## 1 Introduction  
3-Methylthio-1-propanol (3-Met), a sulfur-containing higher alcohol, is a crucial aroma component of many foods and beverages, such as soy sauce, cheese, _Baijiu_, beer and wine and is naturally present in apples, tomatoes and other foods [1]. 3-Met has a low odor threshold of 1-3 ppm, with a recommended dosage in food of 0.1-10 mg/kg [2]. It has been classified by the US Federal Emergency Management Agency in the class of safe and edible flavor substances. It is widely used in flavor and fragrance formulation and food flavoring because it imparts cauliflower-like, meaty, savory or toasted cheese flavor notes [3].  
At present, 3-Met is produced by chemical synthesis and biotechnological methods [3]. Chemical synthesis is mainly used because of its lower costs but it produces undesired side products, resulting in the formation of racemic mixtures and environmental pollution [3, 4]. With the increasing demand of consumers for natural products, developing healthier and more eco-friendly natural products have become more important [1, 3, 4]. Microbial fermentation is a necessary process that uses microorganisms to transform raw materials into products needed by human beings through specific metabolic pathways under appropriate conditions [5]. Recent studies have shown that microorganisms, especially yeasts, can biotransformation L-methionine (L-Met) to produce 3-Met by fermentation, an effective method of natural production [3; 5]. L-Met is transaminated and then decarboxylated to the corresponding aldehydes via the Ehrlich pathway, then reduced to higher alcohols or oxidized to fused acids [1]. In recent years, biological conversion methods have become an important development direction for producing natural 3-Met, because these methods could avoid undesired by-products and are considered as more eco-friendly [3; 5]. Thus, interest in the bioproduction of 3-Met by fermentation has been growing in recent years.  
However, relatively few studies have reported on the ability of microorganisms to produce natural 3-Met, which mainly included a few types of yeast, such as _Saccharomyces cerevisiae_, _Kluyveromyces lactis_, _Debaryomyces hansenii_, _Yarrowin lipolytica_, _Zygosaccharomyces rouzi_ and _Williipsis_ species [6; 7; 8; 9]. The ability of these natural strains to produce 3-Met is limited with yields generally less than 0.5 g/L (Table 1), therefore their application as a microbial factory in industrial fermentation has been restricted [6; 8; 10].  
With the demand for natural 3-Met increasing and the rapid development of genetic engineering technology, most studies have begun to use molecular biology techniques to modify genes to engineer strains with a high yield [1; 16; 17]. After determining the key enzymes that affect the synthesis of 3-Met in yeast strain, the yeast genome was modified through gene overexpression or knockout [1; 16; 18]. The yield of 3-Met from certain genetically modified strains has increased compared with the yield of unmodified strains, with yields of up to 4.38 g/L after fermentation condition optimizations [18; 19]. As some consumers may consider 3-Met produced by genetically modified strains unnatural, natural strains with a high 3-Met yield need to be identified and their fermentation conditions should also be optimized to obtain acceptably natural 3-Met production capability.  
Therefore, the objective of this study was to obtain a strain with a high 3-Met yield from the _Baijiu_-making environment and to optimize its fermentation conditions. In this study, a yeast identified as _Hyphopichia burtonii_ with a high yield of 3-Met was obtained from the _Baijiu_-brewing environment. Several statistical methods will be combined to optimize the fermentation conditions for this strain to further enhance its 3-Met production. To the best of the author's knowledge, the strain discovered in this study is among the highest 3-Met yielding natural strains previously reported.