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Food Microbio > AAB > Flashcards

AAB Flashcards

1
Q

aim of food fermentation

A

extend shelf-life

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2
Q

fermentation of barley

A

beer

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3
Q

fermentation of mead

A

honey wine

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4
Q

fermentation of milk

A

cheese

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5
Q

fermentation of cabbage

A

sauerkraut

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6
Q

Benefits of fermented foods (4)

A
  • improves organoleptic profile (taste)
  • extends shelf-life
  • improves availability of nutrients (vitamins) [bacteria break down nutrients]
  • develops health-promoting properties (probiotics)
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7
Q

what are 3 ways to start fermentation reactions?

A
  • spontaneous fermentation
  • inoculated fermentation
  • back slopping
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8
Q

explain spontaneous fermentation + example

A

based on the action of wild MO that are already naturally on the food or in the area => you encourage the growth of certain microbes by creating optimal conditions for them (selection for the organism: high nutrient, high salt -> anaerobic conditions => selects for LAB)
e.g. sauerkraut

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9
Q

explain inoculated fermentation + example

A

based on introducing a single pure culture or several (mixed culture) into an environment that is favorable to their proliferation. The advantage is that it stabilizes fermentations which are sensitive to contamination
e.g. wine (yeast) or yoghurt (specie strain of LAB)

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10
Q

explain back slopping + example

A

based on adding the product of a previously successful fermentation reaction to start a new reaction
e.g. kombucha/sour dough bread

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11
Q

name 5 different types of fermentation

A
  • acetic
  • lactic
  • alcoholic
  • amylolytic
  • proteolytic
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12
Q

Info on acetic fermentation

A
  • acetic acid bacteria
  • Produces acetic acid products such as kombucha or apple cider vinegar
  • Alcohol and oxygen must both be present (basically alcohol is being oxidized into acetic acid)
  • If acetic acid bacteria find their way into a container that is no-longer airtight they will convert wine, beer, cider or any other alcoholic product into vinegar (can be a spoilage reaction)
  • In Kombucha (a symbiotic fermentation) the yeasts and bacteria work together to make alcohol (yeast) and convert it to vinegar (bacteria). This is why kombucha has a low alcohol content and a vinegary flavor.
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13
Q

info on lactic fermentation

A
  • lactic acid bacteria
  • Bacteria feed on sugars (lactose or other in vegetable) and produce lactic acid which quickly acidifies the environment
  • Lactic acid bacteria are robust and are naturally present in the environment.
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14
Q

What is LAB fermentation used for?

A

used for vegetables [spontaneous fermentation] (sauerkraut, kimchi, pickles, capers, miso), meat (dry sausage), and milk [inoculated fermentation] (yogurt, kefir, cheese)

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15
Q

info on alcoholic fermentation

A
  • Uses pure yeast cultures (or wild yeast but not consistent flavour)
  • Yeast convert sugar into alcohol and produce beer, wine cider, sake, whiskey, or bread
  • Alcoholic fermentation is an effective means of preservation since alcohol has a toxic effect on many other microorganisms
  • Alcoholic fermentations must be carried out in an anaerobic environment, therefore airlocks are required
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16
Q

what environment is required for alcoholic fermentation?

A

anaerobic fermentation

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17
Q

info on amylolytic fermentation

A
  • Uses molds (Aspergillus oryzae)
  • break down of Starch: a polymeric carbohydrate consisting of a large number of glucose units joined by glycosidic bonds
  • Amylolytic fermentation simplifies these complex carbohydrates into more simple sugars, generally before they are used in a secondary fermentation to produce alcohol
  • This is not a common fermentation reaction in North America, but it is popular in Asia where it is used to make koji and nuruk
  • Amylolytic fermentation can transform cooked rice into a sweet syrup in approximately a day and then the syrup is used in an alcoholic fermentation to make sake
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18
Q

what organism is used in proteolytic fermentation

A

Mainly uses molds on high protein foods

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19
Q

info on proteolytic fermentation

A
  • Requires a starter culture
  • Tempeh is a good example where Rhizopus oryzae is used to ferment soybeans into a kind of pancake
  • Penicillium grows on the rinds of soft cheeses and digests the proteins to make a runnier cheese with very complex flavors
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20
Q

what organism is used in amylolytic fermentation

A

Uses molds (Aspergillus oryzae)

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21
Q

common name for acetic acid

A

vinegar

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22
Q

what can also be used other than acetic acid

A

ethanolic acid

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23
Q

water free acetic acid (laboratory grade)

A

glacial acetic acid

24
Q

which is more bactericidal: weak acid or strong acid, and why?

A

weak acid, in ionized state and diffuses in cell. once in the cytoplasm where pH is neutral -> dissociation

25
Q

according to food purity laws, vinegar has to be of …

how are traditional vinegars produced?

A
  • biological origin (by bacterial fermentation)
  • via a two-step process the first is production of ethanol from a carbohydrate by a yeast (if starches (i.e. rice) are used, the starch is broken down by a fungi first), the second step is oxidation of ethanol to acetic acid using Acetobacter sp., Gluconobacter sp., or Gluconacetobacter sp.
26
Q

Acetic acid bacteria are … bacteria. what environment do they require for fermentation?

A

gram-negative

aerobic

27
Q

how do acetic acid bacteria ferment?

A

they carry out oxidative fermentation to produce acetic acid from ethanol

28
Q

what is acetogenesis?

A

anaerobic fermentation creating acetic acid, but does not involve AAB

29
Q

which AAB makes vinegar?

A

ACETOBACTER spp.

30
Q

common sources of ethanol

A

wine,
apple cider,
fermented grain, malt, rice or potato mashes

31
Q

how do industrial vinegar making plants accelerate this process

A

improving oxygen supply to the bacteria, stirring

32
Q

which bacteria is used to convert sugar to AA without ethanol intermediate? under which conditions do they ferment?

A

anaerobic bacteria including Clostridium and Acetobacterium

- anaerobic environment

33
Q

how do acetogenic bacteria produce acetic acid anaerobically? how fast is this process?

A

they produce acetic acid from one carbon compounds including methanol or carbon dioxide, and hydrogen
-> slow process because they cannot tolerate te accumulating acetic acid

34
Q

how tolerant are Acetobacter sp. to acetic acid

A

VERY tolerant

35
Q

4 genera of AAB

A
  • acetobacter
  • gluconobacter
  • gluconoacetobacter
  • komagataeibacter
36
Q

In which environment to AAB live in?

A

sugar-rich niches where they utilize either sugar or sugar alcohol for their energy sources and produce the corresponding sugar acids

37
Q

which sugars AAB are able to use?

A
  • Gluconobacter can use several sugars or sugar alcohols such as D-glucose, D-sorbitol, and glycerol in addition to ethanol
  • Acetobacter and Komagataeibacter are only able to use ethanol
38
Q

AAB are … …

A

obligate aerobes

39
Q

oxidative fermentation

A

AAB use oxygen to oxidize ethanol, sugars, and sugar alcohols to produce the corresponding sugar acids.
At the periplasmic side of the cytoplasmic membrane AAB partially oxidize (don’t produce CO2) EtOH by two successive catalytic reactions:
Alcohol dehydrogenase (ADH) takes electrons from the ethanol (or other sugar alcohol) and then transfers them to the terminal oxidase via ubiquinone.
The oxidized alcohol become acetylaldehyde.
Aldehyde dehydrogenase (ALDH) transfers electrons via ubiquinone (UQ) to oxygen as a final electron acceptor and the acetylaldehyde becomes acetic acid.
AcOH accumulates in the media
Membrane bound ADH and ALDH are linked to the respiratory chain which transfers electrons via ubiquinone (UQ) and terminal ubiquinol oxidase to oxygen as the final electron acceptor (generating water as a biproduct)

40
Q

When strains oxidize acetic acid this is called ….. via which cycle?

A

overoxidation

via the TCA

41
Q

AAB can have a diauxic growth curve when the initial ethanol concentration is below 1%. what happens in this case?

A

The first log phase is due to oxidation of ethanol to acetic acid
The bacteria then transition to a second growth phase that relies on using the acetic acid as energy (overoxidation) via the TCA cycle
This results in decreasing acetic acid concentrations outside of the cell

42
Q

which AAB strains can oxidize acetic acid via the TCA cycle?

What can they also oxidize which leads to a butter-like flavor and smell to spoiled wine.

A
  • Acetobacter and Gluconacetobacter

- lactic acid oxidized to acetoin

43
Q

what are the two ways of carrying out acetic acid fermentations

A
  1. Surface Static Processes

2. Submerged Processes

44
Q

which AAB strain do Surface Static Processes use? explain this process.

A

commonly use Acetobacter, and final concentrations can reach 8-9%. In this process AAB form a thin film on the surface of the growth media, which becomes thicker and more gelatinous with time. Bacteria (mother of vinegar)are embedded in this layer

45
Q

which AAB strain do Submerged Processes use?

A

commonly use Komagataeibacter strains and may have acidities up to 15-20%. This involves rapid mixing with forced aeration in a bioreactor.
These bacteria have efflux-pumps which eliminate excess dissociated acetate and protons from the cytoplasm
Synthesis of EPS has been observed in submerged fermentation. These likely serve as a physical barrier to acetic acid diffusion
The composition of the lipid membrane is shifted to decrease the area available for passive transport of lipophilic molecules like acetic acid. The amount of glycolipids also increased which probably strengthens the hydrophobic barrier.

46
Q

acetic acid is bacter….

why?

A

bactericidal
Acetic acid is a weak lipophilic acid that can readily diffuse though the cytoplasmic membrane of bacteria
Concentrations as low as 0.5% can readily kill some bacteria
The toxic effect is caused by the dissociation of the proton when it encounters the higher pH of the cytoplasm
The release of the proton decreases the internal pH in the cytoplasm and causes the uncoupling of oxidative phosphorylation resulting in disruption of the proton gradient, resulting in poisoning of the cell
Lower pH can also cause protein mis-folding

47
Q

what are the four primary mechanisms responsible for AAB resistance to acetic acid:

A
  1. Prevention of acetic acid influx into the cell (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)
48
Q

what are general stress proteins?

A

Molecular chaperones have been suggested as a way in which AAB deal with stress (ethanol and acetic acid) by:

  1. restoring denatured/aggregated proteins
  2. degrading denatured/aggregated proteins
49
Q

what are the functions of molecular chaperons? (3)

A
  1. Prevent denatured proteins from aggregating
  2. Resolubilize aggregated proteins
  3. Facilitating the degradation of seriously damaged proteins.
50
Q

which systems are representative molecular chaperones in bacterial cells?

A

The GroES-GroEL system and DnaK-DNAJ-GrpE system

51
Q

about GroES-GroEL chaperon

A

chaperone that is representative of heat-shock proteins that are found in all bacteria
This chaperone prevents the aggregation of denatured proteins and provides the space and time required for proteins to properly fold
GroEL forms a heptamer ring structure, and GroES forms heptamer lid-like structure.
Together they make a cage like space, referred to as a central cavity in which denatured proteins are separated from outer conditions so that unfolded proteins can be safely re-folded into proper conformations
When AAB are suddenly exposed to ethanol or acetic acid GroES-GroEL is expressed, and therefore it is considered an important factor which allow AAB to perform acetic acid fermentation

52
Q

about DnaK-DnaJ-GrpE chaperone

A

This chaperone has various functions including folding nascent proteins, refolding denatured proteins, and degrading seriously denatured proteins
DnaJ binds to improperly folded proteins and targets them to DnaK
DnaK places the protein into a cleft, when in it’s open conformation
DnaJ binds to DnaK and promotes the hydrolysis of ATP, which causes the DnaK to close and then the substrate protein is tightly bound
While being held by DnaK the protein is prevented from aggregating with other proteins, which could be fatal for the cell
GrpE can release proteins from DnaK by removing the ADP from it and providing it with an ATP
Expression of this system is linked to ethanol and not acetic acid

53
Q

AAB also produce …

what is its biological function?

A

cellulose
the formation of biofilms that allow the retention of bacterial cells on the culture surface to that they have access to the large amounts of oxygen that they need for oxidative fermentation

54
Q

what are some traditional fermented foods involving AAB?

A
  • nata de coco (composed of bacterial cellulose by Komagataeibacter xylinus at the liquid-air interface of coconut water)
  • kombucha (produced by fermenting tea with a symbiotic culture of acetic acid bacteria and yeast. Generally Gluconacetobacter xylinus is the culture. Since the AAB is converting the alcohol produced by the yeast into acetic acid the alcohol content of the beverage is very low)
55
Q

microbial production of vitamin C uses what AAB and what process?

A

one microbial step (M1) the conversion of Sorbitol to sorbose was preformed by Komagataeibacter xylinus

Food Microbio (11 decks)

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