In situ ¹³C solid‐state polarization transfer NMR to follow starch transformations in food

Abstract

Convenience food products tend to alter their quality and texture while stored. Texture‐giving food components are often starch‐rich ingredients, such as pasta or rice. Starch transforms depending on time, temperature and water content, which alters the properties of products. Monitoring these transformations, which are associated with a change in mobility of the starch chain segments, could optimize the quality of food products containing multiple ingredients. In order to do so, we applied a simple and efficient in situ 13C solid‐state magic angle spinning (MAS) NMR approach, based on two different polarization transfer schemes, cross polarization (CP) and insensitive nuclei enhanced by polarization transfer (INEPT). The efficiency of the CP and INEPT transfer depends strongly on the mobility of chain segments—the time scale of reorientation of the CH‐bond and the order parameter. Rigid crystalline or amorphous starch chains give rise to CP peaks, whereas mobile gelatinized starch chains appear as INEPT peaks. Comparing 13C solid‐state MAS NMR experiments based on CP and INEPT allows insight into the progress of gelatinization, and other starch transformations, by reporting on both rigid and mobile starch chains simultaneously with atomic resolution by the 13C chemical shift. In conjunction with 1H solid‐state MAS NMR, complementary information about other food components present at low concentration, such as lipids and protein, can be obtained. We demonstrate our approach on starch‐based products and commercial pasta as a function of temperature and storage.

Influence of Air Humidity Level on the Structure and Mechanical Properties of Thermoplastic Starch-Montmorillonite Nanocomposite during Storage

“…The best resolution was obtained in the TPS85 spectrum indicating the highest mobility of TPS components due to the highest water content. Moreover, signal at 3.8 ppm shows splitting in several lines associated with starch hydrogen nuclei [20,34]. In the TPS-MMT85 spectrum this line is prevented from splitting due to additional interactions between starch chains and nanofiller.…”

“…The water content estimation was carried out for TPS and nanocomposite samples stored at 55 and 85% RHs. First, their BL 1 H NMR spectra were deconvoluted in one broad line related to hydrogen nuclei located in rigid starch chains and bound water molecules, and three narrow lines at chemical shifts of ~5.3, 4.8 and 3.8 ppm whose position was derived from MAS 1 H NMR spectra (Figure 5); free water hydrogens form signal at 4.8 ppm, glycerol OH groups at 5.3 ppm, glycerol CH and CH 2 groups at 3.8 ppm, and hydrogen nuclei in mobile starch chains at 5.3 and between 3 and 4 ppm [20,34]. Signal of the mobile starch chains might thus overlap with glycerol signal in our spectra.…”

Solid-State NMR Spectroscopy: Towards Structural Insights into Starch-Based Materials in the Food Industry

“…On a molecular level, these heterogeneities appear in the forms of granule swelling, starch gelatinization and granule disintegration [ 73 , 77 , 97 ]. However, studying such heterogeneous structures is challenging, with few techniques being capable of holistic analysis.…”

“…Polarization transfer 13 C ssNMR spectra based on CP and insensitive nuclei enhanced by polarization transfer (INEPT) experiments are considered an alternative approach. CP-based experiments exhibit better efficiency on rigid crystalline and amorphous structures, while INEPT-based ones depend directly on the mobility of polymeric segments and flexible gelatinized chains [ 97 , 98 ]. The CP spectrum for dry uncooked pasta (represented in Figure 4 a) reveals broad peaks of starch’s rigid structure, while INEPT shows no peaks of starch except for some visible peaks related to lipids and proteins.…”

“…The INEPT experiment appears to work better for soaked starch; however, only relatively small peaks were observed from the starch region which could be assigned possibly to mobile fractions of dissolved starch or amylose leakage. The INEPT experiment showed peaks related to lipids and proteins with higher intensities compared to the dry starch pasta [ 97 ].…”