On-line dynamic HS-SPME for monitoring endogenous aroma compounds released during the baking of a model cake

Abstract

This work shows that using a dynamic SPME device combined with an instrumented oven, it is possible to monitor the release of a large amount of volatile compounds generated during the baking process of a real cereal product (sponge cake model) by directly sampling its baking vapours. The steam assisted dynamic SPME device made it possible to extract volatile compounds with very different volatility and hydrophobicity, such as 5-hydroxymethylfurfural and 2-methyl-propanal. Time dependent analyses of baking vapours made it possible to simultaneously follow the release of several odour compounds and thermal reaction markers at different stages of their generation in the sponge cake model. The release of newly formed aroma compounds during baking significantly affected the odour quality of baking extracts as shown by odour profiles and sensory preferences evaluated by Direct-GC–Olfactometry. GC–Olfactometry analysis was carried out on the final baking fractions to gain an understanding of the compounds which could contribute to overall odour quality.

Introduction

During a thermal process such as baking, food products undergo intense transformation due to heat and mass transfer. Such changes are also partially governed by the initial composition of food ingredients which constitute the medium of the reactions (Chevalier, Colonna, Valle, & Lourdin, 2000). The baking of cereal products leads to a huge quantity of newly formed volatile compounds which play a major role in developing the flavour of the final product. These compounds are mainly the result of the Maillard reaction which occurs between reducing sugars and the -NH₂ function of amino acids, peptides and proteins (Baltes, 1982). However, depending on raw material composition and processes, the Maillard reaction is not necessarily the only thermal reaction occurring during baking. In fact, both caramelisation and lipid oxidation can also take place. These reactions are known to be involved in the generation of odour active compounds such as aliphatic aldehydes and furfurals (Grosch and Schieberle, 1997, Kroh, 1994, Whitfield, 1992).

Over the past few years numerous studies have dealt with the identification and quantification of the molecular compounds which have an impact on the flavour of the most common cereal products such as bread (Schieberle and Grosch, 1985, Schieberle and Grosch, 1992), the French baguette (Zehentbauer & Grosch, 1998), and cookies (Prost, Lee, Giampaoli, & Richard, 1993). The goal of such work has been to gain a deeper understanding of the role played by fermentation, enzyme activity and food components such as sugars and fat. Pozo-Bayon, Ruiz-Rodriguez, Pernin, and Cayot, (2007) recently showed that changing the formula of a sponge cake by substituting eggs with a leavening agent, produced important changes in some key aroma compounds such as methional. Few interesting studies have been made on the impact of specific thermal processes, such as extrusion, on the generation of flavour compounds and its sensory impact (Heiniö et al., 2003). An extensive work about baked cereal products reviews bibliographic data about aroma generation and summarises the possible elaboration methods that can be used to control or modify flavour in this type of product (Pozo-Bayón et al., 2006a, Pozo-Bayón et al., 2006b). However, it has recently been pointed out that in addition to having a strong impact on organoleptic quality (i.e. colour and flavour development) some thermally driven reactions can have an effect on nutritive value, produce antioxidative components and have toxicological implications. These findings explain why numerous research groups are today involved in research projects aimed at gaining increased understanding of the reaction mechanisms occurring in food and determining “global” food quality (Cost action 927, xxx, Martins et al., 2001, van Boekel, 2006).

Moreover, it now appears necessary to have multi-compounds and dynamic methods to follow not only flavour compounds but also some sensitive reaction products (i.e. furanic compounds). For cereal products, the amount of aroma compounds and/or of some volatile Maillard Reaction Products (MRP) is classically measured in the final food by extracting molecules by solvent extraction (Ramirez-Jimenez, Garcia-Villanova, & Guerra-Hernandez, 2000) or using headspace techniques (Poinot et al., 2007, Pozo-Bayón et al., 2006a, Pozo-Bayón et al., 2006b). These approaches are useful for evaluating the impact of such compounds on food quality but give no kinetic information. It is, in fact, worth stressing that many chemical and physical parameters (i.e. Aw, concentration of reactants, pH, etc.) are simultaneously changed during thermal processing of food, thus affecting the generation of aroma compounds. A novel approach to treating this problem is to monitor volatile compounds (flavour or interesting reaction markers) during their development, that is to say during the thermal process. From an analytical point of view, an interesting dynamic approach has been applied to a roast-beef model by Rochat and Chantreau, (2005). Using headspace trapping and GC–Olfactometry, the authors analysed molecular compounds generated during the baking of roast-beef which affected the odour of the in-oven roast-beef top note. A similar approach was adopted by Rega, Guerard, Maire, and Giampaoli (2006) who showed that using an on-line dynamic SPME device combined with an instrumented oven it was possible to sample volatile compounds released throughout the baking of a model sponge cake. The authors also used Direct-GC–Olfactometry (D-GC–O) to assess the odour representativeness of dynamic SPME extracts of baking aroma obtained by different SPME fibres.

The aim of the present work was to investigate the release of the endogenous volatile compounds formed during the baking process of a model sponge cake and specifically examine whether by monitoring baking vapours during the thermal process information could be obtained about their evolution. Volatile compounds with interesting flavour characteristics and/or which are good thermal reaction markers were followed in the vapours generated at different stages of baking using an on-line steam assisted dynamic SPME device. Both sensory and instrumental analyses were carried out to identify the fraction of baking generating the best sensory appreciation and then to identify the corresponding odour active compounds. The volatile compound composition was also followed in the cake matrices. In future developments, such a dynamic approach could help to gain an understanding of the reaction pathways and kinetics occurring in a real food system during physical and chemical thermal transformation.