is illustrated in Figure 7.14c. Here Line 1 indicates a relation that would be
observed for a single pure protein of limited solubility. Line 2 indicates a
relation for a protein mixture of which 10%is very soluble, the rest being
fully insoluble. It may be clear that the results of these tests are not
unequivocal quantities.
A second remark is that the test result may depend on conditions, such
as the manner and intensity of stirring and of centrifuging. Stirring may
disrupt large protein aggregates, but it may also cause copious beating in of
air, and some proteins become denatured and aggregated upon adsorption
onto air bubbles. Small aggregates of proteins may escape centrifugal
sedimentation, whereas large aggregates may not. Time and temperature
during the test may determine to what extent an equilibrium situation is
reached.
Finally, the result may depend on conditions during manufacture
(especially when these have caused denaturation), and the presence of other
components in the product (e.g., phenolic compounds), as discussed above.
Moreover, some of the other components may contain nitrogen (10%
nonprotein N is not exceptional), which will be reckoned as soluble protein,
if only N content is determined.
FIGURE7.14 ‘‘Solubility’’ of protein preparations as a function of pH. (a)
Turbidity (expressed as absorbancy) of solutions of a whey protein isolate, heated at
708 C for various times (indicated, minutes). (From results by S. Damodaran, see
Bibliography). (b) Solubility (percentage of protein in supernatant after centrifuging)
of various protein products: sodium caseinate (C), peanut (P), and soya (S) proteins.
(Approximate results after various sources.) (c) Solubility (as in b) of the protein in a
potato juice extract (pH¼7.0,I¼0.2 molar) as a function of solvent volume (v,
in ml). See text for lines 1 and 2. (After results by G. A. van Koningsveld. Ph.D.
thesis, Wageningen University, 2001.)