3.3. Properties Analysis of Corn Flour

Determination of starch content in corn flour by 1%-hydrochloric acid optical rotation method. Protein content of the corn flour was measured using AACC Approved method 46-13A (AACC International, 2000, Method 44-13A). Moisture and fat content were determined according to Approved method 44-15.02 (AACC International, 1995, Method 44-15.02) and GB/T 5512-2008, respectively.

Corn flour was first sprinkled on double-sides adhesive tape, which was attached to aluminum stubs, and the sample were then plated with a thin layer of gold. By using a high vacuum bench top scanning electron microscope (Quanta250 FEG; FEI, Hillsboro, OR, USA), the sample were observed and photographed.

The size distribution of corn flour was determined by laser diffraction particle size analyzer (SALD-301V; SHIMADZU, Kyoto, Japan). Prior to analysis, the samples were dispersed in distilled water to form a suspension (2%, w/v).

The morphology and birefringence of corn flour were observed in a digital camera belonged to the optical microscope (ECLIPSE 50i POL; Nikon, Kanagawa, Japan). Corn flour suspension (2%, w/v) were placed on the microscope slide with a glass coverslip. Then the samples was observed under the polarized light microscope.

Crystalline properties were assessed using an X-ray diffractometer (Rigaku MiniFlex600, Tokyo, Japan), which was operated at 30 mA and 40 KV. The diffraction angle (2θ) scanned was from 5° to 45° with a scanning speed of 4°/min. The crystallinity of corn flour was obtained by integrating the diffraction pattern with software MDI JADE 6.0.

The thermal properties of the corn flour were determined using a differential scanning calorimeter (DSC) (Q20; TA Instruments, New Castle, DE, USA), and nitrogen purge gas was used in the experimental work. Corn flour samples (3.0 mg, db) were wetted with distilled water (7 μL) in an aluminum pan and immediately sealed. The sealed pans were allowed to stand for 24 h at room temperature before the measurements. The sample pans were scanned from 30 to 100 °C with a heating rate of 10 °C/min and an empty pan was used as a reference [27]. The DSC equipment software was applied to analyze onset temperatures (To), peak temperatures (Tp), final temperatures (Tc) and enthalpy gelatinization enthalpy (∆H).

Pasting properties of corn flour (8.0% db, 28 g total weight) were determined using a Rapid Visco-Analyser (RVA) (Perten Instruments Ltd.; Sydney, Australia). The heating and cooling cycles were programmed in the following manner. The slurry was held at 50 °C for 30 s, heated to 95 °C within 2.5 min and then held for 20 min. It was subsequently cooled to 50 °C within 3 min and then held for 9 min [46]. Peak viscosity (PV), trough viscosity (TV), final viscosity (FV), breakdown (BD) and setback viscosity (SV) were evaluated.

Rheological experiments were carried out using a DHR-1 rheometer (TA Instruments, New Castle, DE, USA). A 40 mm diameter parallel plate was used and the gap between the plates was set to 1 mm. The free surface of the sample edges was covered with silicone oil to prevent evaporation of the sample during the measurements [47]. The temperature was controlled by a water bath connected to the peltier system.

Flour suspensions (5%, db) was prepared in deionized water and gelatinized by a Rapid Visco-Analyser in order to obtain starch paste and the experimented method was described in Section 3.3.7. The starch pastes was loaded on a peltier-plate carefully. The steady shear tests were performed at 25 °C over the shear rate range of 0.1–100 s⁻¹ and apparent viscosity was recorded as a function of shear rate. The relationship of shear stress and shear rate were fitted to the Herschel-Bulkley model [48].

where τ is shear stress (Pa), τ₀ is yield stress (Pa), K is the consistency coefficient (Pa·sⁿ), γ is shear rate (1/s), and n is the flow behavior index (dimensionless).

For frequency sweep experiments, the samples (6%, db) were prepared according to the method depicted in Section 3.3.7. The starch pastes were then standing 1 h at room temperature before the test. A resting time of 5 min was allowed before the frequency sweep was performed from 0.1 to 100 rad/s at 1% strain (within the linear viscoelastic region) at 25 °C. The viscoelastic parameters obtained with frequency were the storage modulus (G′), loss modulus (G″).