## 4 Discussion
### Trace Amounts of Mycotoxins found during Composting of Biowastes  
Our finding of only trace amounts of AFB1, HT-2, FUMs, and ZEN in just a few samples (Supplementary Table S1) is similar to the findings of other studies, which reported little or no mycotoxin content in compost from large-scale composting facilities [12; 13; 14]. There are two likely explanations for the fact that only traces of mycotoxins were found in the substrate collected from different stages of the process and their content did not change markedly during the time of the experiment. First, any mycotoxins that were produced may have been biodegraded immediately subsequent to their production. Numerous studies have shown that pure or heterogenous microbial cultures isolated from different environments are capable of irreversibly transforming DON and other mycotoxins into less toxic compounds [15]. Composting alone or in combination with anaerobic digestion has also been shown to be an effective method of removing mycotoxins from contaminated substrates [16; 17; 72]. For example, 90 days of co-composing the organic fraction of municipal solid waste with corn artificially contaminated with AFB1 almost completely removed the mycotoxin [17]. On the other hand, noticeable amounts of mycotoxins may not have been produced under the conditions used in this study. Importantly, ZEN was the mycotoxin that was most frequently detected in the samples taken in our study, and once, at the end of the biowaste composting process, its content in the substrate was high enough to be precisely quantified (>5 ug\(\cdot\)kg\({}^{-1}\); LOQ), unlike the contents of the other mycotoxins (Supplementary Table S1). These findings suggest that the production of some mycotoxins during composting could be a selective process, depending on specific bioreactor conditions that favour the development of particular taxonomic groups of moulds and/or the production of specific mycotoxins. In general, the high moisture of the kitchen waste and the conditions in which it was stored would have favoured dynamic mould growth, which was confirmed by our macro- and microscopic observations. However, the population of fungal species and strains can vary depending on the composition of the substrate and the specific conditions of the technological process, e.g., the nutrients, temperature, humidity, and pH [5; 10; 11; 73]. Moreover, the presence of mould does not necessarily correspond to the presence of toxicogenic strains that are capable of producing mycotoxins [74]. This is supported by the fact that, even though moulds of the genera _Aspergillus_ and _Rhizopus_ were present in the composting substrate in our study (Figure 5), and some strains of these moulds can produce AFB1 and OTA [9], no more than trace amounts of these two fungal metabolites were found throughout the entire experiment. This is consistent with the understanding that there is no simple relationship between the growth of mould and whether it produces mycotoxins [75; 73]. On the other hand, ZEN is primarily produced by moulds belonging to _Fusarium_ genera, and high humidity and fluctuating moderate and low temperatures are considered the main factors that stimulate the production of this mycotoxin in grains [76]. Although the conditions that are most favourable to the production of ZEN during composting of biowastes have yet to be established, the bioreactor conditions in our study are consistent with those that are known to favour the formation of ZEN in grains. Finally, it is also possible that there could have also been toxicogenic strains of moulds that produce mycotoxins other that the ones that were analysed in our study. For example, _Penicillium_ species have been reported to produce hundreds of toxic metabolites [11; 20], but analytical standards are available for only some of them. In this study, only eight of the most commonly occurring mycotoxins were investigated; thus, a broader search might have detected more mycotoxins.