The quality, safety, and authenticity of our food is of the utmost importance in society today. The ability to rigorously test these factors is therefore critical for scientists working within the food industry. One technique particularly suited to this task is fluorescence spectroscopy.
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Fluorescence spectroscopy is a non-destructive technique that is able to give rapid and sensitive results. The technique uses a light beam (usually ultraviolet) to excite electrons, thereby causing them to fluoresce. This light is then directed towards a filter and onto a detection mechanism where the molecule, or changes in it, can be measured and identified. This is measured by what is termed a fluorescence excitation spectrum.
For a given fluorophore, emission and excitation wavelengths are mirror images of each other. The spectral intensity and/or peak wavelength of fluorophores is dependent upon variables including concentration, interactions with other molecules, pH, and temperature. Many organic molecules fluoresce under light and can thus be identified by spectroscopic methods. This includes amino acids, fluorescent proteins (FPs) and chlorophylls.
Two types of fluorescence spectrometry instruments exist: filter fluorometers (which use filters to isolate the incident light) and spectrofluorometers (which use diffraction grating monochromators.) Sources of incident light used include LEDs, lamps, and lasers. Lamps particularly used in the technique are xenon arcs and mercury-vapor lamps.
Fluorescence spectroscopy is employed as an analytical tool in many different industries besides the food industry including the chemical and pharmaceutical industries, sewage treatment, and the mining industry. It is eminently more sensitive than absorption techniques.
Use in the food industry
The term “fluorescence” was first coined by Sir George Gabriel Stokes in 1852 in his paper “On the Change of Refrangibility of Light” where he explained what he termed dispersive reflection.
In one of his famous experiments, Stokes placed quinine under the region of light corresponding to UV, causing the solution to glow with blue light. A few years later in 1858, Erasmus Bond used quinine in tonic water, making it one of the first food and drink components to have its fluorescence studied in a scientific manner. This experiment is still used to teach the fundamentals of fluorimetry.
In recent decades there has been a notable increase in the use of fluorescence in food studies. With the development and improvement of computers and spectroscopic instrumentation, fluorescence spectroscopy has helped to improve food quality and security for billions of people. The field has made significant progress from the 1980s onwards. In that decade multivariate data analysis methods were applied and reported.
Food is a complex system, with many different chemical components. For this reason, multidimensional measurement techniques are employed in studies. Conventional single emission/excitation spectra cannot fully characterize the fluorescent patterns of food samples. By utilizing multidimensional measurement techniques, a fingerprint of the food sample can be constructed.
Major food components are usually non-fluorescent but many minor components, conversely, are. These components affect factors such as nutrition and composition. Process-derived compounds can also fluoresce, as can food contaminants. All these compounds can affect food quality and authenticity.
Fluorescence spectroscopy for food quality analysis
There has been a growth in spectroscopic applications for food quality analysis over the past decades due to its non-destructive, rapid, and sensitive nature. This has been due to the widespread application of chemometric tools and improvements in the technical and optical aspects of spectroscopic equipment.
Food quality is one of the most important aspects of the food industry. Nutritive, physical, and chemical aspects of the food sample must be measured and understood to provide a quality product that helps consumers make an informed choice.
The quality of perishable foods (meat, eggs, dairy, fish and so forth) in particular is of utmost importance to the food industry. These foodstuffs are considered to be an intrinsic part of a healthy diet, providing important vitamins, minerals, and essential oils that help the body and brain function healthily.
Fresh food has a limited shelf life and is affected by time-limited supply chains and storage processes, which means accurate analysis of their components improves freshness, safety, and profit. Microbial growth can also spoil food which can lead to outbreaks of harmful diseases such as listeria and salmonella.
Additives and preservation techniques for non-perishable and long shelf-life foods (for example, canned, pickled, and dry food products) can also be studied for quality using spectroscopic techniques.
Using fluorescence spectroscopic methods in studies of food quality has helped the food industry improve the overall standard of the food we consume.
Recent studies have also helped to improve food preservation techniques and reduce harmful chemical and microbial adulteration.
Fluorescence spectroscopy for food authenticity analysis
Accurate labeling is of paramount importance for the food, leisure, hospitality, and retail industries. Consumers may have specific dietary, ethical, or religious needs which means they rely on knowing what their food contains. Therefore, food authenticity is of particular concern for these industries. Inaccurate food labeling may cause fatalities if a consumer is allergic to one of the ingredients.
Fluorescence spectroscopy is one of a series of physical analytical techniques that have been explored for the task of measuring the authenticity of food. The use of synchronous fluorescence and conventional excitation-emission matrix spectroscopy is of huge potential in this field. Combining the technique with multivariate analysis tools such as factorial discriminant analysis can improve results across the board.
A healthy diet and the safety of food we consume helps improves our overall standard of living. The food industry needs a wide-ranging toolkit of analytical techniques to ensure this. Fluorescence spectroscopy is just one part of this toolkit, albeit one which is growing in importance within the industry.
As we move into the future, fluorescence spectroscopy will no doubt continue to play an important role in this field. By utilizing new and complementary technologies such as deep learning and neural networks, fluorescence spectroscopy will continue to play a significant part in ensuring the highest standards of food quality.