Water Bath Canning Diced Tomatoes: The Ultimate Beginner’s Guide

There is a specific kind of magic that happens in a kitchen during late August. The air is thick with the scent of vine-ripened fruit, and the counters are buried under mounds of crimson globes. But for the home preserver, this isn’t just a harvest—it’s the birth of a “grocery store” right in your own home.

If you are new to canning, diced tomatoes are your perfect entry point. Unlike complex jams that require pectin or low-acid vegetables that require a pressure canner, diced tomatoes are straightforward, versatile, and incredibly forgiving. By the time you hear that first “ping” of a sealing lid, you’ll be hooked on the security and flavor of home-preserved food.

The Science of Safety: Why Tomatoes Need a “Boost”

Before we pick up a knife, we have to address the most important rule of canning: Acidity.

In water bath canning, the high heat of boiling water kills most bacteria. However, it isn’t hot enough to kill botulism spores. To keep those spores from growing, the environment inside the jar must be highly acidic (a pH of 4.6 or lower).

While tomatoes are naturally acidic, modern hybrids have been bred to be sweeter, which often pushes their pH levels into the “danger zone.” To ensure every jar is safe, we use a process called acidification. You must add bottled lemon juice or citric acid to every single jar.

Note: Always use bottled lemon juice, not fresh. Bottled juice has a consistent, regulated acidity level, whereas fresh lemons vary wildly.

The Essential Gear

You don’t need a professional kitchen to can tomatoes, but you do need a few specific tools to ensure success:

1. A Water Bath Canner: This is simply a very large pot with a fitted lid and a wire rack at the bottom. The rack is vital; it keeps the jars from touching the bottom of the pot, preventing breakage.
2. Canning Jars, Lids, and Rings: Use only jars specifically made for canning (like Ball or Kerr). Ensure your lids (the flat discs) are brand new; the rings can be reused as long as they aren’t rusted.
3. The “Big Three” Tools: A jar lifter (to move hot jars safely), a wide-mouth funnel (to keep jar rims clean), and a bubble remover (a plastic spatula to release trapped air).
4. The Ingredients: 25 lbs of tomatoes (for about 7 quarts), bottled lemon juice, and optional canning salt.

Choosing and Prepping Your Tomatoes

For dicing, the variety of tomato matters. While a juicy Beefsteak is great for sandwiches, it contains too much water for canning.

The Best Varieties: Look for “Paste” tomatoes like Roma, San Marzano, or Amish Paste. These have thick walls, meaty interiors, and very few seeds. They hold their shape beautifully during the boiling process.

The Blanching Ritual: You cannot leave the skins on. They become tough and stringy in the jar. To peel them effortlessly:

1. Cut a small “X” in the bottom of each tomato.
2. Drop them into boiling water for 30–60 seconds until the skin starts to wrinkle.
3. Immediately plunge them into a bowl of ice water. The skins will slip off in your hands like a loose glove.

The Master Recipe: Step-by-Step Diced Tomatoes

We will use the Raw Pack method. It is the easiest for beginners because you pack the cold, diced tomatoes into hot jars and let the canner do the cooking.

Step 1: Prep Your Jars
Wash your jars in hot, soapy water. Keep them warm in a low oven ($180^\circ F$) or in your simmering canner until you are ready to fill them. Cold tomatoes in a cold jar entering boiling water will cause the glass to shatter (thermal shock).

Step 2: Add the Acid
Before the tomatoes go in, add your acid to the empty, hot jars:

• For Quarts: 2 tablespoons of bottled lemon juice OR 1/2 teaspoon citric acid.
• For Pints: 1 tablespoon of bottled lemon juice OR 1/4 teaspoon citric acid.

Step 3: Pack the Jars
Dice your peeled tomatoes into 1/2-inch chunks. Pack them tightly into the jars. As you press down with a spoon, the tomatoes will release their own juice. You want the tomatoes to be submerged in their own liquid.

Step 4: Headspace and Bubbles
Leave 1/2 inch of headspace (the gap between the top of the food and the rim of the jar). Run your bubble remover tool around the inside of the jar to release trapped air. If the level drops, add more tomatoes.

Step 5: Clean and Seal
Wipe the rim of the jar with a clean, damp paper towel. Any speck of tomato or salt will prevent a seal. Place the lid on and screw the ring until it is “finger-tip tight.” Do not over-tighten; air needs to escape during processing.

The Processing Phase

Lower your jars into the boiling water bath. Ensure there is at least one to two inches of water covering the tops of the jars.

Processing Times:

• Pints: 35 minutes
• Quarts: 45 minutes

Crucial: Start your timer only when the water returns to a full, rolling boil.

Altitude Adjustment:
If you live above 1,000 feet, you must increase your processing time.

• 1,001–3,000 ft: Add 5 minutes.
• 3,001–6,000 ft: Add 10 minutes.

The “Ping” and Storage

When the time is up, turn off the heat and let the jars sit in the water for 5 minutes. This stabilizes the pressure and prevents “siphoning” (liquid leaking out).

Use your jar lifter to remove them and place them on a towel-lined counter. Do not touch the lids. As they cool, the vacuum will pull the lid down, creating a loud, metallic ping!—the most satisfying sound in a kitchen.

The “Rings Off” Rule: After 24 hours, check the seals. If the lid doesn’t move when you press the center, it’s sealed. Remove the metal rings. Storing jars with rings on can hide a “false seal” if the food spoils and gases push the lid up. Wipe the jars, label them with the date, and store them in a cool, dark place.

Quick Reference: Yield and Acidification

Container Size	Bottled Lemon Juice	Citric Acid	Processing Time (0-1,000ft)
Pint (16 oz)	1 Tablespoon	1/4 Teaspoon	35 Minutes
Quart (32 oz)	2 Tablespoons	1/2 Teaspoon	45 Minutes

Yield Guide:

• 1 Quart = Approx. 3 lbs of tomatoes.
• A 25 lb box = Approx. 7–9 Quarts.

Your First Step Toward Food Independence

There is a profound sense of security that comes from looking at a pantry shelf lined with ruby-red jars. You know exactly what is in them—no preservatives, no BPA, just the taste of summer.

This weekend, head to your local farmer’s market, grab a “canner’s box” of Romas, and take your first step. Your winter self—and your Sunday night chili—will thank you.