Attached
This thesis describes the moisture biI1ding
capacities and gelatinization characteristics of several
commercial native starches and some of their derivatives.
The main techniques uEed in the investigation were
Differential Scanning Calorimetry and Scanning Electron
Microscopy.
The introduction presents a general review of
starch-water relati0~s and the significance of these
relations to food systems. A brief discussion of the aim
of t.he project is also pre sent.ed to show the relevance of
the research. 'I'he literature review attempts to introduce
the nature of water , its involvement in ce rt.ai,n forms of
food spoilage, the concept of bound wat.er and t.he COI!lmOn
methods used in t.he determination of bound water. The
chemistry, structure and properties of the starch substance
ara also considered, with brj.ef references ~n outstanding
contributions to the understanding of the behaviour of this
industrially useful cornmod Lty . The process of starch
gelatiniza·tion and the usual methods of its estimat.ion are
also described, and in doing so brief discussion of water
behav i.o o r in dough has been inel ude d .
'I'hestarch samples used in the st.udywere ana l.y sed
for their moisture, protein, fat and ash contents. EC2h
starch was ana lysed for eight metals using atomic observation spectrometry. Calcium, po t.e.s sLum and lTli:ignesimwen rc
present in all samples. J.'>. few of t.he starches contained traces of zinc and copper. Aluminium was present in the
al~~inium octinyl derivatives of both wheat and maize
starches. This metal was also found in the canna and
maize starches.
Calorimetric studies investigating the moisture
binding capacities of the starches showed native starches to
have different moisture binding capacities. Substitution,
cross-linking, oxidation, dextrinization, and acid
hydrolysis of starches resulted in lower levels of moisture
binding capacities compared with the parent starch. The
only exception was the pre-gelatinized form which could bind
more water than the parent starch. The time of mixing of
wheat flour doughs was found to have no effect on the level
of moisture bound in doughs. Sucrose was found to increase
the level of bound water in starch-water system whereas
salt had no effect'on the moisture binding level of the
starches.
Using the D.S.C. procedure, each native and
derivatised starch had a characteristic gelatinization
endotherm from which an enthalpy of gelatinization as well
as a gelatinization range for which temperatures of on se t ,
peak and conclusion were measurable. In all cases, modification reduced the qelatinization energy. A minimum level
of water in the available or free state was needed to
initiate gelatinization. Sucrose reduced the gelatinization
energy with increasing concentration, however wi h salt the
effect was complex. The geJ,atinization energy decreased
initially and then bGg~n to rise slowly with increase& salt concentration. The extent of gelatinization was found to be
dependent on both temperat.ure and time of axpo sur e ,
Studies using scanning electron microscopy showed
marked differences of granular disruption and solubilisation
between the native starches. The processes of cross-linking
were seen to retard granular disruption and dispersion of
the starch materials. Substitution increased ease of
dispersion and the _dried out gel appeared as a strandular
network. Starches which had been dextrinized or oxidized
or hydrolysed showed different behaviour again except for
the oxidized tapioca form which gave a gel st.ruccu re
similar to a substituted starch. Conditions such as
temperature of heating and also time of heating were seen
to affect each modification type differently.
In general, the substituted forms were found to
r
be disrupted and dispersed after :lower heat t.r-e a.t.mnet; t.han
the cross-linked forms with the cross-linked form being most
resistant of all to dispersion. Studies using a sedimentation procedure indicated the same situation wifh
substi tuted forms swelling g:t~eatlyand dispersing easily,
while the cross-linked forms showed limited swelling.
Electron microscopy showed that dextrinized, hydrolysed and
the oxidized forms svreLl.ed and dispers8·4 but may have
undergone some degree of retrogradation compared to the
substituted fOL~s.
