Recently on egullet I attempted to help a few people troubleshoot their limoncello. They were experiencing cloudiness and did not know what to attribute it to. I suspected pectin while others thought terpenes. The conversation turned to terpene removal which made me look for other research out there.
As it turns out, limoncello is economically significant enough that a few PhDs have looked at it in depth. Some of the studies try to characterize limoncello chemically so that consumers can be protected from fraud and adulteration. Though some aspects of the studies are intimidating, there is still great practical stuff that home producers can learn from these studies.
I found a few papers and I’ll give a bullet point run down of the good bits.
“Analysis of volatiles in Limoncello Liqueur and Aging Study wiht Sensory” by Neil C. Da Costa and Theodore J. Anastasiou
**”it certainly does not contain any lemon juice”
**”but must be transferred to the freezer before serving” These guys posit the idea that limoncello is fairly perishable. Homemade is best because it can be freshest. Their study wonders how limoncello changes as it ages.
**They keep it in the freezer to minimize reactions of the extracted oils with water and ethanol as well as with air and light.
**Due to shipping, warehousing, etc. “there is a higher likelihood of commercially available Limoncellos having less of a true flavor profile than their homemade counterparts”. This is their hypothesis anyhow which they will test by looking at how significantly limoncello changes as it ages.
**They present a very long list of compounds they were going to cover but one I was concerned about was pectin which I couldn’t find in their list.
**They prepare their own limoncello recipe by following a Washington Post article: Grisco, J. Limoncello Recipe. The Washington Post, December 7, 2009 (this link was missing for a while but provided by an awesome commenter. thanks KM!)
**”seventeen large store-bought organic lemons (approx. 175g. each, approx 3000 g total) were washed, dried, and carefully peeled as to minimize the amount of pith. the peels were submerged in 1500ml of grain alcohol (95% abv, 190 proof). This mixture was allowed to stand at room temperature for 14 days with brief stirring every other day. After 14 days the spent lemon peels were removed by filtration leaving a dark yellow solution. 1700g of a simple syrup solution consisting of equal parts (1350g) water and granulated sugar were then added to the extract, resulting in an opaque yellow emulsion. this emulsion was allowed to sit at room temperature for an additional 21 days, again stirring every other day. …. the approximate alcohol concentration was calculated to be 50% (100 proof).”
**They didn’t use any terpene separation!
**One cool part of the technical stuff is the descriptors that accompany the analysis of “key volatile components”.
**After aging, some of the terpenes hydrolized/oxidized
**Aldedydes in such a high alcohol content formed acetals (which also happens to distillates when they age)
**There was ester formation (again this happens to distillates! and of course it is going to inspire another blog post!)
**”organoleptically, the fresh limoncello concentrated extract was described as strongly citral, fresh, lemon curd. the aged concentrate extract was more oxidized lemon, less fresh, heavy lemon, missing lower volatiles, lower citral. … the biggest difference detected was a reduction in the concentration of the highly volatile monoterpenes as they became oxidized, which gave rise to the loss of fresh citrusy notes. in addition the harsh gasoline, oxidized terpene note of p-cymene was increased, which had a negative impact on the aged sample.” … “the various ethyl ethers formed were not perceived as giving significantly negative notes to the aged sample.”
**They present two sensory “spider graphs” which are pretty cool and really intuitive. hopefully I can paste the spider graphs in here because they are a tool we should probably see more of.
**They preferred the fresh but they didn’t find the aged stuff too terrible.
“Analysis of Some Italian Lemon Liqueurs (Limoncello)” by Versari Andrea, Natali Nadia, Russo Maria Teresa, and Antonelli Andrea
**For starters those names sound like people that would know limoncello…
**”a total production of 15 million liters of Limoncello per year is estimated.” wow.
**When they detail the process they state that peels are infused in 95% ethanol for 2-7 days. Then it is diluted with syrup to an average alcohol content of 32%.
**”…Limoncello contains several volatile and nonvolatile minor compounds (ca. 2%), which are fundamental for its sensory characteristics. The former are terpenic compounds, which form the essential oil, and the latter include several classes of nonvolatile compounds with potential health-related properties, such as flavanoids, coumarins, and psoralens.”
**The presence of these nonvolatile constituents are sometimes used to tell whether an essential oil was cold pressed or steam distilled the latter being seen as inferior.
**They compare 12 samples by a variety of criteria
**The sugar contents were a surprise: 182, 186, 186, 185, 202, 277, 265, 223, 264, 199 g/L so there seems to be two different styles; a low and high sugar content.
**The samples also had notable citric acid contents that were all over the map: 2044, 438, 190, 209, 475, 152, 83, 1059, 172, 395, 846, 301 mg/L. I suspect the acid is added as a powder as opposed to being from fresh lemon juice. They are listed in the same order so the style that has a high sugar content in most cases also has a high acid content. Unlike the previous paper that sees limoncello as not being sour, quite a few commercial producers interpret it as a sour liqueur. I have no opinion personally.
**They also cover the ethanol content, acetaldehyde, ethyl acetate, methanol, propanol, and i-butanol content. For ethanol: 32, 31, 31, 31, 30, 30, 29, 40, 30, 27, 29, 28.
**A lower alcohol content than 31-32% might affect stability and lead to essential oil separation (those terpenes again!) while “higher values might not fit the consumer’s preference”. “in fact, Limoncello is considered a beverage, not a spirit.” On egullet someone mention 30% as being a key number for terpenes coming out of solution. This data makes me wonder if they are correct, and what would happen to the stability of the product that had 27%. Is that low number the producers intention or an accident?
**Speaking of accidents.. one sample had a declared ethanol content of 35% but 40% was found…
**”methanol showed values below the legal limit, whereas acetaldehyde content was above the regulatory limit for neutral ethanol which is set at 5 mg/L of anhydrous alcohol.”
**They find strange congener values that make them wonder if low quality grain alcohol was used or if the syrup in some cases had started to ferment. The occurrence of glycerol and high acetaldehyde level plus low methanol levels supports the hypothesis that high quality ethanol was used but some of the syrups likely started to ferment. Very weird. Kind of gross. It sounds like some producers do not have control of their product.
**from their conclusion: “Citric acid content indicates the addition of lemon juice.” I do not think this is conclusive. Citric acid powder is easy enough to buy and the juice has all sort of oxygen sensitive compounds. Granted they have a lot of juice laying around…
**”Ethyl acetate, acetaldehyde, 2-methyl-1-propanol, and glycerol are most probably related to the occurence of microbiological activity in the sugar syrup used in the limoncello formulation” -Gross
Conclusion. Terpene separation is not a part of limoncello production. But the pectin issue wasn’t resolved. They might even be using a pectic enzyme but in my understanding that might produce methanol that would make the authors note the phenomenon so maybe they are not? In the mean time just peel it thin! And please don’t let your syrup start to ferment.