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Khan, Intisar Chowdhury (2015) Spreads microbiology in association with product matrix, structure and chemistry. PhD thesis, University of Nottingham.

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Abstract

The overall aim of this study was to eliminate the root cause of the ‘cheese flavour’ in spread caused by four key microbes Bacillus spp, Staphylococcus spp, yeasts and moulds. The major sources of these bacteria were in the product ingredients mainly sweet cream buttermilk and skimmed milk along with environmental aerosols. The causative organisms were present in about 63% of the products and mainly ‘feed’ on the oil element of the recipe, containing high level C12 that generates the distinctive cheese flavour when broken down by bacteria.

The key hurdle factor in spread preventing microbial growth is water droplet size. The spread showing cheese off flavour had a droplet size distribution of 95% <10 micron. To achieve finer droplet size distribution, trial products were made in the Scrape Surface Heat Exchanger (SSHE) over the current churning method with a distribution of droplet size 98% <5 micron. The trial product showed a 50% reduction in the generation of the ‘cheese flavour’ methyl ketones. The Staphylococcus spp cross contamination source where from personnel with direct food contact processing area. Further education on personal hygiene helped to reduce the level of Staphylococcus spp contamination in the product. The trial product from the SSHE was further challenge tested with Listeria monocytogenes over a 10 week shelf life period to evaluate product robustness against microbial growth and spoilage. The organism did not show any growth over the period of time. The liquid phase of the emulsion was further modified with various salts at different concentrations and challenged with L. monocytogenes isolated from various parts of the dairy environment. It was observed that a pH range of 5.5 or lower with added 0.063% potassium sorbate showed significant antibacterial affect compared to the nutrient enriched MPC-broth and the unsalted liquid phase of the emulsion with no added potassium sorbate.

To understand L. monocytogenes survival within a dairy process, the organism was further challenged by exposure to pasteurisation heat treatments and the standard CIP cycle of acid and caustic treatment. No recovery rate of the organism was observed.

Therefore it could be concluded that the contamination within the industry is more likely to be post process or environmental contamination rather than survival through the plant itself as per RASFF alert of Listeria spp outbreak in dairy. Therefore, reducing the available water in the liquid phase of the spread and achieving a <5 μm droplet size and a finer distribution within the product will be limiting factors to microbial growth.

An air purifier system BAXX has reduced the level of environmental contaminants, especially yeast and mould.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Dodd, C.E.R.
Subjects:	Q Science > QR Microbiology > QR100 Microbial ecology
Faculties/Schools:	UK Campuses > Faculty of Science > School of Biosciences
Item ID:	28996
Depositing User:	Khan, Intisar
Date Deposited:	18 Sep 2015 09:00
Last Modified:	14 Oct 2017 08:35
URI:	https://eprints.nottingham.ac.uk/id/eprint/28996

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