Conventional Washing Technology 1721 Washing Agents 17211 Chlorine

Most freshly harvested fruits and vegetables are washed by the grower, packer, or processor to remove soil, plant debris, pesticide residues, and microorganisms from the commodity surface. This may be accomplished by spraying or immersion in water or solutions containing one of a number of cleaning or sanitizing agents, using equipment designed for each particular commodity type, e.g., leafy vegetables, root vegetables, fruit vegetables, tree fruits, or melons. Chlorine is the most widely used sanitizing agent for fresh produce. It may be added to wash water as Cl2 gas or, more commonly, as sodium or calcium hypochlorite. In water, at pH levels and concentrations used on produce, these chlorine sources are converted to hypochlorous acid and hypochlorite ion in a ratio determined by the solution pH [4,5]. At pH 6.0, roughly 97% of the unreacted chlorine is hypochlorous acid, whereas, at pH 9.0, 97% is hypochlorite ion. The antimicrobial activity of these solutions is due largely to hypochlorous acid rather than to hypochlorite.

The concentration of chlorine in a wash solution is sometimes expressed as total available chlorine (or total residual chlorine = combined residual chlorine + free residual chlorine), based on the calculated amount present in the added hypochlorite or chlorine, or determined by oxidation of KI to I2, which may not be indicative of the actual potency as a sanitizer because of the inclusion of reaction products such as monochloramine which are not very effective as sanitizers. Preferably, the chlorine concentration can be expressed as free available (or residual) chlorine, the sum of hypochlorous acid and hypochlorite ion concentrations [5]. The total or free chlorine concentration can be monitored by means of test kits, based on colorimetry (www.chemetrics.com; www.emscience.com,www.hach.com), or by measurement of the oxidation-reduction potential (ORP). Chlorine is highly reactive with certain types of compounds in organic materials and soils that are leached or washed from fruits and vegetables. If this chlorine sink is excessive, the free chlorine concentration will be depleted rapidly. Computerized ORP systems that monitor the pH and chlorine concentration can be used to control the level of chlorine in a wash tank in such situations (www.pulsein struments.net; numerous other suppliers listed on www.globalspec.com).

Use levels of chlorine will depend on allowable levels, the commodity, and the anticipated microbial load. The U.S. Food and Drug Administration (FDA) specifies a use level for washing fruits and vegetables not to exceed 0.2% when followed by a potable water rinse [6]. The U.S. Environmental Protection Agency (EPA) exempts calcium hypochlorite "from the requirement of a tolerance when used preharvest or postharvest in solution on all raw agricultural commodities" [7]. The concentration range of 50 to 200 ppm is commonly used for most commodities. However, as much as 20,000 ppm calcium hypo-chlorite may be used to sanitize alfalfa seeds intended for sprout production because of the failure of other treatments to disinfect adequately seeds and sprouts, and the high risk that sprouts grown from contaminated seeds may be a source of salmonella or Escherichia coli O157:H7 outbreaks [8-10].

Chlorine is highly effective for inactivating planktonic cells of bacteria, yeasts, molds, and viruses, although bacterial and fungal spores are considerably more resistant [5]. However, chlorine is less effective for inactivating bacteria attached to produce surfaces or embedded within the product [11-18]. Typically, population reductions of native microflora on produce surfaces or of human pathogens on inoculated produce are no greater than 2 logs (99%). While such reductions can greatly reduce spoilage, they are insufficient to ensure safety in the event of contamination with human pathogens.

The activity of chlorinated water may be increased by the addition of an acidulant or buffer so that the pH is shifted from an alkaline value (about pH 9) to the neutral to slightly acidic range (pH 6 to 7), thereby increasing the proportion of hypochlorous acid in the equilibrium mixture. Organic acids such as citric acid or mineral acids such as phosphoric or hydrochloric acid can be used for this purpose. If the solution pH is too low (e.g., below pH 4), hypochlorous acid may be converted to free chlorine which is subject to off-gassing. This will result in a loss of activity and may be potentially hazardous. Additionally, equipment corrosion is enhanced as pH levels drop below as well as rise above neutrality. Unpublished data obtained at the Eastern Regional Research Center indicated that hypochlorite solutions acidified with a mineral acid were more stable than solutions acidified with citric acid [19]. Buffers for hypochlorite solutions are available commercially (www.cerexagri.com).

The effectiveness of chlorine in inactivating microorganisms on produce may be enhanced by adding a surfactant to the solution so that it can penetrate into the irregular crevices and pores on produce surfaces where microorganisms may lodge and escape contact with a sanitizer. Several commercial surfactant formulations have been developed for this purpose (www.cerexagri.com/usa/ Markets/Cleaners). Addition of a nonionic surfactant improved the efficacy of chlorine against decay fungi in pears [20,21]. Washing formulations containing sodium hypochlorite, buffers, and surfactants have been described by Park et al. [22] and marketed by Bonagra Technologies under the name Safe-T-WashedTM (www.bonagra.com). The efficacy of chlorine in reducing the microbial flora of shredded iceberg lettuce was increased by elevating the solution temperature to 47°C [23]. However, no greater reduction of nonpathogenic E. coli (ATCC 25922) populations on inoculated apples was obtained when apples were washed at 50 or 60° C compared to 20° C using 200 ppm Cl2 (added as sodium hypochlorite), adjusted to pH 6.5 with citric acid [19].

Chlorine's major advantages are its broad spectrum of antimicrobial activity, ease of application, and low cost. However, chlorine is highly corrosive and may damage stainless steel equipment after prolonged exposure. Its other major disadvantages are rapid depletion in the presence of a high organic load [24], and the potential carcinogenicity and mutagenicity of its reaction products with organic constituents of foods [25-27]. This is a matter of concern to processors, regulators, and consumers [28]. For these reasons, and the desirability of obtaining greater population reductions, the development of alternative sanitizing agents has been an active area of research, and a limited number of agents suitable for use on fresh produce have been commercialized.

Electrolyzed water, a technology developed largely in Japan [29,30], is really a special case of chlorination. This technology is discussed in detail in Chapter 22.

17.2.1.2 Alternatives to Chlorine

A number of commercial detergent formulations have been developed for washing fruits and vegetables. In addition, three approved sanitizing agents

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