My mother used to grow “low acid” tomatoes because high acid levels gave her mouth cankers, or so she thought. There is a concern that canning low acid tomatoes causes botulism because the acid level is too low.
Lots of seed companies and garden blogs talk about low acid tomatoes and usually identify yellow, orange and small fruited varieties as low acid. Some claim that modern breeding has increased the acidity of tomatoes and that heirlooms have less acid. Others claim that there is no such thing as low acid tomatoes.
It turns out that this story starts as a myth. People tried to correct the myth only to create a new myth in the process. I’ll have to debunk the debunkers.
Acidity vs pH
The terms acidity and pH are a bit confusing. pH is a range of values from 0 to 14, with 7 being neutral and anything below 7 is acidic. That is simple enough. The problem comes in when we talk about acidity. A lower acidity is actually a higher pH, and conversely, a higher acidity is a lower pH.
For example, a pH of 6 is close to neutral (i.e. pH = 7), so it is a “low” acidity, meaning it is not very acidic. A pH of 2 is a high acidity since it is “very acidic”. It’s easy to get these terms mixed up.
What is a Low Acid Tomato?
Before we can discuss low acid tomatoes we have to define what this means. The topic of acid tomatoes started with a concern over canning tomatoes. If the canned tomatoes had a pH above 4.8, the organism Clostridium botulinum could grow causing botulism. To err on the safe side, food scientists use a cut off point of 4.6. Foods with a pH above 4.6 are considered to be “low acid foods” and foods with a pH below 4.6 are considered “high acid foods”.
What do gardeners mean when they talk about low acidity? I looked at lot of garden sites and seed companies and nobody defines what they mean. In the absence of a gardening definition we have to go with the one used by food scientists.
What is the pH of Tomatoes?
An important study from 1977 compared the pH of 58 tomato cultivars grown in one Maryland location. The pH ranged from 4.06 to 4.68.
The same study also collected pH values for 356 cultivars and 212 current breeding lines, from research facilities across the US, covering 57 different locations in 23 states. Both sets of data were used to reach the following conclusions.
Do Yellow or Small Tomatoes Have Less Acid?
The above mentioned study concluded that “The public has been told repeatedly that light-colored and small tomato cultivars are low in acid but it is a myth …. The small and light-colored varieties tend to be higher in acid (lower pH) than other tomato types”. This was the knowledge in 1977 and it is still the accepted knowledge. It is hard to believe that this myth is still very common today. I copied this image yesterday from an online seed company, still promoting the myth! That is sad!
Are Heirloom Tomatoes Less Acidic?
There is no pH trend between older cultivars and newer ones. The idea that heirloom tomatoes are less acidic is a myth. The idea that modern breeding is producing more acidic tomato is also a myth.
Are Some Tomatoes Less Acidic?
Based on the above data, some cultivars are more acidic than others, but the range seems small with all cultivars being between 4.1 and 4.7. This narrow range has caused some gardening experts to claim that “all tomatoes have about the same pH” and that there is no such thing as low acid tomatoes. Craig LeHoullier, in his book Epic Tomatoes says, “the acid level in just about all tomato varieties lies within a very narrow range”.
I don’t fully agree with this statement because pH is a logarithmic value. A pH of 5 does not sound much higher than 4, but since it is a log scale it represents a 10 fold decrease in acidity. That means that a tomato with a pH = 4.7 is quite a bit less acidic than one with a pH of 4.1.
If acid is causing you digestion problems, or mouth cankers, you can certainly select a cultivar that is less acidic. The problem is finding such data. The above scientific references can help, but they don’t contain values for newer cultivars. Seed catalogues don’t provide this kind of data and their labels of “low acidity” can’t be trusted. So lower acid tomatoes do exist – you just don’t know which ones they are unless you can find their pH value in a research paper. The list below is taken from the above study.
Do Low Acid Tomatoes Exist?
If we define a low acid tomato as one with an average pH over 4.6, then they do exist, but only if grown in areas that produce low acid tomatoes. The table above shows data for the lower acidic tomatoes grown in Maryland at the test site and none would qualify as low acid tomatoes. The data in the table below was collected from across the US and several of these would qualify as low acid tomatoes. The cultivar Ace grown in Maryland (table above) had a pH of 4.5, while the same cultivar grown in Michigan (table below) had a pH of 4.8.
Do Acid-free Tomatoes Exist?
I found this question asked a lot. The simple answer is NO! Every living organism contains acids.
Taste vs Acidity
Do low acid tomatoes taste sweeter? You would think so, but they probably don’t taste sweeter. For one reason, I doubt humans can easily taste the difference between a pH of 4.1 and 4.7 – let me know in the comments, with references, if I am wrong.
The second reason is that taste is a function of several compounds and sugars (fructose and glucose) are important ones since they mask the acid taste. When people are asked to pick the least acidic tomato in a taste test they usually pick the ones with higher sugar levels. The Super Sweet 100 cherry tomato was introduced as a “low-acid” Sweet 100 variety not because of its acidity level, but because it tasted like a low acid tomato. Several orange varieties are sweet tasting even though they have average pH and I think that is why seed companies label them as low acid tomatoes.
Minerals in the fruit also affect taste. Both potassium and phosphate affect acid levels.
Bottom line; you can’t determine acidity by taste.
What Affects Acidity
Genetics is a main factor but climate and soil are also very important. When the above mentioned data was sorted by state, Iowa, California, and Michigan produced exceptionally high pH values, but this probably reflects highly localized areas rather than area-wide conditions.
Overripe tomatoes have a higher pH than ripe tomatoes. Tomatoes harvested from dead vines late in the season have lower pH values.
Fertilizer type also affects acidity. Nitrate fertilizer resulted in a lower acidity than ammonium fertilizer, chicken manure or grass-clover mulch. To know how much fertilizer you should use on tomatoes, have a look at: How To Fertilize Tomato Plants.
Should You Acidify Tomatoes Before Canning?
The research shows that most tomatoes have an average pH below the critical 4.8, but individual fruits of some cultivars can be above this. There are very few cases of botulism from home-canned tomatoes which means the risk is low. However, adding a bit of acidity with lemon juice or citric acid is a good safety measure.
Correction to: What is a Low Acid Tomato?”
“To air on the safe side…” should be “To err on the…”
Thanks.
Great entry. I always add lemon juice to tomatoes I can. It’s suggested by our extension office in Illinois.
Nanny always said “a spoonful of sugar helps the medicine go down”. Nanny also used to put a spoonful of sugar on her tomatoes when eating a salad, much to the chuckling delight of us children.
pH is a measure (logarithmic as you clearly state) of the hydrogen ion concentration. It is not measure of the amount of acid present. To measure this you need to do a tritration. An organic acid of a given concentration will usually have a much lower pH than the same amount of a mineral (non-organic) acid because mineral acids are less ionised in solution than the same amount of a mineral acid
You are correct. The 1977 study did measure both pH and acidity and found close correlation. I simplified things a bit for a topic that most people will find complicated enough.
I don’t think it changes any of the conclusions?
Bravo! Bravo, again! I grew up with women who “canned,” and those extra spoons of vinegar in each jar of tomatoes or sauce was one of my tasks as a wee girl who like to “help.” Later, I canned my own and dropping that vinegar into each quart jar was an instant of satisfaction. “No botulism in OUR home!” 🙂 There were women my age who scorned home-canned foods, but “putting up” the garden and the gathering was as much a part of my culture as any other tradition that insured that I was a “good woman.” Capable and smart.
Those of us who held to these traditions, also relied on the Extension Office to keep us informed, to provide the best recipes, and to tell us each summer, where to go to get our pressure canner gauges checked. Agri. Extension Service is a MARVEL, and I’m so glad I grew up with its benefits. (in N. California)
Thank you for another fine article to set the record straight.
I suspected as much. Years ago, my grandma said she could only eat “low-acid” tomatoes. Once at a cook-out, my cousin Jimmy gave her a Cherokee Purple and told her it was low-acid. She loved it and asked Granddad to grow it next year in their garden. When I looked it up, it was not listed as low-acid. I think the awesome flavor and smoky note made it taste less acid to her.
Robert: Early in your post you state: “If the canned tomatoes had a pH below 4.8, the organism Clostridium botulinum would not grow to cause botulism.”
Then, at the end, you state: “The research shows that most tomatoes have an average pH below the critical 6.8…”
Did you mean 4.8 instead of 6.8?
It is 4.8 – thanks for catching that.