Abstract
A novel approach based on headspace solid-phase microextraction (HS-SPME) combined with comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry (GC×GC–ToFMS) was developed for the simultaneous screening of microbial and mite contamination level in cereals and coffee beans. The proposed approach emerges as a powerful tool for the rapid assessment of the microbial contamination level (ca. 70 min versus ca. 72 to 120 h for bacteria and fungi, respectively, using conventional plate counts), and mite contamination (ca. 70 min versus ca. 24 h). A full-factorial design was performed for optimization of the SPME experimental parameters. The methodology was applied to three types of rice (rough, brown, and white rice), oat, wheat, and green and roasted coffee beans. Simultaneously, microbiological analysis of the samples (total aerobic microorganisms, moulds, and yeasts) was performed by conventional plate counts. A set of 54 volatile markers was selected among all the compounds detected by GC×GC–ToFMS. Principal Component Analysis (PCA) was applied in order to establish a relationship between potential volatile markers and the level of microbial contamination. Methylbenzene, 3-octanone, 2-nonanone, 2-methyl-3-pentanol, 1-octen-3-ol, and 2-hexanone were associated to samples with higher microbial contamination level, especially in rough rice. Moreover, oat exhibited a high GC peak area of 2-hydroxy-6-methylbenzaldehyde, a sexual and alarm pheromone for adult mites, which in the other matrices appeared as a trace component. The number of mites detected in oat grains was correlated to the GC peak area of the pheromone. The HS-SPME/GC×GC–ToFMS methodology can be regarded as the basis for the development of a rapid and versatile method that can be applied in industry to the simultaneous assessment the level of microbiological contamination and for detection of mites in cereals grains and coffee beans.
Highlights
The food industry suffers enormous financial losses due to complains arising out of off-flavours, which leads to the loss of consumers and suppliers confidence [1]
The GC-qMS analysis was only applied to the five chemical standards, and their identification in all assays were confirmed by their retention times and mass spectra, which were compared with the library data system of the gas chromatography–quadrupole mass spectrometry (GC–qMS) equipment (Wiley 275)
In order to optimize the solid phase microextraction (SPME) procedure, a full-factorial design was implemented, which comprised the evaluation of three extraction temperatures (30.0, 40.0 and 50.0 C), two extraction times (10 and 30 min) and four coating SPME fibres (PA, polydimethylsiloxane coating (PDMS), PDMS/DVB, and DVB/CAR/PDMS)
Summary
The food industry suffers enormous financial losses due to complains arising out of off-flavours, which leads to the loss of consumers and suppliers confidence [1]. Apart from offflavours, serious health problems can arise, due to the biological deterioration as a possible consequence of storage of those grains. While in storage, this biological deterioration can result of several pests such as insects, rodents, mites and microorganisms (especially fungi). The most common volatiles associated to microbial contamination in cereals and other types of foods are 2-methyl-1-propanol, 3-methyl-1-butanol, 1-octanol, 1-octen-3-ol, 2-butanone, 3-octanone, 2-hexanone, 2-heptanone, 2-methylisoborneol, geosmin, limonene, dimethyl disulfide and 3methylfuran [2,6,7,8,9,10,11,12]. The information on the volatile profiles of other organism associated to food spoilage such as microscopic invertebrates is extremely scarce, mites are known to produce volatile compounds associated to specific ‘‘minty’’ odours in contaminated foods [13]
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