An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 37

Molds are filamentous fungi and have a mycelial structure. The mycelium is a highly
branched system of tubes that contains mobile cytoplasm with many nuclei. Long, thin fila-
ments on the mycelium are called hyphae. Certain branches of mycelium may grow in the air,
and asexual spores called conidia are formed on these aerial branches. Conidia are nearly
spherical in structure and are often pigmented. Some molds reproduce by sexual means
and form sexual spores. These spores provide resistance against heat, freezing, drying,
and some chemical agents. Both sexual and asexual spores of molds can germinate and
form new hyphae. Figure 2.10 describes the structure and asexual reproduction of molds.
Molds usually form long, highly branched cells and easily grow on moist, solid nutrient
surfaces. The typical size of a filamentous mold is 5 e 20 m m. When grown in submerged
culture, molds often form cell aggregates and pellets. The typical size of a mold pellet varies
between 50 m m and 1 mm, depending on the type of mold and growth conditions. Pellet
formation can cause some nutrient-transfer (mainly oxygen) problems inside the pellet.
However, pellet formation reduces broth viscosity, which can improve bulk oxygen transfer.
On the basis of their mode of sexual reproduction, fungi are grouped into four classes.

(1) The phycomycetes are algalike fungi; however, they do not possess chlorophyll and cannot
photosynthesize. Aquatic and terrestrial molds belong to this category.
(2) The ascomycetes form sexual spores called ascospores, which are contained within a sac
(a capsule structure). Some molds of the genera Neurospora and Aspergillus and yeasts
belong to this category.
(3) The basidiomycetes reproduce by basidiospores, which are extended from the stalks of
specialized cells called the basidia. Mushrooms are basidiomycetes.
(4) The deuteromycetes (Fungi imperfecti) cannot reproduce by sexual means. Only asexually
reproducing molds belong to this category. Some pathogenic fungi, such as Trichophyton,
which causes athlete's foot, belong to the deuteromycetes.

Conidio
(spores)

Germination

Conidiophore
Aerial hyphae

Subsurface hyphae

Hyphae

FIGURE 2.10 Structure and asexual reproduction of molds.