Methods to characterize the structure of food powders – a review

Figures & data

Fig. 1. An illustration of molecular arrangement in crystalline, amorphous and mixed structural powders (Adapted from Bhandari and Roos¹⁹⁾).

Table 1. Differences about structure and properties of amorphous and crystalline food powders.

Properties	Amorphous solids	Crystalline solids
Structure	• Irregular and fairly random arrangement of molecules • Loosely packed structure • Without having any symmetry • Without having interfacial angles	• Regular, repeating, long range order, three dimensional arrangement of molecules • Highly packed structure • Having symmetry • Having interfacial angles
The strength of bonds between various ions, molecules or atoms	• Different and small	• Equal and large
Heat of fusion	None	Defined
Melting point	Having a wide range of glass transition	Extremely sharp melting point
The physical properties (thermal conductivity, electrical conductivity, refractive index or mechanical strength)	Normally non-linear along different directions	Normally linear along different directions
Scattering produced by incident X-ray	• Weak • Spread throughout reciprocal space (essentially continuous diffraction diagram)	• Strong • Concentrated into a few sharp diffraction peaks (essentially discrete diffraction diagram)
Adsorption capacity of external molecules	• High • dsorption in porous structure	• Low • Primarily adsorption on the surface
Water dissolution ability	High and fast	Low and slow
Compressibility	High	Low
Density	Low	High
Hygroscopicity	High	Low

Fig. 2 Sketch of solid encapsulation using amorphous cyclodextrin powders.

Fig. 3. XRD (a) and SEM (b) of amorphous and crystalline α-cyclodextrin powders (adapted from Ho et al.¹⁶⁾).

Fig. 4. Typical curves of conventional DSC for amorphous and crystalline powders scanned with open and closed pans.

Fig. 5. Typical water adsorption isotherm of amorphous and crystalline powders determined by the gravimetric vapor sorption; ^(*) a_w = RH/100 as the stored powders achieve a moisture equilibrium with surrounding environment.

Table 2. A comparison on static and dynamic water adsorption techniques.

Characteristics	Water adsorption techniques
	Static	Dynamic
Time required for analysis	Slow (3–4 weeks)	Fast (several hours)
Sample amounts	Large	Small
Cost	Low	High
Sensitivity	Low	High
Accuracy	Low	High
Labor requirement	High	Low
Operating mode	Manual	Automatic

Fig. 6. The electromagnetic radiation region applied in various spectroscopic techniques (Adapted from Taday¹²⁵).

Fig. 7. Differences between amorphous and crystalline powder under various analytical techniques: (a) ¹³C NMR of amorphous and crystalline α-CD powders (adapted from Ho et al. ¹⁶⁾); (b) Raman spectroscopy of amorphous and crystalline lactose powders (adapted from Murphy et al. ¹⁴⁶); (c) FTIR spectra of amorphous and crystalline sucrose (adapted from Mathlouthi and Cholli¹⁶⁰); and (d) Absorption coefficient (0.5–4 THz) of amorphous and crystalline glucose (Adapted from Walther et al. ¹⁶⁸⁾).

Table 3. Differences between Fourier transform infrared and Raman spectroscopy¹⁴¹⁾.

Characteristics	FTIR	RS
Operating principle	• Result of absorption of light by vibrating molecules	• Result of scattering of light by vibrating molecules
	• Depending on changes in dipole moment of a molecule	• Depending on changes in polarizability of a molecule
Sensitivity	Hetero-nuclear functional group vibrations and polar bonds (OH stretching in H2O)	Homo-nuclear molecular or non-polar functional groups bonds
The excitation wavelength on electromagnetic spectrum	A monochromatic beam in the infrared region	A monochromatic beam or laser, in the visible, near-infrared, or near ultraviolet range
Interference with fluorescence	No	Yes
Sample preparation	Very elaborative and having some constraints on thickness or uniformity of sample	Not really required
Instrumental cost	Relatively cheap	Expensive
Spectra	Irregular absorbance (or transmittance) lines	An elastic scattered light line (Rayleigh) and two equally distanced lines Stokes and anti-Stokes
Use of water as solvent	No, due to IR adsorption by water	Yes

Table 4. A comparison of various analytical techniques for qualitative and quantitative investigation on the structure of food powders (Adapted from Shah et al. ⁹⁾ and Sheokand et al. ¹⁰⁶⁾.

Characteristics	XRD	SEM	DSC	DVS	^13C NMR	FTIR	Raman	IGC
Analytical time	6-60 min	30-60 min	Seconds to hours	Several hours	30–6000 min	10-60 min	6-120 min	2000-4000 min
Sample weight	300-400 mg	A few particles	4-10 mg	5-50 mg	500-700 mg	5-50 g	2000 mg	300-400 mg
Sample destruction	No	No	Yes	Yes	No	No	No	Yes
LOD/LOQ (%) ^(*)	10	Not applicable	5-10	0.05	0.5	1-2	1	1
Calibration requirement	No	Not applicable	Yes	Yes	Yes	No	No	No
Internal standards	Yes/No	Not applicable	Yes/No	No	No	No	No	No
Measured parameters	• Qualitative and quantitative • Sharp peaks for crystalline and broad peaks for amorphous	• Only qualitative • Spherical shape particles for amorphous and irregular shaped particles for crystalline	• Quantitative and qualitative • Sharp peaks for crystalline and broad peaks (hump) for amorphous	• Quantitative and qualitative • High water adsorption for amorphous and low adsorption ability for crystalline • Phase transitions observed on amorphous • Differentiation of surface/bulk amorphous content	• Quantitative and qualitative • Broad peaks for amorphous and sharp, splitting or even overlapped peaks for crystalline	• Quantitative and qualitative • Sharp or low intensity peaks for crystalline, less defined and higher intensity peaks for amorphous • Complementary quantification technique each other		• Quantitative and qualitative • High sensitivity to surface changes • Differentiation of surface/bulk amorphous content

^* LOD/LOQ (%): Level of detection/level of quantification of amorphous/crystalline content (%).

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