I. Martínez-Castro

Derivatization of carbohydrates for GC and GC–MS analyses

GC and GC-MS are excellent techniques for the analysis of carbohydrates; nevertheless the preparation of adequate derivatives is necessary. The different functional groups that can be found and the diversity of samples require specific methods. This review aims to collect the most important methodologies currently used, either published as new procedures or as new applications, for the analysis of carbohydrates. A high diversity of compounds with diverse functionalities has been selected: neutral carbohydrates (saccharides and polyalcohols), sugar acids, amino and iminosugars, polysaccharides, glycosides, glycoconjugates, anhydrosugars, difructose anhydrides and products resulting of Maillard reaction (osuloses, Amadori compounds). Chiral analysis has also been considered, describing the use of diastereomers and derivatives to be eluted on chiral stationary phases.

Changes in flavour and volatile components during storage of whole and skimmed UHT milk

Abstract Six batches of commercial UHT milk, submitted to direct treatment, three whole and the other three skimmed milk, were stored at 25±2°C for 4 months. Non-casein nitrogen (NCN) and sensorial analysis were carried out on packs opened every month. Volatile composition was analysed every 15 days, using a purge-and-trap concentrator coupled on-line to a GC–MS instrument. NCN increased during storage; the increase was greater in skimmed milk samples. Sensory characteristics were slightly better in the whole samples, although the scores decreased for both groups in the third month. Quantification of about 40 volatile components in whole milks showed no changes until 90 days (the legal shelf-life in Spain); the main change was the increase of methyl ketones. New components appeared in skimmed samples after 65 days storage; they could be related to both proteolysis and Maillard reaction. This is consistent with the poorer sensory quality found in skimmed milk samples.

Chemical changes during microwave treatment of milk

Abstract Raw milk was heated in a microwave oven at 2450 MHz in order to study the effect on the main chemical changes taking place during heating processes: lactose isomerization, Maillard reaction and protein denaturation. Lactulose, epilactose, furosine and undenaturated whey proteins were measured as indicators of the heat damage in milk. Comparison with control samples treated by conventional heating showed a rate enhancement of the studied reactions during microwave treatment. These differences are supposed to be due, at least to some extent, to uneven heating of the milk in the microwave oven.

Characterization ofO-trimethylsilyl oximes of disaccharides by gas chromatography-mass spectrometry

SummaryTwenty-three disaccharides have been analysed by GC-MS as theirO-trimethylsilyl oximes. The relationships between their mass spectral fragmentation and their structural characteristics have been studied in order to use GC-MS information for qualitative purposes. The presence of some structural features, such as the 1–2 or 1–6 linkages, can be estimated from the relative intensity of fragment ions. The method has been applied to the determination of the components present in the disaccharide fraction obtained from lactose treatment with β-galactosidase fromK. fragilis.

Gas chromatographic analysis of free monosaccharides in milk

SummaryThree silylation methods for the gas chromatographic analysis of ketohexoses have been compared for yield and tautomeric composition of trimethylsilyl ethers. One of them has been applied to the analysis of milk monosaccharides and evaluated for precision and accuracy. Of the different stationary phases employed, OV-215 and AT-1000 were the best suited to resolve the TMS ethers of the studied sugars.

Evaluation and optimization of the automatic thermal desorption method in the gas chromatographic determination of plant volatile compounds

Abstract The automatic thermal desorption (ATD) method was used in the gas chromatographic determination of the volatile components of plants. The reproducibility of the method was evaluated for several operating conditions; the results were better than those obtained with other sample preparation methods (simultaneous distillation-extraction and solvent extraction). Some applications of the ATD method in the gas chromatographic determination of volatile components of Umbelliferae seeds are also presented, including the determination of the enantiomeric forms of limonene.

Chemical and sensorial changes in milk pasteurised by microwave and conventional systems during cold storage

Raw milk was submitted to a continuous-flow microwave treatment at 80 or 92°C for 15 s or in a conventional heat exchanger under the same conditions. After pasteurisation, samples were bottled in sterile containers and stored at 4.5±0.5°C for up to 15 days. Samples were taken every 2 days and analysed for pH, sensory properties, volatile compounds and monosaccharides. Volatiles (including aldehydes, ketones, alcohols, esters and aromatic hydrocarbons) in freshly pasteurised samples were very similar to those found in raw milk. Evolution of all analysed parameters during storage was similar for the two series of samples; microwave heating had no adverse effects on flavour.

Kinetics of galactose and tagatose formation during heat-treatment of milk

Abstract The kinetics of the formation of galactose and tagatose during the heating of milk at various temperatures (115–135°C) was studied. A first-order kinetic model was satisfactorily used to describe the formation of both carbohydrates. The Arrhenius plot enabled calculation of the activation energies to be made (113 kJ/mol for galactose and 115 kJ/mol for tagatose). The formation of tagatose in appreciable amounts takes place only under sterilization conditions, and its presence in commercial milks may be an indication of the severity of the heat-treatment.

Rapid identification of volatile compounds in aromatic plants by automatic thermal desorption — GC-MS

SummaryThermal desorption is a valuable method for the fractionation of plant volatile components, which can be carried out on-line with GC analysis. The use of coupled GC-MS affords additional qualitative information, of special interest for plant species whose composition has not been previously studied. Some examples of the application of automatic thermal desorption, coupled to GC-MS to the identification and characterization of volatile components of plants of different families are given.

Simultaneous distillation-extraction (SDE) method in the qualitative and quantitative GC analysis of cheese volatile components

SummaryA simultaneous distillation-extraction apparatus proposed by Godefroot has been used for the GC study of volatile components of cheese. 5–10 g of cheese provide a fraction that can be directly injected into a GC or a GC/MS. Two capillary columns (SE-30 and SP-1000) were evaluated, the second one being adequate for quantitative determinations. Camphor was used as internal standard. The coefficients of variation ranged from 1.4 to 11.8. The technique can be used as a fast method for the characterization of cheeses from their volatile component concentrations.