When potatoes are stored for a longer period of time in a hot or very cold climates outside air ventilation only is not suitable to maintain potatoes at storage temperature only. In these situations, mechanical cooling is an essential tool to extend the storage period and keep potato quality high.

This article by Product Manager Jan van Maldegem of storage technology specialist Tolsma-Grisnich will focus on how systems should be designed in relation to the specific needs of the potato and gives practical tips for the storage operator to optimize settings in the climate control computer.

A cooling system can be just supporting, which means its capacity is limited or with full capacity. An optimal storage design for processing potatoes should be designed in advance both with ventilation und cooling. Also, a smooth integration of both systems in the climate control computer will enable storage operators to fine tune on minimal weight loss and energy consumption.

A physical approach to potato cooling with ventilation or cooling

The choice for a specific system can also be taken from a more theoretical approach. The cooling process can be physically split up into the convection part and the evaporation part and a to be neglected part caused by conduction between tubers. The reason why we use air as a heat transfer medium for potatoes is that it can be transported easily through a pile or a box and can easily exchange heat with the potatoes. This convection is forced by fans.

Another thing that happens is that due to fact that cold air contains less water than warm air and by taking up heat from the tubers by the air the relative humidity of the air will drop (vapor pressure deficit will increase) and moisture from the tubers will evaporate more easily and is removed from the storage by the ventilation or cooling system. For this evaporation of moisture energy is needed which is extracted from the potatoes. Luckily a healthy potato has a protective skin/peel to protect the tuber from losing moisture too quickly. So, one can imagine that there is an optimum ratio between these processes where cooling down is done quickly and, in the meantime, limiting the extraction of moisture from the potatoes with a high energy efficiency of the storage system.

What are the differences for ventilation and cooling systems in relation to the principles discussed above?

Ventilation: depending on settings like “difference air duct to product” and “difference outside/product” and the suitability of the outside climate itself (cold and dry compared to warmer and humid) cooling down can be done quickly because of a high ventilation rate of the fans. This means that cooling down by convection goes so fast that the time the potatoes are evaporating moisture is relatively short. At bigger duct temperature differences that maximum evaporation rate of moisture through the potato skin is reached. So, the shorter this time takes the less weight loss will be the result. Of course, this will result in some more internal ventilation afterwards.

Cooling: when using a cooling system, it’s depending very much on the design of the system with what temperature difference over the air cooler is running. Many cooling systems (direct expansion) have a fixed temperature difference where some systems (indirect) have an adjustable temperature over the air cooler. So, a well-designed cooling system can result in a very low dehydration of the potatoes but a (low cost) cooling system designed for e.g., onions will dehydrate the potatoes excessively. With buying a cooling system the dehydration percentage for the future of the potatoes is already fixed.

You can read the rest of this article in your complimentary e-copy of the March/April Issue of Potato Processing International magazine, which you can access by clicking here.