Propionibacteria and their role in the biological control of aerobic spoilage in silage

Abstract

Silage fermentation has been the subject of much research both on factors influencing the efficiency of preservation and biotechnological approaches to manipulate what is essentially an uncontrolled process. However, although the predictability of the fermentation and, consequently, silage quality has been improved, there is increasing evidence that high quality silages are extremely susceptible to spoilage by the activity of aerobic micro-organisms on opening the silo. The main biological approach to manipulate the fermentation has involved the use of homofermentative strains of lactic acid bacteria as inoculants to direct the fermentation towards lactic acid production and rapid pH decline and there is some evidence that this exacerbates the problem of aerobic spoilage. Several microbial groups have been implicated in spoilage including yeasts, moulds, acetic acid bacteria and Bacillus species. The consensus of opinion, however, is that the yeasts are in most cases the primary initiators of spoilage, with bacteria having an involvement depending on conditions. Species of the genus Propionibacteria have been singled out as potential silage inoculants to combat aerobic spoilage because propionic acid is a very effective antimycotic at the low pH of good silage when a high proportion of this acid is in the undissociated state. Several Propionibacteria species have been isolated from silage and a limited number of experiments carried out using them as silage inoculants, but the findings are equivocal. This paper reviews the area of silage fermentation, aerobic stability of silage and the potential of propionibacteria as biocontrol agents for silage. © Inra/Elsevier, Paris.