How many ‘chemicals’ are there in a fresh mushroom? These simple experiments reveal the hidden chemistry within natural foods.
Everything around us and in our own bodies consists of chemical substances, which react with each other to keep us alive. However, many students do not see chemistry as part of their everyday lives; in fact, chemistry is often seen as the opposite of ‘nature’ – and even as something dangerous to be avoided. For example, many foods are advertised as ‘free of chemicals’, as if the substances they contain consist not of molecules and compounds, but of something else entirely.
This of course is an important misunderstanding, which is quite likely to lead to mistakes and unscientific decisions. In these ‘natural experiments’ articles (see also Höfer, 2017), we use chemical methods to analyse natural, raw foods. Whatever their results, carrying out these experiments confronts students with the indisputable fact that healthy, natural foods are also substances within the realm of chemistry.
Food is a major part of our daily lives, so it makes sense to use foods as a way of connecting chemistry with everyday life. Mushrooms are an interesting example, because they contain a variety of substances that can be quite easily identified. This may surprise students, who are likely to begin the experiments with few expectations about the composition of mushrooms. These substances include carbohydrates, fats and proteins, as well as vitamins and minerals as more minor components, as students can discover for themselves.
In these experiments, we apply standard laboratory chemistry tests to edible mushrooms. The experiments are suitable for chemistry classes in secondary school. They can be carried out by the students themselves in around 1 hour including discussion. Some additional time is needed for teachers to prepare the materials needed in experiments 2 and 3.
Mushrooms are fungi, which are not plants or animals, but form their own taxonomic kingdom. They are eukaryotic organisms, which distinguishes them from bacteria. Like plants, mushrooms absorb organic nutrients from surrounding substrates by osmosis, but their structure and way of reproducing is quite different from plants. And like animals, mushrooms are heterotrophic organisms: they need to take in food and cannot make it themselves. Fungal cells form a net of fibres called a mycelium, hidden in the soil. This is branched like a root system and is responsible for the nutrient uptake. Fungi also have a fruiting body, which grows above ground and is the (sometimes edible) part we call a mushroom (see figure 1).
Nutritionally, mushrooms are a valuable part of our diet. Among other nutrients, they contain sodium, chloride and potassium ions, which are needed to maintain water balance; plus phosphate ions for the stability of bones and teeth and the formation of red blood cells; and vitamin C, which acts as a protective agent for cells and helps the body to incorporate iron.
In the following experiments, we used mushrooms from commercially grown Agaricus bisporus fungi as the sample materials, in either fresh or dried form. In culinary terms, these are known as champignons, button mushrooms or (when mature and brown) portobello mushrooms.
For all the experiments, students should wear safety glasses and follow the usual safety rules for chemistry classes, as some of the reagents are harmful or corrosive. Please also see the general safety note on the Science in School website.
Allow approximately 5 minutes for the Biuret test, and 10 minutes for the ninhydrin test (see procedure section below).
Proteins can be detected using both the Biuret and the ninhydrin tests. These procedures are described here separately.
If protein is present, the colour will change from light blue (the colour of copper(II) sulfate solution) to dark blue or violet (see figure 2, middle) This colour change is due to the formation of a copper-protein complex.
If protein is present, a deep violet compound will be formed (see figure 2, right) . The deep violet compound is called Ruhemann’s purple. It is produced when ninhydrin reacts with the amino acids found in proteins.
Duration: approximately 15 minutes. Teachers need to allow 45 minutes beforehand to prepare the iron(III) chloride filter paper. To do this, soak the filter paper in 1% w/w iron(III) chloride solution and then leave until well dried (about 30 minutes).
Each student (or group) will need:
The potassium hexacyanoferrate(III) solution makes the paper turn light blue, as the soluble form of Prussian blue (or Turnbull's blue) is produced. If vitamin C is present, the Fe(III) ions will be reduced to Fe(II) ions, forming a darker blue compound (see figure 3, right).
Duration: approximately 10 minutes. Prepare the dried mushrooms in advance by slicing the mushrooms and drying them overnight at 105 °C in a drying cabinet.
Potassium and sodium can be detected via their characteristic flame colours. Using cobalt glass to filter out the yellow sodium flame makes the pale lilac potassium flame more visible.
Duration: approximately 45 minutes (15 minutes for the experiment plus 30 minutes for the ash preparation). Teachers will also need 15 minutes to pre-prepare the ammonium heptamolybdate solution.
For the ammonium heptamolybdate ((NH4)6Mo7O24) solution:
For the experiment, each student (or group) will need:
Prior to the lesson, teachers should prepare a solution of ammonium heptamolybdate, ((NH4)6Mo7O24) solution: place 10 g of ammonium molybdate tetrahydrate and 20 g of ammonium nitrate into the volumetric flask. Add 7 ml of concentrated ammonia solution (using a pipette), and fill up the flask to 100 ml with distilled water, dissolving completely.
The rest of the procedure is suitable for students.
Duration: approximately 5 minutes
A white precipitate indicates the presence of chloride ions (see figure 6). The silver chloride (AgCl) precipitate is finely dispersed, so the liquid has a cloudy white appearance.
Using these experiments, students themselves can identify some important compounds in mushrooms – which are also nutrients. This can provide a basis for discussing aspects of chemistry that have an impact on daily life. For example:
The following information may be useful for teachers to use in discussions following the experiments.
Starch is not present in cultivated mushrooms, and glucose is almost completely absent. Fungi contain mainly chitin and cellulose as structural components of the cell walls. They also contain trehalose (a sugar) and mannitol (a sugar alcohol).
Lipids are found in very fine droplets within the cytoplasm of fungi, and in the fungal cell wall (as lipid bilayers). Mushrooms contain 0.3 g fat per 100 g of fresh mushrooms, which is a relatively low proportion.
Mushrooms are rich in minerals – especially potassium, with a surprisingly high level of 390 mg per 100 g of fresh mushrooms. Phosphorus is also present (about 60 mg per 100 g), mostly as phosphate. The sodium content of around 5 mg per 100 g fresh mushrooms, however, is low, as are the calcium and iron contents. Fresh mushrooms contain about 93% water, although the water content depends on the age of the fruiting body, with young fruiting bodies generally containing less water.
Mushrooms contain significant amounts of vitamin C (about 2.1 mg per 100 g of fresh mushrooms), plus some B vitamins (including B1, B2, B6 and the amide of niacin, B3). Mushrooms are also rich in nicotinamide (5.2 mg per 100 g of mushrooms).