If you’ve discovered mold growing in your canned homemade pesto, you’re not alone. This guide is designed for home canners and pesto lovers who want to understand how to safely address mold in their homemade preserves. We’ll cover practical steps to fix or prevent mold, ensuring your pesto stays fresh and safe to enjoy.
1. Identify the Type of Mold and Assess Safety
Not all mold on canned pesto is the same, but any visible mold on low-acid, oil-based foods like pesto is a serious red flag. Mold growth in canned foods often indicates improper sealing or storage. Since pesto contains basil, nuts, and oil with low acidity, mold can produce harmful mycotoxins. If you see mold inside the jar or on the surface, it’s safest to discard the entire jar to avoid foodborne illness.
2. Understand Why Mold Forms in Canned Pesto
Mold growth usually results from oxygen exposure, inadequate sterilization, or contaminated ingredients. Pesto is traditionally high in oil and fresh ingredients, which creates an environment where mold can thrive if not canned properly. Using a pressure canner is essential since pesto includes low-acid ingredients like basil and nuts, which require temperatures above boiling to kill spores.
3. Prevent Mold by Using Proper Pressure Canning Techniques
Water bath canning is unsafe for pesto because it doesn’t reach the temperatures needed to eliminate Clostridium botulinum spores or mold spores. Always use a pressure canner, processing jars at 10-15 psi for 20-25 minutes depending on altitude and jar size. This ensures that mold-causing organisms and bacteria are destroyed, preventing spoilage and health risks.
4. Store Canned Pesto Correctly to Minimize Mold Risk
After canning, store pesto jars in a cool, dark place between 50-70°F (10-21°C). Avoid temperature fluctuations and direct sunlight, which can degrade oil and encourage mold growth. Refrigerate opened jars promptly and consume within a week to limit mold development on exposed pesto.
5. Use Acidification to Inhibit Mold Growth
Adding acid such as lemon juice or vinegar to pesto before canning can help lower the pH and inhibit mold growth. The USDA recommends a pH below 4.6 for safe canning, but pesto’s natural pH is usually higher. Incorporating a tablespoon of lemon juice per pint can help, but acid alone is insufficient without proper pressure canning.
6. Avoid Using Oil as a Sealant Layer in Jars
Some pesto recipes suggest adding a thin layer of olive oil on top before sealing to prevent air exposure. While this may slow mold growth temporarily, it doesn’t replace proper canning methods. Oil can trap air underneath, creating anaerobic conditions that actually encourage botulism if not canned correctly. Rely on proper sterilization and sealing instead.
7. Inspect Jars Before and After Canning
Check jars for cracks, chips, or rusted lids before canning, as damaged containers can compromise seals and allow mold spores in. After processing, ensure lids have sealed properly by pressing the center; it should not flex up or down. Any jars with unsealed lids should be refrigerated and consumed quickly or reprocessed immediately.
8. Reprocess Unsealed Jars Promptly
If you find jars that did not seal after canning, do not leave them at room temperature. Reprocess these jars within 24 hours using the pressure canner to prevent mold and bacterial growth. Alternatively, refrigerate unsealed jars and use within a few days to avoid spoilage.
9. Discard Moldy Pesto Safely
Never attempt to scrape off mold from canned pesto and consume the rest. Mold roots penetrate deeper than the surface, and harmful toxins may be present throughout. The safest option is to discard the entire jar, including the contents and the jar itself if possible, to prevent cross-contamination.
10. Consider Freezing Pesto Instead of Canning
Because of pesto’s low acidity and oil content, freezing is often the safest preservation method. Freezing pesto in airtight containers or ice cube trays maintains flavor and prevents mold without the risks associated with canning. Frozen pesto can be stored for up to 6 months and thawed as needed.
How to Choose
When deciding how to handle mold in canned pesto, prioritize safety above all. If mold is present, discard the jar. For future preservation, choose pressure canning over water bath methods to meet USDA safety standards. Consider whether you want long-term storage (pressure canning) or shorter-term convenience (freezing). Also, evaluate your equipment availability and experience with pressure canning, since improper processing is the main cause of mold growth. Finally, select recipes that include acidification steps and avoid oil layering as a substitution for proper sealing.
Frequently Asked Questions
Can I salvage pesto if there is mold on top?
It is unsafe to eat pesto with any mold growth. Mold can penetrate the entire jar and produce toxins, so discard the jar completely.
Is water bath canning pesto safe?
No. Pesto contains low-acid ingredients that require pressure canning to safely kill spores and prevent mold and botulism.
Why does mold grow even if the jar is sealed?
Mold may grow if the jar was not processed properly, if the seal was compromised, or if contamination occurred before sealing.
How long can homemade canned pesto last?
Properly pressure-canned pesto stored in a cool, dark place can last 1 year. Once opened, refrigerate and consume within 7 days.
Can I add extra lemon juice to prevent mold?
Adding lemon juice lowers pH and helps inhibit mold but does not replace the need for pressure canning and proper sterilization.
Is freezing pesto a better option than canning?
Freezing is often safer and easier for pesto because it avoids the risks associated with low-acid canning and mold growth.
Final Thoughts
Mold in canned homemade pesto signals a serious safety issue that should never be ignored or worked around. Proper pressure canning, correct storage, and ingredient handling are essential to prevent mold and keep your pesto safe and delicious. If mold does appear, discard the jar entirely and consider freezing pesto as a safer alternative. Following these guidelines will help you enjoy homemade pesto without the risks of spoilage or illness.
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