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Food Microbio > LAB > Flashcards

LAB Flashcards

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1
Q

how is lactic acid produced in food fermentation?

A

from the break-down of hexoses such as glucose, mannose, fructose and galactose or disaccharides such as lactose, maltose, sucrose by lactic acid bacteria (LAB).

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2
Q

what gives LA its acidic properties?

A

the separation of hydrogen atoms from the rest of the molecule

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3
Q

what are the two classes of LAB? aka 2 primary hexose fermentation pathways that are used to classify LAB genera

A
  • homofermentative

- heterofermentative

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4
Q

mechanism of homofermentative bacteria + which pathway do they use?

A

produce two moles of lactate from one mole of glucose (+2 ATP)
=> glycolytic pathway

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5
Q

mechanism of heterofermentative bacteria + which pathway do they use?

A

produce one mole of lactate from one mole of glucose, and produce carbon dioxide and acetic acid or ethanol as biproducts (+ 1 ATP)
pentose phosplhoketolase pathway

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6
Q

which medium constituents supply all the necessary nutrients for the growth of lactic bacteria?

A

casein enzymic hydrolysate, papaic digest of soybean meal and yeast extract

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7
Q

which fermentable carbohydrate is present in HHD agar?

A

fructose

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8
Q

what is the pH indicator of HHD agar?

A

bromo cresol green

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9
Q

homofermentative lactic acid bacteria produce lactic acid from …. and is indicated by …. colour formation

A

glucose/fructose

yellow

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10
Q

Homofermentative bacteria cultivated on HHD agar medium form …. colony on agar while heterofermentative bacteria form …. colonies on agar surface

A

bluish-green

white

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11
Q

example of foods using LAB fermentation

A
  • cheese
  • yogurt
  • miso
  • sour dough bread
    => LAB are involved in dairy, vegetable, and cereal fermentations
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12
Q

Lactic Acid Bacteria (LAB) are a group of ….. bacteria that produce lactic acid during the fermentation of carbohydrates
These bacteria are …. forming and can be both …. or ….

A

Gram-Positive
non-spore forming
rods or cocci

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13
Q

what are 5 genera of LAB?

A

Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Streptococcus

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14
Q

…. is often considered a LAB, however it is phylogenetically unrelated, and has a unique mode of sugar fermentation

A

Bifidobacterium

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15
Q

LAB are very adapted to environments that are rich in … and ….

A

nutrients and energy sources

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16
Q

what are some strategies Lab have to compete with other organisms?

A

acid production and acid tolerance

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17
Q

LAB are the most popular and widely available ….

A

probiotics

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18
Q

Only one Streptococcus species is associated with food: …

where is it use?

A

S. thermophilus

manufacture yogurt

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19
Q

which Lactococcus species is intimately associated with dairy products

A

Lactococcus lactis

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20
Q

which are the tetrad-forming LAB

A

Aerococcus,Pediococcus, and Tetragenococcus

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21
Q

… is not used in the food industry but is responsible for greening of cooked meat products

A

Aerococcus

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22
Q

… is a spoilage organism in the beer industry and is responsible for the buttery taste in beer

A

Pediococcus damnosus

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23
Q

… and … are both used as starter for sausage making and silage inoculants

A

Pediococcus acidilactici and Pediococcus pentosaceus

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24
Q

… are extremely salt tolerant (>18% NaCl) and are important in high-salt containing food like soy-sauce

A

Tetragenococcus

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25
Q

…, … and … are each coccoid LAB

A

Leuconostoc, Oenococcus, and Weisella

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26
Q

… are important for spontaneous vegetable fermentations like sauerkraut

A

Leuconostocs

27
Q

…. species are associated with meat and can proliferate at low temperatures

A

Weissella

28
Q

The genus … is the largest genera included in LAB

A

Lactobacillus

29
Q

Lactobacillus is divided into three groups

A

Group 1 - Obligately homofermentative
Group 2 - facultatively heterofermentative
Group 3 - Obligately heterofermentative

30
Q

Lactobacilli are the most … of the LAB, and will therefore be the final successors of many lactic acid fermentations

A

acid-tolerant

31
Q

… mostly found in spontaneous sourdough bread reactions

A

Lb. sanfransiscensis

32
Q

… (previously Lb. bulgaricus) is mostly yogurt associated

A

Lb. delbrueckii

33
Q

…, …, and … can be found in many habitats

A

Lb. brevis, Lb. casei, and Lb. plantarum

34
Q

LAB metabolism is characterized by efficient …

A

carbohydrate fermentation ending in lactic acid production

35
Q

LAB metabolism mechanism

A
  • A high-energy phosphate bond is required for sugar activation, before glycolysis
  • Some species use the phosphoenolpyruvate : sugar phosphotransferase system (PTS), in which phosphoenolpyruvate (PEP) is the phosphoryl donor
  • The main function of the PTS is to translocate sugar across the membrane with simultaneous phosphorylation
  • Since there are two different molecular species outside (sugar) and inside (sugar phosphate) the membrane the translocation process does not involve any concentration gradients
36
Q

LAB metabolism mechanism involving lactose

A

Lactose can be cleaved by B-galactosidase into glucose (glycolytic pathway) and galactose-6-phosphate and then enter one of several major metabolic pathways

37
Q

LAB metabolism mechanism involving maltose

A

Maltose fermentation starts when maltose is cleaved into glucose and B-glucose-1-phosphate. The glucose is used in glycolysis, and the B-glucose-1-phosphate is a precursor to cell wall synthesis

38
Q

LAB metabolism mechanism involving sucrose

A

Sucrose is cleaved into glucose and fructose which can then enter major pathways (glucose can be used in glycolysis, and fructose can be the terminal electron acceptor)

39
Q

explain Obligately Homofermentative (group 1)

A

sugars are only fermented by glycolysis (group I Lactobacilli and some other species from other genera)

40
Q

explain Obligately Heterofermentative (group 3)

A

only the 6-PG/PK pathway is available for fermentation (group III Lactobacilli, Oenococci, and Weissellas) the difference is that key enzymes of the glycolysis pathway are missing

41
Q

explain Facultatively Heterofermentative (group 2)

A

use glycolysis for hexose fermentation, but some sugars induce a heterolactic fermentation reaction to take place (pentoses) (group II Lactobacilli, Enterococcus, Lactococcus, Pediococcus, Streptococcus, Tetragenococcus, and Vagococcus)

42
Q

There is debate about if LAB should be classified as …, or as …
why?

A

aerotolerant anaerobes
facultative aerobes
because LAB can use oxygen as a terminal electron acceptor, but are not able to protect themselves against the toxic effects of oxygen the way genuine aerobic organisms would

43
Q

Lb. brevis is unable to ferment glucose…, but will ferment it …

A

anaerobically

aerobically

44
Q

When grown anaerobically several organic compounds can be used by … LAB as terminal electron acceptors. Which are they? (3)
=> contribute to flavour

A

heterofermentative

  • citrate (cleaved into acetate and oxaloacetate, and oxaloacetate can be a terminal electron acceptor in several pathways)
  • glycerol (terminal electron acceptor in anaerobic glucose fermentation)
  • fructose (electron acceptor for heterofermentative LAB)
45
Q

In the fermentation of plant materials (e.g. sauerkraut or kimchi) the main sugars are … (3). …. can be used as the energy source and … can be used as the terminal electron acceptor

A

glucose, fructose, and sucrose
Glucose
fructose

46
Q

LAB have limited …

what does this entail

A

biosynthetic capability
LAB have a very limited ability to synthesize amino acids from nitrogen sources, and are therefore dependent on obtaining preformed amino acids from their environment

47
Q

All dairy lactobacilli have …, which have been shown to be required for rapid growth in milk

A

proteolytic activity

48
Q

LAB require … (2)

A
  • nucleotides or nucleotide precursors

- several external vitamins

49
Q

When a fermentation relies on active LAB that were added as a starter culture, …. can result in major commercial problems. This is a particular problem in the … industry

A

bacteriophage infection

dairy

50
Q

Phage attacks on LAB can result in ….

A

unacceptably low LAB and flavor compounds produced along with decreased proteolysis

51
Q

what do we call a complete failure of the starter culture due to phage attacks?

A

‘dead-vat’

52
Q

what are 3 phage control plans?

A
  1. Characterization of the phage population involved (including genomic analysis)
  2. Analysis of “natural” and “intelligent” bacterial systems of phage defence
  3. Identification of phage counter-defense mechanisms
53
Q

what are 4 naturally occurring phage defence mechanisms?

A
  1. Absorption Inhibition
  2. Blocking DNA Penetration
  3. Restriction Enzymes/ Modification Systems
  4. Abortive Infection Mechanisms (bacterial cell traps the phages from emerging)
54
Q

what are 4 artificial phage-resistance mechanisms?

A
  1. Antisense RNA Strategies (Clone in antisense RNA which binds to phage DNA and stops viral replication)
  2. Cloned ORI (Clone the ORI for the phage into the bacterial genome, it competes with the phage one and slows viral growth)
  3. Clone in a Phage Repressor (Phage have a repressor for cell lysis, this can be constitutively expressed by the bacteria, which traps the phage)
  4. Phage Triggered Death (Bacterial suicide genes are placed under the control of a phage inducible promoter)
55
Q

…. has been shown to have the greatest ability to remove mycotoxins from foods

A

Lactobacillus rhamnosus

56
Q

about mesophilic LAB cultures

A

grow in temperatures of 10–48C, with the optimum around 38 C. Mesophilic starter cultures, composed of acid-forming lactococci, are often flavor producers, and are used in the production of many cheese varieties, fermented milk products, and ripened cream butter.

57
Q

about thermophilic LAB cultures

A

have their optimum growth temperature between 48C and 58C. Thermophilic starters are used for yogurt and for cheese varieties with high cooking temperatures (e.g. Emmental or Gruyere).

58
Q

how does LAB fermentation strongly influence the final organoleptic qualities of cheeses? (6)

A
  • fermenting sugars, leading to a pH decrease important in the clotting phenomenon
  • reduction or prevention of the growth of adventitious micro-flora
  • protein hydrolysis which causes the texture and, partially, taste of cheese
  • Synthesis of flavor compounds
  • Synthesis of texturing agents, which may influence the consistency of the product
  • Production of inhibitory components
59
Q

Proteolytic activity of LAB in chess production

A
  • LAB must get amino acids from their surroundings, therefore most have several proteinases and peptidases which provide the bacteria with free amino acids
  • All milk proteins, including whey proteins, are available for hydrolysis at the start of the fermentation reaction
  • The hydrolysis of milk proteins is responsible for bitter tastes in cheese
60
Q

what are the 2 categories of flavour compounds produced by LAB

A
  • Compounds produced by fermenting milk (lactic acid, acetic acid, acetaldehyde, diacetyl, and acetoin)
  • Compounds produced during cheese maturation (LAB have less of a role here)
61
Q

what are the inhibitory components of LAB? (2)

A
  • Very few bacteria can grow at the pH that LAB produce

- LAB also produce inhibitory substances including: hydrogen peroxide, diacetyl, and bacteriocins

62
Q

benefits of vegetable fermentation with LAB (5)

A
  • High degree of hygienic safety from pathogenic bacteria
  • Products are still “clean label”
  • Interesting and appealing flavours
  • Less energy input than other methods of preservation
  • Storage without refrigeration
63
Q

how is sauerkraut made? what is the science behind it and the LAB involved

A

Sauerkraut:

  • The concentration of fermentable sugars in cabbage is between 3 and 9% in fresh matter
  • Before processing the cabbage is shredded and salt is mixed in, brine begins to form immediately
  • The containers are sealed and pressed with a weight to maintain anaerobic conditions
  • The lactic acid fermentation is initiated by Leuconostoc mesenteroidesand followed by Lactobacillus brevis, Pediococcus pentosaceus , and finally by Lb. plantarum
  • Ln. mesenteroides produces lactic and acetic acids and CO2, which rapidly lower the pH, thus limiting the activity of undesirable microorganisms and enzymes that might soften the shredded cabbage
  • The CO2replaces air and creates an anaerobic atmosphere, which is important to prevent the oxidation of ascorbic acid and to avoid darkening the natural color of the cut cabbage

Food Microbio (11 decks)

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