From experience we know that grain spoils more readily when it is hot and wet, and that it is the most secure, the safest, when it is dry and cool. But can we quantify spoilage? There have been attempts at observing mold and fungus, but this was just too subjective. In 1981 Fraser and Muir suggested that the reduction in germination capacity was related to spoilage and they arbitrarily declared that when the germination capacity was reduced to 95%, it constituted the first step or first stage of spoiling. The number of days it took to have the grain deteriorate to this point was declared the number of SAFE days and it was empirically found to be:
Safe Days = 10^(6.234 -0.2118 MC – 0.0527 T) wheat and cereals
= 10^(6.224 – 0.302 MC – 0.069 T) canola and oilseeds
The objective for safe storage, is to maximize the number of safe days by lowering the temperature T, and the moisture content, MC. Look at how the number of safe days varies with temperature for ‘dry’ wheat:
14.5% @ 30⁰C = 38 safe days
14.5% @ 20⁰C = 128 safe days
14.5% @ 0⁰C = 1458 safe days
New Definition: Spoilage Index = Ʃ ( 1/ safe days) x 100
The value for spoilage index is equivalent to the amount of spoilage. Anything less than 100 is very good, and essentially constitutes very little spoilage. A value of 100 suggests that we have achieved our first level or stage of spoilage, and if the value is several hundred we are getting into some serious spoilage.
Example, if the calculation above yields the number of safe days to be six, then after 3 days we will have an accumulated deterioration of: (1/6 + 1/6 + 1/6) x 100 = 50%,
after 6 days : (6 x 1/6) x 100 = 100% of the way to 95% germination capacity
With our system we read the sensors for temperature and can calculate the moisture content, MC, and temperature, T, every hour so we can refine the spoilage index to accumulate on an hourly basis to catch the dynamic changes in the bin, especially for the first few days. So when accumulating every hour the Spoilage Index would be:
Spoilage Index = Ʃ ( 1/ safe days)/24 x 100
Again if the spoilage index approaches 100; it means that we have reached the first stage of spoilage, the germination capacity has been reduced to 95%.
Spoilage index is an important tool in being able to measure spoilage such that different control strategies can be compared for the efficacy of safe grain storage.