Thesis (PhD (Food Science))--University of Stellenbosch, 2007. / Thermal pasteurisation failures in the dairy industry have often been found to cause
end-products of poor quality and short shelf-life. Therefore, alternative methods to
eliminate microbial contaminants in raw milk are being studied. Ultrasonication is one
such non-thermal technology that could offer the dairy industry an alternative to
traditional pasteurisation.
The main objective of this dissertation was to evaluate the use of high-power lowfrequency
ultrasound (20 kHz, 750 W, 124 μm) applied in batch mode to eliminate a
selection of spoilage and potentially pathogenic microbes, commonly associated with
milk. These included Gram-positive and negative microbes, comprising of rods and
cocci, an endospore-former, and a yeast (Escherichia coli, Bacillus cereus,
Chryseobacterium meningosepticum, Lactobacillus acidophilus, Lactococcus lactis,
Listeria monocytogenes, Micrococcus luteus, Pseudomonas fluorescens and
Saccharomyces cerevisiae).
Three strains of E. coli (1 x 106 cfu.ml-1) tested, viz. ATCC 11775, a wild strain
from raw milk, and an O157:H7 strain from milk were sensitive to ultrasonication.
Complete elimination of viable cells occurred within 10 min. Viable counts of
P. fluorescens were reduced by 100% within 6 min of ultrasonication and
L. monocytogenes was reduced by 99.0% within 10 min. Lactococcus lactis was
reduced by 97.0% and M. luteus, B. cereus and C. meningosepticum by 88.0%, 87.0%
and 85.0% respectively. Lactobacillus acidophilus showed the most resistance to
ultrasound with only 78.0% of viable cells being eliminated. Under similar conditions,
S. cerevisiae was reduced by 99.7%. Microbial cell morphology, size and Gram status
did not necessarily influence the efficacy of ultrasonication. Sterile saline solution and
UHT milk were used as the suspension media, and the reputed protective effect of milk
fat was not observed under the parameters used in this study. A higher wave amplitude
(100%; 124 μm) was found to be more efficient in eliminating microbes than a lower
wave amplitude (50%; 62 μm). Pulsed-ultrasonication did not enhance the efficiency of
ultrasonication indicating that standing waves were absent.
Limited success was achieved by ultrasonication itself, and the long batch
treatment time (10 min or more) was found to be unrealistic for industrial
implementation. Hence the simultaneous application of ultrasound and heat (thermoultrasonication)
was examined. Thermo-ultrasonication proved to be more effective than either an ultrasonic or
heat treatment with all viable M. luteus cells being eliminated within 4 min (100%
amplitude at 72°C). Similarly, to eliminate E. coli and Lb. acidophilus from milk, only 2
min and 4 min thermo-ultrasonication was required, respectively. Bacillus cereus
endospores remained resistant and after a 10 min thermo-ultrasonic treatment only
78.04% were eliminated.
During this investigation both extensive surface (SEM) and internal (TEM) cell
damage caused by ultrasonication were observed in E. coli, Lb. acidophilus and
S. cerevisiae. Hence ultrasonication physically/mechanically damages these microbial
cells causing cell death/injury.
Microbial proteins and DNA released from cells into the environment after an
ultrasonic treatment was measured and an increase in released microbial proteins and
DNA was found to be indicative of a decrease in the number of viable cells, providing
that the initial cell concentration was high enough. It was, however, not possible to
correlate the concentration of released microbial proteins and DNA with the exact
number of viable cells eliminated, rendering it an ineffective quality indicator for the
industry.
Ultrasonication had no statistically significant influence on the protein, fat and
lactose content of both raw and pasteurised milk. The somatic cell count of raw and
pasteurised milk was found to decrease after ultrasonication. Unlike with heating,
activity of alkaline phosphatase and lactoperoxidase were not reduced by
ultrasonication. Hence neither enzyme can be used to indicate a successful ultrasonic
treatment of milk.
This study has demonstrated that ultrasonication offers a viable alternative to
pasteurisation as it is effective in eliminating microbes, and does not alter native milk
components. However, to attain a more effective killing, thermo-ultrasonication is
recommended for the treatment of milk to be used for the production of different dairy
products.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1163 |
| Date | 12 1900 |
| Creators | Cameron, Michelle |
| Contributors | Britz, T. J., McMaster, L. D., University of Stellenbosch. Faculty of Agrisciences. Dept. of Food Science. |
| Publisher | Stellenbosch: University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 23028312 bytes, application/pdf |
| Rights | University of Stellenbosch |
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