Effects of heating and processing

Question 1

Heating can result in …………………… and ………………….

Answer 1

Unfolding and aggregation

Question 2

Reason to heat protein preparation

Answer 2

1. Inactivation: bacteria/spores, proteases, polyphenol-oxidases, protease inhibitors.
2. Product modification : increased gelling properties.

Question 3

Effect of heat on protein preparations:

Answer 3

• Chemical modifications: maillard reactions, cross-link reactions ( fidulfide bdridge shuffling, formation of LAl) dephosphorulation (caseins)
• Physical changes: denaturation (globular proteins), dissociation(micellar proteins), precipitation of Ca-phosphate(caseins).

Question 4

Polar aminoacids outside protein , hydrophobic amino acids inside. Draw the steps of the heating of globular proteins and micellar proteins

Answer 4

Globular–> unfolding, increase the hydrophobic attraction –> aggregation –>soluble aggregates or insoluble aggregates.
Micellar caseins–>dissociation all the free molecules will be present in the water and if i cool down i can get back the same micelle. Calcium phosphate and K-casein is separated.

Question 5

HEATING IN MILK: on casein macro micelle. Not fully reversible.

Answer 5

Small micelles of alpha s1, s2 and beta casein they are cluster together an interactin with calcium phosphate. And on the outside of the macro micelle we have the kappa caseins. Heating–> dissociation of the complexes and as results when i cool down the calcium phosphate can precipitate and the micelles will reform with a different structure. Irreversible process.

Question 6

MEASURING PROTEIN UNFOLDING:

Answer 6

1. Differential scanning calorimetry (DSC): amount of heat that we need to reach the temperature. Enthalpy of unfolding (peak area) can be used to quantify fraction of `native ´proteins.
2. Precipitation at iso-electric point: transmittance (turbidity of samples). At high turbidity (soluble). If i heat for one minute a big decrease of the solubility at the pI. The solubility at the pI can be used to quantify the amount of remaining native proteins.

Question 7

SPRY DRAYING: proteins do not always denature when heating

Answer 7

Evaporation of water:
-Cools down product.
-Decreased water content.
-Increased denaturation temperature. 
Never have denaturation during spry drying, because you will always make sure the water activity is lower when the temperature of the product becomes higher (below this curve)

Question 8

The use of DSC and solubility at pH 4.6 to determine if proteins have undergone severe heat treatment. How applicable are these methods to determine the extent of heat treatment in a set of caseinates and whey protein concentrates?

Answer 8

For the whey proteins, DSC and solubility at pH 4.6 may be used to indicate the severity of heat treatment. For this you need to have a reference whey protein which is not heat treated. However, for casein sample neither method can be used since the caseins are random coil and therefore cannot really unfold . In addition, caseins are not soluble at pH 4.6.

Question 9

PROPERTIES OF HEATED PROTEINS:
Proteins: alfa-lactoalbumin, whey proteins, casein micelle and casein macro-micelle.
Mention the changes during/ after sufficient heating:

Answer 9

Changes during after sufficient heating:
1. Alfa-lactalbumin: Unfolding, exposure of hydrophobic groups, Aggregation.

2. Whey proteins :Unfolding, exposure of hydrophobic groups, Aggregation.
3. Casein micelle: dissociation
4. Casein macro-micelle (calcium bridges) : dissociation