Document Type

Thesis - Open Access

Award Date

2017

Degree Name

Master of Science (MS)

Department

Dairy Science

First Advisor

Sanjeev Anand

Keywords

biofilms, modified stainless steel, sporeformers, stainless steel

Abstract

Aerobic sporeformers can be traced in a variety of dairy products such as milk powders, evaporated milk, and canned products, which demonstrates their capability of resisting high temperature treatments such as pasteurization and Ultra high temperatures. These bacilli also actively attach to the stainless steel surfaces, consequently resulting in the formation of biofilms. Product quality as well as its safety is undesirably affected by the growth of these sporeforming bacteria. Therefore, creating an ideal environment for the processing of dairy products is a critical challenge for the dairy industry. Hence, the objective of this research was to analyze various surface modifications of the conventional Stainless Steel (SS) and to study the extent of bacterial adhesion in order to develop a surface that is least vulnerable to bacterial attachment, thus reducing the formation of biofilms. The first part of the study analyzed the adhesion tendency of aerobic sporeforming bacteria on native and modified Stainless Steel surface (AMC 18, Dursan, Ni-PPTFE and Lectrofluor 641). Heat resistant aerobic spore-forming bacteria were specifically picked for this study as these can survive pasteurization, Ultra High Temperature (UHT) treatment and can also develop Heat Resistant Spores (HRS), which can potentially contaminate the dairy processing lines. The modified SS coupons were manufactured using spin coating and dip coating method. Biofilm development on native and modifies SS coupons were compared for three common aerobic sporeformers namely G. stearothermophilus, B. licheniformis and B. sporothermodurans. Various surface properties including surface energy, surface hydrophobicity and surface roughness of the coupons were compared for their role on the adhesion tendency of the sporeformers. Bacterial attachment was observed to be directly proportional to the surface energy, whereas it was inversely proportional to the surface hydrophobicity. Biofilm development studies indicated that Ni-P-PTFE modified surface was least vulnerable to bacterial attachment whereas native SS surface was highly susceptible. Scanning Electron Microscopy showed the extent of bacterial attachment and biofilm formation. The second part of the study compared native SS surface and Ni-P-PTFE modified surface plate heat exchangers (PHEs) for the extent of biofouling and shedding of biofilms. Milk was allowed to flow continuously for 17 hours through both the pasteurizers to mimic the conditions encountered in a dairy plant that have the potential to create a conducive environment for biofouling. 3M quick swabs and ATP swabs were employed for sampling from both the pasteurizers, for studying the biofilm formation and evaluating the efficacy of CIP, respectively. Milk samples were collected at the start of pasteurization run and at hourly intervals after the 10th hour of the operation from both the balance tanks (raw milk sample) and outlets (pasteurized milk sample) of both PHEs. It was observed that after the 15th hour, there was a sudden increase in the standard plate counts (SPC) of the native PHE. Consequently, the SPC of the native PHE turned out to be far higher, as compared to the modified PHE, as the experiment reached the 17th hour. Also, there was more biofilm formation in the regeneration section of native pasteurizer as compared to the modified pasteurizer. The third part of the study compared the adhesion tendency of spores and vegetative cells on both native and Ni-P-PTFE modified SS surfaces of various sporeforming bacteria including G. stearothermophilus, B. licheniformis and B. sporothermodurans. The adhesion tendency of the sporeformers was observed to be also influenced by cell surface properties viz. cell surface hydrophobicity and cell surface charge (zeta potential). As per the results from the study, spores exhibited a far greater attachment tendency as compared to the vegetative cells of the same spore-forming bacteria. Amongst different sporeformers, B. sporothermodurans demonstrated greatest adhesion tendency followed by G. stearothermophilus, with B. licheniformis exhibiting least adhesion tendency. The tendency to adhere varied with the variations in cell surface properties as it decreased with lowering cell surface hydrophobicity and increasing cell surface charge. All of the above studies provide useful information regarding the various factors (both contact surface and cell surface properties) that play a significant role in influencing the adhesion tendency and biofilm formation by the aerobic spore forming bacteria.

Description

Includes bibliographical references

Format

application/pdf

Number of Pages

160

Publisher

South Dakota State University

Rights

Copyright 2017 Shivali Jindal

Available for download on Thursday, February 08, 2018

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