Anaerobic Bacteria
Anaerobic spore-forming bacteria are a potential threat to contaminate raw milk because of the spores' resistance to heat and other adverse environmental conditions. The source of most spore-forming bacteria is soil contamination of forages.
Whether a spore population will increase during ensiled storage is dependent on the specific bacteria and the micro-environments that exist in the silage mass.
It is known that spores consumed by cows will pass through the digestive tract unaffected and will be excreted in the manure. Manure-fertilized crops or dirty bedding are possible sources of contamination (Driehuis, 2013).
Clostridium species can be segmented based on their protein and carbohydrate fermentation properties. Clostridia sporogenes are the predominant proteolytic clostridia found in silages and can ferment both protein and carbohydrates. Clostridium butyricum can ferment a wide range of carbohydrates but are unable to utilize proteins. Clostridium tyrobutyricum can ferment some limited carbohydrates but are the classic "butyric acid" bacteria in their ability to ferment lactic acid to acetic acid and butyric acid at a low pH (Driehuis, 2013).
C. tyrobutyricum can be transferred from silage to milk and can grow equally well in the low-pH, low-water activity, lactic acid-rich, nitrate-poor environments of either silage or cheese. Their potential to produce off-flavors and gas formation that leads to texture defects is why silage feeding is prohibited in certain regions of Europe where milk is destined for making hard cheeses (Driehuis, 2013).
The pathogen Clostridium botulinum (botulism) is rarely found in silage since it is much more sensitive to low pH than C. tyrobutyricum. It usually is not a contaminant unless the silage is contaminated with carcasses of birds or small mammals or the crop was fertilized with poultry manure, which is notorious for containing C. botulinum spores (Driehuis, 2013).
European research shows that concentrations of butyric acid bacteria spores in fresh crops vary with the level of soil contamination from 10 to 100 spores per gram. Data from the Netherlands show that the average concentrations in corn silage are about 0.5 log units lower than in grass, likely due to less soil contamination during harvest and the low buffering capacity, rapid lactic acid formation and low pH (3.8-4.0) in corn silage that discourages the growth of C. tyrobutyricum (Driehuis, 2013).
European researchers now think that increased concentrations of butyric acid bacteria are related to aerobic instability problems rather than anaerobic instability caused by insufficient pH decline during the initial fermentation that allowed the growth of C. tyrobutyricum. This is because more recent investigations show that high spore counts are from sections of the silage where yeast and molds were already actively increasing the silage temperature and pH.
The growth of strictly anaerobic C. tyrobutyricum in aerobically deteriorated sections of the storage structure is likely due to the various aerobic and anaerobic micro-environments that can exist simultaneously in the silage mass.
In the cascade of events leading to aerobic instability, acid-tolerant, lactate-assimilating yeast grows slowly as oxygen penetrates the silage face (Feedstuffs, Dec. 10, 2007). As the yeast population increases, their consumption of oxygen also increases, resulting in anaerobic niches with increased pH where C. tyrobutyricum can thrive (Driehuis, 2013).
