Yeast/Mold Flashcards
The main species of yeast implicated in food science:
S. cerevisae
Where is S. cerevisiae normally found in nature?
surface of fruits (high sugar)
cell morphology of S. cerevisiae:
budding yeast
large size (5-10 microns)
round shape
What is the main metabolic process of yeast?
convert carbs into ethanol and byproducts
The metabolism of yeast is described by: ____ equation. Is it accurate?
Gay-Lussac equation: 180g sugar -> 92g ethanol, 82g CO2
real yield usually about 90-95%
What are the two types of proliferating yeast cells?
haploid or diploid
What happens to yeast when nutrients are depleted?
cells enter stationary phase; cease growing
How do stationary yeast cells differ from proliferating ones? (3)
- appear round and bright
- higher levels of storage carbs (trehalose and glycogen)
- increased resistance to env. stresses
describe the sexual and asexual life cycles of yeast:
asexual: budding (haploid or diploid)
sexual: diploid produce ascus (4 haploid) -> haploids mate -> new diploid
The 2 types of haploid yeast cells:
How do they differ?
alpha; a produce pheromone (alpha or a type) contain receptor for OPPOSITE type
What causes a haploid yeast to stop proliferating? (2)
- nutrient depletion (die or become stationary)
2. surface receptor stimulated -> mating process
What happens once a surface receptor on a haploid is stimulated?
stop proliferating in G1 phase -> start growing towards each other -> cell contact -> fusion -> forms diploid
The shape 2 haploids form during the mating process is known as a ____
schmoo
____ cells that are starved of ____ will begin the first step of sexual reproduction, ____.
diploid
NITROGEN
sporulation
True/False: stationary yeast are the most resistant form of yeast
false; ASCUS produced during sporulation is even more resistant to stresses
When does an ascus germinate?
When nutrients are present again
What is the “chitin ring” in yeast cell division?
form between mother and daughter cell at beginning of budding
How are cell materials transferred from mother to daughter cell? Why is this significant?
vesicles
limiting factor for rate of growth (growth of cell wall can only occur with necessary materials)
the 4 stages of yeast asexual proliferation:
G1, S, G2, M
What is the resulting polarity of cells in haploid division? What about diploid division?
haploid: faces each other
diploid: faces away, or in same direction
True/False: in the stationary phase of yeast cells, metabolic activity stops.
False: stationary cells use ethanol formed previously -> TCA cycle, glyoxalate cycle
Growing yeast cells ferment sugar into ____ via ____.
ethanol; glycolysis
Yeast is extremely notable for its ____ metabolism systems
carbohydrate
___ and ___ can easily diffuse across the yeast cell membrane. Other substances require ______
ethanol, glycerol
special transport systems
How are disaccharides processed by yeast?
secrete INVERTASE or GALACTOSIDASE -> monosaccharide -> taken up by specific transport system
What disaccharide is taken up by yeast? What occurs?
actively transported across membrane
broken down by MALTASE
2 glucose -> glycolysis
What disaccharide may be taken up by yeast or hydrolyzed first?
sucrose
broken down by periplasmic or cytoplasmic INVERTASE
fructose + glucose -> taken up by hexose transporter
-> glycolysis
The yeast cell is surrounded by 3 outer layers:
plasma membrane
periplasmic space
cell wall
The cell wall confers ___ to the yeast. What compound is responsible?
mechanical strength
chitin
What is the composition of the yeast cell wall?
50% glucans
40% mannoproteins
10% chitin
What microbes are necessary to degrade starch? How does it accomplish this?
Mold: Aspergillus oryzae
secrete various amylases
What products utilize mold for fermentation?
soy sauce, meso (soybeans)
sake (rice)
A oryzae is a ___ fungi.
filamentous
Describe the process of reproduction in A oryzae:
produce CONIDIUM (spore) at tip of specialized hypha
conidium swells -> germ tube
forms HYPHAE, partitioned by septa
What happens if one cell on the hyphae is damaged?
sensed by other hyphae through communication PORES
pores are “plugged” by WORONIN BODIES (cut off communication, nutrient exchange)
what is the organelle in A oryzae involved in degrading starch? where is it located, and what is its role?
spitzenkorper
tip of hyphae
accumulate/secrete vesicles with secretory enzymes
What makes A oryzae so efficient at degrading starch? (2)
- . large variety of amylases/debranching enzymes
2. re-uses enzymes (uptake into vesicles after release)
What is the specialized hypha for sporulation?
Conidiophore
What is the process of sporulation?
nutrients passed up from hyphae to forming conidia
germinate: conidia swell
vacuoles “push” cytoplasm into forming germ tube
What are other molds used in food production?
Geotrichum candidum: moist cheese, prevent unwanted mold (diff flavors available)
Penicillium candidum (fuzzy white rind on cheese, diff flavors available)
Penicillium roqueforti: blue cheese (diff colors available)
How might AAB, LAB, molds, and yeasts coexist symbiotically within an environment?
AAB and LAB poor at biosynthesis - feed off of substrates made by mold and yeast
Why would AAB, LAB, mold, and yeast grow in different areas of a fermented product?
mold and AAB on surface - AEROBIC
yeast and LAB internal - ANAEROBIC
what are some examples of symbiotic fermented products?
sake
kombucha
fukuyama pot vinegar
kefir
How is sake produced and with what microbes?
Koji (mold) -> break down rice to sugars
LAB -> ferment sugar, kill wild yeast
brewing yeast -> second ferment -> produce sake
Describe the process of fukuyama pot vinegar:
similar process to sake; pot left exposed to air
eventually levels of ethanol rise, mold taken over by AAB -> convert ethanol to acetic acid -> becomes vinegar, no longer alcoholic
what microbes are found in kefir grains?
LAB, yeast, AAB, sometimes mold, possibly Bifidobacter
(L. kefiranofaciens, Lb spp, Lactococcus, Streptococcus, Leuconostoc
S. cerevisiae)
What microbe is mainly responsible for forming the kefir grain? What microbe improves the quality of the kefir grain?
L. kefiranofaciens (forms EPS: KEFIRAN)
yeast will improve kefiran quality
