Fermented Dairy Products Flashcards
TRUE OR FALSE: Molds, yeasts, AAB, and LAB are each used in dairy fermentations
TRUE
Which LAB (homo or hetero) is of greatest importance in the fermented dairy inductry?
Homofermentative
What is the main function of LAB
To ferment lactose into lactic acid, they are also involved in flavor development
Lactose is the primary CHO in milk, it is a disaccharide composed of ____________ and __________
glucose and galactose
Some bacteria (Lb. helveticus) are able to transport galactose and utilize the ________ pathway to metabolize it
Leloir
What are the main (3) volatile and flavor components of fermented milks:
- acetic acid
- acetaldehyde
- diacetyl
LAB are amino acid __________ and typically require several amino acids for growth
auxotrophs
TRUE OR FALSE: Free amino acids in milk are not sufficient for LAB to grow to high cell density so they must have a proteolytic system capable of utilizing the peptides present in milk and hydrolyzing milk proteins to obtain amino acids
TRUE
What are the 3 categories of proteolytic systems in LAB?
- Enzymes outside the cytoplasmic membrane
- Transport systems
- Intracellular enzymes
PrtP degrades caseins into _________
oligopeptides
What are the 3 different transport systems of AA in LAB?
- Amino acid transport system
- Di-and-tri-peptide transport system
- Oligopeptide transport system
Milk is composed of _____ water and water is a polar molecule
85%
Explain coagulation
Coagulation is the first pivotal step in cheese making, where all of the proteins are converted into a non-polar form and when this occurs they separate from the water phase though a process that entraps fat and minerals (curd contraction and whey expulsion)
What are the 3 different ways for coagulation to occur?
1) Rennet coagulation
2) Acid-coagulation
3) Acid/heat coagulation
Talk to me about casein micelles?
- Casein micelles are soluble coagulations of the casein protein
- K-casein is a protein at the surface of the casein micelle that is polar and keeps the micelle soluble
- The core of the molecule is hydrophobic, which is how the micelle retains its structure
- Calcium phosphate helps to stabilize the micelle
Explain acid coagulation
- Acid coagulation occurs when LAB ferment lactose to lactic acid while growing in milk
- As pH decreases, the isoelectric point of casein (pH 4.6) is reached, hydrogen atoms accumulate and neutralize the polar surfaces of casein micelles, forcing them into chains of micelles
- This network of casein micelles entraps all of the water and solid components
- Over the course of several hours the milk is transformed into a soft fragile gel also called a coagulum
What’s bad about the high moisture content of acid coagulated cheeses?
They are vulnerable to spoilage and need to be consumed fresh (cottage cheese, cream cheese and quark
What are the main concepts behind acid/heat coagulation?
-Casein micelles in fresh milk are highly heat stable and remain soluble up to 140C
However, if milk is moderately acidified (pH 6.2-5.4) it becomes susceptible to heat-induced coagulation at relatively low temperatures (85C)
-Coagulation occurs because whey proteins unfold, loose their ability to interact with water
-This causes the denatured whey proteins to attach onto the micellar surfaces and the micelles aggregate into clusters that entrap fat globules
-The curds are allowed to drain and in some cases are pressed
-Ricotta is a well-known example of an acid/heat coagulation; queso blanco is a pressed version
-Acid/heat coagulation cheeses are also high in moisture (50-80%) and are therefore also consumed fresh
Talk to me about rennet coagulation
- Rennet is a group of proteinases that cleave casein at the surface of casein micelles when added to milk
- Rennet preferentially cleaves the K-casein at the surface of the micelles and initiates coagulation
- Rennet enzymes can be derived from a number of animal, plant, and microbial sources
When the rennet cleave the K-casein it causes micelles to aggregate in the
form of a net like matrix. Rennet coagulation may occur at a high pH 6.6-6.3,
before any extensive acidification by starter LAB.
Why is cutting required in cheeses?
- Cutting is required to break up the coagulum and initiate separation of whey from curd
- The new surface area created when the coagulum is cut will allow the rapid release of whey
Explain knitting and pressing
1) Knitting is the term used to describe the fusion of curds together as the whey drains to produce once continuous mass of fused cheese curd
2) Pressing is closely related to knitting and involves the application of external pressure to the curd during knitting (or in some cases after knitting)
What is salting?
The addition of salt to the cheese curd creates an osmotic driving force that draws whey to the surface of the curd where it is released
What are the 2 categories of cheese ripening?
- interior ripening
- surface ripening
Explain Finishing
Finishing is the process though which “green” cheese is transformed into “ripened” cheese
Finishing requires a specific combination of environmental conditions (temperature, humidity, physical surroundings, presence of microflora, and sometimes manipulations like rubbing, scraping, turning, or washing)
Blue cheeses are an example of ____________ ripening
interior
Brie and Camembert are examples of __________ ripening
surface
Most yogurt starters have approximately equal numbers of __________ and ______________
Streptococcus thermophilus
Lactobacillus bulgarius
The starting temperature of milk used in the the average yogurt fermentation is _______ when the inoculation of LAB occurs
42-43∘C
Explain the relationship between L.bulgaricus and S.thermophilus
L. bulgaricus is known to have several cell-bound proteases (PrtP) that cleave proteins into amino acids extracellularly
The extracellular peptides produced by L. bulgaricus are utilized by S. thermophilus for their growth, the overall proteolytic abilities of S. thermophilus are much weaker than the abilities of L. bulgaricus
Tell me everything you know about S.thermophilus and L.bulgaricus (think about the growth curve)
S. thermophilus produces purine, pyrimidine, CO2 , formic acid, oxaloacetic acid, and fumaric acid
Formic acid and CO2 are growth factors for L. bulgaricus
After inoculation S. thermophilus grows the fastest until it accounts for ~90% of the total bacterial cells, then the release of lactic acid, CO2, and formic acid synergistically stimulates L. bulgaricus to grow faster
L. bulgaricus grows better than S. thermophilus at low pH
The growth of L. bulgaricus in turn produces peptides and amino acids that then stimulate the growth of S. thermophilus
At the end of the fermentation
_________ treatment and ________ are key in texture development of yogurt
heat + acidification
Explain what happens during the acidification of yogurt
1) During acidification, at pH 5.0, the calcium phosphate is completely released from the casein micelle, and the integrity of the micelle is reduced
2) The denatured whey proteins also interact with k-casein increasing hydrophobicity, consequently, when the micelles become unstable aggregation starts which leads to gelation
