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Functional Additives
Tuesday, May 25, 2004 3:35:00 PM
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Protection Of Feed And Grain From Mould Contamination

 
J.I.X. Antony, Ph.D, Research Manager & H.M. Tan, Ph.D, Research Director

Kemin Industries (Asia) Pte Limited

 

 

Mould inhibitors benefit the feed industry by increasing the microbiological stability of grains and feed ingredients (Figure 1). The main active ingredients of mould inhibitors are generally short chain organic acids with propionic acid being the most important. However, these short chain weak acids can be corrosive. Myco CURB® Extend contains propionic acid in combination with a plant-based ingredient. This formulation combines the established anti-fungal activity of propionic acid with the anti-microbial properties of a plant extract with the added benefit of corrosion protection.

 

Introduction
 

The widespread occurrence and multiplication of microorganisms in the environment leads to degradation of organic material including grain and feed ingredients. Cereals constitute an important source of energy and protein in animal diets. Managing raw materials under adverse climatic conditions is crucial for retaining acceptable feed quality. Grains can develop moulds that produce mycotoxin compounds either prior to harvest or while in storage. High moisture content in the grain promotes the development of mould and mycotoxin production.                                                                      Figure 1: Untreated corn (left) and corn treated with

                                                                                      a mould inhibitor (right)
An approved anti-mould agent is generally nontoxic,

noncarcinogenic and does not alter the identity of the product being treated. Propionic acid is Generally Recognised As Safe (GRAS) by the United States Federal Department of Agriculture (USFDA). While the use of propionic acid-based mould inhibitors has become widespread in many countries, it is well recognized that feed mill managers are sensitive to corrosive problems associated with the use of mould inhibitors. Corrosion is an oxidation process and is dependent on factors such as moisture, air and pH. pH is a critical parameter for a mould inhibitor formulation and a new mould inhibitor formulation (Myco CURB® Extend) has been designed to have a near neutral pH, a natural colour and a pleasant odour whilst retaining high efficacy. The product contains propionic acid in combination with a plant-based ingredient and did not produce any visible residue when testing for corrosivity. Hence this formulation combines the established anti-fungal activity of propionic acid with the anti-microbial properties of a plant extract with the added benefit of corrosion protection.

 

Antimicrobial Mechanism
 

The inhibitory effect of acids on microorganisms will depend on a range of factors including: dissociation constant of acid, molecular size of the acid, acid solubility in aqueous medium, concentration of acids, etc. Organic acids can inhibit the growth of microorganisms by different pathways: The energy needed for the microorganism to grow is derived from certain biochemical pathways. The transport of the nutrients required for energy production is influenced by the pH difference across the cell's membrane. The pH balance across the cell membrane can be influenced by the addition of acids. The acid may enter the cell in its undissociated form and then dissociate at the pH of the cell's cytoplasm. The cell will then try to eject the H+ ions produced in order to restore the pH balance across the cell membrane. This ejection process disrupts the coupling of the cell's energy production with the transport of the nutrients into the cell. Besides this disruption of nutrient transport into the cell, the increase in the cell's cytoplasmic pH due to the acid's dissociation and release of the H+ ion, will also inhibit the synthesis of certain molecules such as cell wall components and genetic material within the cell. The net effect is the cell expires due to progressive energy depletion.

 

Mould Inhibitor Formulations
 

Mould inhibitor formulations should be a balanced combination of various ingredients with each individual ingredient serving a specific purpose. An optimized mould inhibitor formulation must be judged on its physical properties, chemical properties, and efficacy. Items to be considered are total acid content, solubility, pH, dispersability, evaporation rate and efficacy.

 

It is preferable to have a formulation with a high propionic acid content. However, it is also desirable to use a product with a low risk of corrosion. It has been a challenge to the industry to achieve a high level of mould    

inhibition with a low corrosion risk. Kemin have always developed their mould inhibitor products by an innovative partial neutralization process to reduce the risk of corrosion. They have now developed a new improved mould inhibitor Myco CURB® Extend Liquid which has a new plant-based ingredient, that effectively combines improved mould inhibition with corrosion protection.

 

          Figure 2: Carbon dioxide production over time for untreated corn and corn treated with mould inhibitors

 

 

Carbon Dioxide Test: The carbon dioxide test is an accelerated microbial stability test designed to measure the efficacy of mould inhibitors. More carbon dioxide production indicates more mould growth. An efficient mould inhibitor will produce less carbon dioxide in this test and protect the samples from mould. The figure below (Figure 2) shows the results of a carbon dioxide test done on Myco CURB® Extend.

 

The "Plate Diffusion Test" has sometimes been suggested to test for the efficacy of mould inhibitor formulations. In this test the area of inhibition is measured with a larger area equating to a better mould inhibition. This test method is, however, considered at best a qualitative method because the zone of inhibition is influenced by other factors such as the rate of diffusion of the active molecules, type of microorganism tested, nutrient medium and other variables. For these      Figure 3: Partial immersion corrosion test of mould

reasons, the results of the test may not necessarily           inhibitor formulations

reflect the true efficacy of the molecules and need to be

interpreted with some caution.

 

Corrosion Test: The corrosiveness of mould inhibitor formulations can be tested by partial or full immersion methods.  The partial immersion method is a quantitative method. This method measures the weight loss from a metal plate after partially immersing it in formulations for 11 days at 65 ºC. High weight loss indicates a higher corrosive formulation. Figure 3 shows the pictures of the test samples before and after the test period.


The full immersion corrosion test (also called "Nail-in-Mould Inhibitor Test") is a qualitative test. This test does not involve any instrument and can be easily carried out.  A metal nail is immersed in the liquid mould inhibitor at room temperature with continuous observation for any residue deposition.  The appearance of residue is an indication of corrosion. Figure 4 shows a nail in the Myco CURB®  Extend formulation after 10 days.  The solution remained clear with no visible corrosion of the nail. 

                                                                                                                     Figure 4: Myco CURB® Extend

Corrosion is an oxidation process where the metal ion combines with oxygen       after 10 days of the "Nail-in- 

derived from a plant source has also been included in from the surroundings to    Inhibitor-Test"

form the metallic oxides. The factors affecting the corrosion process are the pH

of the medium and oxygen. Therefore, the corrosion process may be controlled by adjusting the pH of the medium and also by quenching the available oxygen from the system. The corrosion process rate is minimal at near neutral pH. Myco CURB® Extend is formulated to a near neutral pH (pH 6). An antioxidant ingredient the Myco CURB® Extend formulation. The inclusion of this innovative ingredient helps in two ways to reduce the corrosion process:

 

Action 1: Reduction of the oxidation process due to the antioxidant property. This novel, natural ingredient is a known antioxidant.

 

Action 2: Reduction of the localized corrosion process due to better dispersion. This novel ingredient has a low surface tension and a high diffusion property. This leads to better spreadability of the formulation across surfaces. This property prevents pitting (localized corrosion) on metal surfaces which can accelerates the corrosion process.                                                                   

 

Evaporation of the Mould Inhibitor Formulations: Mould are propionic acid & mould inhibitors expected to have

prolonged action against mould. This test is a quantitative method. The mould inhibitor formulations are allowed to stand in open air at different temperatures and the weight loss is measured over a period of time. Figure 5 shows the results of the evaporation test. The results indicate that Myco CURB® Extend has a particularly low evaporation rate. This would contribute to prolonged action against mould when Myco CURB® Extend is applied to grain or feed.

 

 

 

              Figure 5: Evaporation rates of water, inhibitor formulations with a low evaporation rate


Surface Tension: The surface tension of a liquid determines its degree of spreadability over a solid. Liquids with less surface tension are expected to spread more evenly over a solid particle. The comparison of mould inhibitors for this spreadability can be done by diluting them in water and measuring the reduction in the water's surface tension. Lower water surface tension is an indication of better dispersion and spreading of the mould inhibitor over the grain. From Figure 6 it can be seen that addition of Myco CURB® Extend reduced the surface tension of the aqueous solution. This is an indication that Myco CURB® Extend can spread effectively over grain and feed to enhance protection against mould growth.

 

 

          

           Figure 6: Surface Tension of Water with Mould Inhibitor Formulations

 

 

 

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