Abstract:
Presently, thermal treatment is widely applied in the dairy industry to preserve and extend milk
products' shelf life, including pasteurization and ultra-high temperature (UHT) treatment. Each
thermal treatment method is unique, rendering different microbiological grades and storage
conditions depending on the time-temperature combination during the treatment. However, the
resurgence in demand from consumers for more fresh and improved quality and nutritious food
products has prompted researchers to investigate the potential of emerging technologies as an
alternative to the current thermal treatments.
Therefore, the main objective of this study is to achieve microbial inactivation at a reduced
temperature or shorter treatment time by a combination of effects to attain minimally impacted
treated milk products. The first part of this study investigates milk sterilisation using pulsed electric
field (PEF) processing in combination with heat with the optimised operating parameters. Results
showed 4.02 and 3.37 log reductions in skim milk at 300 Hz, 28 kVcm-1 and 100 Hz, 50 kVcm-1,
respectively, at an operating temperature of 115°C. As the microbial inactivation kinetics is associated
with the residence time in a particular system, the residence time distribution (RTD) in the PEF unit
was determined. Findings presented that the mean residence times obtained experimentally in all five
flowrates tested are slightly more significant than the calculated residence time.
Further, the toxicological aspect of food safety is an essential criterion in human acceptance,
prompting an investigation of electrode corrosion occurrence in the PEF unit. Experimental results
presented all metal elements studied obtained at least ten times lower than the maximum allowable
values for human consumption set by the health standards. In the last chapter, this thesis also focuses
on the application of ultra-high-pressure homogenisation (UHPH) processing to study its efficacy in
microbial spore inactivation and the shelf-life stability of raw milk in a refrigerated condition. Log
reductions of 1.368 and 0.864 were achieved at valve temperatures of 129 °C and 127 °C for both
bovine and goat milk, respectively, at 250MPa. Additionally, raw sheep milk reached at least 36 days
of total bacterial count under the safe limit in a refrigerated condition after being UHPH-treated at
250MPa and 118 °C. Minimal variation in pH and conductivity of UHPH-treated sheep milk were
observed throughout the 36 days period.
The findings from the research are expected to be the primary basis for an in-depth investigation of
novel technologies for sterilisation and extended shelf-life application in the dairy industry to
complement the current thermal treatment technology.