Chemistry
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PTR-ToF-MS Coupled with an Automated Sampling System and Tailored Data Analysis for Food Studies: Bioprocess Monitoring, Screening and Nose-space Analysis
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Summary May 11th, 2017
Proton Transfer Reaction Time of Flight Mass Spectrometry allows high-sensitivity, rapid and non-invasive analysis of volatile organic compounds. To demonstrate its potential, we give three examples: lactic acid fermentation of yogurt (on-line bioprocess monitoring), different apple genotypes (large-scale screening), and retronasal space after drinking coffee (nosespace analysis).
Transcript
The overall goal of this paper is to demonstrate the potential of PTR-ToF-MS for direct volatile organic compound analysis in food science and technology and in particular the importance of automated sampling and tailored data analysis. Volatile organic compound play a key role in food science and technology both because they control flavor and odor perception and because they provide a tool for the monitoring of food quality. The high sensitivity and the rapid and direct injection make PTRMS one of the best choices for the monitoring of volatile organic compound release for bio process monitoring, for the screening of large data sets and for nose space analysis.
We also develop proper data and the analysis tools which are necessary for proper exploitation of large and complex data sets such as there was used by PTRMS in a very short time. Future demonstration of our methodology can inspire searches from different statistical and scientific fields such as SENSOMICS, metabolomics, path analysis, and phonemics. To begin the first experiment, add five milliliters of pasteurized milk to 20 milliliter glass vials equipped with PTFE silicone septa.
Then transfer the samples to a multi functional GC auto sampler equipped with a temperature control tray. Use the robotic arm of the auto sampler to inoculate the vials with the microbial starter cultures, set the incubation time according to the typology of the desired process and the specifications reported by the starter culture manufacturer. Then set the auto sampler to analyzer one sample after the other obtaining an online volatile organic compound monitoring of lactic acid fermentation during yogurt preparation.
The second experiment select at least five apples at the desired phase of ripening without any visible damage for each variety. Store the apples for the desired period at four degrees celsius. Following this, collect five cylindrical disks from each apple with a flesh blade sampler including part of the cortex tissue and avoiding the core portion with seeds.
After freezing the samples and liquid nitrogen homogenize them. Place three replicates of 2.5 gram apple samples from each variety into 20 milliliter glass vials. Mix each sample with an antioxidant solution obtained by dissolving one gram of sodium chloride, 12.5 milligrams of ascorbic acid and 12.5 milligrams of citric acid in 2.5 milliliters of deionized water.
Incubate the samples at 40 degree Celsius. Then set the auto sampler to automatically analyze the volatile organic compounds. For the last experiment, prepare brewed coffee from ground coffee samples with a six cup stove top coffee maker using 450 milliliters of water and 30 grams of coffee powder.
Transfer the brewed coffee to a bottle and place the bottle in a 60 degree celsius thermostatic water bath. Report the water powder ratio, the type of mineral water used the type of coffee maker and the procedure adopted to obtain the coffee beverage. For each coffee brew, transfer 7.5 milliliter aliquots to a 40 milliliter polystyrene cup with a plastic cap.
Have each panelist taste the beverage according to the protocol. While PTR-ToF-MS continuously samples the Nose space concentration via a single use ergonomic silicon rubber nose piece at the nose of each panelist. This nose piece is connected to the PTR-ToF-MS by a PIC tube with the part in contact with panelist at room temperature and the remaining tube heated at 110 degree Celsius and an inlet hose that connects the sampling interface with the instrument.
To perform head space measurements of the samples directly inject air into the PTRMS drift tube head space by inserting the end of the PTRMS inlet into the sample head space. Before and during measurements, set and constantly verify the following ionization conditions in the drift tube to lead to an E/N ratio of 140 Townsend. If an auto sampler is used, set the minimum inlet flow to 40 standard cubic centimeters.
Set the sampling time per channel of ToF acquisition to a master charge ratio range. Use the self developed software to correct the count losses due to the ion detector dead time and perform internal calibration, compound annotation noise reduction, and peak extraction. Mine the data by performing principle compound analysis analysis of variance Tukey's post-hoc test and other statistical analysis adapting existing packages developed using Matlab.
The three experiments provide examples of applying our methodology bio process monitoring, screening of sample sets, and in vivo nose space analysis. The online volatile organic compound profile of samples gives a complete mass spectrum every second and more than 300 mass peaks can be identified in the acquired average spectra during the yogurt fermentation. Fermentation kinetics of nine selected mass peaks during yogurt fermentation using four different starter cultures are shown here.
Most of these volatiles displayed classical microbial like kinetics with an initial lag phase followed by a growth phase and a post-log phase. Interestingly the online analysis highlighted particular depletion kinetics for four sulfur containing compounds which can have a sensory effect even at a very low concentration. The dendrogram based on volatile organic compounds associated with a large apple collection represented by a 198 sessions highlights the presence of six main clusters mainly determined by esters and alcohols.
The cumulative profiles for five coffee testers represented by means of radial plots are shown here. The results demonstrate the presence of reproducible and relevant differences between panelists as evident for panelist P one and P two. The difference of samples, the most tried out methodology can provide reliable data on volatile organic compound release, which when compared with our methods can be obtained in a shorter time.
The possibility of measuring many samples the key feature of our approach. Because it allows controlling and tracing viability of biological samples. After this development, this methodology paid well for the researchers in different fields related to volatile organic compound monitoring to explore the link between their release and genetics and the screening of populations.
After watching this video, you should have a good idea of how to explore the potential of PTR-ToF-MS in food and technology analysis to analyze volatility by fruits. Please remember that samples and measuring line are exposed to air during your experiment. So avoid to introduce some sources of volatile organic compounds.
Another advantage of PTRMS is that it doesn't involve the use of solvents or hazardous dyes and it is a green chemistry technique. However don't forget that your samples can be extremely hazardous.
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