Numerous publications have described measurements of breath isoprene in humans, and there has been a hope that breath isoprene analyses could be a noninvasive diagnostic tool to assess blood cholesterol levels or cholesterol synthesis rate. However, significant analytic problems in breath isoprene analysis and variability in isoprene levels with age, exercise, diet, etc., have limited the usefulness of these measurements. Here, we have applied proton transfer reaction-mass spectrometry to this problem, allowing on-line detection of breath isoprene. We show that breath isoprene concentration increases within a few seconds after exercise is started as a result of a rapid increase in heart rate and then reaches a lower steady state when breath rate stabilizes. Additional experiments demonstrated that increases in heart rate associated with standing after reclining or sleeping are associated with increased breath isoprene concentrations. An isoprene gas-exchange model was developed and shows excellent fit to breath isoprene levels measured during exercise. In a preliminary experiment, we demonstrated that atorvastatin therapy leads to a decrease in serum cholesterol and low-density-lipoprotein levels and a parallel decrease in breath isoprene levels. This work suggests that there is constant endogenous production of isoprene during the day and night and reaffirms the possibility that breath isoprene can be a noninvasive marker of cholesterologenesis if care is taken to measure breath isoprene under standard conditions at constant heart rate.

The evolution of the microbial spoilage population for air-and vacuum-packaged meat (beef and pork) stored at 4°C was investigated over 11 days. We monitored the viable counts (mesophilic total aerobic bacteria, Pseudomonas spp., Enterobacteriaceae, lactic acid bacteria, and Enterococcus spp.) by the microbiological standard technique and by measuring the emission of volatile organic compounds (VOCs) with the recently developed proton transfer reaction mass spectrometry system. Storage time, packaging type, and meat type had statistically significant (P < 0.05) effects on the development of the bacterial numbers. The concentrations of many of the measured VOCs, e.g., sulfur compounds, largely increased over the storage time. We also observed a large difference in the emissions between vacuum-and air-packaged meat. We found statistically significant strong correlations (up to 99%) between some of the VOCs and the bacterial contamination. The concentrations of these VOCs increased linearly with the bacterial numbers. This study is a first step toward replacing the time-consuming plate counting by fast headspace air measurements, where the bacterial spoilage can be determined within minutes instead of days.Meat is one of the most perishable foods, and its composition is ideal for the growth of a wide range of spoilage bacteria. Public concern has risen due to numerous food scandals such as those surrounding bovine spongiform encephalopathy and foot-and-mouth disease epidemics (8,9,19), and food-borne diseases remain a substantial burden (21). We can meet these challenges with an improved and global food safety control system. One possible improvement would be a rapid and accurate detection system for microbial spoilage. This technique should ideally also be nondestructive and give results in real time for application in highly automated food-processing environments. Current methods are time-consuming, labor intensive, and, therefore, give retrospective information (8). The common method used for determining the status of meat, with respect to spoilage, is analysis of the counts of total viable bacteria and/or specific spoilage bacteria. An obvious drawback with this method is the incubation period of 1 to 3 days that is required for colony formation. For enrichment cultures several days are needed. Molecular methods have been described as useful approaches to type bacteria and monitor community development in meat (27); quantification of microbial numbers, however, is not yet feasible. Therefore we explore here a novel and very fast method to determine the status of a meat sample within a few minutes to make real-time meat controls possible. Volatile organic compounds (VOCs) produced by meat bacteria have been analyzed by gas chromatography-mass spectrometry (MS) (5, 7) and have been detected by an electronic nose and a sensory panel (3), suggesting the helpfulness of VOC measurements in order to analyze spoilage. The objective of the present work was to evaluate the ability of the proton transfer reaction (PTR)-MS...

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Proton transfer reaction-mass spectrometry (PTR-MS) measurements on single intact strawberry fruits were combined with an appropriate data analysis based on compression of spectrometric data followed by class modeling. In a first experiment 8 of 9 different strawberry varieties measured on the third to fourth day after harvest could be successfully distinguished by linear discriminant analysis (LDA) on PTR-MS spectra compressed by discriminant partial least squares (dPLS). In a second experiment two varieties were investigated as to whether different growing conditions (open field, tunnel), location, and/or harvesting time can affect the proposed classification method. Internal cross-validation gives 27 successes of 28 tests for the 9 varieties experiment and 100% for the 2 clones experiment (30 samples). For one clone, present in both experiments, the models developed for one experiment were successfully tested with the homogeneous independent data of the other with success rates of 100% (3 of 3) and 93% (14 of 15), respectively. This is an indication that the proposed combination of PTR-MS with discriminant analysis and class modeling provides a new and valuable tool for product classification in agroindustrial applications.

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