NMR-Based Approaches in the Study of Foods

Abstract

In this review, the three different NMR-based approaches usually used to study foodstuffs are described, reporting specific examples. The first approach starts with the food of interest that can be investigated using different complementary NMR methodologies to obtain a comprehensive picture of food composition and structure; another approach starts with the specific problem related to a given food (frauds, safety, traceability, geographical and botanical origin, farming methods, food processing, maturation and ageing, etc.) that can be addressed by choosing the most suitable NMR methodology; finally, it is possible to start from a single NMR methodology, developing a broad range of applications to tackle common food-related challenges and different aspects related to foods.

Keywords: NMR methodology; food composition; food structure.

Figure 1

(a) PCA score (b) and loading plots of metabolite profiles (30 metabolites/metabolite classes) obtained from NMR data related to hydroalcoholic and organic extracts of kiwifruit at different postharvest stages (T0–T3). Results associated with malic acid (MA), citric acid (CA), ascorbic acid (AA), lactic acid (LA), quinic acid (QA), α- and β-glucose (AGLC and BGLC), sucrose (SUCR), β-fructopyranose (BFRUPY), Ala, Thr, Glu, Asp, Val, Ile, Trp and γ-aminobutyric acid (GAB), choline (CHN), uridine (URI), myo-inositol (MI), phosphatidylcholine (PC), phosphatidylethanolamine (PE), digalactosyl diacylglycerol (DG), stigmast-7-en-3β-ol (S7), stigmasterol (ST), poly-unsaturated fatty acids (PUFA), di-unsaturated fatty acids (DUFA), mono-unsaturated fatty acids (MUFA), saturated fatty acids (SFA), and β-sitosterol plus campesterol (bSC) are shown (Reprinted from [18] with permission from Frontiers).

Figure 2

The spin–lattice relaxation (T1) images (a) through a longitudinal equatorial plane and spin–spin relaxation (T2) images (b) through a transverse equatorial plane of kiwifruit (A. deliciosa) during development and ripening. Numbers correspond to weeks from fruitset, with commercial harvest occurring at week 23. Images at 23 and 27.5 weeks are of the same fruit at harvest and after 4.5 weeks ripening. Image resolution varies from 0.12 in the youngest fruit to 0.33 mm in the older samples (256 × 256 pixel data arrays). The circular internal standard tubes (4 mm inner diameter) serve as a scale marker for each panel. Image intensity is based on a continuous grey-scale setting ranging from 0 ms = black to 2500 ms = white (Adapted from [20] with permission from Elsevier, Copyright 1999).

Figure 3

(a) Measurement of intact kiwifruit with a portable unilateral NMR instrument. (b) Section of kiwifruit showing the depth of measurement with a portable NMR instrument. Average T_2a (c) and T_2b (d) were measured on nine kiwifruits vs. the harvesting month; the error bars represent the maximum error calculated with the error propagation theory (Adapted from [17] with permission from Elsevier, Copyright 2010).

Figure 4

¹H NMR resonances selected for statistical analyses in the 600.13 MHz ¹H spectrum of an (a) olive oil (top trace) and (b) a hazelnut oil. Peaks: 1, diallylic protons of linolenic acid, 2.82 ppm; 2, diallylic protons of linoleic acid, 2.78 ppm; 3, a signal due to squalene, 1.69 ppm; 4, methylenic protons of palmitic and stearic fatty chains, 1.27 ppm; 5, methyl-18 of β-sitosterol, 0.70 ppm. The reference peak at 2.32 ppm is also reported (∗) (Reprinted with permission [26] from American Chemical Society, Copyright 2009).

Figure 5

(a) Score plot of PCA performed on the sugars matrix. Sucrose and erlose were not considered in this analysis because of their large concentration variability. PC1 and PC2 accounts for 81% of the total variance (i.e., 72.6% + 8.64%). Grey circles: European honey; grey triangles: commercial Chinese honey; black triangles: Chinese honey analyzed with SCIRA; black star: Chinese honey purchased in China; white hexagons: honey from bee-feeding experiments. (b) Loading plot that highlights the metabolites responsible for the cluster separation (Reprinted from [32], with permission from Elsevier, Copyright 2020).

Figure 6

A region of the ¹H NMR spectra of chloroform extracts where the linden and ailanthus markers resonate: (from top to bottom) monofloral linden honey; honey containing both linden and ailanthus; monofloral ailanthus honey (Reprinted with permission from [35]. Copyright 2016 American Chemical Society).

Figure 7

Expanded region of the ¹H NMR spectrum showing the diagnostic resonances of protons 17 and 21: (a) 100% Robusta sample, (b) 100% Arabica sample, and (c) 16-OMC standard. In the insets on the top of the figure, the signals of protons 17 and 21 of 16-OMC ester are enlarged. They are clearly visible in the Robusta sample (top), but they are absent from the Arabica sample. The arrow shows the weak methyl signals in position 21 of 16-OMC in its free form (Adapted with permission from [39]. Copyright 2014, American Chemical Society).

Figure 8

Comparison of 60 MHz (benchtop) and 600 MHz (high-field) ¹H NMR spectra obtained from an extract of saffron in DMSO-d6, with annotations of the main features identified in the 60 MHz spectrum. The benchtop spectral profile is dominated by resonances attributed to picrocrocin, a major saffron metabolite, the structure of which is given in the inset panel. The isolated peak at 10.05 ppm is in a generally uncrowded region of the spectrum and provides a useful indicator of the presence of saffron in a sample. Reprinted from [50], with permission from Elsevier, Copyright 2020.

Scheme 1

Classification of high-resolution ¹H NMR spectroscopy’s application types for food analysis.