Cereal Aroma: Effects of thermal and enzymatic processing on volatile compound formation

Abstract

Oats are common ingredients in many food products. They are valued for their nutritional composition and functional properties. At the same time, oat-based products can develop off-flavours during processing. Lipid oxidation is one important cause of this problem. Aldehydes such as hexanal and nonanal are especially relevant because they can give rancid, grassy, and stale notes. In this project, selected thermal and enzymatic processing conditions were tested in oat-based systems. The aim was to see how these treatments affected volatile compound formation. Headspace gas chromatography–mass spectrometry (HS-GC-MS) was used for volatile analysis. The experimental work was divided into four rounds. These rounds covered thermal treatment of whole oat grains, enzymatic starch hydrolysis under screening conditions, enzymatic treatment under corrected enzyme-specific conditions, and the effect of post-treatment heating after glucoamylase treatment.

Thermal processing at 200°C under dry roasting conditions produced a broader range of volatile compounds than kilning at the same nominal temperature. Selected Maillard- and heat-associated compounds, including pyrazines and furans, were detected mainly after dry roasting at 200°C, whereas kilning at 200°C produced a narrower volatile profile with a stronger contribution from lipid-derived compounds. Hexanal was detected across all Round 1 treatment conditions and matrices, which supports its use as the principal lipid oxidation marker in this study. Comparison between dry powder and slurry samples indicated that water addition and sample matrix substantially influenced apparent headspace recovery, with slurry samples generally yielding lower hexanal signals than dry powder samples at lower treatment temperatures.

Across the enzymatic treatment rounds, the clearest increase in the hexanal-associated signal in oat slurry relative to matched no-enzyme controls was observed when glucoamylase treatment was followed by post-heating at 100°C. Glucoamylase treatment without post-heating did not produce the same increase, and pre-heating the slurry before enzyme addition suppressed the response. These findings point to heating sequence as an important factor in the glucoamylase-associated effect. A possible interpretation is that enzymatic modification of the oat slurry increased the availability or release of lipid-derived precursors before heating promoted volatile formation or release. However, starch–lipid interactions, lipid release, and oxidation intermediates were not directly measured, and this mechanism therefore remains unconfirmed.
Analytical limitations were encountered during the enzymatic treatment rounds. In Round 3, several enzyme-treated slurry samples showed dominant diagnostic-ion peaks outside the expected hexanal retention-time region, and the later signals could not be confirmed as hexanal without authentic reference standards or further spectral validation. Variation in internal-standard behaviour also indicated that fixed retention-time processing and direct cross-matrix normalisation were not reliable across all datasets. These observations were interpreted as peak-assignment and method-related limitations — potentially involving co-elution, retention-time instability, or system-related variation — rather than as direct evidence of matrix-driven chromatographic shifts.

Processing conditions and sample matrix both affected the volatile profiles observed in this project. In the glucoamylase-treated oat slurry, the heating order had a clear effect. The strongest hexanal-associated response appeared when the slurry was heated after enzymatic treatment. However, the analytical results also showed several limitations. Matrix effects, retention-time variation, and uncertain peak assignment made some comparisons less reliable. Future studies would need more replicates, authentic reference standards, retention-index confirmation, and targeted measurements of lipid oxidation and starch-related changes.

Popular Abstract

Oat-based drinks have become a common sight on supermarket shelves, but getting them to taste good is harder than it looks. Oats contain a lot of fat, and fat goes rancid. When that happens, the result is the kind of grassy, stale, slightly cardboard-like smell that makes you put a carton back on the shelf. Understanding where that smell comes from — and what makes it worse — is an important step toward producing oat drinks that stay fresh.
This project tracked a specific chemical called hexanal, which forms when oat fats break down and is one of the main contributors to rancid off-flavours. The central question was how different processing steps change the amount of hexanal and other aroma-active compounds that end up in oat samples, as measured by a technique called headspace gas chromatography-mass spectrometry — essentially a very sensitive way of sniffing a sample and identifying what is in it.
The first set of experiments looked at heat treatment. Roasting oats dry at 200°C produced a wide range of smell-active compounds, including some associated with the same chemistry that makes coffee and bread smell pleasant when heated. Kilning at the same temperature but with steam was different: the moisture suppressed those pleasant roasted notes and left mostly the rancid-type compounds instead. A simple chemical called hexanal turned up in every single condition tested, making it a reliable marker for tracking what was happening to the oats.
The more unexpected finding came from a different set of experiments. Oat drinks are commonly made by mixing oat flour with water and adding enzymes to break down the starch, which reduces the thick, gluey texture. When one such enzyme — glucoamylase — was added to the oat mixture and the mixture was then heated afterwards, the hexanal signal jumped to about twelve times what was seen without the enzyme. Adding the heat before the enzyme, however, made almost no difference at all. The order mattered more than the amount of enzyme used.
A possible explanation is that starch in oats physically traps some of the fat. When the enzyme breaks the starch down first, it may release fat that was previously locked away, making it available to oxidise once heat is applied. If confirmed, this would mean that a standard processing step used to improve the texture of oat drinks could also, under certain conditions, be making them smell worse.
The results are preliminary and the sample numbers were small, so firm conclusions have to wait. But the findings suggest that both heat type and enzyme treatment sequence deserve closer attention when designing oat beverage processes — not just for efficiency, but for how the final product smells.

• author: Lou, Xiaohua
• supervisor: Lars Dahlgren
• organization: Biotechnology (MSc); Biotechnology (M.Sc.Eng.)
• course: KBTM01 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Biology and Life Sciences
• keywords: Biotechnology, Oats, Volatile compounds, HS-GC-MS
• language: English
• id: 9236736
• date added to LUP: 2026-06-12 15:56:24
• date last changed: 2026-06-12 15:56:24