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Food Microbiology > LAB > Flashcards

LAB Flashcards

1
Q

Lactic acid is also known as:

A

milk acid

2-hydroxypropanoic acid

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2
Q

What organism makes lactic acid, and what substrate is used?

A

lactic acid bacteria

breaks down SIMPLE CARBS: glucose, sucrose, galactose

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3
Q

Where can lactic acid be found in the human body? (2)

A

produced by LAB in mouth (causes cavities)

L-lactate produced by muscles during exertion

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4
Q

True/false: lactic acid is the most common organic acid produced in fermentations

A

true

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5
Q

What sugars can be used for lactic acid fermentations?

A

almost any C5 or C6 sugar (sucrose, glucose, raw sugar, etc)

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6
Q

The 2 classes of LAB:

A
  1. homofermentative

2. heterofermentative

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7
Q

What is the main difference between the 2 classes of LAB?

A

homofermentative makes 2 MOLES of lactic acid per mole glucose

heterofermentative makes 1 MOLE lactic acid + CO2 + byproduct (ethanol, acetic acid, etc)

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8
Q

Describe the cell morphology and characteristic of LAB:

A

GRAM POSTIVE
Rods or cocci
non-spore forming
ferment simple sugars into lactic acid

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9
Q

name 5 common LAB types:

A 
Enterococcus
Leuconostoc
Lactobacillus
Lactococcus
Streptococcus
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10
Q

Why are LAB so useful in the food industry? (3)

A

fermentation process

prevent pathogen/spoilage by outcompeting other microbes (and acidity)

Use as probiotic

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11
Q

True/False: LAB are very good at surviving in low nutrient environments.

A

FALSE: adapted to nutrient/sugar rich environments; limited biosynthetic activity (cannot make many compounds)

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12
Q

True/False: LAB are acid tolerant

A

True

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13
Q

What is the streptococcus species used in food, and for what purpose?

A

S. thermophilus

used in yogurt; or in co-culture for cheese

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14
Q

Is enterococcus useful for food production?

A

No; not commonly used, and some species PATHOGENIC

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15
Q

what species of lactococcus are used by the food industry?

A

L. lactis

used in dairy technology

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16
Q

What are the tetrad forming species of LAB? What is the significance of each in food products?

A

Aerococcus (greening/off color of meat): BAD
Pediococcus: (beer spoilage): BAD; (silage or sausage inoculant): GOOD
Tetragenococcus: (used in soy sauce): GOOD

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17
Q

What is a particular feature of Tetragenococcus?

A

very salt tolerant (up to 18%)

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18
Q

What are the coccoid LAB? What are their associated uses?

A

Leuconostoc: spontaneous veg. fermentations
Oenococcus
Weisella: used in silage; found in meat

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19
Q

What is the largest genus in LAB? What is their defining feature?

A

lactobacillus

most acid tolerant (final survivors of fermentation)

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20
Q

What are some LAB species that thrive in many habitats?

A

brevis, casei, plantarum

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21
Q

What are the 3 groups of lactobacillus?

A

Group 1: obligately homofermentative
Group 2: facultative heterofermentative
Group 3: obligately heterofermentative

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22
Q

What groups of Lactobacillus will produce CO2?

A

Groups 2 and 3

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23
Q

True/False: a ferment done with Group 1 lactobacillus will become acidic at a faster rate than using Group 3

A

True

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24
Q

What is the effect of changing the growing conditions for Lactobacillus?

A

different chemical environment (O2, diff. substrate) -> change metabolism to produce different products -> affect flavor and other qualities

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25
Q

Why is a sugar/nutrient rich environment necessary to support LAB?

A

they cannot synthesize their own macromolecules; must obtain from environment

26
Q

What mechanism allows for efficient uptake of sugar in LAB?

A

Incoming sugars are PHOSPHORYLATED using 1 ATP

prevents concentration gradient so cell can keep uptaking sugars

27
Q

the 2 primary hexose fermentation pathways in LAB:

A

heterofermentative

homofermentative

28
Q

Describe the homofermentative metabolic process:

A 
GLYCOLYTIC PATHWAY (only)
glucose -> 2 lactate, 2 ATP
29
Q

Describe the heterofermentative metabolic process:

A

6-PG/PK PATHWAY

glucose -> 1 lactate, 1 ATP, CO2, ethanol

30
Q

Do all sugars undergo glycolysis in LAB?

A

NO; some converted to acetic acids, organic acids, amino acids, etc

31
Q

What step is required prior to glycolysis in metabolism? What other function does this have?

A

forming high energy phosphate bond (phosphorylate)

prevents concentration gradient

32
Q

What is a metabolic system used to phosphorylate sugars?

A

PTS: phosphotranferase system: translocates across membrane, adds P
P donor is phosphoenolpyruvate

33
Q

true/false: LAB cannot ferment disaccarides

A

False

34
Q

How are lactose, sucrose, and maltose metabolized by LAB?

A

broken down by B-galactosidase -> monosaccarides
glucose enter pathway
other component used in cell wall synthesis, as electron acceptor, or secreted

35
Q

True/False: maltose can be cleaved into 2 glucoses, both of which can be metabolized for energy

A

False; produces 1 glucose; other half is B-glucose-1-P

can use in cell wall synth

36
Q

Why can’t heterofermentative LAB use glycolysis?

A

key enzymes missing

37
Q

How might using a pentose affect the metabolism of some LAB?

A

cause facultative heterofermentative LAB to do heterolactic ferment

38
Q

Most genera of LAB are ____ (metabolism type)

A

heterofermentative

39
Q

What evidence argues that LAB are aerotolerant, and not facultative aerobes?

A

does not possess protective mechanisms against oxidation from O2 like most aerobic organisms

40
Q

What is the difference between an aerotolerant anaerobe vs a facultative aerobe?

A

aerotolerant: can withstand presence of air, does not utilize it or benefit from it

facultative aerobe: will use air if it is present (grows better)

41
Q

How can LAB use O2, and when are aerobic conditions necessary?

A

terminal electron acceptor

required for certain substrates; Lb. brevis can only ferment glucose aerobically

42
Q

what is an effect of oxygen exposure on a LAB fermentation?

A

oxidize lactic acid -> acetic acid and CO2

43
Q

What compounds act as terminal electron acceptors in LAB if oxygen is not present?
This is especially common in:

A

citrate (cleave to oxaloacetate)
glycerol
fructose
heterofermentative (produce a lot of these byproducts)

44
Q

How does the composition of cabbage make it very suitable for LAB fermentation?

A

high in sucrose, glucose, fructose

use glucose as energy, fructose as TEA

45
Q

How do LAB obtain their amino acids?

A

proteolytic enzymes - break down proteins in nutrient rich environment (ie: milk)

46
Q

What nutrients do LAB require?

A

vitamins
nucleotides or precursors
sugars
amino acids

47
Q

What is the main concern in commercial LAB ferments? What occurs?

A 
Phage infection
kills LAB (usually target specific taxa) -> decimates population -> complete failure of ferment (dead vat)
48
Q

What do control plans of phage infection involve? (3)

A

characterize phage population

analyze natural bacteria defenses

identify phage counter mechanisms

49
Q

What can be done if phage infection occurs?

A

Ferment is discarded
Can attempt to decontaminate; usually not feasible
usually switch to new strain that is resistant

50
Q

What are some natural phage resistance mechanisms? (4)

A
  1. absorption inhibition
  2. block DNA penetration
  3. restriction enzymes/modification systems
  4. abortive infection mechanisms (prevent phages from emerging from cell)
51
Q

What are some artificial phage resistance mechanisms? (4)

A
  1. Antisense RNA: clone in/add to phage DNA -> stop replication
  2. Cloned ORI: (origin of replication) into bacteria: compete with phage ORI -> slower phage rep
  3. Clone in phage repressor: stop cell from lysing (trap phage)
  4. Phage triggered death: put suicide genes next to phage induced promoter: phage enter -> cell dies
52
Q

How could LAB be used to rectify spoiled grain?

A

if spoilage is mycotoxin (highly carcinogenic), could use L. rhamnoses to remove

53
Q

In the dairy industry, how are LAB classified?

A

according to best growing temperature:
MESOPHILIC (10-48)
THERMOPHILIC (48-58)

54
Q

A fermentation process using lactococci to make cheese is best at what temperature?

A

10-48 (best at 38C)

mesophilic

55
Q

What activities occur during cheese production? (6)

A 
sugars decrease
pH decrease -> clotting, prevent other microbes
protein hydrolysis -> texture/flavor
flavor compound synthesis
texture agent synthesis
produce inhibitory components
56
Q

what is responsible for the bitter taste in cheese?

A

proteolysis

57
Q

what are the 2 categories of flavor compounds in cheese? Which involve LAB?

A
  1. produced during milk ferment (INVOLVES LAB)

2. produced during ripening

58
Q

What flavor compounds do LAB contribute to cheese? (5)

A

lactic acid, acetic acid, acetaldehyde, diacetyl, acetoin

59
Q

What are some inhibitory compounds LAB produce?

A

acid (low pH)
H2O2
diacetyl
bacteriocins

60
Q

What is the importance of adding salt to cabbage for LAB fermentation?

A

breaks down cabbage to release sugars to feed LAB

61
Q

What is the initial LAB present in sauerkraut, and what important role does it play?

A

Ln. mesenteroides: produce lactic and acetic acid, CO2 -> rapidly lower pH (prevent microbial growth and oxidation)

Food Microbiology (21 decks)

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