Lab: Salmon Gravlax

Salmon Gravlax

Salt curing has been used for centuries to preserve fish caught at sea. It’s also easy to do at home! Surrounding fish with a sufficient quantity of salt draws out the moisture; this is called dry brining. But salt doesn’t just dry out the food (along with any bacteria and parasites). At sufficient concentration, dry brining actively disrupts a cell’s ability to function and kills it, rendering bacteria and parasites nonviable.

In a bowl, mix together:
     5 teaspoons (30g) kosher salt
     1 tablespoon (12g) sugar
     3 tablespoons (12g) finely chopped fresh dill
     1 teaspoon (5 mL) vodka
     1 teaspoon (2g) crushed peppercorns (ideally, use a mortar and pestle)
On a large piece of plastic wrap, place:
     1 pound (450g) salmon, washed and bones removed; preferably a center cut so that its shape is rectangular
Sprinkle the salt mixture over the fish and massage it in. Wrap the fish in plastic and store it in the fridge, flipping and massaging it twice a day for a day or two.
Store it in the fridge and consume within a week.
Remove the skin from your salmon fillet.
Remove the skin by placing the fish skin-side-down on a cutting board and carefully running a knife along the surface between the skin and flesh while using your hand to keep the fish from sliding around.
     • Vodka is used here as a solvent to dissolve some of the non-water-soluble aromatic compounds. You can substitute other spirits, such as cognac or whiskey, to bring additional flavors in. And in place of dill, try using coriander seed, loose tea leaves (e.g., Earl Grey or Lapsang Souchong), shallots, or lemon zest. The Scandinavians traditionally serve salmon gravlax on top of bread with a mustard dill sauce.
      • You can substitute other fatty fish, such as tuna, for the salmon and obtain a similar texture.
     • This recipe is a bit heavy on the salt—6% by weight—to err on the side of safety. You can reduce the saltiness before eating the fish by rinsing the finished product in fresh water. Curing above 3.5% salt prevents most common bacterial growth, but not all. Modest concentrations of salt prevent Gram-negative bacteria—which are the most common ones found in food—from growing, but won’t handle the few that are Gram-positive, such as Listeria.
     • Salt curing—as is done in salmon gravlax— is the first step in making lox. After curing, lox is also cold-smoked, which is the process of exposing a food to smoke vapors that have been cooled down. You can approximate the flavor of lox by adding liquid smoke to the rub.

The Sweet Way To Calibrate Your Oven

What if you don’t have a digital thermometer and need to check an oven? It’s common practice to calibrate thermometers with ice water and boiling water because those have temperatures based on the physical properties of water. Water isn’t the only chemical in the kitchen with known temperature dependent properties, though: you can also calibrate your oven’s thermometer using sugar!
Mankind has been harvesting sugar for millennia, but only in the past few hundred years have we industrialized it. The sugar you buy most likely comes from either the sugarcane or sugar beet plant, which is soaked in hot water to dissolve out its sugar into a syrup that’s then crystallized. The white table sugar that you’re familiar with is ~99% sucrose—a pure substance (C12H22O11)—with the rest being water and a tiny percentage of stuff like trace minerals and ash that come along for the ride.
First, grab these supplies:
• Aluminum foil
• Sugar
• A timer
• A plate (for hot sugar samples)
• And, obviously, an oven!
Here’s what to do:
The sucrose in table sugar melts at 367°F / 186°C. It turns from the familiar white granulated substance to something resembling glass. (Sucrose undergoes a chemical breakdown at low temperatures; see page 221.) A properly calibrated oven won’t melt sugar when set to 350°F / 180°C but it will when set to 375°F / 190°C.
We’re going to bake two different samples of sugar at two different temperatures, one hopefully below and the other above sugar’s melting point, to check your oven’s temperature.
1. Preheat your oven to 350°F / 180°C.
2. Make two aluminum foil “sample containers”:
     a) Tear the aluminum foil into 5” × 5” (12 cm × 12 cm) squares.
     b) Fold the edges of each piece up, making a miniature pan that’s about 4” / 10      cm square and ½”/ 1 cm high.
3. Add a spoonful of sugar into each sample container.
4. Put the first sample container in the preheated oven (350°F / 180°C). Set a timer for 20 minutes and wait.
5. After 20 minutes, remove the first sample and transfer it to your plate. Remember, the sugar is hot, even if it doesn’t look it!
6. Set your oven to 375°F / 190°C and wait 10 minutes for it to adjust.
7. Put the second sample container in the oven. Set a timer for 20 minutes and wait.
8. After 20 minutes, remove the second sample and transfer it to your plate.
Investigation time!
What differences do you see between the two samples? Why do you think that happened? Compare the 350°F / 177°C sample with some uncooked sugar; what do you notice? Why might that be happening? And the best part of the investigation: once the samples have cooled down, taste them! What does the 375°F / 190°C sample remind you of?
Sugar at 350°F / 177°C. Sugar at 375°F / 190°C.
Check back soon for my next post: How To Cook A High Heat Pizza

Taste Aversions

My friend Dawn hates the taste of eggs. As a little kid, she ate eggs that had been cooked in burnt butter. Her brain linked the revolting acrid taste of the burnt butter with the taste of eggs, and to this day that link is stuck in the basal parts of her brain to the point that she can’t eat eggs. A taste aversion—a strong dislike for a food, but not one based on an innate biological preference—typically stems from prior bad experiences with food, often occurring in childhood like Dawn’s burnt-butter eggs experience. A foodborne illness is a common cause.

Taste aversions are fascinating because they’re entirely learned associations. The food that triggers the illness is correctly identified only part of the time. Typically, the blame is pinned on the most unfamiliar thing in a meal, known as sauce Béarnaise syndrome. Sometimes the illness isn’t even food-related, but a negative association is still learned and becomes tied to the suspected culprit. This type of conditioned taste aversion is known as the Garcia effect, named for psychologist John Garcia, who determined that he could create taste aversions in rats by invoking nausea when they were exposed to sweetened water. As further proof that we’re at the mercy of our subconscious, consider this: even when we know we’ve misidentified the cause of an illness (“It couldn’t be Joanna’s mayonnaise salad—everyone else had it and they’re fine!”), an incorrectly associated food aversion will still stick.

Sometimes only a single exposure that results in foodborne illness is all it takes for your brain to create the negative association. One of the cleverest examinations of taste aversion was done by Carl Gustavson as a grad student stuck at the ABD (all but dissertation) point of his PhD. Reasoning that taste aversion could be artificially induced, he trained free-ranging coyotes to avoid sheep by leaving (nonlethally) poisoned chunks of lamb around for the coyotes to eat. They quickly learned that the meat made them ill, and thus “learned” to avoid the sheep. As tempting as it may be, I don’t recommend this method for kicking a junk food habit, but it does hold an odd appeal.

What can you do to overcome a taste aversion? To start with, you have to be willing and open. You may feel that eggs are disgusting, and if you’re unwilling to unwire that association, your chances of eating an omelet are rather low. Repeated exposures to small quantities of the offending item, in situations where you feel comfortable, will eventually remove the association between the food item and negative memory (called extinction). Remember, start with small quantities and use consistent repeated exposures in a supported environment. If it’s too much at first, try changing some aspects of the food, such as its texture or the cooking technique, so that the flavor association isn’t as strong.

*See Also: What is Flavor: A Lesson in Orthonasal Olfaction

What is Flavor – A lesson in Orthonasal Olfaction

Flavor is a Jedi trick of the mind, a combination of the gustatory sense of taste and the olfactory sense of smell that your brain fuses into a new sensation. To give you an idea of just how clever your brain is about flavor, consider this: your brain detects odors differently based on whether you are breathing in or out. This is crazy! It’s like saying swiping your hand left to right on a cold countertop causes you to feel temperatures differently than swiping right to left. Our brains are wired to process smell signals in two different ways; flavor uses the second way.

Some definitions will make this easier to discuss. Orthonasal olfaction is defined as what your nose detects from sniffing something that exists in the world. Sniffing a rose, unless you’re also chewing it, uses the orthonasal route for smell. Retronasal olfaction is what your nose detects in the foods you eat when air is taken in from the mouth and circulated up to your nasal cavity. Even if you don’t notice it happening, it is! Try chewing food with your nose pinched: cut off the airflow, and poof, the flavor sensation’s gone.

To unravel this trick of the brain, a researcher, Paul Rozin, gave subjects unfamiliar fruit juices and soups via the orthonasal route—“Here, sniff this; remember this odor”—and then gave the foods to the subjects again via the retronasal route (through a plastic tube), asking them to identify the previously remembered odor. They did horribly. Same compound, same sensory apparatus, completely different experience. As I promised, smell is simple in the abstract but complicated in the details, so it follows that flavor is no different.

From a practical perspective, which flavors you’ll like or dislike is a matter of exposure and preference. Rozin started studying the orthonasal and retronasal issue when stumped by stinky cheeses—how is it that we have a different experience of flavor for something that smells disgusting? There’s a lot that psychologists and physiologists are still exploring. Fortunately, you needn’t be one to cook a good meal. When working with food, keep in mind that flavor is a specific combination of the two senses of taste and smell, but not a straightforward summation of the two. Taste the food to adjust its flavor before serving it! Smelling alone isn’t enough.

Here are some tips for great flavor when cooking:

  • Chew! Admittedly an odd suggestion for good flavor, chewing food crushes, mixes, and kicks up a bunch of compounds for your olfactory system to detect, adding smells that fold into flavor sensation. Remember, for a compound to activate an odor receptor, it has to be present at the point of detection. This raises the question: does chewing food with your mouth open lead to a different flavor experience? (If animals always chew with their mouths open…)
  • Use fresh herbs. Most dried herbs have weaker flavor because the volatile oils that are responsible for the aromas oxidize and break down, meaning that the dry herbs are a pale substitute. Dried herbs have their place, though; it makes sense to use them in the dead of winter when annual plants like basil aren’t in season. Store dry herbs in a cool, dark place (not above the stove!) to limit their exposure to heat and light, which contribute to the breakdown of organic compounds in spices. Grind your own spices. Don’t used preground black pepper; it loses much of its flavor over time as many of the volatile compounds change. Fresh-grated nutmeg is also much stronger than preground nutmeg. The aromatics in a preground spice will have had time to either hydrate or oxidize and disperse, resulting in flavor changes. Most dried spices also benefit from being bloomed—cooked in oil or a dry skillet under moderate but not scorching heat—as a way of releasing their volatile chemicals without breaking them down.
  • Don’t discount frozen ingredients. Commercially frozen vegetables and fruits are convenient and work fine in some dishes. Freezing produce right when it is harvested has advantages: nutritional breakdown is halted, and the frozen item is from the peak of the season with maximal flavor (whereas the fresh version in your store may have been harvested early or late). Frozen produce is especially useful if you’re cooking for just yourself: you can pull out a single portion as needed. Want to freeze your own crop or a surplus from a CSA (community-supported agriculture) food share? See page 365 of my cookbook (you can buy it here) for how to use dry ice. (Freezing in your home freezer takes too long and leads to mushy veggies.)
  • Use alcohol in cooking. My favorite restaurant in San Francisco uses kirschwasser in its fruit soufflés, and adding a splash of wine in sauces or to deglaze a pan to make a quick sauce is standard practice. Using alcohol changes flavors because of its chemistry: it takes the place of water molecules normally attached to compounds, resulting in lighter molecules that are more likely to evaporate, and with higher evaporation rates there are more volatiles for your nose to detect.

*See Also: Taste Aversions


Popcorn and the Ideal Gas Law, PV = nRT — The Science of Popcorn!


Did you know popcorn pops at roughly nine times atmospheric pressure? The inside of a popcorn kernel is about ~13% water. When that water heats up—trapped inside the confined space of the kernel’s pericarp—the pressure goes up until the pericarp ruptures and the insides, now melted, spew out.

You’ve probably never thought about the physics of popcorn, or even what temperature popcorn pops at. Snag some oil, a digital thermometer, and a pan. Try popping some popcorn kernels at various temperatures. You’ll soon figure out that popcorn doesn’t really pop well until ~350°F / 177°C. (For photographs, see page 307 of the second edition of Cooking for Geeks—click for free PDF of that page.)

But how do we know popcorn kernels rupture at nine times atmospheric pressure? Because as temperature changes, the volume of a gas changes, and knowing popcorn kernels are roughly 13% water allows us to use the ideal gas law (click to see UC Davis’s ChemWiki entry), which is:

PV = nRT

What I didn’t include in the second edition of Cooking for Geeks was any discussion of the ideal gas laws—it didn’t seem culinarily useful, even if the geek in me loves these sorts of details. An old magazine, The Physics Teacher, has a lovely writeup on The Physics of Popping Popcorn from April 1991. Thankfully, things like the laws of thermodynamics haven’t change much in the last… oh, ever. If you’re teaching science, or want to really geek out, check out his writeup for details.

P.S. Our CSA share last week included 4 popcorns-on-the-cob—popcorn corn is a variety of corn that has a really tough pericarp. That’s what the photo up top is of!

Sugar Recommendations—And What We’re Missing About Them

Today’s New York Times has a Well Blog post on the FDA’s proposal to change how sugars are labeled in foods. There’s lots that can be said about this (primarily “Yay!”), but here are some quick thoughts about what I think people are missing about the sugar recommendations.

  • Are you aware that the current food labels in the United States that say stuff like “Total Fat   4.5g   3%” don’t list a percentage for sugar? Most of my friends, when I point this out, don’t believe it. (Inattentional blindness.) Go check it out—snag a container of something and look at the food label.
  • The U.S. FDA’s proposed labeling law would require food labels to list a “percent daily value” on a line of “Added Sugars”, similar to the other lines already there. That’s it. Small change; big fuss from industry.
  • Forcing food companies to list “added sugars” (i.e. doesn’t include sugars present from ingredients like fruit), in my opinion, will lead to food companies changing the ingredients in manufactured foods. My bet is we’ll see a decrease in added sugar and an increase in things like fruit puree, without any real change in health outcomes.

While there’s lots of evidence that bad diet and obesity go hand-in-hand, don’t jump up and down thinking that by avoiding junk food and “added sugar” foods that you’re healthier. It’s your overall diet that matters. Brian Wansink’s Food & Brand Lab at Cornell University has an interesting study out last month that claims junk food and fast food aren’t correlated with obesity (David Just and Brian Wansink (2015). Fast Food, Soft Drink, and Candy Intake is Unrelated to Body Mass Index for 95% of American Adults. Obesity Science & Practice, forthcoming). I ran it by one well-respected nutritionist and her reaction was to agree: “the overall diet is what counts.”

P.S. If you’re curious for more thoughts on sugar, see Marion Nestle’s blog post from today.

Does Baking Soda Make Omelets Fluffier?

I’ve found a few recommendations floating around the internet that suggest you could use baking soda to make fluffier omelets, to tenderize meat or even as an additive to beans to reduce “bean bloat”. Do you know if there might be any truth to these? If so, why would they work? I’d also love to know if you have any other ideas for unexpected ways someone could use baking soda in cooking.


Hi Jade —

Both baking soda and baking powder can be mysterious, but they’re actually pretty easy to understand once you view them with some science in mind.

First, baking soda: it’s a single compound — sodium bicarbonate — that reacts with other ingredients and produces carbon dioxide. When sodium bicarbonate gets added into your other ingredients, it dissolves into just sodium and bicarbonate. The sodium adds a salty taste (just like sodium chloride does), but it’s the bicarbonate that’s useful: it can react with other acid compounds (vinegar, lemon juice, buttermilk) to generate gas, or when heated up, will break down on its own and generate carbon dioxide.

Baking powder is like baking soda, but is made of more than one compound. It’s what I call a “self contained leaving system” — it’s got everything you need to lighten food. Baking powder is usually composed of baking soda and an acid such as cream of tartar or monocalcium phosphate. Add water, and poof! The ingredients dissolve and can react with each other. (In powder form, they don’t interact very quickly — but can, over time — so if your baking powder is a few years old, it won’t work very well.)

So now that we have the definitions out of the way, what can we do with them? Baking soda, as a single compound, can be used anywhere there the chemical reaction between the bicarbonate and another compound leads to a change. (Of course, if you add too much baking soda, it won’t completely react — there’ll be left over baking soda that then interacts with your taste buds and tastes nasty.)

You’d asked how baking soda would make an omelet fluffier. Eggs are surprisingly basic; I wouldn’t think of them as having acids that the baking soda can react with. Baking soda does decompose into carbon dioxide and water with heat, though — it looks like this begins around the boiling point of water — so it’s possible that that would be a mechanism. Honestly, though? If I wanted a light, fluffy omelet, I would separate the eggs white and the egg yolks, whisk the egg whites some, and then mix the yolks back in. “Fluffy” means air, and using baking soda is just one way of doing it (chemically). You can “add” air in mechanically, and avoid the potential change in flavor from the baking soda.

As for adding baking soda to beans: according to the US Dry Bean Council — this is a private industry group of folks who grow and sell beans — adding baking soda will make beans more tender (which you’d only want to do if your beans are coming out too tough). They don’t mention anything about baking soda and bloat; however their answer on “gas-causing properties of dry beans” looks reasonable, See:

You might skim and — I don’t know how much I’d trust these, since they’re recent ebook of low quality, but it’s an interesting place to see what’s commonly believed (true or not) that you might be able to then dig into. For example, baking soda in the fridge to absorb odors is not something that I believe works — and here’s a link to “Ask a Scientist” saying as much at the Department of Energy (who I would believe):

Hope this helps! I’m going to wander off to my kitchen to make two different omelets — one with baking soda — to see if there’s any truth to the idea.


Meat: Is It Done?

Ever wonder when your steak is going to be done cooking? Fear no more, Cooking For Geeks has the definitive answer! Step right up! Free, no charge at all!

“An equation for the temperature change with meat layer and time was derived. Heat is added during thermal treatment by radiation from energy generators, thermal conductivity, and phase transitions. The proposed dynamic mathematical model is written as follows:

Meat Equation

Continue reading Meat: Is It Done?

Google Trends and Food Seasonality

I’ve been looking at seasonality in foods, and realized Google might have the answer. It’s so rewarding to see theory line up with data!

California users are shown on the top graph and Massachusetts users are on the bottom. Note the difference in amplitudes: the growing season in Massachusetts is much shorter than in California, so it follows that searches for perishable ingredients would trend up and then back down over a much shorter time period.

Google Trends for MA and CA for tomato, basil, and peach
Google Trends for MA and CA for tomato, basil, and peach

(The spike in June, 2008 – “D” – is from the Salmonella scare.)

SuperTasting at The Awesome Foundation

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Armed with my trusty Nikon and a pack of supertaster test kits, I used The Awesome Foundation’s September event as a testing ground for my Ignite Boston talk.

Here are a few links to resources that I thought covered the topic particularly
well, if you want to learn more about supertasting.

Genetic info:

Interview with Dr. Linda Bartoshuk of Yale School of Medicine:

Activity worksheet:

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