The Maillard-metal complexes share some similarities with the Maillard mixtures described above, but a few key differences exist. The Maillard mixtures generally lead to the production of dark, insoluble materials which chemically resemble the black, insoluble materials found on the Murchison meteorite (Kolb et al., 2005, 2006). On the other hand, the Maillard-metal complexes are generally water soluble
(Trusovs, 2006). The Maillard-metal complexes are formed via a multi-step process in which chelation of metal ions, such as Fe+2, Fe+3, Ca+2, Mg+2, Zn+2 and Cu+2, by the Maillard organic components occurs (Trusovs, 2006; Liesch and Kolb, 2007a).
The Maillard-metal complexes consist of fine brown or orange powders resembling finely ground cinnamon. Upon addition to sodium silicate, these complexes result in polymerization, which, in some cases, can be very rapid or which can take 20-30 minutes in others. The resulting colored gels were not rubbery like the amino acid gels, but instead were soft like the sugar and sugar alcohol gels, which will be discussed later in the chapter. These gels resembled those of the Maillard-mixtures, but were somewhat lighter in color.
As mentioned earlier, the gels resulting from the Maillard-mixtures consisted of small white chunks and powder after being processed and dried. In cases where the metal complexes contained chelated Ca2+, Mg2+, or Zn2+, the dried gels also consisted of small whitish chunks and powder. An interesting trend was observed while processing the gels containing chelated iron. These gels originally possessed a dark brown or blackish color. Upon pulverizing and washing these gels, the color changed to olive and finally a blue-gray color. Drying these gels ultimately yielded small gray chunks. A notable observation was that this trend was observed for complexes that had been formed from both Fe+2 and Fe+3 ions. The Fe+2 and Fe+3 ions are typically associated with reddish or orange colors. Thus it is interesting that both ions result in the same gray color.
Nonetheless, the isolation of these colored gels was a significant event in our research. Of primary importance is the fact that these were the only gels produced in our laboratory that possessed a distinct color after being processed and dried - readily indicating the incorporation of some type of material into the silica gel. The IR analysis did not indicate the presence of organics, which suggests that the color is due to the presence of metal ions in the gel. Therefore, this discovery is important because it suggests a method for the introduction of metal ions into silicates. This brings new light to the debate on the formation of rock coatings and desert varnish (Perry and Kolb, 2004; Perry et al., 2005), which are of astro-biological significance since their presence is suspected on Mars.
Like the gels from the amino acids and the Maillard mixtures, noticeable sol-gel-sol transformations were observed. Often these transformations were accompanied by gels that became very compact, as well as an increase in the amount of liquid present in the experimental vials. Overall, the Maillard-metal complexes appear to catalyze the polymerization of sodium silicate solution. While it appears that the entombment of metal-ions is possible, it does not appear that the organics follow this mechanism.
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