Grape marc, consisting of grape skins and seeds which are left over from wine processing, were composted in windrows. In order to investigate the various changes occurring in grape marc during composting, Fourier transform infrared and cross-polarization magic-angle spinning C-13 nuclear clear magnetic resonance spectroscopy as well as chemical analyses such as C/N ratio, cation exchange capacity, and crude fibers analyses were employed to bulk grape marc seeds, skins, and combined samples without extraction. The main changes in the organic matter were: (i) a decrease in C/N ratio both in the skins and the combined sample but not in the seeds; (ii) an increase in cation exchange capacity; (iii) a decrease in total polysaccharides content; (iv) an increase in lignin content; and (v) an increase in cellulose content. All the methods showed that the main changes took place in the skins while the seeds changed only slightly. The nuclear magnetic resonance and Fourier transform infrared spectra revealed an increase in aromaticity and carboxyl groups and a decrease in polysaccharides as the decomposition proceeded. These findings were verified by crude fiber analysis and changes in cation exchange capacity. This study demonstrated the feasibility of measuring cross-polarization magic-angle spinning C-13 nuclear magnetic resonance and Fourier transform infrared spectra directly on the bulk organic matter without extractions. This procedure was shown to provide useful information on the decomposition process. The resulting information was in good agreement with that derived from conventional chemical parameters of compost maturity.
