Browning reactions hydrophilization

The hydrothermal carbons obtained in the end from soluble, non-structural carbohydrates are micrometer sized, spherically shaped particle dispersions, containing a sp2 hybridized backbone (also responsible for the brown to black color) decorated with a dense layer of polar oxygenated functionalities still remaining from the original carbohydrate. The presence of these surface groups offers the possibility of further functionalization and makes the materials more hydrophilic and well-dispersible in water. The size of the final particles depends mainly on the carbonization time and precursor concentration inside the autoclave, as well as additives and stabilizers potentially added to the primary reaction recipe. An SEM image of a model reaction illustrating this dispersion state is shown in Fig. 7.1. 

Aqueous solutions of permanganate will slowly oxidize, forming a brown film [not clear if this is Mn203, Mn02, or Mn(0)OH] on the walls of the vessel in which it is stored. Increase of either acidity or alkalinity of the solution can accelerate this decomposition reaction. This reaction has been used to treat polymer substrates prior to chemical deposition, for improved adhesion [32]. hi that work, the Mn—O film was dissolved before the CD process, and the improved adhesion was probably dne to some increase in the hydrophilic character of the mostly hydrophobic polymers. 

Amadori compounds (N-substituted-l-amino-l-deoxy-2-ketoses) are potential precursors to the formation of many of these heterocyclic volatile products. The secondary nitrogen in most Amadori compounds is weakly basic and is therefore a likely site for rapid nitrosation reactions via normal reactions with nitrous acid, under mildly acidic conditions. However, purified Amadori compounds are usually obtained only after tedious isolation procedures are invoked to separate them from the complex mixtures of typical Maillard browning systems. Takeoka et al. ( 5) reported high performance liquid chromatographic (HPLC) procedures to separate Amadori compounds in highly purified form on a wide variety of columns, both of hydrophilic and hydrophobic nature. They were able to thus demonstrate that reaction products could be followed for kinetic measurements as well as to ensure purity of isolated products. 

Although a maximum rate of formation of pyrazines occurred at approximately aw 0.75 in the NFEM sanples in this study, one cannot assume that this would be the case in all types of foods or reaction systems. Addition of glycerol and hydrophilic polymers to sugar/amino acid systems has been demonstrated to shift the aw at which visible browning will occur (29, 34). It is more likely that the rate of pyrazine formation increases up to a maximum at an intermediate aw range and then decreases again, but not necessarily at aw 0.75. 

No bubbling, damage, or break-up of the films were observed when immersed in acid as shown in Figure 8.6(a). After the specified reaction period, the films were taken out of the acid bath, washed with deionized water, and dried. The reduction with hydrohalic acid turned the color of GOl and G02 thin films from brown to black (Figure 8.6(b)), and the films lost the characteristic hydrophilicity of GO but did not disintegrate or deteriorate when suspended in water for a period as long as 14 days (Figure 8.6(c)). 

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