Citrus: The Super-Geeky Guide to Storage, Use, and Hackery

citrus

flickr user ginnerobot

You probably know that a squeeze of lemon helps to “liven up” a dish right before service. But, did you know that limes are almost 1.5 times more sour than lemons? Or that pineapples contain more acid than oranges? How about that many bartenders prefer “aged” lime juice over fresh for mixed drinks? Would you ever drink week-old grapefruit juice over freshly-squeezed?

In this post, we’ll look at the science behind the chemical properties, use, and storage of oranges, limes, lemons, and grapefruits and challenge a few long-held assumptions in the process.

Acid and Sugar Content of Common Citrus Fruits

If you were to take all the juice from an orange and dehydrate it down so only the solids were left, you would be left with a pile of sugar and organic acids. It’s important to take note of both sugar and acid levels, as acid content is only one part of perceiving a food as sour. Just as important is sugar content, as sugar has a strong suppressive effect on acidity. Below is a rough guess at the total acid and sugar contents of the most widely available citrus fruits, based on published scientific work and nutritional databases.* More precise numbers are difficult to come by due to the variability within and between varieties of fruit.

acid sugar citrus

The sugar and acid amounts in the above chart are given in % of total mass of juice. Total sugar accounts for sucrose, fructose, and glucose, corrected for sweetness.

The specific type of acids in a fruit also contributes to the flavor and perceived sourness of a juice. Citrus fruits are predominantly citric acid, with some malic and succinic acid thrown in. The latter two are best known for the flavor of apples. Ascorbic acid also appears in citrus, but contributes less flavor than the other acids. The below charts illustrate the differences between citrus fruits.

acid proportions citrus pie chart

Why do these differences matter?

  1. People can taste the difference between acids. Without delving too deeply into a subject worthy of its own post, citric acid tastes fruity, tartaric tastes clean and bright, malic tastes tingly and lactic acid tastes, well, weird.
  2. Aroma is more important than acid content. Notice the similarity between the acid contents of lemons and limes. Both are pretty sour and contain mostly citric acid. So how come they have such distinct flavors? The essential oils trapped in the flesh and peel of the fruit define their flavors. For more on the dominance of smell, see our related articles.

*Kelebek, Sugars, organic acids, phenolic compositions and antioxidant activity of Grapefruit (Citrus paradisi) cultivars grown in Turkey (2010).
Ladaniya, Citrus Fruit: Biology, Technology, and Evaluation (2008), pp. 129, 136.
Albertini et al., Changes in Organic Acids and Sugars during Early Stages of Development of Acidic and Acidless Citrus Fruit (2006).

Peak Quality: Is Fresh-Squeezed Really Better?

squeezed lemon

flickr user kerotab

If the taste of fresh citrus juice was just about sweet and sour, it would be easy for food companies to reproduce and market the flavor. Yet every good cook knows that commercial citrus juices are a far cry from the fresh-squeezed version.

Much of the tell-tale flavors we use to differentiate between, say, an orange and a grapefruit is stored in the plant’s outer peel, or zest. The fruits’ oils are produced by special oil glands that deposit the material into the epidermis. The oil glands also deposit a small amount of these same oils right into the juice vesicles of the fruit, but much more of it ends up on the outer skin.

Hand-squeezed citrus juice is considered to have the best flavor not just because it hasn’t been processed, but also because hand-squeezing ruptures the oil sacs in citrus peels and some of that oil makes it into the juice.

Or as Las Vegas-based bartender Kate Gerwin succinctly put it:

ABOVE ALL OTHER THINGS HAND JUICING IS MOST IMPORTANT, THROW AWAY YOUR JUICER NOW AND IF YOU DON’T BELIEVE ME DO YOUR OWN TEST!!!

“Aged” Juice?

Back during Tales of the Cocktail 2010, well-known technology-inclined bartender/chef Dave Arnold noticed something odd about fresh lime juice. In his eloquently titled blog post, “Fresh Lime Juice: WTF?”, he summarized his findings:

At 2PM we separated 1.5 cases of limes into 3 equal piles. I juiced 1 pile in the Sunkist juicer and 1 pile with the hand juicer. We were done by 2:15. We weighed the samples—the machine juicer yielded 26 ounces of juice, the hand juicer 21.5. I then put the juice in covered quart containers and left them out of the fridge.

At 6:15pm I juiced the third pile. We then made limeade by mixing the same amount of each lime juice with measured amounts of water and simple syrup. We served it in a blind tasting at 7pm.

Results:

The overwhelming favorite was the hand-squeezed lime juice that was 4 hours old. The distant second place was 4-hour-old machine-pressed juice. Almost no one chose the fresh hand squeezed juice. Before I revealed what the samples were, I asked those who chose the 4-hour hand-pressed juice to choose a second favorite. They all chose the 4-hour machine juice. I was flabbergasted, and so was the audience.

A handful of fellow geeks replicated Dave’s experiment in various conditions and came up with the same or similar results. Most agreed that 4-hour-old juice tasted less acidic, more “round” or “mellow”, with more “nutty” or “madeirized” flavors, or simply that it tastes more “like lime.”

So it appears the majority of the bartending community agrees that aged lime juice does indeed taste better than just-squeezed. But that still leaves the question Dave originally asked unanswered…

WTF?

Researching the how’s and why’s of lime juice’s flavor change proved more challenging than expected—the food science community rarely explores the chemistry of extremely fresh products. Instead, it focuses on optimizing packaging and storage methods to maintain quality over the long term. So I don’t have a definitive answer, but we can look at a few possible theories and the evidence behind them.

  • Oxidation. Harold McGee says that lime juice oxidizes very quickly. I’m not usually one to disagree with Harold, but the evidence for oxidation is weak. Citrus juices contain exceptionally high amounts of the powerful antioxidant ascorbic acid (commonly known as vitamin C). Two different studies**, showed that even less-acidic orange and grapefruit juices keep over 90% of their ascorbic acid for weeks when refrigerated.
  • Change in pH. This one’s easy. I tested lemon, lime, orange, and grapefruit juices over three weeks. pH does not change significantly.
  • Enzymatic Bittering. My favorite explanation for the decrease in tartness of aged lime juice. Bartender Stephen Shellenberger explained to me that all citrus fruits contain an enzyme that will produce the bitter compound limonin in the hours immediately after juicing. As we’ve covered before, a very small amount of bitter flavor can mildly suppress sourness.
  • Changes in volatile aromatic compounds. This one’s tough to measure, but probably contributes a great deal to flavor perception. First off, some volatile aromatics will obviously escape into the air after juicing. But even if you quickly cap freshly squeezed juice, chemical interactions continue to occur. Calling these reactions “oxidation” isn’t fair; oxygen plays a role, but so do water and acid. I haven’t found any papers that look specifically at the changes in citrus juice in the hours after juicing; measurements usually look at quality after days or weeks of storage. See next section for more details.

**Fellers, Shelf Life and Quality of Freshly Squeezed, Unpasteurized, Polyethylene-Bottled Citrus Juice (2006);
Kabasakalis et al., Ascorbic acid content of commercial fruit juices and its rate of loss upon storage (2000).

The Peak Quality Chart for Citrus Juices

75_quality_citrus

Notes:

  • I tested all of the juices in multiple tastings, using a combination of straight juice, mixed with water, mixed with water and sugar, etc. The tests were not truly scientific and I could have used more tasters, but given my time constraints, I think these tests were at least a good starting point.
  • I noticed increased enzymatic bitterness in each juice. In the case of orange juice, it was noticeable and unpleasant after just an hour of storage. In lime and lemon, the bitterness seemed to mellow out some of the acidity at first, but became overpowering after 10 hours. In grapefruit, the bitterness actually made the juice taste more grapefruity and didn’t become overpowering for several days.
  • Aroma degradation becomes a problem after a day of storage. This is most relevant to grapefruit juice, as it otherwise maintains peak quality for several days. I recommend squeezing the oil from a fresh peel just before service to replace the lost/degraded the aromas.
  • I continued to test grapefruit juice for several weeks after I first juiced it. It continued to be useable, if not at peak quality, throughout this time. At two weeks, I noticed a faint off-taste and by three weeks there was noticeable carbonation, which suggested yeast was growing, so I chucked it.
  • I didn’t test the shelf life of lemon, lime, and orange juices since they’re simply not very tasty after being stored for just a few days. According to research, orange juice develops off-flavors after 16–22 days of refrigerated storage, though it will not be dangerous to consume.
  • Various sources*** list a refrigerated shelf life of anywhere from 4 months to 27 months for refrigerated lime juice. My guess is that the high acidity of both lemon and lime juices prevents the growth of microbes enough that any contamination is dependent upon acid-resistant strains present in particular test environments. Regardless, you probably won’t want to use either for cocktails after 1 to 2 months in the refrigerator because of the development of off-flavors.
  • On fresh fruit: Citrus fruits keep best stored in the refrigerator, in a humid area if possible. I recommend storing them in a fruit drawer along with a wad of damp paper towels. While under optimal conditions some fruits can last as long as six months, while a good rule of thumb for fruit bought from the supermarket is 3 weeks.

*** For a review, see Hui (ed.), Handbook of Fruits and Fruit Processing (2006), pg. 345.

So what have we learned?

  1. The perceived sourness of a citrus fruit depends on both acid and sugar content.
  2. Aroma is much more important than acid content when it comes to differentiating between fruits
  3. Citrus fruit juices get bitter over time; in a few cases, that’s actually desirable.

Got any other citrus tips & tricks you’d like to share?

Disclaimer: much of the text in this post was adapted from my book, Craft Cocktails at Home: Offbeat Techniques, Contemporary Crowd-Pleasers, and Classics Hacked with Science

About Kevin Liu

Kevin Liu is a hopeless food geek obsessed with bacon, kale, and cocktails (but not usually all at once). You can follow him at @kevinkliu. Kevin is an editor at Science Fare.

13 Responses to Citrus: The Super-Geeky Guide to Storage, Use, and Hackery

  1. Neel M. May 31, 2013 at 1:34 pm #

    Hey Kevin. I have been using your book for about a month now and have loved understanding the deeper mechanics of cocktails when crafting my little experiments.

    Dumb question: what is the definition of hand-squeezing as opposed to machine-squeezing? It seems to be a grey area to me, since there are mechanical juicers that simply add a motor to a “reaming-dome.” These don’t seem to be fundamentally different from a stationary tabletop juicer where you rotate the fruit with your wrist.

    • Kevin Liu May 31, 2013 at 9:05 pm #

      Neel,

      In this context “hand-juiced” means either squeezing the fruit with nothing but your hands or using a contraption like this:



      The key is to twist/bend/break the skin of the fruit in such a way that the peel oils are expressed into the juice.

      Alternatively, you could peel the fruit, juice it with a commercial juicer, and then express the peels into the juice by hand afterward.

      Does that make sense?

  2. Carolyn Tepolt May 31, 2013 at 2:56 pm #

    Have you ever tried freezing citrus juice? I frequently freeze tablespoonsful of lemon / lime juice in an ice cube tray, since I often need zest for baking and don’t want to just throw away the naked citrus post-zesting. It seems to work fine for future baking / cocktails, but I’ve never done a side-by-side comparison.

    • Kevin Liu May 31, 2013 at 9:10 pm #

      Carolyn,

      To tell you the truth, I also never found any papers covering what happens to fresh juice when you freeze it. So the following are just guesses and I would love to hear back if you did a blind side-by-side taste test.

      Orange juice: wouldn’t recommend freezing since the quality goes rapidly downhill in just an hour or so, much faster than your freezer can probably freeze the juice solid.

      Lemon, lime, grapefruit: Should be ok, but I would recommend splitting the juice into a cube-style ice tray so the juice freezes faster. I would also recommend peeling the citrus before juicing into the tray, as the peel oils might develop off flavors when mixed with the acid in the juice, probably even when frozen. You’ll have to use some fresh peels or extract when serving the juice to replace the lost aroma. I also don’t know whether enzymatic bitterirng will still be an issue, or how long frozen juice will last. Finally, I’d recommend freezing the juice and then storing the cubes in an air-tight container to prevent sublimation and the cubes from picking up any off-flavors from the rest of the freezer.

      Like I said though, I haven’t found any research that specifically addresses this question and I haven’t done any blind taste tests myself, so take this advice with a grain of salt. umm, not literally.

  3. stephen May 31, 2013 at 5:40 pm #

    hi Kevin.

    I still think that the changes in aroma are due to oxidation. I’ve experimented widely with the reflux de-aeration of citrus juice where pressure via CO2 is used to force oxygen out of solution so it can be quickly vented. you can de-aerate the juice in just a few seconds. when you store juice this way the aroma doesn’t change any where near as quickly (many, many days). you can sense the bitterness changing but the aroma doesn’t follow suit as usual.

    reflux de-aeration is a useful tool to explore the properties of juices with and purposely create unique expressions because you can disjoint enzymatic bittering from oxidative aroma changes.

    http://bostonapothecary.com/?p=494 (“reflux de-aeration and what it can do for you”)

    people that keg citrus cocktails should learn to reflux de-aerate. once the oxygen comes out of solution cocktail keggers need to remember to vent it!

    cheers! -stephen

    • Kevin Liu May 31, 2013 at 9:13 pm #

      Stephen,

      As usual, you bring up a great point.

      So for folks who can’t reflux deaerate, you think it might work to use something like argon gas to preserve the peels?

  4. stephen June 1, 2013 at 12:13 am #

    argon gas just creates a blanket but does not force oxygen out of solution. you need significant pressure to do that. anyone with a soda siphon or whipped cream canister can explore aspects of reflux de-aeration.

    just don’t fill the canister all the way to prevent gas from dissolving into the liquid. maybe 1/4 full. use room temp juice to make the gas less soluble. add the liquid, charge, shake for 10 seconds, relieve the pressure by very slowly twisting open the cap. the oxygen will be vented with the excess escaping CO2 or NO2. the lid can then be tightened again. you can come back to the citrus juice however many days or hours later and see how it compares to normally processed juice of the same age.

    a daiquiri with bittered but not oxidized lime juice is kind of cool.

    cheers! -stephen

  5. lucas Ranzuglia June 19, 2013 at 3:55 am #

    Stephen, your experiment sounds interesting, what other uses do you see it useful for? What are the benefits of this extra step?

    Kevin: great article, being looking for this information for a while. Tks for sharing.

  6. stephen June 19, 2013 at 9:24 am #

    lucas, the extra step becomes valuable as you have to organize yourself to do large volumes or deal with unpredictable business swings.

    besides citrus you can de-aerate all sorts of liquid prep. high volume juice bars making juice in advance should probably use the technique because many juicing techniques whip air into the juice which speeds up oxidation.

    certain seldom used liqueurs are very much subject to oxidation like sloe gin and other berry liqueurs. these liqueurs can be transferred to clear champagne bottles so they can be de-aerated but still be displayed.

    and the step takes less than 10 seconds.

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