Storage & handling of Raw material in detail
FORMS OF CONTAMINANTS, DETECTION AND REMOVAL
First, it is important to be aware of the different forms of contamination that may be present in the
raw materials brought in for processing.
Second, everything possible must be done to ensure detection of such contamination before it
enters the facility.
And, third, everything possible must be done to avoid, remove or destroy the contaminants before
using these raw materials.
Among others, insects, rodents and birds and their droppings, stone, cobs, weed seeds, glass, wood,
rags, tramp metal, residual insecticides and fumigants, water, bacteria, mold and mycotoxins. Some
of these are from unclean grain handling and transport. Others have been added as fillers, adding
weight without producing usable substrate. Some, like stone and tramp metal, are dangerous and
may cause fires and explosions in the grain handling system. The more difficult contaminants to
detect such as residual insecticides and fumigants as well as mycotoxins, can be present at levels
that can result in unmarketable DDGS(distillers dried grains with solubles).
*Distillers grains are a cereal byproduct of the distillation process.
*Dried distillers grains with solubles (DDGS) is WDG that has been dried with the
concentrated thin stillage to 10–12% moisture. DDGS have an almost indefinite shelf life and
may be shipped to any market regardless of its proximity to an ethanol plant.
Bushel weight, moisture, damaged kernel and broken kernel/foreign material tests are run
immediately. High insect infestation may be determined by visual inspection, but low infestations
may be more difficult to find. Also, the true grain weevils, because they bore into kernels, may not
be visible on the surface of the grain. Only a few plants use a black light to detect aflatoxin presence.
Black light is also usable for the detection of rodent urine contamination. This should be a routine
test for all shipments in all plants. NIR technology offers a solution to many of the grain testing
needs for quick decision making by distillers.
Do not neglect the senses. Examine the grain, checking for insect presence, rodent droppings, bird
feathers as well as foreign seeds and extraneous matter such as cobs, straw, wood and metal. Smell
the material for any hint of mold presence or chemical contamination. Feel the grain for moisture
and slime presence.
High moisture is the most common reason for rejection of a shipment.High moisture contributes to
difficulties in milling and handling and to lower yields. High moisture also contributes to elevated
mold growth, which typically leads to contamination with mycotoxins that become concentrated in
the DDGS.
STORAGE CAPACITY
Financial and logistic experts must contribute to this decision.
Storage capacity must accommodate both the processing schedule and cash flow consideration.
Local supplies reduce storage needs.
If shipments must come from afar, more capacity is needed.
Sugary/Saccharine Raw Material
The disaccharide sucrose is readily fermented to ethanol by S. cerevisiae , after hydrolysis into
glucose and fructose by the enzyme invertase, which is naturally present in this yeast. When ethanol
is produced from sugar cane, either molasses, the viscous saccharide-rich residue left after sucrose
crystallization, or cane juice, are used as the raw material. Due to its high osmolality, molasses has
the advantage that it can be stored for extended periods of time without microbiological spoilage. It
is diluted prior to fermentation to facilitate pumping and to avoid inhibitory ethanol concentrations
in the fermentation step.
Another disaccharide that can be used for ethanol production is lactose, which is present in whey, a
byproduct from cheese production. Lactose cannot be fermented by S. cerevisiae since it lacks β -
galactosidase activity. It can, however, ferment the hydrolysis products of lactose: galactose and
glucose. Strains of Kluyveromyces marxianus are commonly used in commercial ethanol production
from whey. Two methods for hydrolyzing lactose to galactose and glucose have been evaluated:
enzymatic hydrolysis and acid hydrolysis. Enzymatic hydrolysis with β -galactosidase is hardly
possible because of the high enzyme costs, whereas acid hydrolysis of lactose is a cheaper option.
Starchy Raw Material
Ethanol is produced from corn starch by wet mill or by dry mill processes. In a wet mill process corn
is initially steeped with water at 49 – 53 ° C, which softens the hulls and causes the grains to swell.
Subsequently, the wet grain is milled and germs, fibres and gluten are separated, after which the
starch is dried. In a dry mill plant, the raw material is initially ground to increase the surface area
without separating the different grain components. Starch itself is composed of amylose and
amylopectin, the former a linear α -1,4 polymer of glucose and the latter α -1,4 polymer of glucose
with α -1,6 branches. Its hydrolysis is rather similar in wet mill and dry grind plants.
During the liquefaction step, performed at elevated temperature, swollen starch is hydrolyzed by α -
amylase to dextrins of varying chain-length. Subsequently, dextrins are hydrolyzed to glucose by
glucoamylase. After an initial hydrolysis stage, the temperature is lowered to 30 – 35 ° C and yeast is
added, the glucoamylase will continue to hydrolyse dextrins during the fermentation.
Fig. Raw materials and their processing for ethanol production
1 Slicing and extraction
2 Milling (Dry-mill process), steeping (Wet-mill process)
3 Chipping or milling
4 Glucose, mannose, xylose, galactose, arabinose, cellobiose and oligosaccharides
Cellulosic Raw materials
Lignocellulose can be obtained from agricultural and forestry by-products. In addition, the indirect
use of fossil fuels for ethanol production can be significantly reduced when lignocellulose is the raw
material compared to current ethanol production based on saccharide and starch-rich raw materials.
Challenges: (i)development of enzymes for hydrolysis of lignocellulosics,
(ii) ethanologenic microorganisms with a wide substrate utilization range and
(iii)innovative approaches to process integration.
Method of Hydrolysis: (i) dilute-acid hydrolysis process (yield: 50 – 60 % ) e.g.H2SO4
-corrosion and costly acid recovery constitute major disadvantages.
- high hydrolysis temperature causes corrosion and saccharide degradation. Saccharide degradation
can be reduced by applying a two-stage process in which hemicellulose is hydrolyzed in a first step
(150 – 190 ° C) and cellulose subsequently is hydrolyzed in a second step at more severe conditions
(190 – 230 ° C). The two-stage process may also be considered a thermo-chemical pre-treatment,
followed by an acid hydrolysis of the cellulose fraction.
(ii) enzymatic hydrolysis (yield: 80 – 90 %)
-Thermo-chemical pretreatment followed by enzymatic hydrolysis is considered the most promising
method for the saccharification of hemicellulose and cellulose. The specificity of the enzymes and
the mild conditions of hydrolysis minimize saccharide degradation, and enables high hydrolysis yields
and reduced by-product/inhibitor formation.
-To enable efficient enzymatic cellulose hydrolysis, the lignocellulose material has to be pretreated
to disrupt the cellulose-hemicellulose-lignin matrix. The material is first mechanically processed to
reduce size and to improve mass transfer, and is then thermo-chemically treated to hydrolyze
hemicellulose to its monomeric saccharides. Methods involving highpressure steam are frequently
employed, and referred to as steam-explosion. Acid and alkaline catalysts accelerate hemicellulose
hydrolysis, whereas lignin largely remains non-solubilised. Steam-pretreatment generates a fibrous
slurry, of which the liquid phase mainly contains hemicellulose mono-, di- and oligosaccharides, and
the solid phase mainly contains cellulose and lignin. In the organosolv process the material is treated
with an organic solvent – mostly ethanol - at elevated temperature, which in contrast to steam-
pretreatment solubilises lignin.
-Cellulases degrade cellulose. Cellulose hydrolysis is product-inhibited by both cellobiose and
glucose. Therefore the accumulation of these end-products of hydrolysis results in lower conversion
of cellulose. Most commercial cellulase preparations contain insufficient activity of the enzyme β -
glucosidase, which hydrolyses cellobiose to glucose, and hence additional β -glucosidase has to be
supplied to prevent cellobiose accumulation.
Table: Methods for pre-treatment/fractionation of lignocellulosic raw materials
