Free vs. bound Strecker aldehydes - Impact on chocolate aroma perception

Strecker aldehydes are a key component of chocolate aroma, with low odour thresholds and contributing a malty note. They are formed by the Strecker degradation reaction in which amino acids react with various alpha-dicarbonyl compounds as part of the thermally induced Maillard reaction, which occurs during roasting. Recently, studies have shown that the concentration of Strecker aldehydes in various food products is significantly increased after the addition of water, suggesting that another pathway exists for Strecker aldehyde formation, which is induced by water. A group of compounds known as 2-substituted-5-methyl-3-oxazolines were identified as potential precursors of Strecker aldehydes that are hydrolysed upon addition of water. They are thought to be formed as intermediates of Strecker degradation in the absence of water and remain stable in a non-aqueous environment. 2-isobutyl-5-methyl-3-oxazoline has indeed been detected in chocolate, albeit in low quantities, and chocolate products enhanced with 3-oxazolines have been rated for a more intense malty aroma. Various combinations of amino acids and alpha-dicarbonyls are possible, meaning that a wide range of 3-oxazolines exist, each with different substituents, and some 3-oxazolines may be more prevalent in chocolate than others. Therefore, it is important to measure the levels of several different types of 3-oxazolines in chocolate.
This study aims to quantify the importance of 3-oxazolines as precursors to Strecker aldehyde formation in chocolate and, consequently, their contribution to chocolate aroma. A range of 3-oxazolines will be chemically synthesised, characterised, and quantified. Various analytical methods will be tested, such as gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR). The levels of 3-oxazolines will be measured in different chocolate products: unfermented cocoa, fermented cocoa and roasted cocoa beans, as well as chocolate made from unroasted and roasted cocoa. This will give an indication to the formation pathways of 3-oxazolines, as Strecker aldehydes are already known to be formed during fermentation of cocoa beans. Amadori rearrangement products (ARPs), the condensation product of an amino acid and reducing sugar, may also be studied since an alternative proposed mechanism for 3-oxazoline formation involves the oxidative decarboxylation of ARPs. To assess the importance of 3-oxazolines as Strecker aldehyde precursors, isotope labelling experiments will be used to show what proportion of 3-oxazolines are converted to Strecker aldehydes upon addition of water and what proportion of Strecker aldehydes are formed by this route. Factors influencing Strecker aldehyde release will also be studied. The hydrolysis has already been shown to be favoured under acidic conditions but the effects of temperature, 3-oxazoline structure, fat content and hydrolysis time are not yet fully elucidated. Chocolate model systems may be created, utilising human saliva to induce hydrolysis. The conditions identified for optimum flavour release may be different for different food products, such as chocolate bars or powder-based drinks. Finally, sensory tests will be carried out to determine whether enhanced 3-oxazoline hydrolysis and Strecker aldehyde release results in a significant improvement in perception of the chocolate, whether orthonasal or retronasal. If the effects of 3-oxazoline hydrolysis are found to be significant, methods such as oxazoline enrichment, maintaining a dry food matrix or compartmentalisation of water-labile precursors may be used by future food product developers to preserve and/or enhance flavour in chocolate.