Melanoidins

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Melanoidins are Maillard reaction products. They are responsible for food browning / food coloring and aroma. Melanoidins may have a suppressive effect on allergic reactions.[1] Key aroma compounds are often present only in trace concentrations of 1 microg/kg to 1 mg/kg. Nevertheless, they contribute to the respective flavor because of their low odor-perception thresholds.[2] In roasted coffee, the level of melanoidins correlates with the level of phenolic compounds.[3]

Precursors

As melanoidins are Maillard reaction products, they are principally formed from the reaction of amino acids and sugars. Some sugars are more likely to yield melanoidins than others. Compared to glucose, glucose-6-phosphate generates more melanoidins, 'stabilizing' deoxyosones (1-DG and 3-DG; highly reactive Maillard intermediates).[4]

Inhibitors

As melanoidins are Maillard products, compounds that inhibit the Maillard reaction, also inhibit formation of melanoidins. Odorous thiols, for example, bind to melanoidins, and, consequently, are responsible for the decrease in the sulfury-roasty odor quality observed shortly after preparation of coffee brew.[5]

Intermediates

Deoxyhexosuloses (eg 1,4-dideoxyhexosulose), formed from Amadori compounds, may give way to the formation of volatile flavour substances and melanoidins. Oligomeric carbohydrates may yield alpha-dicarbonyls such as 3-deoxypentosulose [6]

  • (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. Degradation of hexoses yields formic acid, acetic acid and the splitting off of carbon units, which (when recombining to acetaldehyde and glycolaldehyde) may render furan in the presence of alanine, threonine or serine. In the absence of amino acids, furan may also be formed under roasting conditions, from the intact sugar skeleton. The total furan levels in cooked vegetables were increased by spiking with hexoses. However, in pumpkin puree, only about 20% of furan was formed from sugars[7]
  • Furfural (an aldehyde) is an important intermediate compound of the Maillard reaction of pentose or vitamin C. Furfural may react with lysine to form furpipate.[8]
  • 2-acetylpyrrole
  • 5-methylfurfural
  • 2-carboxaldehydefuran and (from glyoxal) glycolaldehyde are by far the most effective color precursors in heated mixtures of alanine and carbohydrate degradation products. They are readily transformed into hydroxylated 1,4-dialkyl-1, 4-dihydropyrazines [9]
  • 2-methylfuran may be formed from threonine and degraded hexoses.[10]
  • 2-acetylfuran may be formed from glycine and ribose (or other reducing sugars).[11]
  • 3-amino-4,5-dimethylfuran-2(5H)-one; from pyruvic acid and glycine, or from glyoxylic acid and alanine.
  • 4-hydroxy-2,5-dimethyl-3-(2H)-furanone (Furaneol); an aroma volatile of cooked beef.[12]
  • (Norfuraneol); an aroma volatile of cooked beef.[13]
  • 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF; a caramel-like aroma compound). Formed from methylglyoxal and glycine or cysteine (depending on pH) at 120 degrees C for 1 h.[14]
  • 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone (HEMF); a fragrant reductone.[15]
  • 5-hydroxymethylfurfural (HMF; from glucose-glutamic acid) Furfurals are aroma compounds. HMF is one of the major intermediate products in the Maillard reaction, present in a wide variety of foods. It may become more mutagenic after activation.[16][17] HMF is considered the most important intermediate product of the acid-catalyzed dehydration reaction of hexoses and/or Maillard reaction. It is used as an indicator of quality deterioration in a wide range of foods. It is cytotoxic, weakly genotoxic and has tumoral effects but studies suggest that HMF does not pose a serious health risk.[18]
  • 5-(hydroxymethyl)-2-furaldehyde from glycose-glycine.[19]
  • Furosine

End-products

Common flavor compounds may have protein cross-linking properties (contributing to aging). 2-hydroxy-3-methylcyclopent-2-enone (cyclotene), which is a common food flavor, has such cross-linking properties.[20]

  • 5-hydroxymethyl-3(2H)-furanone
  • 5-(1,2-dihydroxyethyl)-3(2H)-furanone (which may further react with amines)[21]
  • 1,6-dimethyl-2,4-dihydroxy-3-(2H)-furanone (acetylformoin)
  • 4,5-dimethyl-3-hydroxy-2(5H)-furanone (Sotolone; a naturally occuring taste enhancer)
  • 3-Hydroxy-2-methyl-4H-pyran-4-one (Maltol); A naturally occuring flavor enhancer in the bark of larch tree, pine needles, and in roasted malt, produced from glucose and glycine.[22]
  • 2,3-dihydro-3,5-dihydroxy-4H-pyran-4-one
  • 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one
  • 2-Alkyl-(2H)-thiapyrans and 2-alkylthiophenes; Cows and lambs fed supplements high in n-3 polyunsaturated fatty acids will yield more 2-Alkyl-(2H)-thiapyrans and 2-alkylthiophenes (with only low odor potency) when their meat is cooked.[23]

Author

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Author of this article is Thijs Klompmaker, born in 1966