Fungi and Fungal Media Preparation

Study Notes

Fungi are eukaryotes.
Fungi are achlorophyllous, meaning they lack chlorophyll.
Fungi are heterotrophic, obtaining nutrients through parasitism, saprophytism, or symbiosis.
Fungal cell walls are composed of cellulose or chitin.
Fungi can be multicellular (e.g., Aspergillus) or unicellular (e.g., Saccharomyces).
The fungal body is a network of microscopic filaments called hyphae.
The mycelium is a root-like structure of branching, thread-like hyphae.
Fungi are found in air, soil, and water.
Fungi are commonly used in food production and can cause spoilage.

Media is a nutrient-rich material that provides food for microbes.
Potato Dextrose Agar (PDA) and Sabouraud Dextrose Agar (SDA) are common fungal media examples.
A liter of PDA solution comprises: 200g of potato, 20g of agar, 20g of dextrose, and 0.1g/L of Choramphenicol.
Sterilization occurs using an autoclave at 121°C at >15 psi for 15 minutes.
Isolation and inoculation of fungi are completed in laminar air flow.

Kingdom: Fungi (Mycota)
Division: Eumycotae
Subdivisions: Mastigomycotinae, Zygomycotinae, Ascomycotinae, Basidiomycotinae, Deuteromycotinae

Penicillium species can cause fruit rot (e.g., oranges) and spoil refrigerated foods (e.g., cheese).
Aspergillus species are common in grains, nuts, oil seeds, and cured meats.
Aspergillus species contaminate starchy foods such as bread and potatoes.

Microbial food analysis is conducted to meet certain standards, estimate shelf-life, determine food quality, and for public health purposes.

Good/Helpful: some microbes naturally exist in foods or are added to ferment them for unique flavors and textures (e.g., cheese, yogurt, bread, sauerkraut, pickles).
Bad/Spoilage: microbes change foods, causing spoilage, and affecting food quality but not necessarily food safety. Examples include discolored, mushy or fuzzy vegetables, sour milk, or slimy, putrid meat.
Ugly/Disease-causing: microbes capable of causing illnesses ranging from mild to life-threatening. Examples include foods contaminated with Salmonella or E. coli. Symptoms include nausea, vomiting, and diarrhea.

Intrinsic factors: inherent characteristics of the food itself including pH, moisture content, oxidation-reduction potential, nutrient content, antimicrobial constituents and biological structures.
Extrinsic factors: environmental factors surrounding the food (temperature, relative humidity, gases in the environment, and presence of other microorganisms).

Traditional methods: plate counts, membrane filtration, most probable number, direct microscopic count, dye reduction tests, indicator tests
Rapid methods: direct epifluorescent filter technique (DEFT), electrical impedance, enzyme-linked immunosorbent assay (ELISA)

Removal of microorganisms: filtration
Low temperature: refrigeration retarding, but not stopping microbial growth. Microorganism growth can occur below 10°C.
Pasteurization: kills pathogens and reduces spoilage organisms. Procedures vary in heat and duration.
Water availability: dehydration, freeze-drying (reduces free water, eliminating bacterial growth).

Prevent foodborne disease through destroying or removing pathogens, preventing multiplication of pathogens, controlling time/temperature, and promoting proper food handling, employee hygiene, equipment sanitation, packaging integrity, and consumer education.