Eva Almenar and Christopher T. Wilson of Michigan State University review progress in the development and uptake of modified atmosphere packaging, active packaging and intelligent packaging to extend the shelf life of fresh produce.
Harvested fresh produce is still alive, which leads to enormous difficulties when shipping and retailing fruits and vegetables. In the postharvest period, biochemical processes, such as respiration, transpiration and ethylene autocatalysis, continue, shortening the shelf life of the produce.
whole or cut fresh produce is the most challenging food product category to be packaged.
Cutting whole fresh produce to obtain a ready-to-eat format to fulfill consumer demand causes stress and stimulates the aforementioned reactions due to the decompartmentalisation of enzymes and substrates, which notoriously accelerate senescence. Additionally, spoilage and pathogenic microorganisms can proliferate in both whole and ready-to-eat produce, with greater intensity in the latter as processing increases microbial access to nutrients. The speed of these spoilage mechanisms is significantly affected by environmental conditions, such as temperature, relative humidity, oxygen and light. Consequently, whole or cut fresh produce is the most challenging food product category to be packaged.
The role of packaging in fresh produce has changed over the years from containing and protecting produce from physical hazards during distribution to a broad spectrum of shelf life-extending technologies. The many intrinsic and extrinsic factors that affect produce shelf life have spurred the development of a large variety of packaging technologies to maintain quality and nutritional value, as well as ensure safety.
In addition, packaging technologies can:
add value to produce,
strengthen brands,
monitor produce and environment changes during storage,
be a tracking tool,
provide anti-theft prevention,
allow unique identification and communication with consumers,
increase consumer satisfaction,
reduce food waste.
Consumers, industry and government have driven these innovations in packaging over the years; as consumer lifestyles and preferences have changed, concerns for quality, safety and food waste, among others, have grown with desire for fresher, more convenient produce.
Although many different packaging technologies are currently being used to extend produce shelf life, this article focuses on modified atmosphere packaging (MAP), active packaging (AP), and intelligent packaging (IP) due to the growing demand for these packaging technologies in recent years. While the term ‘smart packaging’ is being used to refer to both AP and IP or to IP solely, the term will not be used in this article to avoid confusion.
MAP is a packaging technology based on the replacement of the ambient air (78.09% nitrogen (N2), 20.95% oxygen (O2), 0.03% carbon dioxide (CO2) and 0.93% Argon (Ar) plus others) inside the package with a single gas or a specific mixture of gases that can lead to produce shelf-life extension. Gas mixtures containing low O2 (15%), high CO2 (10-20%) and N2 as make-up gas are normally used. Low O2 is desirable as it slows down respiration rate, ethylene production, enzymatic browning and growth of aerobic microorganisms. However, the amount of O2 needs to be carefully controlled since concentrations of this gas below 1% lead to produce fermentation and growth of harmful microorganisms, such as Clostridium botulinum.
CO2 has direct antimicrobial capacity, but excessive levels of CO2 can cause a reduction in pH, flavour tainting and drip loss. N2 is used as a make-up gas to avoid the collapse of the package and, by displacing O2, it has an indirect antimicrobial capacity. Other gases (e.g. Ar, ozone) [1,2] and gas mixtures (e.g., 95% O2 + 5% N2) [3] have been proven to enhance produce quality and safety as well.
The most adequate gas or mixture of gases is selected based on the type of produce to be packaged, due to the differences in respiration rate, ethylene production and/or sensitivity, etc. between different fruits and vegetables.
Depending on how the ambient air inside the package is replaced with a desired gas composition, MAP is classified into two types: active MAP (AMAP) and passive MAP (PMAP). In AMAP, the ambient air in the package is replaced with a desired mixture of gases or a single gas by direct flushing prior to package sealing (Figure 1).
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Figure 1 - Cantaloupe cubes packaged in a modified atmosphere (AMAP) for shelf-life extension. Photo by C. Wilson.
In PMAP, air free of contaminants, such as medical air, replaces the ambient air inside a package by direct flushing and then after package sealing, the composition of the air is modified. This occurs due to the interplay between produce respiration, which consumes O2 and replaces it with CO2, packaging characteristics (e.g., film permeability controls gas exchange between the inside and outside of the package) and the storage conditions. Very little effect of both produce and package on gas composition occurs in AMAP since the flushed mixture of gases slows down the produce respiration and the packaging material has been tailored in order to maintain a constant gas composition over time.
Both continuous and microperforated films can be used in PMAP to control gas exchange. Microperforated films perform better than continuous films since they allow a faster entrance of O2 into the package that mitigates the high/low concentrations of CO2/ O2 that develop in continuous film packages containing produce. A desired gas composition can easily be obtained with microperforated films by varying the number, area and length of the microperforations and thereby optimising the gas exchange [4]. In either AMAP or PMAP, produce is sanitised prior to packaging. Latest research findings show that the washing sanitiser type and the in-package atmosphere need to be carefully selected since interactions between sanitisers and gas compositions that affect microbial growth and quality occur 5]. Therefore, optimum combinations need to be used to deliver high quality and safe packaged produce.
Retail chains in the United States use AMAP to extend the shelf life of a variety of fresh-cut leafy greens, vegetable salads, sliced apples and sliced peaches, among others.
Retail chains in the United States use AMAP to extend the shelf life of a variety of fresh-cut leafy greens, vegetable salads, sliced apples and sliced peaches, among others, and PMAP with microperforations to commercialise produce including baby spinach, ready-to-eat blueberries and sliced apples. In the United States, AMAP is widely used to extend the shelf life of bulk produce during warehouse storage and transportation, including strawberries, blueberries and cherries. While MAP in combination with refrigeration can definitively delay produce deterioration and health risk, this packaging technology is not always sufficient to maintain produce quality and safety for a desirable marketing period. In addition, meeting new market needs, including produce tracking, monitoring of produce and environment changes during storage and responding to environment changes during storage, is not possible using MAP. As an alternative, AP and IP are able to match the produce shelf life to new market needs for a desirable marketing period.
AP can be defined as a packaging technology where certain additives, known as ‘active compounds,’ are incorporated into the packaging material or placed within the packaging container in order to interact directly with the perishable product and/or its environment to extend its quality and/or safety [6]. In contrast, IP records, regulates or controls the condition of the perishable product by sensing the environment inside or outside the package to ensure quality and safety during transportation, distribution and/or retail [6]. IP can ensure shelf life extension indirectly as well if it is simply used to check the effectiveness and the integrity of MAP or AP systems. The EU legal definitions relating to active and intelligent materials and articles are given in Article 3 of Regulation (EC) No. 450/2009 (currently no US equivalent is available) [7].
The active compound in AP can be placed inside the package along with the product to be packed (e.g. in sachets or labels) or can be part of the materials that form the package itself (e.g. blended in the bulk polymer matrix, applied to the package as a coating, integrated in the ink used for printing) (Figure 2).
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Figure 2 - Schematic views of an active compound as a part of the material that forms the package: blended in the bulk polymer matrix and applied as a coating ((a) and (b), respectively); or placed inside the package (c). Figure by M. Hausmann.
Active compounds that control moisture content, scavenge the gases oxygen and ethylene or have an antimicrobial capacity are being used to extend produce shelf life. For example, U.K. retailers Tesco and Marks & Spencer have used a laminated ethylene remover composite material inserted into packages of fruit to extend shelf life. Soft fruits and berries stored in this type of active package had a shelf life extension of 2 days. The laminated material was created by chemicals firm Johnson Matthey and was supplied by Cranfield-based Food Freshness Technology [8]. As for antimicrobial AP, Sirane’s Dri-Fresh ABV pads contain a blend of natural bioflavonoids and organic acids that are activated by moisture when inserted into packages containing fresh-cut fruit. This antimicrobial AP has shown shelf life extensions of several days [9].
In IP, the intelligent device or system is always part of the material that forms the package and is never placed inside the package along with the product. Examples of systems used currently to communicate produce quality and/or safety or to extend shelf life include indicators (time-temperature indicators, quality indicators, leak detectors), tracking devices (radiofrequency identification devices) and temperature-compensation membranes. An example of a timetemperature indicator (TTI) that has been used in retail is Fresh Code™, a smart barcode label manufactured by Varcode USA Inc. The contents of the barcode change as it is exposed to temperature abuse, providing data that can be automatically read at any stage of the cold chain and interpreted without ambiguity [10]. Such TTIs give data in addition to marked best-by dates, alerting as to when these dates no longer correlate to freshness. An example of IP that is able to sense breaks in the cold chain and change gas permeability as necessary to ensure produce shelf life is the packaging offered by Apio Inc. [11]. This type of packaging, commercialised in the United States, is capable of varying its permeability to O2 and CO2 in response to changes in temperature occurring during storage, distribution and marketing of packaged produce. This occurs due to the presence of a membrane – by the trade name of BreatheWay® – attached to a cut-out section of a flexible bag or tray. The membrane is customised according to the type of produce and the package size to passively maintain the ideal O2 and CO2 levels independently of the surrounding temperature (Figure 3).
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Figure 3 - Intelligent packaging offered by Apio, Inc. It contains the BreatheWay® membrane, which varies its permeability to O2 and CO2 in response to the changes in temperature occurring during the storage, distribution, and marketing of the packaged produce. Photo by E. Almenar
Despite the successful results obtained under retail conditions and the large amount of academic research supporting the value of AP and IP for produce shelf life extension [12-14], these technologies have not yet been widely adopted. This is most likely due to the need for consumer and retailer awareness of the benefits associated with the use of these packaging technologies, as well as proving the technologies offer value for money to the party paying for them. This shows that there is still a great deal of untapped potential in packaging for extending the shelf life of produce. The use of MAP, AP and IP will continue to rise due to the growing consumer demand for high quality, safe produce, the industry need for differentiation and brand protection and the new e-commerce trends.
Dr Eva Almenar and Christopher Wilson, School of Packaging, Michigan State University; East Lansing, MI, USA. Dr. Almenar is an associate professor at the School of Packaging at Michigan State University working on packaging materials made from renewable feedstock with a focus on active packaging materials (antimicrobial, scavenging, and temperature-sensitivity materials) for the delivery of high-quality and safe food products. She is currently the co-chair of the Food Packaging Division of the Institute of Food Technologists (IFT) and the past chair of the S-294 Multi-State Project on Postharvest Quality and Safety in Fresh Cut Vegetables and Fruits. Tel: (517) 355-3603, Email: ealmenar@msu.edu, Twitter: https://twitter.com/EAlmenar_PKG Christopher Wilson is pursuing an M.S. degree in Packaging at Michigan State University. His thesis applies active packaging and modified atmosphere packaging technologies to safety and quality challenges in ready-to-eat cantaloupe. Tel: (517) 355-9580, Email: ctwilson@msu.edu
References
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3. González-Buesa, J., Page, N., Kaminski, C., Ryser, E., Beaudry, R., Almenar, E. 2014. Effect of non-conventional atmospheres and bio-based packaging on the quality and safety of Listeria monocytogenes-inoculated fresh-cut celery (Apium graveolens L.) during storage. Postharvest Biology and Technology, 93: 29-37.
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