Canadian Grain Commission
Symbol of the Government of Canada

Using ultrasound to characterize fresh yellow alkaline noodles



Experiment to test ultrasonic characterization

Materials used in the experiment

To evaluate the sensitivity of ultrasound to changes in the mechanical properties of fresh yellow alkaline noodles, researchers prepared noodles with different ingredients, including:

  • 2 different classes of Canadian wheat flour (hard white spring and durum wheat flour)
  • 3 different additive treatments including transglutaminase

Transglutaminase catalyzes acyl-transfer reactions between peptide-bound glutamyl residues and ε-amino groups of lysine residues, producing non-disulphide covalent cross-links between proteins. This enzymatic protein cross-linking is expected to stiffen the gluten protein structure (Basman, Koksel and Ng, 2002; Autio et al. 2005).

Flour

The researchers used durum flour and hard spring wheat flour.

To make durum flour, the researchers:

  • Milled AC Strongfield, a variety of Canada Western Amber Durum (CWAD) wheat into semolina
  • Reduced the semolina into straight-grade flour (Hatcher et al., 2008b)

To make hard white spring wheat flour, the researchers milled a composite (more than 99%) of multiple samples of the Snowbird variety of Canada Western Hard Spring wheat (CWHWS) into straight-grade flour of 74% extraction (Hatcher et al., 2008b). To minimize the effect of flour particle size on noodle quality, they milled and bolted both flours over 132 μm sieves to ensure comparable particle size distributions (Hatcher et al., 2008b).

The researchers determined the protein content (%N x 5.7) with combustion nitrogen analysis. Using American Association of Cereal Chemists Approved Methods (Hatcher et al., 2008b). they determined gluten index, ash content and Rapid Visco Analysis flour pasting parameters.

Noodles

Using an asymmetrical speed mixer, the researchers mixed the following:

  • 50 grams of flour
  • Sodium chloride (NaCl)
  • Transglutaminase enzyme with kansui (depending on the treatment)

The levels of salt, kansui and transglutaminase in the experimental noodle formulas (w/w, flour basis) were:

  • SK1 = 1% NaCl + 1% kansui
  • SK2 = 3% NaCl + 1% kansui
  • SKT = 1% NaCl + 1% kansui + 2% transglutaminase

To make the noodle specimens, the researchers:

  • Sheeted the dough on a laboratory noodle machine with an initial gap setting of 3.0 mm, folded longitudinally, and re-sheeted to duplicate the lamination process used by commercial noodle manufacturers.
  • Took the resulting sheet through 7 further reductions (Kruger, Anderson and Dexter, 1994) on the machine with rolls maintained at 28°C. They incorporated a 45-second delay between passages to improve sample reproducibility.
  • Cut disks (5 cm diameter) from the noodle sheet using a sharp-edged metal borer. They obtained 9 specimens from each noodle sheet and divided these into 3 groups:
    • 5 specimens – used to obtain 5 ultrasonic measurements at different thicknesses
    • 1 specimen – used for stress relaxation measurements
    • 3 specimens – used for density measurements
  • Allowed the specimens rest at room temperature (22°C) for 1 hour. This allows the transglutaminase enzyme to activate when incorporated into wheat flour dough (Alava et al., 2007).

Noodle temperature during rheological measurements was 23°C.

Methods used in the experiment

In order to understand how effective ultrasonic testing was at discriminating the texture of fresh noodles, the researchers compared the measurements from the ultrasonic testing with the results from a stress relaxation test. To interpret the results, the researchers conducted a statistical analysis.

Rheological measurements using ultrasound

The experimental set-up was

  • An ultrasonic pulse generator/receiver
  • A pair of transducers with a central frequency of 40 kHz
  • A digital oscilloscope

Figure 1.

Experimental ultrasonic set-up

Experimental set-up: ultrasonic pulse generator/receiver, transducers and digital oscilloscope.

To measure ultrasonic properties, the researchers:

  1. Obtained noodle samples with varying thicknesses by stacking
  2. Excited the transmitted transducer with a short positive voltage spike generated from the ultrasonic pulse generator/receiver to produce an ultrasonic pulse that was transmitted through the noodle specimen. The second transducer (receiver) detected the pulse.
  3. Amplified the transmitted pulse with a signal amplifier in the ultrasonic pulse generator/receiver, and averaged the transmitted pulse (120 times) using the average function of the oscilloscope. This reduced random noise and increased the signal-to-noise ratio.
  4. After being downloaded to a computer, the pulse was filtered (bandwidth 30 – 50 kHz) and the pulse’s transit time and amplitude was taken from the first dip of each filtered waveform.
  5. Determined the ultrasonic velocity from the plots of the transit time of the ultrasonic pulse against dough thickness.
  6. Obtained the signal attenuation coefficient from a plot of the intensity of ultrasonic signal as a function of dough thickness.

Stress relaxation test

The researchers carried out stress relaxation measurements using a texture analyzer.

  1. Dough specimens were compressed between compression platens.
  2. Stress relaxation was measured under uniaxial compression.
  3. Fresh noodles were decompressed to 20% of original thickness and the stress required to keep this degree of deformation for 1 minute (Bellido and Hatcher, 2009b).
  4. From the stress relaxation curves, the maximum compression stress (σmax) in kPa, the dimensionless parameters S* (area under the stress relaxation curve) and overall residual stress (P*) were obtained (Bellido and Hatcher, 2009b).
    • (1/S*) is the reciprocal of S*and is indicative of the overall rate of relaxation.
    • (P*) is the overall residual stress and is indicative of the residual stress in the sample.
    • A higher overall rate of relaxation and a higher overall residual stress are typical of soft solid foods with a greater elastic-like behaviour (Bellido and Hatcher, 2009b).

Density measurements

Density is needed to calculate:

  • Modulus of noodles
  • Ultrasonic velocity
  • Ultrasonic attenuation

The researchers measured density based on the principle of water displacement (Bellido et al., 2009).

Statistical analysis

The researchers completed a statistical analysis of their findings to understand how the results from ultrasonic testing compared to those from stress relaxation. To complete their statistical analysis, the researchers:

  1. Replicated all of the tests 3 times (n=3) using a batch of freshly prepared noodles for each test
  2. Determined analysis of variance (ANOVA) using the General Linear Model (GLM) of SAS
  3. Used Fischer’s Lease Significant Difference (LSD) test to compare the mean values of parameters obtained from:
    • Density and ultrasonic measurements (ultrasonic velocity, attenuation, storage modulus, loss modulus, ratio of loss to storage modulus)
    • Stress relaxation measurements (maximum stress, average rate of relaxation and residual stress)

Note: The use of significance was p<0.05, unless stated otherwise.