Abstract
The main objective of this PhD thesis is to understand the effect of industrial processing stages and subsequent storage on the volatile flavour of oat-based products using an integrated GC-MS-based (gas chromatography-mass spectrometry-based) fingerprinting and chemometric data analysis.
In this study, methods for GC-MS volatile fingerprinting, lipid profiling and enzyme activity measurement of oats have been developed. The optimised GC-MS fingerprinting method detected more than 130 volatile compounds, such as alkanals, ketones, alcohols and pyrazines. The fingerprinting method was combined with chemometrics to differentiate 12 commercially available dry oat-based products based on integrated volatile and lipid profiles. Partial least squares discriminant analysis (PLS-DA) and variable identification (VID) were employed to select discriminant compounds driving the classification. This approach was subsequently used to compare the volatile fractions of the 12 commercial oat samples after cooking into a porridge. The classification in the bi-plot and discriminant components selected in oat porridges were comparable to that of dry oats. This demonstrated that the developed fingerprinting method effectively detected an increased number of volatiles from a broad range of chemical classes, and chemometrics-based classification models were developed to compare and discriminate commercial oat samples.
This toolbox was further used to study the effect of industrial processing stages (dehulling, kilning, cutting, steaming, flaking and packaging) on the enzymes (lipase and peroxidase), lipids and volatiles of wholegrain and rolled oats. The applied chemometric approach was a valuable tool to assess the evolution of flavour-related compounds during processing, investigate the relationships between the attributes, and select compounds differentiating the impact of each processing step. Oat groats (dehulled oat grain) had significantly (p < 0.05) higher lipase and peroxidase activities, and possessed higher amounts of short-chain volatile fatty acids than the processed oats. The combined effect of kilning and subsequent steaming significantly (p < 0.05) reduced lipase and peroxidase (POD) activities. The combination of kilning and steaming seems to trigger the Maillard and Strecker degradation reactions, leadingto the formation of odour-active volatile compounds, such as pyrazines (e.g., 2,5-dimethyl-pyrazine and 3-ethyl-2,5-dimethyl-pyrazine) and Strecker aldehydes (e.g., 3-methyl-butanal and 2-methyl-butanal). These compounds are commonly associated with the desirable nutty and toasted flavour of oat-based products. However, the amount of these odour-relevant compounds significantly decreased towards the end of the processing stages possibly lost to the environment because of the open processing lines. This finding implies that enzymatic reaction, lipid oxidation and the Maillard reaction have occurred simultaneously during oat processing, leading to significant modification of the volatile profile.
Storage time and temperature conditions (15 °C, 25 °C, 35 °C and 45 °C for up to 42 weeks) affected the volatile fraction of quick-cook oats after processing. Chemometrics was used to explore the volatile changes and understand the corresponding reaction pathways. These changes were linked to multiple reactions such as lipid oxidation, the Maillard reaction and Strecker degradation. The present work identified potential volatile markers for accelerated shelf life testing (ASLT) of oat-based products for the first time. The changes in these markers appeared to occur in steps following a sigmoidal shape. An initial induction phase was followed by a log phase for the majority of storage periods until it plateaued towards the end. To quantitatively discuss the effect of temperature on formation rate, this sigmoidal step-wise volatile formation was adequately modelled using a three-parameter non-linear logistic model. Based on the estimated kinetic parameters, volatiles such as hexanal, pentanal, 2-pentyl-furan, 2-heptanone, 1-pentanol, 2-furanmethanol, trimethyl-pyrazine, and benzaldehyde are proposed as potential ASLT markers. These compounds could be used to monitor volatile changes during storage and generation of rancidity/off-flavour and improve storage stability of quick-cook oat products.
In summary, this thesis has successfully employed an integrated fingerprinting and chemometrics approach to obtain an increased understanding of the volatile changes and associated complex (bio)chemical reactions during the processing and shelf life of oats. The changes in selected odour active discriminate compounds (e.g., hexanal, 2-heptanone, nonanoic acid, furfural and 2,5-dimethyl-pyrazine) were linked to crucial reaction pathways, such as lipid oxidation, enzymatic oxidation and the Maillard reaction. These compounds can be potentially used to monitor flavour-related changes during industrial oat processing and storage.