Read Cooking for Geeks: Real Science, Great Hacks, and Good Food Online
Authors: Jeff Potter
Tags: #COOKING / Methods / General
Cooking for Geeks: Real Science, Great Hacks, and Good Food (54 page)
I asked Dr. This if he had a favorite experiment that could be done at home to learn more about food. His reply:
The most exciting discovery that I did was to put fruits like plums in various glasses full of water but with different quantities of sugar dissolved. In light syrups the fruits sink, but in concentrated syrups they float. This is, of course, linked with density, but when you wait, the fruits in light syrups swell (by osmosis) and explode, whereas they shrink in concentrated syrups.
This experiment is useful to know how to make a syrup of the exact concentration for preserving fruits: put them in concentrated syrup and slowly add water until they begin sinking. The osmotic pressure is then nil so that they will keep their shape and consistency.
Left: cherries in water only; center: cherries in a light sugar syrup; right: cherries in a heavy sugar syrup.
We’ve already discussed chemical reactions that generate air via acid neutralization with baking powder and baking soda in
Chapter 5
(see
Baking Soda
), but there’s more to pH in cooking than just that. Acids and bases are commonly used to adjust the pH level for two reasons: to cook foods and to prevent foodborne illness.
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. (Uncooked rice is dry, and since bacteria need moisture to reproduce, they remain dormant. See the FAT TOM variables from
Foodborne Illness and Staying Safe
for more.) 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.
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.
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).
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.
Making your own cheese is neither a time-saver nor a money-saver, but it’s a great experiment to see how closely related two seemingly different things can be. Cheese is made from
curds
— coagulated casein proteins — in milk. The whey is separated out via an enzymatic reaction, allowing the curds to be cooked and then kneaded, stretched, and folded to create that characteristic structure found in string cheese.
Extra credit for using water buffalo milk or milking the cows yourself.
American string cheese is really mozzarella cheese that’s been formed into long, skinny logs. Other countries make string cheese using goat or sheep’s milk, sometimes adding in cumin seeds and other spices, and often braid several thin strands together.
You’ll need to order a few chemicals to do this. (See the upcoming notes and the sidebar
Buying Food Additives
for how to do so.) In two small bowls or glasses, measure out and set aside:
- ½ teaspoon (1.4g) calcium chloride dissolved in 2 tablespoons distilled water
- ¼ tablet rennet, dissolved in 4 tablespoons distilled water (adjust quantity per your rennet manufacturer’s directions)
In a stock pot, mix and slowly heat to 88°F / 31°C:
- 1 gallon (4 liters) whole milk,
but not
ultra-pasteurized or homogenized - 1½ teaspoon (12.3g) citric acid
- ¼ teaspoon (0.7g) lipase powder
Where it says “not homogenized,” it really means not homogenized. (The milk can, and probably should, be pasteurized, though.) If you use homogenized milk, you’ll end up with a squeaky mess that vaguely resembles cottage cheese but doesn’t melt together. The homogenization process disrupts the protein structures such that they can no longer bind together.
Once the liquid is at 88°F / 31°C, add the calcium chloride and rennet mixtures and continue to
slowly
heat to 105°F / 40.5°C, stirring every few minutes. At this point, you should begin to see curds separating from whey.
Once the liquid is at 105°F / 40.5°C, remove from heat, cover the pot, and wait 20 minutes. At this point, the curds should be fully separated from the whey; if not, wait a while longer.
Transfer the curds to a microwave-safe bowl using a slotted spoon, or strain out the whey and transfer it from your strainer. Squeeze as much of the whey out of the curd as possible, tipping the bowl to drain the liquid. Microwave on high for one minute. Squeeze more of the whey out. The cheese should now be sticky; if not, continue to microwave in 15-second increments until it is warm and sticky (but not too hot to handle).
Add ½ teaspoon flaked salt to the cheese and knead. Microwave for one more minute on high until the cheese is around 130°F / 54.4°C. Remove and stretch, working it just like playing with silly putty: stretch, fold in half, twist, and stretch again, over and over, until you’ve achieved a stringy texture.
Notes
- The addition of acid denatures proteins in the milk, helping curd formation. Citric acid is commonly used. For similar reasons, many cheeses use rennet — traditionally derived from calf stomach — because it has a number of enzymes that break down proteins in the milk.
- The lipase powder is not
chemically
required, especially given that animal-based rennet contains lipase. Your rennet source might not contain it, however, and the lipase enzyme is responsible for the characteristic flavor of mozzarella because of the way it cleaves the fats in milk. For a lacto-vegetarian mozzarella cheese, use vegetable-based rennet and skip the lipase powder, but note that the cheese will not have the traditional flavor. For a source of lipase, try
http://www.dairyconnection.com
or
http://thecheesemaker.com/cultures.htm
. - For a good writeup on making mozzarella following a more traditional, more authentic, and much more involved approach, see
http://fiascofarm.com/dairy/mozzarella.htm
.
