Pulsed Laser Beam Technology for Selective Inactivation of Microbial Contaminants from Tropical Foods

<p>NON-TECHNICAL SUMMARY: <br/>According to the Centers for Disease Control and Prevention (CDC), the average number of annual foodborne illnesses associated with fresh fruits and produce has more than doubled from the 1980s to the 1990s, and the increasing trend continues to this day. Recently, packaged fresh-cut fruits and vegetables have grown in popularity. The major problems associated with the fresh fruits industry are with contamination, spoilage, or decay. Potential microbial contamination sources for fresh fruits include soil, water, manure, domestic animals, human handling, and contaminated equipment. Currently, methods for microbial decontamination of food are limited, and chemical treatment of food products is becoming less popular with consumers due to growing concerns over chemical residues and toxicity. Due to the rejection of chemical treatment, mild heat
treatment has become a potential alternative for fresh fruits and produce. However, conventional heat treatment of foods at high temperatures may be detrimental to the food's flavor, texture, appearance, and overall quality. On the other hand, if thermal treatment at lower temperatures is applied, there may not be adequate decontamination. Therefore, the PD proposes that a pulsed laser beam with far infrared wavelength can deliver the lethal dose of energy to external microbes at their corresponding absorption wavelengths. The concept of selective laser heating was pioneered and proved for medical applications, i.e. selective photothermolysis; however, is new to food processing areas. Besides the thermal effect, the proposed study will find out the lethal mechanism associated with resonant laser-matter interaction. The use of light-absorbing gold nanoparticles is aimed to enhance the
selective laser killing of bacteria on fruit surfaces and liquids. The expected outcomes are: 1. Pulsed laser beam will selectively eliminate targeted microbes on the fruit surfaces or sub-layers and minimize any heat injuries, leading to improved food qualities. The proposed technique will contribute to preservative retention of all the nutrients while minimizing food damage. 2. The proposed technique will be energy efficient and have punctual heating due to little radiative reflection. Selective heating using far infrared radiation coupled with optical filtering that the PD studied in the past resulted in more than 40% energy loss. 3. Numerical simulation will make it possible to implement various types of light-biological matter interaction in terms of thermal selectivity. 4. The project outcomes found in the laboratory are highly likely to find their applications on a massive scale
in numerous segments of food and the US industry.

<p>APPROACH: <br/>Objective 1: Design and optimize pulsed laser beam for selective heating of model foods. A pulsed laser system located in the PD's lab will be used as a base unit for this project. The wavelength of laser beam ranges from 10.57 to 10.63 um, which is invisible infrared, and the pulse width can vary from ultra pulse mode (200 -1000 us) to super pulse mode (1 - 200 ms). Besides laser modes such as pulse width, repetition times, power levels, the photothermal guiding of laser treatment using thermo-lenses will be fabricated and optimized to ensure relatively uniform radiation of the entire fruit samples on the vibrating plate. Objective 2: Conduct elimination of surface and sub-layer contaminants from fruit surfaces using pulsed laser beam. E. coli K12 will be used as representative fruit surface contaminant in this study. Whole fruits (apples and mangoes)
will be washed with tap water and dripped in solutions of E. coli K12 cells (105 CFU/ml). The artificially contaminated fruits will be dried at room temperature for 30 mins to fix the inoculum and subjected to laser treatment. After the treatment, pieces of peeled skins will be aseptically transferred to sterile stomacher bags containing 225 ml PBST (pH 7.4). Then the bags will be folded and stomached at medium speed (100 rpm) for 2 min. The resulting sample liquids from stomacher bags as well as carrier medium will be serially diluted in BPW and plated on non-selective TSA and selective 3M Petrifilm E. coli/Coliform Count Plate. The plates will be incubated at 37 degC for 48 h and then colonies will be counted. Objective 3: Explore the photothermal kinetics with laser chemistry. A mathematical model for selective photothermal kinetics will be developed using commercial software, COMSOL
(v. 4.2, COMSOL, Inc.). Kinetic parameters can be obtained by modeling the microbial lethality vs. pulse numbers using a first order differential equation. The total heat flux absorbed by targeted bacteria and fruit surfaces will be calculated using the energy balance equation developed by the PD. Objective 4: Enhance the photothermal selectivity using gold nanoparticles conjugated with targeted bacteria on fruit surfaces. Gold nanoparticles with high absorption coefficients (Nanoprobes, Inc., NY) conjugated with antibodies will be used to increase the selectivity of the photothermal effects for active targeting. E. coli K12 samples with or without gold nanoparticles will be irradiated on fruit surfaces using CO2 laser with wavelengths of 10.57 um and various pulse widths and repetition times. After irradiation with 100 laser pulses, the samples will be stomached, serial-diluted and
plated on TSA and E. coli/Coliform Count Plate to quantify surviving colonies. Objective 5: Test for likely nutritional loss during laser decontamination for fresh fruits Antioxidants and vitamin C of fruit skins and sub-layers during and after laser excitation will be accessed and analyzed with respect to laser selective chemistry.

<p>PROGRESS: <br/>2013/04 TO 2013/09
<p>Target Audience: Nothing Reported
Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? Nothing Reported How have the results been disseminated to communities of interest? Nothing Reported What do you plan to do during the next reporting period to accomplish the goals? Methods may also be taken to address this concern by exploring ways in which to recollect GNP after treatment. Some methods that may be feasible include using dielectrophoretic (DEP) fields, magnetic fields, sonication, or an additional antibody-antigen reaction in order to concentrate and recollect GNP after laser treatment.