Topic: Lipids are hydrophobic compounds of various types that play important functions in plants and animals. Lipids are an essential component of food and of most biological systems as a concentrated source of energy as well as of fat-soluble vitamins in foods. As structural components of cell membranes in biological systems, lipids fulfill an important function in the transmission of signals. Lipids are a heat transfer medium that gives foods the desired texture and intended taste during processing. Last, but not least, lipids are important macronutrients that are necessary to grow and maintain organisms.
The Challenge: The oxidation of lipids, typically induced by thermal stress and by reactions with atmospheric oxygen, leads to a deterioration in nutritional value, taste, and texture of food, as well as to the appearance. Food processing operations such as cooking and deep-frying increase the level of lipid oxidation. In long term storage, foods containing lipids can exhibit a build-up of oxidation products, which negatively impacts flavor and taste. Further, lipids lose nutritional value when oxidized. The oxidative degradation of lipids leads to the formation of secondary, often unpleasant smelling degradation products, including alkanes, alcohols, esters, aldehydes, and ketones. Tracking them down is typically a task confronting food and beverage quality assurance and quality control departments.
The Strategy: If the task at hand is to determine off-odors or off-flavors and their sources in food and beverages, gas chromatography (GC) combined with mass-selective and olfactory detection (GC-MS/O) is the analytical tool of choice. Application experts from GERSTEL have successfully demonstrated the use of a GC-MSD/ODP or GC-MS/O system to effectively detect and identify off-odors and off-flavors associated with lipid oxidation products in foods.
The System: A GC-MS/O system with integrated autosampler for direct thermal extraction (DTE) in slitted microvials, and for dynamic headspace analysis of the target analytes was used. The analysis system consisted of: GERSTEL-MPS LabWorks platform with DTE and DHS option and a 8890/5977C GC/MSD (Agilent Technologies) with an Olfactory detection port (GERSTEL-ODP) connected in parallel to the MSD.
Sample Preparation: Samples of fresh and aged rapeseed oil, wheat crackers, and cheese crackers were analyzed. The rapeseed oil was stored at 40 °C for several weeks to simulate the aging process; The wheat and cheese crackers were stored at room temperature for a year. One batch of cheese crackers was subjected to an accelerated aging process at 40 °C.
The Extraction: The oils were extracted for 15 minutes at 90 °C with a helium flow of 50 mL/min in the TDU (direct thermal extraction). Analytes were cryofocused in the CIS (PTV-type GC inlet) at -120 °C and then transferred to the column by rapidly heating the CIS to 280°C in split mode (5:1). The crackers were incubated at 40 °C for two minutes. The exhaustive extraction of the analytes (Dynamic Headspace Option, DHS) was performed for 20 minutes at 40 °C under a helium flow of 50 mL/min, corresponding to a gas volume of 1000 mL. The analytes were enriched in thermal desorption glass tubes packed with Tenax-TA. The subsequent steps, i.e. thermal desorption in the TDU, cryofocusing in the KAS and transfer of the analytes to the GC column, were identical to those in DTE.
MSD/Olfactometry: GC/O analysis was performed with a 2:1 split between the ODP and MS detection systems. The ODP transfer line temperature was set to 250 °C. The mixing chamber was set to 150 °C and flushed with humidified nitrogen make-up gas to prevent olfactory fatigue and nasal dehydration.
The result: In this work, Sensory Directed Analysis (SDA) has enabled the identification of important sensory-active compounds, responsible for off-odors caused by lipid oxidation. The data (see GERSTEL AppNote 252) shows clear differences in the chromatograms and in the sensory perception between fresh, aged, and very aged samples. It was shown that the presence or absence of a compound does not necessarily provide information about its sensory effect; many compounds were detected by the MSD but were not registered at the ODP. In contrast, several compounds were smelled at the ODP, although no clear signal appeared in the chromatogram. In other words: Without the combination of MSD and ODP, valuable information would have been missed in the product evaluation and analysis. The SDA approach is suitable for a variety of applications, i.e. to identify sensory active compounds, helping to safeguard the quality and acceptance of high-quality food products and increase consumer satisfaction.