Consumers place a high level of importance on flavour when assessing the acceptability of food. The flavour of dairy products can be affected by heating both during processing and by consumers during food preparation. Of particular importance to the flavour of heated dairy products is the highly complex network of Maillard reactions. Previously, emphasis has been placed on undesirable flavours generated through the Maillard reaction in dairy products and efforts have been made to minimise the formation of these flavours. However, beneficial flavours are also formed by the Maillard reaction. Dairy products such as ghee are formed by heating and are characterised by their unique flavours; these flavours are generated by the Maillard reaction. The objective of this thesis was to unravel the factors that influence the Maillard and caramelisation reactions that occur during the cooking of ghee. Particular focus was placed on the impact that the structure of the cream had on the reaction, and how manipulation of these parameters could provide an avenue for the control of the Maillard reaction. The development and validation of model reaction systems for the Maillard and caramelisation reactions involved the variation of parameters including cooking time, temperature, pH, phosphate buffers and salt. A group of eleven compounds including acetic acid, furfural, 2-acetylfuran, butyrolactone, 2(5H)-furanone, furfuryl alcohol, maltol, 2- acetylpyrrole, hydroxymaltol, hydroxymethylfurfural (HMF) and dihydro-4-hydroxy-2(3H)- furanone (DHHF) were monitored to determine the impact of these parameter changes. These results provided a starting point to assess the impact of food structure on these reactions. To assess the impact of food structure a series of matrix structures were designed starting from an aqueous matrix. The first component that was added to the aqueous matrix was fat to generate a two phase structure. Emulsion structures were then formed from the two phase structure using emulsifiers and high pressure homogenisation. Analysis of the volatile compounds formed as the matrix structures were altered was carried out using headspace solid phase microextraction/ gas chromatography mass spectrometry (SPME/GCMS). Results indicated that fat is a key structural component in flavour generation via the Maillard reaction. This could have implications for low fat foods where the flavour developed during cooking is important. The addition of fat indicated a significant impact on the Maillard reaction with a less significant impact seen on the caramelisation reaction. The formation of two emulsions with inverted structures provided a means to alter the ratio of volatile compounds in the cooked samples. The oil in water emulsion generated a volatile compound profile similar to that of the fat containing matrix, whereas the water in oil emulsion produced a different ratio of these same compounds. The results reported in this thesis shed some light on the relationship between food structure and flavour formation during the cooking of milk fat emulsions. These structures will create future opportunities to manipulate the structure of food to control flavour formation.

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Proximate analysis of the commercial shrimp meat waste samples indicated the presence of 94.6% protein and 4.2% fat on a dry basis. The HPLC analysis revealed the presence of 17 amino acids (proline most abundant) and 7 sugars (ribose being the most abundant). The changes in the concentration of the amino acids and sugars after heat treatment, are explained based on their interaction through the Maillard reaction and by the thermal hydrolysis of proteins and polysaccharides present in tissue. Carotenoid pigments ($ beta$-carotene, canthaxanthin, astacene, and astaxanthin monopalmitate) were extracted and separated by thick layer chromatography. Their structures were identified tentatively by thick layer chromatography (TLC) and subsequently confirmed by Fourier transformed infrared (FT-IR) spectroscopy. Lipids were separated and categorized into glycerides, sterols, and phospholipids by thin layer chromatography while flavor-active components were extracted by different organic solvents into acidic, basic, and neutral fractions and their sensory properties determined. The flavor profile of the raw and cooked shrimp meat waste revealed the presence of 59 compounds in raw shrimp compared to 44 in cooked. The components identified were of different functional groups, including fatty acid esters, long chain alcohols, aldehydes, ketanes and hydrocarbons. Many fractions provided desirable aroma (nutty, fruity, floral, green woody, meaty) indicating the presence of important flavor compounds in the commercial shrimp waste. / Model studies were carried out under different experimental conditions (water content, pH), to observe the influence of these parameters on the formation of Maillard reaction products. Furthermore, a comparative study between direct injection of the aqueous solutions and its organic extracts was conducted. The direct injection of aqueous solutions revealed the presence of many water soluble components that were not detected by organic solvent extraction such as droserone and 3,9-diazatricyclo$ lbrack7.3.0.0 sp{*}.3 sp{*}.7 sp{ *} rbrack$-dodecan-2,8-dione while maltoxazine and acetanilide are example of compounds identified only in the dichloromethane extracts. Results also demonstrated that monosaccharides affected the yield of the compounds produced whereas the amino acids influenced the sensory properties of these models. Fructose and ribose were found to be the most reactive sugars in their respective categories. Thiazoles were found to be more water soluble whereas $ beta$-carbolines were more abundant in the organic extracts. The influence of water content was investigated in model systems containing fructose or glucose. It was found that water free systems decreased the yield of pyrazines and thiazoles while the formation of pyrrole and pyridine derivatives increased under dry heating conditions. The formation of Maillard reaction products were studied under different pH conditions in order to assess the effect of pH on the formation of heterocyclic compounds. Pyrazine and furan derivatives were the most affected by pH and alanine was found to be a better nitrogen source than lysine in the formation of pyrazines under different pH conditions. Finally, based on energy minimization studies using MM2 force field, a plausible mechanism was proposed for the formation of maltol in Maillard model systems.

The process by which foods are browned during baking and roasting is attributed to the Maillard reaction. The interaction of the $ alpha$-hydroxycarbonyl moiety of a reducing sugar with an amino compound can result in a complex series of changes. The identification and isolation of the key intermediates, known as the Amadori rearrangement product (ARP) and Heyns rearrangement product (HRP), can provide a greater understanding of the browning process. / Fourier transform infrared (FTIR) analysis of 1-hydroxyacetane provided qualitative and quantitative information of the behavior of this $ alpha$-hydroxycarbonyl compound in various aqueous and non-aqueous solutions. / The carbonyl peaks (in the 1750-1700 $ rm cm sp{-1}$ absorption region) due to the keto and aldehydo forms of 1-hydroxyacetone (acetol) in the pure state and in deuterium oxide $ rm(D sb2O)$ were assigned. Upon addition of the acid-base catalysts (triethylamine, 5% NaOD and 5% DCl) additional peaks were detected in the alkene region (1700-1650 $ rm cm sp{-1})$ due to the formation of enediols by enolization. The examination of analogous hydroxycarbonyl structures (1-hydroxy-2-butanone, glyceraldehyde, glycoaldehyde and dihydroxyacetone) provided the means to confirm the assignments of the carbonyl and enediol bands. / The integrated intensity of the carbonyl peak of 1-hydroxyacetone centered at 1720 $ rm cm sp{-1}$ was determined for dilute solutions in $ rm D sb2O.$ The integrated molar absorptivity of the carbonyl band was calculated to be 3674 L/mol/cm. In addition, the effect of concentration and temperature on dimer dissociation was investigated. The effect of solvent and temperature on enolization was also studied. Time run analysis of the carbonyl-amine reaction of 1-hydroxyacetone with pyrrolidine provided the basis for a kinetic study of the rearrangement process in the early stage of Maillard reaction.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references (p. 162-202). Also available in print.

Würzburg, Univ., Diss., 2001. / Erscheinungsjahr an der Haupttitelstelle: 2001

Th. doct.-ing.--Sci. agron.--Nancy--I.N.P.L., 1980.

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