Devon Zagory, Ph.D.
Senior Vice President for Food Safety & Quality Programs Food Safety & Quality Programs, Davis Fresh Technologies, LLC
From: Packaging International 117. April 1998
Modified atmosphere packaging (MAP) of fresh produce has posed difficult challenges for a packaging industry accustomed to barrier packaging designed to exclude atmospheric oxygen. Because fresh fruits and vegetables are still living, and still require oxygen for their metabolism, barrier flexible packaging has not been appropriate in most cases. Instead, packages whose gas transmission properties are precisely matched to the gas requirements of the product have become the norm. As the fresh-cut produce industry has grown larger and more sophisticated, so too have packages and package management grown more sophisticated. Where previous packages had to merely create a modified atmosphere while avoiding anaerobic conditions, current packages are often expected to also serve as vehicles for attractive graphics, reclosable seals, antifog coatings, crinkly texture, high transparency and strength. However, package attributes alone are not sufficient to ensure safety and quality of fresh-cut or minimally processed produce. Proper management of the raw materials prior to packaging and of the package during distribution and marketing are essential if the package is to deliver the desired value.
Fresh-cut produce has been successful in the marketplace in part because of the value added to the product through its preparation and delivery in a ready-to-eat condition. The primary guardians of this added value are low temperature and modified atmosphere packaging (MAP). These two factors work in concert to maintain freshness, extend shelf life, ensure safety and promote sale. Neither low temperature nor MAP can act alone to deliver full value to the customer. The reason for this is that both work together to slow the metabolism and aging of the product so that it stays fresher longer.
The plant tissues in fresh-cut produce are still living and deriving energy primarily through the process of respiration. Respiration involves the consumption, using atmospheric oxygen (O2), of carbohydrates and organic acids and the consequent production of metabolic energy, heat, carbon dioxide (CO2) and moisture vapor. Different fruits and vegetables, and even different varieties of a given fruit or vegetable, will vary in their respiration rates. Those that have high respiration rates (such as asparagus, mushrooms and broccoli) tend to be most perishable while those with low respiration rates (such as nuts, onions and potatoes) tend to be least perishable. Furthermore, when fruits or vegetables are cut, sliced, shredded or otherwise processed, their respiration rates increase. This is probably due to the increased surface area exposed to the atmosphere after cutting that allows oxygen to diffuse into the interior cells more rapidly and to the increased metabolic activity of injured cells.
The best way to reduce respiratory metabolism and thus conserve the plants stores of carbohydrate, acids and moisture, is to reduce the temperature. Most produce will maintain its best quality at temperatures near 0C, except some produce of tropical origin which should be kept at 10-13C to avoid chilling injury. Having achieved excellent temperature control, MAP can be used to further reduce respiration rate, loss of moisture, production of metabolic heat, yellowing, browning, decay and sensitivity to ethylene.
The effects of MAP are based on the often observed slowing of plant respiration in low O2 environments. There is about 21% O2 in air. As the concentration of O2 inside the package falls below about 10%, respiration starts to slow. This suppression of respiration continues until O2 reaches about 2-4% for most produce. If O2 gets lower than 2-4% (depending on product and temperature), fermentative metabolism replaces normal aerobic metabolism and off-flavors, off-odors and undesirable volatiles are produced. Similarly, as CO2 increases above the 0.03% found in air, a suppression of respiration results for some commodities. Reduced O2 and elevated CO2 together can reduce respiration more than either alone. In addition, elevated CO2 suppresses plant tissue sensitivity to the effects of the ripening hormone ethylene. For those products that tolerate high concentrations of CO2, suppression of the growth of many bacteria and fungi results at >10% CO2.
The goal of MAP of fresh produce is to create an equilibrium package atmosphere with %O2 low enough and %CO2 high enough to be beneficial to the produce and not injurious. This is accomplished through the proper balance of several variables that affect package atmosphere. The relationship of those variables, at equilibrium, can be expressed as:
PO2 = RRO2*t*W/A*(O2atm - O2pkg)
PCO2 = RRCO2*t*W/A*(CO2pkg - CO2atm)
PO2 = Oxygen permeability of the film (ml-mil/m2-day-atm)
PCO2 = Carbon dioxide permeability of the film (ml-mil/m2-day-atm)
RRO2 = Respiration rate as product consumption of O2 (ml/kg-hr)
RRCO2 = Respiration rate as product production of CO2 (ml/kg-hr)
t = Film thickness
W = Product weight (kg)
A = Film surface area (m2)
(O2atm - O2pkg) = Desired O2 gradient between the outside and the inside of the package
(CO2pkg - CO2atm) = Desired CO2 gradient between the inside and the outside of the package
Produce packaging has progressed in the past several years. Appropriate packaging materials have been developed for most of the more common fresh-cut products. There is consensus as to which films are appropriate for standard size packages of garden salad, broccoli, peeled carrots, etc. Knowledge of how to effectively seal packages and reduce incidence of leakers has developed. And printing capabilities have provided ever more attractive packages. Technical challenges still exist in produce packaging. Some of the technologies currently available to meet those challenges will be described below.
Tailored Oxygen Transmission Rate (OTR)
The flexible packaging industry has become increasingly responsive to the specific gas requirements of fresh produce and are now providing films specifically designed for given produce items. Films for low, medium and high respiration rate commodities are now available from many package vendors and the process of matching OTR to product is being constantly refined. This has allowed fresh-cut processors to begin to provide a much greater diversity of products which now includes artichoke hearts, baby salad greens, sliced strawberries, stir fry mixes and many others. Very high respiration rate commodities such as broccoli, asparagus and mushrooms have always presented a challenge to packagers. New technologies are now allowing the manufacture of very high OTR (> 15000 cc/m2-day) films for these applications.
Technology developed independently by Dow Chemical Co. and Exxon Chemical Co. uses new single site catalysts to produce desired polymer resins. These catalysts, when applied to the manufacture of polyethylene and other polymers, can provide a much narrower distribution of polymer chain length, molecular weight and density. This results in flexible plastic films with very high OTR, low moisture vapor transmission rate, enhanced clarity, superior strength, low seal initiation temperature and very rapid bonding of the seal. These stronger films with stronger seals are finding wide application in produce packaging.
Microperforated And Microporous Films
Alternative approaches to providing high OTRs, especially in applications where there is limited package surface area for gas exchange, have included films with holes or pores. FreshHold, a microporous film technology owned by Albert Fisher, Plc., P-Plus microperforated technology owned by Print Pak, and proprietary microperforation technologies owned by Respire Films in America and Sidlaw in England are finding applications in the rapidly emerging fresh-cut fruit market. Most cut fruit is packaged in rigid, gas impermeable trays with a permeable film lidstock sealed to the tray. Because the tray is impermeable to gases, there is reduced surface area for gas exchange. All the gas exchange must occur through the lidstock. Until recently, few films had high enough OTRs to be useful in these applications. Those films that had high OTRs often would not seal to the trays. Microporous and microperforated films allow much more rapid gas exchange than would normally be possible through plastic films.
Films with pores or small holes have some physical limitations. Carbon dioxide (CO2) diffuses through plastic films 2-6 times faster than oxygen (O2). Therefore, CO2 exits a package much faster than O2 enters. This results in equilibrium atmospheres of low O2 and relatively low CO2. A range of CO2/O2 permeability ratios among plastic films can provide a range of CO2/O2 proportions inside packages. Because fruits and vegetables vary in their tolerance to elevated CO2, this range of gas proportions is useful. Films with holes or pores admit O2 and CO2 at similar rates. Therefore, the ratios of gases that can result inside such packages are limited. It is impossible to achieve low O2 (1-5%) without accumulating high CO2 (15-20%). Thus, these films are applicable only for those products that tolerate high CO2 without experiencing injury.
Most fruits are relatively tolerant of high CO2. In fact strawberries, blueberries and some other fruits are benefited by high CO2 which reduces mold growth and improves firmness. FreshHold, which is marketed as a high permeability label that can be attached to a bag or film overwrap, is gaining acceptance in the fruit packaging market because of its ability to admit sufficient gas through a very limited surface area. In addition, the high CO2 atmospheres associated with FreshHold are generally beneficial to fruit. Microperforated films may also be appropriate for these applications.
The high OTR requirements for lidstocks sealed to impermeable trays has often conflicted with poor sealing properties. Advances in coextrusion technology, coupled with single site catalyst-based plastic resins, have provided better breathing, better sealing films just in time to meet the needs of the fresh-cut fruit industry.
Customizable Packaging Materials
Because each produce item has differing, often unique, packaging requirements, the ability to customize the package to the product has been the aim of produce package development efforts. FreshHold labels can be customized to provide almost any desired OTR, as is true of microperforated films.
Some film vendors provide an array of OTRs by varying the thickness of a given film. Thinner films have higher OTRs. Very thin films do not run well on modern automated packaging machinery and so this approach is limited.
Landec Corporation, Inc. of California, USA, has developed side-chain polymer technology that allows the film OTR to increase rapidly as temperature increases, thereby avoiding anaerobic conditions subsequent to loss of temperature control. In addition, these polymers can provide very high OTRs, an adjustable CO2/O2 permeability ratio, and a range of moisture vapor transmission rates. These polymers are available as attachable patches that can go on bags or overwraps and represent the first truly customizable packaging system.
Potential buyers like to see fresh produce before they buy it. Therefore plastic packages need to be clear and the product visible. Condensation of water inside the package can often occlude the view of the product. Antifog compounds have been developed that, when included in coextruded films, migrate to the inner surface of the film and prevent large water drops from forming. This results in a more attractive package and a better view of the product. However, antifog coatings can interfere with seal integrity and so newer technology relies on register coatings that apply the antifog material only on selected areas of the film away from the seal.
There has emerged an increased appreciation that packaging can only deliver its promised benefits to fresh produce within a specific temperature range. In addition, an emphasis on shelf life extension has shifted to an emphasis on quality preservation. As the marketplace for fresh-cut products becomes more competitive, it is quality that sells, not shelf life. This has resulted in increased attention to maintaining low temperatures and rapid distribution. Such changes in perspective have helped realize the benefits that modern packaging films can provide.