July/August 2025
Postharvest precision
A study from researchers in China is shedding light on a simple way to boost walnut quality after harvest: skip the traditional air drying and go straight to the oven.
Published in the Journal of Food Science and Technology, the research paper “Postharvest drying of walnuts: Effect of drying methods on walnut quality” found that oven drying at controlled, low temperatures outperforms natural air drying when it comes to preserving walnut flavor, appearance and nutritional value.
For growers and processors, that could mean a better product, fewer postharvest losses and stronger market appeal.
A faster, cleaner way to dry
Drying walnuts is a critical postharvest step that reduces kernel moisture to safe levels — typically around 5% to 8% — to prevent mold and spoilage.
In the study, researchers compared two methods: oven drying in a controlled chamber and traditional air drying under natural conditions. Oven drying proved to be faster and more consistent, reliably reaching target moisture levels within 24 to 48 hours. In contrast, air drying often took several days, with results that varied depending on weather. Because oven drying offers greater control, it also reduced the risk of over-drying, which can damage kernel texture.
For processors looking to move product efficiently, the ability to dry walnuts in just one or two days could provide a significant advantage.
Color and texture get a boost
Visual appeal plays a big role in the value of high-grade or retail-packaged walnuts, and the study found that oven drying offered clear advantages in this area.
Kernels dried in a controlled oven stayed brighter and more uniform in color, maintaining a fresh, just-harvested look. In contrast, air-dried walnuts often darkened unevenly, with some samples showing early signs of browning. Texture was also more dependable in the oven-dried batches, which avoided the rubbery or overly brittle textures sometimes seen with natural drying.
In a market where appearance directly impacts pricing and consumer preference, that level of consistency can make a real difference.
Better flavor and less rancidity
When it came to taste, oven drying also proved superior.
Walnuts dried in a controlled oven had a stronger fresh-nut aroma and a more appealing, clean flavor. In comparison, air-dried nuts were more prone to developing musty or grassy off-notes, particularly when dried in warm or humid conditions.
Beyond flavor, the researchers also measured oil oxidation, which is an early indicator of rancidity, and found that oven drying did a better job of protecting the healthy fats and antioxidants that give walnuts their nutritional value.
Oil and nutrients stay intact
Walnut oil is rich in heart-healthy polyunsaturated fats and antioxidants, but these beneficial compounds are sensitive to heat, light and oxygen.
In the study, oven-dried walnuts showed lower levels of oxidation, indicating better oil stability and freshness. By contrast, air-dried nuts had higher peroxide values, signaling the early breakdown of fats. The researchers also found that phenolic compounds, which are key contributors to antioxidant activity, were better preserved in oven-dried samples.
For processors focused on shelf life, nutritional quality or clean-label marketing, these findings highlight the value of controlled drying in maintaining walnuts’ health appeal.
Takeaways for growers and handlers
Controlled oven drying offers growers and processors a reliable way to enhance walnut quality after harvest. It delivers faster, more predictable drying times — helping move product to market efficiently — while preserving the golden appearance, clean flavor and nutritional value that define a premium nut.
By maintaining more of the heart-healthy oils and antioxidants that today’s consumers value, oven drying also supports both shelf life and health-focused marketing.
While the method does require an investment in equipment and energy, the long-term gains can outweigh the upfront costs. Consistent drying reduces spoilage, improves grading outcomes and ultimately supports higher returns. For many operations, it’s a practical upgrade that doesn’t require overhauling existing systems — just a shift in postharvest strategy.
As demand continues to grow for high-quality, shelf-stable walnuts, adopting a more controlled approach could offer a valuable edge in an increasingly competitive market.
Walnut cultivar-specific drying checklist
Use this quick guide to improve drying outcomes across common walnut cultivars.
Chandler
- Drying preference: Oven drying
- Recommended oven temp: 104°F to 113°F
- Target drying time: 24 to 36 hours
- Notes: Sensitive to color changes during air drying; oven drying maintains kernel brightness and nutty flavor.
Serr
- Drying preference: Oven or mixed (pre-air dry, finish in oven)
- Recommended oven temp: 108°F
- Target drying time: 24 to 48 hours depending on size
- Notes: High oil content; benefits from quick drying to minimize oxidation and off-flavors.
Tulare
- Drying preference: Oven drying preferred
- Recommended oven temp: 113°F
- Target drying time: 30 to 40 hours
- Notes: Retains flavor well; even drying helps avoid internal darkening.
Hartley
- Drying preference: Oven drying strongly recommended
- Recommended oven temp: 109°F to 113°F
- Target drying time: Roughly 36 hours
- Notes: Shell shape can trap moisture in air drying; controlled heat ensures safe moisture reduction.
Howard
- Drying preference: Can handle natural drying but benefits from oven finish
- Recommended oven temp: 104°F
- Target drying time: Roughly 48 hours
- Notes: Tends to brown during slow drying; oven preserves color and limits bitterness.
General tips across cultivars
- Never exceed 122°F to avoid cooking kernels.
- Monitor relative humidity. Aim for 55% to 60% relative humidity in drying chamber.
- Check moisture regularly. Target 8% kernel moisture for storage; 5% to 6% for retail packaging.
- Rotate trays or bins for even heat distribution.
