## 1 Introduction  
Consumers, worldwide, are consciously opting to eat more plant-based foods. This shift to a more plant-based diet is driven by a number of factors, including the health benefits of reduced risk of obesity, cardiovascular disease, type II diabetes, hypertension, coronary artery disease, and certain types of cancer, as well as concern for animal welfare, and a wish to reduce the environmental footprint of their diet [1, 2]. This desire to eat more plant-based foods has led to an increase in the number and range of plant-based meat or dairy analogues in the market [3]. Despite their increasing popularity, many consumers still find plant-based foods, especially meat or dairy analogues, to be unsatisfying in terms of flavour [4, 5, 6, 7]. The flavour of a food is perceived via a complex combination of the olfactory (odour/ aroma), gustatory (taste), and trigeminal sensations experienced during eating. The volatile organic compounds (VOCs) are released into the mouth and move to olfactory receptors in the nose and non-volatile organic compounds released from the food are sensed by gustatory receptors on the tongue [8; 9; 10; 11]. A large number of VOCs are responsible for the flavour associated with a food [9; 10]. While plant-based substrates can contain desirable VOCs, direct extraction is difficult due to their low concentration and recovery rate [12]. Fermentation is a promising way to generate increased concentrations of desired VOCs from plant-based substrates [13; 14]. Micro-organisms can metabolise plant substrates and generate secondary volatile metabolites, including desirable meat or dairy flavour notes or precursors [15]. Because of the complex composition of plants, it is challenging to understand what directly influences the production of VOCs. To better understand the specific VOCs produced as compounds present in the plant substrates are altered, a defined medium is essential. Recent publications have highlighted that lactic acid bacteria (LAB) play a significant role in plant-based fermentations and produce an array of VOCs [16; 17]. Defined medium with minimal nutrients have previously been developed to support the growth of LAB [18; 19; 20; 21; 22; 23]. A carbon source (simple sugars/non-sugar) is a key component of the defined medium as it functions as both an energy source and a flavour precursor [13]. Different carbon sources can influence LAB's growth and metabolic processes, which results in the production of different VOCs [24; 25; 26]. The current study investigated the effects of three carbon sources (either glucose, fructose or citrate) on the production of VOCs by LAB in a defined medium. The selection of LAB strain is another important factor in fermentation studies because the production of VOCs is strongly strain dependent [27]. The obligate heterofermentative LAB, _Levilactobacillus brevis_ WLP672 (_Lev. brevis_ WLP672), which comments hexose sugars via the phosphohedolase (PK) pathway [28], was chosen for the current study, as our previous study indicated it grew well in a defined medium and produced a wide range of VOCs [29]. The most commonly used VOC extraction method is headspace solid-phase microextraction (HS-SPME), which provides reasonably high throughput performance without requiring extensive sample preparation [30; 31]. Extracted VOCs are commonly detected and analysed by gas chromatography-mass spectrometry (GC-MS) [32]. This technique, while effective, has a number of drawbacks, including a limited mass range and the presence of artifacts in the chromatogram; it is also labour intensive and time consuming [33; 34; 35]. A more efficient analysis approach is the use of direct injection mass spectrometers (DIMS), which offer quick, direct, and non-invasive VOC identification without the need for prior separation. Proton transfer reaction-mass spectrometry (PTR-MS) is a well-known technique in this class. The fundamental principles of PTR-MS have been well covered in past reviews [36; 37].