# Greenhouse Residues' Potential for Biogas Production
## 4 Discussion
The VS content of cucumber residues (75.7% TS) was similar to the results of the present study (71.6-83.8% TS), but, as regards the tomato residues, the VS content (82.9% TS) was between the VS of the leaves (69.5% TS) and the suckers and stalks (85.9-86.1% TS) of the present study. [PERSON] et al. [44] found a lower methane yield of 140 L CH\({}_{4}\) kg\({}^{-1}\) VS from tomato branches (_Solanum lycopersicum_) collected from a private farm in Portugal. Tomato brancheshad similar VS content (80 \(\pm\) 0.3% TS), higher COD (1592 \(\pm\) 73 g kg\({}^{-1}\) VS), TKN (33.68 \(\pm\) 0.99 g kg\({}^{-1}\) TS), and insoluble lignin (13.83 \(\pm\) 0.59% TS) compared to that found in the tomato stalks of the present study (86.1 \(\pm\) 1.4% TS, 1215 \(\pm\) 7 g kg\({}^{-1}\) VS, 11.7 \(\pm\) 0.5 g kg\({}^{-1}\) VS and 10 \(\pm\) 1.1% on an extractive free solid basis, respectively). Manual stirring and/or the higher lignin content may have been responsible for the lower methane yield. [PERSON] et al. [58] tested the biochemical methane potential of about 54 fruits and vegetables from a vegetable market in India, including leaves and stems of the coriander plant, cabbage, cauliflower, and leaves of turnip, garden beet, and carrot. The VS content of coriander (80% TS in leaves and 85.7% TS in stems), cauliflower (82% TS in leaves and 87.1% TS in stems), turnip (84.4% TS in leaves), and beet (81.4% TS in leaves) was close to the VS of most residues in the present study (81.3-86.1% TS), except for the cabbage (91.2% TS in leaves and 91.8% TS in stems) and carrot (93.1% TS in leaves), in which the VS content was higher. The lowest methane yield was found in cauliflower leaves (190 L CH\({}_{4}\) kg\({}^{-1}\) VS), garden beet leaves (231 L CH\({}_{4}\) kg\({}^{-1}\) VS), and carrot leaves (241 L CH\({}_{4}\) kg\({}^{-1}\) VS), while the highest methane yield was obtained from coriander leaves (325 L CH\({}_{4}\) kg\({}^{-1}\) VS), cauliflower stems (331 L CH\({}_{4}\) kg\({}^{-1}\) VS), and turnip leaves (314 L CH\({}_{4}\) kg\({}^{-1}\) VS). [PERSON] et al. [59] determined that the lignin content of tomato stalks was 6.49 \(\pm\) 0.93%, very close to 10 \(\pm\) 1.1% determined as acid-insoluble lignin in the present work. Although [PERSON] et al. found this material fibrous and did not proceed further with evaluating its BMP, tomato stalks yielded 233.1 \(\pm\) 27.2 NL CH\({}_{4}\) kg\({}^{-1}\) VS in the present work. The diauxic growth lag phase for the biogas production from the agricultural residues indicates the presence of two main fractions of organic matter, which degrade at a different rate [60; 61], or an inhibitory factor, which slows down the process [62]. In most cases of this study, the methane production was faster at the beginning of the BMP test, and, afterwards, it slowed down until the final plateau. Even if the first plateau was not evident during methane production (from the stalks, for example) (Figure S5), the change in the BMP curve slope indicated there were fast and slow degrading compounds in the substrates, which the MGM cannot predict. Moreover, the methane production from the stalks occurred at a higher initial rate than the other residues, which is not compatible with the fact that the stalks contained lignin, while there was no lignin in the leaves and the suckers. It is possible that there are compounds in the leaves and suckers which may have an inhibitory effect. For example, it has recently been found that tomato leaf extracts exhibit antimicrobial properties [63; 64], which may also slow the anaerobic digestion process.