Food is a complex system
comprised predominantly of water, fat, proteins and carbohydrates together with
numerous minor components. The functional properties of these components, which
are governed by their molecular structure and intra- and intermolecular
interactions within food system, and the amounts present define the
characteristics of food products. Quality of food products refers to the
minimum standards for substances to qualify as fit for human consumption or
permitted to come in contact with food. Appearance, colour, flavour and texture
are critical aspects for the sensory quality of food. The food quality includes
also chemical, biological and microbial factors, e.g. instability of food
products, which limits their shelf life and is connected with irreversible
chemical and enzymatic reactions. Recently, public interest in food quality and
production has increased, probably related to changes in eating habits,
consumer behaviour, and the development and increased industrialization of the
food supplying chains. The demand for high quality and safety in food
production obviously calls for high standards for quality and process control,
which in turns requires appropriate analytical tools to investigate food.
Spectroscopic methods have been historically very successful at evaluating the
quality of agricultural products, especially food. These methods are highly
desirable for analysis of food components because they often require minimal or
no sample preparation, provide rapid and on-line analysis, and have the
potential to run multiple tests on a single sample. These advantages
particularly apply to nuclear magnetic resonance (NMR), infrared (IR), and
near-infrared (NIR) spectroscopy. The latter technique is routinely used as a
quality assurance tool to determine compositional and functional analysis of
food ingredients, process intermediates, and finished products. UV–VIS
spectroscopy, fluorescence and mid-infrared (MIR) and Raman spectroscopy are
some of the techniques used in the food quality monitoring.
TECHNIQUES USED IN FOOD QUALITY
The UV-VIS spectroscopy is mainly used to examine the
quality of edible oils regarding a number of parameters including the anisidine
value. Anisidine value is a measurement of the level of fats oxidation, and is
used for the assessment of poorer quality oils. Precisely, it is the measure of
aldehyde production during oxidation of fats. The anisidine value (AV) is
defined as one hundred-fold value of absorbance of a solution of a fat sample containing
aldehydes which have reacted with p-anisidine. The aforementioned aldehydes are
dienals or alka-2-enals and both are one of the final products of lipids
oxidation. The highest permissible value of anisidine value for edible oils is
8. AV is also an element of Totox (total oxidation value), another factor
indicating deterioration level of total fat. The value of Totox is calculated
as the sum of two-fold value of AV and peroxide value. The anisidine value can be also measured by using Flow
Injection Analysis (FIA) combined with UV–VIS spectroscopy. Thanks to the
implementation of FIA, the period of time required for analysis can be
significantly shortened. Additionally, the number of reagents is also
maintained at very reasonable level. Sample of fat dissolved in propanol-2 is
injected into continuous flow of p-anisidine with a mixture of solvents: propanol-2
and glacial acetic acid. Spectrophotometer is used as a detector, and the value
of absorbance is measured at 350 nm. The process of fat deterioration
is also described by the peroxide value (PV). The deterioration takes place
during lipids’ exposition to some external factors including temperature,
daylight and oxygen. It results in production of peroxides and hydroperoxides,
which are regarded as products of fatty acids oxidation. The highest value of
PV for oil produced through cold press extraction is 10 Meq O2 kg-1,
while regarding refined oil it may reach the amount of 5 Meq O2
kg-1. The PV value is measured by employing UV–VIS spectrometer as detector.
Absorbance measurements explore the chemical composition of a food
directly via the wavelengths absorbed when light is transmitted through it.
Absorbance is most often used for liquids, as Beer’s Law simplifies
quantitative analysis. Samples with high absorption or scatter like colloids
(milk) or suspensions (mixed vegetable juice) may require chemometric analysis.
Temperature is also important for aqueous samples, and for oils that might be
solid at ambient conditions. Also, it is possible to look at the light
transmitted through whole foods, which has proven a viable method for
inspecting fruit for ripeness, internal rot, pests and defects.
Fruit juices: soluble solids content, pH, colour,
Milk: fat, protein and
Saffron: ISO 3632 quality method to measure crocin,
picrocrocin and safranal.
Vegetable oils: identity, adulteration, acid
value, peroxide value.
Wine: quality, phenols, tannins, methanol content.
While colour does play a significant role in beer quality and consumer expectations, measuring colour
can tell us a lot more. The versatility of spectrophotometric technology offers
an important method of analysis that can help determine what is known as “batch
baseline specifications for core beer brands is important to carry quality and
consistency throughout the growth of brewing capacity. The UV-Vis spectrophotometer
is an essential tool to determine empirical specifications of beer.
Infrared Spectroscopy is the analysis of infrared light interacting with
a molecule. This can be analyzed in three ways by measuring absorption,
emission and reflection. The main use of this technique is in organic and
inorganic chemistry. It is used by chemists to determine functional groups in
molecules. IR Spectroscopy measures the vibrations of atoms, and based on
this it is possible to determine the functional groups.5. Generally, stronger
bonds and light atoms will vibrate at a high stretching frequency (wavenumber).
The range of Infrared region is 12800 ~ 10 cm-1 and can be divided
into near-infrared region (12800 ~ 4000 cm-1), mid-infrared region
(4000 ~ 200 cm-1) and far-infrared region (50 ~ 1000 cm-1
). The discovery of infrared light can be dated back to the 19th century. Since
then, scientists have established various ways to utilize infrared light.
Infrared absorption spectroscopy is the method which scientists use to
determine the structures of molecules with the molecules’ characteristic
absorption of infrared radiation. When exposed to infrared radiation, sample
molecules selectively absorb radiation of specific wavelengths which causes the
change of dipole moment of sample molecules. Consequently, the vibrational
energy levels of sample molecules transfer from ground state to excited state.
The frequency of the absorption peak is determined by the vibrational energy
gap. The number of absorption peaks is related to the number of vibrational
freedom of the molecule. The intensity of absorption peaks is related to the
change of dipole moment and the possibility of the transition of energy levels.
Therefore, by analyzing the infrared spectrum, one can readily obtain abundant
structure information of a molecule. Most molecules are infrared active except
for several homonuclear diatomic molecules such as O2, N2 and Cl2 due to the
zero dipole change in the vibration and rotation of these molecules. What makes
infrared absorption spectroscopy even more useful is the fact that it is
capable to analyze all gas, liquid and solid samples. The common used region
for infrared absorption spectroscopy is (4000 ~ 400) cm-1 because
the absorption radiation of most organic compounds and inorganic ions is within
this region. FTIR spectrometers are the third generation infrared spectrometer.
FTIR spectrometers have several prominent advantages:
(1) The signal-to-noise
ratio of spectrum is significantly higher than the previous generation infrared
(2) The accuracy of wave number is high. The
error is within the range of ± 0.01 cm-1.
(3) The scan time of
all frequencies is short (approximately 1 s).
(4) The resolution is
extremely high (0.1 ~ 0.005 cm-1).
(5) The scan range is
wide (1000 ~ 10 cm-1).
(6) The interference
from stray light is reduced. Due to these advantages, FTIR Spectrometers have
replaced dispersive IR spectrometers.
spectroscopy is well-suited to non-destructive analysis of bulk, high-moisture
samples like fruit, fish, meat and grains. While challenging to interpret and
analyze, NIR spectroscopy probes the vibrational overtone absorption of
chemical bonds, and is sensitive to most chemical constituents in foods. The
resulting spectra are often broad, overlapping and complex, necessitating
chemometric analysis to unlock their secrets.
Light at NIR
wavelengths penetrates fairly deeply with less scattering, allowing internal
composition to be analyzed via non-destructive reflectance and through-sample
transmission techniques. When processed using a well-developed chemometric
model, a simple NIR reflectance spectrum can be used to predict complex
characteristics, such as an apple’s ripeness, sweetness or storage duration.
beef: detecting mutton, pork, organs and fillers
detecting thawed versus fresh cuts; artificially boosted water content
Fraudulent labeling of
fish: identification of fish species without DNA testing
screening for core rot, internal pests and ripeness
sorting unprocessed grains with NIR and machine vision
spectroscopy is one of the vibrational spectroscopic techniques used to provide
information on molecular vibrations and crystal structures. This technique uses
a laser light source to irradiate a sample, and generates an infinitesimal
amount of Raman scattered light, which is detected as a Raman spectrum using a
CCD camera. On analysis of the frequency of the scattered radiation, a small amount
of radiation is seen which is scattered at different wavelengths, and also the
incident radiation wavelength. This incident radiation wavelength is known as
Rayleigh Scattering. The small amount of the radiations scattered at different
wavelengths are known as Stokes and Anti-Stokes Raman Scattering. Described by
Lord Rayleigh, Rayleigh Scattering is the scattering process without any change
in frequency. Depending on the vibrational state of the molecule, Raman shifted
photons of light can be of higher or lower energy
Non-contact and non-destructive analysis
High spatial resolution up to sub-micron scale
In-depth analysis of transparent samples using a
confocal optical system
No sample preparation needed.
Both organic and inorganic substances can be
not interfered by
Highly specific like a
chemical fingerprint of a material.
Raman spectra are
acquired quickly within seconds.
Laser light and Raman
scattered light can be transmitted by optical fibers over long distances for
In Raman spectroscopy,
the region from 4000 cm-1 to 50 cm-1 can be covered by a
Inorganic materials are easily analysable
with Raman spectroscopy.
Raman spectroscopy is a novel method of food analysis and
inspection. It is highly accurate, quick, and noninvasive. The investigation
and monitoring of food processing is important because most of the foods humans
eat today are processed in various ways. Raman spectroscopy in food processes,
such as fermentation, cooking, processed food manufacturing, and so on, are
explored. The characteristics and difficulties of the Raman inspection of these
processes are also important. According to the various research reports, Raman
spectroscopy is a very powerful tool for monitoring these food processes in lab
environments and is likely to see usage in situ in the future.
Raman Spectroscopy can be used
for analysis of micro to macro.
Raman spectroscopy can be used to examine powders, solids, liquids in
different geometries. Quantitative analyses based on Raman spectroscopic data
of bulk material, representative of the whole sample are then possible. On the
other hand, micro Raman brings information on micrometer level, which is
required when assessing the distribution of components in grains, particles
within powders, or micro-organisms present in foods.
absorption spectroscopy (AAS)
is a spectroanalytical procedure for the quantitative determination of chemical
elements using the
absorption of optical
radiation (light) by free atoms in the gaseous state.
The technique makes use of absorption
spectrometry to estimate the concentration of an analyte in a given sample. It
requires standards with the known content to get the relation between the absorbance and the analyte concentration. It
relies on the Beer-Lambert Law. In short, the electrons of the atoms in the
atomizer can energised to higher orbitals for a very short period of time by absorbing a
defined quantity of radiation of a given wavelength. This wavelength, is specific to a
particular electron transition in a particular element. In general, each
wavelength corresponds to only one element, and the width of an absorption line
is extremely narrow, which makes the technique highly selective. The radiation
a sample and with a sample in the atomizer is measured using a detector, and
the ratio between the two values (the absorbance) is converted to analyte
concentration or mass using the Beer-Lambert Law.
It is very simple to understand and extremely easy to use.
It is very precise and selective for a large number of elements
It is a relatively cheap technique in comparison to other techniques
AAS method has used to be extremely effective for determining the
presence of trace elements(elements present in negligible quantity) in foods. This can be helpful in determining
any sort of toxicity in foods due to presence of a certain toxic element.
AAS method can be used to easily and quickly determine the presence of
most adulterants in foods.
Different parts of avocados have been analysed to find out all of the
trace elements in the food. Similarly one can find out the amount of nutrients
a person gains by eating certain foods.