This course provides an overview of the issue of postharvest loss of grains by exploring essential physical, technical, and social dimensions of postharvest supply chains and loss prevention methods globally. Each year, estimates suggest that 1/3 of all food produced is lost or wasted, making postharvest loss a critical global food security and sustainability issue of today. Key knowledge areas are presented including: -An overview of postharvest loss -Supply chain activities such as harvesting, drying, and storage -Economics and markets -An introduction to the network of actors working in this field We face the immense challenge of feeding over 9 billion people by the year 2050. To meet these demands, yields will have to more than double using the same amount of natural resources. In recent years, postharvest loss has been recognized by major institutions including the US government, the United Nations, the CGIAR Research Consortium, and several others as a significant opportunity to impact food security and improve livelihoods. Despite this increased attention, a lack of knowledge, technical capacity, and resources remain obstacles for stakeholders worldwide to act on these issues. This course will, for the first time, provide you as professionals, practitioners, and students, with a comprehensive introduction to postharvest loss processes and begin building capacity for loss prevention worldwide.
2.2.1. Grain Drying: Basic Principles of Drying
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En provenance du cours de University of Illinois at Urbana-Champaign
Global Postharvest Loss Prevention: Fundamentals, Technologies, and Actors
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University of Illinois at Urbana-Champaign
25 notes
À partir de la leçon
Week 2: Causes: Major Causes & Encounters for PHL
This module will introduce you to technical, social, and economic factors and situations that lead to postharvest losses. Harvesting, drying, and storage phases will be specifically covered as stages that are both vital in the grain supply chain and in which a great deal of postharvest losses are experienced.
Rencontrer les enseignants
Dr. Prasanta KalitaDirector of the ADM Institute for the Prevention of Postharvest Loss
Department of Agricultural and Biological Engineering
Hello, my name is Kent Rausch.
I'm on the faculty at the University of Illinois, I work in the agriculture and
biological engineering department here at the U of I.
The first lecture is on grain drying,
we're going to talk about the basic principals of drying grain.
The outline of the first lecture here is why is drying needed in the first place?
What is grain moisture shrink during drying?
The concept of equilibrium moisture.
Content, and we'll also talk about drying air properties and
the effect of drying on grain quality.
So grain drying is a process to reduce the moisture content of grain to a safe level
that allows us to efficiently handle store grain after harvest.
It's a critical step because the quality will go down if the proper
moisture content is not maintained.
So this gives you an idea of why drying is needed for various grains.
Typically, we harvest grains at a high moisture content which simplifies
the harvesting process and reduces losses in the field so for example
paddy rice might be harvested around 20% but it needs to be reduced to 10-13% for
a long term storage or 12-14% for short term storage.
And the safe storage moisture contents vary based on the cereal grain that you're
working with.
High moisture grain will have problems such as building up of heat
due to the molding or respiration of grain.
Microorganisms and the grain itself actually give off CO2 and
also heat during the respiration process.
Because grain is a good insulator,
the heat does not move out of the grain mass and develops heat in localized areas.
This can further speed up mold growth and reduce grain quality but
mold growth is to be avoided for obvious reasons it creates a great deal of
difficulties downstream of the storage facility.
It can damage the grain and create odors and go canals.
So potentially produce mycotoxins which are toxic to humans and animals.
High moisture grain will also reduce the germination rate if a grain is being
stored and will be later used for a seed for further planting mold growth will
reduce the percent of kernels or grains that will germinate after planting, it
also reduces the nutrients stored in the germ so if your wanting to use the germ or
the grain for nutrition, the nutrient content is reduced.
And as I mentioned earlier due to molding you'll lose the nutrient content and
also the flavors will change, if the mold is occurring.
This will also give a musty odor to the grain which is not desirable.
It increases the fatty acid content of the grain which leads to rancidity.
And it also reduces the starch and sugar content of the grain.
So when grain is being dried there are two fundamental processes or
steps that are occurring during the drying process.
The first is, is that water in the interior of the grain
migrates from the center or the interior regions towards the surface of the grain.
The second step involves evaporation from the surface of the gain.
Both of these have to occur in order to decrease the moisture content.
Drying mechanisms vary moisture content decreases rapidly at the beginning but
slows down the grain gets dryer, generally speaking drying rate
decreases with moisture content, and increases with air temperature.
This leads us to another concept during commerce and trade of grain.
This is called grain moisture shrink, as the water is removed during the drying
process the amount of grain, the weight of the grain, will reduced.
This can be calculated from this formula, which relates the initial moisture content
and the final moisture content of the grain and
it's typically called moisture shrink, or grain shrink, or simply shrink.
An example calculation is shown here.
If you're drying a sample of grain from 20% to 15%, the percent shrink is 5.9%.
Or, put another way, the weight of 100 grams of corn for example.
If you drive from 20% to 15% would lose 5.9 kilograms.
Another important concept is equilibrium moisture content, during drying and
storage the grain moisture content depends on its surroundings, temperature and
relative humidity interact with the grain during storage.
If the air is drier than what is at equilibrium,
the grain will lose its moisture.
And when the air is more moist or humid, the grain will absorb moisture.
This is usually shown on a typical curve called an equilibrium curve,
or an isotherm.
It's a function of the equilibrium moisture content.
And the relative humidity of the drying air, or the air surrounding the grain.
Each grain has a unique equilibrium moisture curve and
it looks something like an s.
For low relative humidities, you have low moisture contents and
as the relative humidity is increased, the moisture content also increases.
Temperature affects this equilibrium moisture content relationship.
Generally speaking with higher temperatures
you have lower moisture contents and equilibrium at the same relative humidity.
So this needs to be taken into account wherever grain is being stored.
The moisture content varies by grain as well.
For example shelled maize or
shelled corn being stored at a typical relative humidity of 70%.
We'll have an equilibrium moisture of 14% moisture, and
this can take several weeks to reach an equilibrium.
The relative humidity of the air increases to 80%,
the rain will increase 15.6% for shell maze.
Some important properties that come into play during drying and storage is vape,
one of them is vapor pressure which is the partial pressure
exerted by just the water vapor in the surrounding moist air.
Relative humidity is just an expression of the amount of water vapor in the air to
the fraction of water vapor in a saturated or 100% relative humidity air.
And the humidity ratio is just a mass ratio of water vapor
per unit mass of dry air.
These are all describe use to describe the moisture of the air.
For temperatures there are two type of temperatures
dry bulb temperatures is the every day.
Temperature that we, we use to express the warmth of our surroundings
using an ordinary thermometer.
The wet-bulb temperature is indicated by a thermometer who has a bulb that has a wick
surrounding the bulb and reflects how much moisture can actually evaporate from
the wick and it'll actually decrease the temperature indicated the dew temperature,
could also be called the saturation temperature,
where moisture begins to condense as the air is cooled down.
The enthalpy of the air is a measure of the heat content of the moist air
per unit mass of dry air.
And it's referenced to a certain temperature.
Specific volume is the volume per unit mass of dry air.
And the specific heat measures the physical quantity of heat energy
required to change the temperature of air by a given amount.
This can be summed up in what's called a psychometric chart.
It's used to determine how air and its properties change in a drying system and
it's outlined on this slide here, you have lines of constant specific volume,
constant enthalpy, constant temperature, constant wet bulb temperature and so
forth, and this can be a very useful tool when you're estimating how
air properties will change during drying.
So drying will positively affect grain and
result in desirable properties of the grain.
One of those properties is the lowered and
uniform moisture content hopefully we had minimum broken and damaged kernels,
low susceptibility to breakage further on downstream after the drying operation.
Nutrient value should be stable and retained, it'll have a high viability or
germination rate, and the visual and
organoleptic properties of the grain should be preserved.
After drying we should have a moisture content that's low enough for
safe storage.
Different grains can have different safe storage moisture contents.
It's also important to reduce the variability of the grain within
the stored mass.
Minimizing broken and damaged kernels is also important.
If we use too high of a drying air temperature
we can actually damage the grain due to the rapid drying rate.
And this graph shows the different drying temperatures that might be used
varying from about 30 degrees Celsius on up to above 80 degrees Celsius.
And the susceptibility is measured here on the vertical access.
Called the Stein breakage test, but
as we get above temperatures of about 60 degrees Celsius for the drying air,
the susceptibility of various hybrids will increase, which is not desirable.
To make things more complicated,
the response of different hybrids is different to the same drying temperature.
If we dry the grain properly, the nutrients such as protein, sugars, and
gluten will be preserved.
If we use too high of a temperature, those properties may be adversely affected.
Grain viability can also be reduced.
High temperatures reduces the germination rate.
So this needs to be avoided especially if the grain's going to be used for plant.
Delayed, and improper drawing may also reduced the germination rate.
Properly dried grain will have a good appearance, it won't look toasted or
brown.
This is important when you're marketing the grain.
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