Mold growth in feeds occurs when desirable temperature, oxygen, unbound water, and nutrients are available. Mitigation of these four factors prevent mold growth. Management of feed that accounts for these elements is essential. The use of organic acids combination as a mold inhibitor is a widely practiced tool for affecting successful microbiological control feed. Cooling of feed is an important step for ensuring feed safety. Cooling must be done as soon as the pellets leave the die. An efficient cooler should be able to reduce the product temperature within 5-8°C of ambient temperature. As the air is forced through the pellet bed, it is heated, thus increasing its capacity to pick up moisture. The hotter air then removes moisture that has condensed on the surface of pellets. The process of removing moisture with unsaturated air is known as evaporative cooling. When non-saturated air is passed through, it picks up the moisture and at the same time cools pellets.
Nutritional and toxicological implications of microbiological changes in feed ingredients and feeds have begun to be recognized as a critical part of the livestock nutrition. Mold growth is perhaps one of the most important microbiological changes in that it can degrade the nutritional content of the feed and create mycotoxins.
Even though the exact economic impacts of mold growth in feed have not been quantified, progressive farmers and feed manufacturers have discovered that when mold growth is controlled, the microbiological quality of feed increases and production benefits can be realized . The first step in designing a mold control strategy is to understand the basic requirements of mold for growth. Absence in one of these requirements effectively stops mold respiration, thereby, preventing nutritional damage or toxin production by the mold.
Nitrogen and Energy:
Source of nitrogen and energy are most obvious requirements for mold growth. Prevention of mold growth on intact grain relies heavily on this strategy. Intact grains physically protect the nitrogen and energy resources of the seed through use of a cellulose or a polyester covering outside of the seed. If this covering remains intact, mold growth occurs slowly. The shelling of grain especially corn, partially removes this physical barrier, because corn kernels are normally buried in the cellulose barrier of cob.
Second requirement for mold growth is the correct environmental temperature. Temperature at which feed or raw material is stored often dictates whether mold growth will become a significant problem. Mostly, molds grow at a temperature range of 10-40°C. Optimum temperature required for mold growth is 25-35°C. Aspergillus and Penicillium species flourish in warmer temperature, whereas, Fusarium mold prefers cooler temperature.
Image Adopted from: Christensen, C. M., ed. 1982. Storage of cereal grains & their products. Page 1 in Am. Assoc. Cereal Chem., St. Paul, MN.
Mold are obligate aerobic organisms; therefore, their proliferation and growth can be controlled by providing oxygen-free storage. Although this control strategy is not practical for most feed ingredients, it is a principle tool in preservation of silage. Mold can respire successfully at oxygen concentration as low as 4% (5).
Moisture levels in finished feeds have long been an important quality parameter. However, gross moisture content is not the controlling factor in mold growth. True parameter is the actual amount of water in a material that is available to support growth. If the water present is not available, then mold growth cannot occur. Essentially, water activity (aW) is a measure of the degree to which water is bound within a feed and will not be available for further chemical or microbiological activity.
- Water activity (aW) is an important property in feed. Most chemical reactions and microbiological activity are controlled directly by the water activity.
- Value of Water activity (aW) varies from 0-1.
- Value of water activity (aW) below 0.6 is safe from mold & bacterial growth.
Moisture migration within grains is another source of free water. When grains, feed, or both are warmer or colder than surrounding ambient conditions, moisture migrates & condenses because of the air convections within the bags. Thus, even though feed or grain may be stored at a safe moisture, mold growth can occur when condensation of moisture happens.
Mold needs pre-formed organic molecules for their energy supply. Consequently, feed materials and manufactured feeds are highly acceptable substrates for mold growth.
There are basically four strategies to control mold growth in animal feeds
1. Use of edible chemical control agents
Many naturally occurring components of feeds such as organic acids (calcium salt, sodium salt or ammonium salt of propionate and formate), phenols (polyphenols, tannins), flavonoids, herbs and spices possess varying degrees of antimicrobial activity. Organic acids and their salts are well known inhibitors of a variety of hazardous micro-organisms. When choosing a mold inhibitor, it is important that each individual organic acid will have a different effect in controlling mold. A mindful blending of organic acids ensures broad spectrum and highly effective mold inhibitor for feed.
2. Optimize cooler efficiency:
Coolers are designed to extract heat and surplus moisture created during pelleting, thereby, increasing the strength of the pellet. The temperature of pellets leaving a cooler should be not more than 4 to 5°C above ambient temperature. The amount of heat & moisture removed depends on the parameter (Kansas State University) as below.
- Retention time in cooler
- Pellet size
- Initial pellet temperature
- Porosity and fat content of pellet
- Air temperature, relative humidity, and flow rate
- Higher relative humidity means the air can absorb less moisture. Longer retention time is required to dry.
- Ingredients can affect the cooling and drying rate. Fat, particularly that added in the batch mixer, and molasses inhibit drying because they make it difficult for the moisture to migrate at the surface of the pellet.
- Maintenance quality of feed:
Animal feed is asemi-perishable item. Like the bread from the local grocery store or the cheese keptin the refrigerator, the animal feed gets spoiled when stored for a longer period. Groceriescan be purchased from the store every week, but this is not practical oreconomically feasible when purchasing large quantities of animal feed. Thequestion, therefore, is how the shelf life of animal feed can extend overseveral months period. When it is discussed about shelf life, it is important tounderstand what happens to feed under long term storage. The problemsencountered with storage or feed fall into four major categories.
4. Maintenance of storage condition:
A. Bagged Feed:
- Store feed in a cool, dry, and well-ventilated area.
- Rotate stock to use old feed first. “First in, first-out” principle.
- Keep bags stacked neatly on pallets to prevent feed from being in direct contact with damp floors.
- Stacking height should not be more than 10 bags in summer & winter season, whereas, in the rainy season, stacking height should not be more than 8.
- High stacking creates continuous high pressure on the bottom side of bags stacked, due to which high-pressure lumps formation occurs and leads to mold growth.
- Bags should be stacked to allow at least 18 inches between walls and upright supports. This allows for the cleaning and placement of traps/ bait boxes. This also prevents condensation on walls from damaging feed and permits necessary airflow around the bags.
- Maintain different types of feed separately and clearly marked. Be particularly careful not to mix bags of medicated and non-medicated feed together.
- If receiving skids of feed is in plastic wrap, remove the wrap before storing feed in the warehouse. This allows better airflow around the product and helps prevent mold problems.
- Clean up spilled feed immediately and remove torn bags as soon as possible. A good housekeeping policy is the basis of the mold control program.
- Eliminate poor drainage areas that serve as breeding grounds for most insects.
- Do not handle bags more than necessary and handle with care. Pelleted diets are designed to be durable, but they are not indestructible. Offensive handling will increase the dust level in the feed which results in poor water quality and loss to the farmer.
B. Bulk Feed:
- Bin design: Bins should be designed to empty out completely and maintain airflow through the bin, preventing condensation.
- Inspect bins regularly for leaks and repair immediately.
- Allow bins to empty out completely between loads. Many bins have “dead” areas where old feed and dust can accumulate and spoil if the new feed is continually put in on top of old.
- Clean inside of bins regularly, removing encrusted material which acts as mold and insect growth areas.
- Bins can be sealed and fumigated to kill insects.
The realization that mold growth destroys the nutrient content of feed and the possibility of mycotoxin development necessitate that a producer considers strategies for controlling mold growth. Limiting nutrient availability to microorganisms, storing feed at temperatures that do not encourage microbial growth, limiting oxygen, minimizing free water availability, and utilizing a good mold inhibitor and maintaining proper storage condition can ensure the microbiological integrity of the ration.
Note: Reference can be provided on the basis of personal requests.
by Sarwar Ali, Partha Das, Manab Samanta and Jai Prakash Pandey, Kemin Industries South Asia