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Food Microbiology > Fermented Dairy Products > Flashcards

Fermented Dairy Products Flashcards

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1
Q

What microorganisms are used in dairy fermentations? Which is of greatest significance?

A

mold, AAB, LAB, yeast

homofermentative LAB most important

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2
Q

Why are LAB so critical in dairy fermentations?

A

must rapidly produce lactic acid -> drop pH

flavor development

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3
Q

What does pH of a dairy ferment affect (terms of quality)?

A 
moisture control
retention of coagulants
loss of minerals
protein hydration
(affect flavor, texture, body of product)
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4
Q

Dairy starter cultures are classified as ____ or ____.

A

mesophilic (25-30C)

thermophilic (37-42C)

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5
Q

True/False: the most significant threat to the dairy fermentation industry is AAB contamination.

A

False; AAB not issue; phage infection biggest concern

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6
Q

___ is the primary carb in milk, composed of ___ and ___.

A

lactose; glucose and galactose

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7
Q

Describe the process for uptake of lactose by LACTOCOCCI: (3 things occur)

A

uptake by transport system (PEP-PT)
Simultaneous phosphorylation (to prevent gradient)
cleaved (phospho-B-galactosidase)
glucose + galactose-6P

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8
Q

How are the components of lactose metabolized by LAB?

A

Glucose -> glycolysis

galactose-6P -> tagatose pathway

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9
Q

A cheese product has visible cracking, and pink and brown pigments. What is the cause for this and how can it be prevented?

A

Excess excreted galactose from LAB that cannot metabolize it - accumulation in cheese

cracks caused by CO2 from heterofermentative bacteria that eat the galactose
color from rxn with amino groups

Need to include galactose metabolizing strains in culture!

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10
Q

What is the alternative lactose transport system in non-lactococci LAB? What is the difference?

A

Lactose-galactose antiport system
does not add P; cleaved to give glucose + galactose
galactose excreted

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11
Q

____ can metabolize galactose through the ____ pathway, and will prevent ______.

A

Lb helveticus
Leloir
accumulation of galactose in cheese

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12
Q

How does lactic acid in cheese affect aroma?

A

No aroma (not volatile); but contribute to acidity in taste

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13
Q

What compounds give fermented dairy its aroma? Where do they come from?

A

acetic acid, acetaldehyde, diacetyl

from citric acid in milk -> fermented by LAB

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14
Q

besides sugars, what nutrient is required for LAB? How can they obtain it from milk?

A

amino acids

free AA not sufficient, use PROTEOLYTIC SYSTEMS to break down milk peptides

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15
Q

True/False: LAB are considered auxotrophs

A

True (cannot make AA, many other macromolecules)

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16
Q

the 3 systems needed for proteolysis:

A

extracellular enzymes
transport systems
intracellular enzymes

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17
Q

What is the significance of PrtP? Where is it found?

A

envelope associated proteinase (EXTRACELLULAR)
in LACTOCOCCI
broad range of cleaving ability, hacks up milk peptides so can uptake

if not present -> cell grow 10x smaller

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18
Q

What is the major protein type in milk?

A

Caseins (80%)

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19
Q

True/False: dipeptides and tripeptides are taken up by the same membrane transporter

A

True (Dtp)

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20
Q

True/False: oligopeptides must be broken down further to enter the cell

A

False; oligopeptides produced by PrtP can be taken up by Opp -> broken down inside cell

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21
Q

the 3 membrane AA transport systems:

A

AAT
Dtp
Opp

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22
Q

What proteolytic enzyme types are inside the cell?

A

aminopeptidases
dipeptidases
tripeptidases
endopeptidases

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23
Q

How does the proteolysis affect flavor of the milk?

A

bitterness, flavor development

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24
Q

____ is the process in cheese that breaks the milk emulsion/dispersion, and results in separation of _____.

A

coagulation

curds, whey

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25
Q

The 3 coagulation methods:

A

acid coagulation
rennet coagulation
acid/heat coagulation

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26
Q

How do insoluble proteins remain dispersed in milk? What protein is responsible?

A

contained in micelles, surrounded by KAPPA CASEIN (soluble)

stabilized by Ca phosphate

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27
Q

How does acid coagulation occur?

A

LAB ferment -> lactic acid -> pH drop to 4.6
micelle polar surface neutralized, Ca Phosphate solubilized -> casein clump into chains
water, fat globules trapped in casein matrix

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28
Q

acid coagulated cheese form a (strong/weak) gel, which is high in ___.

A

weak

water (70-80%)

29
Q

List some examples of acid coagulated cheese:

A

cottage cheese
quark
cream cheese

30
Q

Are acid coagulated cheese usually aged?

A

No, high moisture -> prone to spoilage

31
Q

What causes the coagulation in acid/heat coagulation? Can it be used alone?

A

heat coagulation

no; micelles are highly heat stable up to 140C (require acid to lower coag temp)

32
Q

Describe the process of acid/heat coagulation

A

acid lowers pH to 6.2-5.4
apply heat 85C
Whey proteins unfold -> clump with caseins -> trap fat
Drain, press curds

33
Q

Ricotta and queso blanco are examples of ____ coagulation. They are relatively high in ____.

A

acid/heat

moisture

34
Q

What are the advantages of rennet vs acid coagulation?

A
  1. firmer curd, expel more whey

2. faster (30-60min)

35
Q

How does rennet coagulation occur?

A

rennet (enzymes) target k-casein

k casein cleaved -> micelle insoluble -> caseins form matrix trapping fat (curds)

36
Q

What is rennet and where does it come from?

A

mix of proteinases that cleave caseins

animal, plant, microbial sources (most common)

37
Q

rennet cheese are low in ___, and can be aged.

A

moisture

38
Q

What is important to consider in early stages of rennet cheese making?

A
  1. correct amount of whey expelled
  2. rate of acidification
  3. rate of salt addition
39
Q

True/False: rennet cheese do not require acidification

A

True; can occur at 6.6-6.3

40
Q

What is “cutting” in cheese making? What is the purpose?

A

break up coagulum

more surface area -> more whey expulsion

41
Q

What step occurs after the whey expulsion in the cheese? What does this step accomplish?

A

cooking

promote curd contraction
high temp -> LAB make more acid -> more curd contraction
contraction squeeze out more whey

42
Q

Cooking the whey and curd for longer at a (higher/lower) temp will produce a curd that has less ____.

A

higher

moisture

43
Q

How are the curds removed from whey and why is this necessary?

A

Dripping: scoop out, put in draining mold
Draining: take out curds from strainer in the vessel, leave whey behind
Industrial: pump out onto draining table, let whey drip

curds need to fuse to form the cheese

44
Q

____ describes the fusion of the cheese curd. ____ is a similar process but involves pressure and produces ____ cheeses

A

Knitting
Pressing
compact (salt brine cheeses, swiss cheeses)

45
Q

Why is salting important in cheesemaking?

A

osmotic force: draws out more whey

46
Q

How is salting done?

A
  1. rub dry salt on surface of cheese
  2. submerge in brine
  3. add to curd before knitting
47
Q

Finishing transforms ____ into finished cheese

A

green cheese

48
Q

True/False: only finished cheese should be salted

A

false; can add to curd

49
Q

What are some considerations in the finishing process?

A

depends on cheese;

temperature, humidity, surroundings, microflora, manipulations, etc

50
Q

The 2 ripening zones in cheese:

Why are they different?

A 
interior and exterior
different environments (aerobic vs anaerobic)
51
Q

What are examples of interior ripening and how is this done?

A

blue cheeses
poke with needles to make air shafts so mold can grow
(P. roqueforti added to milk)

52
Q

Are blue cheeses pressed? Why or why not?

A

no; need to be somewhat permeable to air to allow mold to grow

53
Q

Blue cheeses should be turned regularly to avoid ____

A

rind rot, moisture accumulation

54
Q

The 2 groups of surface ripened cheese:

What category is Brie and Camembert?

A 
Low pH (<5)
High pH (>5)

brie/camembert are low pH

55
Q

What is the microbe in camembert ripening and what does it do?

A

P. camemberti (mold)

break down lactic acid (pH rises)

56
Q

The temperature range of yogurt fermentation is ____, because the cultures used are _____.

A

42-43C

thermophilic

57
Q

What is the optimal endpoint for yogurt pH? What happens if the ferment continues?

A

4.2-4.6

if continue, pH keep dropping -> wheying off

58
Q

yogurt starter cultures usually have ____ and ___. In what proportion?

A

Strep. thermophilus, Lactobacillus bulgarius

equal numbers

59
Q

What are the optimal temps for S thermophilus and L bulgarius?

A

St: 37C
Lb: 45C

60
Q

Describe the symbiotic relationship of S. thermophilus and L. bulgarius in yogurt:

A

Lb has proteases (PrtP), breaks down milk proteins
St not as good at proteolysis, uses peptides made by Lb; St also use intermediate products from Lb

St make formic acid and CO2 -> acts as growth factor for Lb

61
Q

Describe the population dynamics through the yogurt process

A

beginning: St grows fastest, biggest population
Middle: lower pH and growth factors stimulate Lb to grow (Lb more acid tolerant); Lb breakdown proteins to feed St
End: 1:1 population, about 2x10^7 each

62
Q

List some yogurt types

A

stirred yogurt (mixed with sugar, flavorings, fruit)
Greek yogurt (drained)
Set yogurt (set in container, not disturbed)
drinking yogurt: (added milk and flavors)
yogurt cheese (drained)
frozen yogurt

63
Q

What gives yogurt its gel-like texture?

A

interaction between acid-destabilized k-casein and heat destabilized whey proteins (similar to acid/heat coagulated cheese)

64
Q

At pH ___, Ca phosphate is _____, causing the micelle to lose its structure.

A

5.0

soluble; released from casein micelle

65
Q

Flavor compounds in yogurt:

Which is most important?

A

Lactic acid
Diacetyl
Acetaldehyde (most important) - 40mg/kg
Acetic acid

66
Q

How can using Bifidobacteria affect the taste of yogurt?

A

makes more acetic acid -> more vinegary taste

67
Q

Inclusion of citrate positive bacteria will influence yogurt flavor in what way?

A

create diacetyl -> buttery taste

68
Q

True/False: fresh fruit will contribute to yogurt positively

A

False; usually use dry/frozen/processed fruit to avoid spoilage from yeast/mold

69
Q

Activia yogurt is marketed as containing a strain called _____.

A

Bifidobacterium animalis

Food Microbiology (21 decks)

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