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Peroxyacetic acid

Advantages

Easy to apply, inexpensive, effective against all microbial forms, not affected by hard water, easy to monitor, FDA approved

More potent antimicrobial than chlorine, no chlorinated reaction products formed, economical to operate, self-affirmed GRAS, but FDA review possible, activity not pH-dependent

More potent than chlorine, activity not pH-dependent, fewer chlorinated reaction products formed than with Cl2, effective against biofilms, FDA approved, residual antimicrobial action, less corrosive than Cl2 or O3 Broad spectrum antimicrobial action, no pH control required, low reactivity with soil, effective against biofilms, FDA approved, no hazardous breakdown products, no on-site generation required, monitoring not difficult, available at safe concentration

Disadvantages

Decomposed by organic matter, reaction products may be hazardous, corrosive to metals, irritating to skin, activity pH-dependent, population reductions limited to < 1-2 logs Requires on-site generation, requires good ventilation, phytotoxic at high concentrations, corrosive to metals, difficult to monitor, higher capital cost than chlorine, no residual effect, population reductions limited to < 1-2 logs Must be generated on-site, explosive at high concentrations, not permitted for cut fruits and vegetables, population reductions limited to < 1-2 logs, generating systems expensive

Population reduction limited to < 1-2 logs, strong oxidant, concentrated solutions may be hazardous are available as alternatives to chlorine: chlorine dioxide (or acidified sodium chlorite), ozone, and peroxyacetic acid. The advantages and disadvantages of the agents described in the following sections are compared in Table 17.1.

17.2.1.2.1 Detergent Formulations

Among the detergents approved by the FDA for washing produce are sodium «-alkylbenzenesulfonate, sodium dodecylbenzenesulfonate, sodium mono- and dimethyl naphthalenesulfonates, sodium 2-ethylhexyl sulfate, and others [6]. These formulations may be neutral in pH, acidic due to the presence of citric or phosphoric acid, or alkaline because of the addition of sodium or potassium hydroxide. Major suppliers of detergent formulations for produce cleaning include Cerexagri (formerly Elf Atochem N.A., Inc., source of Decco products) (800-221-0925; www.cerexagri.com), Microcide, Inc. (www.microcideinc.com), and Alex C. Fergusson, Inc. (800-345-1329; www.afcocare.com).

These products are designed to remove soil and pesticide residues from produce and do not contain antimicrobial agents per se. Relatively little information is available concerning the ability of these products to remove or inactivate microorganisms attached to produce surfaces. However, their use can result in significant population reductions. Sapers et al. reported that some commercial washing formulations could achieve population reductions as great as 1 to 2 logs in decontaminating apples inoculated with a non-pathogenic E. coli, comparable to reductions obtained with hypochlorite [16]. When these products were applied at 50°C instead of at ambient temperature, a 2.5 log reduction was obtained. Wright et al. [31] reported similar efficacy with a commercial phosphoric acid fruit wash and with a 200 ppm hypo-chlorite wash, each applied to apples inoculated with E. coli O157:H7. Kenney and Beuchat [32] compared the efficacy of representative commercial cleaning agents in removing or inactivating E. coli O157:H7 and S. muenchen on spot-inoculated apples. They obtained reductions as great as 3.1 logs with an alkaline product and as great as 2.7 logs with an acidic product, reductions generally being greater with salmonella. Raiden et al. [33] compared the efficacy of water, sodium lauryl sulfate, and Tween 80 in removing Salmonella spp. and Shigella spp. from the surface of inoculated strawberries, tomatoes, and leaf lettuce. They obtained high removal rates but concluded that the detergents were no more effective than water. However, this result may have been a reflection of the brief time interval (1 hour) between inoculation and treatment, which may have been insufficient for strong bacterial attachment. In nature, the interval between preharvest contamination and postharvest application of a wash may be days or weeks, sufficient time for strong attachment and even biofilm formation.

In a study of cantaloupe rind decontamination, Sapers et al. [34] reported reductions in the total aerobic plate count of about 1.3 logs when the rind was washed with a 1% solution of a commercial produce wash containing dodecylbenzene sulfonic acid and phosphoric acid (pH 2) at 50°C. Sequential washing with this product followed by treatment with 1% hydrogen peroxide, both at 50°C, resulted in a 3.1 log reduction. Both washes extended the shelf life of fresh-cut cantaloupe prepared from the treated melons. No significant population reductions were obtained when the cantaloupe rind was washed with aqueous solutions of sodium dioctyl sulfosuccinate or sodium 2-ethylhexyl sulfate.

17.2.1.2.2 Chlorine Dioxide

Solutions of chlorine dioxide and acidified sodium chlorite have been used commercially as alternatives to chlorine for sanitizing fresh produce.

Chlorine dioxide is considered to be efficacious against many classes of microorganisms [5]. Chlorine dioxide and acidified sodium chlorite are approved by the FDA for use on fresh produce [35,36], but chlorine dioxide is not permitted for use on fresh-cut products. Chlorine dioxide must be generated on-site, usually by reaction of sodium chlorite with an acid or chlorine gas. Information concerning various proprietary generating and stabilizing systems are available from suppliers such as Vulcan Chemical (800-873-4898), Alcide Corp. (Sanova®; www.alcide.com/sanova), CH2O Inc. (Fresh-PakTM; www.ch2o.com), Rio Linda Chemical Co., Inc. (916-443-4939), Bio-Cide International, Inc. (Oxine®; www.biocide.com), International Dioxcide (www.idiclo2.com), Alex C. Fergusson (800-345-1329; www.afco care.com), CDG Technology, Inc. (www.cdgtechnology.com), and others. Unlike chlorine, chlorine dioxide is claimed to be effective over a broad range of pH levels, more resistant to neutralization by the organic load, and unlikely to produce trihalomethanes (see Oxine Technical Data Sheet; www.bio-cide.com). Chlorine dioxide also is claimed to be less corrosive than chlorine and to be effective against bacteria in biofilms. However, generation of chlorine dioxide by reaction of sodium chlorite with acid or Cl2 must be carefully controlled to avoid production of high concentrations of ClO2 gas which can be toxic and explosive (MSDS for IVR-San 15 sodium chlorite; www.ch2o.com). Additionally, unlike chlorine, chlorine dioxide dissolves in water as a gas and is subject to off-gassing if the water is moving or used in washers. In that situation, special venting would be required to prevent worker discomfort.

The efficacy of chlorine dioxide in disinfecting produce is comparable to that of chlorine. Published reports indicate that chlorine dioxide and related products were potentially effective in preventing potato spoilage by Erwinia carotovora [37], reducing populations of E. coli O157:H7, S. Montevideo, and poliovirus on inoculated strawberries [38], reducing the population of E. coli O157:H7 on inoculated apples (but at a treatment level 16 times the recommended concentration) [39], and suppressing decay in pears [40]. Treatments were less effective in suppressing microbial growth on the surface of cucumbers [41]. Fett obtained only a 1 log reduction in alfalfa sprouts irrigated with acidified sodium chlorite [42]. Population reductions of L. monocytogenes on uninjured surfaces of inoculated green bell peppers, washed with ClO2 solution (3mg/l), were about 2 logs greater than could be achieved with a water wash, but reductions were negligible on injured surfaces [43]. In contrast, these investigators obtained population reductions of 7.4 and 3.6 logs on uninjured and injured surfaces of peppers, respectively, using a ClO2 gas treatment (see Chapter 18).

The efficacy of ozone in killing human pathogens and other microorganisms in water is well established [44], and it is widely used as an alternative to chlorine in municipal water treatment systems and for production of bottled water

[45]. Ozone is effective in killing food-related microorganisms [46] and has been approved for use on foods by the FDA [47]. Potential applications of ozone in disinfecting foods have been reviewed [48,49]. Ozone is effective in reducing bacterial populations in flume and wash water and may have some applications as a chlorine replacement in reducing microbial populations on produce [50,51]. Ozone treatment was effective in suppressing decay of table grapes by Rhizopus stolonifer [52]. Use levels of 0.5 to 4.0 mg/ml are recommended for wash water and 0.1 mg/ml for flume water [53,54].

However, not all ozone treatments show high efficacy. Ozone treatment of fresh-cut lettuce, inoculated with a mixture of natural microflora, yielded reductions of only 1.1 logs [18]. Treatment of lettuce, inoculated with Pseudomonas fluorescens, with 10 mg/ml of ozone for 1 minute achieved less than a 1 log population reduction [50]. While ozone treatment of apples inoculated with E. coli O157:H7 was effective in reducing populations on the surface (3.7 log reduction), reductions were <1log in the stem and calyx regions [55]. Ozone treatment of pears (5.5 mg/ml water for 5 minutes) was ineffective in reducing postharvest fungal decay [56]. Population reductions obtained by ozone treatment of alfalfa seeds inoculated with E. coli O157:H7 were only marginally better than those for water-treated controls [57]. In another study, ozone treatment of alfalfa seeds, inoculated with L. monocytogenes, was ineffective in reducing the population of this pathogen, while treatment of inoculated alfalfa sprouts reduced the L. monocytogenes population by < 1 log and was phytotoxic to the sprouts [58]. These results are probably a reflection of the difficulty in contacting and inactivating bacteria attached to produce surfaces in inaccessible sites (see Chapters 2 and 3).

One of the major advantages claimed for ozone is the absence of potentially toxic reaction products. However, ozone must be adequately vented to avoid worker exposure [48]. Ozone has to be generated on-site by passing air or oxygen through a corona discharge or UV light [48]. A number of commercial systems for generating ozonated water for produce washing are available. Information about commercial ozone generators is available on-line from Air Liquide (www.airliquide.com), Praxair, Inc. (www.praxair.com), Novazone (www.novazone.net), Pure Ox (www.pureox.com), Osmonics, Inc. (www.osmonics.com/food), Ozonia North America, Inc. (www.ozonia.com), Lynntech, Inc. (www.lynntech.com), Clean Air & Water Systems, Inc. (360-394-1525), Electric Power Research Institute (EPRI; www.epri.com), and others. For information about ozone gas disinfection treatments, see Chapter 18.

17.2.1.2.4 Peroxyacetic Acid

Peroxyacetic acid (peracetic acid) is an equilibrium mixture of the peroxy compound, hydrogen peroxide, and acetic acid [59-61]. The superior antimicrobial properties of peroxyacetic acid are well known [59]. Peroxyacetic acid is approved by the FDA for addition to wash water at concentrations not to exceed 80ppm [6]. Under EPA regulations, an exemption from the requirements of a tolerance was established for peroxyacetic acid as an antimicrobial treatment for fruits and vegetables at concentrations up to 100 ppm [62]. Much higher concentrations are permitted for sanitizing food contact surfaces [63]. Peroxyacetic acid decomposes into acetic acid, water, and oxygen, all harmless residuals.

Peroxyacetic acid is recommended for use in treating process water, but Ecolab, one of the major suppliers, is also claiming substantial reductions in microbial populations on fruit and vegetable surfaces [64]. However, company literature provides insufficient information on methodology to assess treatment efficacy (www.ecolab.com/initiatives/foodsafety). Population reductions for aerobic bacteria, coliform bacteria, and yeasts and molds on fresh-cut celery, cabbage, and potatoes treated with 80 ppm peroxyacetic acid were less than 1.5 logs [65]. Addition of 40 ppm Tsunami 100 (the Ecolab peroxyacetic acid product) to the irrigation water used during sprout propagation did not suppress the outgrowth of the native microflora [42]. Treatment with 100 ppm Tsunami reduced the population of E. coli O157:H7 and S. Montevideo on inoculated strawberries by about 97% [38]. Several published studies have looked at the efficacy of peroxyacetic acid against E. coli O157:H7 on inoculated apples. Attempts to disinfect apples, inoculated with E. coli O157:H7, by washing with 80 ppm peroxyacetic acid 30 minutes after inoculation resulted in a 2 log reduction compared to a water wash [31]. However, in another study where inoculated apples were held for 24 hours before washing (allowing more time for attachment), an 80 ppm peroxyacetic acid treatment reduced the E. coli O157:H7 population by less than 1 log; at 16 times the recommended concentration, a 3 log reduction was obtained [39]. Sapers et al. [16] reported similar results with apples inoculated with a nonpathogenic E. coli. Like ozone and chlorine dioxide, low concentrations of peroxyacetic acid are effective in killing pathogenic bacteria in aqueous suspension [59]. Addition of octanoic acid to peroxyacetic acid solutions increased efficacy in killing yeasts and molds in fresh-cut vegetable process waters but had little effect on population reductions on fresh-cut vegetables [65].

Peroxyacetic acid is a strong oxidizing agent and can be hazardous to handle at high concentrations, but not at strengths marketed to the produce industry. Peroxyacetic acid is available at various strengths from Ecolab, Inc. (www.ecolab.com), FMC Corp. (www.fmcchemicals.com), and Solvay Interox (www.solvayinterox.com).

17.2.2 Washing Equipment 17.2.2.1 Types of Washers

Washing equipment for produce is designed primarily for removal of soil, debris, and any pesticide residues from the harvested commodity. The design of most commercial equipment has not taken into account requirements for the reduction of microbial populations on produce surfaces although this is a desirable goal of washing.

FIGURE 17.1 Commercial washing equipment for fruits and vegetables: (a) flat-bed brush washer; (b) U-bed brush washer; (c) rotary washer; (d) pressure washer; (e, f) flume washers; (g) helical washer.

Numerous types of washers have been developed for cleaning fresh fruits and vegetables, varying in complexity from a garden hose used for cleaning apples prior to farm-scale cider production (an unsatisfactory procedure due to lack of control) to sophisticated systems employing rotating brushes and applying heated water under pressure with agitation. The more common types of commercial washers for produce include dump tanks, brush washers, reel washers, pressure washers, hydro air agitation wash tanks, and immersion pipeline washers (Figure 17.1). Major suppliers of such equipment are listed on the Postharvest Resources website of the University of Florida (http:// postharvest.ifas.ufl.edu). The choice of washer for a particular commodity will depend on such characteristics of the commodity as shape, size, and fragility. It is obvious that equipment requirements are quite different for cut lettuce than for tomatoes or potatoes.

17.2.2.2 Efficacy of Washers

The efficacy of commercial flat-bed and U-bed brush washers in removing or inactivating a nonpathogenic E. coli on artificially contaminated apples was investigated by Annous et al. [66] and Sapers [3]. These studies demonstrated that the E. coli population could be reduced by about 1 log (90%) by passage of the apples through a dump tank with minimal agitation (Table 17.2). However, further cleaning of the apples in a flat-bed brush washer had little further effect on the E. coli population, irrespective of the cleaning or sanitizing agent used (water, 200ppm Cl2, 1% acidic detergent, 8% trisodium phosphate, 5% H2O2). Similar results were obtained with a U-bed brush washer. Subsequent studies by the investigators showed that the bacteria that had attached in the

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