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.
Here’s the video we made last weekend for KidScience app that shows you how to stand on a carton of raw eggs without breaking them:
Remember, Force is pressure per unit of area. In the video, you’ll see what happens when you try to stand on eggs in high heels and the force isn’t evenly distributed.
Does soap really get rid of germs? On Wed. morning, Feb.27th, I’ll stop by Kare11 Sunrise around 6:20 to demonstrate a cool hand-washing experiment and talk about Citizen Science, which lets you help real scientists with their research by collecting data on everything from microbes to dog behavior.
Here’s how to make petri plates for the hand-washing experiment, where you touch labeled sections of the plates with your fingertips before washing, after washing with water alone, and after scrubbing with soap. You can also use the plates to grow microbes from around your house or school!
Click here for a link to the written recipe and more science. I made this video for both the free and premium version of KidScience app, where it will appear on next week’s update, along with the recipe. I’ll post details for the hand-washing experiment early next week.
“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!
I’m re-posting this project we did two years ago, since I’m making plates today for a hand-washing experiment that the kids and I will do after school. Stay tuned!
Did you know that every surface in your home is teeming with microorganisms? Culturing microbes from your home on petri dishes lets you grow some of them as colonies that you can see with your naked eye. You might already have what you need in your kitchen cupboard. If not, the ingredients are readily available at most grocery stores. I demonstrated this experiment on Kare11 and you can watch it here, following the yeast experiment.
To make petri plates, you’ll need disposable containers (see below), beef bouillon cubes or granules, plain gelatin or agar agar*, water, sugar and Q-tips. (*Agar-agar can be found with Asian groceries in some grocery stores.) **Gelatin will melt if it gets too warm, and some strains of bacteria can liquify it, which is why scientists in labs use agar to make their plates. The idea to use agar for plates originally came from Angelina Hess, who used agar for canning food.
For containers, you can use foil muffin tins, clear plastic cups covered with plastic baggies, clear Tupperware with lids, or real petri dishes. We’re going to use clear deli containers, so that we can recycle while we learn. (Containers must be heat-resistant enough to pour warm agar into.)
Start by making microbial growth medium (or microbe food, as we like to call it.) Mix together:
-1 cup water
-1 Tbs. agar-agar (OR one and one half packages gelatin, which is about one and a half oz or 12g)
-1 bouillon cube (or 1 tsp. granules)
-2 tsp. sugar
The next step requires adult assistance, since it involves very hot liquid. Bring the mixture to a boil on the stove or in the microwave, stirring at one minute intervals and watching carefully until the gelatin or agar is dissolved. Remove the boiling liquid from heat and cover. Let cool for about fifteen minutes.
Pour the medium carefully into clean containers, until 1/3 to 1/2 full. Loosely place lids, foil or plastic baggies over containers and allow dishes to cool completely. The geltin or agar should make the growth media hard like jello. When your plates have hardened, store them in a cool place, like a refrigerator, before using. Plates should be used in 2-3 days. When you are working with the plates, try to keep the lids on loosely whenever possible, so that they are not contaminated by microorganisms floating around in the air. If you’re planning to use muffin tins, simply place them in a muffin pan, fill them with agar, and when they’re cool, put them in individual zip-lock baggies. With other containers, put the lids on tightly once the plates harden.
When the plates are ready, shake the condensation (water droplets) off the lids of the containers and put them back on. If you have a clear container, you can draw a grid of four sections on the bottom of the plate with permanent marker. (If using muffin tins, label each bag with the surface you are checking.) Decide which surfaces you’d like to test.
Label your plates with the names of the surfaces you want to test. Be sure to label the bottom of the plate since the lid will move. You should be able to see through the agar to see your lines and your writing. If you want to, you can label a separate plate for each surface, but we had three kids and three plates, so we made sections. TV remotes, kitchen sinks, computer keyboard, doorknobs and piano keys are great surfaces to check. You can even touch your finger to the plate, cough on a plate, or leave one open to the air for half an hour to see what’s floating around! (See the photo at the top of this post for a better picture of how your plate might look.)
Rub a clean Q-tip around on the surface you want to test. Then, remove the lid from the plate and gently rub the Q-tip across the section of the plate labeled for that surface. If you’re careful, the agar shouldn’t break. If it does, it’s no big deal. When you’re done, set the plates on a flat surface with their lids loosened and taped on (do not invert them.)
See what grows! You will mostly see fungi (molds), but you may also see some tiny clear or white spots that are colonies formed by millions of bacteria. Record and draw how your plates look in your science notebook. Keep track of how long it takes things to grow and the shapes, sizes and colors of the microbial colonies that grow on their plates. Sciencebuddies.org has this great page on interpreting what you find growing on your plates! If you want to learn more about microbes, search for the words fungi and bacteria on the website cybersleuthkids.com and it will give you some great links to microbiology websites. Microbes are everywhere, but that very few of them are harmful, and many of them are essential for good health.
Wash your hands after handling the plates, and throw the plates away when you are done. Remember, if you washyour hands with regular hand soap for the length of time that it takes to say the ABCs, you’ll remove most of the harmful bacteria and viruses on them. (One side effect of this experiment is the sudden urge to disinfect computer keyboards and remote controls.)
One of the first things you learn when you do science is that experiments don’t always turn out the way you hope they will. And that’s OK. If at first you don’t succeed….
We tried to measure how far a sneeze would throw visible droplets by putting grape juice in our mouths and tickling our noses with feathers dipped in pepper. Sadly, no matter how hard we tried, we couldn’t seem to sneeze with grape juice in our mouths. Laughing so hard we spit the juice out was one unexpected outcome. We decided to try it again in the future using petri dishes spaced at intervals to avoid the grape juice problem.
Since there was no school today and it was ten degrees below zero (F), we tried throwing boiling water into the air to see if it would freeze before it hit the ground. Once again, our experimental conditions were less than perfect, and although some of the water froze into an icy cloud, the majority hit the ground with a splash. The experiment would have worked better at -20F, but it was still fun!
This season’s manifestation of the flu (H3N2) is currently burning it’s way though the Twin Cities, sending droves of tough Minnesotans coughing and shivering to their beds. Five people have already died from the flu, two of them teenagers. Stomach bugs, caused by other viruses, are making their rounds too, but when health care workers and scientists talk about flu, they’re talking about viral influenza.
How can a shot help prevent a horrible case of the flu and why do you need a new one every year? Can you avoid the flu by washing your hands?
It helps to understand a little bit about the influenza virus. Like I tell my nursing students: know your enemy and you’ll have a better chance of outsmarting it!
Viruses are tiny packages of genetic material (DNA or RNA) that can infect every type of life on Earth, from bacteria to humans. They’re not considered living, because they can’t make new copies of themselves outside their host organism. In fact, to replicate themselves, they have to hijack their host’s cellular machinery. Essentially, viruses are extremely tiny parasites, so small you can’t even see them under a microscope.
To cause an infection and make you sick, the influenza virus has to get into your cells, use your cells to make lots of new copies of itself, and then escape from those cells so the new viral particles can go out and infect other cells.
The influenza virus is made up of eight segments of RNA, surrounded by a protein coat and an envelope. Two types of protein “spikes” that jut out of the viral envelope are called hemagglutinin and neuraminidase. Hemagglutinin (H) helps the influenza virus attach to host cells, and neuraminidase (N) helps burst the host cell open so the new viral particles can escape. Flu strains are names for these proteins, which is why you see influenza called H1N1, and so on.
These H and N proteins undergo small changes all the time. Each year, health workers and scientists study flu strains around the world by looking at their H and N proteins, among other things. If one of the proteins has changed a little bit, they will give it a new strain name (like H3N2.) Pharmaceutical companies make new vaccines every year so that your body can recognize the new strains with the new H and N proteins to fight them off.
A flu shot primes your body’s immune system to fight the flu virus and get rid of it. How well the vaccine works each year depends on a number of thing, including how well scientists guessed about which strains would be circulating and whether those strains have undergone other changes. How well your vaccine protects you personally depends on variables including what flu strain you’re exposed to, your age, whether you have another viral or bacterial infection at the same time, and even how strongly your individual immune system reacts.
What can you do to minimize your risk of getting sick with the flu? Get your flu shot! Even if it doesn’t work perfectly, it will give your immune system a boost against the virus. The flu is spread by coughing and sneezing and on surfaces like doorknobs and other surfaces. Try to stay away from people who are coughing. Wash your hands frequently and keep them away from your eyes, mouth and nose, which are portals of entry for microbes. Eating right and getting enough sleep will bolster your immune system so that if you do get the flu, you may be able to fight it off faster and better.
Although you can’t grow viral cultures outside a lab, you can make homemade petri dishes to grow fungi and bacteria from your hands. Try a hand-washing experiment where you label four petri dishes #1-#4. Touch plate #1 with unwashed fingers, wash those fingers with water alone and touch plate #2, wash the same fingers with soap and water and touch plate #3, and finally wash one last time with hand sanitizer and touch plate #4. Let the cultures on the plates grow for several days and you’ll see for yourself why it’s so important to wash your hands.
Grab an extra bag or two of cranberries and you can stir up a Thanksgiving science project: Spy Juice and invisible ink!
Click here for detailed directions.