Amor, D.R.
(2024)
Characterising the Structure and Functionality of Insect Proteins.
PhD thesis, University of Nottingham.
Abstract
Protein ingredients are added to foods not just for nutrition, but to exploit the functional properties of the protein. A key functional property of protein ingredients is the stabilisation of air surfaces, the foaming properties of the protein. Surface-active protein molecules stabilise foams by adsorbing at the interface during foam generation. The formation of a viscoelastic film which surrounds each air bubble prevents immediate coalescence of the air bubbles.
The utilisation of insects as high-protein food ingredients has gained increasing interest as a sustainable alternative to traditional livestock, as the environmental impact of rearing insects is considerably lower. This thesis examines the protein composition of Tenebrio molitor (yellow mealworm) and Gryllus bimaculatus (black cricket) and appraises the functional properties of Osborne fractions extracted from these insect species in both liquid and solid foam systems.
The protein composition of both insect species was studied, and it was discovered that, when utilising the same extraction methods, the Osborne fractional distribution of the insect species was not significantly different. Three different fractionation procedures were investigated, and it was discovered that different defatting methodologies resulted in different Osborne fractional distributions. However, when a sequential and parallel technique were compared, there were fewer significant differences. As Osborne fractionation separates proteins based upon solubility, the similarity in Osborne fractional distribution was attributed to the amino acid composition of the insects, which was also not significantly different. Regardless of extraction technique, alkaline-soluble glutelins presented as the primary Osborne fraction for both insect species.
The foaming properties of crude insect powders, defatted insect powders, and Osborne fractions were measured and compared. Removal of the lipid component from insect powders enhanced the foaming properties, due to the antifoaming nature of lipids. The order of Osborne fractions from highest to lowest foam capacity was consistent between the insect species. Infrared spectroscopy showed that the protein fractions which consisted of greater proportions of random coil structure stabilised a greater initial volume of foam, interpreted as greater foam capacity. Greater foam stability was attributed to Osborne fractions which presented lower surface hydrophobicity. Proteins which exhibited very high surface hydrophobicity were hypothesised to preferentially interact with other protein molecules rather than adsorb at the air-water surface and stabilise the foam.
The effect of insect ingredients in a solid foam system was also studied. Namely, the expansion of starch during compression popping. Increasing the inclusion of crude insect powders reduced the specific volume of popped cakes, interpreted as reduced expansion. It was shown that this was not the result of starch dilution by non-starch ingredients, but an effect of a component of the insect powders. Thermal analysis demonstrated that the addition of insect powders did not affect starch conversion, but increased lipid and chitin content upon the addition of insect powders adversely affected expansion, hypothesised to be the result of amylose-lipid complexation and the premature rupture of air cells due to fibre.
The inclusion of different Osborne fractions on starch expansion was also tested. The addition of high foam capacity and low foam capacity Osborne fractions did not affect the expansion of the starch differently. This suggested that the surface activity of proteins, although greatly affecting liquid foam systems, did not affect the solid foam system. Overall, this indicates that in the solid foam system the surface activity of protein does not play a role in the expansion of starch, and that protein does not play a role at the surface of these air cells.
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