# Forging Prawn and Salmon Flavours with Non-Animal-Based Ingredients
## 1 Introduction  
In the global plant-based (PB) protein market, the PB seafood alternative (PBSA) sector remains relatively small compared to meat and dairy analogues. However, despite its global market size being valued at only USD 42.1 million in 2021, it is projected to reach USD 1.3 billion by 2031, making it the fastest-growing sector within the PB protein alternative industry [1]. Despite this optimistic forecast, both person- and product-related barriers inhibit the wider acceptance of PBSAs. Factors such as flavour, dietary preferences, product pricing, product availability, and naturalness all play a role [2]. Amongst these barriers, sensory qualities, particularly flavour, are fundamental to product acceptability and largely depend on the use of flavouring agents, whether chemically synthesised or derived from natural sources. The first step in an effective flavouring process for PBSAs is to gain a thorough understanding of the flavour profile of the target seafood. Numerous studies have been conducted to profile inherent flavour molecules and precursor compounds, both of whichplay a crucial role in shaping the flavour profile of cooked seafood. Due to their high abundance and contribution to umami and sweetness [3], free amino acids (FAAs) together with 5\({}^{\prime}\)-nucleotides significantly influence seafood taste profiles [4; 5; 6]. In addition to directly contributing to taste, FAAs also serve as precursors to seafood odour formation, especially through thermal processing. Strecker degradation of FAAs generates a range of aldehydes that are important for the seafood flavour profile [7]. For instance, methional, derived from methionine, is a common odourant found in cooked crustaceans, molluscs, and finfish [8]. 2-Acetyl-1-pyrroline, a well-established key odourant of cooked prawns [9; 10], can be generated during thermal food processing through the Maillard reaction between proline or ornithine and carbohydrates [11]. Furthermore, trimethylamine oxide (TMAO) and betaine (_N_,\(N\),_N_-trimethyl glycine) are key osmolyte molecules in seafood species--present at relatively high concentration--that protect cells from osmotic stress, which may contribute to sweet and umami tastes [12]. Both TMAO and betaine can be degraded to trimethylamine (TMA) through microbial activity [13; 14], or for betaine, potentially through thermal degradation during cooking. While a high concentration of TMA is considered a biomarker of seafood spoilage, a low concentration of TMA contributes to the fresh fish-like odour, which is essential for shaping fundamental seafood flavour [15]. The thermal degradation of omega-3 polyunsaturated fatty acids (n-3 PUFAs) produces a range of aldehydes and methyl ketones, followed by acids, hydrocarbons, lactones, and alcohols [7]. These compounds contribute to the characteristic volatile profile of cooked oily fish, such as salmon [6; 16]. Even with this information available, the formulation of seafood flavour is still far from straightforward, and the flavour quality of commercial PBSAs currently does not live up to expectations of regular seafood consumers and is often disappointing. Recent research has shown that most PBSAs available in the Australian market fail to replicate the flavours of their seafood counterparts, with the exception of PB sashimi products, where flavour plays a relatively minor role [17]. This may be due to the lack of ideal PB seafood flavouring agents and/or flavouring strategies that can accurately introduce the complex and subtle flavour attributes to the PB substrate [17]. Seafood typically has a very mild odour when raw, but its flavour profile changes significantly upon cooking. In contrast, in the production of commercial PBSAs, flavouring agents are generally pre-added during the structuring process and homogenised into the food matrix [18]. However, manufacturers often add excessive amounts of flavouring agents at this stage, one of the important reasons being to mask undesirable flavours like beny odour and bitterness from plant ingredients [19].