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Food Microbiology > Fermented dairy products > Flashcards

Fermented dairy products Flashcards

1
Q

What type of products are dairy foods, why they are prone to spoilage?

A

Dairy - high moisture high nutrient
Homofermentive use to bring ph down as quickly as possible, so it does not spoil

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2
Q

what is the function of homofermentative LAB in dairy foods

A

The main function of LAB is to ferment lactose into lactic acid, it is not their only function, they also are involved in the development of flavor

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3
Q

Two types of LAb cultures based on temperature

A

•Starter LAB cultures can be mesophilic (optimal growth between 25-30C), or thermophilic (optimal growth between 37-42C)

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4
Q

Wh starter culture is so important in dairy fermented foods

A

1) •The key to a fermented milk product is a consistent and predictable rate of acid development
2) •The rate and extent pH decrease is critical

→pH has a profound effect on moisture control, retention of coagulants, loss of minerals, hydration of proteins

→These things in turn have an effect on the flavor of the cheese and physical properties such as body and texture

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5
Q

What is the biggest threat to fermented milk products

A

Phage

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6
Q

What are the two ways to get lactose into the bacteria cell

A

Phosphorylation of lactose → passive diffusion

Active transport and just lactose

Lactococci can translocate lactose into the cell via a phosphoenolpyruvate phosphotransferase system where the lactose is phosphorylated during translocation

•Some bacteria transport lactose via a lactose-galactose antiport system

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7
Q

What is the ultimate pathway for lactose once it gets into the bacteria cell?

A

Lactose→ in glucose and galactose or galactose-6-P

Glucose into glycolytic pathway

Galactose→ leloir pathway and then into glycolytic pathway→ lactic acid

galactose-P→ tagatose pathway→ lactic acid

  • 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
  • The glucose moiety enters the glycolytic pathway, and the galactose-6-phosphate is converted into tagatose-6-phosphate via the tagatose pathway
  • In this second case, lactose is not phosphorylated, but is cleaved by B-galactosidase to yield glucose and galactose
  • The glucose moiety enters the glycolytic pathway, but the galactose is either excreted from the cells or left to accumulate in the milk or cheese
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8
Q

What type of bacteria uses Leloir pathway

A

Lb. helveticus

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9
Q

what problems galactose can cause if it is accumulated in the medium

A
  • If heterofermentative bacteria metabolize it rapidly and produce CO2 it can lead to cracks in the cheese
  • Or residual galactose can react with amino groups causing pink or brown pigments to form
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10
Q

Aromatic components in the end of milk fermentation and how they are synthesized (mention % citric acid in milk)

A
  • Lactic acid is the main metabolite at the end of a diary fermentation, but while it is responsible for the acid taste, it is not volatile and does not contribute to aroma
  • The main volatile and flavor components of fermented milks are acetic acid, acetaldehyde, and diacetyl
  • Milk contains 0.15-0.2% citric acid and can be metabolized to diacetyl, acetic acid, and carbon dioxide by certain LAB
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11
Q

Why proteolytic systems are essential in LAB, what is auxotrophs, how the flavor is connected to it

A
  • Proteolytic systems are essential for LAB survival in milk, and are required for flavor development in ripened cheeses
  • LAB are amino acid auxotrophs and typically require several amino acids for growth
  • 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
  • The flavors of the fermentation are dependent upon which proteolytic systems were present in the LAB present
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12
Q

•Proteolytic systems in LAB can be divided into three categories:

A
  • Enzymes outside the cytoplasmic membrane
  • Transport systems
  • Intracellular enzymes
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13
Q

What system is there in Lactococci? how it is attached, specificity, what happens if lactococci misses this system, casein % and how it is related

A
  • PrtP is the only envelope associated proteinase in Lactococci
  • It is attached to the cell via an anchor sequence
  • It has broad cleavage specificity
  • Loss of PrtP results in bacteria only capable of about 10% of the cell density
  • Caseins compose 80% of all proteins present in milk, PrtP degrades caseins effectively into oligopeptides
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13
Q

Lactose content in milk

A

45-50 g/L

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14
Q

•Transport of amino acids across the cytoplasmic membrane takes place via three different transport systems:

A
  • Amino acid transport systems (AAT)
  • Di- and tri-peptide transport systems (Dtp)
  • Oligopeptide transport systems (Opp)
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15
Q

What transport system is connected to PrtP

A

•Opp is essential for uptake of peptides from the breakdown of casein via PrtP

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16
Q

Once inside the cell peptides are hydrolyzed by one of a number of peptidases:

A
  1. •Aminopeptidases (AP)
  2. •Dipeptidases (DP)
  3. •Tripeptidases (TP)
  4. Endopeptidases (EP)
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17
Q

How proteolysis and flavor development is connected?

A
  • Even where the process behind flavor development remains poorly characterized, it is generally agreed that proteolysis is essential for flavor development during ripening
  • The proteolytic enzymes present in the milk matrix determine which peptides and amino acids accumulate and how flavor develops
  • A major effect of proteolysis is bitterness, but the pathways are unknown
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18
Q

Characterize milk

A

•Milk is composed of 85% water, water is a polar molecule, because of it’s polar nature the water keeps the solid constituents of milk dispersed throught the water phase because they are either polar in nature (soluble), or they are packaged with macromolecular structures that help them interact with water

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19
Q

A first pivotal step in cheese making

A

•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)

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20
Q

There are three different ways for coagulation to occur, which give rise to three very different cheese families

A
  • Rennet-coagulated
  • Acid-coagulated
  • Acid/heat-coagulated
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21
Q

Characterize casein micelle (mention kappa casein and solubility)

A
  • Casein micells 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
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22
Q

Why there is acid when making cheese

A

•Acid coagulation occurs when LAB ferment lactose to lactic acid while growing in milk

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23
Q

Describe acid coagulation

A

•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 solid components•Over the course of several hours the milk is transformed into a soft fragile gel also called a coagulum

Acid coagulation occurs when micellar calcium phosphate (MCP) is extensively

converted to the soluble form, resulting in a matrix that is depleted of MCP

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24
Q

Examples of acid coagulated cheeses, what is the general characteristic of acid coagulated cheeses

Are they aged?

A
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25
Q

Are milk proteins susceptible to heat?

A

•Casein micelles in fresh milk are highly heat stable and remain soluble up to 140C

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26
Q

How high temperature of casein micells coagulation can be decreased?

A

•However, if milk is moderately acidified (pH 6.2-5.4) it becomes susceptible to heat-induced coagulation at relatively low temperatures (85C)

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27
Q

How acid/heat coagulation occurs

A
  • 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
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28
Q

Examples of acid/heat coagulated cheeses and their examples/ characteristics

A
  • 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
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29
Q

Rennet: Although this is similar to acid coagulation there are two important difference

A

1- Rennet curd is more resilient (less fragile), and better able to expel whey

2- Rennet coagulation occurs quickly (30 -60 minutes)

30
Q

What is rennet

A

•Rennet is a group of proteinases that cleave casein at the surface of casein micelles when added to milk

31
Q

How rennet coagulation occurs

A
  • 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.

32
Q

•For Rennet coagulated cheese three objectives must be achieved during the first few days of cheese making:

A
  1. The 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
33
Q

What cheeses come out of rennet coagulation

A
  • Rennet Coagulated cheese can be dried and permitted to age
  • However, they are much more complicated to make than acid- or acid/heat coagulated cheeses
34
Q

what is cutting? How it is done, why

A
  • 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
  • Cheese makers have developed a variety of cutting utensils and techniques, but you can also just use a knife at a 45 degree angle
  • The LAB are trapped in the matrix of the curd and are not lost with the whey
35
Q

After curdling and cutting, what step

A

Cooking

36
Q

Why cooking is done, its role

A
  • Cooking involves the application of heat and stirring to the mixture of curds and whey
  • Cooking to a higher temperature, for a longer duration and with more stirring promotes curd contraction
  • Cooking also influences the rate of lactic acid production by LAB, which also causes the curds to contract and expel more whey as the pH decreases
  • Therefore, cooking and cheese moisture have a complex relationship

Different cooking→ different cheeses

37
Q

•The curd must be separated from the whey so that curd particles can fuse together and form a larger entity that will become the cheese: methods

A

Dripping: where curd-whey mixture is scooped out and placed in a draining vessel (ceramic sieve, wicker basket, or actual plastic draining mold)

Draining: were there is a fitted valve or a strainer in the vessel used for coagulation and the whey is simply drained off

38
Q

How curd separation from whey is performed in industrial practice

A

•In industrial practice the curds are pumped from the vat onto a draining table fitted with a perforated base that drains the whey and retains the curd

39
Q

What is knitting

A
  • 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
  • During knitting the curds continue to expel whey in a temperature dependent manner (higher temperature equals more whey expulsion)
40
Q

What is pressing? what is unpressed cheese, what devices can be used and why

A
  • Pressing is closely related to knitting and involves the application of external pressure to the curd during knitting (or in some cases after knitting)
  • Some cheeses can be referred to as “unpressed” because gravity draining alone is used to knit the curd particles together, pressing helps to expel the whey and promotes a more complete fusion of the curd particles
  • Pressing devices capable of high pressure, that produce very closed and compact textures in the cheese are required for the entrapment of microbiologically produced gases (i.e. the holes in swiss cheese), or the tough and impervious rinds on large brine-salted cheeses
41
Q

why do salting

A
  • 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
  • Thus salting is yet another step to expel moisture from the cheese, the greater the uptake of salt the greater the release of whey
42
Q

Three ways of salting

A
  1. Dry salt can be rubbed onto the surface of the finished cheese
  2. The cheese can be submerged in a concentrated salt brine (large cheeses)
  3. Salt can be applied directly to the curd particles before knitting
43
Q

What is done after salting?

A
  • 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)
44
Q

Two types of ripening depending on the region of the cheese

A
  • There are two distinct ripening zones the body (interior of the cheese) and the surface, from a microbiological perspective these represent radically different environments
  • Interior ripening involves obligate anaerobes such as molds cannot grow in the cheese interior unless the exterior is broken
  • Surface ripening is that takes place at the cheese surface, since the molds readily have access to an aerobic environment. The inadvertent or deliberate additions of mold are made to the surface
45
Q

Example of interior ripened cheese

A

Blue cheese

46
Q

Type of organism to make Roquefort cheese

A

P. roqueforti

47
Q

Describe the process of making Roquefort cheese

A
  • Require mold growth throughout the anaerobic cheese
  • For this reasons cheeses such as Roquefort promote internal mold growth by piercing the cheese with needles at the start of ripening to create shafts where carbon dioxide can vent out and oxygen can diffuse in•

These varieties are also made without pressing to that the structure is not impermeable to air

  • Also other LAB are often used as co-cultures to enhance the open texture of the cheese through production of carbon dioxide gas
  • Milk is generally inoculated with P. roqueforti prior to adding the rennet
  • Blue cheeses must also be turned regularly to prevent moisture accumulation and rind rot on one of the sides
48
Q

What are all the steps of making cheese- flow chart

A
49
Q

•Surface ripened cheeses include two groups

A
  1. Low pH at surface (pH<5)
  2. High pH at surface (pH>5)
50
Q

What are the examples of low pH surface ripened cheeses

A

Brie and Camembert

51
Q

Microorganism to make brie and camembert

A

The mold (Penicillium camemberti)

52
Q

How Brie and camembert are made (mention humidity)

A
  • Low pH surfaces and high humidity produce the right conditions for mold growth favoring bloomy rind cheeses such as Brie and Camembert
  • The mold (Penicillium camemberti) can be added before renneting or sprayed onto the surface before ripening
  • P. camemberti rapidly catabolizes lactic acid and results in deacidification of the cheese surface (making the cheese prone to contamination by some bacteria that produce orange or yellow pigments)
  • Well made Camembert will be white and only white
53
Q

Why cheeses made from non-pasteurized milk is not recommended for pregnant women

A

To avoid lysteria

54
Q

Maxx temperature to cook cheese at

A

42C, otherwise bacteria will be killed

55
Q

Why finishing cultures are added at the very first step

A

Finishing culture at the very first step, before the rennin, because you do not want to be pushed out during whey expulsion

56
Q

How parmesan cheeses are aged?

A
  • Cheeses are then aged in warehouses operated by consortia
  • They are aged for different amounts of times some are aged for up to 30 months
  • While aging, they are turned and rubbed by large robots that are controlled by a computer.
57
Q

•The starting temperature of milk used in the the average yogurt fermentation is ___ when the inoculation of LAB occurs

What is the incubation temperature

A

42-43∘C

•Yogurt is generally incubated at 42C which is compromise between these two microorganisms (S. thermophilus 37C, and L. bulgarius 45C)

58
Q

How inoculation culture for yogurt can be achieved?

A

•Inoculation can be achieved by either growth the cultures on-site, or incorporating concentrated freeze-dried or frozen cultures (the latter is the most common)

59
Q

Why the end point is important for yogurt? Final pH, what if it goes lower

A
  • The determination of the end point is of critical importance to the textural properties of the yogurt
  • The optimal water retention pH is 4.2-4.6. If pH decreases below 4.2 wheying off will occur
60
Q

Types of microorganisms in starter culture for yogurt

A
  • Most yogurt starters have approximately equal numbers of Streptococcus thermophilus and Lactobacillus bulgarius
  • Some will have additional microbes included
61
Q

Starter cultures used for yogurt in our labs

A

S. thermophilus, L. bulgarius, Bifidobacterium infantis, B. longum, and B. bifidum

62
Q

Why a mix of microorganisms is used in starter culture?

A

•The key to a successful yogurt starter is producing an obligate symbiotic relationship where the acid and flavor produced by the blend of the mixed culture is considerably higher than by either of the two organisms separately

63
Q

How symbiosis work between bacteria in the starter culture for yogurt?

A
  • 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
  • However, S. thermophilus has several peptidases that can hydrolyze the intermediate products from L. bulgaricus, which is important in their symbiosis
64
Q

why the growth of bacteria is like that for yogurt?

A
  • 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
65
Q

The end concentration of microorganisms in yogurt and pH

A

•At the end of the fermentation, a balance of almost 1:1 of the organisms will be reached. The pH will be 4.2-4.3, and the cell count will be 2x107 cells/mL for each starter organism

66
Q

Why yogurt has such a texture

A
  • The ”gel” like texture of yogurt relies on the interaction of between acid destabilized k-casein (just like in cheese) and heat-denatured whey proteins
  • Depending on the environmental conditions of the fermentation, yogurts with different physical properties can be obtained•Heat treatment and acidification are key in texture development
67
Q

How heat and acid influences yogurt texture

A
  • During acidification, at pH 5.0, the calcium phosphate is completely released from the casein micelle, and the integrity of the micelle is reduced
  • The denatured whey proteins also interact with k-casein increasing hydrophobicity, consequently, when the micelles become unstable aggregation starts which leads to gelation
68
Q

What stabilizers can be added to yogurt and why

A

•Stabilizers, such as, pectin or gelatin, are often added to the milk base to enhance or maintain the appropriate yogurt properties including texture, mouthfeel, appearance, viscosity/consistency and to the prevention of whey separation

69
Q

•Taste in yogurt is determined by:

A
  • Lactic acid
  • Acetaldehyde
  • Acetic Acid
  • Diacetyl
70
Q

How flavor is developed in yogurt?

A
  • Acetaldehyde is probably the most important contributor to taste and LAB have a variety of pathways to produce acetaldehyde
  • Acetaldehyde levels in yogurt can reach 40 mg/kg
  • Bifidobacteria produce a little more acetic acid than other LAB and will produce a more ”vinegary” taste
  • Diacetyl is created via citrate metabolism and gives a buttery flavor
71
Q

the most popular yogurt sold in Western societies

A

Stirred fruit yogurt

72
Q

Is fresh food is sued in stirred fruit yogurts and why

A
  • In the manufacture of fruit yogurt, fresh fruit is not used due to a high risk of spoilage, processed fruit is usually used, but frozen fruits are available to “all-natural” yogurt processors
  • Advantages to using processed fruits in yogurt are:
  1. •Entirely free from yeasts and molds
  2. •Level of color can be adjusted•
  3. Degree of sweetness can be adjusted by adding sucrose
73
Q

What is so special about acitvia?

A
  • Is a brand of yogurt that is classified as a functional food, and is designed/ marketed to improve digestive health
  • In the 1980’s Danone researchers began investigating Bifidobacteria and eventually developed a strain able to survive the lactic acid in yogurt Bifidobacterium animalis DN 173,010
  • This strain is proprietary and is marketed under the names: Bifidus Regularis, Bifidus Actiregularis, and Bifidus Digestivum

Food Microbiology (19 decks)

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