ARTICLE: The Science of Modified Atmosphere Packaging for Fresh Produce

by Jeffrey Brandenburg

Introduction of MAP for Produce

By definition Modified Atmosphere Packaging is when the package internal atmosphere is something other than ambient atmosphere. This type of packaging falls into two broad categories: Modified Atmosphere Packaging (MAP) and Controlled Atmosphere Packaging (CAP). For barrier packaging of products such as meat, cheese, snack food etc., MAP refers to enclosing a product within a barrier package and modifying the atmosphere, either by or a combination of drawing a vacuum or filling it with a gas(es). CAP is a process where gases are added or removed to maintain a desired balance. MAP barrier packaging allows meat to bloom, extends shelf life, prevents soft products from being crushed, and retains moisture.

MAP for fresh produce is more complex because the produce continues to respire, consuming O2 and giving off CO2; therefore when designing modified atmosphere packaging for fresh produce the convergence of three unique and separate sciences must take place, namely produce physiology, polymer engineering and converting technology.

In fresh produce package design the internal atmosphere of a MAP package is achieved by the relationship between the respiration rate of the live produce and the transmission rate of the breathable package. Therefore in order to properly design and optimize MAP one first must quantify the produce respiration rate values. Using published respiration rates is not as effective as measuring the respiration rate of your product.

Jeffrey Brandenburg (The JSB Group) and Deirdre Holcroft (HPC) have developed a relatively simple respiration rate (RR) quantification procedure that includes:

  • Ability to perform tests at either customer or our own facilities,
  • Testing RR on multiple produce raw materials simultaneously,
  • Testing RR of complex produce blends,
  • Testing RR on ready to eat meals,
  • Improved test materials,
  • Updated data collection spreadsheets directly linked to JSB HPC proprietary package design and shelf life determination system.

When the package is designed properly this relationship will allow the internal atmosphere to modify passively until an optimal final modified atmosphere is achieved. Active gas flushing in produce packaging is an augmentation technology which is only used in certain situations e.g. to prevent enzymatic browning (pinking) reactions in Iceberg or Romaine lettuce.

Modified atmosphere packaging when combined with proper post-harvest handling procedures and temperature control management can have a positive impact on the quality and shelf life of fresh produce. MAP does have some limitations:

  • MAP is only effective if there is consistent temperature management throughout the entire life cycle of the produce product from processing throughout entire distribution channel. Lack of temperature control will result in produce physiological variations, particularly respiration, which will impact the effectiveness of the packaging system. For example if MAP is designed for 10C but held at 5C then it will take longer to achieve an effective atmosphere because the respiration rate is lower. If it is stored at 20C then respiration rate increases and the product can go anaerobic which affects quality, causes off-flavours and decreases shelf life.
  • Modified atmosphere packaging (MAP) will never improve the quality of the incoming raw material product. Under ideal circumstances the best that can be achieved is to maintain the existing quality level throughout the desired shelf life. In real world applications often MAP will maintain quality for the majority of the targeted shelf life, but due to parameter variations during distribution, quality will suffer at the very end of the desired shelf life. Since modified atmosphere packaging will never improve incoming product quality the need for optimal post-harvest handling procedures is paramount.
  • If the package leaks then then the target modified atmosphere cannot be achieved and or maintained. In effect a package with a leaker will no longer be in control, therefore modified atmosphere packaging must be designed to minimize and ultimately eliminate leakers. This can be accomplished through proper polymer, and film selection as well as packaging equipment parameters.

Effects of MAP on Product Physiology and Safety

Most of the reactions with food constituents involving oxygen are degradative reactions involving the oxidative breakdown of foods into their constitutive parts. Because of this, many packaging strategies seek to exclude oxygen and thus slow these degradation processes. Many spoilage microorganisms require oxygen and will grow and cause off odors in the presence of sufficient oxygen. Oxygen is necessary to the normal respiratory metabolism of fresh fruits and vegetables and normal atmospheric concentrations of oxygen encourage and facilitate senescence and degradation of quality.

Reducing O2 concentrations below about 10% around many fresh fruits and vegetables slows their respiration rate and indirectly slows the rates at which they ripen, age and decay. Reducing the O2 concentration can, in some cases, reduce oxidative browning reactions which can be of particular concern in precut leafy vegetables. Reduced O2 can delay compositional changes such as fruit softening, pigment development, toughening of some vegetables (such as asparagus and broccoli), and development of flavor (Kader, 1986). However, O2 is required for normal metabolism to proceed. O2 concentrations below about 1-2% can lead to anaerobic (sometimes called fermentative) metabolism and associated production of ethanol and acetaldehyde resulting in off flavors, off odors and loss of quality. Of even greater concern is the potential growth of anaerobic bacteria, some of which are pathogenic to humans, under these low oxygen conditions. The proper O2 concentration will depend upon the fruit or vegetable and its tolerance to low O2, the temperature (which will affect the product’s tolerance to low O2), and the time that the product will be exposed to low O2.

Carbon dioxide is present in the atmosphere in low levels, typically about 0.03%, but is an important product of combustion and so is easily produced. It is very soluble in water, especially in cold water, (179.7 cm3/100 ml @ 0°C), and will thus be absorbed by high moisture foods. When CO2 dissolves in water it produces carbonic acid which will cause a drop in pH and an acidifying effect. This acidification, as well as direct antimicrobial effects, can suppress the growth of many spoilage microorganisms and for this reason is essential in many extended shelf life packages.

Elevated CO2 can, like reduced O2, slow respiration thereby extending shelf life. Although the effects of elevated CO2 on respiration are not as dramatic as those of low O2, high CO2 and low O2 together can, in some cases, reduce respiration more than either gas alone (Kader et al., 1988). CO2 at relatively high concentration (> 10%) has been shown to suppress the growth of a number of decay-causing fungi and bacteria.   However too much CO2 can be damaging to plant tissues and individual fruits and vegetables differ in their tolerance to CO2.

Oxygen (O2) and CO2 permeate through plastic polymers at various rates depending on the polymer, but it generally O2 permeates through more slowly than carbon dioxide. The permeability rate of oxygen (and all gases) in plastics increases as temperature increases. Similarly, the chemical reactivity of oxygen with food constituents increases as temperature increases.

When creating MAP from microperforated films the transmission rate of the gases through the holes are directly related to the geometry, size and consistency of the perforated holes. Unlike gas transmission through polymer films which creates different rates for O2 and CO2, gas transmission through perforations are equal. In other words, O2 and CO2 have the same transmission rate through a perforation. This means that if you target a low O2 level (e.g. 2% O2) you will have a resulting high CO2 level (e.g. 19%), whereas with a non-perforated polymer film when you target a low O2 level the resulting CO2 level can be at low to moderate levels.

In addition there is a transmission rate ceiling with non perforated films and a lower level transmission rate limit with perforated films. It is therefore important to understand the different properties of the two types of packaging transmission technologies so that the optimal technology can be used.

Finally it is imperative that MAP always work in conjunction with an excellent program of sanitation and quality assurance in order to optimize quality and safety.

Through the relationship between the Global Fresh Technology Group and companies such as Packaging World we can provide services and technical support to optimize packaging, produce quality and shelf life.

Specific services and support includes:

  • Optimization of product quality and shelf life; for example 10 days of shelf life on packaged salad is obtainable with the correct film and good postharvest handling
  • Quantify product respiration rates through property RR test method to design optimal packaging films
  • Modified atmosphere package design based upon quantified produce physiological properties
  • Postharvest physiology resource
  • Food safety resource
  • Sustainable packaging resource
  • Independent MAP resource and expertise
  • Global technology resource for both rigid and flexible packaging
  • Customized training programmes for your staff.

 

Jeffrey Brandenburg of The JSB Group, jbrandenburg@jsbgroup.com and Deirdre Holcroft of Holcroft Postharvest Consulting, Manuel Madrid of Fruit Profits, and David Barney of Geofresh, are the founding members of the Global Fresh Technology Group. They have joined forces to provide technical and educational services to the global fresh produce, ornamental and food industries.

 

 

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