by John W. Jett, WVU Extension Specialist

Tomatoes are generally considered a high-acid food, with a pH below 4.6. Unfortunately, much misinformation has circulated about “low acid” tomatoes, referring to those with a sweet taste.

Some tomato varieties (mostly yellow and pink) are slightly less acidic than others. But the difference is so small that there is no real difference in taste when they are grown under similar conditions. The ratio of sugar to acid in different varieties accounts for flavor differences

Most of the tomato fresh weight is water; solids constitute only 5 percent. These solids consist of water-insoluble substances such as cell walls and soluble components such as sugars and acids.

A tomato’s flavor is determined by the amount of sugar (generally about half of the solids) and the amount of acids (generally about one-eighth of the solids) it contains. The best combination for good flavor is high amounts of both sugars and acids.

Low-sugar/high-acid tomatoes are tart; high-sugar/low-acid tomatoes are bland; and low-sugar/low-acid tomatoes are tasteless.

Two amino acids, glutamate and aspartic, are essential to the taste of tomatoes. The ratio of these two amino acids is also important. A glutamate-to-aspartic acid ratio of 4:1 makes the tomato taste the best and brings out the genuine tomato flavor. The amount of both acid and sugar increases when tomatoes ripen on the vine in plenty of sunshine. That is why vine-ripened tomatoes always taste superior.

Acid content is also a major factor when you process tomatoes. Researchers at the U.S. Department of Agriculture and the University of Minnesota have found that most underripe to ripe cooked tomatoes have a pH below 4.6.

Unfortunately, a few varieties are low in acid, having a pH above or close to 4.6. These varieties include Ace, Ace55VF, Beefmaster Hybrid, Big Early Hybrid, Big Girl, Big Set, Burpee VF Hybrid, Cal Ace, Delicious, Fireball, Garden State, Royal Chico, and San Marzano. To can these varieties safely, you’ll need to add acid for water-bath processing or use the pressure canning process like you do for other low-acid vegetables.

The new USDA Home Canning Guides recommend that you add lemon juice or citric acid to each canning jar of tomato product to be certain of correct pH. This procedure is recommended if you use one of the varieties listed above, use unknown varieties, or wish to minimize the potential risk of botulism.

—WVU-ES—