Thermal aroma compounds, model

Model Reactions on Generation of Thermal Aroma Compounds... 

It is demonstrated that a great many flavor compounds are formed in both model systems. On the other hand, phenylalanine formed by aldol condensations some special aroma products. Furthermore, the generation of thermal aroma compounds depend on the pH, the sugar/amino acid ratio and the temperature. 

Thermal aromas result from the Maillard reaction. By heating carbohydrates with amino acids degradation is accelerated yielding reactive compounds which, by new reactions with amino acids, are converted to heterocyclic products. Results of model investigations of glucose or its degradation compounds with the amino acids serine and phenylalanine are discussed. 

We have carried out some model reactions on the formation of thermal aromas in order to test the conditions for the analysis of such aromas and to study the mechanisms of their formation and their dependence on concentration and temperature. Last but not least we were interested to get an overview about the compounds which can be formed by generation of thermal aromas. 

Interactions between mannoproteins from yeast cell walls and aroma compounds have been studied by Langourieux and Crouzet (1997). They performed the experiments with crude mannoproteins extracts and observed no effect on the activity coefficient of isoamyl acetate, and a slight decrease on the activity coefficients of ethyl hexanoate and limonene. However, when they purified the mannoproteins or when they used a model glycopeptide, they did not observe any effect on limonene volatility. If the synthetic peptide was heat treated (50 °C), they observed a slight reduction on the activity coefficient of limonene. This was explained by an increase in the hydrophobicity of the glycopeptide after the thermal treatment. 

The Maillard reaction between reducing carbohydrates and amines is among the most important flavor generating reactions in thermally processed foods (5). Thus, it might be expected that in foods treated with HHP, but at low temperatures, some of the typical aroma compounds might not be formed. Only two studies about the influence of HHP on the formation of volatiles in Maillard model systems are currently available (6, 7). Bristow and Isaacs (d) reported that at 100°C, the formation of volatiles from xylose/lysine was generally suppressed when HHP was applied. Hill et al. (7) confirmed this observation for a glucose/lysine system. However, it has to be pointed out that the samples analyzed were not reacted in a buffered system and, also, the reaction time of the pressure-treated and untreated sample were not identical. 

In heated foods the main reactions by which flavors are formed are the Maillard reaction and the thermal degradation of lipids. These reactions follow complex pathways and produce reactive intermediates, both volatile and non-volatile. It has been demonstrated that lipids, in particular structural phospholipids, are essential for the characteristic flavor development in cooked meat and that the interaction of lipids with products of the Maillard reaction is an important route to flavor. When model systems containing amino acids and ribose were heated in aqueous buffer, the addition of phospholipids had a significant effect on the aroma and on the volatile products. In addition a number of heterocyclic compounds derived from lipid - Maillard interactions were found. The extent of the interaction depends on the lipid structure, with phospholipids reacting much more readily than triglycerides. 

Thermal characteristics of encapsulates are important, especially for aroma active compounds, from the viewpoint of their release in thermally processed food. In one of the recent publications, the thermal release of vanillin encapsulated in Carnauba wax microcapsules was studied by isothermal thermogravimetric analysis at a temperature range of 170°C-210°C. Kinetic studies revealed that the release is not a single-step reaction but a complex kinetic process that can satisfactorily be described by the Avrami-Erofe ev kinetic model A3. More importantly, thermal release of vanillin encapsulated into Carnauba wax proceeded with an activation energy lower than 40 kJ moL, indicating that the Carnauba wax microcapsules release vanillin relatively easily and thus suggesting that the Carnauba wax can be suitably used as a carrier for aromas especially in the food industry. 

The investigation of characteristic flavors associated with cooked meats has been the subject of much research over the past four decades but, although compounds with "meaty" aromas had been synthesized, compounds with such characteristics were not found in cooked meats until recently (1). In the search for compounds with characteristic aromas it was found that furans and thiophenes with a thiol group in the 3-position possessed meat-like aromas (2). The corresponding disulfides formed by oxidation of furan and thiophene thiols were also found to have meat-like characteristics, and exceptionally low odor threshold values (3). A number of such compounds are formed in heated model systems containing hydrogen sulfide or cysteine and pentoses or other sources of carbonyl compounds (4,5), The thermal degradation of thiamine also produces 2-methyl-3-fiiranthiol and a number of sulfides and disulfides (6J). 

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