How To Preserve Tomato Paste Through Canning

Tomato paste is a kitchen staple—rich, concentrated, and deeply savory—but its high water activity and low acidity make it a risky candidate for home canning if not handled correctly. Unlike whole tomatoes or salsas, which have established safe guidelines, preserving pure tomato paste in sealed jars demands precise attention to pH, processing methods, and equipment. Many home cooks assume that because tomato paste is thick and cooked down, it’s inherently shelf-stable. That misconception can lead to dangerous outcomes, including the risk of botulism. Proper preservation requires understanding food science, acidification protocols, and pressure canning procedures. This guide provides a comprehensive, safety-first approach to canning tomato paste at home, ensuring flavor retention, microbial safety, and long-term storage without refrigeration.

Definition & Overview

Tomato paste is a highly concentrated form of tomatoes achieved by cooking down ripe tomatoes for several hours until most of the water evaporates. The result is a thick, viscous product with an intense umami-rich flavor, deep red color, and natural sweetness balanced by acidity. Commercially, it’s sold in cans or tubes, but homemade versions offer superior freshness and control over ingredients. While refrigeration or freezing are common preservation methods, canning allows for extended shelf life at room temperature—provided it's done using scientifically validated techniques.

Canning involves sealing food in glass jars and applying heat to destroy microorganisms and create a vacuum seal. For low-acid foods like tomato paste (pH typically between 4.1 and 4.6), boiling water bath canning is insufficient. Only pressure canning achieves the temperatures needed—above 240°F (116°C)—to neutralize spores of *Clostridium botulinum*, the bacterium responsible for botulism poisoning. Therefore, successful preservation of tomato paste hinges on correct acidification and use of a pressure canner.

Key Characteristics of Tomato Paste

• Flavor Profile: Deeply savory, slightly sweet, with tangy undertones from natural acids.
• Aroma: Cooked tomato, caramelized, earthy; intensifies when heated.
• Color/Form: Thick, smooth paste ranging from brick red to dark maroon.
• pH Level: Naturally borderline acidic (4.1–4.6); must be adjusted below 4.6 for safe water bath canning, though pressure canning remains recommended.
• Culinary Function: Base ingredient for sauces, soups, stews, braises, marinades; enhances depth and color.
• Shelf Life (Unpreserved): Up to 7 days refrigerated, 6 months frozen.
• Shelf Life (Canned): 12–18 months when properly processed and stored in a cool, dark place.

Expert Tip: Never attempt to can plain, unacidified tomato paste using a boiling water bath. The density and low acidity create ideal conditions for anaerobic bacterial growth. Always use a pressure canner or add citric acid to lower pH if adapting recipes.

Why Canning Tomato Paste Requires Caution

The primary concern with canning tomato paste lies in its physical and chemical properties:

• Density: Thick consistency impedes heat penetration during processing. Cold spots may remain where pathogens survive.
• Low Acidity: Tomatoes hover near the threshold between high- and low-acid foods. Processing adjustments are non-negotiable.
• Oxygen Deprivation: Once sealed, jars create anaerobic environments—perfect for *C. botulinum* spore germination if under-processed.

The USDA and National Center for Home Food Preservation (NCHFP) do not publish a standalone recipe for canned tomato paste due to these risks. However, they endorse pressure canning for tomato products with proper preparation. This means following tested protocols, avoiding improvisation, and prioritizing safety over convenience.

Practical Usage: How to Use Canned Tomato Paste

Once safely preserved, canned tomato paste functions identically to store-bought versions but with richer, more nuanced flavor. A spoonful dissolves easily into hot liquids, making it ideal for:

• Building flavor bases for ragùs, curries, and chili
• Enhancing roasted vegetable glazes
• Strengthening the body of bean stews and lentil soups
• Creating quick pan sauces for meats or grains

Recommended Ratio: Start with 1 tablespoon per serving of sauce or soup. Adjust to taste, as homemade paste can vary in intensity based on tomato variety and reduction level.

To incorporate into dishes, scoop directly from the jar and whisk into simmering liquid. If the paste has settled or darkened slightly at the top (due to oxidation within the seal), stir thoroughly before use. Discard if there are signs of spoilage: bulging lids, off-odors, mold, or spurting liquid upon opening.

Variants & Types of Tomato Paste

While “tomato paste” refers broadly to reduced tomato puree, variations exist in preparation and concentration:

Type	Description	Suitable for Canning?
Homemade Slow-Cooked	Fresh tomatoes simmered 4–6 hours until reduced by 75%. No additives.	Yes, with acidification and pressure canning.
Double-Concentrated (Commercial)	Industrial evaporation produces thicker texture; often contains citric acid.	Not recommended for re-canning; already processed.
Sun-Dried Tomato Paste	Dehydrated tomatoes blended with oil; higher fat content.	No—oil-based pastes cannot be safely canned at home.
Tomato Puree vs. Paste	Puree is less reduced (~27% solids); paste is ~36%+ solids.	Puree can be canned similarly, but requires longer processing.

For home canning purposes, only plain, additive-free tomato paste made from fresh or peeled canned tomatoes should be considered. Avoid adding herbs, garlic, onions, or oils unless following a tested mixed-product recipe (e.g., tomato sauce).

Comparison with Similar Ingredients

Tomato paste is often confused with other tomato-based products. Understanding the differences ensures proper usage and safe preservation.

Product	Texture	Acidity (pH)	Canning Method	Storage Life (Canned)
Tomato Paste	Very thick, paste-like	4.1–4.6 (must acidify)	Pressure canner only	12–18 months
Tomato Sauce	Smooth, pourable	~4.3 (with lemon juice)	Boiling water bath or pressure	18–24 months
Whole Peeled Tomatoes	Firm chunks in juice	~4.3 (acidified)	Boiling water bath acceptable	18 months
Tomato Ketchup	Medium-thick, sweet-sour	~3.9 (high vinegar/sugar)	Not typically home-canned	Commercial only

The key distinction: thickness increases thermal resistance, requiring higher processing temperatures. Thus, while tomato sauce can be safely processed in a boiling water bath with added acid, tomato paste cannot.

Step-by-Step Guide to Safely Can Tomato Paste

This method follows best practices from the NCHFP and USDA guidelines for low-acid food preservation. It assumes the use of fresh, ripe tomatoes and standard home canning equipment.

1. Select and Prepare Tomatoes: Choose disease-free, blemish-free ripe tomatoes. Roma or San Marzano varieties are preferred for lower moisture and higher solids. Wash thoroughly, remove stems, and quarter.
2. Cook Down the Tomatoes: Simmer tomatoes in a heavy-bottomed pot over medium-low heat for 3–5 hours, stirring frequently to prevent scorching. Add no water. Strain through a food mill or sieve to remove skins and seeds. Continue reducing until the mixture holds its shape briefly when drawn across the bottom of the pot (“sheeting” test).
3. Acidify the Paste: For every quart of finished paste, add 1 tablespoon bottled lemon juice or ¼ teaspoon citric acid. This lowers the pH to a safe level (<4.6) and supports microbial inhibition even under pressure canning.
<4> Prepare Jars and Lids: Use standard Mason jars (half-pint or pint size). Inspect for chips or cracks. Heat jars in simmering water (180°F) to prevent thermal shock. Keep lids warm in a separate container.
4. Fill Jars: Ladle hot tomato paste into hot jars, leaving 1-inch headspace. Remove air bubbles with a non-metallic utensil. Wipe rims clean with a damp cloth. Apply two-piece lids (flat disc and ring), tightening fingertip-tight.
5. Process in Pressure Canner:
   • Place jars on rack in pressure canner with 2–3 inches of water.
   • Secure lid, vent steam for 10 minutes, then close vent to build pressure.
   • Process at 11 PSI for dial-gauge canners or 10 PSI for weighted-gauge canners.
   • Processing Time: 35 minutes for half-pints, 40 minutes for pints (adjust for altitude above 1,000 ft).
6. Cool and Store: Turn off heat, wait for pressure to drop naturally (do not force-cool). Remove jars and place on a towel-lined counter. Let sit undisturbed for 12–24 hours. Check seals: lids should be concave and not flex when pressed. Label with date and store in a cool, dark pantry.

Safety Checklist:

• Always use a calibrated pressure canner—not a stockpot.
• Never reduce processing time, even for smaller jars.
• Do not thicken paste with flour or starch before canning.
• Discard any jar that fails to seal or shows spoilage signs.

Practical Tips & FAQs

Can I use a boiling water bath instead?

No. Boiling water baths reach only 212°F (100°C), insufficient to destroy *C. botulinum* spores in dense, low-acid foods. Even with acidification, the internal temperature of thick paste may not rise adequately. Pressure canning is mandatory.

What if I want flavored tomato paste?

Add-ins like garlic, basil, or olive oil introduce additional variables that affect safety. These ingredients are low-acid or fat-based, increasing spoilage risk. To can flavored versions, follow a tested recipe such as “tomato-garlic sauce” with documented processing times. Do not improvise.

How do I know if my canned tomato paste has gone bad?

Check for:

• Bulging or swollen lids
• Spurting liquid when opened
• Foul, yeasty, or sulfur-like odor
• Mold on surface or inside lid
• Discoloration beyond normal darkening at the top layer

If any of these signs appear, discard the entire contents immediately—do not taste.

Can I freeze tomato paste instead?

Yes—and it’s often safer and simpler. Freeze in ice cube trays (1 tablespoon per cube), then transfer to freezer bags. Thaw as needed. Freezing preserves flavor better and avoids canning risks altogether.

Does home-canned tomato paste lose nutrients?

Lycopene, the antioxidant in tomatoes, becomes more bioavailable after cooking and canning. Vitamin C decreases slightly due to heat exposure, but overall nutritional value remains high. The trade-off is convenience and shelf stability.

Why use lemon juice instead of vinegar?

Lemon juice provides consistent acidity (citric acid) without altering flavor as dramatically as vinegar (acetic acid). Bottled lemon juice is preferred over fresh-squeezed for standardized pH levels.

“The margin for error in canning tomato paste is extremely narrow. Thickness matters as much as acidity. If you wouldn’t eat it raw, don’t assume heat will save it during processing.” — Dr. Elizabeth Andress, Director Emerita, National Center for Home Food Preservation

Storage, Shelf Life & Best Practices

Properly canned tomato paste retains quality for 12 to 18 months. Beyond that, it remains safe if sealed and stored correctly, but flavor and color degrade. To maximize longevity:

• Store jars in a cool (50–70°F), dark, dry location away from appliances or sunlight.
• Rotate stock using FIFO (first in, first out) principles.
• Label each jar with processing date and contents.
• Inspect seals before each use.

Once opened, refrigerate and use within 7 days. Scoop with a clean utensil to avoid cross-contamination.

Summary & Key Takeaways

Canning tomato paste is possible—but only when strict food safety protocols are followed. Unlike other tomato products, its density and marginal acidity require pressure canning and deliberate acidification. Boiling water baths are inadequate and pose serious health risks. By selecting quality tomatoes, reducing them properly, adding citric acid or lemon juice, and processing in a calibrated pressure canner, home cooks can preserve this versatile ingredient safely.

Remember: There is no shortcut in low-acid canning. Every step—from headspace to processing time—affects microbial safety. When in doubt, freeze instead. But for those committed to pantry self-reliance and flavor mastery, properly canned tomato paste offers unmatched convenience and culinary depth.

Preserve wisely: Review updated guidelines from the National Center for Home Food Preservation before every canning session. Share this knowledge with fellow home preservers—safety starts with education.