Antifungal properties of dairy propionibacteria
Abstract: Today global agriculture is facing major challenges. The world's population is growing rapidly and we need to meet the increasing need for food with new strategies, including environmentally sustainable practices. Production must be enhanced with optimised resource utilisation and improved storage and distribution techniques. In addition, consumer demands for a reduction in artificial preservatives require innovative research thinking. The use of microorganisms as biopreservatives has many advantages. They often originate from environments similar to those where they will be used, making their presence natural. Bacteria such as lactic acid bacteria and dairy propionibacteria produce weak organic acids with good preserving capacity and several other antimicrobial compounds. Dairy propionibacteria possess additional advantageous features such as high production of vitamin b₁₂ and folic acid. Some strains also produce exopolysaccharides, useful for texture enhancement in dairy products. This thesis focuses on the antifungal properties of dairy propionibacteria. An important part of their antifungal capacity is due to the production of organic acids. The effect of these acids at different ph and different concentrations against eight spoilage fungi was investigated. A ph-dependent growth inhibition effect was detected, with Penicillium roqueforti being the most sensitive fungal species. In addition, dairy propionibacteria produce a number of other compounds with antifungal activity. Identification of 3-phenyl lactic acid and several antifungal peptides present in the growth medium was made after separation with a solid phase extraction column and hplc. The antifungal activity was monitored by screening against Aspergillus fumigatus. Strong enhancement of the antifungal activity from dairy propionibacteria in combination with glycerol was demonstrated. Addition of 500 mm glycerol to the upper layer in an overlay assay reduced the growth of some spoilage fungi completely. An antifungal strain of Propionibacterium jensenii was combined with Lactobacillus fermentum in a biopreserving culture used in moist grain preservation. The addition of glycerol to the system enhanced production of propionic acid in some cases, and an earlier decline in ph was detected. However, this combination of bacteria was not able to reduce the growth of P. roqueforti.
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