Tomatoes are a garden favorite, and home canning is a wonderful way to enjoy your harvest year-round. But when it comes to canning tomatoes, you might wonder: Why is lemon juice so important? The short answer is that adding lemon juice ensures safety by increasing the acidity of the tomatoes. Let’s dive deeper into why this step is crucial and how to do it correctly.
The Importance of Acidity in Canning Tomatoes
Tomatoes are considered a borderline high-acid food. This means their natural acidity is sometimes not high enough to prevent the growth of *Clostridium botulinum*, the bacteria that causes botulism, a severe and potentially fatal form of food poisoning.
Over the years, tomato varieties have changed, with many modern types having milder flavors and lower acidity than their older counterparts. Testing has revealed that some tomatoes now have pH values at or above 4.6, and some even higher. A pH of 4.6 is the critical threshold; above this, *C. botulinum* can thrive.
Adding lemon juice (or citric acid) lowers the pH of all tested tomato varieties enough to allow for safe water bath canning. Acidifying all whole, crushed, or juiced tomatoes is recommended to ensure the canned product’s safety after processing in a water bath canner, and it’s also essential for a safe, short process in a pressure canner.
How to Acidify Tomatoes Properly
The acidification process is straightforward, but precision is key. Here’s how to do it:
- Measure accurately: For every quart of tomatoes (whole, crushed, or juiced), add two tablespoons of bottled lemon juice or ½ teaspoon of citric acid. For pints, use one tablespoon of bottled lemon juice or ¼ teaspoon of citric acid.
- Add directly to the jar: You can add the lemon juice or citric acid directly to the jars before or after filling them with the tomato product. Just make sure each jar gets the correct amount.
- Use bottled lemon juice: It is crucial to use bottled lemon juice rather than freshly squeezed. Bottled lemon juice has a standardized acidity level, whereas fresh juice varies too much to be reliable.
- Don’t reduce the acid: You can add sugar to offset the acid taste after opening and processing the jars, but never reduce the amount of acid before canning.
By acidifying tomatoes as recommended, you can be confident that you have safely preserved them to enjoy throughout the year.
Water Bath vs. Pressure Canning
Tomatoes can be safely canned using either a water bath canner or a pressure canner, but acidification is still necessary in both methods.
- Water Bath Canning: This method involves submerging sealed jars of food in boiling water for a specific time. The high heat kills most harmful microorganisms. Because tomatoes are borderline in acidity, adding lemon juice ensures the pH is low enough to prevent botulism.
- Pressure Canning: Pressure canning uses a specialized canner to create high pressure and temperatures above boiling point. While pressure canning can kill *C. botulinum* spores, the USDA recommendations for tomatoes rely on having sufficient acidity to prevent the spores from germinating.
Even when pressure canning, acidifying tomatoes provides an extra layer of safety. USDA researchers based the pressure canning process for raw packed tomatoes on killing spoilage organisms in acidified tomatoes, not on inactivating *C. botulinum* spores. If a pressure canning process were developed for unacidified tomatoes, it would require a much longer processing time.
Additional Tips for Safe Tomato Canning
- Use current, research-based recipes: Always use up-to-date recipes from reliable sources when canning tomatoes or any other produce. Recommendations and preservation methods change over time, so using current methods is critical for a safely preserved product.
- Recommended resources: Consult resources such as the MSU Extension Michigan Fresh, the USDA Guide to Complete Home Canning, the National Center for Home Food Preservation, the So Easy to Preserve book, or a current Ball Blue Book.
- Adjust for altitude: If you live at a high altitude, you may need to adjust the processing time in your canning recipes. Water boils at a lower temperature at higher altitudes, so it takes longer to kill harmful microorganisms.
- Inspect jars carefully: After processing, check that the jars have sealed properly. The lids should be concave and not flex when pressed. If a jar doesn’t seal, you can reprocess it within 24 hours or refrigerate the contents and use them soon.
- Maintain proper headspace: Headspace refers to the space between the top of the food and the lid of the jar. For tomatoes, leave ½ inch of headspace. This allows for proper expansion during processing and helps create a good seal.
- Remove air bubbles: Before sealing the jars, remove any trapped air bubbles. You can use a non-metallic spatula or bubble tool to gently release air bubbles from the sides of the jar.
- Wipe jar rims: Before placing the lids on the jars, wipe the rims clean with a damp cloth. This ensures a good seal between the jar and the lid.
Why Trust the Recommendations?
When it comes to food preservation, following tested recipes and guidelines is essential. Recommendations are based on data from specific recipes that consider factors like heat transfer and acidity. These factors determine whether the recipe is safe for everyone to use.
Acid in tomatoes acts as a safeguard, ensuring that regardless of geography, elevation, tomato variety, or local growing conditions, the canned tomatoes will be safe to eat.
Other Options for Acidifying Tomatoes
While bottled lemon juice is a popular choice, citric acid and vinegar are other options for acidifying tomatoes.
- Citric Acid: A natural acid found in citrus fruits, citric acid is a good alternative to lemon juice. Use ½ teaspoon of citric acid per quart of tomatoes.
- Vinegar: Vinegar can also be used to acidify tomatoes, but it may slightly alter the flavor. Use 4 tablespoons of 5% acidity vinegar per quart of tomatoes.
Choose the method you prefer, but always adhere to the recommended amounts to ensure safety.
Addressing Taste Concerns
Some people worry that adding lemon juice or other acids will make their canned tomatoes taste too tart. While it’s true that the added acid can affect the flavor, there are ways to mitigate this.
- Add Sugar After Opening: If you find the canned tomatoes too acidic after opening a jar, you can add a pinch of sugar to balance the flavor. However, do not add sugar before canning, as this can interfere with the acidification process.
- Choose Low-Acid Tomato Varieties: Some tomato varieties are naturally lower in acid. While you still need to add lemon juice for safety, starting with a less acidic tomato can result in a milder flavor.
- Balance with Other Flavors: When using canned tomatoes in recipes, balance the acidity with other flavors. Ingredients like onions, garlic, and herbs can help create a more complex and balanced taste.
The Bottom Line
Adding lemon juice to tomatoes when canning is a critical safety measure. It ensures that the acidity is high enough to prevent the growth of *Clostridium botulinum*, protecting you and your family from botulism. While it may seem like an extra step, it’s a small price to pay for the peace of mind that comes with knowing your home-canned tomatoes are safe to enjoy. Always follow current, research-based recipes and guidelines, and happy canning!