Food is mostly comprised ofwater, fat, proteins and carbohydrates together with numerous trace elements.The functional properties of these components, which are governed by theirmolecular structure and intramolecular and intermolecular interactions withinfood system, and the amounts present define the characteristics of foodproducts.
Quality of food products refers to the minimum standards for foods tobe considered as fit for human consumption. The food quality also includes chemical, biological andmicrobial factors, which significantlyreduces their shelf life and is connected with irreversible chemical and enzymaticreactions. Nowadays, public interest in food quality increased immensely due tochanges in eating habits, consumer behaviour, and increased industrialization of the food supplyingchains. The demand for high quality and safety in food production calls for high standards for quality andprocess control, which in turns requires appropriate analytical tools toinvestigate food. Spectroscopic methods have been shown to be very successfulfor evaluating the quality of agricultural products, especially food. Thesemethods are highly desirable for analysis of food components because they oftenrequire minimal or no sample preparation, provide rapid analysis, and have thepotential to run multiple tests on a single sample.
These advantagesparticularly apply to nuclear magnetic resonance (NMR), infrared (IR), andnear-infrared (NIR) spectroscopy. UV–VIS spectroscopy, fluorescence andmid-infrared (MIR) and Raman spectroscopy are some of the techniques used inthe food quality monitoring. TECHNIQUES USED IN FOOD QUALITYANALYSIS UV-VISspectroscopyThe UV-VIS spectroscopy is mainly used to examine thequality of edible oils regarding a number of parameters including the anisidinevalue. Anisidine value is a measurement of the level of fats oxidation, and isused for the assessment of poorer quality oils. Precisely, it is the measure ofaldehyde production during oxidation of fats.
The anisidine value (AV) isdefined as one hundred-fold value of absorbance of a solution of a fat sample containingaldehydes which have reacted with p-anisidine. The aforementioned aldehydes aredienals or alka-2-enals and both are one of the final products of lipidsoxidation. The highest permissible value of anisidine value for edible oils is8. AV is also an element of Totox (total oxidation value), another factorindicating deterioration level of total fat. The value of Totox is calculatedas the sum of two-fold value of AV and peroxide value. The anisidine value can be also measured by using FlowInjection Analysis (FIA) combined with UV–VIS spectroscopy.
Thanks to theimplementation of FIA, the period of time required for analysis can besignificantly shortened. Additionally, the number of reagents is alsomaintained at very reasonable level. Sample of fat dissolved in propanol-2 isinjected into continuous flow of p-anisidine with a mixture of solvents: propanol-2and glacial acetic acid. Spectrophotometer is used as a detector, and the valueof absorbance is measured at 350 nm.
The process of fat decompositionis also described by the peroxide value (PV). The decomposition takes placeduring 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 ofPV 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 O2kg-1. The PV value is measured by employing UV–VIS spectrometer as detector. ApplicationsAbsorbance measurements explore the chemical composition of a fooddirectly via the wavelengths absorbed when light is transmitted through it.Absorbance is most often used for liquids, as Beer’s Law simplifiesquantitative 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 besolid at ambient conditions. Also, it is possible to look at the lighttransmitted through whole foods, which has proven a viable method forinspecting fruit for ripeness, internal rot, pests and defects.
· Fruit juices: soluble solids content, pH, colour,adulteration.· Milk: fat, protein andcasein content.· Saffron: ISO 3632 quality method to measure crocin,picrocrocin and safranal.
· Vegetable oils: identity, adulteration, acidvalue, peroxide value.· Wine: quality, phenols, tannins, methanol content.· While colour does play a significant role in beer quality and consumer expectations, measuring colourcan tell us a lot more. The versatility of spectrophotometric technology offersan important method of analysis that can help determine what is known as “batchquality.”· Establishingbaseline specifications for core beer brands is important to carry quality andconsistency throughout the growth of brewing capacity. The UV-Vis spectrophotometeris an essential tool to determine empirical specifications of beer.INFRARED SPECTROSCOPY INTRODUCTIONInfrared Spectroscopy is the analysis of infrared light interacting witha molecule. This can be analyzed in three ways by measuring absorption,emission and reflection.
The main use of this technique is in organic andinorganic chemistry. It is used by chemists to determine functional groups inmolecules. IR Spectroscopy measures the vibrations of atoms, and based onthis it is possible to determine the functional groups.
5. Generally, strongerbonds and light atoms will vibrate at a high stretching frequency (wavenumber).The range of Infrared region is 12800 ~ 10 cm-1 and can be dividedinto 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.
Sincethen, scientists have established various ways to utilize infrared light.Infrared absorption spectroscopy is the method which scientists use todetermine the structures of molecules with the molecules’ characteristicabsorption of infrared radiation. When exposed to infrared radiation, samplemolecules selectively absorb radiation of specific wavelengths which causes thechange of dipole moment of sample molecules. Consequently, the vibrationalenergy levels of sample molecules transfer from ground state to excited state.The frequency of the absorption peak is determined by the vibrational energygap. The number of absorption peaks is related to the number of vibrationalfreedom of the molecule.
The intensity of absorption peaks is related to thechange of dipole moment and the possibility of the transition of energy levels.Therefore, by analyzing the infrared spectrum, one can readily obtain abundantstructure information of a molecule. Most molecules are infrared active exceptfor several homonuclear diatomic molecules such as O2, N2 and Cl2 due to thezero dipole change in the vibration and rotation of these molecules.
What makesinfrared absorption spectroscopy even more useful is the fact that it iscapable to analyze all gas, liquid and solid samples. The common used regionfor infrared absorption spectroscopy is (4000 ~ 400) cm-1 becausethe absorption radiation of most organic compounds and inorganic ions is withinthis region. FTIR spectrometers are the third generation infrared spectrometer.FTIR spectrometers have several prominent advantages: (1) The signal-to-noiseratio of spectrum is significantly higher than the previous generation infraredspectrometers. (2) The accuracy of wave number is high.
Theerror is within the range of ± 0.01 cm-1. (3) The scan time ofall frequencies is short (approximately 1 s).
(4) The resolution isextremely high (0.1 ~ 0.005 cm-1). (5) The scan range iswide (1000 ~ 10 cm-1). (6) The interferencefrom stray light is reduced. Due to these advantages, FTIR Spectrometers havereplaced dispersive IR spectrometers. APPLICATIONSNear infraredspectroscopy is well-suited to non-destructive analysis of bulk, high-moisturesamples like fruit, fish, meat and grains.
While challenging to interpret andanalyze, NIR spectroscopy probes the vibrational overtone absorption ofchemical bonds, and is sensitive to most chemical constituents in foods. Theresulting spectra are often broad, overlapping and complex, necessitatingchemometric analysis to unlock their secrets.Light at NIRwavelengths penetrates fairly deeply with less scattering, allowing internalcomposition to be analyzed via non-destructive reflectance and through-sampletransmission techniques. When processed using a well-developed chemometricmodel, a simple NIR reflectance spectrum can be used to predict complexcharacteristics, such as an apple’s ripeness, sweetness or storage duration.
· Adulterated groundbeef: detecting mutton, pork, organs and fillers· Chicken quality:detecting thawed versus fresh cuts; artificially boosted water content· Fraudulent labeling offish: identification of fish species without DNA testing· Fruit quality:screening for core rot, internal pests and ripeness· Gluten screening:sorting unprocessed grains with NIR and machine vision RamanSpectroscopyIntroduction Ramanspectroscopy is one of the vibrational spectroscopic techniques used to provideinformation on molecular vibrations and crystal structures. This technique usesa laser light source to irradiate a sample, and generates an infinitesimalamount of Raman scattered light, which is detected as a Raman spectrum using aCCD camera. On analysis of the frequency of the scattered radiation, a small amountof radiation is seen which is scattered at different wavelengths, and also theincident radiation wavelength. This incident radiation wavelength is known asRayleigh Scattering.
The small amount of the radiations scattered at differentwavelengths are known as Stokes and Anti-Stokes Raman Scattering. Described byLord Rayleigh, Rayleigh Scattering is the scattering process without any changein frequency. Depending on the vibrational state of the molecule, Raman shiftedphotons of light can be of higher or lower energy ADVANTAGES- · Non-contact and non-destructive analysis· High spatial resolution up to sub-micron scale· In-depth analysis of transparent samples using aconfocal optical system· No sample preparation needed.· Both organic and inorganic substances can bemeasured· not interfered bywater.· Non-destructive.· Highly specific like achemical fingerprint of a material.· Raman spectra areacquired quickly within seconds.
· Laser light and Ramanscattered light can be transmitted by optical fibers over long distances forremote analysis.· In Raman spectroscopy,the region from 4000 cm-1 to 50 cm-1 can be covered by asingle recording.· Inorganic materials are easily analysablewith Raman spectroscopy. Applications Raman spectroscopy is a novel method of food analysis andinspection. It is highly accurate, quick, and noninvasive. The investigationand monitoring of food processing is important because most of the foods humanseat today are processed in various ways. Raman spectroscopy in food processes,such as fermentation, cooking, processed food manufacturing, and so on, areexplored. The characteristics and difficulties of the Raman inspection of theseprocesses are also important.
According to the various research reports, Ramanspectroscopy is a very powerful tool for monitoring these food processes in labenvironments and is likely to see usage in situ in the future.· Raman Spectroscopy can be usedfor analysis of micro to macro.· Raman spectroscopy can be used to examine powders, solids, liquids indifferent geometries. Quantitative analyses based on Raman spectroscopic dataof bulk material, representative of the whole sample are then possible. On theother hand, micro Raman brings information on micrometer level, which isrequired when assessing the distribution of components in grains, particleswithin powders, or micro-organisms present in foods.
ATOMICABSORPTION SPECTROSCOPYIntroductionAtomicabsorption spectroscopy (AAS)is a spectroanalytical procedure for the quantitative determination of chemicalelements using theabsorption of opticalradiation (light) by free atoms in the gaseous state. The technique makes use of absorptionspectrometry to estimate the concentration of an analyte in a given sample. Itrequires standards with the known content to get the relation between the absorbance and the analyte concentration. Itrelies on the Beer-Lambert Law. In short, the electrons of the atoms in theatomizer can energised to higher orbitals for a very short period of time by absorbing adefined quantity of radiation of a given wavelength.
This wavelength, is specific to aparticular electron transition in a particular element. In general, eachwavelength corresponds to only one element, and the width of an absorption lineis extremely narrow, which makes the technique highly selective. The radiationflux withouta sample and with a sample in the atomizer is measured using a detector, andthe ratio between the two values (the absorbance) is converted to analyteconcentration or mass using the Beer-Lambert Law. Advantages· 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 Applications· AAS method has used to be extremely effective for determining thepresence of trace elements(elements present in negligible quantity) in foods. This can be helpful in determiningany 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 ofmost adulterants in foods.
· Different parts of avocados have been analysed to find out all of thetrace elements in the food. Similarly one can find out the amount of nutrientsa person gains by eating certain foods.