Jean-Luc Le Quéré

Volatile components of ripe fruits of Morinda citrifolia and their effects on Drosophila

Abstract The only larval resource of the specialist species, Drosophila sechellia , is ripe fruits of Morinda citrifolia . The chemical composition of this fruit, which is very toxic to most Drosophila species, was investigated and 51 compounds were abundant enough to be identified by GC-MS. The ripe fruit is characterized by a large amount of carboxylic acids, especially octanoic and hexanoic acids. The biological effects of the ripe fruit and its main acids were investigated with behavioural studies. Octanoic acid is responsible for the general toxicity of the fruit to most Drosophila species; D. sechellia is the only species which is resistant to this acid. Hexanoic acid has a unique effect, causing reversible coma but no mortality. Decanoic acid is inactive. A mixture of these three acids in proportions similar to those found in the fruit, mimics the effects of ripe fruits of M. citrifolia .

Representativeness of coffee aroma extracts : a comparison of different extraction methods

Analyses for the investigation of aroma components are routinely performed on coffee aromatic extracts. Various extraction methods exist. Ideally, the extraction method used should provide an extract with sensory characteristics as close as possible to the complete product. This is particularly relevant in the case of coffee, as no single key compound has been demonstrated as being responsible for the typical flavour of roasted and ground coffee. The purpose of this study was to compare various methods to see which provided an aromatic extract most representative of coffee. Five different extraction methods were compared: supercritical fluid extraction with carbon dioxide, simultaneous distillation extraction, oil recovery under pressure and vacuum steam-stripping with water or with organic solvent. In addition, Arabica Colombia coffee was used at three different roasting levels, i.e. green coffee as well as the same coffee light-roasted and medium roasted. Sensory testing of the extracts showed that vacuum steam-stripping with water provided the most representative aroma extract, for all three coffees.

Novel Scheme for Biosynthesis of Aryl Metabolites from L-Phenylalanine in the Fungus Bjerkandera adusta

ABSTRACT Aryl metabolite biosynthesis was studied in the white rot fungusBjerkandera adusta cultivated in a liquid medium supplemented with l-phenylalanine. Aromatic compounds were analyzed by gas chromatography-mass spectrometry following addition of labelled precursors (14C- and 13C-labelledl-phenylalanine), which did not interfere with fungal metabolism. The major aromatic compounds identified were benzyl alcohol, benzaldehyde (bitter almond aroma), and benzoic acid. Hydroxy- and methoxybenzylic compounds (alcohols, aldehydes, and acids) were also found in fungal cultures. Intracellular enzymatic activities (phenylalanine ammonia lyase, aryl-alcohol oxidase, aryl-alcohol dehydrogenase, aryl-aldehyde dehydrogenase, lignin peroxidase) and extracellular enzymatic activities (aryl-alcohol oxidase, lignin peroxidase), as well as aromatic compounds, were detected in B. adusta cultures. Metabolite formation required de novo protein biosynthesis. Our results show that l-phenylalanine was deaminated to trans-cinnamic acid by a phenylalanine ammonia lyase and trans-cinnamic acid was in turn converted to aromatic acids (phenylpyruvic, phenylacetic, mandelic, and benzoylformic acids); benzaldehyde was a metabolic intermediate. These acids were transformed into benzaldehyde, benzyl alcohol, and benzoic acid. Our findings support the hypothesis that all of these compounds are intermediates in the biosynthetic pathway froml-phenylalanine to aryl metabolites. Additionally,trans-cinnamic acid can also be transformed via β-oxidation to benzoic acid. This was confirmed by the presence of acetophenone as a β-oxidation degradation intermediate. To our knowledge, this is the first time that a β-oxidation sequence leading to benzoic acid synthesis has been found in a white rot fungus. A novel metabolic scheme for biosynthesis of aryl metabolites froml-phenylalanine is proposed.

Mechanisms of Extraction of Aroma Compounds from Foods, Using Adsorbents. Effect of Various Parameters

Due to their high adsorbing capacity, different solids (carbonaceous adsorbents, zeolites, and polymers) are used to extract and concentrate aroma compounds from foods. Adsorption mechanisms are described in two phases, a kinetic stage that involves the diffusion of the analytes within the adsorbent pores and a thermodynamic stage that can be described by adsorption isotherms. These two phases determine the extraction time and the capacity of the adsorbent for a given aroma compound. Several applications involving adsorbents or sorbents have been developed, including purge and trap, solid-phase microextraction (SPME), and stir bar sorptive extractions (SBSE), headspace sorptive extraction (HSSE), which now are widely used for aroma extractions. Different extraction modes (in the headspace or by immersion in the sample) can be used to recover aroma compounds in foods. The same adsorption mechanisms take place in both cases. Various parameters affecting the extraction kinetics and the capacity of the adsorbent have to be optimized when developing an extraction method. They can be divided into three groups: physicochemical characteristics of the adsorbent, including its porosity and hydrophobia; physicochemical characteristics of the aroma compounds (among them, two parameters play an important role, the sample/adsorbent partition coefficient, which determines the affinity for the adsorbent and the volumetric mass, which influences the diffusion in the adsorbent pores); and the extrinsic parameters that depend on the sampling conditions such as pH, temperature, gas or solvent flows, time, and composition of the sample. Several models used to determine the diffusivity and partition coefficient of aroma between the food sample and the adsorbents have been developed. They are useful to understand the behavior of aroma compounds in regards to various adsorbents and in selecting the adsorbent material with the greatest affinity for the target aroma compounds. Quantitative studies based on adsorbent extraction of volatiles have been carried out. Nevertheless, competition between analytes and the saturation of these materials are limiting parameters. Thus, new strategies (SBSE, HSSE) are being developed to avoid these limiting aspects of adsorbents in order to use them in a quantitative way.

Isolation and identification of (3Z,6Z,8E)-3,6,8-Dodecatrien-1-ol in Reticulitermes santonensis Feytaud (Isoptera, Rhinotermitidae): Roles in worker trail-following and in alate sex-attraction behavior

Abstract (3Z,6Z,8E)-3,6,8-Dodecatrien-1-ol was isolated and identified by gas chromatography-mass spectrometry in workers and alates of Reticulitermes santonensis Feytaud (Isoptera, Rhinotermitidae). This unsaturated alcohol is about 10 times more abundant in alates than in workers. Synthetic (3Z,6Z,8E)-3,6,8-dodecatrien-1-ol is highly active in eliciting trail-following in workers, and the threshold concentration is about 0.1 pg/cm trail. In alates, it elicits trail-following in both sexes at low concentration and sex attraction in male alates at high concentrations. It is suggested that (3Z,6Z,8E)-3,6,8-dodecatrien-1-ol is the main component of the trail-following pheromone as well as the sex pheromone in Reticulitermes santonensis .

2-Methylthiazolidine and 4-ethylguaiacol, male sex pheromone components of the cockroachNauphoeta cinerea (dictyoptera, blaberidae): A reinvestigation

InNauphoeta cinerea, male calling behavior is associated with sex pheromone release by the sternal glands. The male pheromone that attracts females from a distance is a mixture of 2-methylthiazolidine and 4-ethylguaiacol. It is active at very low concentrations, 0.05 and 0.01 ng, respectively. Two other compounds, 3-hydroxy-2-butanone and 2-methyl-2thiazoline, act at close range, keeping the female in the vicinity of the male. The function of the volatile pheromone and those of previously described contact pheromones are discussed in regard to their possible involvement in the establishment of male dominant-subordinate relationships.

2. In vivo nonvolatile release during eating of a model cheese: relationships with oral parameters.

This study deals with the release kinetics of nonvolatile compounds (NVC) (leucine, phenylalanine, glutamic acid, citric acid, lactic acid, propanoic acid, sodium, potassium, calcium, magnesium, chloride, and phosphates) during the eating of a model cheese and the relationships to some oral (salivary and masticatory) parameters. The aroma release has previously been characterized in similar conditions [Pionnier, E.; Chabanet, C.; Mioche, L.; Le Quéré, J.-L.; Salles, C. J. Agric. Food Chem. 2004, 52, xxx-xxx (1)]. Saliva samples were collected from the tongues of eight assessors at different times during and after the chewing sequence. Atmospheric pressure ionization-mass spectrometry and/or high-performance liquid chromatography analyses have been performed on these samples in order to quantify the 12 NVC released in saliva. The maximum concentration (C(max)) in saliva varied significantly according to the compound. However, there was no significant effect of the compound on the time to reach maximum concentration (T(max)). Interindividual differences were observed for most of the parameters and for all of the NVC studied. The parameters extracted from the release profiles of the NVC were closely correlated. High T(max) and AUC (area under the curve) values could be related to high chewing time and low saliva flow rates, low chewing rates, low masticatory performances, and low swallowing rates.

Determination of taste-active compounds of a bitter Camembert cheese by omission tests

The taste-active compounds of a Camembert cheese selected for its intense bitterness defect were investigated. The water-soluble fraction (WSE) was extracted with pure water and fractionated by successive tangential ultrafiltrations and nanofiltration. The physicochemical assessment of these fractions led to the construction of a model WSE which was compared by sensory evaluation to the crude water-soluble extract, using a panel of 16 trained tasters. As no significant difference was perceived, this model WSE was then used directly or mixed with other cheese components for omission tests. Among the main taste characteristics of the WSE (salty, sour, umami and bitter), bitterness was found to be due to small peptides whose mass distribution was obtained by RPHPLC-MS (400-3000 Da) and whose taste properties are discussed.

Normal-phase high-performance liquid chromatography of volatile compounds Selectivity and mobile phase effects on polar bonded silica

Abstract Liquid chromatographic retention data of various volatile alcohols, phenols, ethers, aldehydes, ketones, esters, nitriles, isothiocyanates, and heterocyclic compounds were measured on diol-, cyano- and amino-bonded silica columns. The mobile phase was pentane-diethyl ether in varying compositions. The Snyder theory was applied to retention of polycyclic aromatic hydrocarbons and to selected other compounds. The role of diethyl ether in the retention process was examined. Restricted-access delocalization was observed on all three bonded phases, and was pronounced on diol and amino silica. Secondary solvent effects were important for most compounds, indicating differences in specific solvent-solute interactions between adsorbed and non-adsorbed mobile phase. Principal component analysis was performed on a large set of retention data. Esters and ethers showed clear affinity for diol silica, while alcohols and phenols were preferentially retained on amino silica.

Relevance of omission tests to determine flavour-active compounds in food: application to cheese taste

One of the main problems to overcome in determining the flavour-active compounds of a food product is to point out a causative linkage which may exist between its composition obtained by physico-chemical analysis, and its flavour properties determined by a trained sensory panel. Starting with an overview of the strategies classically used to solve the problem and their limitations, this paper presents an alternative methodology based on omission tests. Through different applications concerning cheese taste studies, the main interests of this methodology are shown and discussed.