Some species of fungi and yeasts are useful in the processing of certain foods, but they are also responsible for the spoilage of many others. When there is little bacterial competition in food, fungi can develop favorably.
Some of the most common fungi that occur in food belong to the genus Penicillium, which is characterized as a fast-growing filamentous fungus, appearing white at first and later turning blue-green, green, or olive-gray. Depending on the species, its texture can be flat, filamentous, velvety, or cottony; additionally, its spores are resistant to desiccation (moisture loss).
The optimum temperature for Penicillium growth is between 20 and 30 °C, although some species of this fungus can grow at intervals of 5 to 37 °C, which can cause alterations in foods that are stored under refrigeration. These fungi can live in pH ranges from 3.5 to 10, but they grow best at pH 4.
Another fungus of great importance in food is Rhizopus stolonifer, which is characterized by having a visible cottony white mycelium and abundant black sporulation that gives the appearance of blackening to food; it is the causal agent of soft rot of fruits and vegetables and causes significant economic losses, as it is capable of causing the total rotting of fruit in just four days. It is found all over nature and can live off of decaying organic matter (saprophytically) in soil and organic waste, which it can use to get into plant tissues.
R. stolonifer spores can be maintained for long periods without water, withstand high temperatures, and be spread by wind. When they encounter favorable conditions, such as a temperature of 20 °C and high humidity, they germinate on damaged plant tissues, rapidly generating fruit rot.
How do fungi grow on food?
Fungi reproduce using spores, which are transported by water and wind; these microscopic particles are spread everywhere and, due to their size, are not perceptible to the naked eye. Spores can get on food when it comes in contact with spore-filled air, after it has been cooked and packaged, in packaging that hasn't been properly cleaned, or when food is stored in a place with too much humidity and heat.
When we leave food on a surface, the spores land on it, and if the right temperature and humidity conditions exist, they will germinate, giving rise to the fungus. Fungi obtain the nutrients for their growth from the components of food.
Bread, fruits, and vegetables are the most common foods contaminated by fungi. Warm environments, such as those generated when space heaters are used during the winter, favor the growth of fungi in food.
Is it possible to eliminate the part of the food that has fungi and consume the rest?
It is not advisable to consume foods such as bread with fungi, even if the damaged part is removed because it is not safe to eat. It's important to remember that mushrooms make mycotoxins, which are harmful to both people and animals.
According to the United States Department of Agriculture, some fungi such as Alternaria, Aspergillus, Botrytis, Cladosporium, Fusarium, Geotrichum, Mucor, Penicillium, and Rhizopus, among others, are capable of producing mycotoxins, and some of them, such as aflatoxins, are carcinogenic.
Mycotoxins can be impregnated in bread or other foods such as grains and cause intoxication. Therefore, it is safer to discard bread or other foods such as grains or seeds that show fungal growth, as this is an indicator that they are unfit for consumption.
Are all fungi harmful to food?
Not all fungi are harmful to food. Some are beneficial, such as some species of Penicillium used in the production and maturation of certain cheeses, to which they provide flavor and aroma, such as Roquefort, Brie, or Camembert cheese. Different species of Aspergillus are also used to ferment soy sauce and produce citric acid or gluconic acid, as well as Saccharomyces cerevisiae, which is used to make bread, beer, and wine.
What precautions should be taken with foods that have fungus?
In the case of bread, do not cut the piece where the fungal growth is visible and eat the rest. If the slice of a whole loaf of bread has a slight fungal growth, likely, the other slices of bread are also contaminated, even if it is not visible at the time. Porous foods such as bread, biscuits, and other baked goods allow easy penetration of the fungus and spore dissemination, so it is best to discard the food.
It is important to know that food with fungal growth should not be inhaled, as the spores could be inhaled and cause respiratory problems.
To dispose of food contaminated by fungi, it is recommended to place it in a plastic bag and close it tightly. It should then be placed in the garbage container, out of the reach of animals.
If the contaminated food was stored in the refrigerator, it is advisable to clean the refrigerator surfaces, trays, rags, and utensils that have been in contact with the food contaminated with fungi with a 0.5% (5 g/L) sodium hypochlorite solution.
Finally, it is important to remember to store food in appropriate containers and places with good ventilation, adequate temperature, and humidity. Also, an important measure to be taken into account is to avoid buying food in quantities greater than what is going to be consumed. This prevents food from spoiling and wasting.
Authors: María del Carmen Bermúdez Almada and Angélica Espinosa Plascencia, researchers of the Biological Analysis Laboratory of the Food Science Coordination of CIAD.