Low-Field NMR Testing Experiment on Dried Squid

Objective

 

Test squid slices at different drying stages to observe moisture migration during the drying process.

2. Materials & Methods

 

2.1 Materials

Seven samples dried with the same method but for different durations: T5-1h, T5-2h, T5-3h, T5-4.5h, T5-5.5h, T5-7h, and T5-8.5h. (T5 indicates the drying method; “1h” indicates a drying time of 1 hour.)

2.2 Instrumentation

NMI20 low-field NMR imaging analyzer with a 15 mm probe coil diameter.

2.3 Sample Preparation

Cut each sample into cubes of approximately 2–3 mm per side and place them into a 15 mm OD test tube; target packing height: 15–20 mm.

2.4 Experimental Parameters

Data acquired using the CPMG sequence.

3. Analysis & Results

 

(i) Background

The state of water—its binding and mobility—is critical in foods, directly affecting rheology and stability. Low-field NMR has been widely used to study bound water in biological systems by measuring the transverse relaxation time T₂ of hydrogen nuclei, which reflects molecular mobility. T₂ shortens when water is tightly bound to substrates (restricted mobility), while free water shows longer T₂. Thus, LF-NMR reveals the strength and extent of water binding.

(ii) Test Results

(1) Moisture migration (T₂ spectra)

With one drying method and varying times, post-dried samples can be grouped into two stages:

Stage 1: Samples T5-1h to T5-3h. Common features: muscle structure intact; gel-like appearance; white and semi-transparent.

Stage 2: Samples T5-4.5h to T5-8.5h. Common features: muscle structure disrupted; dry texture; yellowish; reduced transparency.

Relaxation analysis — Stage 1

The Stage 1 T₂ spectra show two prominent components: a larger early peak and a smaller later peak. Both shift to shorter T₂ with longer drying. The larger peak’s fraction decreases over time, while the smaller peak’s fraction increases. As drying proceeds, squid muscle contracts, water-holding capacity declines, and water is expelled—yet water that remains becomes more tightly bound. Hence, the larger peak reflects muscle–water binding: its T₂ shortens (stronger binding) and its fraction drops (less retained water). The smaller peak reflects expelled water: its T₂ also shortens (expulsion becomes harder), while its fraction increases (bound water converting to free water).

Relaxation analysis — Transition

By inspection, T5-3h appears gel-like, while T5-4.5h is dry. The former indicates intact muscle; the latter indicates structural damage. A sharp change in water binding is therefore expected across this interval.

As shown, the expelled-water and muscle-bound-water peaks present at 3 h disappear by 4.5 h. Even the shortest expelled-water T₂ exceeds the initial muscle-bound-water T₂; yet beyond ~65 ms no peaks remain at 4.5 h. Once no further water is expelled, the muscle has lost water-holding capacity—its bound-water signature vanishes. From Stage 2 onward, the spectra no longer represent expelled water vs. muscle-bound water.

Relaxation analysis — Stage 2

Following the transition, muscle structure is compromised and cannot retain water. Continued moisture loss in Stage 2 therefore relates to components binding water more tightly than the original muscle matrix—likely proteins. The observed doublet resembles spectra from cross-linked elastomers, suggesting signals from hydrogens on protein backbones. Crosslink theory assigns the shorter T₂ to crosslink sites and the longer T₂ to dangling chains; higher crosslink density shortens both T₂ values, increasing the short-T₂ fraction while decreasing the long-T₂ fraction. Here, with longer drying, both T₂ components decrease; the short-T₂ fraction rises while the long-T₂ fraction falls—consistent with denaturation and densification of a protein network and increasing sample rigidity.

Conclusion

 

1) With longer drying, muscle-bound water in squid progressively expels (Stage 1).

2) As drying continues, proteins denature and the texture hardens (Stage 2).

3) A critical point of complete muscle destruction and loss of water-holding capacity occurs between 3 h and 4.5 h.

4) Protein denaturation begins between 4.5 h and 5.5 h.