Packaging Technology for Fresh Produce

William R. Romig and Nazir Mir
Central Research, EPL Technologies, Inc., Philadelphia, PA, USA

Summary

The fresh produce industry in U.S. generates $100 billion annually. The singular characteristic of fresh products is that they are alive and therefore continue to function metabolically even after harvest. As a general practice, the harvested plant products are moved from sites of production, stored, and distributed to sites of consumption in a living state as close to that of harvest as possible, i.e., ‘fresh-like’ condition. Because of the rigors of handling and distribution, postharvest maintenance of fresh-like quality during storage and distribution is a challenge. Quality loss is minimized by a number of means including restricting metabolism with low temperature, low O₂, high CO₂, altering C₂H₄ production or perception, and avoiding moisture loss and physical damage to the tissue. Packaging systems have been developed that provide containment, protection, metabolic benefits and ease in marketing for fresh produce.

The bulk of the fresh produce is marketed as whole produce all of which is packaged in one way or another. In addition to whole produce, fresh-cut products represent a rapidly growing segment of the fresh produce industry. These products often have significantly different packaging requirements than the whole product. The primary driving force behind their use is the combination of fresh-like quality and convenience offered to the food service industry and to consumers. Of the total fresh produce sales of $100 billion per year, sales of fresh-cuts average $8-10 billion. A three-fold increase in the market share of lightly processed products is projected within the next three years.

For whole produce, packaging is primarily designed to avoid bruising during post storage handling. For relatively more perishable products such as fresh-cut products that are not so subject to physical damage, the primary challenges are to provide a gaseous environment that reduces product metabolism significantly compared to ambient environment yet avoid fermentation and post processing microbial contamination. Target package environments can be modeled but their success often requires strict temperature control during shipment, distribution and retailing. As we do not have sufficient temperature control at the commercial level, one can encounter situations where the packaged product environments become over modified, especially for high respiring produce, resulting in product fermentation, generation of a conducive environment for human pathogenic microorganisms, off-flavor development, and consequently termination of shelf life. The technological challenges become more complex when one accounts for varied responses of commodities to the package environment. Considering the product needs and matching them with packaging systems may require a combination of technologies to preserve quality and extend shelf life.

Overview

Long transport distances for fresh produce from production areas to domestic and overseas consumer markets subject shipped fruit to extended periods of compression and vibration. Excessive vibration causes damage to the sensitive skin of some fruits that is visible as brown lesions on the fruit surface, and compression and impact cause injury detectable as bruises. Thus maintaining a defect-free fruit surface during the market chain with good consumer quality is a significant packaging challenge but by and large, a major objective for the marketing of whole fresh produce. Corrugated box packaging seems to work well for the majority of fruits and vegetables. For small fruits, such as blueberry, raspberry and strawberry, clam-shell packaging is widely employed. Contrary to this, for fresh-cut products, increased rates of global metabolism induced by wounding and rupturing during cutting and shredding operations warrant that packaging systems be developed that minimize loss of product quality due to these undesirable changes in product physiology. All fresh-cut products are packaged under modified atmosphere (MA) packaging in some form to modify the level of O₂, CO₂ or water vapor. MA packaging is also being evaluated and used on a restricted scale for small fruits.

The MA packaging technique consists of the enclosure of respiring produce in polymeric films in which the gaseous environment is actively or passively altered to slow respiration, reduce moisture loss and decay and/or extend the shelf life of the products. MAP techniques provide low O₂/high CO₂ regimes similar to those achieved by using controlled-atmosphere storage and are theoretically able to maintain desired atmospheres throughout the marketing chain. The degree to which atmospheric modification takes place in packages is dependent upon several variables such as film permeability to O₂ and CO₂, product respiration and the influence of temperature on both of these processes (Beaudry, 1999; Beaudry et al., 1992; Cameron et al., 1994).

Temperature abuse during transportation, storage and marketing of fresh products is a primary concern in the fresh produce industry, because poor temperature control can lead to the deterioration of a packaged product due to increase in product metabolism and growth of food spoilage organisms. Consequently, O₂ levels decline and CO₂ levels increase as temperature increases (Beaudry et al., 1992; Cameron et al., 1994). The reason for this is that the permeability of O₂ and CO₂ through most packaging films does not increase with increasing temperature as rapidly as the rate of respiration increases. More specifically, O₂ permeation has a low activation energy when compared to that of the respiratory process (Mir and Beaudry, 2001). Thus, when there is an increase in temperature, the resultant increase in film permeability cannot keep pace with the increase in the demand for O2 by the product, leading to the observed decline in O₂ as temperature increases. In perforated packages, this effect is more pronounced because there is only a minimal increase in O₂ transport through perforations with increasing temperature. Thus, O₂ levels are even more responsive to temperature shifts in packages that rely on perforations for gas exchange. It is also worth mentioning that in perforated packages the CO₂ will rise to higher levels for a given level of O₂ relative to semipermeable film packages.

The concentrations of O₂ and CO₂ within a package can be modeled. Elaborate and useful models have been developed that would allow fresh produce processors to choose packaging materials most suited to the enclosed product (Cameron et al., 1994, Cameron et al., 1995; Joles, 1993; Lakakul, 1999). A common mathematical model involves the use of what is known as a Michaelis-Menton type respiratory model to describe the influence of temperature, O₂ (and potentially CO₂) on respiration. This approach has been used for blueberries (Cameron et al., 1994), strawberries (Joles, 1993), raspberries (Joles et al., 1994) and apple slices (Lakakul et al., 1999). Respiratory models are then coupled with an equation describing the temperature sensitivity of film permeability to gases (known as the Arrhenius equation) to predict package O₂ partial pressure as a function of temperature, product mass, surface area and film thickness (Cameron et al., 1994; Cameron et al., 1995; Lakakul et al., 1999). MAP has been shown to preserve the quality of a number of perishable products (Ballantyne et al., 1988a; Ballantyne et al., 1988b; Barmore, 1987; Beaudry and Gran, 1993; Cameron et al., 1994; Kader et al., 1989 Lakakul et al., 1999).

The general dogma of extending storability by MAP is that storability will improve in response to low-O₂ package atmospheres. Plant responses to modified oxygen levels have generally been well characterized and include responses at the levels of primary and secondary metabolism (Kader, 1997a). Of the primary metabolic responses to low O₂, beneficial reactions include a reduction in respiration (i.e., O2 uptake), which can be manifested as a reduction in starch degradation and sugar consumption. Reduced respiration is often interpreted as reflecting a reduction in global metabolism (Kays, 1997). With regard to secondary metabolism, low O₂ reduces synthesis and perception of ethylene (which can also be manifested as reduced respiration and starch conversion, and reduction in degradation of chlorophyll and the cell wall) and reduced oxidation of phenolic compounds. In general, partial pressures of O2 below 5 kPa significantly reduce ethylene effects (Sfakiotakis and Dilley, 1973). However, it has been shown that metabolic responses to C₂H₄ and O₂ combine to modulate storability.

In addition to MAP, ethylene effects can also be modified during storage and handling for many horticultural products of commercial interest by 1-methylcyclopropene (1-MCP) and its analogues (Sisler and Blankeship, 1996). 1-MCP is a relatively new plant growth regulator that exists as a gas and can be used to render plant material completely insensitive to ethylene. 1-MCP is presently approved for use in greenhouses and its approval for use on fresh produce is actively being pursued. Of the few published studies describing its action, 1-MCP was found to reduce ethylene related responses in apple (Fan et al., 1999a; Mir et al., 1999), banana (Golding et al., 1998), broccoli (Ku and Wills, 1999a), carrot (Fan et al., 1999b), lettuce (Fan et al., 1999b), tomato (Mir et al., 1999b) and strawberry (Ku and Wills, 1999b).

Using polymeric films, MA packaging systems for products with low to medium respiration rates have to some extent been successfully developed. Products such as broccoli, mushrooms, leeks, etc. exhibit very high rates of respiration such that conventional films can potentially over modify the pack atmosphere and thus result in fermentation. Accordingly, in the recent past, there has been a lot of commercial interest to develop films with high gas transmission rates (Lange, 2000). High gas transmission films are obtained by modifying the film manufacturing process so that gases such as O₂, CO₂ and water vapor exit or enter the package in a controlled manner such that aerobic respiration needs are met and desirable CO₂ and moisture levels are maintained. Three categories of such films have been developed: 1. films with novel chemistry; 2. films with intentional additives; and 3. perforated films.

Films that have improved rates of gas transmission by virtue of their polymeric nature are usually blends of two or three different polymers, where each polymer of a blend performs a specific function such as strength, transparency and improved gas transmission to meet certain product descriptions. Furthermore, films can be laminated to achieve needed properties. Among this class are high (6-18%) ethylene-vinyl acetate content, low density polyethylene (Elvax, DuPont, Wilimington, DE), oriented polypropylene laminates (OPP, BP Amoco, Lisle, IL), styrene butadiene block copolymer films (K-Resin, Phillips Chemical Company, Houston, TX) and ultra low density ethylene octene copolymer films (Attane series, Dow Chemical Company, Midland, MI) and polyolefin plastomer octene copolymer films (Affinity series, Dow Chemical Company, Midland, MI)

The plastic polymer can also be mixed with an inert inorganic material such as CaCO₃ and SiO₂ to generate micoporous films. The gas permeabilities can be manipulated by adjusting the filler content, particle size of the filler and degree of stretching. The average pore size ranges from 0.14 to 1.4 µm (Mizutani, 1989). The most publicized microporous materials are FreshHold, developed by Hercules (Hercules, Wilmington, DE) and now marketed by River Ranch (River Ranch, Salinas, CA) and Landec Corp.’s Intellipac breathable membrane (Landec Co., Menlo Park, CA).

Films using microperforations can attain very high rates of gas transmission. The diameter of microperforation generally ranges from 40 to 200 µm and by altering the size and thickness of microperforations, gas permeability through a package can be altered to meet well defined product requirements. After development of microperforated package materials such as P-Plus from Sidlaw (Sidlaw Packaging P-Plus, Bristol, UK), now marketed in the US by Printpack (Printpack Inc, Atlanta, GA), much commercial interest has developed in the package industries. EPL Technologies, Inc. (Philadelphia, PA), a leader in perforating technology, has established a ‘Center of Excellence’ in perforating to develop novel packaging material for a wide range of products.

Based on the rates of respiration and gas transmission through the microperforations and the base film, packages have been developed that maintain desired levels of O2 and moisture for high respiring mushrooms (Gosh et al., 2000). Microperforated films have also been used to extend the storability of strawberries and nectarines (Meyers, 1985), leeks, asparagus, parsnips, cherry tomatoes and sweet corn (Geeson, 1988) and apple (Watkins et al., 1988).

Using high gas transmission films can extend the application of MAP to high respiring produce for controlling package O₂. However, controlling O₂ alone may not provide sufficient protection to preserve quality under all situations. Additional factors such as inhibition of C₂H₄ effects, moisture loss, and decay may contribute to preserving quality. For instance, broccoli responds favorably to low O₂. Mir and Beaudry (1999, unpublished results) found that storability of broccoli was extended by 1.3-fold by 3% O₂ as compared with ambience in a flow-through system. The same study showed that when broccoli was rendered insensitive to C₂H₄ with 1 ppm 1-MCP and stored under 3% O₂, the storability relative to untreated controls was extended by 1.7-fold. This demonstrates that a combination of technologies may be needed to extend storability and preserve quality of various horticultural products. At the commercial level, it may one day be possible to achieve beneficial concentrations of package O₂ by using microperforated film and the product can be made insensitive to C₂H4 by 1-MCP exposure prior to packaging.

To date, the major aim of MAP technology has been to maintain the visual and textural characteristics that define quality (Kader et al., 1989). Recent outbreaks of food-borne illness due to consumption of contaminated fresh packaged fruit and vegetable products in the U.S. has raised concerns that MAP effects on produce microbiology need to be evaluated (Nguyen-the and Carlin, 1994). In the U.S., major microbial pathogen outbreaks in fresh-cut fruits and vegetables include Shigella sonnei, Listeria monocytogenes, Escherichia coli, Clostridium botulinum, and Salmonella sp. (Hurst, 1995). The growth of these organisms on fresh produce has been linked to the overmodified package environments (Hintlian and Hotchkiss, 1986) and in certain situations, physical decomposition and physiological degradation as affected by storage duration and temperature abuse may be serious concerns as well (Farber, 1991).

The type of microorganisms and growth rate will depend on the product, O₂, CO2, water vapor and temperature levels (Brackett, 1989). Low O₂ and/or high CO2 partial pressures used in MA packages can retard the growth of gram-negative bacteria and favor the development of gram-positive organisms such as lactic-acid bacteria (Brackett, 1989; Carlin et al., 1990). In some cases, atmospheric modification creates environments where pathogenic microorganisms can proliferate while inhibiting growth of spoilage microorganisms (Farber, 1991). Thus, contaminated food that lacks off odors or other signs of decomposition may be unwittingly consumed. Temperature abuse during distribution may lead to the development of certain microorganisms (Carlin et al., 1990). It is likely that fresh-cut vegetables are more susceptible to bacterial growth than fresh vegetables due to mechanical processing damage and associated plant cell leakage. King et al. (1991) reported higher bacterial counts on processed lettuce when compared to lettuce heads after storage for 15 days at 2.8°C.

It is important to recognize that while atmosphere modification can improve storability of some fruits and vegetables, it has the potential to induce undesirable effects as well. If O₂ levels decline below concentrations required to sustain aerobic respiration, fermentation and off-flavors may result (Kays, 1997; Richardson and Kosittrakun 1995). Risks include not only the loss of product quality through fermentative metabolism, but also the growth of potential human pathogens that thrive under anaerobic conditions (Hintlian and Hotchkiss, 1986; Nguyen-the and Carlin, 1994). Similarly, if CO₂ concentrations exceed a level tolerated by the plant tissues, injury is likely to occur. Ranges of non-damaging O₂ and CO₂ levels have been published for a numbers of fruits and vegetables (Kader 1997a, Kupferman 1997, Richardson and Kupferman 1997, Saltveit 1997, Beaudry 1999, 2000), minimally processed products (Gorny 1997) and flowers and ornamentals (Reid 1997). Tolerance limits for O₂ and CO₂ are given in Tables 1 and 2.

CONCLUSION

Packaging of fresh produce is needed not only to provide containment for ease of handling but to preserve post storage product quality during distribution and in certain cases to add value to the commodity during marketing. Packaging systems are product specific but may benefit quality retention through protection from handling abuse and moisture loss and restriction of metabolism. In most of the packaging systems, moisture control is often a more important issue in produce than the control of package O₂ and CO₂ atmosphere. MAP is employed to modify the atmosphere for some bulk and consumer sized products. While non-perforated films are generally used for atmosphere modifications, perforated films may provide the additional option of controlling moisture loss as well. However, not all plant materials benefit from MAP and those that do may differ in their responses to atmospheres generated. Moreover, temperature control is of critical importance and, by itself, has a greater impact than atmosphere modification.

The use of MAP, to some extent, is beset with some technical and physiological challenges. Technical challenges remain, for instance, in developing packages that maintain O₂ partial pressures within tolerance levels as packages undergo changes in temperature and humidity. Challenges also remain in avoiding adverse physiological responses to modified atmospheres. Progress is demanded if the fresh-cut segment of the industry is to continue to expand its market share and the variety of its offerings.

Table 1. O₂ limits below which injury can occur for selected horticultural crops held at typical storage temperatures (adapted from Beaudry 2000). Those commodities in bold are considered to have very good to excellent potential to respond to low O₂.

O2 (kPa)	Commodities
0.5 or less	chopped greenleaf, redleaf, Romaine and iceberg lettuce, spinach, sliced pear, broccoli, mushroom
1	broccoli florets, chopped butterhead lettuce, sliced apple, Brussels sprouts, cantaloupe, cucumber, crisphead lettuce, onion bulbs, apricot, avocado, banana, cherimoya, atemoya, sweet cherry, cranberry, grape, kiwifruit, litchi, nectarine, peach, plum, rambutan, sweetsop
1.5	most apples, most pears
2	shredded and cut carrots, artichoke, cabbage, cauliflower, celery, bell and chili pepper, sweet corn, tomato, blackberry, durian, fig, mango, olive, papaya, pineapple, pomegranate, raspberry, strawberry
2.5	Shredded cabbage, blueberry
3	cubed or sliced cantaloupe, low permeability apples and pears, grapefruit, persimmon
4	sliced mushrooms
5	green snap beans, lemon, lime, orange
10	asparagus
14	orange sections
Table 2. CO2 partial pressures above which injury will occur for selected horticultural crops (adapted from Watkins, 2000).
CO2 (kPa)	Commodity
2	Lettuce (crisphead), pear
3	Artichoke, tomato
5	Apple (most cultivars), apricot, cauliflower, cucumber, grape, nashi, olive, orange, peach (clingstone), potato, pepper (bell)
7	Banana, bean (green snap), kiwi fruit
8	Papaya
10	Asparagus, brussels sprouts, cabbage, celery, grapefruit, lemon, lime, mango, nectarine, peach (freestone), persimmon, pineapple, sweet corn
15	Avocado, broccoli, lychee, plum, pomegranate, sweetsop
20	Cantaloupe (muskmelon), durian, mushroom, rambutan
25	Blackberry, blueberry, fig, raspberry, strawberry
30	Cherimoya

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