The Fine Art and Engineering of Walnut Drying

How you process your walnuts has a decisive effect on their final quality. Drying is perhaps the single most important stage of this process.

I have witnessed countless cases where a good batch of product (tens of tonnes) had to be thrown away because of improper drying. In this part, I want to focus on the walnut drying process and set out the key parameters.

Why Do We Need to Dry Walnuts?

Freshly harvested walnuts generally have a moisture content of over 25%. This can be even higher in the first half of the harvest or during a wet harvest season. At this high moisture level, the product is not stable. If left like this, mould and fungus will begin to form within a week and will spread over time, making the product completely unfit for consumption.

According to the FDA (U.S. Food and Drug Administration), a moisture content of 8% or lower (in-shell) is a stable level that prevents mold from forming on the product. This corresponds to a moisture level of approximately 5-6% for the kernel.

Fresh walnut consumption — sales before drying — is a very limited opportunity that exists in certain markets. However, this type of sale has a short life, so the vast majority of harvested walnuts must be dried to extend shelf life before and after sale.

Walnut drying is also, quite accurately, called dehumidification. In essence, the goal of this process is to bring the product's moisture down to a safe level.

Why Precision Matters

The result must be very precise. If you over-dry, you lose not only flavor and the sense of freshness, but also reduce your final product quantity for every percentage point you over-dry. On the other hand, if you under-dry or fail to achieve uniform moisture levels across the product, you will certainly face serious quality problems and inventory losses.

How Do We Dry Them?

Walnuts can be dried in various ways — including simply leaving them on the orchard floor after they fall from the trees. However, if you are approaching this with a commercial mindset, you need to do it in a more controlled, standardized and cost-effective way.

Commercial growers and processing facilities generally dry walnuts in cylindrical bins made of steel. Apart from a few cases where different drying methodologies are used — for example (French) rack systems — almost all commercial operators do this in metal bins placed in parallel, also known as Parallel Fixed (Stadium) Bins.

How the Stadium Bin System Works

The walnuts are loaded into bins that have an air distribution mechanism (louvres) at the bottom. Pressurised, high-volume dry air is pumped in through these inlets. As the air passes through the walnut mass, it absorbs moisture and exits the bin from the top. In this way, moisture is gradually drawn out of the walnuts and collected in the drying room.

This process must be carried out as quickly and precisely as possible, using the least amount of energy (electricity and gas). And the results must be uniform.

What Not to Do — 7 Critical Mistakes

The "how" of this process is far more complex than the "why" and the "what." Throughout the entire drying process, airflow, air pressure, humidity and heat are held in a critical balance within a constantly changing formula. Simply pumping in more air and heat without setting these parameters correctly will not get you where you want to be.

1. Don't Leave Walnuts Wet

Do not leave walnuts wet before placing them in the drying process — wet walnuts start to lose colour. Try to stick to this principle: Everything shaken from the trees should be hulled the same day. Every walnut hulled in a day should go straight into drying. Avoid leaving the nuts on the orchard floor overnight.

2. Don't Use High Temperatures

Do not use high temperatures for drying. With a well-designed air distribution system, 32-34 degrees Celsius will generally give you the best results in terms of flavour and quality. High drying temperatures cause bitterness, an astringent taste, changes in the chemical composition of the essential walnut oils, and a reduction in nutritional value.

3. Don't Over-Dry or Under-Dry

Remember that once walnuts have been dried to the desired moisture level, they will still exchange moisture with their surroundings. They therefore need to be moved out of the high-humidity chamber and into a less humid storage area where they can keep their moisture levels stable.

4. Mind the Bin Depth

Avoid drying bins deeper than 1.40 m — or, in such cases, do not fill them above this level, in order to minimise moisture differences between the bottom and top of the bin.

5. Correct Airflow Rate and Pressure

Make sure the bins receive an airflow rate high enough, and the correct pressure, to move air through the large mass of walnuts. The design of the air inlet should also allow the air to be distributed evenly within the bin.

6. Prevent Moisture from Building Up

Drying walnuts converts about a quarter of their weight into water vapor, which is released into the drying room environment. Do not allow moisture to build up in the drying room — otherwise you will not be drying your walnuts.

7. Use a Moisture Analyzer

Built-in moisture sensing systems are very useful. However, you should also use a moisture analyzer. The Shimadzu Mo 63u is a very reliable choice. Make sure you learn to use it well.

A Common Misconception About How Walnuts Dry

Of the two elements we use in drying — air and heat — heat has only an indirect, secondary effect on drying. Remember, we are not roasting or frying the walnuts — so it is not the heat that removes moisture from them.

Air molecules moving through the walnut mass remove the moisture. The more air molecules pass through, the more effective the process. A good analogy is a colony of ants moving through a small pile of bread crumbs. Each one carries off a millimeter-sized piece of the pile. The more ants there are, the faster the process.

Heat, when applied to the air, makes the air expand and so lowers its relative humidity — enabling each air molecule to absorb more moisture from the walnut mass.

Automatic Drying Process Control

While the same principles always apply, every drying process design is unique — it must take into account the atmospheric conditions, day and night, over the 30 days of the harvest season. With the right design, you can build a highly efficient drying process that dries your walnuts quickly (24 hours on average per batch), uniformly and precisely, using very low amounts of energy. Such drying systems also produce noticeably higher-quality products.

There are various drying parameters that must be adjusted during the drying process. A drying process controller computer can make autonomous decisions around the clock, 24 hours a day, and ensure that the right settings are maintained throughout the process.

Exhaust and Fresh Air System

The drying chamber's exhaust/fresh air system is also a vital part of the process. The drying process controller can select the right balance between retaining air that has already been heated and venting excess humidity. It can also run the operation autonomously, even when you are not at the processing line.

Safety: CO and CO₂ Monitoring

Finally, because the process involves real propane gas combustion, the drying chamber produces toxic gases: carbon dioxide and carbon monoxide. With the help of integrated sensors, the process controller can autonomously start and stop the chamber exhaust system to maintain healthy operating conditions.