Lecture 5: Fermented Dairy Products Flashcards
what is the important of pH in fermented dairy products?
profound effect on moisture control, retention of coagulants, loss of minerals and hydration of proteins. These then have an effect on the flavour and physical properties like body and texture
what is the most significant threat to fermented milk products in the industry
phage
describe the process of lactose metabolism in dairy products
Lactococci can translocate lactose into the cell via a phosphoenolpyruvate phosphotransferase system where the lactose is phosphorylated during translocation and cleaved by phospho-B-galactosidase into glucose and galactose-6-phosphate. glucose then enters the glycolytic pathway.
what problems can galactose accumulation in the milk or cheese cause?
- cracks in the cheese
2. can react with amino groups to cause pink or brown pigment
what are the main volatile and flavour components of fermented milks?
- acetic acid
- acetaldehyde
- diacetyl
The 0.15-0.2% citric acid in milk can be metabolized to ___, ____ and ___ by certain ____
diacetyl, acetic acic and carbon dioxide by certain LAB
LAB are amino acid ___ and typically require several amino acids for growth
auxotrophs
what is the meaning of auxotrophs?
is the inability of an organism to synthesize a particular organic compound required for its growth
why are proteolytic systems important for LAB survival in milk?
free amino acids are not sufficient for LAB growth to high cell density so they must have a proteolytic system capable of utilizing the peptides in milk and hydrolyzing the milk proteins to obtain amino acids.
3 categories of proteolytic systems in LAB
- enzymes outside the cytoplasmic membrane
- transport systems
- intracellular enzymes
what is PrtP
only enveloppe associated proteinase in Lactococci. It is attached to the cell via anchor sequence. Has broad cleavage specificity. Loss of Prtp result in bacteria only capable of 10% cell density.
Prtp degrade caseins into oligopeptides
Proteolytic systems in LAB; transport of amino acids across the cytoplasmic membrane takes place via three different transport systems: name them
- amino acid transport system
- di and tri peptide transport systems
- oligopeptide system
essential for uptake of peptides from the breakdown of casein via Prtp
Oligopeptide transport system
what happens to peptides once they reach the cell?
they are hydrolyzed by a number of peptidases (AP, TP, DP, EP) to release FAA which LAB require for growth
what is a major effect of proteolysis with respect to flavor?
bitterness
explain the process of cheese making
raw milk is pasteurized then standardized and filtered. The fermenting microorganisms are then introduced as well as rennet to initate coagulation/ cuurdling process. Then the curds are cut, sitrred and cooked. milled and pressed to encourage expulsion of whey and left to ripen.
what is coagulation in cheese making?
1st pivotal step; proteins are converted into non-polar form, separate from water phase and entrap fat and minerals (curd contraction and whey expulsion_
what are the 3 different ways that coagulation can occur?
- rennet coagulated
- acid coagulated
- acid/heat coagulated
explain the structure of 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 molecule is hydrophobic which is how the micelle retains its structure. The calcium phosphate helps stabilize the micelle.
explain the process of acid coagulation?
acid coagulation occurs when LAB ferment lactose into lactic while growing in milk. as pH decrease the iso point is reached and hydrogen atoms accumulate and neutralize the polar surface of casein micelles, forcing them into micelles.
network–> entraps al the water and solid components.
soft fragile gel = coagulum.
what is particular about the curd made during acid coagulation?
- casein matrix = fragile and has limited ability to expel whey
- high water content
- susceptible to spoilage
examples of acid coagulated cheeses
cottage cheese, quark and cream cheese
explain the process of acid/heat coagulation
casein micelles in fresh milk are highly heat stable and remain soluble up to 140 deg C.
1.acid + heat = coagulation as low as (85 deg C).
- whey proteins unfold & lose ability to interact with water.
- Denatured whey proteins attach onto micellar surfaces aggregate into clusters that entrap fat globules.
example of acid/heat coagulated cheese
ricotta
acid/heat coagulated cheese are high in moisture between ___ to __% and are therefore consumed fresh
50-80%
What is rennet and rennet coagulation?
rennet = group of proteinases that cleave casein at the surface of casein micelles when added to milk. rennet cleaves kappa casein at the surface of micelles and initiates coagulation.
Difference between rennet and acid coagulation?
rennet curd is more resilient (less fragile) and better able expel whey. rennet coagulation occurs quickly.
what are the 3 objectives that must be achieved in the first few days of rennet coagulated cheese?
- the correct amount of whey must be expelled from the curd. 2.controlled rate of acidification
- salt incorporated at the correct rate
what is cutting?
breaking up the coagulum to encourage release of whey
what is cooking?
heat + stirring mixture of curds and whey to promote curd contraction. also influences rate of LAB which causes pH decrease curds to contracts and expel more whey
2 methods of seperating whey so that curd particles can fuse together?
- dripping
2. draining
Knitting
term used to describe the fusion of curds as the whey drains to produce once continuous mass of fused cheese curd
Pressing
involved application of external pressure to the curd during knitting, helps to expel whey and promotes complete fusion of the curd.
salting
the addition of salt to the cheese curd to create an osmotic driving force that draws whey to the surface of the curd where it is released
3 ways of salting cheese
- dry salt can be rubbed onto the surface of finished cheese
- cheese can be submerged in brine
- salt can be applied directly to the curd particles before knitting
finishing
green cheese to ripened cheese
2 types of ripening
interior ripening and surface ripening
what type of ripening is used in blue cheese
interior ripening
how is internal mold growth promoted in cheeses like Roquefort
by piercing the with needles at the start of fermenting to create shafts where CO2 can vent out and oxygen can diffuse in.
- these varieties are not pressed (air)
- Lab cocultures
- inoculation with P roqueforti
2 types of surface ripened cheeses
low pH and high pH (>5)
Brie and Camenbert are an example of what kind of ripened cheese
surface ripened, low pH and high humidity to promote blomy rind cheese
Starter cultures used in yogurt production
Streptococcus thermophillus and Lactobacillus bulgarius
Explain the process of yogurt making
- heat milk 42-43 deg C and inoculate with LAB
- incubate at 42 deg C (compromise temp between S thermo and L bulgarius)
- optimal pH =4.2-4.6
what happens to yogurt is pH decrease below 4.2
wheying off
what is the key to a sucessful yogurt ferment?
symbiotic relationship between mixed culture.
explain the symbiotic relationship between S thermophillus and L bulgarius
L bulgaricus; has sveral cell bound proteases (PrtP) cleave proteins into AA
L bulgaricus are utilized by S thermophillus for their growth.
S. thermophilus produces purine, pyrimidine, CO2 , formic acid, oxaloacetic acid, and fumaric acid
Formic acid and CO2 are growth factors for L. bulgaricus
explain the growth curve of S thermophillus and L bulgaricus over time
after inoculation S thermophilus grows the fastest until it accounts for 90% of the cells then release of lactic, formic and Co2 stimulate L bulgaricus which grows better at low pH. L bulgaricus produces peptides and AA’s that stimulate S thermophillus. Balance of 1:1 is then reached.
What are the 7 types of yogurt?
stirred, set, drinking, yogurt cheese, frozen yogurt, strained yogurt and dadiah
the gel like structure of yogurt relies on?
interaction between acid destabilized k-casein and heat denatured whey proteins
what is added to milk base to maintain the yogurt properties
pectin or gelatin
Taste in yogurt is determined by
- lactic acid
- acetaldehyde
- acetic acid
- diacetyl
