Full text
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| Author(s): |
Natália Ellen Castilho de Almeida
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Carlos. |
| Institution: | Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT) |
| Defense date: | 2011-02-18 |
| Examining board members: |
Daniel Rodrigues Cardoso;
Quezia Bezerra Cass;
Rodrigo Ramos Catharino
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| Advisor: | Daniel Rodrigues Cardoso |
| Abstract | |
The iso-α-acids are the main responsible constituents for the bitter taste of beer, they are easily degraded during the aging and light exposed process, specially the trans- diastereoisomer. The 1-hydroxyethyl radical is the major radical produced during the beer aging process. The present work describes the reactivity of iso-α-acids towards the 1-hydroxyethyl radical as probed by the spin-trapping technique and detected by electron paramagnetic resonance (EPR) and mass spectrometry (ESI-(+)-MS/MS). It was observed that both diastereoisomers cis- and trans-iso-α-acids are degraded in the presence of 1-hydroxyethyl radical with apparent rate constant of 1.8 108 e 9.2 109 L mol-1 s-1, respectively. The reactivity of dihydro-iso-α-acids towards the studied radical was similar to the reactivity of the iso-α-acids diastereoisomeric mixture, showing apparent rate Constant of 1.5 109 L mol-1 s-1. The tetrahydro-iso-α-acids analogues did not observed reactivity towards the 1-hydroxyethyl radical suggesting the allilic hydrogens as the reaction sites. In addition, the ab initio DFT calculations demonstrated that the BDE values for the allilic hydrogens of the prenyl and isohexenoyl side chains are equivalents and according to that suggesting the higher reactivity of the trans- diastereoisomer to be accounted to an entropic factor since both goups are in the same plane of the space. The oxidation products were determined by LC-ESI-MSn and its was verified the formation of hydroxyl-allo-iso-α-acids. The data colected allows a mechanism to be proposed for the oxidation process and loss of bitter acids of beer during the beverage aging. (AU) | |
| FAPESP's process: | 08/08773-0 - Reativity of iso-alpha-acids and its hydrogenated derivatives towards the 1-hydroxyethyl radical: implications in the sensorial quality loss of beer |
| Grantee: | Natália Ellen Castilho de Almeida |
| Support Opportunities: | Scholarships in Brazil - Master |