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REMOVAL OF PATHOGENS FROM PRODUCE
Most processors and consumers have assumed that washing and sanitizing fresh fruits and vegetables will reduce the microbial load. However, published efficacy data indicate that these methods cannot reduce microbial populations on produce by more than 90–99% (Beuchat, 1998). While such population reductions are useful and not to be over looked, they are insufficient to assure microbiological safety. Conventional washing technology was developed primarily to remove soil from produce, not microorganisms. Even with newer sanitizing agents such as chlorine dioxide, ozone, and peroxyacetic acid, improvements in efficacy have been made with shortcomings, such as the inability of chlorine dioxide to reduce the population of E coli O157:H7 on inoculated apples (Beuchat, 1998). Alternatives to chlorine were limited in their ability to kill bacteria when realistic inoculation and treatment conditions were used (Sapers, 2001; Fonseca, 2006; Abadias et al., 2008). Nozomi, Matasume and Kenji (2006) showed that a combination of sodium hypochlorite, fumaric acid and mild heat was very effective in killing aerobic bacteria, E. coli O157:H7, Salmonella typhimurium DT 104 and S. aureus on fresh-cut lettuce but it caused browning. Because of these limitations, it is preferable, wherever possible, to avoid contamination of fruits and vegetables by following good agricultural and manufacturing practices rather than by depending on decontamination (Sapers, 2001; Bihn & Gravani, 2006).
Factors that limit the efficacy of washing are: contamination conditions, interval between contamination, attachment in inaccessible sites, biofilms and internalization (Bhagwat, 2006). According to Sapers (2001), Salmonella sp survived washing to a greater extent when attached to cut surfaces of apple and green pepper than on unbroken external surface. Fresh produce such as apples, pears, cherries, grapes, potatoes, carrots and lettuce were reported to often have punctures, cuts or splits, providing space for attachment and internalization of foodborne pathogens (Sapers, 2001). E. coli was also reported to grow in wounds on apples and was difficult to kill after it was established within the wounds and puncture (Sapers, 2001).
Chlorine is routinely used as a sanitizer in wash, spray and flume waters used in the fresh fruit and vegetable industry (Beuchat & Ryu, 1997; Beuchat, 1998; Hagenmaier & Baker, 1998; Seymour et al., 2002). Antimicrobial activity depends on the amount of free available chlorine in water that comes in contact with microbial cells. Francis et al. (1999) studied the effect of chlorine concentration on aerobic microorganisms and faecal coliforms on leafy salad greens. Total counts were markedly reduced with increased concentrations of chlorine up to 50 ppm, but a further increase in concentration up to 200 ppm did not have a substantial additional effect. The effectiveness of treatment with water containing up to 200 ppm chlorine in reducing numbers of naturally occurring microorganisms and pathogenic bacteria is minimal, often not exceeding 2 log on lettuce (Adams, Hartley & Cox, 1989; Beuchat & Brackett, 1990; Beuchat et al., 1998; Beuchat, 1999; Weissinger et al., 2000) and tomatoes (Beuchat et al., 1998; Weissinger, Chantarapanont & Beuchat, 2000). Several workers have emphasized that chlorine cannot be relied upon to eliminate pathogenic microorganisms such as L. monocytogens (Hagenmaier & Baker, 1998; Nguyen-the & Carlin, 1994; Beuchat & Ryu, 1997).
The hydrophobic cutin, diverse surface morphologies and abrasions in the epidermis of fruits and vegetables limit the efficacy of sanitizers (Burnet & Beuchat, 2001). The inaccessibility of chlorine to the microbial cells in the crevices, pockets and natural openings in the skin of the fruits and vegetables contributes to the overall lack of effectiveness of chlorine in killing pathogens (Lund, 1983).
Use of electrolyzed water as a sanitizing agent is a type of chlorination. Electrolysis of water containing a small amount of sodium chloride generates a highly acidic hypochlorous acid solution containing 10–100 ppm available chlorine and was effective in reducing pathogens in apple and lettuce leaves (Sapers, 2001). Other authors have also reported on the application of electrolyzed water in the produce industry (Koseki et al., 2004; Huang et al., 2008). However, the reaction of chlorine with organic residues can result in the formation of potentially mutagenic or carcinogenic-reaction products (Hidaka et al., 1992; Simpson et al., 2000). A number of alternatives to chlorine such as chlorine dioxide, iodine compounds, ozone and hydrogen peroxide have been examined and some are in commercial use (Sapers, 2001, Zhao, Zhao and Doyle, 2009). Chlorine dioxide has a higher biocidal activity than chlorine but there are still some difficulties in its large-scale application by the fresh-cut produce industry (Bhagwat, 2006). Hydrogen peroxide has been shown to be a promising alternative to chlorine (Ukuku, et al., 2001, Bhagwat, 2006). It was shown that it increased the shelf life of fresh-cut melons by 4 to 5 days compared to that of chlorine-treated melons. However, commercial application of hydrogen peroxide in the produce industry still requires FDA approval (Bhagwat, 2006).
Another potential replacement for chlorine as a sanitizer is ozone (Graham et al., 2004). In 2001 the FDA approved the use of gaseous and aqueous ozone for application as an antimicrobial agent for foods (FDA, 2001). Garcia et al. (2003) determined the effectiveness of using ozone in combination with chlorine as a sanitizer in the treatment of minimally processed lettuce. They found that lettuce treated with chlorine, ozone or a combination had a shelf life of 16, 20, or 25 days respectively, indicating that chlorine-ozone combinations may have beneficial effects on shelf life and quality of lettuce salads as well as on the water used for rinsing or cleaning the lettuce. However, ozone treatment was ineffective in reducing decay of pears and foodborne pathogens (Spotts, 1992; Sapers, 2001). Iodine compounds are also more effective sanitizers than chlorine but they predispose surfaces and products to discolouration (Beuchat, 1998).
Mechanism of action of chlorine
Chlorine is normally used for sanitizing produce in three forms: chlorine gas (Cl2), calcium hypochlorite (CaCIO2), and sodium hypochlorite (NaOCl) (Fonseca, 2006). Chlorine is able to reduce microbial population on produce and other surfaces because it is a strong oxidizing agent (Bhagwat, 2006).
The efficacy of chlorine, however, is affected by the amount of free available chlorine in solution, the pH, the temperature and the amount of organic matter (Fonseca, 2006). According to Stopforth et al. (2004), low pH of internal tissues of fruits and vegetables and high loads of organic matter in the sanitizing solution significantly reduce the antimicrobial activity of chlorine. Also, according to Suslow (2007), “for optimum antimicrobial activity, the pH of the water must be between 6.5 – 7.5 because at this pH range, most of the chlorine is in the form of hypochlorous acid which produces the highest rate of microbial kill and reduces the release of irritating and potentially hazardous chlorine gas.”
CHAPTER 1: GENERAL INTRODUCTION
1.1 Problem statement
CHAPTER 2: LITERATURE REVIEW
2.1 Importance of fresh and minimally processed vegetables
2.2. Economy of vegetables in South Africa
2.3 Food pathogens associated with vegetables
2.4 Sources of contamination
2.5 Water situation in South Africa
2.6 Quality of South African surface water
2.7 Water for agricultural use
2.8 Irrigation water and pathogen transfer
2.9 Attachment and internalization of pathogens into produce
2.10 Removal of pathogens from produce
2.11 Control and prevention measures against fresh produce contamination
2.12 Hypotheses and Objectives
CHAPTER 3: RESEARCH
3.1 Irrigation water as a potential pre-harvest source of bacterial contamination of vegetables
3.2 Effect of attachment time followed by chlorine washing on the survival of inoculated Listeria monocytogenes on tomatoes and spinach
3.3 Bacterial pathogens in irrigation water and on produce are affected by certain predictor variables
CHAPTER 4: GENERAL DISCUSSION
4.1 Introduction
4.2 Review of Methodology
4.3 Overall discussion
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
CHAPTER 6: REFERENCES
