The production of hydrogen sulphide and other aroma compounds by wine strains of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> in synthetic media with different nitrogen concentrations

The aim of this study was to evaluate the combined effect of initial nitrogen content on the production of hydrogen sulphide and other volatile compounds during alcoholic fermentation. For that propose, three commercial wine strains of Saccharomyces cerevisiae were used to ferment synthetic grape juice media under different nitrogen concentrations. H₂S was measured throughout fermentations and other aroma compounds were analyzed at the end of the experiments. The trigger levels at which an inverse relationship between the initial nitrogen present in media and total H₂S production varied among the three strains tested. For UCD522 and PYCC4072, the highest H₂S levels were produced in media with 267 mg N l⁻¹ of initial nitrogen, whereas the lowest levels were detected with nitrogen limitation/starvation conditions (66 mg N l⁻¹). Moreover, 21 other aroma compounds belonging to different chemical classes were identified and quantified by solid phase micro-extraction (SPME) coupled to gas chromatography–mass spectrometry (GC–MS). The initial nitrogen concentration more than yeast strain had a decisive effect on the final aroma composition, suggesting that modulation of nutrients emerges as a useful tool for producing desired flavour and odour compounds.     

Identification of the possible new odor-active compounds “12-methyltridecanal and its analogs” responsible for the characteristic aroma of ripe Gouda-type cheese

The aroma concentrates of the three maturation stages of Gouda-type cheeses were prepared by combining the solvent extraction and the solvent assisted flavor evaporation techniques. The aroma extract dilution analysis applied to the volatile fraction revealed 31 odorants that were identified or tentatively identified from the 38 odor-active peaks with FD factors between 4³ and 4⁸. By comparison with the FD factors in the three maturation stages of the cheeses, 16 odorants, including 12-methyltridecanal, which is a newly identified odorant from the cheese, increased with the increasing maturation stage of the cheese. In addition, many iso- and anteiso-methyl-branched long-chain aliphatic aldehydes could be identified as the analogs of 12-methyltridecanal, which have a unique odor note. It may be then expected that these aldehydes were able to influence the flavor of the highly ripened Gouda cheese, since these compounds also increased with the increasing maturation stage.     

An isotope dilution gas chromatographic-mass spectrometric method for the simultaneous assay of estrogens and phytoestrogens in urine

The metabolism of endogenous estrogens is complicated and certain metabolic patterns may reflect an individual risk of estrogen-dependent diseases such as breast cancer. Since the 1960s we have been constantly involved in developing estrogen profiling methods, in the beginning using gas chromatography and later gas chromatography–mass spectrometry (GC–MS) in the selected ion monitoring mode (SIM) and finally utilizing isotope dilution (ID–GC–MS–SIM). The addition of the dietary phytoestrogens to the profile rendered the method even more complicated. The present work presents the final estrogen profile method for 15 endogenous estrogens, four lignans, seven isoflavonoids and coumestrol in one small urine sample (1/150th of a 24 h human urine sample, minimum 2.5–5 ml) with complete validation including investigations as to the precision, sensitivity, accuracy and specificity. The method does not include the minimal amounts of unconjugated estrogens in urine. It may also be used for animal (e.g. rat and mouse) urine using a minimum of 2 ml of usually pooled sample. Despite its complexity it was found to fulfill the reliability criteria, resulting in highly specific and accurate results.     

On the study of plant defence and herbivory using comparative approaches: how important are secondary plant compounds

Species comparisons are a cornerstone of biology and there is a long tradition of using the comparative framework to study the ecology and evolution of plant defensive traits. Early comparative studies led to the hypothesis that plant chemistry plays a central role in plant defence, and the evolution of plant secondary chemistry in response to insect herbivory remains a classic example of coevolution. However, recent comparative work has disagreed with this paradigm, reporting little connection between plant secondary chemicals and herbivory across distantly related plant taxa. One conclusion of this new work is that the importance of secondary chemistry in plant defence may have been generally overstated in earlier research. Here, we attempt to reconcile these contradicting viewpoints on the role of plant chemistry in defence by critically evaluating the use and interpretation of species correlations as a means to study defence–herbivory relationships. We conclude that the notion that plant primary metabolites (e.g. leaf nitrogen content) are the principal determinants of herbivory (or the target of natural selection by herbivores) is not likely to be correct. Despite the inference of recent community‐wide studies of herbivory, strong evidence remains for a prime role of secondary compounds in plant defence against herbivores.