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3. Acids and Bases

When it comes to cooking with acids, ingredients such as lime juice can be used to essentially “cook” the proteins in items like shrimp and fish, resulting in similar changes to those that happen when applying heat. On the molecular level, a protein in its native state is structured so as to balance the various attracting and repulsing charges between both its internal regions and the surrounding environment. Portions of proteins are nonpolar—flip ahead a few pages to the section on “When a Molecule Meets a Molecule” to read about polarity—while water is polar. Because of this, proteins normally contort and fold themselves up so that the polar regions of their structures are arranged in a stable shape. Adding an acid or base denatures a protein by knocking its charges out of balance. The ions from an acid or base are able to slip into the protein’s structure and change the electrical charges, causing the protein to change its shape. For dishes like ceviche (citrus-marinated seafood), the acid from the lime or lemon juice literally causes a change on the molecular level akin to cooking. And this change doesn’t just happen on the surface—given sufficient time, acidic and basic solutions will fully penetrate a food.

When it comes to food safety, adjusting the pH level of the environment can both destroy any existing bacteria or parasites and also prohibit their growth. Ceviche is a classic example of this. Vibrio cholerae—a common seafood-borne pathogen—rapidly dies in environments with a pH level below 4.5, even at room temperature. With sufficient lime juice, V. cholerae will not survive. Or consider cooked white rice. Left out at room temperature, it becomes a perfect breeding ground for Bacillus cereus: it’s moist, at an ideal temperature, and has plenty of nutrients for bacteria to munch away on. But drop the pH level of the rice by adding enough rice vinegar—down to about 4.0—and the rice falls well outside a hospitable range for bacteria to grow. This is why proper preparation of sushi rice is so critical in restaurants: failure to correctly adjust the pH level can result in sickening diners.


Note:

Spores for B. cereus are highly prevalent in soil and water; they’re essentially impossible to get rid of. They’re heat-stable, too—you can’t boil them away. Whoever joked about cockroaches being the only thing to survive a nuclear blast clearly hadn’t read up on these things.


Scallop Ceviche

This scallop ceviche is a simple dish to prepare, and surprisingly refreshing on a warm summer day. It’s also a good example of how acids—in this case, the lime and lemon juices—can be used in cooking.

In a bowl, mix:

½ cup (130g) lime juice

¼ cup (60g) lemon juice

1 small (70g) red onion, sliced as thinly as possible

2 tablespoons (20g or 1 bulb) shallot, thinly sliced

2 tablespoons (18g) olive oil

1 tablespoon (15g) ketchup

1 clove (7g) garlic, chopped or run through a garlic press

1 teaspoon (4g) balsamic vinegar

Add and toss to coat:

1 lb (500g) bay scallops, rinsed and patted dry

Store in fridge, toss again after two hours, and store overnight to give sufficient time for acid to penetrate scallops. Add salt and pepper to taste.

Notes

  • Try slicing one of the scallops in half after two hours. You should see a white outer ring and a translucent center. The outer ring is the portion that has had time to react with the citric acid, changing color as the proteins denature (just as they would with heat applied). Likewise, after marinating for a day or two, a sliced scallop should show a cross-section that’s entirely white.

  • Keep in mind that the pH of the marinade is important! At least 15% of the dish should be lime or lemon juice, assuming the remaining ingredients are not extremely basic. Lime juice is more acidic than lemon juice (pH of 2.0–2.35 versus 2.0–2.6).

  • Try adding minor quantities of herbs like oregano to the marinade or adding cherry tomatoes and cilantro to the final dish (after marinating).


Note:

What, you’re worried that the scallops are still “raw” and full of bacteria? To quote from the literature: “In the face of an epidemic of cholera, consumption of ceviche prepared with lime juice would be one of the safest ways to avoid infection with [Vibrio] cholerae.” (L. Mata, M. Vives, and G. Vicente (1994), “Extinction of Vibrio cholerae in acidic substrata: contaminated fish marinated with lime juice (ceviche),” Revista de Biologiá Tropical 42(3): 472–485.)

Still, since some types of bacteria can withstand more extreme environments, if you really want to play it safe, avoid serving this to anyone in an at-risk group.

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