2013-03(Mar)-31: scCO2 Extractions of Szechuan Peppercorns

I'm starting to get the hang of how to use the SFT-100XW. The 220V outlet has been installed, and I got a heating pad for the CO2 cylinder. But every material requires different conditions for optimal extraction, and that's going to be an ongoing challenge.

Besides coffee and cardamom, I've extracted grand fir (a friend's Christmas tree) and more recently, Szechuan peppercorns. I'll get to the extraction in a moment, but first a little background on Szechuan peppercorns. This exotic spice is known by many names, including alternate spelling Sichuan peppercorns and prickly ash. I have no idea the Chinese name, but even if I did know it I'd have trouble reproducing the Chinese characters in HTML. Szechuan peppercorns are not related to black pepper at all, but related to rue and citrus plants. The Latin family name is Zanthoxylum, and there are several possible species sold as Szechuan peppercorns, including Zanthoxylum simulans, Z. piperitum, and Z. bungeanum. Here's the wikipedia page for more info.

Without getting into a whole dissertation, why am I interested in Szechuan peppercorns? The first reason is their scarcity--or more specifically, the scarcity of their essence. I've done plenty of shopping for plant essences, including essential oils, absolutes, and of course CO2 extracts, and I've never seen Szechuan peppercorn essence for sale. This means there's more of a potential market for any extract I obtain. The second reason is that Szechuan peppercorns are used in a common spice blend called Chinese Five Spice, which includes also star anise, cloves, Chinese cinnamon, and fennel. Other possibilities are anise, ginger, nutmeg, turmeric, orange peel, licorice, black pepper, cardamom, and galangal. I happen to have lots of star anise on hand (it's a long story, involving the bird flu scare a few years back), so making a Chinese Five Spice soap is a good way to use up that star anise in a sensible way.

Another reason I decided to extract Szechuan peppercorns concerns their chemical properties. One of the components of the plant is a chemical called sanshool, which causes a numbing, tingling sensation unlike the "hot" sensation of capsaicin. I haven't tasted any of the extract, so I don't know if I extracted much sanshool, but there are many other aromatic components of the peppercorns including: beta-myrcene, limonene, 1,8-cineole, and Z-beta-ocimene for species simulans, and citronellal, citronellol, and Z-3-hexenal for piperitum.

Now on to the extraction details. I ordered two pounds of Szechuan peppercorns from The Spice House for $56 including shipping, coming to $28/lb. in total. Recently I found a pound bag at my local Asian market for $15, so I'll probably be buying from them in the future. I decided to grind 1/3 lb. (~151 g) based on other spices, but I could probably fit another 20-30 g in the nylon sack. In the first few experiments, I extracted the sample almost exhaustively. Way longer than I would ordinarily extract something. From those experiments I found out that I could extract ~6.5% from the sample, but that last percent took forever to obtain. Also, while the first ~5% of extract were a clear, yellow color, the last bits were thicker and olive colored.

There are several practical questions that need to be answered for any extraction, and I've already alluded to a couple: the mass of sample per extraction, and the expected yield of extract for that type of sample. In addition, I want to know when I should cut my losses and stop an extraction. To help answer this question, I extracted a sample for seven hours, and periodically weighed the collection vial to see how much I'd gotten. Here were the results:

1 hr: 1.38%; 2 hrs: 2.62%; 3 hrs: 3.54%; 6 hrs: 5.18%; 7 hrs: 5.34%

I also plotted the results so I could analyze the behavior mathematically:

Why bother fitting the data to a mathematical model? Well, among other things it makes it much easier for me to figure out how long I'll need to extract to get 5%, or 5.2%, or 6%. If the feedstock (peppercorns) are much cheaper than CO2, it might be worthwhile to continue extracting from 6 hours to 7 hours to get that last 0.16%. But if the feedstock is relatively cheap, it's better to stop extracting sooner and just move on to the next batch.

By the way, I used ZunZun to fit my data for free, online, powered by open source software. The model I selected is a simple exponential function that starts at zero and increases to some maximum / plateau. The height of this plateau is reflected in parameter "a," which is 6.47 for the fit, meaning if I were to let the extractor keep going indefinitely, the most I would get would be 6.47% by weight of extract. The parameter "b" is related to the curviness of the fit. -1/b is something called the "time constant," usually indicated by the Greek letter tau, and for this fit it would be ~1 hour. Since I want the extraction to be done quickly, I want to try to change experimental parameters to reduce the time constant to less than an hour. Perhaps adding some ethanol to the ground peppercorns at the beginning might help reduce tau.

In the future, I hope to analyze the different Szechuan peppercorn extracts and perhaps identify some of the components. But that's a topic for another day.

Last Modified:
10 November 2013

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