Odors In the Food Industry (eBook)

SERIES ACKNOWLEDGMENTS 6
SERIES PREFACE 7
PREFACE 9
CONTENTS 11
Odor Problems in the Food Industry 15
Odor Measurement 28
Preconcentration Prior to Gas Chromatography 53
The Application of Intelligent Sensor Array for Air Pollution Control in the Food Industry 58
Electronic-Nose Technology: Application for Quality Evaluation in the Fish Industry 68
Odors Prevention in the Food Industry 86
Odors Treatment: Physicochemical Technologies 116
Odors Treatment: Biological Technologies 135
Index 169

3 Preconcentration Prior to Gas Chromatography (S. 41-42)

Elefteria Psillakis

1. PRECONCENTRATION TECHNIQUES

There are several obstacles impeding complete characterization of odorous samples when using gas chromatography (GC). In many cases identifi cation remains ambiguous or questionable as a result of the presence of unknown components at very low concentration levels. This is due to the fact that direct injection of odorous samples into a GC is in most cases impossible due to the low concentrations of malodorous compounds in these samples. This problem can be overcome if a preconcentration step prior to analysis is introduced. In general there are three possibilities for enriching components in an air sample: absorbing the compounds in a suitable liquid, condensing them at low temperatures (cryotrapping), and adsorbing them on a porous solid material. In the case of liquid absorption, a liquid solution is used to absorb or react with the target volatile compounds. An aliquot of the solvent solution containing the entrapped odorous compounds is then injected into the analytical device for further analyses. There are many drawbacks inherent to liquid absorption. A solvent evaporation step is usually required, leading to large losses of the volatiles (Pillonel et al., 2002). Furthermore, sample contamination due to contact with glassware is possible, therefore, a thorough cleaning of all laboratory equipment coming into direct contact with the sample is required. In cases where impingers are used, the liquid containing the entrapped analytes may be subject to spillage, and in these cases, a liquid trap should be used in order to prevent the solution from getting into the pump. Finally, during analysis, interferences due to solvent impurities also may obstruct quantifi cation. It also should be mentioned here that it is generally accepted that it is easier to precondition a solid phase than to purify a liquid phase.

In cryogenic trapping, volatile compounds are trapped on an inert surface, such as glass-fi ber wool, glass beads, Tenax®, Porapak Q®, or even activated carbon. According to this method, during sampling the trap is most frequently immersed in liquid nitrogen (-196°C) or in liquid argon (-186°C) (Wardencki, 1998). The main advantages of cryogenic trapping versus other trapping techniques are that there are (1) no artifact from thermal desorption, (2) no carryover between runs, and (3) no breakthrough problem (Pillonel et al., 2002). However, the additional equipment needed to handle the cryogen is quite expensive and very sensitive to water. Furthermore, there are another two concerns when using this preconcentration method. First, if the cooling temperature is too low, liquid oxygen also may be trapped, which can readily oxidize organic compounds, thus altering the composition of the sample. To overcome this problem, Peltiercooling devices can be used providing temperature control within the required range for primary and secondary refocusing or concentrating a sample (Hobbs, 2001). Second, when samples are too large, then moisture can condense, forming ice and blocking/restricting the fl ow of the compounds trapped in the sample. To prevent clogging of the trap, water has to be removed effi ciently from the charged carrier gas before entering the trap (Pillonel et al., 2002).

More commonly preconcentration is achieved by sampling odorous compounds on a porous material packed in a cartridge or in short columns. In general, adsorbents have proved a successful and relatively inexpensive means of trapping volatile analytes and the associated sorbent tubes are easy to condition and small in size, facilitating collection, transport, and storage. The most commonly used sorbents were recently reviewed by Harper (2000) and the general types are inorganic adsorbents and porous materials based on carbon and organic polymers. Various adsorbents may be used individually or in combination and their selection depends on the compounds to be sampled (e.g., concentration, species and mixtrure) as well as the boiling point involved. The surface area of an adsorbent also has an impact on the amount of a given substance that can be withheld by the medium as well as the surface polarity. Carbon-based adsorbents with a large area are useful to trap very low-boiling compounds, whereas it gets diffi cult to desorb substances with higher boiling points. Porous polymers with a comparatively small surface area allow adsorption and desorption of lesser volatile components from gaseous samples.

The different characteristics of adsorbents show the need to carefully choose the right adsorption material for given mixtures (Peng and Batterman, 2000). For trapping volatile compounds with very different properties, multibed adsorbents can be helpful. Typical combinations include Tenax TA® or graphitized carbon and carbon molecular sieve (Harper, 2000). The weaker sorbent (Tenax TA®) is placed fi rst to trap the heavier molecules and the lighter compounds are retained on the stronger sorbent located in second position. Desorption always takes place in the reverse direction to the adsorption step (Pillonel et al., 2002).