PROCESSED BY NEWER TECHNOLOGIES
Products processed by nonthermal technologies such as high- pressure processing (HPP), pulsed electric fields (PEFs), ultraviolet-C (UV-C), irradiation, microfiltration, active packaging (oxygen scavenging or antimicrobial packaging), or biopreservation (antagonistic culture) require right pack- aging material and processes to be successful (Devlieghere et al. 2004).
To maintain the sterility of nonpumpable foods, the foods should be packaged before nonthermal processes such as pulsed light emission, irradiation, batch high-pressure pro- cess, or antimicrobial packaging. Therefore, similar to the heat resistance of packaging materials to thermal treatment, the packaging materials for nonthermal processing should have appropriate resistance to high-energy light, irradiation, pressure, or chemicals. This means that the chemical and
physical requirements of packaging materials for nonther- mal processing are different from those for thermally pro- cessed foods. To identify these special requirements of pack- aging materials for nonthermal processing, it is necessary to understand the process parameters and microbicidal mech- anisms/kinetics of the nonthermal process and their effect on mechanical and physical properties of packaging materi- als. Besides the mechanical and physical characteristics of packaging materials, various other factors of food packaging systems should be considered in designing the package for the nonthermal processes, which may include, as in, for example, high-pressure process, volume of the package, headspace gas, dissolved oxygen in foods, and deformation characteristics of packaged foods (Balasubramaniam et al. 2004).
Characteristics of Packaging Materials for Nonthermally Processed Foods
Packaging materials for nonthermal processing should have strong resistance (physical and mechanical properties) to the nonthermal process mechanisms. For example, packaging materials for HPP should be restored from the deformation under pressure to their original shape without any mechanical and physical change of properties. The packaging materials for irradiation should be chemically stable under the radi- ation dose without depolymerization or significant changes in elastic modulus of the packaging materials. For pulsed UV-C/white-light emission process, the packaging material must be transparent during pulsed light emission. There is no general requirement for packaging materials for all non- thermal processes. However, from the above examples, most characteristics of packaging materials required for the vari- ous nonthermal processes are related to the barrier properties of the packaging materials (Min and Zhang 2005).
This is due to the satisfaction of the primary functions of packaging systems: containment, protection, and preserva- tion. Nonthermal processes do not utilize increased tempera- ture to inactivate decomposing microorganisms and enzymes.
This is the biggest advantage of nonthermal processes be- cause this low-temperature pasteurization does not overcook food and/or degrade foods thermally. Furthermore, this low- temperature treatment also widens the selection of packaging materials and systems. Owing to the low-temperature treat- ment, the packaging system does not require high melting temperature for heat seal. Low-temperature sealing meth- ods can utilize various polymers and sealants, if required, or cold sealing using adhesives. These methods produce far less volatile odor of plastics, additives, and printing solvent. This is very beneficial to high-fat foods and frozen/refrigerated foods (Min and Zhang 2005).
Antimicrobial Edible Films and Coatings for Nonthermally Processed Foods
Nonthermal food-processing technologies comprise an im- portant area of study and application in food science and en-
gineering. These technologies are being developed to satisfy consumer demand for fresh-like foods. They are intended to inhibit both spoilage and the growth of pathogenic microor- ganisms in foods without significant loss of flavor, color, taste, nutrients, viscosity, and functionality of the food by minimizing thermal effects on foods (Min and Zhang 2005).
Fresh-like food products optimally processed by these tech- nologies require appropriate packaging to preserve their qual- ities for desired shelf life during storage. Antimicrobial ed- ible films and coatings draw attention from the food and packaging industry because of increasing consumer demand for minimally processed products. Antimicrobial edible films and coatings can control microbial contamination occurring on the surface of the food during restorage after opening or because of package defects. In addition, since antimicrobial films and coatings have self-antimicrobial abilities, the need for chemical sanitization or sterilization of packaging mate- rials may be obviated and aseptic packaging processes may be simplified (Hotchkiss 1997).
A combination use of nonthermal food processing and antimicrobial films and coatings is suggested because the bioactive films and coatings are expected to provide an ad- ditional barrier for the contamination and the growth of both spoilage and pathogenic microorganism in nonthermally pro- cessed food products. The benefit of nonthermal processing will not be altered because the antimicrobial films and coat- ings do not add either heat or synthetic chemicals to the nonthermally processed food. Both production of minimally processed foods and extension of their microbial stability can be made possible by combining these technologies (Appen- dini and Hotchkiss 2002).
Packaging for High Pressure Processing Packaging materials for HPP are required to be flexible enough to withstand the compression forces while maintain- ing physical integrity. They must recover their initial volumes after the pressure is released (Caner et al. 2004). This is a reason why metal cans, glass bottles, and paperboard-based packages are not well suited for HPP (Lambert et al. 2000;
Caner et al. 2004). The presence of headspace must be kept as small as possible (Lambert et al. 2000). Kubel et al. (1996) investigated the effect of HPP on the sorption of aroma com- pounds,p-cymene, and acetophenone into plastic films and found that the sorption of aroma compounds was lower in films exposed to 500 MPa pressure compared with nonpres- surized films.
Packaging for Irradiated Foods
Foods are generally prepackaged before irradiation to prevent recontamination. The use of irradiation is also becoming a common treatment to sterilize packages in aseptic processing of foods and pharmaceuticals (Ozen and Floros 2001). Any packaging materials must be accepted by FDA before use
in food irradiation because gases (e.g., hydrogen) and low- molecular-weight hydrocarbons and halogenated polymers formed during irradiation at doses accepted for food use have a potential to migrate into foods (Kilcast 1990; Lee et al.
1996; Olson 1998). Some chemical and physical properties of polymeric packaging materials can be changed by irradiation (Ozen and Floros 2001). The changes depend on the type of polymer, processing exposure, and irradiation conditions (Crook and Boylston 2004).
Predominant reactions during irradiation in most plastics used for food packaging [e.g., polyethylene, PP, polystyrene (PS)] are cross-linking and chain scission (Ozen and Floros 2001). Cross-linking can decrease elongation, crystallinity, and solubility and increase the mechanical strength of the plastics. Chain scission can decrease the chain length of plastic materials, providing free volume in the plastics, and can produce hydrogen, methane, and hydrogen chloride for chlorine-containing polymers under vacuum. In the pres- ence of oxygen, additional chain scissions would be able to form peroxide, alcohol, and various low-molecular-weight oxygen-containing compounds (Ozen and Floros 2001).
A 25 kGy irradiation (Caesium137) had a significant ef- fect on increasing the volatile compounds in crystalline and oriented semirigid PET homopolymer (Komolprasert et al.
2001). Major volatile compounds that evolved from the PET specimens are formic acid; acetic acid, 1,3-dioxolane; and 2-methyl-1,3-dioxolane (Komolprasert et al. 2001). Irradia- tion on LDPE, HDPE, PET, and polyvinyl chloride (PVC) packaging materials can release hydrogen, carbon dioxide, carbon monoxide, and methane gases and form volatile ox- idation products including peroxides, alcohols, aldehydes, ketones, and carboxylic acids (Crook and Boylston 2004).
Packaging for Pulsed Electric Field Processed Food Products
The permeation values are very important in determining packaging materials for PEF-processed food products. Asep- tic food packaging is considered the most appropriate way of packaging for PEF-processed food products (Qin et al. 1995).
The effect of packaging materials on the quality preservation of orange juice treated by PEF (35 kV/cm, 59 s, pilot plant scale PEF system) has been studied by Ayhan et al. (2001).
Some chemical and physical properties of PEF-treated or- ange juice packaged in four different packaging materials, sanitized glass, PET, HDPE, and LDPE bottles, were evalu- ated. Glass bottles and PET bottles were effective at retarding degradations of flavor compounds, vitamin C, and color of PEF-treated orange juice during storage at 4◦C for 112 days.
Vitamin C and the flavor compounds were labile in polyethy- lene (HDPE and LDPE) bottles, which might be due to their relatively low barrier property of polyethylene to oxygen.
The degradation of lycopene of PEF-processed tomato juice in a PP tube was found most significant during the first 7 days of the storage at 4◦C (Min et al. 2003). The main cause
of carotenoid degradation in foods is oxidation (Thakur et al.
1996), and thus the significant reduction was caused by the oxygen available in the headspace of the PP tubes (Min et al.
2003). The MAP, which limits oxygen in the headspace, may be applied as a complement to PEF to reduce oxidation of PEF-processed food products.
Jin and Zhang (2002) reported that total operational cost for PEF-treated orange and tomato juices was 6 to 7 cents per liter, which was considered competitive with a conventional thermal processing of the juices.
FUTURE RESEARCH NEEDS
In selecting food products, consumers look for attributes such as convenience, portability, health benefits, and great taste.
They demand clean and safe products with good economic value and tamper-evident packaging made with ecofriendly materials and fewer preservatives. Consumers expect labeling to clearly educate about food safety, food ingredients, compo- sition, nutrition, storage, and instructions for use. Consumers would generally switch brands for attractive packaging with complete labeling. However, convenience is the most impor- tant attribute, especially among time-pressed working adults, as well as among elderly buyers who like single-serve, easy- to-open, resealable containers and which have label which is easy to read.
Processors have different interests from consumers, and this translates into a conflict of priorities in package design and production. From the processors point of view, in con- trast, the strongest drivers of packaging design include sim- plicity of operation in filling, sealing, efficiency of materi- als and labor usage, consumer convenience, product safety, faster line speeds, shelf life extension, size flexibility and cus- tomization, improved automation, improved graphics, cost of materials, environmental concerns, and labeling and coding improvements. Of these, only convenience and safety are re- ally key concerns of consumers. The rest are more responsive to the manufacturers and distributors.
In addressing the current global mega trends of convenient and healthy retail and foodservice, processors must incor- porate all of the demands of the consumer base, as these evolve and accumulate over time. Packaging innovation is a key factor in ensuring continued product success, while new graphics are needed to differentiate and self-sell the novel products regardless of the category under consideration. In future, the focus should be on developing functional pack- ages which are more in trend among consumers as function- ality outweighs nearly everything else, including design and graphics considerations.
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