Melanoidins, free

Monosaccharides are probably involved in the browning reactions that occur during the roasting of coffee. Caramelization involving the sugars alone, and Maillard reactions, between sugars and free amino acids, produce polymeric yellow to dark brown substances, known as melanoidins. These melanoidins can be extracted into hot water, separated and characterized.105... 

In summary, it may be stated that the reaction of acyl esters of aldoses and aldobioses (see Section III, p. 92) with ammonia consists of a set of competitive pathways, including intramolecular O —> N migrations of acyl groups, deacylations, and transesterifications, with formation of aldose amides and variable proportions of the free sugar as the principal products. Significant proportions of basic or insoluble polymeric substances were not observed with aldose acetates or benzoates, although occurrence of extensive browning indicates the probable formation of soluble melanoidins. 

On the other hand, the acyl esters of 2-ketoses follow a completely different pathway, as heterocyclic, nitrogenated compounds and melanoidins are the principal products of their reaction with ammonia. The free 2-ketoses show similar behavior, but they present a significant difference in the extent of formation of these compounds, the presence of the esterifying acyl groups in the molecule being decisive in increasing their yields. 

The formation of melanoidins in yields exceeding 20% (on the basis of the free sugar involved in the reaction) is a noteworthy feature of this reaction in the 2-ketose field. Direct reaction of L-sorbose with ammonia gave only 2-3% of melanoidins, showing the special influ-... 

The formation of melanoidins amounted to a 20-25% yield (based on free sugar), whereas prolonged reaction (eight days) of ammonia with D-fructose gave only a 2.5% yield of these polymers. 

With the single exception of the stable free radical observed in melanoidin prepared from the glycine-glucose reaction ( 7 ) > there have been no reports of free radical formation in the early stages of the Maillard reaction. We have observed the... 

Cammerer et al.H2 have prepared melanoidins from glycine and a range of sugars. When prepared under water-free conditions (170 °C, 20 min), melanoidin from... 

The presence of stable free radicals in melanoidins has been known for some time Mitsuda et alm have detected a relatively stable free radical as a broad singlet by ESR in the solid melanoidin from glucose-glycine (1 h, 100 °C). Namiki and his colleagues89,90 went on to show that free radicals developed at an early stage of the... 

Electron-spin resonance (ESR) indicated free radicals in a brown polymer (mean ca 1 kDa by field-desorption MS) from a glucose-4-chloroaniline model system.185 Such free radicals (see Chapter 2) could be a source of visible colour. Exposure of the melanoidin to nitric oxide, a radical-trapping agent, diminished the ESR signal by 48% and changed the colour to red-brown. 

Scavenging free radicals may also play a role in the antimutagenicity of melanoidins. 

For a number of carbohydrate-Gly melanoidins (170 °C, 20 min, dialysed), free-radical content (EPR) and oxygen scavenging ran parallel,81 except for maltotriose, where oxygen scavenging was greater than expected on the basis of such a relationship. Yet, the overall implication is that the oxygen-consuming properties are based on a radical mechanism. 

A considerable amount of work has been done on comparing free radicals in humic substances and melanoidins by ESR,539 as well as by 13C-CP/MAS NMR spectra and <513C and <515N values. These studies have not been able to define the role played by the melanoidins, but do not exclude their participation either. 

Free melanoidins (glycine + U-14C-glucose) high-molecular-weight fraction... 

A great deal of work has been devoted to recognition of the structure of melanoidins. A wide variety of methods has been employed for this purpose. First, melanoidin shows a hyperfine structure in the e.s.r. spectrum and that means that stable free-radicals are present in caramel. Amino acids and ammonia were also detected in nondialyzable melanoidin after acid hydrol-ysis, indicating that amides are present in melanoidin. 

Dihydroxyacetone (DHA) acts as a self-tanning agent to give sunless tans. The browning action probably involves the reaction of DHA with free amino acids to form melanoidins. Melanoidins probably offer only slight UV protection. 

Indirect evidence of the involvement of free carbonyl groups in the browning of fruit juices is provided by the work of Markh (1950). He showed that apple and grape juices, when heated with added amino acids, became colored, presumably owing to formation of melanoidins. Color production could be prevented by fermentative removal of sugars or by their inactivation by reaction with sulfurous acid. 

Studies on the non-enzymatic browning of carbohydrates and amino acids evidenced that, besides ionic condensation reactions (3,4), also mechanisms involving free radical formation produce browning compounds in the Maillard reaction (5-7). Because radicals have been detected in model melanoidins (8,9), similar reactions might be involved in the formation of melanoidins during thermal food processing. 

Although extensive investigations have been perfohned for more than 35 years, the chemical nature and the formation of the free radical in roasted coffee is still unknown. The objective of the present investigation was, therefore, (i) to characterize the chemical structure of the free radical formed during roasting of coffee beans, and (ii) to study whether this radical is somehow involved in melanoidin formation. 

They fractionated the melanoidins by gel filtration to determine which molecular weight melanoidins were responsible for this loss. They found that aU of the fractions reacted with the sulfur-containing volatiles. Attanpts to free the melanoidin-bound volatiles through the addition of other free thiols were unsuccessful, suggesting that the sulfur compounds were not simply involved in disnlfide interchange as observed for proteins. Further work strongly supports the hypothesis that the thiols are covalently bound to pyrazinium ions which are oxidation products of l,4,-bis-(5-amino-5-carboxy-l-pentyl) pyrazinium radical ions. 

The importance of Strecker degradation is mainly in the formation of reactive and often sensory active aldehydes and ammonia (free or bound in reactive a-amino carbonyl compounds). Strecker aldehydes, together with a-amino carbonyl compounds, contribute significantly to the formation of many heterocyclic compounds (which are important flavour-active components of processed foods) and the brown pigments melanoidins. Aldoses simultaneously produce the corresponding 2-deoxyaldoses or 1-amino-1-deoxyalditols and 2-oxoacids. 

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