Formation of phenazines, phenoxazines, and benzoxazoles in the browning reactions of o-quinones

Quinone-induced browning is widely produced in foods and is mostly considered a consequence of quinone/nucleophile reactions. However, even in the absence of amino acids or proteins, o-quinones develop browning. In an attempt to better understand the reaction pathways involved in this browning development, this study describes the reactions of 4-methyl-1,2-benzoquinone with alcohols, ammonia, and short chain aldehydes. These reaction mixtures developed browning at 37 °C and the main produced compounds were isolated by semipreparative HPLC and characterized by NMR and MS as phenazines, phenoxazines, and benzoxazoles. A reaction pathway that explains the formation of all these compounds is proposed. The formation of phenazines is responsible, at least partially, for the produced browning, and the formation of benzoxazoles inhibits such browning. Browning development seems to be a consequence of a competition among the reactions of formation of phenazines, phenoxazines, and benzoxazoles, which appear to be produced from a single intermediate.

Addition of olivetol to crackers decreases malondialdehyde content and produces malondialdehyde-olivetol adducts

This study was undertaken to investigate the malondialdehyde-trapping ability by m-diphenols and the consequent decrease of malondialdehyde in foods. Olivetol was added to crackers, which were prepared with wheat flour and either oxidized or fresh sunflower, linseed, and camelina oils. When crackers were prepared with oxidized oils, olivetol-containing crackers contained less malondialdehyde (∼30%) than control crackers. This decrease of malondialdehyde content was parallel to the formation of malondialdehyde-olivetol adducts (250-1300 ng/g). When fresh oils were employed, storage produced more malondialdehyde (300-700%) in control than in olivetol-containing crackers. This decrease of malondialdehyde content was also parallel to the formation of malondialdehyde-olivetol adducts (10-90 ng/g). In both cases, the formation of adducts required the contribution of either formaldehyde or acetaldehyde. Obtained results suggest that olivetol not only removed malondialdehyde, but also short chain aldehydes, therefore contributing to the decrease of the content of these toxic aldehydes in phenolic-enriched crackers.

Malondialdehyde trapping by food phenolics

The reactions between malondialdehyde and 2,5-dimethylresorcinol, orcinol, olivetol, and alkylresocinols were studied in an attempt to investigate both if this lipid oxidation product is trapped by phenolics analogously to other reactive carbonyls and to elucidate the chemical structures of the produced adducts. After being formed, malondialdehyde is both partially fractionated to acetaldehyde and oligomerized into dimers and trimers. All these compounds react with phenolics producing three main kinds of derivatives: 5(or 7)-alkyl-7(or 5)-hydroxy-4-methyl-4H-chromene-3-carbaldehydes, 7-alkyl-9-hydroxy-6H-2,6-methanobenzo[d][1,3]dioxocine-5-carbaldehydes, and 4-(3-formylphenyl)-7-hydroxy-4H-chromene-3-carbaldehydes. A total of twenty-four adducts were isolated by semipreparative high-performance liquid chromatography (HPLC) and characterized by mono- and bi-dimensional nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Reaction pathways to explain the formation of all these compounds are proposed. Obtained results show that phenolics can trap malondialdehyde producing stable derivatives. The function(s) that such derivatives can play in foods remain(s) to be elucidated.

Carbonyl-trapping abilities of 5-alkylresorcinols

In an attempt to investigate the carbonyl-trapping abilities of 5-alkylresorcinols, this study describes the role of these compounds in inhibiting the formation of the 2,5-dialkylpyridines (5-ethyl-2-methylpyridine, 5-butyl-2-propylpyridine, and 5-hexyl-2-pentylpyridine) produced by 2-alkenals (crotonaldehyde, 2-hexenal, and 2-octenal) in the presence of ammonia. 5-Alkylresorcinols (as well as orcinol and olivetol) inhibited the formation of pyridines to an extend that depended on the 2-alkenal involved and the reaction conditions. This inhibition was consequence of the trapping of 2-alkenals by the phenolics. Thus, the major adducts produced between the C21:0 alkylresorcinol and crotonaldehyde were isolated and characterized by nuclear magnetic resonance (NMR) and mass spectrometry (MS). These results confirm that, in addition to their free radical scavenging abilities, 5-alkylresorcinols also trap reactive carbonyls. Because trapped carbonyls are involved in the formation of flavors and processing-induced antioxidants, 5-alkylresorcinols might be implied in some of the observed differences between whole and refined grain products.

Formation of naphthoquinones and anthraquinones by carbonyl-hydroquinone/benzoquinone reactions: A potential route for the origin of 9,10-anthraquinone in tea

The reaction of 2-alkenals (crotonaldehyde and 2-pentenal) with hydroquinones (hydroquinone and tert-butylhydroquinone) and benzoquinones (benzoquinone, methylbenzoquinone, and methoxybenzoquinone) was studied as a potential route for the endogenous formation of naphthoquinones and anthraquinones in foods. Polycyclic quinones were produced at a low water activity, within a wide pH range, and in the presence of air. 9,10-Anthraquinone formation had an activation energy of 46.1 ± 0.1 kJ·mol^-1, and a reaction pathway for the formation of the different naphthoquinones and anthraquinones is proposed. These reactions also took place in tea, therefore suggesting that the common tea pollutant 9,10-anthraquinone is also a process-induced contaminant. In fact, when four commercial teas (from a total of eight studied teas) were heated at 60 °C for 72 h, they significantly (p < 0.05) increased the amount of this toxicant. Reduction of 9,10-anthraquinone formation in teas is suggested to be carried out by reducing/scavenging its precursors.

Identification of acrolein as the reactive carbonyl responsible for the formation of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)

Reaction mixtures of reactive carbonyls and creatinine were submitted to high temperature and studied to identify the reactive carbonyl(s) responsible for 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) formation. MeIQx was produced by reaction of acrolein and creatinine within a wide pH range and with an activation energy of 81.1 ± 1.4 kJ/mol. No additional reactants were required, although methylglyoxal, ammonia, and formaldehyde also participated in the reaction. Nevertheless, these additional reactants were produced in situ from either acrolein or creatinine. A reaction pathway that both explains the formation of MeIQx and is valid for the formation of other heterocyclic aromatic amines (HAAs) with the structure of quinoxaline is proposed. Obtained results demonstrate the key role of reactive carbonyls present in foods (the food carbonylome) on HAA formation. Because creatinine is ubiquitous in proteinaceous foods, the control of the food carbonylome seems to be the key point to control HAA formation in foods.