Six trays of 20 kg beef forequarter, stacked directly on top of each other in a 2 °C chiller with the fans running, should reach 0 to 2 °C core in about 22 hours. In one plant they took 26 at the top of the stack and 31 at the bottom. The room was at temperature, the probe was calibrated, and nobody could explain the difference until someone put a data logger between trays two and three and found 6.4 °C air sitting there for eleven hours.
Stacking is the forgotten variable in thaw planning, and it interacts directly with pads, because a tray that thaws slowly releases drip slowly and a pad that was sized for 22 hours is now sitting under product for 31. This article covers what a stack does to airflow and what to change.
What a Six-High Stack Does to Air Movement
Chiller air is designed to move at 0.3 to 0.8 metres per second across the product. A stack of solid-walled trays at 600 by 400 mm, six high and 1.2 metres tall, presents a nearly continuous vertical wall. Air flows around the stack, not through it. The gaps between trays are 5 to 15 mm where the tray bases nest, which is not enough for convective exchange.
The consequence is a vertical temperature gradient inside the stack. Typically the top tray runs 1 to 2 °C above room, the middle trays 3 to 4 °C above, and the bottom tray 4 to 6 °C above, sustained for most of the thaw. The bottom tray therefore thaws last and, in a room with a fixed release time, is released early.
The Four-Hour Gap and What It Costs
A four-hour delay on the last tray in every stack is not a scheduling inconvenience. It means the release decision for the whole stack is set by the worst tray, so either everything waits, or the bottom tray goes to the line at 3 to 4 °C core and the temperature record shows it.
For drip, the effect is the opposite of what most people expect. Slow thaw at a lower average temperature releases less total drip, because there is less protein denaturation and less cell damage. The bottom tray may show 30 percent less liquid than the top one, which reads as a good tray and is actually an under-thawed one.
Stack Height, Spacers and Rotation
Anything that breaks the wall helps. Reducing to four high cuts the gradient roughly in half. Inserting 40 mm rigid spacers at every second tray allows cross flow and flattens the gradient within 1 to 1.5 °C. Alternating tray orientation by 90 degrees creates edge channels that air can enter.
The simplest change is stack height, because it costs nothing but floor area. Where floor area is the constraint, spacers cost about 4 to 7 per tray and last years.
Timing the Release Off the Coldest Point
Probe the bottom tray, not the top, and not the room. One probe per stack, inserted into the thickest part of the largest piece, read at hour 18 and hour 22, tells you when the stack is actually ready. If the spread between top and bottom exceeds 2 °C, fix the stack before adjusting the schedule.
Pads should be specified against the longest thaw in the room, not the average, because the bottom tray is the one that will be in contact with its pad for the extra hours. Our extended-dwell SAP pad grades for long thaw cycles hold capacity past the 24-hour mark under stack load.
Da maohua
Zhengzhou Flor New Materials Co., Ltd. specializes in SAP super‑absorbent polymer products. Serving personal hygiene, agriculture & horticulture, pet supplies and other industries, we leverage supply‑chain resources to provide particle‑size selection, performance matching and customized solutions based on clients’ practical requirements.
