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

straker醛是巧克力香气的关键成分，具有较低的气味阈值和麦芽味。它们是由streker降解反应形成的，其中氨基酸与各种α -二羰基化合物反应，作为热诱导美拉德反应的一部分，发生在焙烧过程中。最近有研究表明，在各种食品中加入水后，streker醛的浓度显著增加，这表明存在另一种由水诱导的streker醛形成途径。一组被称为2-取代-5-甲基-3-恶唑啉的化合物被确定为在加水时水解的斯特莱克醛的潜在前体。它们被认为是在没有水的情况下作为streker降解的中间产物形成的，并且在非水环境中保持稳定。2-异丁基-5-甲基-3-恶唑啉确实在巧克力中被检测到，尽管含量很低，而且添加了3-恶唑啉的巧克力产品被认为具有更强烈的麦芽香气。氨基酸和α -二羰基的各种组合是可能的，这意味着存在各种各样的3-恶唑啉，每种都有不同的取代基，有些3-恶唑啉在巧克力中可能比其他的更普遍。因此，测量巧克力中几种不同类型的3-恶唑啉的含量是很重要的。本研究旨在量化3-恶唑啉作为巧克力中streker醛形成的前体的重要性，因此，它们对巧克力香气的贡献。一系列3-恶唑啉将被化学合成、表征和定量。将测试各种分析方法，如气相色谱-质谱（GC-MS），液相色谱-质谱（LC-MS）和核磁共振（NMR）。3-恶唑啉的含量将在不同的巧克力产品中进行测量：未发酵的可可、发酵的可可和烘焙的可可豆，以及由未烘焙和烘焙的可可制成的巧克力。这将为3-恶唑啉的形成途径提供指示，因为已知在可可豆发酵过程中会形成斯特雷克醛。Amadori重排产物（ARPs）是氨基酸和还原糖的缩合产物，也可以研究，因为3-恶唑啉形成的另一种机制涉及ARPs的氧化脱羧。为了评估3-恶唑啉作为Strecker醛前体的重要性，将使用同位素标记实验来显示在加水后有多少比例的3-恶唑啉转化为Strecker醛，以及通过这一途径形成的Strecker醛的比例。研究影响streker醛释放的因素。在酸性条件下水解已被证明是有利的，但温度、3-恶唑啉结构、脂肪含量和水解时间的影响尚未完全阐明。巧克力模型系统可以创建，利用人类唾液诱导水解。对于不同的食品，如巧克力棒或粉状饮料，确定的最佳风味释放条件可能是不同的。最后，将进行感官测试，以确定增强的3-恶唑啉水解和streker醛释放是否会显著改善巧克力的感知，无论是正鼻还是后鼻。如果发现3-恶唑啉水解的影响是显著的，那么未来的食品开发人员可能会使用诸如恶唑啉富集、保持干燥的食品基质或水不稳定前体的区隔化等方法来保存和/或增强巧克力的风味。

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.