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Title:Investigation on Factors Influencing Ultrasound-assisted Surface Decontamination of Fresh and Fresh-cut Vegetables
Author(s):Zhou, Bin
Director of Research:Feng, Hao
Doctoral Committee Chair(s):Padua, Graciela W.
Doctoral Committee Member(s):Feng, Hao; Kushad, Mosbah M.; Miller, Michael J.
Department / Program:Food Science & Human Nutrition
Discipline:Food Science & Human Nutrition
Degree Granting Institution:University of Illinois at Urbana-Champaign
fresh-cut vegetables
surface decontamination
continuous flow system
surface characteristics
leafy green produce
Abstract:Although strict practices for controlling the safety of leafy green produce have been implemented in the produce industry, current commercial operations rely on a wash treatment with an antimicrobial agent as the only step for reducing microbial populations on fresh produce. However, washing with a sanitizer has been demonstrated to achieve no more than 1-2 log reduction in pathogen populations. In recent years, much research effort has been put into the development of multiple-hurdle techniques to enhance produce safety. Ultrasonic waves ranging from 20 to 100 kHz in frequency have been successfully used as a surface cleaning technique in the medical and precision processing industries for a number of years. The use of ultrasound in fresh produce sanitation as a hurdle technique is a relatively recent endeavor, however. The reported studies have been sporadic and inconsistent, and have only dealt with batch operations. There are few reports documenting the effects of ultrasound and washing tank operational parameters on the efficacy of combined treatments of ultrasound and sanitizer, or on the removal of bacteria from fresh or fresh-cut vegetable surfaces. In this study, a systematic approach, starting from understanding the interactions among acoustic energy, produce, sanitizers, and bacteria, as well as the distribution of the ultrasonic field in the treatment channel, was employed to find the answer to the important question: "Can an ultrasound-assisted treatment indeed enhance microbial reduction in a continuous-flow pilot-scale produce washing system?" The first question that has to be answered for any ultrasound-assisted produce wash is if ultrasound as a form of physical energy will cause produce quality degradation. To address this issue, a determination was made of the threshold of acoustic power density (APD) in an ultrasound treatment allowable without causing unacceptable produce damage immediately after sonication. All subsequent produce wash tests were performed with an APD below this threshold. Next, the effect of ultrasound on the reduction of Escherichia coli O157:H7 inoculated on the surfaces of selected produce was measured. The interactions between ultrasound and sanitizers were examined by monitoring the concentration changes of the sanitizers in washing solutions used for fresh-cut lettuce washes. The effects of ultrasound parameters and operational conditions on the uniformity of the acoustic field in the ultrasonic channel, and on surface decontamination of leafy green produce, were also examined. A continuous-flow pilot scale ultrasonic washing system was designed, fabricated, and tested in this study. Further, produce surface characteristics and infiltration pathways were studied. Finally, the application of ultrasonication for surface decontamination of lettuce coring knives was also investigated. The overall quality scores of baby spinach, lollo rosso, loose leaf lettuce, iceberg lettuce, and romaine lettuce remained unchanged during a 2-week storage period when sonicated at an APD of less than 100 W/L for less than 2 min., and were above the acceptable level as determined by a sensory panel. The electro-conductivity rate of all four of these salad leaves increased over the treatment time. The ultrasound treatment significantly enhanced the removal of E. coli O157:H7 cells from the spinach surfaces in all of the sanitation treatments, and the enhancement of the ultrasound increased with the treatment time and APD. Ultrasonication accelerated the degradation of free chlorine and total chlorine during a 4-min treatment, compared to a wash without ultrasound, while concentrations of acidified sodium chlorite and peroxyacetic acid remained unchanged. An ultrasound treatment at 75 kHz was significantly less effective in the removal of E. coli O157:H7 from spinach leaves than treatments at 25 and 40 kHz (p < 0.05). The removal of microbes from baby spinach leaves by ultrasonication decreased when the ultrasonic channel width increased from 304.8 to 609.6 mm. The blockage of ultrasound due to the overlapping of spinach leaves dramatically decreased the decontamination efficacy, and hence care should be taken to avoid the overlapping of produce leaves. A 20-min degassing prior to ultrasonication only resulted in a marginal increase (0.16 log cycle) in the E. coli count reduction. In the tests with the pilot-scale ultrasonic produce washer, ultrasound in combination with chlorine enhanced the reduction of E. coli inoculated on spinach by a 0.53 log cycle for batch-leaf washes. Additionally, batch-leaf washes with chlorinated water enhanced the reduction of aerobic plate count by a 0.50 log cycle over a chlorine-only wash. No significant effect of ultrasound was observed for yeast and mold reduction. Ultrasonication enhanced the removal of E. coli O157:H7 from baby carrots by 1.24 log cycles and 0.65 log cycle when treated for 1 min and 3 min, respectively. A batch wash with ultrasound also significantly (p < 0.05) increased the aerobic plate count reduction for roma tomato surfaces by a 0.75 log cycle over a sanitizer-only wash, while there was only a marginal enhancement in yeast and mold reduction. A number of new techniques were developed or applied to elucidate the relationships between produce surface characteristics and microbial attachment and removal. In the inactivation tests, the underside of spinach leaves (rough side) was found to provide a better shelter for E. coli O157:H7 than the upside. Scanning electron microscope (SEM) and optical profiler mapping were used to provide some insight into this observation. The SEM images and surface profiles of a spinach leaf showed valleys, deeper in the underside than in the upside, harboring and even protecting the cells attached in them. In the effort to quantify the surface roughness of produce, a freeze-drying sample preparation method for confocal laser scanning microscopy was developed which worked well. There were no significant differences in surface roughness among selected vegetables. Moreover, the surface hydrophobicity of the inner surface of spinach leaves was not significantly different from that of the outer surface, and was not affected by ultrasonication. For the first time, the MicroXCT technique was employed in this study to examine the internal structure of a produce sample on the micrometer scale. The MicroXCT images provided a detailed view of the potential infiltration pathways inside a produce tissue that may be used by human pathogens to penetrate fresh and fresh-cut produce. In the test to examine the interactions of ultrasound with a hard metal surface, the ultrasound treatment reduced the E. coli O157:H7 count to below the detection limit on coring knives on both the blade and welding joint when treated for 30 s in 1 mg/L chlorinated water. The two newly designed prototype knives harbored significantly fewer E. coli cells than the current commercially used ones, enabling high disinfection efficacy. In summary, prerequisites for the application of acoustic energy for produce surface decontamination are a good understanding of the underlying physics of acoustics, as well as of the interactions among ultrasound, sanitizers, produce, and bacteria. The operational parameters of both the ultrasound generator and produce washer are also critical in improving the washing efficacy. With a carefully designed ultrasound system and a uniform acoustic field distribution in the washer and on each side of a produce sample, a significantly enhanced reduction in microbial count over that of a sanitizer-only wash can be achieved.
Issue Date:2011-01-21
Rights Information:Copyright 2010 Bin Zhou
Date Available in IDEALS:2013-01-22
Date Deposited:2010-12

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