Chapter 6
Irradiation of Food and Packaging Downloaded from pubs.acs.org by UNIV OF SYDNEY on 10/23/15. For personal use only.
Improving Safety and Extending Shelf Life of FreshCut Fruits and Vegetables Using Irradiation 1
Anuradha Prakash and Denise Foley 1
2
2
Departments of Physical Sciences and Biological Sciences, Chapman University, Orange, CA 92866
Irradiation can serve as a hurdle step in an overall safety plan that enhances safety while preserving quality of fresh-cut fruits and vegetables. Extension of shelf-life using irradiation is primarily due to the decrease in spoilage organisms, thus the effectiveness of irradiation depends on initial quality of the product. Irradiation at the levels optimal for shelf-life extension is also effective against pathogens found in fresh produce. Vegetative pathogens are destroyed while background flora is reduced but not eliminated. Combining irradiation with other technologies such as calcium treatment, warm water dips, and modified atmosphere packaging can further enhance shelf-life and mitigate adverse effects on quality.
90
© 2004 American Chemical Society
Irradiation of Food and Packaging Downloaded from pubs.acs.org by UNIV OF SYDNEY on 10/23/15. For personal use only.
91 Fresh-cut or minimally processed (MP) products are also referred to as "lightly processed", "partially processed", "fresh processed", and "preprepared." Fueled by demands for convenience and freshness, sales of ready-toeat vegetables have grown rapidly in the last decade and are expected to reach $19 billion in 2003 (/). Fresh-cut or minimally processed fruits and vegetables are defined as those trimmed, peeled or cut into 100% usable form and then packaged. The initial preparation and preservation treatments are often followed by some kind of modified/controlled atmosphere or vacuum packaging. Fresh-cut products are then subjected to low temperatures (above the freezing point) during storage, distribution, marketing, and just prior to preparation for consumption. Shredded lettuce, mixed salads, peeled carrots, cauliflower and broccoli florets, sliced mushrooms, sliced and diced tomatoes, cut bell peppers, and peeled garlic are examples of fresh-cut vegetables. Peeled and cored pineapple, peeled citrus fruits, sliced apples, cantaloupe chunks, and fruit salads are examples of freshcut fruits.
Shelf-life of Fresh-Cut Produce Unlike most processing techniques that extend shelf-life, minimal processing increases perishability. Shelf-life of fresh-cut vegetables is generally 10-14 days and slightly less for fresh-cut fruit. The behavior of plant tissue that has been minimally processed is similar to that of tissue that has been wounded or stressed. This behavior includes increased respiration and ethylene production, and sometimes wound healing. Other consequences include oxidative browning reactions, lipid oxidation, and enhanced water loss. Several factors such as species and variety, stage of physiological maturity, extent of wounding, temperature, oxygen and carbon dioxide concentrations, water vapor pressure, and various inhibitors affect the intensity of the wound response. Microbial spoilage is also enhanced due to the presence of cut surfaces or damaged plant tissues that facilitate microbial colonization, active metabolism of the plant tissue, and methods used to extend shelf life that allow longer periods for microbial multiplication.
Microbiological Concerns with Fresh-Cut Produce Fresh fruits and vegetables are often considered to be among the most healthful and safe foodstuffs available, yet, according to the Centers for Disease Control and Prevention, the number of produce-related outbreaks doubled between the period between 1973-1987 and 1988-1992 (2). Outbreaks with identified causes were primarily of bacterial origin with Salmonella topping the list (2, 3) although viruses (such as Hepatitis A) and parasites (such as Giardia)
Irradiation of Food and Packaging Downloaded from pubs.acs.org by UNIV OF SYDNEY on 10/23/15. For personal use only.
92 have also been implicated. Buck and others (2) note that enteric pathogens are among the greatest concern for fresh-cut fruits and vegetables because they have a low infectious dose and have the potential to grow on the product prior to consumption. At any stage in the growth and production cycle of the product, the potential for contamination exists. The field or orchard, harvesting, sorting, washing, cutting, distributing and handling in the home or for food service all have the potential for introducing pathogenic organisms. Not surprisingly then, numerous isolations of a wide variety of pathogenic organisms from fresh fruits and vegetables have been documented (2, 4, 5, 6, 7). The increase in fresh produce consumption, the trend towards eating away from home, a proliferation of salad bars, centralized processing, and an increase in global trade have been suggested as possible reasons for the increase in the number of food borne outbreaks related to produce (2, 8). Increased scrutiny of outbreaks and improved detection methods may also be contributing factors.
Preservation of Fresh-Cut Fruits and Vegetables There is much interest in the fresh-cut industry to find effective strategies to extend shelf-life and minimize contamination by pathogens. The most common methods used to preserve the quality of fresh-cut fruits and vegetables are temperature control, chemical sanitation, and modified atmosphere packaging. Maintaining low temperature is critical to reduce respiration rate, microbial growth, and other deteriorative reactions. However, psychrotrophic organisms such as Listeria monocytogenes, Yersinia enterocolitica, and Aeromonas hydrophila are capable of growing at refrigeration temperatures used to store fresh produce. Various sanitizers can be used to wash raw fruits and vegetables. Sodium hypochlorite, calcium hypochlorite, chlorine dioxide, chlorine gas, hydrogen peroxide, ozone, and organic acids such as lactic acid, acetic acid and peroxyacetic acid are approved by the US Food and Drug Administration (FDA) for food contact (9). Antimicrobial activity depends on the amount of sanitizing compound that comes in contact with microbial cells. The inaccessibility of sanitizers to microbial cells lodged in crevices and natural openings in the skin contributes to the lack of effectiveness of available sanitizers in killing pathogens. Modified atmosphere packaging ( M A P ) is used widely for fresh-cut produce spurred in part by advances in packaging material. High CO2 levels and low 0 levels are highly effective in delaying spoilage but have minimal effect on some pathogens such as Listeria monocytogenes, Yersinia enterocolitica, or Aeromonas hydrophila (10, 11, 12). Furthermore, although the incidence or Clostridium botulinum spore presence is low (0.36% in one study of 1118 precut M A P samples (13)), the low oxygen conditions may allow toxin formation (14, 2
93 15). However, toxin is usually found in samples considered spoiled but is infrequently detected in a sample that would be otherwise considered edible (15).
Irradiation of Food and Packaging Downloaded from pubs.acs.org by UNIV OF SYDNEY on 10/23/15. For personal use only.
Irradiation can serve as a hurdle step in an overall safety plan that enhances safety while preserving quality of fresh-cut fruits and vegetables. This non thermal treatment is highly effective against many pathogens found in fresh produce and offers the potential, in combination with other treatments, of improving shelf-life.
Effect of Irradiation on Fresh-Cut Fruits and Vegetables
Effect on Microorganisms Irradiation like other physical processing can be applied in various ways, at various times, and at various intensities. The process and the product will determine the level of effectiveness of irradiation treatment. For example, a noted decrease in sensitivity to irradiation is shown at temperatures at or below freezing. Furthermore, the food matrix with variable water availability, differing ionic concentrations, oxygen concentrations, and other factors will affect the dose needed to achieve the desired sanitizing effect (16, 17). Most food pathogens including Campylobacter jejuni, A. hydrophila, Y. enterocolitica, L. monocytogenes, and E. coli 0157:H7, have a low tolerance for irradiation whereas the spores o f pathogens such as Clostridium botulinum, Clostridium perjringens, and Bacillus cereus are more resistant. Few studies provide D-values (the dose needed to achieve a 90% reduction in counts) for various pathogens on fresh-cut produce, but the high moisture content would suggest lower D-values. For example, D-value of L. monocytogenes on endive is only 0.20 (18) and D-value for E. coli on various lettuce samples is less than 0.12 (19) but on alfalfa sprouts, the D-value for Salmonella and E. coli 0157:H7 is higher, 0.5 kGy and 0.32 kGy, respectively (20). Since the level of contaminating pathogens in produce is usually low (< 10 CFU/g), low levels of irradiation (
