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5) Food Microbiology > 8) Fermented Dairy > Flashcards

8) Fermented Dairy Flashcards

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1
Q

what LAB is most imp in dairy fermentations?

A

homofermentative LAB

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2
Q

what is the main function of LAB in dairy fermentations?

A

to ferment lactose into lactic acid

also imp for flavour development

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3
Q

starter LAB cultures can be ____ or ____

A

mesophilic (lower optimal growth temp)

htermophilic (higher optimal growth temp)

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4
Q

what is the key to a fermented milk product?

A
  • consistent and predictable rate of acid development
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5
Q

what is the most significant threat to the fermented milk products industry?

A

phage

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6
Q

lactose breaks down to what?

A

glucose + galactose

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7
Q

what system is used for lactococci to translocate lactose into the cell?

describe this system

A

phosphoenolpyruvate phosphotransferase system

  1. lactose is phosphorylated during translocation
  2. cleaved by phospho-B-galactosidase into glucose and galactose-6-phosphate
  3. glucose moiety enters glycolytic pathway
  4. galactose-6-phosphate is converted into tagatose-6-phosphate (via tagatose pathway)
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8
Q

what is the lactose-galactose anti-port system?

A

allows bacteria to transport lactose

  1. lactose is not phosphorylated, but cleaved by B-galactosidase to yield glucose and galactose
  2. glucose moiety enters glycolytic pathway
  3. galactose is excreted from cells or left to accumulate in the milk or cheese
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9
Q

what problems with galactose can occur if the starter culture doesn’t have the right strains to metabolize it?

A
  1. if heterfermentative bacteria metabolize it rapidly and produce CO2, it can lead to cracks in the cheese
  2. residual galactose can react with amino groups, causing pink or brown pigments to form
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10
Q

what is the main metabolite at end of dairy fermentation?

A
  • lactic acid
  • responsible for acid taste
  • does not contribute to aroma
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11
Q

what are the main volatile and flavor components?

A

acetic acid
acetaldehyde
diacetyl

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12
Q

what are proteolytic systems in LAB essential for?

A
  1. LAB survival in milk

2. flavour dev in ripened cheeses

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13
Q

LAB are AA auxotrophs. What does this mean?

A

they require additional AA for growth

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14
Q

why are proteolytic systems necessary for LAB to grow?

A

since free AA in milk are not enough for LAB to grow, so they use proteolytic systems to hydrolyze peptides in milk to obtain free AAs

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15
Q

what are 3 categories of proteolytic systems in LAB?

A
  1. enzymes outside the cytoplasmic membrane
  2. transport systems
  3. intracellular enzymes
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16
Q

what is PrtP?

what are functions and characteristics?

A

envelope associated proteinase in Lactococci

function: degrades caseins into oligopeptides
characteristics: attaches to cell via anchor sequence, has broad cleave specificity

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17
Q

what are 3 transport systems of AA across the cytoplasmic membrane?

A
  1. AA transport systems (AA)
  2. Di- and tri-peptide transport systems (Dtp)
  3. Oligopeptide transport systems (Opp): essential for uptake of peptides from breakdown of casein via PrtP
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18
Q

what are 4 peptidases that hydrolyzes peptides inside the cell?

A
  1. aminopeptidases (AP)
  2. dipeptidases (DP)
  3. tripeptidases (TP)
  4. endopeptidases (EP)
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19
Q

how does proteolysis affect flavour development?

A
  • proteolytic enzymes in the milk matrix determine which peptides and AAs accumulate, which impacts what flavours are developed
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20
Q

what is the first step in cheese making?

A

coagulation

proteins are converted into a NP form; causes separation from the water phase through entrapping fat and minerals (curd contraction and whey expulsion)

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21
Q

what are 3 coagulation processes?

A
  1. rennet coagulated
  2. acid coagulated
  3. acid/heat coagulated
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22
Q

what is the casein-micelle?

A
  • soluble coagulations of the casein protein with hydrophobic core
  • involves kappa-casein which helps keep the micelle soluble
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23
Q

what is kappa-casein?

A
  • protein at surface of the casein micelle
  • polar nature
  • keeps micelle soluble (since micelle is not soluble on its own)
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24
Q

what is the casein-micelle stabilized with?

A

calcium phosphate

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25
Q

describe the acid coagulation process

A
  1. LAB ferments lactose to lactic acid
  2. pH decreases until isoelectric point of casein (pH 4.6) is reached)
  3. H atoms accumulate and neutralizes polar surfaces of casein micelles
  4. micellar calcium phosphate (MCP) is converted to the soluble form (soluble calcium phosphate)
  5. micelle chains are created, which entraps H2O and solid components
  6. after several hours, a coagulum is formed (soft gel consistency)
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26
Q

describe the coagulum formed from acid coagulation

A
  • fragile
  • limited ability to expel whey, which slows drainage
  • has high water content (spoils easily)
27
Q

describe the coagulation process of acid/heat coagulation

A
  1. whey proteins unfold and loose ability to interact with H2O
  2. denatured whey proteins attach onto micellar surfaces
  3. micelles aggregate into clusters that entrap fat globules
  4. curds are drained and sometimes pressed
28
Q

how does acid or heat cause coagulation of milk?

A

when milk os moderately acidified (pH 6.2-5.4), it becomes susceptible to heat induced coagulation at lower temps

29
Q

what is rennet

A

group of proteinases that cleave casein at surface of casein micelles when added to milk

30
Q

what is the role of rennet?

A

cleaves the kappa casein at surface of micelles and initiates coagulation

31
Q

what are the coagulation steps of rennet coagulation?

A
  1. rennet cleaves kappa-casein

2. micelles form a matrix net (at high pH 6.6-6.3) before acidification by starter LAB occurs

32
Q

whats the diff with rennet coagulation vs acid coagulation?

A
  1. rennet curd is more resilient (less fragile) and better able to expel whey
  2. rennet coagulation is faster
33
Q

describe rennet-coagulated cheeses

A
  • can be dried and aged
  • more complicated and unforgiving to make than acid or acid/heat coagulated cheeses
  • process can take up to months/years
34
Q

what are 3 objectives of the first few days of rennet coagulated cheeses?

A
  1. correct amount of whey must be expelled from the curd
  2. the rate of acidification must be controlled
  3. salt must be incorporated at the correct rate to attain the target salt content
35
Q

what are post-coagulation processes that occur?

A
  • cutting of the coagulum
  • cooking
  • dripping and draining
  • knitting
  • pressing
  • salting
  • finishing
36
Q

describe cutting of the coagulum

A

required to break up the coagulum and initiate separation of whey from curd

  • creates new surface area for faster whey release
  • LAB are trapped in matrix of the curd (not lost with whey)
37
Q

describe the cooking process of the coagulum

A
  • applies heat and stirring
  • higher temp and longer duration = more curd contraction
  • influences rate of lactic acid production by LAB
  • causes more pH decrease (curds contract and expel more)
38
Q

describe the dripping and draining process of the coagulum

what are the 2 methods?

A
  • separates curd from whey and allows curd particles to fuse together to form a larger curd

methods:

  1. dripping: curd whey mixture is placed in a draining vessel
  2. draining: uses a fitted valve or strainer
39
Q

describe the knitting process of the coagulum

A

fusing curds together as the whey drains to produce once continuous mass of fused cheese curd

higher temp = more whey expulsion

40
Q

describe the pressing process of the coagulum

A
  • external pressure to curd after knitting
  • helps expel whey
  • promotes more fusion of curd particles
  • uses pressing devices (for more compact textures; needed for trapping microbiologically produced gases or rinds on some cheeses)
41
Q

what is an unpressed cheese?

A

uses only gravity draining to knit curds together

42
Q

describe the salting process of the coagulum

A
  • function: creates osmotic driving force to draw whey to curd surface to be released
  • more salt uptake = more whey released
  • 3 ways of applying salt: dry salt rubbed on surface, submersion in salt brine, applied directly to curd
43
Q

describe the finishing process of the coagulum

A

where “green” cheeses are transformed into “ripened” cheeses

- requires specific combination of environmental conditions (temp, humidity, microflora, etc…)

44
Q

what are two ripening zones (in finishing process)

A
  1. body (interior of cheese): involves interior ripening; obligate aerobes can’t grow in the cheese interior unless exterior is broken
  2. surface: involves surface ripening; aerobic
45
Q

what are two types of ripening?

A
  1. interior ripening

2. surface ripening

46
Q

describe interior ripening in blue cheeses

A
  • P. roqueforti
  • mold growth occurs throughout the anaerobic cheese
  • ## Roquefort: promotes internal mold growth by piercing cheese with needles to allow CO2 out and O2 in
47
Q

what is an example of surface ripened cheese?

A

bre

caember

48
Q

what is an example of interior ripening cheese?

A

blue cheeses

49
Q

what are 2 groups of surface ripened cheeses?

A
  1. low pH at surface (pH < 5)

2. high pH at surface (pH > 5)

50
Q

how is penicillum camemberti added in cheese production?

how does it function?

what does it result in?

A

mold added before renneting, or sprayed to surface before ripening

function

  • catabolizes lactic acid
  • results in de-acidification of cheese surface, which makes the cheese prone to contamination by bacteria that produces orange/yellow pigments
  • well
51
Q

what pigments are produced in surface ripened cheese when bacteria causes spoilage?

what color would a well-made cheese be?

A

orange/yellow pigments

white

52
Q

what is milk innoculated with in yogurt production?

A

LAB

53
Q

what is the optimal water retention pH in yogurt production?

what occurs if the pH is below this?

A

optimal: 4.2-4.6

below 4.2: wheying-off

54
Q

yogurt starters will have which strains of bacteria? how much of each?

A

equal amounts of streptococcus thermophilus and lactobacillus bulgarious

55
Q

why is yogurt incubated at 42C?

A

42C is in between the optimal growth temp for streptococcus thermophilus and lactobacillus bulgarious

56
Q

what conditions are acheived at the end of fermentation?

A
  1. 1:1 balance of the starter organisms
  2. pH 4.2
  3. cell count of 2x10^7 for each starter organism
57
Q

describe the role of L. bulgaricus in the starter culture of yogurt

A
  • has cell bound proteases that cleaves proteins into AAs, which are used by S. thermophilus for growth
  • grows better at low pH than S. thermophilus
  • growth produces peptides and AAs that help growth of S. thermophilus
58
Q

describe the role of S. thermophilus in the starter culture of yogurt

A

has peptidases that hydrolyze intermediate products from L. bulgaricus
- produces purine, pyrimidine, CO2, formic acid, oxaloacetic acid and fumaric acid (growth factors for L. bulgaricus)

59
Q

what types of yogurt are there?

A
  • stirred yogurt
  • set yogurt
  • drinking yogurt
  • yogurt cheese
  • frozen yogurt
  • strained yogurt (greek)
  • dadiah (buffalo milk)
60
Q

what causes the gel-like texture of yogurt?

A

due to interactions between acid destabilized k-casein and heat denatured whey proteins

61
Q

describe the gelation process of yogurt

A
  • during acidification at pH 5
  • calcium phosphate is released from casein micelle
  • denatured whey proteins interact with k-casein
  • hydrophobicity is increased
  • leads to aggregation and gelation
62
Q

what are 4 imp compounds for the taste of yogurt?

A
  1. lactic acid
  2. acetaldehyde: most imp; can be produed by LAB
  3. acetic acid
  4. diacetyl: created via citrate metabolism; results in a butter flavor
63
Q

why don’t yogurt manufacturers use fresh fruit?

what is used instead?

A

due to high spoilage rate

uses processed or frozen fruits instead since they are:

  • free from yeasts and molds
  • easy to adjust color and sweetness
64
Q

in activia, what bacteria is important? why?

A

bifidobacterium animalis

  • able to survive lactic acid in yogurt

5) Food Microbiology (21 decks)

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