by André Meeusen, application manager and Yvonne van der Horst, technical manager, Kemira ChemSolutions b.v., The Netherlands
Moulds are ubiquitous and unavoidable contaminants in all animal feeds. Virtually all animal feeds contain moulds and viable mould spores which continue to pose a threat to grain quality.
Moulds are fungi which are distinguished by the formation of mycelium (a network of filaments or threads), or by spore masses. Conditions that favour moulds include moisture levels higher than 12 percent, warm temperatures, the presence of oxygen, and prolonged storage time.
Many moulds are toxigenic and produce mycotoxins, a secondary metabolite created by moulds that is toxic to organisms other than the mould itself. The growth of moulds and production of mycotoxins by these moulds in feed ingredients can cause significant economic losses. They consume valuable feed nutrients such as vitamins and amino acids and they convert energy into water and CO2 and can cause temperature increase as a result of their carbohydrate metabolism (see below)
C6H12O6 + 6 O2 => 6 CO2 + 6 H2O + Heat
Mould growth depletes the nutrient density and affects feed palatability and consequently decreases feed intake. Moulds cause lipid oxidation and pigment deterioration and are detrimental to animal health, performance and reproduction. The most frequently found mycotoxins are aflatoxins and ochratoxin produced by Aspergillus, the latter are also produced by Penicillium and zearalenone and trichothecenes produced by Fusarium moulds.
Toxin binders vs. organic acids
Animal feed is susceptible to mould growth. The microbiological quality of feed is a comparatively unexplored area but is receiving more attention due to the recognition of mycotoxins as a widespread economic threat.
The use of toxin binders in feed is a widespread application, however, this is not an efficient way to tackle the problem as they damage the nutritional quality of feeds. These toxin binders, which usually contain different types of clay minerals, are not that efficient and may even compromise nutrient digestibility. Moreover, mycotoxin contaminated feeds can impair the animals’ health and productivity due to loss of appetite, feed refusal, allergic reactions, reproductive failure, suppression of the immune system and even mortality.
Contamination by moulds and consequently, the production of mycotoxins, can be greatly reduced by using organic acids as inhibitors. Organic acids effectively inhibit the growth of moulds, yeast and bacteria in different types of feedstuffs and prevent recontamination after production of the compound feed. This extends shelf life, maintains nutritional value, and prevents formation of mycotoxins.
Mould and moisture
Development of moulds in feed depends on the interaction of several factors, including the presence of spores, the availability of nutrients, storage time, temperature and moisture. Water activity, i.e. the presence of free water, is the most important factor in the growth of moulds. Indeed, microbial spoilage of food and feeds occurs at different levels of moisture and the water activity (aw) concept describes the water available for microbial growth.
Most feed mills optimise or maximise moisture levels during feed production to compensate for losses that occur during grinding, pelleting and cooling processes. Moreover, a sufficient moisture level reduces the energy usage during the pelleting process and results in better pellet quality. The drawback of increasing moisture levels is that increasing levels of free water creates ideal conditions for rapid mould growth and the development of mycotoxins. Moulds and yeast grow at aw > 0.75 and aw > 0.85 respectively.
Products that protect feeds against mould growth and at the same time lower the aw are based on calcium or sodium propionate. Propionic acid is reacted with calcium or sodium to produce a salt with high propionic acid level, 76-78 percent depending on the salt form. The acid is completely buffered, has a good solubility, is safe to use and easy to handle. Their efficiency in shelf life extension when used in feeds is dose related and can be easily demonstrated by an in vitro accelerated method, increasing moisture content and storage temperature, by measuring the CO2 production over time. An efficient preservative effect is obtained up to the moment that CO2 production starts to increase (Figure 1).
Animal feeds generally contain mould spores which originate from raw materials used. The three genera of moulds – Aspergillus, Penicillium and Fusarium cause most cases of mycotoxin contamination in many grains and their byproducts and in vegetable proteins. Their optimal growth is mostly influenced by temperature and this determines their global presence.
Aspergillus and Penicillium species will grow better in warmer-tropical climates whereas Fusarium moulds prefer cooler temperate climates. Moulds are obligate aerobe and their proliferation can thus be controlled by oxygen free storage, such as silage. They consume carbohydrates and provoke fat hydrolysis leading to nutritionally low quality grains. Organic acids are known in the feed industry as an effective and affordable tool to control mould growth in grains and their byproducts during transport and storage.
Anaerobic preservation of grains usually applies when moisture is very high, from 25 – 45 percent. Grains are crimped before ensiling with formic acid based products.
Aerobic preservation is usually done with whole grains with moisture content between 15 – 25 percent. Typically, blends of different acids or acids with other active compounds are used, with propionic acid being the principle active component. The level of propionic acid needed under local conditions depends on kernel quality, initial mould counts, storage conditions and time.
Straight vs. buffered acids
The mechanism of inhibition of growth of moulds by organic acids is generally not considered a pH phenomenon. It is the propionate ion or radical (CH3CH3CO0-) that is the active mould inhibiting ingredient in propionic acid, so attempts have been made to use salts of propionic acid to overcome the odour and corrosion problems.
To enter into the mould cell, the acids have to pass a double barrier, the cell membrane and the outer cell wall of these moulds. Inside the moulds the organic acids dissociate decreasing the intracellular pH and compromising the cell metabolism. The three-dimensional structure and the lipophilic character of propionic acid seems to play an important role for the acids to pass through this double barrier.
Kemira has developed several mould control products containing appropriately buffered acids avoiding the typical drawbacks of straight acids. The organic acids in the liquid Kemira Mould Control product range are buffered with ammonium or sodium, ensuring reduced corrosivity and volatility and a long lasting preservation effect.
Ammonium buffering has the advantage of delivering a proton H+ supporting a more effective mould inhibitory effect. The inclusion of a lipophilic compound assures a better surface contact with grains and an easier penetration into meals and feeds. It will also improve the water binding capacity of feed materials and lower the water activity of feeds. This extends the shelf life, maintains the nutritional quality, and prevents the formation of mycotoxins in feeds and feedstuffs.
Formic acid based mould inhibitor
Traditionally propionic acid is used against mould and mycotoxin formation. The relative shelf life depends to a great extent on the propionic acid content. Indeed, the lowest survival rates for Fusarium spp. and Aspergillus niger were achieved with the highest actual propionic acid contribution, irrespective of the type of mould inhibitors tested.
As the leading global producer of formic acid-based products for the animal feed industry, Kemira has developed a new liquid mould inhibitor for grain preservation based on formic and propionic acid with an excellent ammonium-sodium buffering system to minimise volatility and corrosivity and ensuring proper handling properties. It is activated by lipophilic compounds. Formic acid does not have this lipophilic characteristic but it contains the highest antimicrobial properties as this is the smallest molecule of all the organic acids and has a > 60 percent higher number of active organic radicals per kilogram of pure substance.
The efficacy of such a novel ammonium-sodium buffered formic acid based product (Kemira Mould Control LF1) on reduction of Aspergillus niger in grinded whole wheat grains was assessed in a laboratory study done in the Kemira R&D center in Espoo-Finland and compared to ammonium buffered propionic acid (Kemira Mould Control LP1 NC). Figure 2 shows that the initial inoculation with Aspergillus niger in grains without preservatives resulted in significant growth during the first week. Both Kemira Mould Control LP1 NC and Kemira Mould Control LF1 at 0.2 w/w-%, inhibited growth of Aspergillus niger in grinded whole wheat grains over a 2 weeks period and total kill off was obtained with both products at 0.4 w/w-%.
The same laboratory tests have been repeated in Wessling Laboratories. Also here the inclusion of 0.4 percent with both products showed to be efficient in killing off both types of moulds during a two weeks incubation period. The Fusarium moulds seemed somewhat more sensitive to the formic acid based mould inhibitor with total kill off at seven days already (Table 1).
The efficacy of Kemira Mould Control LF1 was further assessed in a simulated field trial at Wageningen UR, The Netherlands. The trial measured mould and temperature development of fresh harvested grains during a four month storage period at ambient temperature and results were compared to Kemira Mould Control LP1nc.
The treatment with the formic acid based Kemira Mould Control LF1 at 0.7 percent showed the biggest effect in preventing the temperature to increase. It was significantly (P<0.05) lower than both the positive control (propionic acid based) and negative control. Moreover, results showed that Kemira Mould Control LF1 at 0.4 percent is as effective in inhibiting mould growth and preventing temperature increase in freshly harvested grains with high moisture content (17.6%) as the propionic acid based Kemira Mould Control LP1nc. Mould growth was reduced from 5 log platable fungal colonies per gram in the non-treated grains down to 1.34 log/gram and 1.15 log/gram respectively.
This was also confirmed in a field trial in UK, monitored by Aberystwyth University in which freshly harvested wheat with a moisture content of 17.9 percent was rolled and treated with different dosages of the ammonium-sodium buffered formic-propionic acid based product or with the ammonium buffered propionic acid only and stored for eight months in 10 ton bins.
At nine litres per ton of grains, both products allowed easy storage without any temperature increase in the bins during the eight months. Initial mould counts at three weeks showed both products to be effective, compared to an untreated sample.
This makes Kemira Mould Control LF1, a formic acid based blend with a novel ammonium – sodium buffering and activated by lipophilic compounds, an interesting alternative to the traditional propionic acid based products, assuring the fast killing of any mould that appears on grains and assuring a save and long preservation.
Kemira ChemSolutions b.v.