Tendencia uno: La ira como forma de expresar el descontento por ciertas situaciones presentadas en la escuela

Edible coatings can be applied by dipping or spraying after solubilization or disper- sion of proteins� Typical methods for forming a coating include spraying and dip- ping� In the spraying operation, the coating is applied on food surface and then, a separate operation to coat the bottom surface of the product is required� Spraying is preferred for items possessing a large surface area� On the contrary, dipping is more convenient for irregularly shaped food objects� However, final coatings might be less uniform and multiple dipping might be necessary to ensure full coverage (Dangaran et al�, 2009)� In both cases, the coating operation time is usually fixed to control the coating thickness (Zhong et al�, 2014)�

Formation of films also involves the previous solubilization or dispersion of pro- teins� The solubility of proteins depends on the type of protein and its isoelectric point� Type A gelatin, produced from acid-treated collagen, has an isoelectric pH of 6–9, while the isoelectric point of type B gelatin is about 5 (Eysturskaro et al�, 2009; Stainsby, 1987)� Physicochemical properties of some animal and plant proteins are summarized in Table 3�2�

Proteins normally show lower solubility close to the isoelectric point (Massani et al�, 2014)� The isoelectric point of whey protein is around 5�2 and at pH lower than 4 and higher than 6, transparent whey protein films can be obtained, although only films prepared at basic pH are flexible (Moditsi et al�, 2014)� In relation to wheat gluten, its isoelectric point is 7�5 and thus, films do not form at pH 7–8, and dis- persions at pH 5–6 are also very poor� Nevertheless, according to Gennadios et al� (1993), wheat gluten films can be obtained at basic or acidic conditions, although alkaline conditions result in stronger films (Kayserilioglu et al�, 2001)� Regarding

TABLE 3.2

Physicochemical Properties, Including Solvent Solubility and Isoelectric Point (pI), of Some Animal and Plant Proteins

Protein Solvent pI References

Casein Water 4�6 Creamer and MacGibbon (1996)

Egg albumen Water 4�5 Mine (1995)

Gelatin Water 5–9 Stainsby (1987)

Soy protein Water 4�6 Kinsella (1979)

Wheat gluten Alcohol–water 7�5 Wieser (2007)

Whey protein Water 5�2 Kinsella and Whitehead (1989)

soy proteins, films cannot be formed close to pH 4�5 (Sian and Ishak, 1990) and are mostly formed at alkaline conditions (Brandenburg et al�, 1993; Jiang et al�, 2012)� These changes related to pH can be analyzed by FTIR spectroscopy�

The most characteristic peaks associated with peptide bonds are the band cor- responding to the stretching of C = O (amide I) and the band corresponding to the stretching of C–N and to the vibration of N–H (amide II) (Lodha and Netravali, 2005; Subirade et al�, 1998)� As shown by Guerrero and de la Caba (2010) for soy protein films (Figure 3�7), the relative intensity between the band at 1630 cm−1 _(amide

I) and the band at 1530 cm−1 _{(amide II) can change depending on pH� The intensity}

of those two bands was similar at the isoelectric point, but the relative intensity between amide I and amide II bands became higher at pH 1�4 and lower at pH 10�0, indicating that the pH value affected protein structure and consequently, functional properties� Mauri and Añón (2008) showed that soy protein films formed at basic conditions (pH 8 and 11) exhibited 70% higher deformation than the films prepared at acidic conditions (pH 2) due to the presence of protein fractions in native state, allowing macromole cules to unfold during mechanical tests�

The isoelectric point of proteins and thus, their sensitivity to pH are associated with the content of ionized polar amino acids in proteins� In soy proteins, the high content of ionized polar amino acids (i�e�, 25�4%) limits film formation at low pHs (Kim and Netravali, 2012)� However, zein and keratin films form over a wide pH range due to their low content of ionized amino acids, 10�0% and 10�7%, respectively (Cabra et al�, 2007; Cuq et al�, 1998)� Therefore, the structural diversity of proteins

4000 3500 3000 2500 Wavenumber (cm–1₎ pH 1.4 pH 4.6 Tr ansmittanc e pH 10.0 2000 1500 1000

highly influences their solubility, processing conditions, and thus, the properties of the resulting films and coatings�

Although the thermal stability of proteins is dependent on their amino acid com- position, temperature is a key factor in the solution processing of proteins since their conformation changes and thus, their degree of denaturation determines the type and proportion of covalent and noncovalent interactions between protein chains� Hoque et al� (2010) investigated the effect of temperature, from 40°C to 90°C, on the properties of glycerol-plasticized fish gelatin films� They found that thickness and transparency of the films were not affected by the heat treatment, while mechani- cal properties changed� The films prepared from solutions heated at 60–70°C showed higher tensile strength, whereas those films prepared by heating solutions at 80–90°C had higher elongation at break� These results were related to the melt- ing transition temperature and enthalpy measured by differential scanning calorim- etry (DSC)� The higher melting transition temperature and enthalpy observed for the films heated at 60–70°C was related to the greater interchain interactions, mainly via hydrophobic interactions and hydrogen bonding, as observed by FTIR, causing stronger film network and higher tensile strength� In a similar way, Denavi et  al� (2009) evaluated the influence of drying conditions, both temperature and relative humidity (RH), on mechanical properties, solubility, and barrier properties of soy protein films prepared by solution casting� They found that 70°C and 30% RH were the optimal drying conditions to achieve higher tensile strength and lower solubil- ity and water vapor permeability (WVP)� This behavior was attributed to a greater unfolding degree at these conditions, which allowed higher number of interactions between protein chains and led to a more compact structure, as observed by trans- mission electron microscopy (TEM)�

To date, heat treatment is the most common method for protein processing in solution, but application of ultrasound technology is currently attracting much atten- tion, since it can represent an alternative method to process proteins at lower tem- peratures using less time (Soria and Villamiel, 2010)� Ultrasonic waves can be used to change specific surface area, rheological properties, and thus, physicochemical properties of proteins (Jambrak et al�, 2009)� Recently, Hu et al� (2013) examined the effect of low-frequency ultrasonication at different powers (200, 400, and 600 W) and times (15 or 30 min) on soy protein dispersions� The structural changes observed in this study were related to the decrease in noncovalent interactions after ultrasonic treatments� Changes in the secondary structure of soy proteins by ultrasonication were analyzed by circular dichroism� This analysis indicated that ultrasonic treat- ment resulted in an increase in α-helix component and a decrease in β-sheet compo- nent, in accordance with previous results obtained by other authors for whey proteins (Chandrapala et al�, 2011)� This effect was more pronounced at higher powers (400 and 600 W) and longer times (10 min)� Therefore, controlling treatment conditions could lead to different structures and thus, to different functional properties�

Solvent casting has been the preferred method used by researches to prepare films due to the simplicity of the equipment required, which can consist of simple casting dishes� However, there are more advanced and sophisticated equipment that can produce larger films by mechanically spreading the solution to a fixed thick- ness� Kozempel and Tomasula (2004) developed a continuous process, where feed

system and dryer parameters are the key factors to obtain films with good properties� Furthermore, the combination with emergent technologies in the food industry, such as high power ultrasound, is becoming more efficient for large-scale applications at lower temperatures (Patist and Bates, 2008)�