Jacobus Myburgh

Plasminogen is an inactive protease in milk, which can be activated to plasmin. Whole milk contains fat and in response to lipase activity, free fatty acids are released. Free fatty acids are not known to act as an activator for plasminogen. The aim of this study was to determine whether commercial bovine plasminogen could be activated to plasmin by commercial free fatty acids present in milk, and also to determine if plasminogen activators to plasmin are present in abnormal milk using milk agar plates and the Merck protease assay. Fatty acids and activators present in abnormal milk had the ability to activate plasminogen to plasmin, which could, after weeks, harm milk protein stability during extended storage time (ultra-high temperature milk). The milk agar plate technique containing plasminogen could detect plasmin activators present in abnormal milk. This can act as an indicator of high-risk milk being used within the ultra-high temperature milk industry.

Staphylococcus species are amongst the bacteria that cause bovine mastitis worldwide, whereby they produce a wide range of protein toxins, virulence factors, and antimicrobial-resistant properties which are enhancing the pathogenicity of these organisms. This study aimed to detect Staphylococcus spp. from the milk of cattle with subclinical mastitis using MALDI-TOF MS and 16S rRNA PCR as well as screening for antimicrobial resistance (AMR) and virulence genes. Our results uncovered that from 166 sampled cows, only 33.13% had subclinical mastitis after initial screening, while the quarter-level prevalence was 54%. Of the 50 cultured bacterial isolates, MALDI-TOF MS and 16S rRNA PCR assay and sequencing identified S. aureus as the dominant bacteria by 76%. Furthermore, an AMR susceptibility test showed that 86% of the isolates were resistant to penicillin, followed by ciprofloxacin (80%) and cefoxitin (52%). Antimicrobial resistance and virulence genes showed that 16% of the isolates carried the mecA gene, while 52% of the isolates carried the Lg G-binding region gene, followed by coa (42%), spa (40%), hla (38%), and hlb (38%), whereas sea and bap genes were detected in 10% and 2% of the isolates, respectively. The occurrence of virulence factors and antimicrobial resistance profiles highlights the need for appropriate strategies to control the spread of these pathogens.

Neste estudo, o tempo de fermentação do iogurte foi acelerado usando a enzima proteolítica Flavourzyme para produzir peptídeos suficientes. Flavourzyme foi adicionado diretamente ao leite utilizado para fermentação em diferentes concentrações (0,1 g/L, 0,01 g/L e 0,001 g/L) e antes do tratamento térmico (90˚C por 15 min), onde o Flavourzyme foi inativado. A libertação de peptídeos durante o pré-tratamento com Flavourzyme foi confirmada pelo perfil peptídico de RP-HPLC. Foi possível acelerar o tempo de fermentação em 13,33% com Flavourzyme. O iogurte feito com leite pré-tratado Flavourzyme não apresentou alterações significativas na qualidade. No entanto, os consumidores conseguiram detectar uma diferença entre o iogurte feito com leite pré-tratado Flavourzyme e o iogurte feito com leite não tratado (controle). O iogurte feito com leite pré-tratado Flavourzyme obteve menor pontuação em sabor, textura, aparência e aceitabilidade geral, mas geralmente não foi rejeitado. Tempos de fermentação acelerados podem levar a vantagens econômicas na indústria de produção de iogurte.

The aim of this study was to determine the effect of synthetic conjugated linoleic acid (CLA) supplementation on the quality of yoghurt. Yoghurt was prepared by adding CLA to create yoghurt with 0%, 25%, 50% and 100% w/w of the recommended dietary allowance for CLA. CLA inclusion had no effect on the viability of the yoghurt lactic acid bacteria. Physical properties such as viscosity and pH increased with increased CLA levels. CLA inclusion had a significant effect on the fatty acid composition of the yoghurt. Yoghurts from all four treatments were found acceptable by a consumer panel.

Water soluble autolysate of yeast, usually utilised for microbial growth support, was used as additive in yoghurt fermentation. The yeast extract (YE) resulted in a decrease of fermentation time by 21% to reach a pH of 4·6. However, the YE resulted in unacceptable flavour and taste. By size exclusion chromatography, a fraction of the YE was obtained that could account for the observed 21% decrease in fermentation time. The fraction contained molecules of low molecular weight, consisting of minerals, free amino acids and peptides. The acceleration of the yoghurt fermentation was ascribed to the short peptides in the fraction. It is proposed that the application of this extract in industrial yoghurt manufacture would result in savings for both the industry and the consumer.

A new xylanase producing yeast species was isolated from South African soil. A description of the new species, Candida mokoenaii is given.

Both beta-xylanase and beta-xylosidase were purified to homogeneity from a xylose-grown culture of Aureobasidium pullulans. Cellular distribution studies of enzyme activities revealed that beta-xylanase was an extracellular enzyme, during both the exponential and stationary phases, whereas beta-xylosidase was mostly periplasmic associated. The beta-xylanase exhibited very high specificity for xylan extracted from Eucalyptus grandis dissolving pulp, whereas the beta-xylosidase was only active on p-nitrophenyl xyloside and xylobiose. Comparison of kcat/Km ratios showed that the beta-xylanase hydrolyzed xylan from dissolving pulp 1.3, 2.1, and 2. 3 times more efficiently than Eucalyptus hemicellulose B, Eucalyptus hemicellulose A, and larchwood xylan, respectively. The beta-xylosidase exhibited a transxylosylation reaction during the hydrolysis of xylobiose. When applied on acid sulfite pulp, both enzymes released xylose and hydrolyzed xylan to a different extent. Although beta-xylosidase (0.4 U/g pulp) liberated more xylose from pulp than beta-xylanase (4.7 U/g pulp), it was responsible for only 3% of xylan solubilization. Treatment of pulp with beta-xylanase liberated 51.7 microgram of xylose/g and hydrolyzed 10% of xylan. The two enzymes acted additively on pulp and removed 12% of pulp xylan. A synergistic effect in terms of release of xylose from pulp was observed when the enzyme mixture of beta-xylanase and beta-xylosidase was supplemented with beta-mannanase. However, this did not result in further enzymatic degradation of pulp xylan. Both beta-xylanase and beta-xylosidase altered the carbohydrate composition of sulfite pulp by increasing the relative cellulose content at the expense of reduced hemicellulose content of pulp.