Candying citrus zest: fresh vs. frozen

California is not like other places. Having grown up in gun-toting, snowy New Hampshire, I feel this acutely. Lemons, where I grew up, came from the supermarket in yellow mesh bags. Here, they grow on trees in backyards.

Recently, my friend Danna Staaf gave me a shopping bag full of lumpy, aromatic homegrown lemons from her tree. I juiced part of this bounty for cocktails, and the lemons were so bracing and aromatic I wondered if I could stash the hollowed-out rinds in the freezer for later use. I had candied citrus zest on the brain: the homemade stuff tastes like a grown-up gummy bear, miles away from the horrible glace fruits that lurk in plastic tubs in the supermarket near the holidays.

With that in mind, the next batch of Danna’s wonderful lemons were devoted to science: would frozen zests taste any different from the freshly-candied variety?

Why expect a difference?

Freezing often affects the texture of foods, particularly delicate foods like fresh fruits and vegetables. The water inside plant cells freezes into crystals, which pierce cell walls from the inside out. Once thawed, water dribbles out of these holes and the cells deflate, making defrosted plants softer and limper than their fresh counterparts. Fresh spinach salad is fantastic, but use frozen spinach and you’ll end up with soup.

Frozen-food companies try to minimize the leakage by fast, low-temperature freezing. Colder temperatures mean that food freezes faster, and lots of tiny ice crystals are formed, doing less damage to cells when they thaw. In contrast, at higher temperatures, a smaller number of large crystals form and grow over time. While large ice crystals are bad for fruits and veggies, they do have their uses, as in Kevin L’s great (and simple!) guide to freezing perfectly clear ice cubes for your fancy drinks.

But, to repeat: large crystals are bad for fruit texture, and large crystals are what you’re likely to get in a home freezer. So I wondered: would candying frozen lemon zest result in mush?

The Experiment

I split a batch of lemons into two groups, juiced them all, and froze the juice in an ice-cube tray for future cocktails. Half of the squeezed-out rinds went into a ziploc bag in the freezer, while the other half were candied immediately.

After a couple weeks, I had a little time, so I pulled out the bag of frozen rinds and candied them pretty much exactly the same way. I did rinse them off in the sink quickly, to remove the frost, but I didn’t thaw them all the way before trimming and cooking. Actually, they were easier to handle when still a little frozen – the extra stiffness made trimming simpler.

The Method

I sliced off the spent lemon membrane and some of the bitter white pith from the zest. This bitterness is caused by a chemical called limonin, which is a big headache for the citrus industry – no one likes to start the day with a tall, cold glass of bitter orange juice. Industrial debittering methods aren’t really feasible at home, though, unless you make a habit of cooking with polymeric films or fancy bacteria.

Instead, the home method works by repeatedly boiling the zest and tossing out the water. This works because limonin is more soluble at higher temperatures, so when you boil the zests, some of the limonin transfers from the pith to the water. Tossing the water gets rid of the dissolved limonin. Since there’s a lot of limonin (and it isn’t terribly soluble, even at high temperatures), you traditionally repeat this process three times before the actual candying can begin.

Once the zest has been thrice-boiled, it’s dumped into a pan of simple syrup and simmered at very low heat for a very long time. At this point, sugar starts to penetrate the zest tissue. After simmering, the zest + syrup are popped into the refrigerator for at least a week, where the candying process will continue. The ultimate goal is to replace the water in the fruit cells with sugar – because of this, carefully candied fruits can have a room-temperature shelf life of years, if you can keep the ants away from them.

The next part is the trickiest: the zests have to be dried them out so they won’t be sticky and covered with sugar-snot*. Like every other step in the process, this one is slow. I let the zests dry out in a low/off gas oven for at least a day.

When they were no longer sticky to the touch and looked a bit mummified around the edges, I pulled them out.

At this point, and throughout the process, I didn’t notice any differences between the fresh and frozen zests. Texture seemed entirely dependent on dehydration: better-dessicated zests had a texture like fruit jerky, while slightly moister zests were closer to the gummy bear end of the spectrum.

Finally, I tossed the zests in superfine sugar and put them back in the oven for a final round of really thorough dehydration. (Earlier forays into zest-candying have taught me that any hint of moisture leads quickly to a sugar-snot situation.)

The Test

I didn’t carry this out as methodically as I would have liked, since I was in the throes of prepping for 3 months of fieldwork abroad**. Basically, I tossed two unmarked containers of candied zests at a bunch of friends and said, “Eat it and tell me which is better! I have to go order crab food now.”

When I returned, the report was as follows: both preparations were delicious, but no one had strong opinions either way, and no preparation was a clear winner. Which made this experiment a complete winner in my book. If frozen zest is just as good as fresh, you can store spent citrus rinds in the freezer, and candy the zest whenever time allows. I haven’t tried using frozen rinds for anything else, but I’ve heard they can be zested while frozen and the zest used in place of fresh – an experiment for another time.

But wait!

I followed a fairly traditional technique that calls for boiling the rinds three times, starting with cold water each time. But why cold water? If it’s just a solubility issue, I wonder if boiling water would work just as well, and faster.

Do you have any ideas on how to use frozen citrus rinds? Let me know, and I’ll test them out next time I come into a pile of gorgeous California lemons!

 

*I use LibreOffice for word-processing, and it has a feature that tried to predict what you’re typing, based on what words you’ve used in the past. Now every time I type “sugar”, it helpfully suggests that maybe I mean “sugar-snot”? Technology is the best.

**Portugal and Norway – rough, right?

 

RECIPE: Candied citrus zest

This is really more of a set of guidelines than an actual recipe. It’s flexible.

Ingredients

Citrus rinds (orange, lemon, lime, etc.)
Granulated sugar
Superfine sugar

Technique

Cut zest into desired sizes. Long, narrow strips are traditional, but I prefer wedges as they’re simpler and the geometry works out better. Cut off any remaining citrus flesh. If your citrus is especially pithy, cut off some of the pith along with the flesh.

Put all the trimmed bits of zest into a pan, and cover them with cold water. Bring to a boil.

Once boiling, drain the zest in a colander and rinse it with cold water.

Repeat the boiling / draining / rinsing process twice more, for a total of 3 cycles.

Estimate (or measure) how much water it will take to cover the zest chunks in your pan. Add the same amount of sugar (by volume), and bring to a simmer to dissolve the sugar completely.

Once the syrup is ready, add the citrus zests and set to a very low simmer. Simmer for 1.5 – 3 hours. Stir every once in a while, and keep an eye on the level of the syrup – it may evaporate substantially. If this happens, add a bit more water (preferably hot) to bring the level back up, and stir.

After simmering, let the syrup and zests cool. Move them to a sealed container in the fridge, making sure the zest is completely submerged in syrup.

Let the zests sit in their syrup in the fridge for at least a week, and up to a month.

Lay the zests out on a cooling rack placed in a baking sheet (to catch the drips). Put them in a very low oven (160 – 200°F) and leave them until they’re no longer sticky and are slightly curled at the edges, several hours or more. If you have a gas oven, I recommend turning the oven off when you go to bed, and leaving the zests in there overnight. (Gas ovens, even when off, are nice dry environments – great for dehydrating.)

Toss the zests in sugar until well-coated. Superfine granulated sugar is excellent if you can get a hold of it – it’s sometimes sold as baker’s sugar or bar sugar, since it dissolves into liquids more readily than regular sugar does. (It is NOT interchangeable with powdered sugar, which is ground extremely fine and mixed with a substantial dose of cornstarch to keep it from clumping.)

Put the sugared zests back on a fresh rack and into the oven for some more dehydration. If the zest isn’t dried well, the sugar will slowly liquefy and the zests will get sticky and sugar-snotty.

Once you’re sure they’re dry, store them in a sealed container at room temperature. They should last several weeks to several months, depending on how well you dried them and how humid the air is.

About Carolyn Tepolt

As a PhD student in Biology at Stanford University, Carolyn studies adaptation in invasive marine crabs. As part of her degree, she drove 5,000 miles from Newfoundland to California, stopping only for sleep, tea, and Carhenge. For special occasions, she bakes ridiculously elaborate, deeply nerdy cakes. Carolyn is an editor for Science Fare.

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3 Responses to Candying citrus zest: fresh vs. frozen

  1. Isaia Panduri August 8, 2012 at 6:13 am #

    Hi.
    Thanks for your experiment with frozen rinds.
    We live in Sicily, Italy, where citrus plants are endemic. We have plenty of oranges and lemons, so we are used to make candied orange/lemon rinds every year.
    We follow the same procedure as you did, with a couple of differences:
    After the three boiling phase, we weigh the rinds and prepare a 1:1 water-sugar syrup in weight (not volume). The other difference is that we do not leave the rinds into the syrup. After slowly boiling the rinds until all the syrup has almost evaporated (but long before it start to caramelize!) we lay the rinds on cooling rack and let them dry by themselves for a day or two. Then we store them with sugar as you did.
    In order to avoid sugar crystallization (if not consumed in a few days, they become progressively harder) we started to add some glucose (about 10-15%) to the water-sugar syrup, and it worked fine. The candied rinds boiled in sucrose-glucose-water syrup stay tender for longer times.

    • Carolyn Tepolt August 8, 2012 at 6:55 am #

      Isaia, thanks for sharing your methods! Sicily sounds like a citrus wonderland.

      I’ve tried adding glucose (in the form of corn syrup) to the simple syrup before, and didn’t notice any discernible hardening over the course of a few weeks. I live in a very humid area, though, so it’s possible that the sugar is pulling moisture from the air to prevent hardening.

      I haven’t tried your trick of slowly cooking away the water in the syrup rather than soaking the zests. I wonder if this also promotes crystallization and hardening. I’ve had candied ginger made with the method, and thought it had a distinctly (but pleasantly) crystalline texture and bite to it. I never thought of applying this method to citrus, though.

      I’m really intrigued — I’d like to run an experiment testing the water-evaporating vs soaking methods, with a cross-wise test of glucose/no-glucose syrups. Very curious to see what would come of it!

  2. MiraUncut August 8, 2012 at 5:58 pm #

    Uh, yum! I hope your sharing those :)