PTR-MS monitoring of VOCs and BVOCs in food science and technology
Introduction
In environmental studies and atmospheric chemistry, the origin of VOCs (volatile organic compounds) is typically characterized as either “biogenic” or, the opposite, “anthropogenic”. So the term “biogenic VOC” (BVOC) is one of the typical trademarks of these fields and is less used in other contexts. However, VOCs in general and BVOCs, in particular, play a most relevant role in agro-industrial processes, and food science and technology.
The principal reason is that they are at the origin of both the aroma and flavor of food and hence control human sensory perception of food. For example, the smell, also called orthonasal stimulation, is caused by the appealing scent of fresh fruit, while the flavor released during eating or drinking, is part of the long-lasting pleasure (e.g., of a glass of wine that is modified by the changes of the VOCs released in our mouth and reaching our nose, this being a vivid example of retronasal stimulation). Thus, improvement in food quality must also consider its complex flavor profile, the origin of this profile, its evolution in time and the interaction with people [1], [2]. It is worth noting that, among the human senses, olfaction (also termed a chemical sense) is probably the most complex and least understood. In contrast to the physical senses, hearing and sight, or taste, the olfactory system comprises a large number of receptors, more than 900, and it has only relatively recently been deciphered [3]. It is clear that the success of future research in this field will also depend on the available methods to identify, to quantify and to monitor the olfactory stimuli (i.e. VOCs).
A second reason for the relevance of VOCs in food science is that they are produced and released in most stages of the food-production chain “from farm to fork” (i.e. from plants and crops, during fruit ripening and maturation, in food processing and storage and, eventually, during food consumption). They also create an interesting link to atmospheric studies because, in several regions, BVOC emissions are, at least for certain species, dominated by emissions from crops [4]. Direct BOVC monitoring is therefore an important non-invasive tool for product characterization and process monitoring in agro-industrial applications. With regards to the latter, there is also increasing awareness about BVOC emission from food industries, farms or composting plants because of public health and environmental issues [5]. Moreover, food-related BVOCs were recently investigated in connection with emerging themes {e.g., their possible link with health [2], [6] or ecological issues also in view of organic pest management [7]}.
Given this, it does not come as a surprise that there is a huge literature on VOCs in food, mostly of biogenic origin. A general reference for technical aspects, not limited to foods, is the book edited by Berger [8], which also includes interesting chapters on biogenic precursors of flavor compounds and bioprocessing. Two additional references in this context are a recently edited book by Taylor and Linforth [9], and the review of Ross on studies related to sensory analysis and instrumental characterization of food [10].
From a technical point of view, gas chromatography (GC) [11] is the reference method for the analysis of food VOCs. But there is also a growing need to develop rapid, simpler methods [12] to overcome its main limitations, as GC is still a time-consuming procedure. There is therefore genuine interest in the development of methods for rapid, non-invasive and very sensitive monitoring of volatile compounds emitted from food samples. On the one hand, a rapid procedure is necessary to go in the direction of high-throughput analysis that can be of benefit for analytical laboratories, quality control and, in view of possible connections of BVOC profiles, genomics and metabolomics [13]. On the other hand, high time resolution will allow real-time monitoring of fast processes in agro-industry [14], [15], oral processing during food consumption and model mouth experiments in connection with textural changes [16].
Among the various possibilities proposed and investigated for rapid identification and quantification of VOCs in food [9] is proton transfer reaction mass spectrometry (PTR-MS) [17], [18], which allows rapid, direct and highly sensitive on-line monitoring of VOCs.
In this contribution, we review the work of the various groups that have addressed the above-mentioned issues in food science and technology by PTR-MS. In Section 2, we briefly describe the technique and give a methodological perspective, in order to provide a rationale for its use in food science and technology. In Section 3, we review several practical applications, and, in Section 4, we describe recent developments. Section 5 is dedicated to conclusions and future trends.
Section snippets
The PTR-MS technique
PTR-MS has been described in detail in several reviews: the first by Lindinger and co-workers [17]; a more recent, general review by Blake et al. [18]; and, the very thorough one by de Gouw and Warnecke, focusing on environmental applications [19]. Here, we briefly describe its main characteristics and highlight the benefits of applying PTR-MS in food science and technology. In the most common version of PTR-MS apparatus (Fig. 1), a hollow cathode ion source produces an intense, pure H3O+ ion
Applications of PTR-MS in food science and technology
We organize this review along the following methodological steps (Fig. 2). We start off with applications where PTR-MS is used to obtain fingerprints of products and processes (Section 3), without attempting to assign the mass peaks, and leading to food-sample classification. Another possibility is to correlate these fingerprints with data (on the same set of samples) from alternative techniques (e.g., sensory analysis). Furthermore, one can take advantage of the high time resolution and
Recent technical developments
The potential of PTR-MS has been improved by a number of fruitful collaborations of food scientists with companies and other communities (e.g., investigation of ion-molecule chemistry or application of innovative methods for data analysis).
Most PTR-MS studies have been performed with instruments equipped with a quadrupole mass analyzer that provides only the nominal mass of the observed peaks and does not allow for the separation of isobaric compounds. Since the very beginning, food scientists
Conclusions and perspectives
Due to the importance of VOCs, mostly of direct biogenic origin, in food science and technology, there is an increasing interest in the development of simple, fast and high-sensitivity methods for their identification and quantification.
PTR-MS is an accurate, highly sensitive, direct-injection technique that allows for the rapid characterization of food products and for the monitoring of processes in food science and technology, and agro-industry, without any pre-treatment.
Although PTR-MS is
Acknowledgments
FB thanks Alessandra Lucianer and her team for the direct and indirect, but always useful, support. Work partially supported by PAT (APR 2009/2010).
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