Thermal degradation of 1-amino-1-deoxyketoses and their role in flavour development : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University

Abstract

Sugars undergo caramelisation reactions at relatively high temperatures but when amino compounds are present, Maillard browning reactions are possible and these occur under less severe conditions. The reaction conditions and the basic character of the amino compounds determine the range of flavour compounds formed. The first step during Maillard browning is the condensation of a reducing sugar with an amine to form a glycosylamine and this compound may then undergo the Amadori rearrangement to form a 1-amino-1-deoxyketose. The pyrolysis of two 1-amino-1-deoxyketoses (1-deoxy-1-glycino-D-fructose and 1-β-alanino-1-deoxy-D-fructose) was studied in this investigation to examine their participation in a low energy route to aroma formation. Thermal analysis and parallel chemical investigations showed that the formation of these Amadori compounds facilitates the thermal degradation of their sugar and amino acid moieties. In addition increased quantities of various aroma compounds are produced, compared with the controls. In particular, the toxic compound protoanemonin is formed and a degradation pathway leading to its production is proposed. Most of the work involving the elucidation of degradation pathways during Maillard browning have involved studies in aqueous systems. Browning reactions between glucose and amino acids were also observed during heating in the dry-state in this study. These reactions are very vigorous once initiated and this precluded the study of a glucose plus amino acid control by the techniques used to study the pyrolysis of the 1-amino-1-deoxyketoses. Such reactions occur at temperatures below those required for the thermal degradation of the corresponding Amadori rearrangement compound thus questioning the involvement of these compounds in the lowest energy thermolysis pathway in the absence of moisture. The results of experiments designed to investigate the role of Amadori compounds during the browning of sugar-amino acid systems in the dry-state demonstrated however that the reactions reported to occur in aqueous systems can also account for the dry-state processes at temperatures up until the spontaneous decomposition of the 1-amino-1-deoxyketose can occur. That the 1-amino-1-deoxyketose does not brown by itself or in the presence of glucose as readily as a glucose plus amino acid system is presumably a basicity effect. The stronger base (the amino acid) may promote a solid-state enolisation of the glucose and hence initiate browning at a somewhat lower temperature. The results of these experiments also demonstrate the stability of the 1-amino-1-deoxyketoses and show that their formation is not a rate-limiting step during browning. In the third section of this thesis the effect of changing the amine moiety on the degradation pattern of 1-amino-1-deoxyketoses is assessed. Previous research has indicated that glucose by itself and Amadori compounds formed from weak primary bases degrade via an initial 1,2 enolisation step to form mainly 2-furaldehydes and pyrrole derivatives while 1-amino-1-deoxyketoses containing a strong basic moiety (usually formed from a secondary base) degrade via a 2,3 enol intermediate and give rise to fragrant caramel aroma compounds. Several 1-amino-1-deoxy-ketoses were prepared using primary and secondary bases covering a range of pkb values. These compounds were pyrolysed and their decomposition characteristics monitored by thermal analysis methods. Parallel analysis of the volatiles produced and a comparison of the results from previous investigations generally endorsed the reported hypotheses on the degradation of Amadori compounds. It was found that the structure of the base and functional groups present influenced the degradation phenomena as well as the basicity. The thermal decomposition of amino acid - derived Amadori compounds for instance, did not fit into the pattern of that observed for 1-amino-1-deoxyketoses derived from other bases. The amino acid influences the degradation traits by promoting 1,2 enolisation and charring rather than aiding 2,3 enolisation similar to bases of comparable pkb.