Tendencia tres: El asco como censura ante el no control de lo orgánico

Proteins are heteropolymers built from up to 20 different monomers� Those mono- mers are amino acids joined together by peptide bonds, which are amide linkages formed by the condensation reaction of amino acids (Figure 3�2)�

Amino acids differ from each other in the structure of side chains (R), which can be nonionized or ionized polar groups (basic or acid groups) and nonpolar groups

NH CH C NH O R CH C O R′

(McMurry, 2011)� Proteins contain more than 100 amino acids, known as residues� The sequence of those amino acids constitutes the primary structure of proteins� The properties of proteins depend not only on the sequence of amino acids, but also on the way in which protein chains are folded in space (Zhang et al�, 2014)� Owing to their size, the orientation options of proteins could be enormous, but several factors limit the structural options, and it is possible to identify some common structures that appear repeatedly, such as α-helix, β-sheets, or unordered random coil struc- tures, which are referred as secondary structures� Some factors that influence the conformational equilibrium of protein chains are the planarity of peptide bonds, hydrogen bonding of carbonyl groups to amino groups in peptide bonds, steric hin- drance of neighboring groups, repulsion and attraction of charged groups, and the hydrophilic and hydrophobic characters of side groups� Most proteins do not adopt completely uniform conformations, and full descriptions of their preferred three- dimensional arrangements are defined as tertiary structures� In addition to the ter- tiary structure, the way in which polypeptide structures may aggregate constitutes the quaternary structure (Silva et al�, 2014)� This native structure of proteins may be altered by treatments that do not disrupt the primary structure but alter the second- ary, tertiary, and quaternary structures�

According to their shape, proteins can be classified as fibrous or globular pro- teins, which show different behavior in aqueous solutions (Belitz et al�, 2009)� On the one hand, fibrous proteins have fiber-like structures and serve as the structural material in tissues� According to this structural function, fibrous proteins are rela- tively insoluble in water and unaffected by moderate changes in temperature and pH (Bourtoom, 2008)� Fibrous proteins include collagens, which are the proteins of connective tissues, and keratins, which are major components of skin and hair� On the other hand, globular proteins, such as wheat gluten, corn zein, and soy protein, serve maintenance roles in living organisms and either dissolve or disperse in aque- ous solutions (Ghanbarzadeh et al�, 2007)� Such proteins are generally more sensitive to temperature and pH changes than fibrous proteins (Yampolskaya and Platikanov, 2006)�

Proteins from different sources have different characteristics, as they contain dif- ferent amino acid contents, and show different structures� With regard to animal proteins, collagen is the primary protein component of animal connective tissues� All collagens comprise at least two different α-chains (α1 and α2), and their dimers

(β-chain) and trimers (γ-chain)� Collagen is composed of different polypeptides, which contain mostly glycine, proline, and hydroxyproline (Gelse et al�, 2003; Wang et  al�, 2014)� By denaturation of collagen, a high-molecular weight polypeptide is produced, called gelatin, which is water soluble (Cao et al�, 2009; Gómez-Guillén et al�, 2009)� The amino acid composition of gelatin is very close to that of its par- ent collagen� Gelatin contains a large amount of proline, hydroxyproline, and lysine (Dangaran et al�, 2009; Gómez-Guillén et al�, 2002; Karim and Bhat, 2009)� In rela- tion to milk proteins, there are two major protein types: casein and whey protein (Creamer and MacGibbon, 1996)� Casein is the main protein of milk, accounting for approximately 80% of the total protein content� It is a protein that can be separated into various fractions, such as α-, κ-, β-, and γ-casein, which differ in composition and molecular weight (Audic et al�, 2003)� Whey protein is the soluble fraction of

milk protein after alkaline precipitation of caseinate� Various whey protein grades are commercially available, mainly whey protein concentrates (WPCs), containing 35–80wt% protein, and whey protein isolates (WPIs), with protein content above 90wt%� WPC and WPI also differ in the content of other components, such as car- bohydrates and lipids and thus, the final properties of the films and coatings based on these whey proteins may also differ (Ramos et al�, 2013)� The two major constituents of whey protein are β-lactoglobulin and α-lactalbumin (Wang et al�, 2013)� In the case of egg proteins, egg white (albumen) is a complex protein, consisting of ovo- mucin fibers in an aqueous solution of numerous globular proteins (Gennadios et al�, 1998), in which ovalbumin, a monomeric phosphoglycoprotein, represents about 54% of protein content (Fernández-Espada et al�, 2013)�

Regarding plant proteins, wheat gluten contains four main fractions of proteins: albumins (water soluble), globulins (soluble in dilute salt solution), but mainly glia- dins (soluble in 70% ethanol), and high-molecular weight glutenins (partially solu- ble in dilute acids or alkali) (Chen et al�, 2012; Lagrain et al�, 2010)� In relation to corn, it contains a prolamine protein, zein, exclusively found in corn (Anderson and Lamsal, 2011)� Four different fractions of zein have been identified: α-, β-, γ-, and δ-zein, but α-zein is the predominant fraction present in commercial zein (Shi and Dumont, 2014)� Zein comprises a large amount of hydrophobic amino acid residues, which are the basis for its hydrophobic character� In contrast, soy protein contains a high content of polar amino acid residues� Soy proteins are composed of a mix- ture of albumins and globulins, 90% of which are storage proteins with globular structure� Soy protein consists of four major fractions: 2S, 7S, 11S, and 15S, where S stands for Svedberg units, based on the rate of sedimentation� 7S (β-conglycinin) and 11S ( glycinin) globulin fractions make up 70% of the total proteins in soybeans (Kinsella, 1979; Ning and Villota, 1994)�

Proteins have potential properties for their application in the food packaging field due to their ability to form films and coatings with good barrier properties against oxygen (O2), carbon dioxide (CO2), aroma, and lipids (Falguera et al�, 2011; Miller

and Krochta, 1997)� However, protein-based films and coatings have limited resis- tance to water vapor and thus, barrier and mechanical properties are compromised by moisture due to the inherent hydrophilicity of proteins� Therefore, formulations, preparation methods, and modification treatments must be optimized to achieve the desirable functional properties for specific applications�