B.L. Wedzicha

Food chemistry: Acrylamide is formed in the Maillard reaction

Reports of the presence of acrylamide in a range of fried and oven-cooked foods have caused worldwide concern because this compound has been classified as probably carcinogenic in humans. Here we show how acrylamide can be generated from food components during heat treatment as a result of the Maillard reaction between amino acids and reducing sugars. We find that asparagine, a major amino acid in potatoes and cereals, is a crucial participant in the production of acrylamide by this pathway.

Acrylamide is formed in the Maillard reaction

Reports of the presence of acrylamide in a range of fried and oven-cooked foods have caused worldwide concern because this compound has been classified as probably carcinogenic in humans. Here we show how acrylamide can be generated from food components during heat treatment as a result of the Maillard reaction between amino acids and reducing sugars. We find that asparagine, a major amino acid in potatoes and cereals, is a crucial participant in the production of acrylamide by this pathway.

Melanoidins from glucose and glycine: composition, characteristics and reactivity towards sulphite ion

Melanoidins with Mr > 12 000 were prepared by reaction of glucose with glycine (molar ratios 1:9 to 9:1) at 55° and 90°C, initial pH 5·5. Extinction coefficients (E1%) at 450 nm were 36·9 ± 6·9 and 40·5 ± 2·9 at the two temperatures, respectively, and do not depend on reaction time or the molar ratio of glucose to glycine used to prepare the polymer. All the melanoidins had similar degrees of dehydration (c. 3 mol H2O/mol glucose) and stoichiometry with respect to glucose and glycine, but the degree of decarboxylation of glycine varied in the range 0–38% of residues incorporated into the polymer. The melanoidin prepared when [glucose] > [glycine] was more reactive towards sulphite species than when [glucose] < [glycine], and the extent of bleaching of melanoidin at 450 nm was proportional to the number of sulphur atoms incorporated into the polymer. These observations are related to the nature of the chromophores in melanoidins and the mechanism of polymerisation.

Chemistry of sulphiting agents in food

The main reason for the reactivity of sulphites in food is the nucleophilicity of the sulphite ion. The factors which determine the activity of this nucleophile are summarized and critically evaluated for concentrated systems, e.g. dehydrated foods. The distinction between free and bound sulphite is explained, and reversible binding of the additive in beverages and dehydrated foods is discussed with reference to simple theory of chemical equilibrium. The inhibition of non-enzymic browning reactions accounts for a large proportion of sulphite which undergoes irreversible reaction in concentrated foods. The mechanisms of reactions between sulphite species and intermediates in the model Maillard reaction, glucose+glycine, are considered in depth together with supporting kinetic data. An interesting feature is the fact that sulphites seem to catalyse the reactions they are added to control. Implications of this to the level of use of sulphite are discussed. Reaction products from the inhibition of Maillard browning include 3,4-dideoxy-4-sulphohexosulose which is formed initially and polymeric substances which arise from the reaction of sulphites with melanoidins. A proportion of sulphite added to food becomes converted to sulphate. Mechanisms of autoxidation are critically appraised in view of the presence of considerable concentrations of antioxidants in foods. The autoxidation of sulphite involves reactive free radical intermediates which include effective oxidizing agents. Thus, a pro-oxidant effect by the additive is possible and demonstrable in model system experiments.

A critical appraisal of the kinetic model for the Maillard browning of glucose with glycine

A critical analysis of kinetic data for the loss of sulphite species, S(IV), in the glucose-glycine–S(IV) reaction, and the browning (A470) of glucose–glycine mixtures (pH 5.5, 0.2 mol l−1 acetate buffer, 55°C) confirm the accuracy of the 3-step model (proposed by Davies, C.G.A., Wedzicha, B.L., & Gillard, C. 1997). (Kinetic model of glucose–glycine reaction. Food Chemisty, 60, 323–329). Rate constants k1 and k2 are obtained by regression of [S(IV)]-time data to the integrated rate equation for the glucose–glycine–S(IV) reaction. It is confirmed that neither rate constant depends on [S(IV)]. The integrated rate equation for the overall browning reaction is derived, and the rate constant k3 for melanoidin formation as well as the effective extinction coefficient E are obtained by regression. The value of E is confirmed for high molecular weight (Mr>3500) melanoidins by a radiochemical technique based on 14C-labelled glucose. This paper thus presents a protocol to obtain values of the significant rate constants in the browning of aldoses with amino acids.

Kinetic model for the formation of acrylamide during the finish-frying of commercial French fries

Acrylamide is formed from reducing sugars and asparagine during the preparation of French fries. The commercial preparation of French fries is a multistage process involving the preparation of frozen, par-fried potato strips for distribution to catering outlets, where they are finish-fried. The initial blanching, treatment in glucose solution, and par-frying steps are crucial because they determine the levels of precursors present at the beginning of the finish-frying process. To minimize the quantities of acrylamide in cooked fries, it is important to understand the impact of each stage on the formation of acrylamide. Acrylamide, amino acids, sugars, moisture, fat, and color were monitored at time intervals during the frying of potato strips that had been dipped in various concentrations of glucose and fructose during a typical pretreatment. A mathematical model based on the fundamental chemical reaction pathways of the finish-frying was developed, incorporating moisture and temperature gradients in the fries. This showed the contribution of both glucose and fructose to the generation of acrylamide and accurately predicted the acrylamide content of the final fries.

Investigations on the effect of amino acids on acrylamide, pyrazines, and Michael addition products in model systems.

Acrylamide and pyrazine formation, as influenced by the incorporation of different amino acids, was investigated in sealed low-moisture asparagine-glucose model systems. Added amino acids, with the exception of glycine and cysteine and at an equimolar concentration to asparagine, increased the rate of acrylamide formation. The strong correlation between the unsubstituted pyrazine and acrylamide suggests the promotion of the formation of Maillard reaction intermediates, and in particular glyoxal, as the determining mode of action. At increased amino acid concentrations, diverse effects were observed. The initial rates of acrylamide formation remained high for valine, alanine, phenylalanine, tryptophan, glutamine, and leucine, while a significant mitigating effect, as evident from the acrylamide yields after 60 min of heating at 160 degrees C, was observed for proline, tryptophan, glycine, and cysteine. The secondary amine containing amino acids, proline and tryptophan, had the most profound mitigating effect on acrylamide after 60 min of heating. The relative importance of the competing effect of added amino acids for alpha-dicarbonyls and acrylamide-amino acid alkylation reactions is discussed and accompanied by data on the relative formation rates of selected amino acid-AA adducts.

Kinetic model of the glucose-glycine reaction

The kinetics of browning (measured in terms of the absorbance at 450 nm, A450) in the glucose-glycine and 3-deoxyhexosulose (DH)-glycine reactions, and the kinetics of loss of sulphite species (S(IV)) in glucose-glycine-S(IV) reactions, all at pH 5.5 (acetate buffer) and 70°C, were used to derive, from first principles, a kinetic model for the browning of glucose-glycine mixtures. A consecutive 3-step reaction mechanism, in which glucose is converted to DH, which in turn is converted into an unspecified intermediate (I), the precursor of melanoidins, is described by the following three rate equations, d [ D H ] d t = k 1 [ glucose ] [ glycine ] d [ I ] d t = k 2 [ DH ] ( [ glycine ] + 5.1 [ glycine ] 2 ) d A 450 d t = k 3 [ I ] where k1, k2 and k3 are rate constants. The validation of this model is discussed critically. Experiments show that absorbance, in this system, varies with (time)3 consistent with the integrated rate equations, but in contrast with the previously reported dependence on (time)2.

A kinetic model for the sulphite inhibited Maillard reaction

The time-dependent concentrations of free and total sulphur(IV) oxoanion present during the sulphite inhibited Maillard reaction of glucose and glycine have been found to fit a simple kinetic model. The irreversible combination of the additive follows the scheme: In this the sulphur(IV) oxoanion does not participate in the reactions involving k1 and k2. The former proceeds at constant rate when the concentrations of glucose and glycine are both high compared with the concentration of sulphur(IV) oxoanion, whilst the latter is of first order with respect to intermediate 1. When the reaction is carried out at pH 5·5 and 55°C and the initial concentrations of glucose and glycine are 1 m and 0·5 m, respectively, k1 = 3·2 × 10−5 Mh−1 and k2 = 5·5 × 10−3 h−1. Reversible binding of the additive takes place as a result of hydroxysulphonate formation with glucose and with the final product of reaction, 3,4-dideoxy-4-sulphohexosulose. The dissociation constant of the hydroxysulphonate of the latter is deduced to be 5 × 10−3 M under the reaction conditions stated above. The model does not include hydroxysulphonate formation involving intermediates in the reaction leading to irreversible combination of the additive, and fits concentration-time data with an initial sulphur(IV) oxoanion concentration of 0·039 m to 9-% loss of the additive, the latter representing the limit of the available experimental data.

Kinetics of the sulphite-inhibited Maillard reaction: The effect of sulphite ion

Abstract The kinetics of the irreversible and reversible binding of sulphur(IV) oxospecies (S(IV)) in the system: glucose-glycine-S(IV) are reported at pH 5·5 and 55°C. The formation of 3-deoxyhexosulose (DH), a reaction intermediate, is of first order with respect to glucose and glycine, but is also catalysed by S(IV) according to: d[DH] d t = k (1+35·0[S(IV)] free ) DH is subsequently converted to 3,4-dideoxy-4-sulphohexosulose (DSH) in a process which is independent of [S(IV)]. In a system containing [ glucose ] = 1·0 m , [ glycine ] = 0·5 m and [ S(IV) ] = 0·01–0·10 m , reversible binding of S(IV) is a result of the formation of hydroxysulphonates of glucose and DSH. No evidence could be found for the formation of the hydroxysulphonate of DH which had been previously demonstrated to play a role in the reaction between DH and S(IV) when studied in isolation. The effect of pH on the rate of loss of total S(IV) is also reported. Suggestions as to the mechanism of the involvement of S(IV) in the formation of DH are made and reasons for the non-formation of the hydroxysulphonate of DH are considered.