In order to investigate plausible extraction methodology, I wanted to test two different factors: water source and heat of extraction. Previously, I simply soaked charred kelp in room-temperature water and boiled the runoff. That’s great, but we also know that the solute holding capacity of water increases with temperature – so hypothetically, a hot water extraction should allow more salts to dissolve than a room-temperature one.
I also used water from my tap, which is all good and well – but this is a utility endeavor, and it was practiced on beaches isolated from major population centers. Would a salt-karl really haul fresh water from somewhere just to make salt, or would he use the seawater that’s right next to his setup? Remember, Pliny indicates that many cultures (including various Germanic tribes) made salt by evaporating seawater, and some by pouring it over the hot coals of wood. It’s plausible that seawater plus ashed kelp could be used to produce a salt; Atlantic seawater is only 3.5% salt in composition, and the saturation point for a saltwater solution is around 26% (barring any hypersaline water oddities).
Before I could do anything, though, I needed to burn some shit.
As I’ve mentioned before, I bought 50 pounds of Icelandic kelp meal some time ago. Since then, I’ve been trying to figure out a way to effectively burn the stuff. The configuration makes it useless as a fuel item; a friend had suggested burning it as food, and even offered up the above-pictured Lodge cookware for it. The test-run many moons ago was successful but stinky – I figured my 60,000 BTU propane burner could get the job done.
Man, did it ever. My previous efforts never resulted in significant combustion, but this stuff really took off after the initial heavy smoke phase. Interestingly enough, it also burned out and never re-ignited; my guess is that most of the carbon content burned off, leaving behind mostly mineral salts. The fire itself produced a fairly noxious-smelling black smoke, with a chewy oily texture.
Let me just reiterate how awful this shit smells. It’s extremely smokey, takes a while to burn off (I think the pot above smoked for 45 solid minutes before catching fire), and smells like a rotting whale carcass stuffed with fermented shark that is also on fire. Also the whale is on fire. Also the entire ocean is on fire.
It’s really not pleasant.
Seems there’s a good reason that “salt-karl” was an insult, and why the Norse did this on a beach well away from other people. When I came in after 3 hours of burning stuff (during which I reduced 12 lbs of kelp to ~5 lbs of ash), my fiancee could only say “UGH. What’s that smell?” And today, two days later, my peacoat still reeks.
So after I finished standing outside freezing my ass off while inhaling fumes of unknown toxicity, I had a tub of charred stuff that smelled fairly awful. It needed to cool overnight before it could really be useful – ashes tend to stay warm for some time. They never fully ashed, not even when combusted – but again, I believe that to be a byproduct of the configuration. Future experiments will look at trying to use sheet kelp as an actual fuel source, rather than expending heating fuel to make ashes.
Once you’ve got cooled kelp ash, it’s time to extract the mineral content! I’m used water as an extraction medium, and tried both conventional tapwater and a seawater analogue consisting of 3.5% salt.
It’s always prudent to assemble your materials before you proceed with an experiment. Here, I’ve procured my kelp ashes (~2.35 kg), propane burner and propane (set to 50% maximum output), several measuring containers, a strainer and bowl, and of course a kitchen scale.
The faux seawater solution was prepared by combining 5 kg of tapwater with 175 g of kosher salt, giving a final salt concentration of ~3.5%. Note that I weighed the water as opposed to measuring volumetrically – that’s because water has a density of 1 gm/cm^3 (until it gets near freezing, at least), so that 1 gram = 1 mL and 1 kg = 1 L. Convenient! My scale has better resolution (minimum 1 g) than my volume equipment, so this will allow for maximum accuracy.
Controls are crucial in any experiment, and it’s important to identify needed controls at the outset of an experiment – let your hypothesis govern the choices. In this case, I’m specifically looking to assess the difference in extraction efficiencies between 1) salt and fresh water and 2) low-temperature and high-temperature extractions. Because I will ultimately be measuring a mass of solid product, it’s important to know what will be contributing solids to the extracts. In order to provide controls, I boiled down 1.5 kg each of tap water and “seawater” and measured the mass of residue that could be removed from the pan.
On the left, you can see the residue remaining from boiling off tapwater. We’ve got hard water here (perfect for brewing), so it’s not surprising that there is a scale left on the pan. However, it proved to be too little to effectively harvest or measure, failing to register any mass on my scale. Thus, 1.5 kg tapwater contributes less than 0.5 grams of solids to final counts. On the right, we see the residue of the “seawater,” representing the base contribution to the method as well as accounting for the losses that invariably occur when trying to harvest the salt.
1.5 kg of saltwater with a concentration of 3.5% salt by weight yielded a final salt load of 46 grams. Hypothetical yield was 52 grams, but some salt was lost in processing. Still a fairly efficient extraction. The salt was initially rather wet after drying – something like 70 grams and a consistency not unlike brown sugar – but it was heated in the microwave for 1 minute to fully dry.
For sample setup, I basically drew a Punnet square and did the appropriate combinations. 4 500-gram portions of kelp were measured into appropriately-labeled dry containers. 1.5 kg of either salt or fresh water was added to each sample. Two of the samples (one fresh and one sea) were left to steep at room temperature for 30 minutes, while the other two samples (fresh and sea) were heated in a pot on the kitchen stove. Heated samples were brought to a visible boil, and dropped to a simmer for 5 minutes after the first bubbles breached the surface. Following all extractions (whether heated or room temperature), the water/kelp masses were strained through a wire mesh strainer, and the liquid phase collected. The kelp mass was allowed to drain for 5 minutes, ensuring collection of a significant portion of the liquid.
I didn’t measure the volume of runoff from each (though now I’m wishing I had) since I was only focusing on final solid extract generated by the methodology. However, all 4 extract methods appeared to produce roughly the same volume of runoff – roughly 400 – 500 mL. Future experiments will more accurately determine runoff volume generated by these extraction methods.
Once extracts were obtained, they were boiled down as the controls were. The pan was washed and dried in between each boiling (actually, all common equipment was thoroughly cleaned and dried in between samples to eliminate the possibility of cross-contamination), and the same equipment was used to extract the salt from the pan (i.e. a spoon and a spatula). Extract mass was determined using the same scale used to measure all of the ingredients. In order to standardize the moisture level, all samples were microwaved for 1 minute after collection as the saltwater control was.
I devised an abbreviation scheme to represent the four sample configurations. All samples are identified by three consecutive letters indicating their combination of treatments: K/[R or H]/[F or S], indicating [K]elp, [R]oom-temperature or [H]igh-temperature extraction, and [F]resh or [S]altwater extraction. Top row from left to right shows the solids extracted from KRF and KRS; bottom row from left to right shows extracts of KHF and KHS.
Yields for all samples and controls are given below. Compounded uncertainty in measurements is +/- 1.5 g; the scale has no listed uncertainty of its own, and 10 consecutive weighings of identical volumes showed no deviation. Uncertainty is thus half the value of the smallest unit of measure (1 g), added for each step that involves weighing. In this case, 3 different weighings of different components were used to determine the components of the extraction process – their uncertainties add together.
Sample Name Mass of Extract (+/- 1.5 g)
Saltwater control 46 g
Freshwater Control <0.5 g
KRF 42 g
KRS 112 g
KHF 90 g
KHS 120 g
The results are not terribly surprising. Both of the hot water extractions yield more salt content than the lower-temperature extractions. The difference is greatest when using fresh water for the extraction, which indicates that the charred kelp contains quite a lot of salt to potentially extract. It is curious that the yield from KRS is larger than [KRF + Saltwater]; one would expect that the yield would simply be additive and thus the two would be mostly equivalent.
It appears that the high-temperature extraction with salt water yields the largest quantity of salt, but the gain from heating is minimal compared to a simple room-temperature salt water extraction. It appears that the use of salt water for extracting leads to the greatest gains in salt yield. This is unsurprising, as the salt water contributes a significant salt portion. It may be that the addition of charred kelp to salt water allows the solution to approach saturation; assuming 500 mL of final volume, the KHS solution would have had a solute concentration of ~24% prior to boiling.
Recovery efficiency seems to decrease as final salt mass increases. When evaporating KHS, several larger globs of salt “popped” out of the pan in response to heating. This phenomenon was observed in other extracts, and is generally exacerbated as the amount of salt condensing increases. This may also account for the observed decrease in effectiveness of heating in extracting additional salt from the kelp.
Ultimately, this demonstrates the utility of using kelp ash to increase salt yield from boiling seawater. 1.5 kg of seawater, when boiled off, yields 46 g of salt. The addition of kelp ashes to that same mass of seawater, while reducing final liquid volume, can increase the final salt yield by a factor of approximately 2.5, for a maximum yield of 120 g. This has the potential to consume less fuel (boiling a smaller volume of liquid) while simultaneously increasing salt yield.
It should be noted that expending fuel specifically to ash the kelp is likely a fuel-losing prospect. More than likely, sheets of dried kelp were themselves burned as a fuel source, and the ashes collected and used for various home purposes.
So what’s next? Fuel consumption estimation, liquid extract volume yields, experimenting with sheet kelp as fuel, additional experiments for the sake of rigor…
But before that, I’m a Norwegian, and I have salt. Let’s get some cod and see what happens when I apply kelp-ash salt to it. Next time, we’ll see how that works out.
EDIT: UPDATE WITH TOXIC METALS ANALYSIS INFORMATION
Element symbol: amount (ppb)
Note: BDL = Below Detection Limit
1 PPB = 1 ug/kg
As (total): 574.2
Control Salt (Kosher Salt boiled in a pan)
As (total): BDL
The arsenic (As) level was not speciated, as it was not considered a level of general concern for salt.
The FDA sets a level of concern for arsenic in juice of 23 ppb, at which point the arsenic must be speciated. Inorganic arsenic in juice has a tolerance level of 10 ppb.
The US does not set an arsenic standard for any other product.
Codex Alimentarius maintains internationally-recognized standards for contaminants in some products:
The standard for food grade salt is 500 ppb total arsenic, so this slightly exceeds that. However, they also note that marine products (seafood and kelp) routinely have higher levels of arsenic (mostly organic, with ~1 – 3% as inorganic), often up to 50 mg/kg (50,000 ppb).
A 2007 study by Amster et al raised some concern about arsenic in kelp supplements, but was highly criticized because it failed to speciate the arsenic, and thus could not demonstrate the toxic link it claimed. The paper also suffered other severe methodological flaws.
In general, the amount of arsenic observed in the salt is not of concern. 10 g of the salt (twice the RDA for sodium) would contain 5 ug of arsenic, well within the typical human daily consumption range. And it is unlikely that all of the arsenic is inorganic – most is likely the organic (non-toxic) form, rendering the salt largely non-toxic.
But I would not use this salt as a day-to-day table salt, to be on the safe side. As a preservative for fish which is likely to be soaked out, it should be fine.
Thanks to Tom King and the chemistry division of the NYS Department of Agriculture and Markets Food Lab!