# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
###### Abstract  
Health, environmental and ethical concerns have resulted in a dramatic increase in demand for plant-based dairy analogues. While the volatile organic compounds (VOCs) responsible for the characteristic flavours of dairy-based products have been extensively studied, little is known about how to reproduce such flavours using only plant-based substrates. As a first step in their development, this review provides an overview of the VOCs associated with fermented (bacteria and/or fungi/yeast) vegetable and fruit substrates. Following PRISMA guidelines and using two English databases (Web of Science and Scopus), thirty-five suitable research papers were identified. The number of fermentation-derived VOCs detected ranged from 32 to 118 (across 30 papers), while 5 papers detected fewer (10 to 25). Bacteria, including lactic acid bacteria (LAB), fungi, and yeast were the micro-organisms used, with LAB being the most commonly reported. Ten studies used a single species, 21 studies used a single type (bacteria, fungi or yeast) of micro-organisms and four studies used mixed fermentation. The nature of the fermentation-derived VOCs detected (alcohols, aldehydes, esters, ketones, acids, terpenes and norisoprenoids, phenols, furans, sulphur compounds, alkenes, alkanes, and benzene derivatives) was dependent on the composition of the vegetable/fruit matrix, the micro-organisms involved, and the fermentation conditions. volatile organic compounds (VOCs); plant-based substrates; lactic acid bacteria (LAB)  
1,2]S.S.  
means depending on their concentration, they have the potential to induce an odour sensation [8; 9]. Fat, protein (casein, whey/amino acids), sugars (lactose), and citrate are the key components of raw/processed milk, and they are responsible for a wide range of flavour VOCs. The sensory perception of fresh milk is largely determined by a pleasant mouthfeel owing to the physical composition of milk, a slight sweet/salty taste derived from lactose and milk salts, and a delicate aroma due to the VOCs present. In addition, lactic acid bacteria (LAB), when present in fermented dairy products, produce flavour VOCs from sugars or citrate (acetoin, diacetyl, propionic acid, and acetaldehyde), amino acids (benzaldehyde, 3-methylbutanal/3-methyl butanol, methional, methanethiol, phenyl acetic acid, dimethyl sulphide, dimethyl disulphide, and dimethyl trisulphide), and lipids (butanoic acid, butanone, octene-3-ol, hexanal, and 8-decalactone) [10; 11]. As many consumers wish to avoid dairy-derived products completely, dairy-derived flavours are generally not acceptable components of dairy analogues. For this reason, the flavours or the substrates needed to generate appropriate flavours need to originate from other sources, with plants owing to diversity, availability and affordability being the most logical choice. However, as dairy flavours contain specific VOCs, as outlined above, producing these flavours and simulating the desired flavour from plant substrates is challenging. In addition, when using plant-based substrates, the flavours of interest may also be associated with other less desirable flavours contained in the plant material matrix, as plants typically contain complex mixtures of substrates [10; 12]. The use of micro-organisms to produce desirable flavours from plant-based substrates has gained interest owing to the diversity of the micro-organisms, and the wide range of metabolic reactions they can carry out. Micro-organisms growing on plant-based substrates utilise the available sugars, lipids, and proteins for their energy, synthesis, and growth needs and, in this process, produce a wide range of volatile secondary metabolites through various metabolic pathways [13]. Microbial flavours can therefore be produced in a relatively simple and environmentally friendly/food-grade manner, and such approaches are generally scalable and affordable [14].