Home preservers often wonder why USDA preserving recipes calling for lemon juice specify that the juice should come from a bottle. In most grocery stores the only such product used to be ReaLemon, which is made from concentrate and preserved with sulfites. Today a few competing brands of lemon-juice-from-concentrate are available, with similar assortments of preservatives. To most discerning cooks, ReaLemon and its imitators don’t taste quite real, and to people allergic to sulfites these products may be a health hazard. Bottled fresh lemon juice, with juice from Sicily or Peru, is available at some fancy grocery stores, but it also contains sulfites. Why shouldn’t home preservers use fresh lemons, which are inexpensive and available year-round in every supermarket? Is ReaLemon really better than real lemon?
Extension agents explain that lemons vary in their acidity, and that bottled lemon juice does not. To make sure your jam or your salsa–or, especially, your lemon curd— reaches a safe level of acidity, you should always use the bottled stuff, say the home economists. I decided to find out whether they’re right.
I first researched laws regarding bottled lemon juice. The Code of Federal Regulations (Title 21, volume 2, revised April 1, 2010), includes this FDA rule: Lemon juice prepared from concentrate, like ReaLemon, must have “a titratable acidity content of not less than 4.5 percent, by weight, calculated as anhydrous citrus acid.” Citric acid is the main acid in lemons. Lemons also contain some malic acid, but it usually isn’t measured separately. The ascorbic acid, or vitamin C, for which lemons are justly valued is destroyed by heat and so ignored in discussions of food processing. For our purpose here, we can say simply that lemon juice made from concentrate must have an acid level of at least 4.5 percent, and that the law allows this acid level to vary.
Hmmm. Even if the law allows a variable acid level, a manufacturer would settle on a standard, right? In the opinion of my husband, a chemist, that standard would be 4.5 percent. After all, water is cheap! Why would ReaLemon use more lemons than necessary?
I asked the folks at ReaLemon whether they standardized the acidity of their lemon juice and, if so, what their standard was. Here is their reply: “ReaLemon meets or exceeds the FDA standard of identity for lemon juice, which is 4.5% w/w.” This reinforced my husband’s opinion: ReaLemon had a standard acid level, and it was 4.5 percent.
We decided to test this hypothesis. I bought a bottle of ReaLemon, and we titrated the juice (I’ll explain in another post how to do this). ReaLemon tested at 4.9 percent—the “natural strength” of lemon juice, according to the label. The company rose in our estimation. They were exceeding a minimum standard!
If lemon-juice-from-concentrate is at least 4.5 percent acid, and sometimes 4.9 percent acid, what is the natural range of acidity in lemons? I posed this question to David Karp, a fruit researcher at the University of California, Riverside, who also writes for the Los Angeles Times. David referred me to Walton Sinclair’s Biochemistry and Physiology of the Lemon (University of California, 1984), a four-inch-thick summary of all scientific research on Citrus limon.
According to the research, some lemon varieties are more acidic than others. Lemons of a single variety can vary in acidity depending on the local soil and climate, the rootstock on which the tree is growing, the amount of fertilizer applied, and the season in which the lemons were picked. Lemons and other citrus fruits grown in hotter places, for example, are generally less acidic than those grown in cooler places. Both potassium and nitrogen fertilizers tend to increase acidity levels. New Zealand lemons are less acidic than California lemons, and California lemons are less acidic than Sicilian lemons.
Even a single lemon can show variations in acidity, depending on when you do the testing and from what part of the fruit you take the juice. California lemons increase their acid levels almost 25 percent during curing–that is, in the weeks of storage after harvest. One study found that juice from the stem end of a lemon is slightly more acidic than juice from the blossom end, and another study found that juice from the core area is slightly more acidic than juice from the periphery.
If all these variables make you think the home economists are right, think again. Although lemons vary in acidity, they generally don’t vary much. The least acidic lemon found among all those tested in dozens of studies, an uncured Eureka from California, had an acid level of 4.53 percent. The most acidic uncured Eureka tested at 6.50 percent, and cured Eureka lemons ranged from 5.71 to 7.42 percent. Lisbon lemons from California varied less, from 4.79 to 4.86 percent acid before curing and 5.25 to 5.32 percent afterward.
Florida lemons vary no more in their acidity than California lemons. In one Florida study, samples ranged from 5.16 to 6.41, in another from 5.24 to 5.92.
If you live outside the United States, the lemons in your market may be more or less acidic. In New Zealand lemons averaged only 4.9 percent acid, and in Italy lemons tested as high as 8.1 percent acid. But you won’t find lemons from New Zealand or Italy in Safeway or Albertsons.
Note that I’m not counting Meyer lemons as lemons. A cross between a lemon and an orange, the Meyer is relatively low in acid. Meyer lemons sampled in July averaged 2.4 percent acid in one study; those sampled in February and May averaged 4.1 percent acid.
With all this information before me, I guessed that the juice of a lemon from one of my local grocery stores would test at somewhere around 6.0 percent acid. It would almost certainly be a Eureka or a Lisbon (the fruits of these two varieties are hard to tell apart) or a clonal selection of one or the other. If it were a Eureka, it might be a little more acidic than 6.0 percent; a Lisbon might measure only about 5.0 percent.
So I bought a lemon, and my husband and I titrated the juice. It tested at 6.2 percent acid. Eureka! (Probably.) We drank some of the juice, too, and compared the taste with that of ReaLemon. The natural lemon juice was much less bitter (ReaLemon, like other varieties of bottled lemon juice, contains oil from the peel) and noticeably more sour.
Provided you start with regular lemons rather than Meyers, then, substituting fresh lemon juice for bottled in canning should be entirely safe, although the finished product might end up a little more tart than it would with bottled lemon juice.
Are you adding lemon juice to jam or jelly? This is done not for safety, generally—nearly all fruits are acidic enough for safe canning—but to ensure that the jam or jelly will jell. You can add a little less lemon juice than a recipe specifies if your fruit is quite tart, or a little more if you want a stronger gel.
If you’re canning tomatoes, the acidity of your lemons shouldn’t be a concern. Nearly all tomatoes are acidic enough to can without added acid. If yours are unusually dull in flavor, follow the USDA recommendation: Add 1 tablespoon lemon juice per pint of whole, crushed, or juiced tomatoes. Or, if you like, add more.
Recipes for canned salsa call for quite a lot of lemon juice (or lime juice, or vinegar). If you’re using several fresh lemons, their acidity will average out, and the average will almost certainly be higher than 4.5 percent. By using fresh lemons you may risk making your salsa a little too tart, but you can minimize this risk by using low-acid, paste-type tomatoes, such as Roma, which provide the additional advantage of making salsa thicker.
A particular concern of many home preservers is the safety of fresh lemon juice in canned lemon curd, a tart, buttery custard that’s used as a dessert topping and filling and as a spread for toast, pancakes, waffles, and so on. It’s essential to have a high level of acid in a protein-rich food that’s processed in a boiling-water bath. Home economists say that canned lemon curd is safe only if the lemon juice comes bottled, but remember: American store-bought lemons—the regular kind, not Meyers—are at least as acidic as bottled lemon juice. Besides, you may prefer to use more lemon juice in your curd than called for in the USDA recipe, which, I notice, contains proportionally less lemon juice than does my recipe in The Joy of Jams. You can find the USDA recipe here.
Lemon curd doesn’t need canning, of course. If you put it in a jar in the refrigerator instead, it will keep well for several weeks. You can also freeze lemon curd, and thaw it in the refrigerator for a day before you plan to serve it. If you have a lot of lemons and want to juice them right away, you might freeze the juice so you can make lemon curd as you need it. Lemon juice keeps very well in the freezer.
If you want to give your lemon curd as gifts, though, you may be set on canning it. In this case, be sure to follow the USDA processing instructions. Heat the water to no more than 180 degrees F. before adding the jars, and boil them for 15 minutes, or longer if your altitude is over 1,000 feet.
When you give a friend a jar of your homemade lemon curd or another preserve, you can feel proud that you’ve used the tastiest, freshest ingredients, and confident that your gift won’t prompt an allergic reaction to sulfites.
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