| Summary: | This thesis deals with studies on the hydrophobicity of proteins and enzymes and is divided in four chapters summarized separately below.
(1) Traditional methods of coagulating milk for the manufacture of cheese suffer from two major drawbacks, namely, the high cost of the enzyme and the fact that they are batch systems. An obvious solution to both problems would be the use of immobilized enzymes to coagulate milk. Hydrophobic adsorption offers certain potential advantages over other techniques of enzyme immobilization. The objective of this part of the thesis was to immobilize the milk clotting enzymes chymosin and pepsin on various hydrophobic carriers and to assess their suitability for continuous coagulation of skimmilk. All enzyme-carrier preparations exhibited high initial activity on exposure to milk. However, the deactivation rates were very high. The main reason for this rapid deactivation appeared to be the loss of enzyme from the carriers, since soluble activity was detected in all enzyme preparations. The enzyme loss was due to the physical desorption of enzyme from the carriers as well as to the relatively rapid leakage of the ligand from the carrier (phenoxyacetyl cellulose). The best enzyme preparation was obtained with phenoxyacetyl cellulose. However, a study indicated that the continuous coagulation of skimmilk with proteases immobilized on the hydrophobic supports used in this study was not economically feasible.
(2) The fat binding capacity (FBC) of food proteins is an essential functional property. However, fat binding as determined by existing methods has been mainly attributed to physical entrapment of the oil rather than to the binding with proteins. A simple turbidimetric
method, thus, was developed for determining the FBC of various proteins. The turbidity was dependent on wavelength, blending time and volume of oil. The regression equation for predicting FBC was:
FBC (%) = 30.271 + 1.381 S[sub=o] - 0.014 S[sub=o] x s
where S[sub=o] and s are surface hydrophobicity and solubility index, respectively.
A highly significant correlation (R² = 0.802, P < 0.01) was found
between S[sub=o], S[sub=o] x s, and FBC of 11 food proteins tested. Advantages of
the method developed include the small amount of sample required and the fact that the measured values would reflect the true fat binding capacity of proteins by minimizing the fat-entrapping effects.
(3) The objectives of this part of the thesis were to determine
the effects of heating on the emulsifying properties of selected food
proteins, and, to assess the value of Sq as a predictor of these
properties. The results obtained indicated that the emulsifying
properties of the proteins studied were differently affected by heating,
and that heat-denaturation was not always accompanied by loss of
functionality, but, on the contrary, resulted in great improvement. The
emulsifying properties could well be predicted solely on the basis of
S[sub=o] level but not on the basis of solubility level, which indicated that S[sub=o]
is a very important property determining protein functionality. However, the emulsion activity, emulsion stability and fat binding of the proteins studied could be well explained and more accurately predicted by S[sub=o] and solubility together.
(4) The objectives of this part were to evaluate the thermal
properties (thickening, coagulation and gelation) of selected food
proteins and to assess the value of hydrophobicity as their predictor.
The results obtained indicated that the average (S) and not the surface
hydrophobicity was important for these properties. The thermal properties studied could not be explained by either the average hydrophobicity or sulfhydryls alone. Instead, they could well be predicted using average hydrophobicity and sulfhydryls together. === Land and Food Systems, Faculty of === Graduate
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