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Animal food sience - milk > Lecture 29/3 - Rennet and acid induced milk coagulation > Flashcards

Lecture 29/3 - Rennet and acid induced milk coagulation Flashcards

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1
Q

Vad gör micellen hydrophilic?

A

K-casein på ytam surface

– Negatively charged protruding tail (glycosylated)

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2
Q

Är the core of micellen hydrofob eller hydrofil?

Varför är det konstigt?

A

Core is hydrophobic

– trotts att 4 g H2O / g dry casein (mycket vatten i)

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3
Q

Vilka bindningar håller ihop micellen?

A

– Hydrophobic bonds between protein groups
– Cross-links between peptide chains by nanoclusters
– Ionic bonds

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4
Q

Varifrån utvinns chymosin?

A

Originates from calf stomach, related to stomach enzyme pepsin

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5
Q

Vad är skillnaden mellan chymosin och pepsin?

A

Unlike pepsin, chymosin cannot hydrolyze Ig in colostrum (bryter ej ner imoglobinerna)

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6
Q

Vad gör chymosin med mjölken?

  • vad klyver de?
  • varför klyvs de så lätt?
A

– Chymosin splits Phe-Met bond between residues 105-106 of Κ-casein

– Positive charge of Κ-casein in that region accounts for strong affinity to the negatively charged active site of enzyme

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7
Q

Vad händer med chymosin när man gör ost?

obs, frågar ej vad de gör

A

During cheese making some chymosin is adsorbed to para-casein
– chymosin included in the cheese curd
– amount increasing with decreasing pH

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8
Q

Vad inaktiverar chymosin?

vad hindrar inaktiveringen?

A 
Chymosin readily inactivated under certain
conditions
• Low pH and high pH
• Heat (already at 45°C)
• Salt inhibits inactivation
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9
Q

Hur mycket pepsin kan inkluderas utan att osttillverkningen påverkas?

A

Inclusion of 25% pepsin in rennet will not affect renneting, curd making or cheese ripening

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10
Q

Vilka andra enzymer klipper lika som chymosin?

A

Recombinant chymosin
– Insertion of bovine gene in DNA of microorganisms

• Porcine pepsin
– Similar to bovine pepsin but needs lowering of milk pH

• Vegetable rennets
– Moulds
– Plants

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11
Q

Vad sker i fas 1-2 i enzymatisk milk clotting?

A

1a fasen: GMP blir avklippta av enzymerna i rennet

2a fasen: micellerna har förlorat sin neg laddning och spontan aggregation sker

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12
Q

Vad sker i fas 3-4 i enzymatisk milk clotting?

A

3: gel formation - fler o fler börjar sitta ihop
4: syneresis och whey expelling

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13
Q

Hur kommer enzymet nära micellen?

A

• Enzyme approach casein micelle by diffusion

– Diffusion into the hairy layer of micelle, Phe-Met bond close to micelle surface

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14
Q

Hur många enzymer går på varje micell?

A

1 molecule of enzyme per 30 casein micelles

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15
Q

Vad påverkar reaktion rate of enzymatisk clotting

A

Various factors affect reaction rate
– pH most important
– Low pH increase affinity of enzyme for micellen

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16
Q

Hur påverkar olika faktorer bindningen mellan enzym och micell?

A

Enzym är väldigt stabilt så detta ändras en något men genom att öka ip på micellen blir bindningen bättre. Detta görs genom att sänka ph, tillsätta cacl och höja temp

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17
Q

Hur påverkar cooling, heat acid och ca2+ var ca och p finns i mjölken? (enzymatisk)

A
  • cooling och acid gör att mer ca o p finns i the whey

* heating och ca2+ drar in dessa in i micellen

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18
Q

Hur påverkar tillsatt ca2+ var ca och p finns i mjölken? (enzymatisk)

A

det drar in den till micellen

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19
Q

Hur mycket av micellerna är förlorade när GMP klippts av?
Vad blir kvar i gelen?
(enzymatisk)

A 
The glucomacropeptide (GMP; (Met
106 -) is soluble in whey, and about
1/3 of Κ-casein is lost

Para-Κ-casein (-Phe 105) is insoluble
and remains in the gel

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20
Q

När kan micellerna börjar aggregera?
Vilka krav måste uppnås?
(enzymatisk)

A

Micelles only aggregate when greater part

of hairs have been removed (70% of Κ-casein split)

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21
Q

Vad ingår i aggregation? - bindningar

enzymatisk

A

Aggregation involves
• Van der Waals attraction
• Ca2+
-Diminish electrostatic repulsion by neutralising negative charges on the micelles
-Make bridges between negative sites on para-casein micelles
-Lowering pH considerably increase Ca2+ activity

22
Q

Vilka faktorer påverkar aggregation rate?

enzymatisk

A

Factors affecting aggregation rate
– Temperature: rate increases with temperature, no aggregation below 20°C . This is because the hydrophobic bonds will be too low at low temperatures
– pH: rate increase with decreasing pH (Ca2+ activity effect). Provided that Ca2+ is not too low, little effect

23
Q

Vad sker vid gelformation (steg 3)

enzymatisk

A

At the time a gel is formed
• Aggregation has advanced so far that the aggregates occupy the whole volume

• The network becomes more and more dense as more interactions are created and the modulus of the gel increases

  • First because more paracaseinmicelles/ small aggregates are imported in the gel network
  • Later because junctions between any two micelles become stronger due to fusion of the paracasein micelles
  • Modulus at maximum when steric hindrance inhibits further interactions

• Identity between micelles disappear and
micelles ”fuse” into one unit.
– Van der Waals attraction and Ca-bridging between
caseins in micelle and also between paracasein
micelles

24
Q

När är gel modulus som högst i gelen?

enzymatisk

A

Modulus at maximum when steric hindrance inhibits further interactions

25
Q

Vad beror value of modulus på? - stelhet

enzymatisk

A

• Value of modulus depends on
– Number of junctions (particle concentration, geometry of network)
– Number and strength of bonds in one junction
– Modulus of the particles (casein particles are deformable)

26
Q

Vad innebär det att casein micellen är dynamisk?
Hur påverkar det?
(enzymatisk)

A

• The casein micelle is dynamic and allows shuffling of submicelles, re-arrangements and an increase in number of bonds in a junction between between paracasein micelles

27
Q

Hur många andra miceller kan en micell göra junctions med?
Hur många paracasein micelles?
(enzymatisk)

A

• One micelle makes junctions with on average 3 other micelles but its total area is reactive and it can make junctions with additional paracasein micelles

28
Q

Vad sker vid syneresis?

enzymatisk

A
  • Formation of junctions cause a gain in bond energy, which is the driving force of aggregation
  • Not only pores between micelles that shrink, but also micelles themselves
  • Fat globules finally set an end to syneresis due to steric hindrance
29
Q

När sker whey expulsion

enzymatisk

A

• Whey expulsion normally begins upon cutting

– May occur spontaneously but this is rare with rennet gels

30
Q

Hur påverkar cutting, preasure, pH och temp avgivandet av whey?
(enzymatisk)

A

cutting - mer yta mer avges

tryck - pressa ut whey

ph: lowering m ca: fler bryggor håller ihop gelen. till visst gräns tills den sen börjar hålla fast whey. Low med syra: som över fast ej bryggorna - jämnvikten påverkas
temp: trycker ihop, rörelse

31
Q

Hur påverkar cooling bildandet av ost?

enzymatisk

A

K-kasein får mindre verkan pga minskad enzym aktivitet, gel formation och syneres sker inte om temp är lägre ä 20 grader

32
Q

Varför minskar k-kaseinens split med för hög temp?

enzymatisk

A

Denaturerar enzymerna/minskad aktivitet

33
Q

Varför vill man inte tillsätta förmycket ca?

enzymatisk

A

förmycket ca gör gelen för hård så whey kan ej komma ut

34
Q

hur påverkar pH osttillverk?

enzymatisk

A

PH ökar aktiviteten vid k-casein klippning pga ökad net charge.
Senare så ändrar det jämnvikten med ca i/utanför micellen

35
Q

Hur påverkar gener k-kassein content?

A

Det har isolerats och testats - påverkar en hel del

36
Q

Kan all mjölk koagulera?

A

Nej det finns vissa kor som ger mjölk som har svårigheter att koagulera- vet ej varför

37
Q

Vad sker vid acid induced ost making?

A

• At low pH the micelles will lose the colloidal CaP and keep their integrity by internal salt bridges and hydrophobic interactions. The same bonds formed between particles
• The negative net charge of the micelle is neutralized by H+ at pH 4.6 (pI) and casein
precipitates

38
Q

Vad är skillnaden mellan acid och rennet koagulering?

A

lägre ph vid acid
acid gel blir mer viscös
acid har större porer
acid har ej syneresis (ingen ca bryggor, ingen gel kontraktion)

39
Q

Vad sker med whey proteingerna när man värmer?

A

Vid heating binder whey protein till casein micellen. - protein network

40
Q

Vad sker vid aggregation vid acid?

A

No ca bridges

network based on hydrofobic interactions och salt bryggor

41
Q

Hur påverkar denaturering av proteiner cheesen

A

Denatured whey proteins help to form a more extensive protein network, resulting in cheese curds with a firmer texture

42
Q

Formation of whey protein complexes
upon heating
- vad beror det på och vilka komplex bildas?

A

• Micelle bound as well as milk serum complexes of whey protein
are formed during heat treatment.
• Formation of complex pH dependent.

43
Q

Vilka possible pathways for the formation of serum complexes between K-casein and whey proteins finns

A
  1. Interaction between K-casein and whey protein takes place before dissociation
  2. Dissociation of K-casein takes place before interaction
  3. Both pathways result in higher viscosity of the gel
44
Q

Differences between rennet and acid

induced gels?

A

Rennet gels are elastic gel
• Hydrophobic interactions + calcium phosphate (CaP) bridges

Acid induced gels are viscous
• Hydrophobic interactions + electrostatic interactions

45
Q

vad inhiberar enzymatisk clotting

A
  • Enzymatic coagulation typically inhibited due to steric hindrance of enzyme - enzymet kan ej göra sitt jobb
  • Also hindrance for formation of CaP bridges
  • Pasteurization at 72-75°C, 15-20 sec OK
46
Q

Vilka koagulents finns det i osttillverkning?

A
  1. Rennets:
  2. Fermentation produced chymosin (FPC)
  3. Microbial coagulants
  4. Plant coagulants
47
Q

How can we determine the activity of

an unknown rennet?

A
  • The activity (strength) of a rennet is expressed as international milk clotting units (IMCU) per ml or g
  • The activity of a rennet can be assessed, using standardised methods, against an international reference rennet powder (=1000 IMCU/ g)
  • also the enzymatic composition plays an important role
48
Q

Hur påverkar ålder och feed av chymnosin

A

Ung och matad med mölk ger mer chymosi

49
Q

Vad är skillnaden mellan chymosin och pepsin när det kommer till ph känslighet

A

Chymosin är mindre känsligt för ph än vad pepsin är

50
Q

varför är det viktigt att veta compositionen av rennet?

A 
• Note that pepsin and chymosin
have different relative milk
clotting activites at lower and
higher pH
– Addition of CaCl2 or fermentation by
LAB will lower pH. Pepsin activity will
increase more than chymosin activity

• Therefore, important to know the
composition of a rennet

51
Q

why not measure activity of rennet

at pH 6.6?

A 
At pH 6.6, activity of pepsin
is low and therefore also
the accuracy of the
determination. Therefore
standards are described at
pH 6.5

Animal food sience - milk (14 decks)

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