Synopsis: Understanding the role of water activity in maintaining the vital nutrients and protecting it from the microorganisms will be probably the most significant advancement made by poultry feed manufacturers. Water activity (a_w) is one of the most critical factors for determining the quality and safety of feed and grain. It quantifies the amount of “free” water available in materials for use by microorganisms and chemical agents.

About the author

Dr Naveen Kumar , B.V.Sc & A.H (Gold Medalist), M.V.Sc (IVRI, Bareilly) is a food & oil seed grains storage specialist and a finished feed quality expert for Asian and other tropical countries. He also Business Director of Delst Asia and is located in Faridabad, India. He can be reached at naveensharma21@gmail.com.

To face up to the extremely challenging tropical hot and humid weather condition, maintaining overall grain quality over storage requires an in-depth understanding of the sciences involving:

• Water activity, Moisture movement
• Good silo design and good silo management program
• Intake grain quality and length of storage.

This article focusses on Water activity (a_w), reasons for rot, Bio-deterioration, Moisture migration and Corrosion, Grain respiration, Shrinkage, Fungus and Mold and Insect infestation.

Charting Water Activity in Silos

Over the entire storage period, good quality grain will continue to degrade in quality. Grains degrade 10 times faster in the tropics due to the very adverse weather condition posed by hot scorching afternoon sun, the occasional burst of rain, high humidity and temperature difference between hot days and cold nights. At best, proper grain storage management can only help minimise the damage.

It only takes about 2-3 weeks for an onset of the many negative elements to start degrading the stored corn, and thereafter, rapidly worsening over time. Degradation in grain quality relates to nutrient degradation, bio-degradation, insect and mite infestation, physical structure damage, mold contamination and grain respiration, which leads to weight or moisture shrink. All these damaging losses has an economic impact and cannot be taken lightly.

What actually starts the rot?

Water activity (a_w)

The most important property of water in food systems is the water activity (a_w) of food. Water activity is the ratio of the vapor pressure of water in equilibrium with a food to the saturation vapor pressure of water at the same temperature. The water activity of a food describes the degree to which the water is “bound” in the food and hence its availability to act as a solvent and participate in chemical or biochemical reactions and the growth of microorganisms. It is an important property that can be used to predict the stability and safety of food with respect to microbial growth, rates of deterioration, chemical and physical properties (Mathlouthi, 2001).

At a_w of 0.70 and above, mold starts to grow which inflicts great damage to the grain quality. The water activity and the propensity for mold growth increases with temperature (Tapia et al., 2020). Shelled corn can be safely stored for a year at a moisture content of 13% and a temperature of 10℃. However, the same corn stored at 32℃ can be substantially be damaged by mold within 2 months. This is why it is so much easier to store grains in the cold regions but extremely challenging in tropical regions.

Bio-deterioration

The inherent enzymes present in the seed causes bio-deterioration. The extent of deterioration depends upon the rate of enzyme activity, which is affected by free water and temperature. Biodeterioration results in degradation of nutrients and contamination with anti-nutritional factors.

Nutritional impairment in degraded corn during storage is of great consequence to the health, nutrition and performance of the animals, while directly affecting the profitability of an organization.

Moisture migration

This is the spark that starts the fire and all other problems we face with grain degradation. Moving free water that leached out of grains is the primary cause of grain degradation. This is a natural phenomenon due to the huge temperature difference between day and night throughout the storage period. The difference in temperature causes moisture to move from a region of higher temperature to lower temperature. Moisture will start to migrate if the difference of temperature variance is more than 5℃. The movement of free water is further assisted by convective air current flow inside the silo. This convection air flow moves free moisture to localise spot, and consequently increases water activity, promoting mold growth, insect, and mite infestation.

As steel is a good conductor of heat, the silo wall is easily affected by weather conditions, impacting grain temperature nearer to the wall.

In a cold weather scenario, the cold temperature outside the silo results in the grain and air nearer to the wall to be colder while the center is relatively warmer. This forms a convection air flow that moves downward alongside the wall through the grain. As the air current moves downward and up again through the warmer center, the air starts to warm up from the warm grains and picks up moisture. As warm air rises, it cools because of the colder temperature near the roof, resulting in condensation on the top of grain mass.

In a hot tropical weather scenario, the hot temperature outside the silo results in the grain and air nearer to the wall to be warmer while the center is relatively colder. This forms a convection air flow that moves upward alongside the wall through the grain. As the air current flows upward and down through the cooler center, the air starts to pick up moisture along the passage, leading to moisture condensation towards the bottom silo floor. This phenomenon is most commonly seen in tropical countries, where the silo external surface can be heated to approximately 60℃ in the afternoon and quickly cooled down to about 22-25℃ at night.

Moisture migration occurs quite readily in regions with extreme day and night temperature, as in the Middle East, harsh tropical and equatorial climates. Storing grains at 13-14% in cooler climates is fair game, but however, in hot climates, it is very challenging which involve a totally different approach.

Typically, free water in the grain mass tend to migrate towards the cooler areas in a silo, which is usually the shaded part from the sun, the floor and the lower half of the silo. This is where you will observe sweated patches clinging to the silo wall and at the bottom of silo after gravity unloading.

The increased a_w from moisture migration supports microbial and micro-flora growth, leading to spontaneous heating and eventually causes grain respiration. Heavily contaminated caked layer adhering to the silo sidewall must be properly cleaned and disposed of to control corrosion, contamination in feed production and future seeding of problems.

Corrosion

Corrosion of galvanized steel silos is due primarily to moisture ingress and the consequent degradation of the sweated corn fermenting and producing a complex mixture of chemicals amongst which are formic and acetic acids, both of which are extremely aggressive and damaging on the galvanized coating protection layer, and eventually corroding the unprotected exposed steel surface.

Worst- and best-case scenarios for continued exposure to damp corn predict that the total useful service life of the silos can be reduced by half if corrosion is left unchecked or proper remedial maintenance to the corroded surface is not looked into. Many silos failed because of corrosion and neglect.

Grain respiration

Grain respires once it detects sufficient heat and moisture. This is the reason why feed mills dread storing corn above 14% moisture content. The hot pounding tropical sun with its strong UV rays in the middle of noon will drastically increase the surface temperature of the upper part of silo to a temperature way above ambient temperature. Just like the car roof which gets heated up to a very high temperature from the hot afternoon sun which burns upon touching.

The increase in temperature leads to an increase in respiration rate and consequently shrinkage, especially in the tropics. When grain respire, starch and oxygen are converted to carbon dioxide as well as water and heat, leading to the onset of even more aggressive and uncontrollable respiration.

Shrinkage

Shrinkage is physically noticeable and contributes to financial losses as weight loss in grain storage. However, in addition to weight loss, shrinkage causes irreversible changes to starch molecules, especially to amylose and protein matrixes that encapsulate individual starch granules within the endosperm of the corn kernel.

Depending on the grain intake moisture and storage time, moisture shrink can range from 0.5-3.0%, which is a substantial weight loss.

Moisture loss causes starch retrogradation, which limits digestibility and nutrient availability to the animal. This is an important factor why animal fed with fresh high moisture corn performs better than old dry corn. Shrinkage of corn due to moisture loss also has a negative impact on feed pelleting, since less moisture is being relayed to the compounded meal, resulting in poor steam conditioning and cooking of starch, a low degree of starch gelatinization and hence affecting the overall pellet quality.

Fungus and Mold

Fungus refers to a group of unicellular or multicellular organisms, which feed on organic matter. It includes mold, mushrooms and yeast. Fungus that are usually involved in deterioration of grain have been classified as field fungi, storage fungi, and advanced decay fungi depending on the time of their invasion and colonization of grains before or post-harvest (Afsah et al., 2013). Mold, are multicellular microscopic fungi, typically characterized by the presence of hyphae filaments. Their life cycle is divided into four phases: sporulation, germination, hyphal growth (vegetative growth) and reproduction. The spore stage is where a mold is dormant, allowing mold to endure harsh environments such as extreme temperature and dry conditions. Once mold spores obtain adequate nourishment and moisture, they will germinate and form hyphae. From this time onwards, fungi metabolize the grains by extracting the necessary nutrients and retaining moisture needed for growth, which ultimately poses a biosecurity threat to feed and animal (D’orazio, 2012).

In tropical conditions, Aspergillus spp. are prolific storage fungi in grains as it favours hot conditions with 13-20% moisture and relative humidity of 65-90% depending on the species. They can spontaneously produce heat up to 55°C, resulting in spontaneous heating in grain mass (Atanda et al., 2011). These molds are abundant in the environment and present on all corn kernels surface. It is important to control moisture and water activity to prevent mold proliferation and its damaging consequences. This mold is also often associated with granary weevil activities, which is usually an issue in long period storage.

Mold is a living organism and its growth is influenced by moisture, temperature, oxygen, and substrates. Moisture is the most critical among these factors. Mold unable to grow when the moisture in grains is less than 12%. When the moisture is increased above 12%, molds will start to germinate and grow. Molds proliferate when moisture is above 17% (D’orazio, 2012). Humidity affects grain moisture which makes it extremely challenging to store corn in bags or bulk in an open warehouse.

Molds not only produce mycotoxins but also damage and reduce the nutritional value of grains. Actively growing molds utilise carbohydrates present in grains to produce carbon dioxide, water and heat, leading to reduction of energy value and nutrients degradation of grains or feed.

It is difficult to control fungal contamination in well-dried shelled corn of 13-14% moisture, stored in a steel silo or as bags in a warehouse in tropical condition. Insect infestation due to moisture migration can also further contribute to temperature rise in grains. The combination of mold contamination and insect infestation increases water activity, resulting in an increase rate of mold growth. If mold growth is not controlled, the degradation of corn continues as the storage duration prolongs. There is simply no end to all of these issues if there is uncontrolled moisture migration and grain respiration since both generates an excessive amount of free water, increasing the water activity.

Insect infestation

Insect infestation is a greater problem in regions with high relative humidity while temperature has the greatest influence on insect multiplication. At approximately 32°C, the rate of multiplication is monthly compounded exponential increase of fifty times the original amount (Yasothai, 2019).

Growth of insect, pests and molds raises both temperature and moisture. Insect infestation is rampant whenever there is heavy sweating in areas where corn layer adheres to the silo wall in patches. Naturally, insects will start to breed and incubate their eggs in a conducive environment which provide a rich food source for its young larvae.

The concluding part of the article will focus on Good Silo Design and good Silo management program, Intake grain quality and length of storage.

Featured Silo Image photo created by standret – www.freepik.com

References
Afsah H., Leili S., Jinap, S., Hajeb, P., Radu, S. and Shakibazadeh, Sh. (2013). A Review on Mycotoxins in Food and Feed: Malaysia Case Study. Comprehensive Reviews in Food Science and Food Safety. 12. 10.1111/1541-4337.12029.
Atanda, S.A., Pessu P. O., Agoda S., Isong I. U., Adekalu O. A., Ehendu M. A. and Falade T. C. (2011). Fungi and mycotoxins in stored foods. African Journal of Microbiology Research. 5(25). 4373-4382. 10.5897/AJMR11.487
D'orazio, M. (2012). Materials prone to mould growth. 10.1016/B978-0-85709-122-2.50012-7.
Mathlouthi, M. (2001). Water Content, Water Activity, Water Structure and the Stability of Foodstuffs. Food Control. 12. 409-417. 10.1016/S0956-7135(01)00032-9
Tapia, M.S., Alzamora, S.M. and Chirife, J. (2020). Effects of Water Activity (aw) on Microbial Stability as a Hurdle in Food Preservation. In Water Activity in Foods (eds G.V. Barbosa-Cánovas, A.J. Fontana, S.J. Schmidt and T.P. Labuza). 1002/9781118765982.ch14
Yasothai R. (2019). Storage Losses in Feed Ingredients by Insects and its Control. International Journal of Science, Environment and Technology. 8(1). 44-49.