Structural Investigation of Edible Zein Films/Coatings and Directly Determining Their Thickness by FT-Raman Spectroscopy

Abstract

Near-infrared Fourier transform Raman (FT-Raman) spectroscopy was employed to study the molecular structure of edible zein films/coatings, which were fabricated directly from zein protein. The secondary structure of zein protein was mainly in alpha-helix and remained unaltered during film formation as evidenced by the vibrational modes of amide I at 1656 cm(-1) and amide III at 1274 cm(-1). Raman results indicated that hydrophobic interaction played an important role in the formation of zein film and disulfide bonding might be responsible for the structural stability of zein protein during film formation. To enhance its antimicrobial property, an antimicrobial zein film was manufactured by incorporating zein protein with benzoic acid whose structure was then characterized by FT-Raman. It showed that physical entrapment or hydrophobic interaction was crucial to the incorporation of benzoic acid with zein protein, and the secondary structure of the antimicrobial film was still maintained in alpha-helical form. In addition, FT-Raman exhibits its preference in directly determining the thickness of zein films/coatings. By correlating the Raman intensity ratio of nu(1003) to nu(84) (I(1003/84)) versus the thickness of zein film, a linear relationship with high coefficient (R(2) = 0.9927) was obtained, which was then used pragmatically to determine the thickness of zein coatings on apple. It showed that the FT-Raman result (thickness = 0.27 +/- 0.01 mm) was consistent with that of classical micrometric measurement (thickness = 0.28 +/- 0.02 mm). Consequently, FT-Raman provides a direct, simple, and reagent-free method to characterize the structure and the thickness of zein films/coatings.

Formation of a viscoelastic dough from isolated total zein (α-, β- and γ-zein) using a glacial acetic acid treatment

“…Both the commercial zein and total zein preparations as well as the films formed from them had predominantly α-helical conformation, as indicated by FTIR in the Amide I region (Figure 5a). The commercial zein data are in agreement with the general finding that it is α-helical in conformation (Shewry and Tatham, 1990) and with Hsu et al (2005) who found that the secondary structure of commercial zein film was predominately α-helical. Figure 5: Effects of zein type, film formation and rinsing residual glacial acetic acid out with distilled water on the FTIR absorbance spectra of a) dry zein powders and zein films and b) wet zein doughs and slurries still predominated, with an α:β ratio of 1.19:1.…”

Physical and structural characterisation of zein and chitosan edible films using nanotechnology tools

“…3A shows the zein film Raman spectrum in the region of 200-1800 cm −1 , where the amide groups I and III displayed bands at 1659 (amide C O stretch) and 1283-1269 cm −1 (N H in plane bend), respectively. These bands are usually related to an alpha helix secondary protein structure [27], which is considered notably stable, even when zein is subjected to a heating process (25-70 • C), because the denaturation it undergoes upon heating is reversible upon cooling [28]. With regards to the amino acid composition [29], zein contains large quantities of glutamic acid (24.2 g/100 g protein) and leucine (17.7 g/100 g protein) followed by proline (8.9 g/100 g protein), alanine (8.4 g/100 g protein) and phenylalanine (6.0 g/100 g protein).…”

“…With regards to the amino acid composition [29], zein contains large quantities of glutamic acid (24.2 g/100 g protein) and leucine (17.7 g/100 g protein) followed by proline (8.9 g/100 g protein), alanine (8.4 g/100 g protein) and phenylalanine (6.0 g/100 g protein). Of these amino acids, only glutamic acid (glutamine), and others presented in the protein at lower concentrations such as cysteine (2.27 g/100 g protein), tyrosine (4.3 g/100 g protein) and lysine (0.46 g/100 g protein), are considered to be reactive and have characteristic Raman bands at 1448 cm −1 (glutamine), 525 cm −1 (cysteine), 848/825 cm −1 (tyrosine) and 1525 cm −1 (lysine) [27,30,31] (Fig. 3A).…”

“…3A). Due to its conformation insensitivity, phenylalanine (998/620 cm −1 ), is useful as an internal intensity standard in Raman analysis [27].…”

Water vapor and oxygen-barrier performance of corn–zein coated polypropylene films

“…This improvement was caused by the excellent oxygen-barrier properties of free-standing zein films, thus higher amount of zein present in coating formulation resulted in lower O 2 permeability of coated PP film. The high oxygen-barrier property of free-standing zein films is well known from several reviews published in literature (Cuq et al, 1998;Rakotonirainy and Padua, 2001;Hsu et al, 2005). It was reported that oxygen permeability values of zein films are lower than those of common synthetic plastic films such as low density polyethylene (LDPE), high-density polyethylene (HDPE), PP, polystyrene (PS), and poly (vinyl chloride) (PVC).…”