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Food Bioscience Research

Current Postgraduate Students

Dong Zhang

Title of research study: The effect of raw milk quality on final dairy product quality

Chief Supervisor: Steve Flint

Co-Supervisors: Dr. Jon Palmer, Dr. Koon Hoong Teh, Dr. Denise Lindsay

Scholarships: Tetra Pak

Raw milk is a fragile ingredient in the manufacture of dairy commodities, and the quality of the final product is affected by many variables. From a microbiological perspective, it is thought that the bacteria, in terms of numbers and types, present in raw milk at the start of the manufacturing process, have a big impact on the quality of the final product. Refrigeration is used to control the microbiological quality of raw milk before processing. This favours the growth of psychrotrophic bacteria that can produce heat-stable enzymes, which may spoil final dairy products. For example, in ultra-high-temperature (UHT) dairy products, shelf-life is affected mostly by the activity of proteolytic enzymes, predominantly native plasmin and bacterial enzymes. For powder products, such as whole milk powder (WMP), adverse flavor issues caused by bacterial lipases are the main customer complaint.  Therefore, this project aims to examine the microbiological factors which lead to damage of raw milk components, and show scientifically the link to poor final product quality.

Emmanuel Kyere

Colonization of lettuce by Listeria monocytogenes

Haoran Wang

Haoran Wang Title of Research Study: Factors contributing to biofilm formation of Yersinia enterocolitica

Chief Supervisor: Prof Steve Flint

Co-Supervisors: Dr. Jon Palmer

Biofilm Formation

Biofilms of pathogenic bacteria are recognised as a threat to food safety. The aim of the present study was to investigate the potential of Y. enterocolitica to form biofilms in pork processing environment and identify the resistance of these biofilms to sanitation. The biofilm formation by Y. enterocolitica was monitored at conditions simulated pork processing environment under daily cleaning routine by an impedance method established in this study. Results showed that Y. enterocolitica had the potential to form biofilm and become resistant to sanitation in pork processing environment. An investigation into the influencing factors on biofilm formation of Y. enterocolitica indicated that the calcium ion increased the level of biofilm

formation. In addition, the presence of the virulence plasmid pYV is essential for the biofilm calcium response. Further analysis of bacterial cell surface properties and EPS composition suggested that the pYV+ cell surfaces are more negatively charged and more hydrophobic than the pYV- cells although no significant difference was observed with the addition of calcium. The pYV+ cells appear to produce more exopolysaccharide than the pYV- cells regardless of Ca concentration. Ca2+ was able to increase the yield of extracellular DNA while the presence of pYV appeared to be dispensable in terms of extracellular DNA release. Analysis of cell wall protein revealed one protein expressed in the pYV+ cells but absent in the pYV- cells.

Figure 1 SEM microscopy (A)-pYV+ without Ca2+ (B)-pYV+ with Ca2+ (C)-pYV- without Ca2+ (D)- pYV- with Ca2+

Jessika Nowak

Title of Research Study:  Identification and understanding the roles of biofilm formation-related genes in Listeria monocytogenes isolated from seafood

Chief Supervisor: Prof Steve Flint

Co-Supervisors:Dr. Jon Palmer, Dr Cristina Cruz, Mr Graham Fletcher

Scholarships: The New Zealand Institute for Plant and Food Research Limited (NZ$25000 pa)

Listeria monocytogenes is a pathogen which can cause severe infections in humans, especially immunocompromised people, and is usually derived from food contamination. The major route of contamination is thought to be due to formation of biofilms in food processing plants. My research focuses on identification of genes that are involved in biofilm formation of a persistent L. monocytogenes strain using a transposon mutant library. Selected mutants are used to study the function of the genes using phenotypic assays and microscopic analysis. Furthermore the influence of cations and environmental stresses (oxidative, heat and acid) on biofilm formation will be investigated in order to find measures to prevent biofilm formation in food processing plants.

Michael Dixon

biofilm formation of bacteria from a dairy plant wastewater irrigator system

Shuyan Wu

Title of Research Study: Persister cell formation of Listeria monocytogenes in response to natural antimicrobial agent

Chief Supervisor: Steve Flint

Co-Supervisors: Pak-Lam Yu

Shuyan Wu Research

The persistence of Listeria monocytogenes (L. monocytogenes) can be defined by its prolonged existence in food environments that pose a risk to food safety. The persistence of cells exposed to antibiotics and chemical sanitisers is well recorded however the persistence following treatment with natural antimicrobials like bacteriocins has not been determined. The mechanism of L. monocytogenes persister formation is unknown however persister isolates do exist as viable and non-culturable cells. Understanding the mechanism of persistence will help in developing strategies to control this food safety risk. In order to study these persister cells, this project will use L. monocytogenes isolates from food environments to obtain a persistent subpopulation following exposure to nisin treatment. Persister cells became a population of cells showing tolerance to the high concentrations of nisin in our tests and proved not to be resistant with nisin re-exposure tests. XTT assays showed that the reduced level of metabolism maybe an important requirement for the L. monocytogenes persistence. The current data will be initial evidence to understand the mechanism of persistence through gene expression and in the future to give more reliable clues in developing strategies to control this food safety risk.

Figure. Killing kinetics of L. monocytogenes treated with nisin. a) Concentration- dependent killing of L. monocytogenes A1 overnight culture cells treated with nisin at concentrations of 0-75µg/ml at 30°C for 24 hrs. b) Concentration- dependent killing of L. monocytogenes M5 overnight culture cells treated with nisin at concentrations of 0-75µg/ml at 30°C for 24 hrs. c) Time-dependent killing of L. monocytogenes A1 overnight culture cells treated with nisin at concentrations of 75µg/ml at 30°C for 24 hrs. d) Time-dependent killing of L. monocytogenes M5 overnight culture cells treated with nisin at concentrations of 75µg/ml at 30°C for 24 hrs. The experiment was performed with three biological replicates and error bars indicate the standard deviation.

Siti Norbaizura Md Zain (Baizura)

Siti Norbaizura Md Zain Title of Research Study: Biofilm formation by B. licehniformis isolated from whey powder (WPC80) and their effect in whey product manufacture.                                                         

Chief Supervisor: Prof Steve Flint

Co-Supervisors: Mr. Rod Bennett, Dr. Tay Hong Soon

Scholarships: Malaysian government

Contamination of whey products with bacteria is an economic problem for the dairy industry. The role of biofilms as the source of the contaminating bacteria in whey products is unknown. Biofilms can form on any surface, but as a large amount of surface area is included in the ultrafiltration membranes used in whey processing, this is one key potential site for biofilm development leading to product contamination. Whey provides nutrients to enable bacterial growth and the temperatures used in certain parts of the dairy industry are suitable for bacterial growth. There is ample opportunity for biofilm development and it is likely that the contaminants seen in whey powder originate from biofilms shedding bacteria into the product. In addition, whey protein, along with other organic or inorganic molecules can form a conditioning layer on surfaces that may make them more likely to attract bacteria and start biofilm colonisation.

Yufeng Zhang

Title of Research Study:  Cell-to-cell communications during meat spoilage and methods for preservation

Chief Supervisor: Prof Steve Flint

Co-Supervisors: Dr Anne Midwinter, Dr Gale Brightwell, Dr Tanushree Gupta, Mr. John Mills

The influence of cell-cell signaling on the interactions of spoilage bacteria on meat will be studied to identify quenching molecules for incorporation into active packaging to significantly reduce the outgrowth of these bacteria

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