Fermented Dairy Products Flashcards
What microorganisms are used in dairy fermentations? Which is of greatest significance?
mold, AAB, LAB, yeast
homofermentative LAB most important
Why are LAB so critical in dairy fermentations?
must rapidly produce lactic acid -> drop pH
flavor development
What does pH of a dairy ferment affect (terms of quality)?
moisture control retention of coagulants loss of minerals protein hydration (affect flavor, texture, body of product)
Dairy starter cultures are classified as ____ or ____.
mesophilic (25-30C)
thermophilic (37-42C)
True/False: the most significant threat to the dairy fermentation industry is AAB contamination.
False; AAB not issue; phage infection biggest concern
___ is the primary carb in milk, composed of ___ and ___.
lactose; glucose and galactose
Describe the process for uptake of lactose by LACTOCOCCI: (3 things occur)
uptake by transport system (PEP-PT)
Simultaneous phosphorylation (to prevent gradient)
cleaved (phospho-B-galactosidase)
glucose + galactose-6P
How are the components of lactose metabolized by LAB?
Glucose -> glycolysis
galactose-6P -> tagatose pathway
A cheese product has visible cracking, and pink and brown pigments. What is the cause for this and how can it be prevented?
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!
What is the alternative lactose transport system in non-lactococci LAB? What is the difference?
Lactose-galactose antiport system
does not add P; cleaved to give glucose + galactose
galactose excreted
____ can metabolize galactose through the ____ pathway, and will prevent ______.
Lb helveticus
Leloir
accumulation of galactose in cheese
How does lactic acid in cheese affect aroma?
No aroma (not volatile); but contribute to acidity in taste
What compounds give fermented dairy its aroma? Where do they come from?
acetic acid, acetaldehyde, diacetyl
from citric acid in milk -> fermented by LAB
besides sugars, what nutrient is required for LAB? How can they obtain it from milk?
amino acids
free AA not sufficient, use PROTEOLYTIC SYSTEMS to break down milk peptides
True/False: LAB are considered auxotrophs
True (cannot make AA, many other macromolecules)
the 3 systems needed for proteolysis:
extracellular enzymes
transport systems
intracellular enzymes
What is the significance of PrtP? Where is it found?
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
What is the major protein type in milk?
Caseins (80%)
True/False: dipeptides and tripeptides are taken up by the same membrane transporter
True (Dtp)
True/False: oligopeptides must be broken down further to enter the cell
False; oligopeptides produced by PrtP can be taken up by Opp -> broken down inside cell
the 3 membrane AA transport systems:
AAT
Dtp
Opp
What proteolytic enzyme types are inside the cell?
aminopeptidases
dipeptidases
tripeptidases
endopeptidases
How does the proteolysis affect flavor of the milk?
bitterness, flavor development
____ is the process in cheese that breaks the milk emulsion/dispersion, and results in separation of _____.
coagulation
curds, whey
The 3 coagulation methods:
acid coagulation
rennet coagulation
acid/heat coagulation
How do insoluble proteins remain dispersed in milk? What protein is responsible?
contained in micelles, surrounded by KAPPA CASEIN (soluble)
stabilized by Ca phosphate
How does acid coagulation occur?
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
acid coagulated cheese form a (strong/weak) gel, which is high in ___.
weak
water (70-80%)
List some examples of acid coagulated cheese:
cottage cheese
quark
cream cheese
Are acid coagulated cheese usually aged?
No, high moisture -> prone to spoilage
What causes the coagulation in acid/heat coagulation? Can it be used alone?
heat coagulation
no; micelles are highly heat stable up to 140C (require acid to lower coag temp)
Describe the process of acid/heat coagulation
acid lowers pH to 6.2-5.4
apply heat 85C
Whey proteins unfold -> clump with caseins -> trap fat
Drain, press curds
Ricotta and queso blanco are examples of ____ coagulation. They are relatively high in ____.
acid/heat
moisture
What are the advantages of rennet vs acid coagulation?
- firmer curd, expel more whey
2. faster (30-60min)
How does rennet coagulation occur?
rennet (enzymes) target k-casein
k casein cleaved -> micelle insoluble -> caseins form matrix trapping fat (curds)
What is rennet and where does it come from?
mix of proteinases that cleave caseins
animal, plant, microbial sources (most common)
rennet cheese are low in ___, and can be aged.
moisture
What is important to consider in early stages of rennet cheese making?
- correct amount of whey expelled
- rate of acidification
- rate of salt addition
True/False: rennet cheese do not require acidification
True; can occur at 6.6-6.3
What is “cutting” in cheese making? What is the purpose?
break up coagulum
more surface area -> more whey expulsion
What step occurs after the whey expulsion in the cheese? What does this step accomplish?
cooking
promote curd contraction
high temp -> LAB make more acid -> more curd contraction
contraction squeeze out more whey
Cooking the whey and curd for longer at a (higher/lower) temp will produce a curd that has less ____.
higher
moisture
How are the curds removed from whey and why is this necessary?
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
____ describes the fusion of the cheese curd. ____ is a similar process but involves pressure and produces ____ cheeses
Knitting
Pressing
compact (salt brine cheeses, swiss cheeses)
Why is salting important in cheesemaking?
osmotic force: draws out more whey
How is salting done?
- rub dry salt on surface of cheese
- submerge in brine
- add to curd before knitting
Finishing transforms ____ into finished cheese
green cheese
True/False: only finished cheese should be salted
false; can add to curd
What are some considerations in the finishing process?
depends on cheese;
temperature, humidity, surroundings, microflora, manipulations, etc
The 2 ripening zones in cheese:
Why are they different?
interior and exterior different environments (aerobic vs anaerobic)
What are examples of interior ripening and how is this done?
blue cheeses
poke with needles to make air shafts so mold can grow
(P. roqueforti added to milk)
Are blue cheeses pressed? Why or why not?
no; need to be somewhat permeable to air to allow mold to grow
Blue cheeses should be turned regularly to avoid ____
rind rot, moisture accumulation
The 2 groups of surface ripened cheese:
What category is Brie and Camembert?
Low pH (<5) High pH (>5)
brie/camembert are low pH
What is the microbe in camembert ripening and what does it do?
P. camemberti (mold)
break down lactic acid (pH rises)
The temperature range of yogurt fermentation is ____, because the cultures used are _____.
42-43C
thermophilic
What is the optimal endpoint for yogurt pH? What happens if the ferment continues?
4.2-4.6
if continue, pH keep dropping -> wheying off
yogurt starter cultures usually have ____ and ___. In what proportion?
Strep. thermophilus, Lactobacillus bulgarius
equal numbers
What are the optimal temps for S thermophilus and L bulgarius?
St: 37C
Lb: 45C
Describe the symbiotic relationship of S. thermophilus and L. bulgarius in yogurt:
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
Describe the population dynamics through the yogurt process
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
List some yogurt types
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
What gives yogurt its gel-like texture?
interaction between acid-destabilized k-casein and heat destabilized whey proteins (similar to acid/heat coagulated cheese)
At pH ___, Ca phosphate is _____, causing the micelle to lose its structure.
5.0
soluble; released from casein micelle
Flavor compounds in yogurt:
Which is most important?
Lactic acid
Diacetyl
Acetaldehyde (most important) - 40mg/kg
Acetic acid
How can using Bifidobacteria affect the taste of yogurt?
makes more acetic acid -> more vinegary taste
Inclusion of citrate positive bacteria will influence yogurt flavor in what way?
create diacetyl -> buttery taste
True/False: fresh fruit will contribute to yogurt positively
False; usually use dry/frozen/processed fruit to avoid spoilage from yeast/mold
Activia yogurt is marketed as containing a strain called _____.
Bifidobacterium animalis
