Potential of processing to modify flavour of single and mixed vegetable systems
Due to the consumer demand, food industries have shown an increased interest in the production of healthier, more natural and more tasty vegetable-based products. In that context, a major challenge is to (re)design the manufacturing processes of such food products in order to comply with the increasingly important requested criteria. Flavour is one of the most important quality attributes in foods affecting consumers’ acceptance. The flavour formation in vegetable-based products is strongly affected by processing and it can be associated with several reaction pathways, resulting in a complex mixture of a large number of volatile compounds which contribute to their aroma. In many cases, vegetable-based foods are submitted to thermal processes aiming to either improve the sensorial characteristics or to increase the stability and extend the shelf-life. Hence, there is a need for an adequate approach to understand the complexity of flavour generation in vegetable-based systems as a function of processing in order to design new/improved vegetable-based foods with better quality.
In this perspective, this work aimed at gaining further insight into the potential of a range of processing conditions to modify the volatile fraction of single and mixed vegetable-based systems. Given that this work was carried out in an industrial research enviroment, the selection of the vegetable matrices was based on their economical relevance for the industry, whereas the processing conditions selected were based on real industrial settings. Processing of mixed vegetable systems adds even more complexity to their volatile profile due to the interactions among the vegetable components. Both chemical and enzymatic reactions can potentially occur between compounds of the respective vegetables when they are processed together which can affect the volatile profile of the mixed vegetable-system. Thus, focus was also given on the investigation of potential (bio)chemical interactions between the vegetable ingredients in more complex mixed-vegetable systems. In addition, sensory analyses was performed to validate the results obtained by the instumemtal analyses.
To achieve the objective, the experimental part was divided into three parts: (i) impact of processing on the volatile fingerprint of a single vegetable-based system; (ii) impact of processing on the volatile fingerprint of a mixed vegetable-based system; (iii) impact of processing on the odour-active volatiles of a mixed-vegetable system.
In all parts, an untargeted analytical approach (fingerprinting) along with multivariate data analysis (MVDA) was applied to detect significant differences in volatile profiles of vegetable purees prepared by different processing conditions in order to obtain an insight into chemical reactions which are influenced by processing. Sensory difference tests were performed to verify whether the differences observed in the instrumentally obtained volatile profiles were perceivable and significant in vivo.
In the first experimental part, the potential of the sequence of different thermo-mechanical treatments (cold break/hot break) to control the off-flavour generation in broccoli puree was investigated. Comparison of the headspace GC-MS fingerprinting of the differently processed broccoli purees revealed that an adequate combination of processing steps allows to reduce the level of off-flavour volatiles. Next, insight was obtained into the interrelationship between processing parameters, microstructural changes and flavour generation. Application of mechanical processing before (cold break) or after the thermal processing (hot break) at 90 °C showed to determine the pattern of broccoli tissue disruption, resulting into different microstructures and various enzymatic and non-enzymatic reactions inducing volatile generation. Difference sensory tests clearly showed that the differences detected in the volatile profile fingerprints of the differently processed broccoli purees were perceivable by the panelists. These results may aid the identification of optimal processing conditions generating a reduced level of off-flavour in processed broccoli.
The second experimental part dealt with changes in volatile fingerprint of a mixed tomato-carrot system as a function of different thermo-mechanical processing conditions. Untargeted GC-MS chemical fingerprinting resulted into a selection of discriminative marker volatile compounds in mixed-vegetable puree, which act as the witness of the process footprint. Besides this, when individual components, in a mixed vegetable system, were processed together ('all-in-one processed') vis-a-vis processed separately and then mixed together ('split-stream processed') different extents of chemical and enzymatic reactions induced changes in their volatile profiles. Results obtained by sensory difference tests showed that the fingerprinting differences are perceivable by humans.
The third experimental part focused on the determination of the odour-active compounds of a mixed tomato-onion puree by using gas chromatography-olfactometry (GC−O) and on the impact of different processing on its aroma profile. Based on the results of two olfactometric methods, i.e. detection frequency (DF) and aroma extract dilution analysis (AEDA), the most potent aroma components of a mixed tomato-onion puree include: dipropyl disulfide, S-propyl thioacetate, dimethyl trisulfide, 1-octen-3-one, methional, dipropyl trisulfide, 4,5-dimethylthiazole, phenylacetaldehyde and sotolone. As a next step, the impact of different processing steps (i.e. thermal blanching, all-in-one and split-stream processing) on the volatile profile and aroma of a mixed tomato-onion puree was investigated using a GC-MS fingerprinting approach. Results showed the potential to control the aroma in a mixed tomato-onion system through process-induced enzymatic modulations for producing tomato-onion food products with distinct aroma characteristics.
The present work showed the potential of a range of different processing conditions to modify the flavour of single and mixed vegetable purees. This knowledge forms the scientific basis to select optimal processing conditions for producing vegetable-based food products with distinct headspace profiles. This knowledge base can be the starting point for more targeted tailoring of the flavour of vegetable-based foods.