Would you imagine being able to determine where a vegetable has been harvested, if it is fraudulent, or even genetically modified by just analysing a tiny amount of it? The truth is that, you actually can! Nuclear magnetic resonance (NMR) is the way. In this article we will discuss how this physical phenomenon operates, and how it is being implemented in the food sector.
Briefly, NMR is explained as a physical phenomenon in which atomic nuclei within a sample (1H and 13C are the most common nuclei targeted in food science-related applications) in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a frequency characteristic of the magnetic field at the nucleus. This allows us to accurately determine quantitatively the molecular structures found in a sample without the need of destroying it in a reproducible way, if the sample preparation, handling and instrument parameters are just the right ones. NMR technology was initially used in the late 1940s by organic chemists, however, its application to food matrices did not happen until the 1980s, almost 2 decades since its introduction.
NMR is advantageous over other chemical analytical techniques in the sense that it is of non-destructive character, accurate and reproducible, generating valuable quantitative data. Why then it took so long for the food scientists to realise the potential of this remarkable analytical technique?!
The reason for the delayed NMR food-based applications had to do with the complexity around this technique. Yes, it is not an easy thing to deal with. It requires high-cost equipment, NMR parts designed specifically for food purposes, safety practices in-place related to the magnetic field maintenance, and especially, strong expertise involved among the NMR operators. And considering that extending the application of NMR techniques beyond research to industrial process and quality control is still to come, food industry strongly requires more NMR-trained staff. Yes, food scientists, NMR expertise might get you a better job nowadays! But before we get into the specifics of the uses that the food sector gives to this tool, I think it is worth discussing who was or were, the geniuses that discovered this technique, when it happened, and how it specifically operates.
Three names and three Physics nobel prizes: Isidor Rabi, Felix Bloch and Edward Mills. They were the first to describe the NMR technology in 1938, and expanded it in 1946 for use on liquids and solids. We could say that these three minds brought humanity a tool that can rapidly determine molecular structures, their arrangements and concentrations in matrices of every kind, without destroying them. A simplified description of NMR is that the sample, dissolved in a liquid, is placed into the instrument, which contains a magnetic field. A radio frequency pulse is then sent through the sample solution in order to orient the magnetic moments of the nuclei in the solution. As the magnetic moments relax, they exhibit a free induction decay. The free induction decay is Fourier transformed into an NMR spectrum. The NMR spectrum displays chemical shifts for the individual nuclei; and from these chemical shifts, the structure of the compound can be determined.
As mentioned before, it was not until the 1980s when NMR spectroscopy started to be included in food science-related operations. Since then, a wide range of NMR food-related research has covered various fields of food science, including food chemistry, food engineering, food microbiology and food packaging. This growth in its use can be demonstrated by looking at the drastic increase in the number of scientific publications that included this technique for food-related determinations.
Before NMR spectroscopy reached the food sector, other characterisation techniques utilised were generally employed for quality assurance and product monitorisation (i.e. HPLC, GC, MS, etc). These techniques, which are still in place and are approved by the AOAC official methods of analysis, have demonstrated to be in good agreement with results from those that involved NMR spectroscopy, thereby demonstrating the good capability and adaptability of this incredible tool within the food sector. Among the most generalised uses of NMR spectroscopy in food industry reported in literature are:
- Food packaging: NMR holds the promise to evaluate the performance of various packaging materials with respect to their ability to preserve food in different storage and processing conditions, and it can be also utilised for selecting desired food package materials.
- Honey: In recent years, honey production has been limited and has risen the prices, and in turn, the number of cases of honey adulteration has increased. 1H-NMR has been reported to accurately identify the botanical origin of unifloral and polyfloral honey, yielding 100 % correct sample identification.
- Salmon: Farmed salmon is differenciated from the wild salmon in the sense that farmed salmon poses a risk of containing much higher concentrations of organocholorine compounds such as polychlorinated biphenyl (PCBs), among other contaminants. Therefore, mislabel the type of salmon as well as its origin is a common occurrence in some fish markets. Well, here 1H-NMR is also able to distinguish between farmed and wild species, including their origins by just looking at the muscle lipid profiles.
- Olive oil: Virgin olive oils (VOOs) also experiences mislabelling in terms of the specific production region and production method. Trust me, I am Spanish, and this is frequent! Here, although several analytical techniques can be utilised, 1H, 13C and 31P NMR analyses are a much more effective method. These analyses look at the unsaponifiable fraction of VOOs and phenolic compounds within their polar fraction.
- Alcoholic beverages: These type of beverages generally command higher prices as their production requires a lot of resources, time and marketing strategies. However, as with other food-based matrices, certain producers tend to deliberately mislabel and adulterate, in order to expand their margin of benefits by selling a product of lower quality. In here, NMR can be used to distinguish the between brewing site and date of production based on the many distinct chemicals observed.
The aforementioned applications have been long used within the food sector, and it is noticed that the majority of applications of NMR are related to food authentication, however, novel and more interesting applications are still emerging in line with the new developments of the NMR technology.
Among the most novel and interesting applications reported in literature we can find:
- NMR-based food metabolomics: Foods are very complex matrices consisting of thousands of compounds produced by the metabolism of plants, bacteria, and animals. This complexity and variabiliy in the metabolism is often reflected in the NMR spectra. This can be utilised, for instance, to evaluate any unintended effect following a genetic modification within food, which is indeed a high-priority task for food regulatory agencies. Also, it allows the differentiation between GMO and non-GMO foods.
- Stock breeding information: Imagine that you are a farmer and you want to breed poultry. I am sure that you would want to select the best chicken that allows you to produce the major amount of meat with the best quality. Well, NMR can give you this information, as it has been already used to genetically select the best chicken based on fat compositional analysis of abdominal fat and total body fat of chicken.
- Food as smmugling carrier: There are reported cases in which dissolved cocaine has been smuggled illegally with the aid of wine bottles. For this cases, NMR has been helpful in accuratelly detecting intact bottles with the aid of an in-line magnetic resonance (MR) scanner. As NMR is rapid and non-destructive, it can be effectively utilised for analysing suspicious food-containing cargo. Cool stuff!
If you are into NMR and its food applications, then you should have heard about Bruker. Bruker Corporation can be regarded not only as one of the giants in the production and implementation of NMR instruments, but also as the corporation who has worked the most to bring their NMR technologies to the food sector. They not only develop NMR instruments but X-ray diffractometers, mass spectrometers, and other devices that we will surely discuss in future articles. In terms of NMR and food, they have developed FoodScreener, an easy-to-use instrument, totally automatic and push-button NMR solution that with a minimal sample preparation and cost per sample, detects unpredicted and even unknown fraud in a wide range of food matrices. Additionally, they continuously post extremely interesting post blog articles and podcast episodes regarding the different areas within food where their NMR solutions can be employed.
Oxford Instruments is regarded as another giant that develop NMR instruments and that strongly focus on bringing this marvelous technology to the agrofood industry. In their link associated to NMR-based food applications, they display a broad range of possibilities to interact with. No matter what your focus is as a client, Oxford Instruments offers solutions for food authenticity and safety, agriculture, R&D, animal feed, and/or food production. And one of the coolest things I have seen so far when doing a bit of research for this article, they develop benchtop NMR spectroscopy instruments, that is, NMR spectrometers that are smaller in size to fit within a laboratory bench and that do not require liquid cryogens such as helium to cool down the NMR magnet, as this has excellent temperature stability.
Another player that called my attention when looking for NMR instruments for food-based applications was Magritek, this company from New Zealand also develops benchtop NMR instruments that are able to perform well in smaller spaces, like their Spinsolve Family. This family of products allow for a variety of things from multinuclear options to sample temperature control, which is interesting for small-sized instruments like these. However, something that seemed quite promising for benchtop NMR products was that their instruments can be equipped with different flow cells to pump mixture continuously through the system for on-line analysis, allowing for reactions monitoring.
In conclusion, extending the application of NMR techniques beyond research to industrial process and quality control is still to come, and for this, more NMR-trained staff will be needed in the aspect of food application. Food scientists face the tremendous challenge of establishing standard operation procedures (SOPs) of NMR analysis for specific categorized food products due to the complex nature of foods. Rather than a impediment, this should be seen by food scientists out their as an opportunity to expand their careers and work opportunities. Besides, would not it be cool to be able to distinguish food fraud from legit food practices?