Welcome to Savor the Science! In each Savor the Science, RENDER’s resident chemist, Claire Lower, will explore culinary questions through a scientific lens, perfecting recipes and demystifying techniques. Theories and reactions will be discussed and experiments will be performed; it’s like your high school chemistry class, only edible. Twice a month, Claire will take a scientific concept (such as the acid-base reactions in baking, macerating, or Maillard browning), explain it in a way that would make Bill Nye proud (hopefully), and then provide an edible experiment which allows you to demonstrate your new scientific food knowledge.
It’s science you can eat!
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Ceviche is an ideal summer dish.
Requiring nothing more than a citrus marinade and the freshest of fish, this dish is always light, refreshing, and cold; all things that are good when the temperature gets above 80 degrees. And, not only is it delicious and super easy to make, it is impressive. By soaking seafood in citrus juice, you can produce a restaurant-quality dish with about ten minutes of work and thirty minutes of drinking margaritas while you let the acid do its thing.
Ceviche isn’t quite raw, but it’s not exactly cooked either. “What is this fish paradox?” you ask, “How can something be “not raw” without ever being exposed to heat? What is this, some sort Schrödinger’s fish?”
Though it is never “cooked” in the traditional sense, ceviche goes through a chemical process that effectively rearranges or “denatures” the fish’s proteins, much like the application of heat.
Proteins are large, complex biological molecules made up of long chains of atoms that twist and fold in on themselves in unique, three dimensional structures. These unique arrangements are the proteins “native form.”
When a protein is exposed to a situation that is “stressful” for it, like heat or an acidic environment, it loses its unique structure, and can become thick and dense. When you denature a protein, you are destroying its native form. The change is purely structural; no atoms are gained or lost, they are simply rearranged.
Imagine a wad of rubber bands, all twisted and tangled together. Now imagine someone comes in with scissors and snips a couple of them. You still have the same amount of rubber band; none has been removed, but that wad is never going back to the shape it was in.
Denaturing proteins is a similar process. Proteins are held together by bonds. Heat or acidic substances break some of those bonds, and the protein is forever changed. This is why high fevers are dangerous; once your proteins are scrambled, there’s no folding them back together.
For food, this usually results in coagulation, a process which causes proteins to develop a sort of delicate density, like cooked egg whites. Recall our rubber band: now that it has been unraveled, more of its long chains are exposed, allowing the atoms on these changes to interact with previously unexposed areas, forming new bonds throughout the chains that hold the various lengths of protein close together (McGee, 1984). This new closeness between molecules creates a denser configuration, driving out water and increasing toughness.
In the case of ceviche, lime juice (which has a pH of about 2) denatures the proteins in the fish, freeing the long chains of amino acids from its native form, allowing them to rearrange in a formation that is similar to that of traditionally cooked seafood.
Because of the delicate nature of fish, this process can occur quite rapidly; leave it too long and your ceviche will become tough and rubbery. Just look at the change in color and consistency that occurred in tilapia after being exposed to lime juice for ten minutes.
But what of pesky pathogens? Isn’t heat required to make seafood safe for consumption?
Lucky for us, acid takes care of that as well. Vibrio cholera, a seafood-borne pathogen, cannot exist in an environment that has a pH of less than 4.5 (Potter, 2010) even at room temperature, so marinating seafood in lemon or lime juice should take care of it.
Phew! Now that we don’t have to worry about pathogens and possible death, let’s denature some tilapia!
Deliciously Denatured Tilapia Ceviche
You will need:
· 2 tilapia filets (about 1 lb.) [Note: Partially freeze fish for easy and neat chopping.]
· The juice of 4 limes
· ½ cup of peeled, de-seeded chopped cucumber
· ¼ finely chopped purple onion
· 2 tablespoons fresh cilantro leaves
· ½ cup soy sauce
· 1 tablespoon Sriracha
· A couple of dashes of Worcestershire sauce
· 1 teaspoon chopped ginger
· 1 teaspoon chopped garlic
1. Combine cucumber, onion, and cilantro in a small bowl. Sprinkle with salt to get ‘em sweaty; cover and chill in the refrigerator.
2. Combine soy sauce, Worcestershire sauce, ginger, garlic, and Sriracha in a food processor until smooth. Cover and chill.
3. Cut fish into half-inch cube-esque pieces. Place in a nonreactive bowl and pour lime juice over. Stir around to make sure all the fish pieces are submerged in the juice.
4. Cover and chill until fish is completely opaque (20-30 minutes).
5. Drain lime juice from fish, gently squeezing fish with your hands before transferring to a bowl.
6. Add soy sauce mixture to taste (I used about half of the mixture). It is quite salty, so I like to err on the side of less soy sauce, letting guests add more if they desire.
7. Stir in vegetable mixture and serve with toasted baguette or Cuban bread.
Jeff Potter. 2010. Cooking for Geeks. (Sebastopol: O’Reilly), 286
Harold McGee. 1984. On Food and Cooking: The Science and Lore of the Kitchen. (New York: Scribner), 808-809