Due to its high nutritional value a

Due to its high nutritional value and tasty taste, the consumer
demand for beef increases dramatically during the last decades. In
USA alone the beef industry has a retail value equals to $73 billion
in 2009 alone with about 26.9 billion pounds of beef being
consumed in that year [1]. However, beef is highly susceptible
to spoilage and contamination. The freshness of beef degrades
because microbial spoilage and biochemical reactions occur during
storage. The main ingredients like protein, fat and carbohydrates
will be decomposed by enzymes and bacteria, producing odor: the
protein will be decomposed into ammonia, hydrogen sulfide, ethyl
mercaptan, etc.; the fat will be decomposed into aldehydes and
aldehyde acids; the carbohydrates will be decomposed into alcohols,
ketones, aldehydes, and carboxylic acid gases [2]. Metabolites
such as trimethylamine, aldehydes, ketones and esters, as well as
other low molecular weight compounds responsible for off-flavors
and sensory product rejection are produced. During storage, these
substances and other basic nitrogenous compounds together make
up total volatile basic nitrogen (TVBN). The odor gets more and
more intense with the decrease of beef freshness. Consumption of
spoilage beef could cause serious health hazards [3].
A number of techniques have been used to assess beef freshness.
Traditionally, sensory evaluation, chemical experiments includingTVBN evaluation and microbial population evaluation are three
key techniques [4]. The sensory evaluation attributes usually cover
color, flavor and texture including viscosity and elasticity [5]. This
method provides immediate quality information but suffers from
some disadvantages, for example, the subjective nature of the
assessment. Errors may arise from fatigue of panelists, and low
threshold concentrations of stale odor compounds may not be
perceived [6]. The TVBN evaluation and the microbial population
evaluation methods are objective and precise. However, these two
methods are destructive and time-consuming.
The developments of electronics and sensor technologies show
promises for developing rapid and nondestructive sensors for meat
quality/safety [7]. Electronic nose (e-nose), also known as artificial
olfactory, is a simulation of biological functions to identify some
simple or complex odor [8,9]. A typical e-nose system contains a
selective chemical sensor array, a signal processing subsystem and
a pattern recognition subsystem. The sensors in the sensor array
are sensitive to different substances, for example, some sensors
can discern ammonia and some can discern aldehydes. So instead
of detecting one or two components of the substances, the e-nose
extracts the whole information for identification.
Since the last decades, some researchers have been studying the
potential of using the e-nose as a non-destructive method for food
detection [10–14]. Zhang et al. [15] reported using a nine nano ZnO
thick film gas sensors based e-nose to analyze 17 kinds of commercial
Chinese vinegars, and the Principle Component Analysis (PCA)
and Cluster Analysis (CA) results showed that characterizing the
Chinese vinegars by the e-nose was highly related to their type,