Low Acid Tomatoes
Tom Spies - Cross Country Nurseries, Dec 2023
Introduction and TLDR
"Low Acid Tomatoes" is one of the most frequent search terms on our site, so I wanted to put forth some effort into the topic.
On review of several studies, it appears that there are not certain varieties that are particularly low in acid. The most likely scenario is that certain tomato varieties are higher in sugar content (measured in Brix), and this masks the natural acidity of the tomato.
The current sentiment is that yellow and other lighter colored varieties are lower in acid, and this is how we define them on our website. I wasn't able to find this trend in the data found. Further research is definitely needed on this topic.
What is Acidity?
Acidity is defined using the pH scale which ranges from 0 to 14. Anything below 7 is considered acidic.
How Acidic is a Tomato?
This study is often referenced to compare the acidity of tomato varieties. It shows that the pH ranges from about 4.1 to 4.7. This is a fairly narrow range, but note that the pH scale is logarithmic. This means that a tomato with a pH of 4 is ten times as acidic as a tomato with a pH of 5.
Below is a chart made from the data in "Tomato Acidity and the Safety of Home Canned Tomatoes". I added the tomato colors and shapes, then averaged the pH. Some of the varieties in the study are extinct hybrids, but most had data still available.
The results are inconclusive based on the limited data (7 yellow varieties, but 41 red), but the data present suggests that yellow varieties are slightly more acidic (a lower pH is more acidic).
Interstingly the paste tomatoes, led by San Marzano, were the least acidic on average.
Taste of Acidity and Sugar Concentration
Sugar concentration is measured in Brix, and is commonly used to judge the quality of a tomato.
Is there any merit to the idea that the higher sugar concentration over powers the fairly standard pH level in a tomato and making it seem less acidic?
Unfortunately, the data here is anecdotal at best. One way to look at it is that orange juice is usually considered acidic, while apple juice is sweet. The USDA lists the pH of apple juice as ranging from 3.20 to 4.33, and orange juice at 4.2. Apple juice has a large pH range, but on average, is more acidic than orange juice. The USDA is citing data from the 1940s, but again, we are working with what's available.
The sugar percentage for apple juice is 13.2%, and the sugar content of orange juice is 8.1%. This supports the argument that a higher sugar content can mask acidity, but it's not exactly case closed.
Tomato Studies in Brix
As I found with studies regarding pH, very few studies are available that research the Brix content in tomatoes. It's likely that most research is commercial in nature: ie a company in Big Tomato will know the sugar content of the varieties they sell to supermarkets, but keep that information internally.
The variety of tomato of course has an impact on Brix, but so does the growing conditions: soil type, average rainfall, and ground vs. container growing to name a few. These factors also effect the pH.
One study available, from 1964 did perform a Brix analysis. Unfortunately, most of the varieties are now obsolete commercial varieties so color and shape information isn't available.
More promising is this research from 2016, which examined Brix and pH for several tomato varieties grown in ground and in containers. Again, lack of variety limits the conclusions we can safely draw from the results. Of the 21 varieties tested, only 4 were orange. Below is a chart with the colors added to the existing data.
Information is difficult to come by, but the tools to measure pH and Brix are accessable and inexpensive. We grow many varieties of tomatoes every year, so we'll start to catalog variety, size, color, pH, and Brix to build up a database of various tomato varieties and post the results when availble.