Effect of spices on food borne pathogens during African milk fermentation
dc.contributor.advisor | O'Gara, Liz | |
dc.contributor.author | Ogwaro, Betty | |
dc.date.accessioned | 2024-10-28T15:02:52Z | |
dc.date.available | 2024-10-28T15:02:52Z | |
dc.date.issued | 2024-04 | |
dc.identifier.citation | Ogwaro, B. (2024) Effect of spices on food borne pathogens during African milk fermentation. University of Wolverhampton. http://hdl.handle.net/2436/625749 | en |
dc.identifier.uri | http://hdl.handle.net/2436/625749 | |
dc.description | A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy. | en |
dc.description.abstract | Recent studies have highlighted an overall consumers’ trend towards less processed and the use of natural antimicrobials for food preservation. There are no studies however on the effect of spices on foodborne pathogens in fermented milk. in this study, the antimicrobial effect of spices and essential oils when combined with milk fermentation acids and/or with each other on food-borne pathogens was evaluated with the aim to find new ways to preserve traditional fermented milk using natural products. It is often assumed that acidic foods such as fermented milk products are intrinsically safe due to their low pH (high acidity), however there are evidences that pathogens do survive in these products. To establish the status of microbial risks associated with the traditional African milk fermentation, the microbiological quality of typical African traditional fermented milk was assessed. The results showed that the traditional fermented milk products indeed have low pH (2.9-3.6) but contained a rich microbial diversity (22 different types of microorganisms according to colony types and Gram stains and biochemical reactions). Irrespective of the country or region collected from, the fermented milk products were dominated by lactic acid bacteria (35%). Yeasts and moulds comprised of 9% with high cell counts (107-1010 CFU mL-1) Pathogenic bacteria such as Escherichia coli, Staphylococcus aureus and Bacillus cereus and other species were common (11%) comprising of 103-106 CFUmL-1, above the recommended safe levels. The impact of fermentation temperature on lactic acid and the test bacteria (E. fergusonii, S. Typhimurium, S. aureus or L. monocytogenes) was observed when milk was fermented at the traditional (25-30oC) or industrial (43oC) temperatures with the test bacteria. The pH of the fermented milk declined from the initial 6.68 (±1) to pH 4.1-4.4 at 43oC and to pH 5.4-5.6 at 25oC after 24h of fermentation. This was reflected in the viable cell counts of the test bacteria which was lower in the samples with the test bacteria in milk fermented at 43oC (106-7 CFUmL-1) than in milk fermented at 25oC (108 CFU mL-1). Growth of LAB were not affected as the cultures increased from 106 to above 109 CFUmL-1 the levels required for milk fermentation. The antimicrobial activities of whole unground spices (clove buds, cinnamon bark, cardamom, cumin, black pepper, and red pepper) were assessed individually at 1-4% (w/v) the levels traditional farmers add in fermenting milk, and incubated at 25, 30, 37 or 43oC for 24h and stored for 144h. The test bacteria increased to from 105 to 108 CFUmL-1 after 24h of fermentation and each LAB bacterium increased from 106 to 109 CFUmL-1 indicating that the quantity of the spice added was not enough to restrict growth of the tested bacteria. An antimicrobial study was carried out with the methanol extracts of clove buds (CL), cinnamon bark (CNN) or black pepper and their essential oils (eugenol, cinnamaldehyde and piperine) individually at the concentrations ranging from 0-1% at a double increment in combination with lactic fermentation milk against E. fergusonii, S. Typhimurium, S. aureus or L. monocytogenes. The milk was incorporated with 1% (v/v) of Lactobacillus delbrueckii (approx. 106 CFUmL-1 each) and incubated at 25 or at 43 °C for 24 h and subsequently, the fermented milk was stored at 25°C for 144 h. (pre-fermentation contamination). Another set was contaminated post fermentation of the milk. Results showed that cell counts of Gram-negative bacteria (E. fergusonii and S. Typhimurium) in milk fermented with clove extracts were approx. 107 CFUmL-1 1-log unit higher than those observed in samples containing Gram positive bacteria (S. aureus or L. monocytogenes) suggesting that G+ve bacteria were more affected by the spice extracts. When CL, CNN, or BP were applied singly or in combination with each other at concentrations based on their minimum inhibitory concentrations as follows: BP alone; ¼BP + ¾CL/CNN; ½BP + ½ CL/CNN; ¾ BP + ¼CL/CNN; CL/CNN alone. results showed that during fermentation for 24h at 25°C all the test bacteria grew to a similar level (approx. 109 CFUmL-1) in samples with the combination of these essential oils, the test bacterium grew to a lower number of 106-107 CFUmL-1 only. During fermentation at 43°C, the test pathogens did not maintain their contamination levels during fermentation as they declined by approximately 2-3 log units depending on the concentrations of the spices. During subsequent storage at 25°C for 5 days resulted in undetectable levels of the bacteria in all the samples treated with the EOs and that were not recovered after 24 h of storage in samples containing CL combined with CNN or BP at different combinations. EOs showed stronger antimicrobial activities in fractional combination (Most combinations ¼Eu + ¾Ci; ½Pi + ½ Eu/Ci; ¾ BP + ¼Eu/Ci showed synergistic or additive interactions) of the essential oils. the combinations led to the total demise of the test bacteria within 4-12h of incubation. The inhibition was stronger against S. aureus, a Gram-positive bacterium and lower against E. fergusonii, a Gram-negative bacterium. The starter cultures were less affected by the concentration of the spice extract applied (0-0.125%) although the cell counts were lower in sampled containing 0.5-1% clove or eugenol extract (107 CFUmL-1) compared to the other concentrations (108 CFUmL-1) after 24h of fermentation. A mixture of spice EOs could be applied to successfully extend the shelf-life of fermented milk products. The combination of spices together with organic acids that are produced in fermenting milk by lactic acid bacteria, albeit in low amounts, may lead to a synergistic effect that renders pathogens susceptible to the combined action of organic acids. This study showed that when spice extracts were combined or incorporated with lactic milk fermentation and other inhibitory factors (low pH and high temperature) safe and good quality traditional fermented milk is maintained. Moreover, the strong aroma of the spice extracts can also be reduced to an acceptable level. Further work is needed to explore the use of combination of active constituents of clove, cinnamon, or black pepper. | en |
dc.format | application/pdf | en |
dc.language.iso | en | en |
dc.publisher | University of Wolverhampton | en |
dc.rights | Attribution-NonCommercial-NoDerivatives 4.0 International | * |
dc.rights.uri | http://creativecommons.org/licenses/by-nc-nd/4.0/ | * |
dc.subject | traditional African milk fermentation | en |
dc.subject | food-borne pathogens | en |
dc.subject | natural antimicrobials | en |
dc.subject | spice extracts | en |
dc.subject | essential oils | en |
dc.subject | temperature | en |
dc.subject | Escherichia fergusonii | en |
dc.subject | Salmonella typhimurium | en |
dc.subject | Staphylococcus aureus | en |
dc.subject | Listeria monocytogenes | en |
dc.title | Effect of spices on food borne pathogens during African milk fermentation | en |
dc.type | Thesis or dissertation | en |
dc.contributor.department | Faculty of Science and Engineering | |
dc.type.qualificationname | PhD | |
dc.type.qualificationlevel | Doctoral | |
refterms.dateFOA | 2024-10-28T15:02:53Z |