When does lactose-free mean lactose-free? The food industry needs ways to answer this question and nuclear magnetic resonance (NMR) spectroscopy could soon help them do so.
The need to quantify lactose content
The ability to accurately characterize the composition of food and liquids is an important tool both for the food industry and the people who regulate it. It has wide-ranging applications; from ensuring food labelling meets legal requirements to detecting food adulteration and contamination.
In the dairy sector, there is a need for reliable methods to confirm the lactose content of the numerous products that claim to be lactose-free. This is important for the 70% of people worldwide who are lactose-intolerant. There is also a growing market for milk-free and lactose-free products in people who choose not to consume cow’s milk for ethical reasons.
However, milk substitutes, such as those based on grains and soy, have not received much attention in the literature and it is unclear how accurately the labelling reflects these products’ true lactose contents.
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The advantages of NMR spectroscopy
There are several methods available for measuring the lactose content of raw milk, the most commonly used being an enzymatic assay based on spectrophotometric measurement. Other techniques include biosensors, colorimetry, near-infrared (NIR) spectroscopy and chromatography.
However, none of these are suitable for rapid screening and the majority are unable to detect low lactose concentrations. In particular, NIR was previously found to be inadequate for detecting the low lactose contents of lactose-free products or soy substitutes.
By contrast, nuclear magnetic resonance (NMR) spectroscopy has several properties that could make it an ideal alternative for this purpose. The method requires very little sample preparation and produces rapid results, allowing many samples to be analyzed over a short time period. And, since the amplitude of each NMR signal corresponds to the concentration of a given molecule, it can simultaneously quantify multiple constituents in a single sample. Furthermore, its limit of detection can be as low as a few mg/L.
Putting NMR into action
A 2012 study by Monakhova et al., found that NMR spectroscopy could be used to place milk products and substitutes into five distinct categories and could quantify lactose at very low concentrations.
The researchers used 1H NMR spectroscopy to analyze 84 milk samples and milk substitutes purchased at local stores. Because much of their data overlapped, they performed a principal component analysis (PCA) in the 6-3 ppm region. The results show that the samples clustered into five distinct groups: milk, lactose-free milk, soy, oat and rice.
Using soft independent modelling by class analogy (SIMCA), the researchers were then able to accurately classify a test set of 12 samples into the correct group with no false-positive or false-negative results.
Previously, NMR had been used to classify milk samples into only two classes. Thus, the researchers showed that by combining NMR with a chemometric technique, such as PCA, it can be used determine the type of both milk and substitutes.
Furthermore, Monakhova et al. used 2-dimensional J-resolved NMR spectroscopy to show that the technique could detect lactose at concentrations between 0.05 and 50 g/L – a wide enough range to quantify lactose in both lactose-free products and ordinary milk samples.
A growing role for NMR spectroscopy
Despite its potential in the industry, NMR spectroscopy is not in wide use in the official analysis of food products or in state control laboratories. However, that is beginning to change due to improvements in both equipment and software.
For example, Bruker products allow for the full automation of the NMR workflow, from sample preparation and sample changing to data analysis and archive.
AVANCE III HD-NanoBay, equipped with CryoProbe Prodigy and autosampler SampleXpress
The Bruker Avance III NanoBay NMR device, part of the same series that Monakhova et al. used for their research, offers highly integrated state-of-the-art spectroscopy, enhancing both productivity and quality. Its compact design makes it suitable for small analytical labs and, when combined with Bruker’s Ascend 300 and 400 MHz magnets, it can be placed even in non-NMR labs.
Meanwhile, Bruker software, such as CMC-assist seamlessly integrates with all Bruker spectrometers for automated data analysis and Icon-NMR supports sample changers and sample preparation robots for high-throughput automated NMR spectroscopy.
- Hu F, et al. Nondestructive quantification of organic compounds in whole milk without pretreatment by two-dimensional NMR spectroscopy. Journal of Agricultural & Food Chemistry 2007; 55: 4307-4311.
- Monakhova Y, et al. NMR spectroscopy as a screening tool to validate nutrition labelling of milk, lactose-free milk, and milk substitutes based on soy and grains. Dairy Science & Technology 2012; 92: 109-120.
- Monakhova Y, et al. Chemometric methods in NMR spectroscopic analysis of food products. Journal of Analytical Chemistry 2013; 68: 755-766.
Bruker is market leader in analytical magnetic resonance instruments including NMR, EPR and preclinical magnetic resonance imaging (MRI).
Bruker's product portfolio in the field of magnetic resonance includes NMR, preclinical MRI ,EPR and Time-Domain (TD) NMR.
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