2) Acetic Acid Bacteria Flashcards
what are general functions of fermenting food?
- improves organoleptic profile of food material
- extends shelf life
- improves availability of nutrients
- develops some health promoting properties
what are 3 ways to start a fermentation rxn?
- spontaneous fermentation
- inoculated fermentation
- back sloping
describe spontaneous fermentation
whats an example?
based on action of wild microorganisms already naturally on the food or in the area
can encourage growth of certain microbes by creating optimal condition
- eg. sauerkraut
describe inoculated fermentation
example?
based on introducing a single (pure culture) or several (mixed culture) into an environment that favors their proliferation
advantage: stabilizes fermentations sensitive to contamination
eg. wine
describe back slopping
example?
based on adding the product of a previously successful fermentation rxn to start a new rxn
eg. kombucha
what does AAB fermentations produce?
acetic acid products
such as kombucha or apple cider vinegar
what does AAB require?
alcohol and O2
what is the LAB fermentation rxn?
bacteria feeding on sugar –> produces lactic acid
what is the most common type of fermentation?
LAB
differentiate the use of starter cultures for vegetable vs dairy fermentations
veg: don’t need a starter culture, just good conditions for growth
dairy: requires starter cultures b/c it’s hard for LAB to overcome other bacteria in the mix
what are the 5 types of fermentation?
- AAB
- LAB
- alcoholic
- amylolytic
- proteolytic
describe amylolytic fermentations
- molds simplifies starches into simple sugars
2. simple sugars are used in secondary fermentation to produce alcohol
what is an example of amylolytic fermentations
koji and nuruk
what does amylolytic fermentations use?
molds (Aspergillus oryzae)
describe proteolytic fermentation
what are examples?
- uses molds on high protein foods
- uses a starter culture
- tempeh (fermented soybean) and cheese (penicillium growth on rinds)
what are other names for acetic acid?
AcOH
ethanolic acid
vinegar
CH3CO2H
what do acidic properties of AcOH come from?
from H+ being able to separate from the rest of the molecule by ionization
what are ways that AAB can be produced?
synthetically
bacterial fermentation
is AAB gram pos or neg?
Gram-neg
what is the most commonly used AAB?
acetobacter sp
are AAB anaerobic or aerobic?
except what type of AAB?
aerobic
except acetogenic bacteria
what are acetogenic bacteria?
what do they produce?
what do they require?
type of AAB that produces AcOH anaerobically
requires one C compound and H
what can acetogenic bacteria NOT tolerate?
accumulating acetic acid
why are acetogenic bacteria slower than oxidative AAB?
b/c they can’t tolerate accumulating AcOH, in comparison to oxidative AAB which are very tolerant
what kind of environment does AAB live in?
high conc of sugar, alcohols or sugar alcohols
what are sugar alcohols?
organic compounds derived from sugars and comprise a class of polyols
what are sugar alcohols used in the food industry as?
thickeners and sweeteners
common for diabetics
what are common sugar alcohols?
glycerol
xylitol
mannitol
sorbitol
compare sucrose to sugar alcohols
- sugar alcohols affect blood sugar levels, but less than sucrose
- sugar alcohols have fewer calories
what 2 enzymes are imp in converting EtOH to AcOH in the periplasm?
ADH: alcohol dehydrogenase
ALDH: aldehyde dehydrogenase
in the periplasm, what AAB rxn occurs?
- AAB oxidizes EtOH to acetaldehyde (by ADH; coupled with UQ to UQH2)
- ALDH converts Acetaldehyde to ACOH
what is overoxidation
- occurs when strains oxidizes AcOH
- causes H to be much lower (3.5) than the pH optimum for oxidation of organic acids (6.0)
- causes production of Co2 and off flavours
what is the pH optimum for oxidation of organic acids?
what is the pH that occurs with overoxidation?
- 0
3. 5
what are the 2 phases of growth in overoxidation?
- first log phase: due to oxidation of ethanol to acetic acid
- second overoxidation phase: uses acetic acid as energy via TCA. Results in decreasing AcOH conc outside the cell
what are two methods of AAB fermentations?
- surface stations processes
2. submerged processes
describe surface stations process
- one method of acetic acid fermentation
- uses acetobacter
- final conc of acid can reach 8-9%
- AAB forms thin film on surface of growth media (containing the bacteria/mother of vinegar), which becomes thicker and more gelatinous with time.
describe submerged process
- method of AAB ferm
- uses Komagateibacter strains
- final conc of acid: 15-20%
- involves rapid mixing with forced aeration in a bioreactor
describe the toxic effects of AcOH
AcOH is a weak lipophilic acid that can diffuse through the cytoplasmic membrane and has a toxic effect on bacteria
toxic effect is b/c of dissociation of proton caused by higher pH of the cytoplasm –> internal pH in cytoplasm decreases –> uncoupling of oxidative phosphorylation –> disruption of proton gradient –> cell poisoning
what are 4 primary mechanisms responsible for AAB resistance to acetic acid
1. prevention of acetic acid influx into cells (CPS, EPS and LPS)
2. acetic acid assimilation (overoxidation)
3. acetic acid efflux (efflux pumps)
4. protection of cytoplasmic proteins against denaturing (general stress proteins)
what are molecular chaperones used for?
to deal with stress due to ethanol and acetic acid
what are 2 categories of molecular chaperones?
- those involved in the restoration of denatured/aggregate proteins
- those involved in the degradation of denatured/aggregate proteins
what are 3 functions of molecular chaperones?
- prevent denatured proteins from aggregating
- re-solubilize aggregated proteins
- facilitate degradation of very damaged proteins
what is GroES-GroEL? what is its function?
chaperone that is representative of heat-shock proteins found in all bacteria
describe the mechanism of GroES-GroEL
GroEL forms a heptamer ring structure; GroES forms a heptamer lid-like structure. Together they create the “central cavity”, where denatured proteins are separated from the outside, to allow them to safely re-fold into proper conformations
when is GroES-GroEL expressed?
when AAB is suddenly exposed to ethanol or acetic acid.
what is DnaK-DnaJ-GrpE?
what functions does it have?
chaperone
folding nascent proteins
refolding denatured proteins
degrading very denatured proteins
what is the expression of DnaK-DnaJ-GrpE linked to?
ethanol, not acetic acid
describe the mechanism of DnaK-DnaJ-GrpE
- DnaJ binds to improperly folded protein and targets them to DnaK
- DnaK places protein into a cleft (in it’s open conformation)
- DnaJ binds to DnaK and promotes ATP hydrolysis
- DnaK closes
- substrate protein is tightly bound
- while being held by DnaK, the protein can’t aggregate with other proteins (can be fatal for cell)
- GrpE releases proteins from DnaK by removing ADP from it and providing an ATP
what is Komagateibacter spp. used in?
submerged industrial vinegar production
how does Komagateibacter spp. avoid build up of AcOH inside the cell?
- efflux pumps: eliminates excess dissociated acetate and protons
- synthesis of EPS (extracellular polymeric substance) is used to serve as a physical barrier to acetic acid in submerged fermentations
- composition of lipid membrane is shifted to decrease the area available for passive transport of lipophilic molecule, like acetic acid.
what are the 2 main steps in producing traditionally produced vinegars?
- production of ethanol from a CHO by a yeast (if starches are used, the starch is broken down by a fungi first)
- oxidation of ethanol to acetic acid by Acetobacter sp., Gluconobacter sp., or gluconacetobacter sp.
what is the function of cellulose produced by AAB?
formation of biofilms that allow the retention of bacterial cells on the culture surface so they have access to O2 for oxidative fermentation
what are examples of fermented foods involving AAB?
nata de coco: liquid coconut water surrounded by bacterial cellulose
kombucha
vit C (the Reichstein process was used to generate vitamin C, which includes one microbial step where Sorbitol is converted to sorbose by Komagataeibacter xylinus)
in wine production, yeast is used as a ___ and sulfur dioxide is used as an _____
starter culture; antibacterial
what does wine spoiled by AAB taste like?
sour, buttery, nutty or like bruised apples
