29-06-2012, 12:48 PM
Electronic Nose to Monitor the Freshness of Red Fish (Sebastes marinus) Stored in Ice and Modified Atmosphere Packaging (MAP)
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ABSTRACT
An electronic nose called FreshSense was used as a rapid technique to monitor the freshness of red
fish stored in ice and modified atmosphere packaging (MAP). Standard compounds were measured
to study the characteristic response of the FreshSense sensors. Volatile compounds produced during
storage of red fish were monitored and the results were analysed by multivariate analysis methods.
The sensors showed good selectivity, sensitivity and repeatability to standard compounds that are
representative of spoilage compounds in fish. The FreshSense could discriminate between standard
compounds and their mixtures and was also able to discriminate between fresh samples and spoiled
samples of red fish. The CO sensor increases earlier than the other sensors and is most likely
responding to short chain alcohols and carbonyls formed during the storage of red fish. The NH3
and H2S sensors are sensitive to amines and sulphur compounds respectively formed in high
concentrations at the end of the storage period.
INTRODUCTION
Odour is one of the most important indicators of fish freshness. Traditionally, analysis of odour has
been performed either by sensory panel or by gas chromatography which are time-consuming and
costly. The electronic nose has proven to be a rapid, non-destructive technique for measuring
volatile compounds which exhibit spoilage odours in fish (Ólafsdόttir et al. 1997a).
The advent of electronic noses has opened a variety of applications in the food industry,
environmental management and medical diagnoses (Keller et al. 1996). Research focusing on fish
has also been done, such as evaluation of fish freshness and monitoring of odour (Di Natale et al.
1998a, Ghosh et al. 1998).
LITERATURE REVIEW
Spoilage process of fish and the influence of packaging techniques
The initial quality loss of fresh fish is due to autolytic changes mainly related to the break down of
nucleotides, while spoilage is due primarily to bacterial action (Huss 1995). The shelf life of fresh
fish depends mainly on storage temperature and the atmosphere around the fish (Gram and Huss
1996).
Temperature changes have a great impact on microbiological growth and activity. Many bacteria
are unable to grow at temperatures below 10°C (Huss 1995). When fish stored in ice aerobically,
Pseudomonas sp. and S. putrefaciens have been identified as specific spoilage bacteria. These
bacteria also played a dominant role in fish spoilage at higher temperature (i.e. 20°C), but other
spoilage organisms particularly Vibrionaceae developed as well (Gram et al. 1987).
Methods of fish odour evaluation
During storage of fish the odour changes from fresh through flat, sweet and stale and ends as
spoilage or putrid odour (Ólafsdόttir and Fleurence 1998). Research has shown that during each
phase of storage different volatile compounds are present and characterise the odour. Fresh fish
odour is mainly contributed by compounds that are oxidatively derived from long chain
polyunsaturated fatty acids such as eicosapentaenoic acid 20:5ω3 (Josephson et al. 1984). These
compounds have low odour thresholds and are present at low concentrations (ppb). Compounds that
contribute to microbial spoilage odours of fish are well known. TMA, ethanol and hydrogen
sulphide that result from microbial degradation of amino and fatty acids exhibit odour such as fishy,
stale, rotten and putrid and are present in high concentrations (ppm) in the fish during storage
(Gram and Huss 1996).
Development of chemical sensors
The most frequently used sensors in electronic noses are metal oxide semiconductor sensors (MOS),
conducting polymer sensors (CP), quartz microbalance (QMB), surface acoustic wave sensors
(SAW), metal oxide field effect transistors (MOSFET) and electrochemical sensors (Bartlett et al.
1997, Haugen and Kvaal 1998).
MOS sensors are made from a metal-oxide film (e.g. tin oxide). The odorant molecules undergo a
reaction on the film surface producing a conductivity change in the sensor. Heater within the
sensors aids in the reaction process. The advantages of MOS sensors include low cost, longevity
and electronic simplicity. The disadvantages are the necessity to operate at high temperatures (200-
500°C), limited selectivity, high power requirements and modest sensitivity (Haugen and Kvaal
1998, Mielle 1996).
CONCLUSIONS
The FreshSense sensors showed good selectivity, sensitivity and repeatability when measuring
standard compounds (TMA, ethanol, acetaldehyde and DMDS) that are representative of spoilage
compounds in fish and the sensors could discriminate between those compounds and their mixtures.
The results indicate that the FreshSense sensors could be used efficiently to measure volatile
compounds that contribute to the spoilage odour in fish.
The FreshSense sensors have the ability to monitor freshness and the onset of spoilage of red fish
stored under various conditions. The CO sensor appeared to increase earlier than the other sensors
and was most likely responding to short chain alcohols (i.e. ethanol) and aldehyde that form during
storage. The responses of the NH3 and SO2 sensors increased at later stages of storage.