You can’t judge an egg by its shell, but you can use science to figure out whether or not it’s fresh.
Imagine an egg. It can be white or brown, since they’re identical except for shell color. There are two membranes inside an eggshell, separating it from the inside of the egg and helping to keep it safe from microbial invaders.
Under the membranes is the egg white, made up of proteins and water, and the yolk, which also contains fat and is enclosed in a sac. Tiny rope-like structures anchor the yolk between either end of the egg. The egg white contains a substance called lysosyme, which is a potent antibacterial. Eleven percent of an egg’s weight is made up by shell, 58% by white and 31% by yolk.
When a hen first lays an egg, the raw egg white contains carbon dioxide, making it look cloudy, and the proteins in the egg white are freshly folded into their correct protein shapes, so it will hold a nice shape in a pan. However, egg shells contain thousands of tiny pores, some big enough to see with the naked eye, and as an egg sits, it changes.
The contents begin to slowly shrink, and a small air pocket forms between the two membranes, usually at the large end. The egg’s pH, about 7.6 when first laid, rises as the egg ages and loses carbon dioxide. In just a few days, the pH may reach 9.7, causing the egg white to look clear and spread out more in a pan when the egg is broken.
The nicer shape and centered yolk of fresh eggs is why they’re recommended for frying. But why are older eggs better for boiling, and why does the yolk turn green sometimes?
Fresh eggs are harder to peel. When you boil an egg, it cooks from the outside to the inside, and its proteins become unfolded, or denatured. The denatured proteins are more likely to stick to the membranes on the eggshell of a fresh egg because the pH is lower. According to “FOODS, A Scientific Approach” by Charley and Weaver, eggs are easier to peel if their pH is greater than 8.7. In other words, old eggs that have lost carbon dioxide have a higher pH (are less acidic) and are easier to peel.
Sometimes, when you boil eggs, you see a greenish/gray/blue layer on the outside of the yolk. It’s the harmless product of a chemical reaction between the iron in the egg yolk and sulfer-containing proteins in the white. You can try to avoid it by using fresh eggs, using hot (not boiling water) to cook the eggs, by plunging eggs into ice water immediately after cooking, and by promptly removing the shells.
If you’ve heard of candling eggs, it involves shining a strong light through a raw egg to look at yolk position, air sac size and white clarity. You can also tell that an egg is older if it floats in water, due to the enlarged air sac.
Ideally, to cook perfect hard boiled (large) eggs, you put them in cold water, bring the water to a boil, remove the heat and let the pan stand with the lid on for 17 minutes before removing the eggs and plunging them into cold water. Alternately, boil large eggs for eleven minutes and put them in ice water to stop the cooking. For perfect eggs, prick the large end of your eggs with a pin to release the air in the air sac.
“Are you a good cook?” was the first thing Dr. Tsneo Suzuki asked when I sat down in the office next to his cancer research lab at the University of Kansas. I stared at the picture of his wife, who I later learned had passed away from breast cancer, and wondered whether I should be offended.
After all, I was in my twenties and had five years of molecular biology experience under my belt. But I understood why he asked the question. Once you figure out how to test a hypothesis, most science experiments involve following recipes, which scientists call protocols. Generally, if you can read directions and mix things together in the correct order, in the right proportions, you can do things like amplify DNA and clone genes into bacteria.
So I truthfully answered “Yes, I’m a pretty good cook,” and got the job.
Food preparation is like a science experiment. If you can follow a recipe, you should get something close to what you set out to make, because often the ingredients will interact with each other to make something new. This is the very definition of a chemical reaction. Everything you cook with, from water to baking soda, is just a collection of molecules.
Here’s a collection of some food science experiments on my website. Since I love to cook, I hope to add more in the future! Leave a comment if you have other favorite kitchen science experiments, and I’ll try to add them to the list.
Testing Foods for Starch- Add a drop of iodine and watch for color change to detect starch.
Crock Pot Microbiology: Making yogurt from scratch is a delicious experiment
Yeast Experiment: Pyramids, Pasteur and Plastic Baggies- Grow yeast in a plastic bag to see how they make bread rise.
Emulsions: Mayonnaise and Vinaigrette- Mix the un-mixable using surfactants.
Curds and Whey: Make glue and plastic from milk and vinegar.
Gluten Ball- Explore the protein that makes bread chewy.
Red Cabbage Juice CO2 experiment- Use the pH-sensitive pigment in red cabbage to illustrate how CO2 can acidify liquids (and why soda is bad for your teeth.)
Homemade Petri Plates: test surfaces around your kitchen and house for microbes. Use to test fingers before washing, after washing with water alone, after washing with soap, and after using hand sanitizer.
So remember, cooking can make you a better scientist, and doing science can make you a better cook.
Microbes are always fighting for space.
Bacteria and fungi try to outnumber other tiny competitors using chemical warfare, among other things. That’s why many antibiotics (which kill certain bacteria) are actually produced by other bacteria. One reason foods like yogurt and cheese, which are made by beneficial bacteria like Lactobacillus acidopholis, don’t easily spoil is that these bacteria can turn milk sugars into lactic acid. This makes their environment toxic to some of their competitors, like pathogenic bacteria. Luckily, we humans aren’t harmed by lactic acid and can enjoy its tangy flavor.
To grow bacteria in labs, scientists have to take care of them the way you’d take care of a pet. You have to give them the type of food they like, the right amount of oxygen and moisture, and keep them at their optimal temperature.
The same principles apply to growing the bacteria that make yogurt. You prepare the bacteria’s food by heating some milk and letting it cool to a temperature that the bacteria can tolerate. Then, you add the bacteria and let them grow for about eight hours. During that time, the bacteria will happily divide, multiply and eat milk sugar. In the process, they’ll produce lots of lactic acid which changes the way the proteins and fats in the milk interact, forming a more solid food product.
We made yogurt in our crock pot, which turned out to be a lovely bacterial incubator. The end product was a little runny, but putting it through cheese cloth (or a coffee filter in a plastic bag with the tip cut off) gives you thicker yogurt. It is delicious! Here’s how we made it, thanks to directions from Stephanie O’Dea:
Ingredients: 8 cups (half-gallon) of whole milk , 1/2 cup grocery store yogurt (must contain live/active culture), thick bath towel, slow cooker
Turn crock pot on to low. Add an entire half gallon of milk. Cover and cook for 2 hours and 30 minute. Unplug your crock pot, but leave the cover on. Let it sit for 3 hours so your bacteria will not be overheated when you add them.
After 3 hours, put 2 cups of your warm milk in a bowl. Whisk in 1/2 cup of the live/active culture yogurt. Dump the bowl contents back into the crock pot and stir well. Wrap a heavy bath towel all the way around the unplugged crock pot as insulation and let your bacteria grow for 4-8 hours or until thickened. Refrigerate and enjoy with fruit, honey, or granola. As I mentioned, you can strain the yogurt if you prefer a thicker consistency, and your homemade yogurt will make a great starter culture for the next batch!
Happy kitchen microbiology!
“When I wasn’t at school, I was experimenting at home, and became a bit of a Mad Scientist. I did hours of research on mayonnaise, for instance, and although no one seemed to care about it, I thought it was utterly fascinating. When the weather turned cold, the mayo suddenly became a terrible struggle, because the emulsion kept separating, and it wouldn’t behave when there was a change in the olive oil or the room temperature. I finally got the upper hand by going back to the beginning of the process, studying each step scientifically, and writing it all down. By the end of my research, I believe, I had written more on the subject of mayonnaise than anyone in history. I made so much mayonnaise that Paul and I could hardly bear to eat it anymore, and I took to dumping my test batches down the toilet. What a shame. But in this way I had finally discovered a foolproof recipe, which was a glory.” Julia Child, from My Life in France
Julia’s secret for fool-proof mayo? Beat the mixture over a bowl of hot water to get the oil and eggs to form an emulsion, which is a mixture of two thing which are normally immiscible, like water and oil. In an emulsion, a bunch of one type of molecule will actually surround individuals or small groups of the other type of molecule (think ring-around the rosy with one or two people in the middle who would rather not be there.) When you’re trying to make an emulsion, it also helps to add a mediator called a surfactant to get between and interact with the immiscible molecules to stabilize the mixture. In a vinaigrette prepared using oil, mustard and vinegar, the proteins in the mustard act as surfactants. In mayonnaise, adding a little water to the eggs before adding the oil helps make some of the proteins in the eggs more available to act as surfactants. Of course, adding a little mustard helps too and tastes great!
You can tell when an emulsion begins to form, because your mayo or vinaigrette will start to look lighter-colored and thicker as the molecules are rearranged and reflect light differently!
Here’s the New York Times recipe we used to make mayonnaise:
- 1 large egg yolk, at room temperature
- 2 teaspoons lemon juice
- 1 teaspoon Dijon mustard
- 1/4 teaspoon kosher salt
- 1 teaspoon cold water
- 3/4 cup neutral oil such as safflower or canola
- In a medium bowl, whisk together the egg yolk, lemon juice, mustard, salt and 1 teaspoon cold water until frothy. Whisking constantly, slowly dribble in the oil until mayonnaise is thick and oil is incorporated. When the mayonnaise emulsifies and starts to thicken, you can add the oil in a thin stream, instead of drop by drop.
Try some variations on these kitchen experiments. Does it work better to use a cold egg, room temperature egg, or warm egg? What happens if you try to make mayo by setting your mixing bowl in a bowl of ICE water? Do you get an emulsion?
As Julia Child would say, “Bon Appetit!”
Ever wonder how much starch is in your Thanksgiving dinner? Click here for a fun experiment that lets you test your favorite foods for starch using iodine from the medicine cabinet.
Remember to supervise small children if you do this experiment, since iodine should not be ingested! Happy Thanksgiving!
Gluten is a hot topic these days, but most people aren’t really sure what it is.
Gluten is a protein found in wheat, rye and barley. It makes bread chewy and helps hold the bubbles that yeast makes in dough so that it can rise. (Here’s a fun yeast experiment!) Although some people have a disorder called celiac disease which impedes them from digesting gluten properly, most people can eat it without any problem.
You can easily extract a ball of gluten from flour using nothing but your hands and cold water, to see for yourself what this stretchy grain protein looks like. Just add a cup of water to a cup of flour, mix it together and knead it for about 5 minutes. (Add more flour if it’s too sticky to handle.)
Now, put the dough under COLD water and start “washing” out the carbohydrates so that mostly the gluten remains. Your hands will freeze, so you may need to take a break. If you keep going until the water coming off the dough is mostly clear, you’ll be left with a gluten ball.
Some types of flour have more gluten than others, and sometimes extra gluten is added to pizza dough to make it chewier! Try this with several types of flour to see what has the most gluten (or no gluten.)
Last week, I wrote about Carbon, Bananas, Coal and You and promised to try to come up with a safe, easy way to see the carbon dioxide in your breath, so here it is! If you prefer a video, click here to see a quick demo of the experiment on Kare 11.
Once again, the star of the show will be red cabbage juice, a safe, natural, easy-to-make acid/base indicator and the same one you can use to make magic potion and red cabbage litmus paper. The trick is to use a very small volume of cabbage juice, since it’s not a very sensitive acid indicator.
You’ll need red cabbage, drinking straws, and very small cups (the ones you measure kids’ medicine with work well) or test tubes. Chop a head of red cabbage, boil it for 5-10 minutes, and collect the juice. It will be purple and turns blue when exposed to a base or pink when exposed to an acid.
Then, pour a very small volume- a teaspoon or two (5 to 10 ml)- of the (cooled) juice into two small cups. Take a straw, put it all the way against the bottom of one cup and blow through the straw repeatedly for a few minutes until you see the cabbage juice turn noticeably pinker than the juice in the control cup. It may take several minutes to see a difference, so be patient! Test tubes are less messy since the juice can’t splatter so much.
What happens? The carbon dioxide in your breath combines with the water in the cabbage juice to form carbonic acid, causing the pH of the solution to drop and the cabbage juice to turn pink.
Why is this interesting? About a quarter of the carbon dioxide released by activities like burning fossil fuels and burning down rainforests is absorbed by our world’s oceans. This results in the ocean water becoming more acidic, like the cabbage juice in the experiment, and can have an effect on sea life, like coral. To learn more about ocean acidification and the chemistry of ocean acidification, check out NOAA’s amazing website.
You can explore the same concept (and see why carbonated drinks are hard on your teeth) by pouring uncarbonated water into one cup of cabbage juice and carbonated water into another. If you can, choose water from the same source, so you know the only difference is the carbon dioxide that’s been added to make it fizzy! Or, you could use dry ice to add carbon dioxide bubbles to water and test it before and after you add bubbles!
What happens if you add yeast to cabbage juice and let it grow for a while?
This is a great science project and results in beautifully colored paper that can be dried and used for art projects like collages.
All you’ll need is a head of red cabbage and some paper towels or white coffee filters. Alternately, you can just use the juice from canned red cabbage. I’d recommend wearing an old tee shirt or a home-made lab coat for this project, since I’m guessing that cabbage juice stains. To make a lab coat, just have kids write their name in permanent marker on the pocket of an old button-down shirt.
Chop half a head of red cabbage into small pieces and add it to a pan with about a cup of water. Boil the cabbage uncovered for about 15 minutes, stirring occasionally, let it cool, and strain the juice into a jar or bowl. (Save the cooked cabbage for your favorite recipe and make cole slaw with the other half!)
If you want to avoid the stove, chop half a head of red cabbage and blend it with about 3 cups of water. Strain the liquid through a colander and then through a coffee filter in a plastic bag with one corner cut off. Blended cabbage juice makes longer-lasting bubbles and turns a slightly brighter shade of blue!
Cut the paper towels or coffee filters into strips about an inch wide and a few inches long and soak them in the cabbage juice for about a minute. Remove them and let them dry on something that won’t stain. I blotted them a little to speed up the drying process. You might even try using a blow dryer!
When dry, your litmus paper will be ready to use for testing acidity. Your can dip the paper into orange juice, soapy water, lemon juice, baking soda in water, baking powder in water, vinegar, and anything else they want to test. The paper will turn red-pink in acids and blue or green in bases.
Everything in our world is made of very tiny pieces called atoms. Atoms are so small that if you blow up a balloon, it will contain about a hundred billion billion atoms of the gases that make up air. Atoms are often bonded to other atoms to form a group of linked atoms called a molecule. A water molecule, for example, has two hydrogen atoms and one oxygen atom, bonded together.
Acids usually dissolve in water to form free-floating hydrogen atoms. Bases are the opposite and take up free hydrogen atoms. The molecules in the cabbage juice litmus paper change when exposed to an acid or base, making the paper change color.
For years the F.D.A. has been threatening to do more rigorous testing of milk supplies for antibiotics. Now they’re going through with it, and many in the dairy industry are furious.
According to an article I read in the New York Times, inspectors have found both illegal levels of antibiotics, and types of antibiotics not regularly tested for, in older dairy cows that have been sent to slaughter. Anyone who doesn’t want their children getting an extra dose of drugs with their glass of milk should find this disturbing.
The fact that one large dairy COOP in the NorthEast told members to dump milk tested by the FDA in order to avoid recalls indicates that many in the industry are worried about what inspectors will find once they begin testing.
Large-scale agriculture is rife with antibiotics that are fed to beef cows to make them grow faster. What do consumers have to do to stop the overuse of these drugs in our food supply?
I plan to keep buying organic milk, hoping the tighter regulation and safer farming practices will keep my food pristine.
And please, test away, F.D.A. It’s your job to protect consumers. Don’t be afraid to do it.
Using cranberries and baking soda, you can create invisible messages that will be revealed to friendly eyes and self-destruct before your enemies have a chance to read them. You’ll see how some pigments in fruit can change color when they’re exposed to an acid or a base.
I showed viewers how to make Spy Juice on Kare11 this morning. Here’s the link, if you want to watch a video demonstration!
You will need half a bag of cranberries, water, baking soda and some printer paper. To write your message, you’ll also want to find a small paintbrush, Q-tip, or something else with an absorbent tip. We made our own pens by wrapping a tiny piece of paper towel around the pointed end of a wooden skewer and winding scotch tape around to secure it. Be sure to put on an apron or wear old clothes for this experiment, since cranberry juice stains!
Have an adult or teenager boil the cranberries in about three cups of water for 15 or 20 minutes. Be sure to put a lid on the pan, since the small pockets of air that help cranberries float can make them explode as they cook. If you listen, you’ll hear some of the them popping! Crush the cooked berries and push the liquid through a sieve or colander to collect the concentrated cranberry juice. Most cranberry juice from the grocery store is diluted with water, corn syrup and other juices and won’t work as well!
Let the juice cool and pour it into a casserole dish or cake pan that the paper you’re writing your messages on will fit into. If your cranberry juice seems thick and syrupy, add a little water (maybe half a cup.) It has to have enough water in it so that it will soak into the paper!
Add a few teaspoons of baking soda to about 1/3 cup of warm water and dissolve it as well as you can. (Don’t worry if you can still see some baking soda.) Using a Q-tip, paintbrush, or your homemade writing tool, use the baking soda solution to write a message on your paper. It may take a little practice, so don’t get frustrated. You’ll get the hang of it!
Let your message air dry, or speed things up with a blow dryer.
To reveal your message, place your paper in the cranberry juice and see what happens!
The science behind the fun:
Cranberries contain pigments called anthocyanins (an-tho-SY-a-nins), which give them their bright color. In nature, these pigments attract birds and other animals to fruit. This is important because animals eat the berries and spread plants seeds from one place to another.
These pigments, called flavanoids, change color when they come in contact with acids and bases. Cranberry juice is very acidic, and the pigment is red in acids. When you add it to a base, it turns purple or blue. Baking soda is a base, so your baking soda message will turn blue when it comes into contact with the pigments in the cranberry juice. Eventually, when enough cranberry juice soaks into the paper, it will dilute the baking soda and make the paper acidic, turning the pigment back to red and your message will disappear!
There are over 300 kinds of anthocyanins which are found in many fruits and vegetables including blueberries, red cabbage, grapes and blueberries. Scientists think they may have many health benefits and some researchers are even making organic solar cells using flavanoids!
What other juices can you use to reveal secret messages? What other bases could you use as ink?
Try making your own recipe for spy juice! I’d love to hear how red cabbage works! Check out my red cabbage litmus paper experiment to find out how to make red cabbage juice.
Let me know what works best!