Biomedical Engineering Reference
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of concentrations and heating times. In relation to this mechanism, Grif
n and Grif
n
(Grif
n et al.,
1993
) and de Kruif et al.(
1995
) proposed that
the aggregation process occurs in two stages. First, a primary linear aggregate is
formed, followed by more general aggregation of these species. The work by Gimel
(Gimel et al.,
1994
), however, suggests that the primary aggregate is globular rather than
linear, with a size independent of the initial protein concentration. This globule then
forms fractal aggregates.
To summarize, a number of workers have tried to establish more speci
n and Grif
n,
1993
; Grif
c assembly
mechanisms, especially for
-Lg. However, the techniques employed, including neutron
and light scattering, valuable though they are (Nicolai and Durand,
2007
), can only
measure such parameters as strand thickness, and cannot address the most signi
β
cant
aspect, particularly for this volume, which is why and how the linear segments branch or
intertwine to give rise to the structure necessary for gel formation. Complications are due
to a number of additional factors, and the precise balance of structural pre-gel inter-
mediates appears controversial. In this respect, and for these systems, lower-resolution
techniques such as TEM and optical microscopy have, in our view, proved more useful.
While the gelation of animal globular proteins, particularly those from milk, has been
studied widely, until recently plant (particularly soybean) protein gelation has been less
extensively investigated, although work carried out on glycinin (11S) and
β
-conglycinin
(7S) soy proteins (Nagano et al.,
1994a
,
1994b
) found that the FTIR band at 1618 cm
−
1
,
normally associated with
-pleated sheet, begins to increase around 80°C for 11S and
around 65°C for 7S. In previous work, it was not possible to measure FTIR in aqueous
solution because of strong interference from water absorption, but in this case the protein
solution in D
2
O was set between two CaF
2
discs using a spacer. These results were
interpreted to suggest that heat-induced gels of glycinin and
β
β
-conglycinin are formed
just like the milk proteins, by the formation of intermolecular
β
-sheet. In FTIR measure-
-sheet peak at 1620 cm
−
1
ments for heated 7S in the gel state (Nagano et al.,
1994a
), the
β
was found to increase with decreasing pH, consistent with
findings of Clark et al.(
1981a
,
1981b
) for the gelation of other globular proteins, including BSA, insulin and lysozyme.
More recently, Samoto et al.(
2007
) reported that earlier descriptions of soybean
protein were insuf
cient and, in addition to the previously known 11S (glycinin) and
7S (
-conglycinin), they reported the existence of a new protein portion which they
termed LP (lipophilic protein). They found that LP itself had no strong gelling ability but
it interfered with the gelling behaviour of 7S and 11S.
β
9.2.2
Rheological measurements
Rheological properties of these systems have been carried out by a considerable number of
workers. Before around 1980, such measurements were often made using home-designed,
sometimes crude, equipment. Experiments reported the dependence of the gels
'
modulus
'
'
on concentration (Clark and Ross-Murphy,
1987
) or the properties of
gels
under various conditions of pH, ionic strength (as NaCl concentration) and BSA concen-
tration (Richardson and Ross-Murphy,
1981
). The essential conclusions were that (a) for a
given set of gelling conditions, a critical gel concentration could be established, although
fully cured
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