Melanoidins are polymeric and coloured final products of the Maillard reaction, which are formed during home- and industrial-processing of foods, and are widely distributed in our diet (i.e. coffee, cocoa, bread, malt, honey, etc.). Melanoidins are responsible for the physical properties (colour, viscosity) and involved in organoleptic properties (i.e. stabilising flavour substances) in cooked and processed foods. Melanoidins shows remarkable binding properties toward low molecular weight substances in the food matrix, which are interesting for technological, nutritional and physiological point of view. This property has been recently used for the stabilisation of aroma compounds, such as furfural or pyrrole derivatives and aromatic thiols, which have overall organoleptic properties of coffee, beer and sweet wine. In the recent years, several studied have mainly been focused on the effect of melanoidins on the human diet, and their feasible nutritional, biological and health implications.
Melanoidins are formed by cyclizations, dehydrations, retroaldolisations, rearrangements, isomerisations, and condensations of Maillard reaction products, but none has been fully characterised yet. Therefore it is necessary to apply indirect strategies to assess structural differences in the melanoidin backbone, such as their ability to chelate metal ions.
Three main proposals for the structure of melanoidins have been put forward:
a.- low-molecular-weight (LMW) substances, which are able to crosslink free amino groups of lysine or arginine in proteins to produce high molecular weight (HMW) coloured melanoidins.
b.- units of furans and/or pyrroles that, thought polycondensation reactions, form melanoidins from repeating units.
c.- melanoidin skeleton is mainly built up from sugar degradation products formed in the early stages of the MR, polymerised and linked by amino compounds.
Water soluble melanoidins can be obtained easily from beverages such as coffee brew, sweet wine or dessert wine, and beer (lager, stout, etc.). However, a previous enzymatic treatment with pronase is necessary to release melanoidins structures linked to the protein backbone (named, melanoproteins).
There is three main ways for melanoidins isolation based on their molecular weight:
- bath dialysis in a cellulose dialysis tubing (i.e. 33 mm of falt width, 12.4 kDa of MWCO, Sigma).
Advantages (procedure is cost-effective)
- Adverse: time-consuming, risk of sample contamination, low yield, limited volume of sample processed
Adverse: low yield, reduced volume of sample
- ultrafiltration (sequential molecular cut-off; from 3kDa to 30kDa)
Advantages: high yield, high rate of sample processed, low contamination.
Against: low specificity (should be accompanied with HPGPC)
- browning at 420 nm and 360 nm
- hunter-lab colour (Lab scale)
- colour dilution analysis (CDA analysis)
- HPGPC. TSK-GEL 3000SW column (60 cm x 7.5 mm id). Void column volume is calculated with a standard solution of 1 mg/ml blue dextran (2000 kDa) diluted in 50 mM N-phosphate at pH 7. Detection at 420 nm.
- CZE. Melanoidins exert a partially anionic character in solution over a wide pH range. Capillary Zone Electrophoresis is usually performed on an uncoated fused silica capillary with 48.5 cm of total length (40 cm effective length), 50 µm internal diameter. Samples are hydrodynamically injected at the anionic end of the capillary. A running buffer is used 50 mM tetraborate pH 9.3. Temperature of the cartridge is maintained at 25ºC, and total separation was set at 15 min. A standard electrophoretic run was performed at a constant voltage of 25 kV with the anode in the inlet side, reaching a current of about 96 µA. Benzyl alcohol is used as marker of the electrophoretic flow (EOF) and a benzoic acid solution (1 mg/mL) as internal standard.
5.1. Antioxidant activity
The specific role of the melanoidins in the overall antioxidant activity of the MRPs has not been addressed yet. On the other hand, relation between structure, functional groups and antioxidant activity is not yet clarified.
Melanoidins may act by:
decreasing oxygen concentration
intercepting singlet oxygen
- preventing first-chain initiation by scavenging initial radicals such as hydroxyl radicals
- binding metal ions, such as iron
- decomposing primary products to non-radical products
- chain-breaking to prevent continued hydrogen abstraction from substrates
5.3. Chelating of dietary components
6. Free radical scavenging by melanoidins
7. Metal ion binding properties of melanoidins