passing through. A fast roll-up door cuts the time the door stays open down to seconds. An air curtain forms a barrier between indoor and outdoor air by blowing air downward across the doorway. Using a dock shelter at the shipping ramp cuts off outside air contact between the vehicle door and the warehouse door. Having an antechamber in frozen rooms also prevents hot air from entering the cold room directly every time the door opens.

Behavioural measures are harder to enforce but more lasting: the door is closed immediately after product is taken out or brought in; the front of the door is never blocked with a pallet; products to be taken from the same room are collected in a single pass rather than going in and out three times; the shipping list is prepared outside the warehouse, not inside it.

Pre-cooling, on the other hand, means leaving the field heat of the product entering the warehouse at the door. Harvested fruit and vegetables arrive at outdoor air temperature and continue to respire; in the summer months, this means a product core temperature above 30°C. Putting this product directly into a +2°C warehouse both heats up the other products in the room and means the product itself takes days to cool down. In the pre-cooling room, the product is brought close to its target temperature using methods such as forced-air cooling, hydro-cooling or vacuum cooling before it is placed in the warehouse. The same logic applies to meat: after slaughter, the carcass is not moved to the storage room until its core temperature has been lowered in the cooling tunnel.

The rule on-site is simple: the warehouse is designed to keep the product cold, not to cool it down. Product temperature is measured with a probe thermometer at intake; if it falls outside the acceptance range, it is diverted for pre-cooling or not taken into storage without agreement from the customer.

FIFO and FEFO: stock rotation and batch tracking

FIFO (first in, first out) and FEFO (first expiry, first out) are the two core rotation rules of warehouse management. FIFO looks at the intake date; FEFO looks at the expiry or best-before date. If one of two batches arriving on the same day has a shorter shelf life, FIFO cannot detect this, but FEFO can. For food warehouses, FEFO is the correct approach; FIFO remains only as a fallback rule for products with no shelf-life information.

Rotation is simple on paper but difficult on the ground. In block stacking, the most recently arrived pallet is placed at the front, while the oldest pallet stays at the back and the forklift operator takes the easy one. This is why warehouses applying FEFO require a racking system, or at the very least an aisle layout; drive-in and gravity flow racks are designed for this purpose. Where racking is not available, the intake date and best-before date are marked on the pallet label in large print; a colour-coded labelling system by week allows the operator to spot it from a distance.

Batch (lot) tracking is the record-keeping side of rotation. Every pallet carries a batch number that matches the manufacturer's batch. Should a recall be required, being able to trace within minutes which pallet went to which customer depends on this record. Warehouse management software automates this, but in a small business a well-maintained spreadsheet can do the same job; what matters is that the record is updated with every intake and dispatch.

Counting discipline completes rotation. A weekly blind count reveals discrepancies between records and the shelf; the source of such discrepancies is most often an unlabelled pallet, a product placed on the wrong shelf, or a shipment whose dispatch was never recorded. An early-warning list is kept for products approaching the end of their shelf life; remaining shelf life is checked before a product is dispatched to a customer, since retail chains generally set a minimum remaining shelf-life requirement.

Daily operating routine and choosing the right partner

Even if everything described so far is correct in isolation, it all needs to be tied into a daily routine to work together. On the morning shift, overnight records for every room are reviewed, the alarm list is checked, and temperature recovery after defrost is confirmed. Throughout the day, the temperature of every product received is measured, non-conformities are checked, and shelf assignment is recorded. On the evening shift, it is visually confirmed that doors are closed, air curtains are working, and the areas in front of evaporators are clear. Weekly, sensor comparisons, evaporator fin cleaning checks, and blind counts are carried out.

Documenting this routine is equally important, because food safety systems don't call for saying it was done — they call for showing that it was done. We've also covered hygiene rules on the production side in our article on food safety and hygiene in production.

For businesses that rent external storage or work with a logistics partner, this routine is a selection criterion: can the provider share temperature records in real time, do they measure product temperature on receipt, do they ask about ethylene incompatibilities, can they prove FEFO with records. At İTS Gıda & Lojistik, we regard answering each of these questions with documented records as a natural part of the job at our cold storage facilities in Milas; you can get in touch with us for your storage and distribution needs.