How you process your walnuts has a detrimental effect on their final quality. Drying is perhaps the single most important stage in this process.
I have seen countless cases in which a good batch of product (tens of tons of them) had to be sent to garbage due to problems caused by improper drying. In this episode, I would like to focus on walnut drying process and outline the key parameters.
Why (do we dry walnuts):
Freshly harvested walnuts usually have in excess of 25 % moisture content. This could be higher during the first half of the harvest or in a wet (rainy) harvest period. The product is not in a stable stage at this high level of humidity. If left this way, Mold and Fungus formation will begin in a week and increase in time, making the product totally unconsumable.
According to FDA (Food and Drug Administration of USA), a Moisture Content (MC) of 8 % (with the shell) or lower is a stable level that will prevent mold formation in the product. This corresponds to approximately 5-6 % moisture level for kernel.
Moisture Content (MC) can also be calculated with the simple formula below:
%MC= [wet_weight(g)−dry_weight(g)] / wetweight(g)×100
Fresh walnut consumption, sales before drying, is a very limited opportunity that exists in certain markets. But the life of this type of sale is short and therefore the vast majority of the harvested walnut must be dryed in order to prolong its shelf life, before and after sale.
Drying walnut, is also (quite accurately) referred to as Dehumifying process. In fact what is targeted in this process is to lower the humidity (dehumidify) of the product to a safe level.
The end result should be very precise. If you dry too much, not only does the taste and feeling of freshness lost, but you also decrease the amount of your end product for every percentage point that you overdry.
On the other side, if you dry too little or do not achieve a homogeneity of moisture level in the product, you will for sure have serious quality problems and loss of inventory.
What (we do to dry them):
Walnut can be dried in a variety or ways, which may include simply leaving them on the orchard floor after they fall off the trees. Having said this, if you are trying to do this in a commercial mentality, you need to do this in a more controlled, standardized and cost effective way. Commercial growers and processing facilities dry walnuts in bins that are usually made from steel. Except for very few cases where different drying methodologies are used like (French) rack systems, almost all commercial players do this in parallel placed metal bins which is sometimes referred to as Parallel Stationary (or Stadium) Bins.
Walnuts are loaded into bins that have an air delivery mechanism (louvers) at their bottom. Pressurized, high volume of dry air is pumped from these inlets. Air molecules travelling through the walnut stacks absorb moisture, leaving the bin from the top. This his way moisture is gradually removed from the walnut, and accummulated in the drying room. This process should be done as quickly and as precisely as possible using the lowest amount of energy (electricity and gas). And the results must be homogenous.
How (do we do that):
Well, the “how” part of this process is a lot more complicated than the “why” and the “what” part as there is a critical balance between airflow, air pressure, mositure and heat in a continuously changing formula throughout the drying process. Simply pumping more air and heat without getting these set of parameters right will not get you where you want to go.
Let’s start by “how not to”:
1- You should not keep walnuts wet before placing the into the drying process as wet walnut will begin losing color. Try to stick to the following principle: Whatever is shaken off the trees should be hulled in the same day. And whatever walnut is hulled in a day should be placed in drying immediately. Avoid keeping fruits on the orchard surface overnight.
2- You should not use higher temperature in drying. With a well designer air delivery system usually 32-34 degrees Celcius will give you the best results in terms of taste and quality. Higher temperatures in drying will cause rancidity, bitterness and change the chemical composition of the essential walnut oils lowering their nutritional content.
3- You should not over or under dry. Keep in mind that the walnut; after dried to the desirable moisture level, will still exchange moisture with the surrounding. So they should be removed from the high moisture room to a less moist storage place that the will maintain their moisture levels at stable levels.
4- Avoid drying bins that have more than 1.40 m height, or in such cases; do not fill them beyond this level in order to minimize moisture differences between the lower and the higher part of the bin.
5- Make sure to provide the bins with enough air flow-rate and with the correct pressure that will overcome the huge walnut stack. Furthermor the design of the air inlet should allow the air to be evenly distributed inside the bin.
6- Drying walnuts will convert approximately a quarter of their weight into water vapor and integrate this into the drying room ambient. Do not allow humidity to build up in the drying room or you will not be drying your walnut.
7- Integrated moisture sensing systems are very useful. But you should also use a moisture analyzer tool. Shimadzu Mo 63u is a very reliable option. Make sure to learn to use it well.
There is a common misconception about how walnuts dry. Within the two elements we use in drying, air and heat, the heat has an indirect secondary effect on the drying. Remember we are not frying or roasting walnuts and therefore it is not the heat that removes moisture from them.
The air molecules that travel withint the stacks of walnut removes the humidity. The more air molecules pass through the more dense effective is the process. A good anology is to think about a colony or ants moving thought a stack of tiny pieces of breadcrumbs. Each removing milimeter size of the stack with them. The more ants the faster the process.
When applied to the air, heat on the other hand helps it to expand, and thus reduce the relative humidity of it. Making each air molecules capable of absorbing more moisture content out of the walnut stack.
While same principles always apply, each drying process design is unique in the sense that it needs to take into account the atmospheric conditions throughout the harvest period, for 30 days and nights. With a correct design, you can build a very efficient drying process that dries your walnut fast (in average in 24 hours each batch), in a homogenous and precise way, using very low amounts of energy. Such drying systems also produce noticeably higher quality products.
There are several drying parameters that need a be adjusted during the drying procees. A drying proceee controller computer can make autonomous decisions on a continuous basis, 24 hours a day making sure that the correct settings are followed along the way. The drying room exhaust/fresh air system is also a very important part of the process. The drying process controlled can select the correct parameters between conserving the already heater air and the discharge of the high moisture level. It can also manage the operation autonomously, even when you are not in the processing line. And finally, since there is an actual burning of Propane gas, the drying room produces poisonous gasses, Carbon Dioxide and Carbon Monoxide. With the help of integrated sensors, the DPC can autonomously start and stop the room exhaust system to maintain healthy working conditions.
Do not forget: The drying is the most important part or your walnut processing that determines your product quality and should not be taken lightly..