Inhibition of Maillard reaction pathways and off-flavor development in UHT milk: structure reactivity of phenolic compounds

Abstract

The Maillard reaction, a carbonyl-amine reaction, is an important food and biological reaction. In food, this reaction is known to impact color and flavor, alter the nutritional content, as well as generate therapeutic and toxic compounds. In biology, Maillard derived reactive carbonyl compounds, generated in vivo or from dietary exposure, have been associated with several pathological conditions. Consequently the ability to control the pathways/products of this reaction would be beneficial to the food and health related industries. Recently phenolic compounds have been demonstrated to suppress the development of Maillard off-flavor compounds and browning in foods, as well as to reduce the levels of Maillard-derived reactive carbonyl species (RCS) and related advanced glycation end (AGEs) products in biological systems. The overall goal of this thesis was to build on this prior work by providing a more in-depth mechanistic study of phenolic structure-reactivity on the pathways of the Maillard reaction and product formation, specifically in UHT processed bovine milk during both thermal processing and storage. In the first phase of this project, five phenolic compounds at an equivalent dose of 1.7mM were examined for structure-reactivity relationships (catechin, genistein, daidzein, 1,2,3-trihydroxybenene, and 1,3,5-trihydroxybenene) on Maillard pathways and product generation. Levels of transient Maillard reaction precursors (C2, C3, C4, and C5 α-dicarbonyls and α-hydroxycarbonyls) and select off-flavor markers (methional, 2- acetyl-2-thiazoline, 2-acetyl-1-pyrroline) were quantified by LC/MS/MS and GC/MSTOF, respectively; stable isotope surrogates were utilized as internal standards. In general, the addition of phenolic compounds prior to UHT processing significantly reduced the concentration of MR precursors and off-flavor compounds compared to traditional UHT sample (p<0.05) and particularly after storage. However phenolic compounds with a more activated ring structure for aromatic electrophilic substitution reactions (1,3,5-trihydroxybenzene, catechin and genistein) were more reactive at suppressing Maillard pathways. Albeit unique structure reactivity was also noted among the different phenolic compounds analyzed. Sensory studies were in agreement with the analytical data; lower cooked flavor intensity was observed for the off-flavor recombination models of the catechin treated UHT milk (versus the control UHT milk). Furthermore consumer acceptability of the catechin treated UHT milk was rated significantly higher than the control sample (Fisher's LSD = 0.728) showing improved palatability. In the final research phase, the application of Response Surface Methodology (RSM) for process optimization was evaluated. A Box-Benhken 3-factor (catechin, genistein and diadzein) 3-level (0.17, 0.645 and 1.12mM) design was employed and dose-response relationships of phenolic compounds/mixtures (added prior to thermal processing) on the levels of reactive carbonyl species (RCSs; glyoxal, methyglyoxal and 3-deoxyglucosone) in UHT milk were examined. In general, a range in RCSs reduction was observed, with the most reactive phenolic mixtures reporting levels of RCS in UHT milk at or below those of pasteurized milk. Predictive models, with no significant lack of fit (p>0.05), high r2-values (0.886-0.979) and good predictive power, were developed. ANOVA analysis for glyoxal levels reported only significant (p<0.05) linear effects of each factor indicating no significant interactions between the different phenolic compounds were observed. However for the levels of methylglyoxal, linear, cross product and quadratic effects were reported (p<0.05), indicating more complicated interactions existed. Similarly significant linear and quadratic effects (p<0.05) were also reported for 3-deoxyglucosone. Overall, based on canonical analysis, catechin was reported to be the most influential phenolic compound for the reduction of RCSs in UHT milk. The observed unique reactivity noted between select phenolic compounds and the suppression of the three RCS furthermore indicates the implementation of different phenolic structures can alter the appropriate dosage for tailored mixtures of phenolic compounds used for various food systems and processing conditions. In summary, phenolic structures with a more activated ring structure for electrophilic aromatic substitution reactions were the most effective at reducing the levels of RCS and off-flavor compounds in UHT milk, particular during storage. The utilization of phenolic compounds as a pre-processing ingredient was demonstrated to improve the quality of UHT milk by increasing the palatability and simultaneously reducing the dietary load of these potentially toxic RCS compounds. Furthermore, statistical RSM provided an improved basis to understand phenolic structure reactivity to control Maillard chemistry as well as to optimize the dose-response and therefore ingredient cost for effective utilization in UHT milk.