Whey protein based edible food packaging films and coatings

Packaging systems are intended to protect the food from its surroundings acting as physical/mechanical, chemical and microbiological barrier to maintain quality, safety, and to prolong the packaged food shelf-life. Food quality and its average shelf-life are decreased when the foodstuff interacts with its environment gaining or losing moisture and aroma, or taking oxygen leading to oxidative rancidity. Additionally, microbial contamination may produce food spoilage, or even food poisoning. In multicomponent foods the quality and shelf life are reduced Correspondence/Reprint request: Dr. Regalado, C, DIPA, PROPAC, Facultad de Química, Universidad Autónoma de Querétaro, 76000. Qro. Mexico. E-mail: [email protected] Regalado, C. et al. 238 when moisture, aroma or lipids migrate from one food component to another. Food packaging also provides important information to the consumer (nutrition facts, ingredients, expiration date, etc.), and makes the food available for a long period of time. Introduction A variety of techniques have been developed to maintain the quality and microbial safety of foods, being food packaging one of these methods. Fresh oranges and lemons were wax coated in China in the 12 and 13 centuries, to reduce water loss [46]. The first packaging materials based on cellulose were developed in 1856, and in 1907 phenol-formaldehyde (bakelite) resins were synthesized. This was the starting point of a series of developments and innovations giving birth to a great diversity of packaging materials which nowadays are employed [74]. Packaging systems are intended to protect the food from its surroundings acting as physical/mechanical, chemical and microbiological barrier to maintain quality, safety, and to prolong the packaged food shelf-life [37]. Food quality and its average shelf-life are decreased when the foodstuff interacts with its environment gaining or losing moisture and aroma, or taking oxygen leading to oxidative rancidity. Alternatively, microbial contamination may produce food spoilage, or even food poisoning. In multicomponent foods the quality and shelf life are reduced when moisture, aroma or lipids migrate from one food component to another. Food packaging also provides important information to the consumer (nutrition facts, ingredients, expiration date, etc.), and makes the food available for a long period of time [60]. Initially, food packaging contributed to easy handling of food products to manufacturers, distributors and consumers. However, this has changed due to a growing consumer demand for minimally processed and easy preparation foods, where natural food additives are favored over their synthetic counterparts [12, 84]. Petrochemical based plastics have been widely used because of their availability in large quantities at low cost and favorable mechanical and barrier properties to oxygen, and heat sealability [105]. Nowadays the use of synthetic packaging materials has considerably raised with a concomitant increase in environmental pollution, since they are recalcitrant. Plastic materials may be degraded by naturally occurring microorganisms in the environment, but the process may take about 150 years (low density polyethylene), while paper can be naturally biodegraded in about one year [93]. Inert and non-biodegradable plastic materials account for about 30% by weight of municipal solid wastes, but due to their low density they represent 2/3 of the wastes volume [52]. There has been a growing interest for edible films and coatings in recent years trying to reduce the amount of wastes, capable of protecting the food once the Food biotechnology 239 primary packaging is open, and because of public concerns about environmental protection [20]. Edible coatings and films can be advantageously used on meat and meat products with the following benefits [39]. Moisture loss reduction during storage of fresh or frozen meats; retention of juices from fresh meat and poultry when packed in plastic trays; oxidation/reduction of lipids and myoglobin; reduction of spoilage and pathogen microorganisms on the surface of coated meats; and restriction of volatile flavor loss and foreign odor pick up. In this chapter it is intended to give an account on the new developments on whey protein edible films. Characteristics of whey proteins, formation of edible films using whey proteins, and the mechanical and physicochemical properties of such films are disclosed. Finally, an account is given on the antimicrobial properties of edible antimicrobial whey protein based films as well as their possible applications in the food industry. Whey proteins Whey is the yellow-green liquid that separates from the curd during manufacture of cheese and casein [96]. In the recent past commercial cheese manufacturers treated whey as sewage or returned it to dairy farms for pig feeding or field spreading. However, since whey generates a biochemical oxygen demand as high as 38,000 mg/L[53], environmental concerns and regulations have become and important issue. Financial costs of disposal have made it profitable to further process whey, especially its protein and lactose for use as food ingredients. Whey protein has been used in confectionery, bakery and ice cream products, infant formula, health foods, and sports bars. Recent investigations aimed to find new uses of whey protein, have made use of the ability of processed whey protein (80 to 90% by weight) to form films and coatings on the surface of food products [7, 108]. Riedel [89], estimated a worldwide annual production of liquid whey of 118 million tons, which is equivalent to about 7 million tons of whey solids. However, in 1995 about 62% of total whey production was used in some application, but the remaining 38% of whey proteins (270,000 tons) were still available as valuable food ingredients. In the USA about 51.2% of the liquid whey produced was used by the food industry [1]. The whey protein is of high quality, since it has all essential amino acids, and a biological value higher than egg or casein proteins [69]. Whey protein has also some functional properties of interest to the food industry, such as solubility, emulsification, foaming, gelation, and viscosity development [2, 110]. Whey proteins represent about 20% of milk proteins, and four proteins represent more than 80% of total protein: β-lactoglobulin (β-LG), α-lactalbumin (α-LA), bovine serum albumin (BSA) and immunoglobulins (Ig) (Table 1). Regalado, C. et al. 240 β-Lactoglobulin β-LG represents about 50% of whey protein and act as carrier of retinol from the cow to the young calf. Under physiological conditions is predominantly dimeric, although it undergoes reversible conformational changes known as the Tanford transition at pH 7 [104]. β-LG can dissociate into monomers at pH<3.5 or at elevated temperatures and higher pH. Unfolding of the protein occurs at 72°C (tertiary level) while dimer-monomer dissociation starts at 52°C. The secondary structure showed 54% β-strand, 15% α-helix, 12% β-turns and 19% nonstructured at 15°C [54]. It has been reported eleven genetic variants, with isoforms A and B more prevalent in bovine milk. They differ by two amino acid substitutions: Asp64 and Val118 in isoform A are replaced by Gly and Ala in isoform B, respectively [36]. Native β-LG possesses two disulfide bonds and a solvent inaccessible free sulfhydryl group which upon conformational changes leads to sulfhydryldisulfide interchange reactions, affecting characteristics such as solubility [101]. The reactivity of the free thiol group increases a pH>6.5 [45]. α-Lactalbumin α-LA is a 123 amino acid globular protein which represents about 20% of total whey protein, and has four intrachain disulfide bridges which confer hydrophobicity and foaming properties to the molecule. It has three genetic variants, the A variant contains a Glu10, while the B variant has Arg substitution at that position. Variant C is not well characterized and has Asn for Asp or a Gln for Glu substitution [36]. The two lobes of the threedimensional structure can be divided into a acidic lobe rich in β-sheet structure, that contains the Ca domain and a basic lobe rich in αhelical structure [29]. The main function of α LA is to participate in lactose biosynthesis as the regulatory component of the lactose synthase complex [34]. It has the ability to interact with lipid membranes and fatty acids [9], and has 72% sequence identity to human α-LA, which makes it an ideal protein for human infants nutrition [49]. α-LA requires Ca for proper folding and disulfide bond formation, and at pH>5 it can bind to metal ions [29] such as Mn, Mg. At pH≤4 unfolds and can be digested by pepsin. α-LA and β-LG interact when heating at 75°C, pH 7, forming soluble aggregates whose composition changes with heating time and relative proportions of these proteins [31]. Co-aggregation of α-LA and β-LG may be attributed to the facilitation of intermolecular disulfide bond formation by hydrophobic interactions [50]. When held at temperatures ≥85 °C, α-LA evolves free thiol groups that form intermolecular disulfide bonded aggregates[31, 50]. At 85 °C the surface exposed, highly reactive C6-C120 disulfide bond, drives initiation of Food biotechnology 241 thiol/disulfide interchange, and, subsequently, a neighboring thiol (C111) is the most reactive in forming intermolecular disulfide bonds [64]. The high reactivity on the C-terminus chain may be a result of its known flexibility and the enhanced reactivity of Cys thiols in the proximity of positive charge density [15]. The aggregation of α-LA was enhanced by the presence of calcium, a high degree of purity, excess ionic screening, and heating at 95 °C or above [70]. Table 1. Composition of major proteins in bovine whey, their relative molecular weight (Mr), isoelectric point (pI) and biological functions (adapted from 11, 34, 36, 47, 48).