Mechanisms of browning development in aggregates of marine organic matter formed under anoxic conditions: A study by mid-infrared and near-infrared spectroscopy

• Relationships between FTIR spectra of organic matter with its colour by spectral crosscorrelation analysis.
• Relationships between FTNIR spectra of organic matter with its colour by spectral crosscorrelation analysis.
• Hypothesis of non enzymatic (Maillard) reactions involved in browning colour development.
• Hypothesis of enzymatic browning involved in colour development.
• Significance of hydrogen bond in colour development evidenced by two-dimensional correlation spectroscopy.

In this paper we analyze some chemical aspects concerning the browning development associated to the aggregation of marine organic matter (MOM) occurring in anoxic conditions. Organic matter samples obtained by the degradation of different algal samples were daily taken to follow the evolution of the aggregation process and the associated browning process. These samples were examined by Fourier transform mid infrared (FTIR) and Fourier transform near infrared (FTNIR) spectroscopy and the colour changes occurring during the above mentioned aggregation process were measured by means of Colour Indices (CIs). Spectral Cross Correlation Analysis (SCCA) was applied to correlate changes in CI values to the structural changes of MOM observed by FTIR and FTNIR spectra which were also submitted to Two-Dimensional Hetero Correlation Analysis (2HDCORR). SCCA results showed that all biomolecules present in MOM aggregates such as carbohydrates, proteins and lipids are involved in the browning development. In particular, SCCA results of algal mixtures suggest that the observed yellow–brown colour can be linked to the development of non enzymatic (i.e. Maillard) browning reactions. SCCA results for MOM furthermore suggest that aggregates coming from brown algae also showed evidence of browning related to enzymatic reactions. In the end 2HDCORR results indicate that hydrogen bond interactions among different molecules of MOM can play a significant role in the browning development.In this study the combination of spectroscopic techniques such as FTIR and FTNIR with Colour Indices measurements shows a peculiar ability to improve the knowledge of the complex mechanisms related to the aggregation of marine organic matter and its colour development under anoxic conditions such like those present in the marine environments at high depth.