# Dynamics of Microbial Community Structure and Metabolites during Mulberry Ripening
## 1 Introduction  
Mulberry, also known as mulberry fruit, mulberry seed, and mulberry juibube, is a kind of juicy berry obtained from the mulberry tree with a long history of cultivation, mainly grown in Sichuan and Hunan provinces [1]. Since ancient times, mulberry has enjoyed the reputation of the \"fruit king,\" its soft and juicy flesh is not only rich in a variety of nutrients but also known to lower blood glucose and cholesterol and reduce symptoms of arthritis [2] and so can be used as both medicine and food [3]. The carbohydrates [4], amino acids [5], vitamins, and minerals [6], such as Se, Ca, and Fe, contained in mulberries are important nutrients required by the human body. Existing studies have shown that mulberries can regulate the body's immune function, promote the growth of haematopoietic cells, and have health effects such as anti-mutagenesis and the lowering of blood lipids [7]. Although the nutritional value of fresh mulberries is extremely high, their shelf life is only 1-2 days [8], and they are prone to decay and deterioration during transport and sale. In order to maximise the retention of the nutrients of the mulberry, increase the added value of itsproducts, and meet consumer requirements, it is particularly important to choose the best picking stage. Mulberry is popular not only for its rich nutritional value and health benefits but also for its unique sweet and sour flavour [9]. During the ripening stage, the colour of the skin will gradually change from an initial greenish colour to deep red, then to deep purple and, finally, to completely black; meanwhile, the taste changes from an initial hard astringency to a delicate and soft flavour. Amino acids [10], organic acids [11], and sugars--as precursors of flavour substances--produce dozens of volatile flavour substances, such as esters, alcohols, and terpen, after complex chemical reactions [12], which give mulberries their strong floral, nutty, and fruity aromas [13]. Therefore, an in-depth study of the unique substance composition of mulberry at different stages of ripening is of great significance for understanding the mechanism of mulberry flavour formation and enhancing its quality. To date, studies on the quality characteristics of mulberries at different maturity levels have mainly focused on their physicochemical properties and mineral content. [PERSON] et al. [14] found that ripe mulberries contain high levels of total sugars and total phenols, at 129.88 \(\pm\) 1.09 mg/g and 3.08 \(\pm\) 0.16 mg/g, respectively, which make them a good choice as a dietary supplement. Comparatively, unripe mulberries contain high mineral and organic acid contents, which allows them to be used as food additives or stabilisers. A study by [PERSON] et al. [15] indicated that the flavonoid content of mulberry increases after ripening. However, there are relationships among the quality indicators of mulberry, which may lead to an overlap of the many pieces of information reflected by the indicators, and systematic evaluation eliminates these overlapping pieces of information. Currently, there are relatively few comprehensive studies on key characteristics such as antioxidant capacity, flavour substances, and microbial community succession in mulberries at different maturity levels. In this study, high-throughput sequencing was used to analyse and characterise the composition and succession of the microbial community during mulberry ripening. Meanwhile, headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) was used for a detailed analysis of the volatile flavour substances in mulberries. In addition, we examined the contents of non-volatile metabolites and minerals during the ripening process. The aim was to study the evolution of basic physicochemical indices, metabolite changes, and microbial community structure during the ripening process of Nucellar Big Ten and to explore the correlation between key flora and physicochemical properties and flavour substance species.