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!”
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.)
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.
I just put together a homemade science kit (for a silent auction at a fundraiser) and it was so cute I had to post a picture.
Click here to see a post with links to homemade science kit experiments. Love giving these as birthday and holiday gifts too! To take it up a notch, pair your science kit with KidScience app for iPhones/iPods!
Over the next few weeks, I’ll be posting some fun biology experiments, so be sure to check back in!
Doctor Frankenstein would have loved this experiment that makes electrons jump from place to place, with a shocking conclusion. You can watch us demonstrate it on Kare11 Sunrise by clicking here.
To make an electrophorus, or charge carrier, all you need is a Styrofoam plate, a cardboard square large enough to tape the plate onto, an aluminum pie pan, a Styrofoam cup, aluminum foil and wool, like an old mitten or stocking cap.
Tape the foam cup to the inside of the pie pan. Then cover the cardboard with foil and tape the plate on to the surface, facing down.
For your Leyden jar, which can make a bigger spark and shock, you’ll need a plastic film canister, a nail longer than the canister, aluminum foil and water.
Cover the outside of the bottom of the film canister with foil, push the nail through the cap, fill it 3/4 up with water and replace the lid/nail so the nail is in the water.
Now rub the foam plate with wool for about a minute. The Styrofoam attracts electrons from the wool, giving the plate a negative charge.
It’s important to do the next steps in order!
1. Put the pie tin on the foam plate. The electrons on the pie tin are repelled by the negative charge on the plate, but they can’t go anywhere.
2. Put your thumb on the foil at the bottom and leave it there while you touch the pie tin with a finger on the same hand. You should feel a small spark as the electrons jump from the pie plate to your hand, leaving the plate with a positive charge.
3. Lift the plate using the foam cup and touch it to the head of the nail on your Leyden jar. Electrons will flow from the nail to the pie plate, leaving the nail and inside of the jar with a positive charge. Repeat steps 1-3 a few times to build up a charge in your Leyden jar.
4. When you’re ready for a shock, put your thumb on the foil on the bottom of your Leyden jar, leave it there, and bring your fingertip close to the nail on the jar. Electrons will jump from the negatively charged foil on the bottom of the jar to the positively charged nail, giving you a shock. If you try it in the dark, you may see a spark as the electrons move through the air to your finger!
When you’re tired of getting shocked, you can always whip up a batch of color-changing, bubbling Magic Potion! Click here for directions and a video.
What could be more fun than creating your own green slime to play with? It’s easy to synthesize your own green goo using only Elmer’s glue (the non-washable kind), Borax (found in the laundry detergent section of Target and some grocery stores), green food coloring and water.
Mix together about 1/3 cup glue and 1/3 cup water with a spoon or Popsicle stick. These measurements don’t have to be exact. Add a few drops of green food coloring and stir.
To make the Borax solution, add around a cup of water to a jar. To the water, add about a Tablespoon of Borax. Shake or stir to dissolve as much of the Borax as possible. You’re making a saturated solution, so it may not all dissolve!
Here’s the fun part: Add a teaspoon at a time of the Borax solution to the glue/water mix. After each addition, stir the mixture. You’ll see long strings begin to form and stick together. Keep adding Borax until the mixture doesn’t feel sticky. It will form a shiny playdough-like substance. If you add too much Borax solution, it will feel wet, but you should be able to squish it around in your hands to absorb the extra water! The slime isn’t toxic, but Borax is soap, so don’t eat it!
What happens? Mixing Elmer’s glue with water forms a substance called a polymer, which is a long chain of molecules, sort of like a string of pearls. (A molecule is the smallest amount of a specific chemical substance that can exist alone, like H2O, a single water molecule). The polymer formed by water and glue is called polyvinyl acetate.
The Borax solution (sodium tetraborate) is a cross-linking substance that makes the polymer chains stick together. As more and more chains stick together, they can’t move around and the goo gets thicker and thicker. Eventually, all the chains are bound together and no more Borax solution can be incorporated.
You can store the slime in plastic bags. If you want to make a larger batch, just remember to mix equal amounts of glue and water and then add as much Borax solution as needed.
Carin Bonder recently wrote a great post for Scientific American’s PsiVid about an autistic boy named Jordan Hilkowitz who is storming YouTube with his fantastic science videos! Parents will enjoy Carin’s post, and kids will love his Doctor Mad Science videos!
If you’re on Twitter, we’ll be chatting about STEM (Science, Technology, Engineering and Math) tonight at 8 Central under the hashtag #STEMchat. Click here for more information if you’re interested in joining us as we talk about kids science at home and in schools.
Here’s a video that I just made for KidScience app about a fun chemical reaction we call fizzy balloons. Hope you have as much fun watching it as we had making it!
If you haven’t downloaded KidScience, it’s a great way to search for experiments based on kids’ ages, what you have around the house, or how much time you have. There’s a free version and a Premium version with extra videos, so you can have science at your fingertips wherever you go!
Got fruit, laundry detergent, coffee filters, salt and alcohol? You can extract DNA!
Here’s a video on how to extract DNA from strawberries. This version was a little long for KidScience app, so I thought I’d post it here.
If you haven’t tried this experiment, you should! It’s a simple version of how scientists in labs really extract DNA for their research. Click here to see my original post on DNA extraction with easy directions and a little more science.