# Discoloration Mechanisms of Natural Rubber and Its Control
## 5 Discoloration Mechanism of Natural Rubber
### Non-Enzymatic Browning
#### 5.2.1 Maillard and Caramelization Reaction  
The reaction between sugars and amino groups was first described in 1908 by [PERSON], who considered color formation in the beer brewing process. In 1912, [PERSON] described a browning reaction between reducing sugars and amino groups. Maillard reaction involves the reaction between carbonyl compounds (reducing sugars, aldehydes, ketones, and lipid oxidation products) and amino compounds (lysine, glycine amine, and ammonia proteins) to produce glycosyl-amino products, followed by Amadori rearrangement. An intermediate step involves dehydration and fragmentation of sugars, amino acid degradation, etc. A final step involves aldol condensation, polymerization, and the formation of colored products.  
The condensation products between syrups and amino acids (Amadori compounds) play a role in discoloration in certain types of systems [42; 43]. Fructose-tryptophan is among the Amadori compounds, the oxygen-dependent browning of which is one of the most important factors contributing to the browning of soya sauce during storage.  
However, little or no attention has been paid to the possible involvement of these Amadori compounds in the darkening processes in NR. Tryptophan was detected significantly in the bottom fraction and _Hevea_ latex [46]. Nevertheless, there was not enough evidence to rule out the role of the Amadori compound tryptophan-fructose and several other compounds in the blackening process [34].  
That was the case until [PERSON] et al. [47] pointed out that proteins and amino acids in NR latex can react with carbonyl compounds, such as reducing sugars, aldehydes, and ketones, in the presence of heat to produce coloring substances via the Maillard reaction.  
Due to banana skin powder being a good source of nutrients, the Maillard reaction occurring in NR latex plays an important role in reducing allergenic protein and increasing the mechanical properties of NR products [48].  
[PERSON] et al. have shown that the formation of brownish compounds by Maillard reaction occurs in glucose-containing NR after vulcanization due to N-cyclohexyl benzothiazole-2-sulfenamide (CBS), a nitrogenous accelerator [49]. The possible reaction is shown in Figure 4. NR latex contains a small amount of sugar, an important prerequisite for Maillard reaction, leading to rubber discoloration.  
Both sulfur and peroxide vulcanizates containing glucose exhibited discoloration and became brown. Moreover, the brown color was darker when the glucose content was increased. Non-enzymatic browning reactions are thought to cause the color change in vulcanizates. Furthermore, the formation of brownish compounds is dependent on the glucose content. In the case of peroxide vulcanization, non-enzymatic browning reactions occur through caramelization only due to the absence of nitrogenous compounds in this system.  
On the other hand, both Maillard and caramelization reactions arise after sulfur vulcanization due to CBS, a nitrogenous compound used as an accelerator in this system. Thus, the Maillard reaction can originate from the reaction between glucose and CBS, and it is expected to be the major reaction because the Maillard reaction can occur at a lower temperature than the caramelization reaction. Based on the results in these experiments, it appears that carbohydrates in NR can also cause discoloration, especially in vulcanized products.