Moisture variability in walnuts
The study “Walnut Moisture Variability,” conducted by Tom Rumsey in 1988, examined how moisture content varies among individual walnuts before and after drying. For most of the trials, random samples of 100 walnuts were taken, and their moisture content was determined individually. The results reveal a fundamental aspect of the process: within the same batch, walnuts with very different moisture contents can coexist. This variability influences both the necessary drying time and the decision of when to end the process.
1. A single batch can contain completely different nuts
The greatest differences were found at the beginning of the season. In one of the samples of the Ashley variety, for example, the average moisture content was approximately 34%, but individual nuts showed values ranging from just 3% to over 70%.
As the harvest progressed, both the average moisture and its dispersion decreased.
CKnowing only the average moisture content can give an incomplete view of the product entering the dryer.
2. Drying reduces moisture and its variability, but does not eliminate it.
Reaching a certain average moisture level does not necessarily mean that all the nuts have reached a suitable condition.
After drying, the differences between nuts are considerably reduced, although they do not disappear.
The study even found batches removed from the dryer too early, with average moisture levels exceeding 8% and a significant number of nuts still above 10%. Possible causes include batch sampling and the measurements used to determine when to end the process.
3. The process continues after the dryer
Moisture equalization continues after active drying is complete.
Some containers were observed for several days after leaving the dryer. The nuts with higher moisture content continued to lose water, and the range of moisture levels within the batch decreased.
In the trials carried out on the farm, the containers were exposed to the environment and the sun, so the authors clarify that part of this reduction could also be due to solar drying.
4. Nuts seek balance with the environment
This demonstrates that the moisture content of the walnut does not freeze when the dryer is turned off, but continues to evolve according to its condition and the surrounding air. In this analysis, both samples were left in an environment with 45% relative humidity. At this relative humidity, the moisture content tends to be around 6% on a wet basis.
The equalization tests showed two opposing behaviors: insufficiently dried nuts continued to lose moisture, while overdried nuts were able to recover moisture from the environment. Furthermore, moisture loss was faster than recovery.
What does this study tell us?
The 1988 study shows that Drying nuts is not simply a matter of bringing a batch to a certain average moisture percentage..
Within that average there is a distribution of wetter and drier fruits, and that distribution continues to evolve after drying.
Therefore, understanding product variability is as important as controlling temperature, airflow, or process time.
The real challenge is to achieve sufficiently uniform drying, while simultaneously avoiding leaving the product damp and unnecessarily prolonging the process by over-drying the rest of the batch.
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