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Mold Facts and Experiments

This Frequently Asked Questions is designed to help students learn more about molds and other fungi. Many students have questions about molds for Science Fair projects, so this page is focused on questions that come up from project work.

LIBRARY RESOURCES:

As a first step, many questions about molds can and should be answered at the library. Any General Biology textbook will have a section on Mycology, which is the branch of science under which molds are studied. You can also find out about molds from the following books:

  • The Advance of the Fungi, E.C. Large: A historical look at the effect of fungi on the world and world events
  • Introductory Mycology, by Alexoupolos: The standard college textbook for mycology

INTERNET RESOURCES:

Recent information on the genetics of orange bread mold, Neurospora crassa

SAFETY:

Caution: try not to breathe in the spores of any mold that you grow. Leave bags or jars closed so the spores don't get loose in large numbers. You are normally exposed to mold spores in the air all of the time, but when they are grown for experiments, you have amplified the number of spores you are growing by much higher numbers than found in the environment.

What are molds?

The word Mold is a general term used for fungi that produce asexual spores. Fungi include such things as mushrooms, athlete's foot, apple-scab, corn smut, wheat rust, wood rots, and so forth. The most famous mold is the genus Penicillium, from which we get the antibiotic penicillin. The name Penicillium comes from the Latin word for brush because under the microscope, the mold has a brush-like appearance. Molds are composed of long strands of cells, resembling spaghetti, which are collectively called mycelium.

How do molds get their food?

Molds carry their stomachs on the outside, unlike people who digest food on the inside.

Molds digest food by producing enzymes outside of their cells. The enzymes break down foods in their environment into small molecules, and then the mold brings back the molecules into its cells for continued digestion and energy. This is the opposite of what people do - we eat foods from outside of our bodies, and the enzymes to break the food down are inside of our stomach.

This is how molds rot things like wood - they literally digest their way through the material!

What is the best food for molds?

Most molds grow well on things like bread that has no preservatives, old coffee-grounds, orange rinds, old fruit (not bananas). Cheeses which are made with molds, such as Brie, Camebert, or true blue cheese, will grow out their molds when left in a warm, moist environment that has fresh air. Wet paper products, such as cardboard, or materials high in cellulose, such as wet hay or grass also will grow molds well.

Molds must have a moist environment to grow, with lots of fresh air. Molds will be overcome by bacteria growth if you put them in a plastic bag with no fresh air. Molds also need some light to produce their spores. Most molds do not grow well at very cold or very hot temperatures. Molds like a slightly acid environment, around pH=5.5 to 6.

Many times preservatives are added to bread and other foods to keep mold and other organisms from growing. Check the labels of the breads you use to see if they have preservatives. If preservatives are present, mold will not grow very well or not at all.

How do molds reproduce?

Usually when the food supply is used up, molds produce upright structures which are full of spores. The upright structures are called conidiophores, and the spores are called conidia. Several types of spores are produced. Dry spores fly off when air currents disturb them. Wet spores are usually moved by small insects, animals, or even raindrops. Some very special spores are produced inside of water bubbles by molds which reproduce under water - the spores have arms which stick out on all sides, similar to toy jacks. These arms get caught in the water surface, and with wave action a bubble forms around the arms of the spore. You can capture these often beautiful spores by collecting the foam that collects in streams during the fall - this foam is full of those spores.

How do I know what mold I have?

Molds can only be positively identified with a microscope. Bluish-green to green molds are usually Penicillium or Aspergillus. Black to brown-black molds can be Aspergillus niger, Alternaria alternata, Cladosporium herbarum, Cladosporium sphaerospermum, or Stachybotrys chartarum (a highly toxic mold). Reddish or pink molds are usually species of Fusarium.

Ideas for Experiments:

Molds grow on many foods, especially foods with starch (like bread). Many times preservatives are added to bread to keep mold and other organisms from growing. Check the labels of the breads you use to see if they have preservatives.

You might want to perform an experiment to see which molds would grow on breads with preservatives as compared to home-made or local bakery bread made without preservatives. Would one of the breads grow mold sooner than the other?

There are many molds that would grow on bread, but some of the most common species are Penicillium and Aspergillus molds. It is well known that a mold called Neurospora also is a mold problem in bakeries. Spores of Neurospora only germinate after being exposed to high temperatures used in baking. It is a very difficult mold to get rid of in a bakery once it has established itself.

Molds need a moist environment with moderate temperatures to grow.

A good way to grow them is to place some kind of food in a jar with holes placed in the lid. Be sure the food is damp but not soaking wet. Keep in a warm place, at room temperature. Too hot or too cold may give poor results. Keep a log or notebook of daily observations.

SOME QUESTIONS AND ANSWERS FROM STUDENTS AROUND THE COUNTRY:

QUESTION: thank you for all your help!!!! i took your advice and went to the library. it was alot of help!!! but in the encyclopedia it was the same info i got in the net so that wasnt any help. for my question: which kind of liquid would make mold grow faster.... im thinking of useing salt water, mouth wash, hydrogen peroxide, and oil. is that the best kind of lquuid to do for this project? if you have any comments please e amil me back,asap!!! thatnk you soooo much for the information!!! jackie

ANSWER: It looks like you are doing an experiment: if you are, why not just go ahead with the liquids you mentioned and give them a try? I think you should also add sugar water to your experiment. The results from your experiment will tell you the answer. That is how science experiments of this kind work, and you can learn a lot from these.

QUESTION: I am developing a science fair project based on identifying types of molds growing in different locations throughout the building for an Advanced Chemistry and Physics class. I would appreciate any suggestions or advice on conducting the experiment (my teacher has arranged protocol with the Microbiology professor at the local community college). Also, any suggestions for the literature search would be greatly appreciated. I am looking forward to hearing from you as soon as possible and thank you for your assistance once again.

ANSWER: I would suggest that you run an experiment to see how mold species differ in different parts of the air systems. Find out where the air filters are for the building and sample at the beginning of the system, and then continue sampling at rooms further and further away from the air supply system. You can sample for mold spores which are mixed up in the dust that accumulates on the surfaces of air-duct grillwork.

The easiest way to sample quickly is to use the tape-lift method. This is safer than trying to grow them, since when you grow them you amplify their presence in a Petri dish of agar: since you don't a priori know which species you have sampled, you may accidentally select toxic fungi which could do you harm when the petri dish lid is opened for examination. Most schools don't have very good safety equipment for working with toxic, airborne molds.

Purchase some ultra-clear scotch-brand tape, and get some microscope slides. At a sample site, pull off about two inches of tape, gently press it against the sample surface, and then attach the tape to the slide (you can put stand-offs on the slide and place the tape sticky-side down or come up with some other method: I just gently place the tape sticky-side down on the slide and gently peel it off back in the lab). Back in the lab, place a drop of 50%aq glycerol on a microscope slide, excise a centimeter square portion of tape, place it sticky-side (mold-side) up on the slide - the glycerol should help hold the tape down, but a scalpel and dissecting needle will help you at this point. Put a small drop of alcohol on the surface of the tape to break surface tension, followed by a drop of 50%aq glycerine, and then top off with a #1 coverglass 22mm square. You can then examine the tape under a microscope for spores. There really aren't too many species of fungi you would find in a building anyway, so you wouldn't be over-taxed for finding too many species. You will probably find Alternaria, Pithomyces, Epicoccum, Aspergillus, Penicillium, and possibly the nasty Stachybotrys. You can find pictures of all of these molds on the web - go to the web site hosted by the University of Minnesota Fungal Tree of Life Project: http://aftol.umn.edu/

QUESTION: How can I grow molds?

ANSWER: There are two ways to go about growing molds. You can grow them on natural occurring things like bread, fruit rinds, or other organic material or grow them on agar gelatin in sterile Petri plates.

To grow them on natural substrates, just moisten the material and place it in a closed container, like a babyfood jar or mayonnaise jar, with holes poked in the lid to allow some air to move in and out. This setup will tell you what kind of naturally occurring molds would grow. Don't let the substrate get soaking wet - it just needs to be moist, and it needs air, otherwise bacteria will overgrow the mold.

To grow molds on Petri plates and use the agar media is expensive and time-consuming. You can order material from Carolina Biological https://www2.carolina.com/webapp/wcs/stores/servlet/StoreCatalogDisplay?storeId=10151&catalogId=10101&langId=-1

which specializes in school materials. You would have to sterilize the agar growth medium in a pressure cooker, then pour the medium into sterilized plastic petri plates, or empty baby food jars with the lids. Sterilize the baby-food jars in the pressure cooker as well.

 

How to make the media: To make growth media is pretty easy. You could buy powdered agar at a health food or Japanese specialty store. It is expensive, but you don't need much (it runs $45/lb, but you only need a few grams). You will also need some potatoes and sugar.

  1. Boil two cut-up medium potatoes in about a quart and a half of water for 30 min (tap water is fine for this, well-water is better as it has minerals the Penicillium needs). Pour off the water into a container and chill in the refrigerator until cold.
  2. Take 20 gm of powdered agar and mix in with 1 liter of cold potato water. Very important not to mix the agar powder into hot water, as it will clump. Stir in about 2 tablespoons corn sugar or 1 tablespoon sugar
  3. Bring the mixture to a boil, stirring constantly over medium heat. Take it off the heat once it boils and pour into empty, clean, babyfood jars or small canning jars. Put about 1/4"-1/2" growth medium into the bottom of your container. Put the lids on the jars, and then cover the tops of the jars and lids with a generous amount of aluminum foil to seal and protect the lid junction with the jar from outside contamination later on.
  4. Sterilize in a pressure cooker for 20 min at 15 lbs. Don't begin timing until 15 lbs. is reached on the pressure cooker.
  5. After 20 minutes, take the pressure cooker off of the burner. Let the pressure go to "0" before opening the lid, or else the growth medium will boil out of the jars and you will have a mess.
  6. Let the jars cool overnight.
  7. Find some Penicillium mold - it is usually greenish to greenish blue in color and lives on rotting vegetable debris and moist read. It also grows on true soft French cheeses, such as Roquefort, blue-cheese and Brie. The outer rind of the cheese contains the mold. When you find some mold, take a very small watercoloring brush, clean it off well, and brush it across the mold to get the spores on the brush. Then take the brush and wipe it across the surface of your growth medium. Keep the whole thing at room temperature and you should get good mold growth. You can also grow other molds on this stuff. Molds also need light to grow and produce colored spores, so an interesting experiment would be to grow cultures in absolute darkness and others in light. Do not open the dark-grown cultures until after about a week and a half to do comparisons, and you should never take a peek in between, as this would let enough light in to interrupt your experiment.

QUESTION: I am doing a science project. It's about mold. The name of my project is called The effect of different solutions on bread: will it have mold? Because bread molds, I thought that by having different things up there it would mold faster. I put milk, hot sauce, ketchup, soy sauce and things like that on the bread. I put them in my basement, its always cold in there. But the bread has been in my basement for over 3 weeks and no mold has occurred. What's wrong with it?

ANSWER: First of all, many prepared foods like bread and ketchup have preservatives in them to keep mold and bacteria from growing, so you are up against that problem. Soy sauce has a very special kind of mold in it that only tolerates high salt contents, but needs higher temperatures to grow. Milk has a lot of protein in it, which molds don't like. To get the most out of your bread experiment, use home-made or organic store-bought bread with no preservatives. Keep it warm, cold and even hot to see differences. You also might want to try other stuff like leaves from the yard. Also with the bread, you might want to add substances to the bread that don't have preservatives: sugar water, honey (which has SOME preservatives made by insects), home-made tomato juice, home-made orange juice, etc. Anything else that comes in a package would have preservatives and ruin your experiment, although you could try the difference between canned tomato sauce and home-made tomato sauce.

QUESTION: Does Penicillium grow on just particular fruit or can it grow on all kinds of fruit (ie. pineapple)? Other than unsuitable growing conditions, what kills penicillium?

ANSWER: Some species of Penicillium can grow on almost any kinds of fruit, even if they are dried and have a very high sugar content. Occasionally, those species are found as food spoilage organisms.

Things that kill Penicillium, besides pesticides and unsuitable growing conditions, are some kinds of fungal viruses, and it is also possible under special conditions for some species of fungi to attack Penicillium. In addition, a number of small, microscopic mites and insects graze on species of Penicillium and other molds.

QUESTION ON MEASUREMENT: ok guys, this'll only take a couple of seconds. i've been cruisin' your site and i have found much information on the topic "bread mould", so i won't waste your time in asking those questions that you have answered one thousand times. i am a school student like many others and i'm doing the same "mould on bread" experiment just like many others have before me. i have just come across one small problem....once you have your beautiful mould on the bread, how would you measure it? would you measure it by it's surface area or something similar?

kind regards, alex

ANSWER: Greetings Alex,

You ask a good question. Things to measure are changes in area, volume, weight, color against time.

Here are some suggestions:

  1. Measure the amount of time it takes to first visually detect mold with or without a lens.
  2. Measure surface area over unit time: Measure the surface area of growth daily, then perhaps 3 times a day and then hourly if necessary as some of these grow pretty fast as they take over the surface.
  3. Measure any change of weight of the bread over time. The mold is decomposing the bread and releasing some of its weight as carbon dioxide into the atmosphere. If you could figure out how to prepare a control piece of bread that has no mold, this would be a good control with which to compare (perhaps put fungicide on the bread). This would work best if the mold is in a closed container that has some small holes in it to vent gases.
  4. Measure the number of different species that appear and their respective surface areas
  5. Measure any changes in volume of the bread - does it shrink or expand as fungi grow on it?
  6. Measure density of the bread before and after the mold is on it.

Those are a few thoughts that may be useful. Good luck!

Last updated 30 October 2005