Maillard reaction

The Maillard reaction is responsible for many colors and flavors in foods. Well known Maillard reaction products include heterocyclic amines (HCAs) (including beta-carbolines), acrylamides and styrene, which may be mutagenic and/or neuroactive, or even neurotoxic and/or carcinogenic.

Maillard precursors

In cooked foods, the Maillard reaction is heat induced, but Maillard reaction products are also formed inside the human body (endogenously). In the Maillard reaction, the amino groups (or the biogenic amines, or ammonium) of amino acids (or as bound in peptides or proteins) react with carbonyl groups of reducing sugars.

• Asparagine may readily form acrylamide.
• Tryptophan, glycine, glutamine, glutamic acid and tyrosine readily form heterocyclic amines (HCAs), particularly in the presence of creatine.
• Phenylalanine may readily form HCAs or styrene.
• Lysine may readily form pent-4-en-1-amine or N(ε)-(carboxyethyl)lysine.

Various reducing sugars vary in glycation activity. Pentoses are more reactive than hexoses, which are more reactive than disaccharides.

• Pentoses: Arabinose, xylose, lyxose, ribose, ribulose, xylulose (a pentosan is a polymere of pentoses)
• Hexoses: Glucose > fructose[1], mannose, galactose, allose, altrose, gulose, idose, talose, sorbose, tagatose, psicose (a hexosan is a polymere of hexoses)
• Disaccharides: Sucrose (glucose-fructose), lactose (galactose-glucose), maltose (glucose-glucose), etc.

Maillard intermediates

Degradation of 1-deoxyhexo-2,3-diulose is a key intermediate in Maillard chemistry, yielding carboxylic acids (glyceric acid, acetic acid and many beta-carbolines; stable Maillard end-products), as well as unstable, reactive Maillard intermediates such as dicarbonyls (3,4-dihydroxy-2-oxobutanal, 1-hydroxybutane-2,3-dione, and 4-hydroxy-2-oxobutanal) and hydroxycarbonyls (acetol).[2]

Methylglyoxal (MGO) is a metabolite of glucose, which readily binds to free (or bound in proteins) arginine, lysine, and cysteine, leading to the formation of Maillard reaction end-products, including advanced glycosylation end-products. MGO is associated with hypertension.[3]

Maillard end-products

Maillard end-products include heterocyclic amines (HCAs), acrylamides, styrene, melanoidins (food-browning) and many other compounds.

Pent-4-en-1-amine

Is considered the lysine-glucose counterpart of acrylamide (instead of asparagine-glucose). In the presence of sugars, lysine, similarly to asparagine and phenylalanine, can undergo carbonyl-assisted decarboxylative deamination reaction to generate pent-4-en-1-amine. Alternatively, decarboxylation of lysine generates cadaverine (1,5-diaminopentane) followed by deamination to form pent-4-en-1-amine.[4]

N(ε)-(carboxyethyl)lysine

Incubation of bovine serum albumine with glucose yields methylglyoxal (a reactive glucose metabolite), which subsequently yields N(epsilon)-(carboxyethyl)lysine (CEL).[5] The latter is inhibited by arginine.[6] (as arginine also readily reacts with methylglyoxal) Endogenously, inhibition of aldolase B prevents accumulation of methylglyoxal and subsequent formation of advanced glycosylation endproducts (AGEs; Maillard reaction products, incl. CEL).[7]

Acetamide

Amides are derivates of ammonia or (carboxylated) amines. Acetamide is a carcinogenic derived from acetic acid, by dehydrating ammonium acetate[8], or by hydrolysis of acetonitrile[9]. Thermal degradation (>200°C) of chitin also yields acetamide.[10] Chitin is a good inducer for defense mechanisms in plants[11], and present in fungi, the exoskeletons of crustaceans such as crabs, lobsters and shrimps, in mollusks, and in the internal shells of squid and octopus. Acetamide is also a byproduct of thermochemical treatment of lignocellulosic biomass.[12]

Chloropropanols

3-monochloropropane-1,2-diol (3-MCPD) is a chloropropanol.

Furan

(Fur)furan (5-oxacyclopenta-1,3-diene or 1,4-epoxy-1,3-butadiene) is a toxic heterocyclic organic compound, readily converted to other compounds. Furan is present in coffee, canned and jarred food, and baby food.

• 5-hydroxymethylfurfural
• furosine