Freezing Walnuts Does Not Destroy Their Bioactive Compounds
Freezing at -20°C is an effective preservation method for walnuts that maintains their bioactive compounds, including phenols, antioxidants, and fatty acids, particularly when stored in-shell for up to 10 months. 1
Evidence Supporting Frozen Storage
Preservation of Bioactive Compounds
Frozen storage at -20°C maintains total phenol content and total antioxidant activity in walnuts, with walnuts stored in-shell showing the best preservation of kernel quality over 10 months compared to fresh kernels or walnuts with green husk 1
Antioxidant enzyme activities (peroxidase, catalase, and superoxide dismutase) are preserved during frozen storage, contributing to maintained kernel quality and acceptable acid and peroxide values throughout the storage period 1
Fatty acid composition, including the critical polyunsaturated fatty acids that make walnuts nutritionally valuable, remains stable during frozen storage 1
Key Bioactive Compounds in Walnuts
Walnuts contain several important bioactive compounds that contribute to their health benefits:
Polyphenols: Walnuts contain the highest total phenolic content among common nuts (432.9 mg/100g), with polymeric procyanidins being the dominant polyphenols (415.1 mg/100g) 2
Alpha-linolenic acid (omega-3 fatty acid): This critical component is metabolized into bioactive oxylipins and has anti-inflammatory and cardioprotective effects 3
Ellagitannins and ellagic acid: These are metabolized by gut microbes into urolithins, which are bioactive anti-inflammatory compounds 4
Phytosterols and phytomelatonin: These contribute to cholesterol-lowering and anticancer effects 3
Optimal Storage Recommendations
Temperature and Duration
For long-term storage (up to 10 months), freeze walnuts at -20°C in-shell to maximize preservation of bioactive compounds, fatty acids, and antioxidant enzyme activities 1
For short-term storage (up to 3 months), refrigeration at 0°C is acceptable, with walnuts stored with green husk showing better quality preservation than fresh kernels alone 1
Storage at 11°C or lower is optimal for preventing oxidative rancidity and maintaining walnut quality 5
Storage Form Matters
In-shell walnuts show superior preservation during frozen storage compared to shelled kernels, maintaining better fatty acid content, total phenols, and antioxidant activity 1
For short-term refrigerated storage, walnuts with green husk provide better protection against oxidation, resulting in lower acid and peroxide values 1
Common Pitfalls to Avoid
Avoid storage at room temperature (21°C), as this accelerates hexanal formation (a marker of rancidity) and degradation of bioactive compounds 5
Do not store at -20°C without proper packaging, as oxygen exposure even at low temperatures can lead to oxidative degradation 5
Avoid repeated freeze-thaw cycles, as this can compromise the structural integrity of the kernels and potentially affect bioactive compound stability (extrapolated from general biospecimen handling principles 4)
Mechanistic Considerations
The preservation of bioactive compounds during freezing relates to:
Reduced enzymatic activity at low temperatures, which slows oxidative degradation of polyunsaturated fatty acids and phenolic compounds 1
Maintained antioxidant enzyme function, which continues to protect against oxidative stress even during frozen storage 1
The protective effect of the shell or husk, which provides a physical barrier against oxygen exposure and moisture loss 1