92  ◾  Fundamental Food Microbiology
Yeast Spores
On the basis of spore-forming ability, yeasts important in food are divided into two groups: Those
that can produce sexual ascospores are designated as Ascomycetes (true yeasts), and those that do
not form spores are called false yeasts. Examples of some yeasts important in food that form ascospores are Saccharomyces, Kluyveromyces, Pichia, and Hansenula. Species in the genera Candida,
Torulopsis, and Rhodotorula do not form spores. Ascospores form by the conjugation of two yeast
cells; in some cases, this can result from the union of the mother cell and a bud (daughter cell)
(Figure 9.1). The number of spores developed in an ascus varies by species. In a suitable environment, each spore develops into a yeast cell.
Bacterial Spores
The ability to form spores is confined to only a few bacterial genera, namely the Grampositive Bacillus, Alicyclobacillus, Clostridium, Sporolactobacillus, and Sporosarcina and the
Sterigmata
Conidia
Conidia
Hypha
Hypha
Hypha
Conidia
Arthrospore
Rhizoid
Sporangiophore
Sporangiospore
Sporangium
(i)
(ii)
(f )
(e)
(d)
(a)
(b)
(c)
Figure 9.1 Schematics of spores of molds and yeasts. Conidiophore with condial head and condia of (a) Aspergillus sp., (b) Penicillium sp., (c) Fusarium sp., (d) arthrospore in Geotrichum sp.,
(e) sporangiophore with sporangium containing sporangiospores of Rhizopus sp., (f) ascospore
formation in yeasts by conjugation of (i) mother–daughter and (ii) two separate cells.