Slide 1

Ong, Law - Hygrothermal Properties of FVFs

Figure 2.3. Colour degradation of paprika during thermal treatment (a) kinetics of deg-

radation at various aw and (b) activation energy as a function of aw (Topuz, 2008)

2.2.3. Proteins

Proteins (also known as polypeptides) are basically polymers of amino acids. Exam-
ples of some important amino acids that are essential for humans are phenylalanine, va-
line, threonine, tryptophan, isoleucine, methionine, leucine, lysine, cysteine, tyrosine,
histidine and arginine (Reeds, 2000; Young, 1994). Heating protein foods can cause loss
of nutritive value due to several deteriorative reactions that occur with the presence of
heat and water, for instances, amino acids destruction, protein denaturation and Mail-
lard reaction. Desrosiers et al. (1987a) reported that heat treatment do not significantly
affect amino acid profile of protein (except some reactive amino acids such as tyrosine,
histidine, lysine and proline) but could change protein digestibility through conforma-
tional changes in the protein structure. The digestibility of individual amino acid in pro-
tein could be affected in different way during heating. For example, digestibility of most
amino acids in diafiltered whey protein may increase when heated at 75°C or 100°C
whilst the digestibility of cysteine, valine, methionine and phenylalanine remain the
same (Desrosiers et al., 1987a). The increment of the digestibility of the amino acids
may due to an unfolding of protein under the mild heating (60°C to 100°C), which fa-
vours the formation of an enzyme-substrate complex and allowed a more rapid cleavage
of peptide linkages. Furthermore, the physical state of water in foods also affects the

(a)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300

Time (min)

Total colour difference, C-C

/C∞

-C 0

∞

aw = 0.459

aw = 0.582

aw = 0.703

aw k (1/h)

0.459 0.018

0.582 0.024

0.703 0.036

(b)

99

101

103

105

107

109

111

113

115

0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75

aw

Ea

(kJ/mol)