Two Food Applications of Biopolymers: Edible Coatings Controlling

Chapter 17

Two Food Applications of Biopolymers: Edible Coatings Controlling Microbial Surface Spoilage and Chitosan Use to Recover Proteins from Aqueous Processing Wastes

Downloaded by UNIV OF ROCHESTER on August 28, 2013 | http://pubs.acs.org Publication Date: March 25, 1999 | doi: 10.1021/bk-1999-0723.ch017

J. A. Torres, C. Dewitt-Mireles, and V. Savant Food Process Engineering Group, Department of Food Science and Technology, Oregon State University, Corvallis, OR 97331-6602

Edible coatings prepared using proteins and polysaccharides can help retain high preservative concentrations on food surfaces to retard microbial surface growth which limits the shelf life of many products. Coating effectiveness is affected by coating formulation, food water activity, food pH and storage temperature. Microstructure deter minations by electron microscopy help interpret coating effectiveness. Chitosan, the deacetylated derivative of chitin, is a versatile molecule with potential applications in diverse fields, including waste water treatments. By electrostatic interactions of chitosan N H3+ groups with COO- or SO3- groups in proteins or polyanions (e.g., alginate, carragee nan and pectin), chitosan can form polymeric complexes. Suspended proteins found in waste effluents, fermentation media and other industrial streams can be recovered as chitosan-protein or chitosan -polyanion-protein complexes.

Edible Coatings to Control Microbial Surface Spoilage Edible coatings developed with a wide range of properties contribute to the stability of many processed foods (Table I). An edible coating or film is defined as a thin, continuous layer of edible material formed or placed, on or between, foods or food components to provide a barrier to mass transfer, to serve as a carrier of food ingredients and additives, or to provide mechanical protection. Polysaccharides such as cellulose, modified cellulose and starch; proteins such as whey, wheat and soy proteins, zein, collagen, gelatin, ovalbumin and serum albumin; plant and microbial polysaccharides such as agar, carrageenan, alginate, pectin, dextrin, gum ghatti, scleroglucan, pullulan, curdlan; and waxes and lipid derivatives have all been considered for food applications (1-29). Restricting the water activity (aj in foods is important to control microbial growth, prevent texture degradation and minimize undesirable chemical and enzymatic

248

© 1999 American Chemical Society

In Biopolymers; Imam, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


249

Table I. Applications for Edible Coatings and Films Control of moisture migration/losses Control of gas exchanges (0 , C0 , C H , etc.) 2

2

2

4

Control of oil and fat absorption and migration Control of solute migration Control offlavorand other volatile migration, exchange, and losses Carrier offlavor,color, antimicrobial, and other food additives Prevention or control of photo degradation - oxidation Improvement of mechanical handling properties of foods Adaptedfrom(57).



250 reactions. Consequently, a functional property of key interest in edible films and coatings is resistance to moisture migration. Most edible films and coatings use polymers to provide structural support, a lipid to improve moisture barrier effectiveness, and a plasticizer to modify mechanical properties. Naturally occurring examples of these multicomponent systems are the glycoproteins, lipoproteins and glycolipids present in cellular membranes (29). Fennema and coworkers have investigated the control of moisture migration between regions with different water activities by methyl and hydroxypropyl methyl cellulose films. Films containing stearic acid, beeswax, paraffin or hydrogenated palm oil had lower water permeabilities than low density polyethylene (4, 31-36). Extensive research has been done also on films and coatings using proteins as the structureforming component with added lipids and plasticizers to improve their functionalities (37-38). Surface microbial stability determines the shelf-life of many products including refrigerated and intermediate moisture foods (IMF) (10-11, 37). During storage and distribution of meats, beef, poultry and seafood, nearly all microbial growth occurs on the surface with Gram-negative bacteria being predominant (39-47). For example, cm , and as a result can be stored refrigerated only for a few days before spoiling (4849). 2

In IMFs, low a and preservatives retard or stop the growth of bacteria, molds and yeasts (50). Progress in the development of these IMFs and other foods can be conceptualized by representing shelf-life and organoleptic quality as a function of the fabrication parameters; i.e., production, formulation and processing steps. The concentrations of stability additives (e.g., salt or K-sorbate) and the intensity of processing steps (e.g., heating) increase product stability but generally decrease perceived quality. An example of this approach, corresponding to the development of an intermediate moisture cheese analog, illustrates the product development difficulties surrounding IMF technology (Figure 1). The search for potentially acceptable formulations was guided by three criteria: (1) texture, evaluated with an Instron; (2) taste, identified by sensory restrictions; and, (3) a minimum a , measured with an electric hygrometer. The limited number of acceptable combinations, represented by the shaded area, points out how difficult it would be to add considerations on product abuse. For example, storage temperature fluctuations (57), product transfers between facilities at different temperatures, or packaging products while still warm affect microbial stability of food surfaces (2, 8). These situations result in surface condensation leading to localized increases in surface a where microbial growth occurs even if the bulk a is acceptable (2, 52). Another source of microbial instability, even for products that are heat processed, is product handling. Slicing and packaging provide many opportunities for surface recontamination (53). Surface counts are highly variable (43-44) and this is particularly important for IMFs since bacteriostatic barriers can be overcome by large localized microbial counts. Furthermore, the minimum a,, for certain microorganisms is oxygen-dependent. Under anaerobic conditions the minimum a for Staphylococcus w

w

w

w

w



251

Protein-Oil Mixture

Solutes Mixture

Figure 1. Product development difficulties surrounding IMF technology. Adapted from (5).


252 aureus is 0.91, whereas under aerobic conditions it is 0.86 (54). Product developers should be more concerned with potential outgrowth of this ubiquitous organism on food surfaces where oxygen is more readily available. To cope with surface microbial problems, food processors treat food surfaces with approved preservatives. Potassium sorbate dips reduce viable bacteria at refrigeration and temperature abuse conditions (48, 55-59). However, the shelf life extension achieved is limited because diffusion reduces preservative concentration on the surface where microbial spoilage must be controlled (8, 60). Sorbic acid diffusion rates (D) can be reduced by lowering food a^ Lowering the a of a model system from 1.0 to 0.88 by use of 40% w/w glycerol or 16% w/w salt reduces the apparent diffusivity at room temperaturefrom6.7 X 10" to 2.0 X 10* cm /s. At 70% glycerol, improved by retaining a higher (initial) concentration of preservative(s) on the surface and by using a coating to maintain a large concentration difference for as long as possible (8). This approach requires coatings to reduce preservative diffusion from food surface into food bulk (Figure 2). For example, films made from zein, a corn protein, combined with potassium sorbate show improved surface microbial stability. The effectiveness of zein films was confirmed in microbial tests using a model food system with a,, = 0.88 coated with zein and S. aureus as the challenge microorganism. The barrier property of zein films was identified as the mechanism for stability improvement (2-3). The diffusion of sorbic acid in zein films was found to be 3-7 X 10" cmVs, i.e., about 100-1, 000 fold slower than in foods. The film is very waterresistant but may add off-flavors (6) and is expensive. w


6

6

2

9

Protected Surface

Uncoated food

Coated food

Figure 2. Preservative diffusion from food surface into food bulk controlled by edible coatings. Adaptedfrom(8).


253 Permeability Measurements Permeability can be determined using a cell consisting of two mechanically agitated chambers separated by the film to be tested (2, 8). The upper chamber is filled with aqueous glycerol or pure water. The lower chamber contains the same solution with 2.5% w/v potassium sorbate. The cell is placed in a controlled temperature chamber. When mounted on the permeability cell the top side (air drying side) of the film faces the high K-sorbate concentration. The use of glycerol solutions is recommended because it allows the inclusion of the food a effect on the coating permeability. Samples are takenfromthe upper chamber and the K-sorbate concentration is measured spectrophotometrically at 255 nm. Films should be inspected before and after every test to assure that results are not affected by cracks or other types of film failures. The permeability test is not a gentle procedure and films are subjected to the mechanical abuse of stirrers and compression between the two permeability cell chambers. Permeability coefficients are calculated by plotting preservative transferred through the film as a function of time (2, 8). After a time lag, the slope of the linear relationship obtained is the steady-state rate of K-sorbate transferred through the film (61, 62).


w

Fl

where K is the apparent permeability coefficient, F is the amount of K-sorbate permeated per unit time, / is the thickness of the film, and c is the K-sorbate concentration in the lower chamber. The determination is confirmed by the following relation between lag time and apparent permeability constant (Equation 4.26 in 61, 62):

£

(2)

where L is the intercept on the time axis by extrapolation of the steady-state rate of Ksorbate transfer through the film. For example, in tests at 0.80 a for a polysaccharide film with thickness 66 u.m the intercept on the time axis was 6 min and the calculated value using Equation (2) was 5.86 min. A film with the same composition and 63 \xm thickness tested at 0.65 a had an observed lag time of 8 hr and a calculated value of 8.2 hr (19). Similar determination and confirmation procedures can be used for other permeating molecules and films. w

w

Factors Controlling Potassium Sorbate Permeability Coating effectiveness is affected by the polysaccharides used in coating formulations,


254 Temperature Effects. The permeability phenomenon is a combination of two physical processes. There are sorption and desorption processes on both sides of the membrane, the adhesive forces at the interface (63). In addition, the permeate must diffuse in the film. In most cases, the latter process is the controlling step and explains why permeability rates follow an Arrhenius activation energy model (30, 63-65):

RT

K = Ke

@)

o

where, K is a frequency constant, E is the permeation activation energy, R is the universal gas constant, and T is the absolute temperature. For example, values for potassium sorbate permeability through polysaccharide films were measured at 5, 24, in the Arrhenius plots (Figure 3) indicates that no film morphological changes occur in the 5 to 40°C range. In addition, no significant differences (a = 0.05) exist between the slopes for films madefromchitosan, methyl cellulose, hydroxypropyl methyl cellulose, solution. However, the slope difference between the chitosan film in water versus all other films in aqueous glycerol was highly significant, with a 45% reduction in activation energy (Table III).


a

a

The observation that permeability values follow the Arrhenius model and that the activation energy is affected by the solvent embedding the film suggests that the diffusion process in the film occurs through the aqueous phase. Consequently, the performance of edible coatings controlling surface preservative concentration will depend strongly on the food a . However, K values are affected by the nature of the polysaccharide. Another approach to estimate the effect of temperature on permeability rate is by use of the following expression (66-67): w

0

where ju is the solvent viscosity at temperature Tand iff is a constant. This expression is based on the Stokes-Einstein equation for the diffusion of a molecule in a medium of known viscosity (5). This equation should be used with caution when the solution viscosity is high because it can overestimate the temperature effect on the diffusion constant (66). Fatty Acid Effects. Permeability decreases as fatty acid concentration increases (Table IV). Film casting difficulties interrupt the tendency towards lower permeability values as the fatty acid concentration increases because of the need for MC or HPMC structural support. Commercial users should explore the highest fatty acid


255 Table II. Potassium Sorbate Permeability Through Polysaccharide Films Evaluated at Various Temperatures 4P£ 3TC 2£C S°C Kxl

Two Food Applications of Biopolymers: Edible Coatings Controlling

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