Interventions Sprouts

A variety of antimicrobial chemicals have been tested as additives to sprout irrigation water for the purpose of preventing or reducing the growth of native microflora and bacterial human pathogens. A study in our laboratory indicated that addition of H2O2, Tsunami®, acidified NaClO2, Aquatize™ (Bioxy, Raleigh, NC), EDTA, NaPO4, and NaOCl at varying concentrations to the irrigation water did not reduce the populations of the native microflora on alfalfa sprouts grown in a commercial-scale tray system by more than approximately 1 log10 without evidence of phytotoxicity [107]. Piernas and Guiraud [108] reported that spray irrigation of tray-grown rice sprouts with chlorinated water (100 mg/l) every 6 hours was not effective in reducing populations of total aerobic bacteria, B. cereus, or L. innocua. Daily spraying of alfalfa sprouts grown from artificially inoculated seed with chlorine (100 mg/l) led to reduction of less than 2 log10 in the population of salmonella at day 4 of sprouting [51]. Daily irrigation with ClO2 (100 mg/l) did not reduce the population of total aerobic bacteria on alfalfa sprouts grown in trays, but did reduce populations of V. cholera up to 2 log10 when sprouts were grown from seed inoculated with the pathogen [32]. A reduction of 4 log10 for total coli-forms was obtained for mung bean sprouts that were subject to irrigation with 0.2 ppm gaseous ozone and 0.3 to 0.5 mg/l of ozonated water at days 4 to 7 of sprouting [105]. Rinsing of inoculated alfalfa seed growing in plastic jars with aqueous ClO2 (25 mg/l) or ozonated water (9.27 mg/l) after 48 or 72 hours of sprouting was ineffective in reducing populations of E. coli O157:H7 [92]. However, rinsing with thyme oil (5.0 mg/l) alone or in sequence with ClO2 and ozonated water led to reductions of up to 2 log10 in pathogen populations when carried out at 24 and 48 hours into the sprouting process. None of the rinsing treatments were effective at 72 hours, however. Rinsing with water was ineffective at all time points. Taormina and Beuchat [110] tested a variety of aqueous antimicrobial chemicals as spray treatments for reducing or eliminating E. coli O157:H7 from the surface of growing alfalfa sprouts. None of the chemicals were effective for reducing pathogen populations and only acidified NaOCl2 (1200 ppm) controlled the growth of the pathogen. A complication of addition of antibacterial compounds to the irrigation water is that any pathogens present in the spent irrigation water may be killed, but viable pathogen populations may remain on the sprouts rendering the testing of spent irrigation water for viable pathogens meaningless [4].

Several postharvest treatments for reducing the populations of native microbes and pathogens have been examined. Water rinses are not highly effective in reducing microbes on sprouts with resultant population reductions of 1 log10 or less [31,39,110,111]. A 2-minute treatment with aqueous ozone (23 ppm) did not reduce the population of aerobic microorganisms on alfalfa sprouts [75]. Dipping in hot water (60°C) for 30 seconds led to a reduction of 2 log10 in the population of total microbes on soybean sprouts [112] and a similar treatment for 5 minutes led to a reduction of 5 log10 in aerobic plate counts on rice sprouts [113]. Blanching in hot water (90°C, 1 minute) was reported to reduce microbial counts by 5 log10 units for mung bean sprouts [111]. Rinsing of mung bean sprouts in 1 and 2% lactic or acetic acid reduced the native microflora by less than 2 log10 [111]. Treatment of rice sprouts with chlorine (100 mg/l) for up to 10 minutes decreased aerobic plate counts by only 1.5 log10 [111]. Treatment (10 minutes) of inoculated mung bean sprouts with chlorous acid (HClO2; 268 ppm), NaOCl (200 ppm), or lactic acid (2%) resulted in a maximum reduction of 1 log10 for total aerobes [114].

Blanching in hot water (100°C, 30 seconds) did not eliminate E. coli O157:H7 from alfalfa sprouts [115]. Treatment (10 minutes) of alfalfa sprouts with EO water (84 mg/l of available chlorine) in conjunction with sonication led to a reduction of 1.5 log10 in the population of salmonella [116]. Treatment (64 minutes) with EO water (50 mg/l of available chlorine) resulted in a reduction of 3 log10 of E. coli O157:H7 on alfalfa sprouts without any reported changes in appearance [117]. Aqueous ozone treatments (maximum concentration of 20 to 23 ppm, treatment time of 20 to 64 minutes) of alfalfa sprouts led to a maximum population reduction of approximately 1 to 2 log10 for L. monocytogenes and E. coli O157:H7, respectively [75,118]. The greatest log reductions reported for a postharvest aqueous chemical treatment were for HClO2. Treatment (10 minutes) of inoculated mung bean sprouts with HClO2 (268 ppm) resulted in a reduction of approximately 5 log10 of salmonella and L. monocytogenes. Lactic acid (2%) was also tested in this study, but was less effective [114]. Exposure of inoculated alfalfa sprouts to gaseous acetic acid or allyl isothiocyanate vapor led to significant reductions in the population of salmonella, but also led to undesirable changes in sensory quality [56].

Most likely the only postharvest treatment able to inactivate pathogens that have been internalized into sprouts during the growing process is irradiation. A postharvest treatment with gamma radiation at 2 kGy extended the shelf life of alfalfa and broccoli sprouts by 10 days due to significant decreases in the native microflora [72,88]. Doses up to 2.6 kGy did not significantly change the appearance or nutrient quality of alfalfa sprouts [118]. Salmonella was eliminated from alfalfa sprouts grown from naturally contaminated seed when exposed to gamma radiation at a minimum dose of 0.5 kGy [120]. Irradiation of inoculated alfalfa sprouts with 3.3 kGy of beta radiation (electron beam) eliminated L. monocytogenes without an adverse effect on quality [121].

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