The Formation of Methyl Ketones during Lipid Oxidation at Elevated Temperatures

Lipid oxidation and the resulting volatile organic compounds are the main reasons for a loss of food quality. In addition to typical compounds, such as alkanes, aldehydes and alcohols, methyl ketones like heptan-2-one, are repeatedly described as aroma-active substances in various foods. However, it is not yet clear from which precursors methyl ketones are formed and what influence amino compounds have on the formation mechanism. In this study, the formation of methyl ketones in selected food-relevant fats and oils, as well as in model systems with linoleic acid or pure secondary degradation products (alka-2,4-dienals, alken-2-als, hexanal, and 2-butyloct-2-enal), has been investigated. Elevated temperatures were chosen for simulating processing conditions such as baking, frying, or deep-frying. Up to seven methyl ketones in milk fat, vegetable oils, and selected model systems have been determined using static headspace gas chromatography-mass spectrometry (GC-MS). This study showed that methyl ketones are tertiary lipid oxidation products, as they are derived from secondary degradation products such as deca-2,4-dienal and oct-2-enal. The study further showed that the position of the double bond in the precursor compound determines the chain length of the methyl ketone and that amino compounds promote the formation of methyl ketones to a different degree. These compounds influence the profile of the products formed. As food naturally contains lipids as well as amino compounds, the proposed pathways are relevant for the formation of aroma-active methyl ketones in food.

Mechanism of Collagen Processed with Urea Determined by Thermal Degradation Analysis*

During the beamhouse process for nappa leather, pelts are usually limed with amino compounds such as urea, ethylenediamine, and triethanolamine. However, the interaction between amino compounds and collagen is not well known. In this work, collagen fibers were soaked in various concentrations of urea and the thermal degradation of collagen fibers were studied by the methods Horowitz-Metzger and Coats-Redfern. The mechanism of the reaction between urea and collagen fibers is discussed, wherein the thermal degradation activation energy first decreases and then increases. The lowest thermal degradation activation energy of urea processed collagen appears at 2-3 mol/L urea, suggesting that the stability of collagen is the poorest when the pelt is processed in the urea solution. At the urea concentration above 6 mol/L, the thermal degradation activation energy of the sample is similar to samples without urea processing and the higher concentrations does not have the same effect as lower concentrations of urea. The collagen fibers with a urea processing history were washed to remove the urea in them, and the samples were studied again for their thermal degradation behavior. The results indicated that the thermal degradation activation energy of the collagen fibers might recover to the unprocessed level. Therefore, it was suggested that the reaction between urea molecules and collagen fibers is reversible. Urea molecules might help to destroy some of the hydrogen bonds between collagen peptides in the urea solution. After the urea is washed out, the structure of the collagen will return to its original state, because the hydrogen bonds might be reconstructed

Ultrasound Assisted Cu(I) Catalyzed Aza-Michael Reactions in Water

The β-amino compounds were synthesized from α,β-unsaturated esters, nitriles and amine using water as a solvent in the presence of 10 mol % Cu(I) catalyst in high yields within 2-5 min under ultrasound irradiation at room temperature. The reaction rate was enhanced tremendously under ultrasound irradiation as compared to conventional methods with improved yields have been recorded.

2-Unsubstituted Imidazole N-Oxides as Novel Precursors of Chiral 3-Alkoxyimidazol-2-ylidenes Derived from trans-1,2-Diaminocyclohexane and Other Chiral Amino Compounds

‘Desymmetrization’ of trans-1,2-diaminocyclohexane by treatment with α,ω-dihalogenated alkylation reagents leads to mono-NH2 derivatives (‘primary-tertiary diamines’). Upon reaction with formaldehyde, these products formed monomeric formaldimines. Subsequently, reactions of the formaldimines with α-hydroxyiminoketones led to the corresponding 2-unsubstituted imidazole N-oxide derivatives, which were used here as new substrates for the in situ generation of chiral imidazol-2-ylidenes. Upon O-selective benzylation, new chiral imidazolium salts were obtained, which were deprotonated by treatment with triethylamine in the presence of elemental sulfur. Under these conditions, the intermediate imidazol-2-ylidenes were trapped by elemental sulfur, yielding the corresponding chiral non-enolizable imidazole-2-thiones in good yields. Analogous reaction sequences, starting with imidazole N-oxides derived from enantiopure primary amines, amino alcohols, and amino acids, leading to the corresponding 3-alkoxyimidazole-2-thiones were also studied.