<p>Multiple studies have demonstrated atmospheric cold plasma (ACP)
as an effective non-thermal technology for microbial decontamination, surface
modification, and functionality alteration in food processing and packaging. ACP
constitutes charged particles, such as positive and negative ions, electrons,
quanta of electromagnetic radiation, and excited and non-excited molecules,
which corresponds to its predominant reactive properties. However, in many of
these applications, the interactions between plasma and the components in food matrix are not well-understood. The <b>overall goals</b> of this dissertation were
to 1) evaluate the interactions between high voltage atmospheric cold plasma (HVACP) and microbes in liquid and semi-solid
food; 2) investigate plasma transfer into semi-solid foods and determine the
relationship between microbial inactivation and plasma transfer; 3) explore the
interactions between plasma and proteins. </p>
<p>The first
study explored the microbial (<i>Salmonella</i>
<i>enterica</i> serovar Typhimurium, <i>S</i>. <i>enterica</i>)
inactivation efficacy of HVACP. The physicochemical interactions between HVACP
and biomolecules, including an enzyme
(pectin methylesterase, PME), vitamin C and other components in orange juice (OJ) under different conditions was
also evaluated. Both direct and indirect HVACP treatment of 25 mL OJ induced
greater than a 5 log reduction in <i>S</i>. <i>enterica</i> following 30 s of
treatment with air and MA65 gas with no storage. For 50 mL OJ, 120 s of direct
HVACP treatment followed by 24 h storage achieved <i>S</i>. <i>enterica</i> reductions of
2.9 log in air and 4.7 log in MA65 gas. An indirect HVACP treatment of 120 s followed
by 24 hours storage resulted in a 2.2 log reduction in air and a 3.8 log
reduction in MA65. No significant (<i>P </i><
0.05) Brix or pH change occurred following 120 s HVACP treatment. HVACP direct
treatment reduced vitamin C content by 56% in air and PME activity by 74% in
air and 82% in MA65. These results demonstrated that HVACP can significantly
reduce <i>Salmonella</i> in OJ with minimal quality degradation.</p>
<p>The second study in this dissertation examined the
penetration process of plasma into semi-solid food and the resulting microbial
inactivation efficacy. Agar gels of various densities (0.25, 0.5, 1.0, and 2%) with
a pH indicator were inoculated with <i>S</i>. <i>enterica</i> (10<sup>7</sup>>CFU) and exposed directly (between
the electrode) or indirectly (adjacent to the plasma field created between the
two electrodes) to 90 kV at 60 Hz for up to 1.5 h. A long treatment time (1.5 h) caused sample temperature to increase
5~10 °C. The microbial analysis indicated a greater than 6 log<sub>10</sub>
(CFU) reduction (both with air and MA65) in the zone with a pH change.
Inactivation of bioluminescence cells in the plasma penetrated zone confirmed
that the plasma, and its generated reactive species, inactivate microbial as it penetrates into the gel. A two-minute HVACP direct treatment with air at 90 kV induced greater than 5 log<sub>10</sub>
(CFU)<i> S</i>. <i>enterica </i>reduction in applesauce. <em></em></p>
<p>The third
study investigated the interactions between HVACP and protein, using bovine serum albumin (BSA)
as a model protein. The physicochemical and structural alteration of BSA and
its reaction mechanism, when subjected to HVACP, were investigated. After
treating 10 mL of BSA solution (50 mg/mL) at 90 kV for 20, 40, or 60 min, we
characterized structural alteration and side-group modification. FTIR spectroscopy, Raman spectroscopy, and circular
dichroism analysis indicated protein unfolding and decreased secondary structure
(25 % loss of α-helix, 12% loss of β-sheet) in HVACP
treated BSA. Average particle size in the protein solutions increased from 10 nm to 113 µm, with a broader
distribution after 60 min HVACP treatment
indicating protein aggregation. SDS-PAGE and mass spectrometer
analysis observed a formation of new peptides of 1 to 10 kDa, indicating that
the plasma triggered peptide bond cleavage.
Chemical analysis and mass
spectrometer results confirmed the plasma modifications on the side chains of
amino acids. This study reveals that HVACP
treatment may effectively introduce structural alteration, protein aggregation,
peptide cleavage, and side-group modification to proteins in aqueous
conditions, through several physicochemical interactions between plasma reactive
species (reactive oxygen species and reactive nitrogen species) and the proteins.
This finding can be readily applied to
other plasma-protein studies or applications in the food system, such as enzyme inactivation or protein-based film
modifications.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7395569 |
| Date | 12 February 2019 |
| Creators | Lei Xu (5930420) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/INTERACTIONS_OF_HIGH_VOLTAGE_ATMOSPHERIC_COLD_PLASMA_WITH_MICROORGANISM_AND_PROTEIN_IN_FOOD_SYSTEMS/7395569 |
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