Charrlng Heo t
Water Caking I
Dry matter
loss1
104 TECHNOLOGY OF CEREALS
are shown in Fig. 5.1, should be borne in mind
35'
0 H r [\
throughout.
Moisture content and storage
E 30'
5
temperature e c7'
Moisture content is expressed as a percentage
of the grains' wet weight. The safe moisture
contents for storage vary according to the type of
cereal but it is widely assumed that they are
equivalent to the equilibrium moisture content
of the respective grains at 75% RH and 25°C
(Table 5.1).
50 io0 I50
?d
TABLE 5.1
Equtlibnum Moisture Contents of Grains at 75% RH and 25°C
Cereal Type Moisture ?LO
Barley
Maize 14.3
Oats 13.4
Rye 14.9 (25"-28"C)
F~~ 5.2 potential storage the (number of days) of wheat
grain as a function of temperature ("C) and moisture content
("h), the germination rate maintained being 70%. From
14.3 (250-280c) Guilbot, (1963) Producteur Agncole Francats, Suppl. mai
ITCF, Paris.
Rice 14.0 40'
Sorghum 15.3 (u) h
??
red 14.7 (rt) +
white 15.0 (rt) s
Wheat e 30"
durum 14.1 (u)
3
g 200
G 100
?!
u
i
Based on values in Bushuk and Lee, 1978.
I
In temperate regions the moisture contents at
which grain is stored are closer to those described
moisture contents cannot be considered alone as
the deleterious effects of excessive dampness are
affected critically by ambient temperature and
the composition of the surrounding atmosphere.
The increase in relative humidity of the interseed
atmosphere with temperature, is slight. It amounts
10°C drop in temperature.
temperature as they affect storage of wheat is
The relationship depicted takes account only
of the maintenance of grain quality as assessed
by grain viability. The relationship is also import-
ant, however, through its effects on infesting
organisms, as Fig. 5.3 shows.
The values used in Figs 5.2 and 5.3 refer
as wet rather than dry. The significance of 0"
5 io 15 20 25
Moisture content %
0 Good conservation
H Insects
m Germination
n Moulds
to about 0.6-0.7% moisture increase for each
FIG 5.3 Risks to which stored cereal grain is exposed as a
%. From Burges, H. D. and Burrel, M. J. (1964) J. Sci
The relationship between moisture content and
function of grain temperature in "C and moisture content in
shown in Fig. 5.2.
Food AFC. 1: 32-50.
to sound clean samples. Broken grains are almost
always present to some extent as a result of
damage during harvesting or transferring to
stores. In broken grains endogenous enzymes and
their substrates, kept separate in the whole grain,
STORAGE AND PRE-PROCESSING 105
can achieve contact and lead to necrotic deteriora- radicle, and leaves and stem develop from the
tion. Further, the most nutritious elements of the plumule (see Ch. 2). Hydrolytic enzymes are
grain, endosperm and embryo, are exposed released into the starchy endosperm, and these
to moisture, micro-organisms and animal pests catalyze the breakdown of stored nutrients into
whereas in the whole grain they are protected a soluble form available to the developing plantlet.
by fruit coat, seed coats and possibly husks. The conditions required for germination are
Impurities can also reduce storage time in that also conducive to other, more serious, hazards
weeds present in the crop ripen and dry at a such as excessive mould growth. They would
different rate from the crop itself. Hence, still rarely occur throughout a well managed store
green plant material - with a relatively high but could develop in pockets due to moisture
moisture content - can carry excessive moisture migration. Deterioration results from loss of
into store even when mixed with dry grain. weight due to enzyme activity and a loss of quality
resulting from excessive enzyme activity in the
products of processing. These problems would
apply even if the germination process were termin-
Changes during storage in the grains
ated through turning the grain. Having germinated
themselves
in store the grain would also be useless for seed
Respiration
purposes as the process cannot be restarted.
In a natural atmosphere gaseous exchange will
Microbial infestation
occur in a stored cereal crop. This is due to
respiration and it involves a depletion in atmos-
pheric oxygen and an increase in carbon dioxide Fungal spores and mycelia, bacteria and yeasts
with the liberation of water, and energy (as heat). are present on the surfaces of all cereal crops.
Respiration rates measured in a store normally During storage they respire and, given adequate
include a major contribution from micro-organisms moisture, temperature and oxygen, they grow
that are invariably present at harvest; neverthe- and reproduce, causing serious deterioration in
less even ripe dry grain, suitable for storing, grains.
contains living tissues in which respiration takes A distinction may be drawn between those that
place, albeit at a very slow rate. The aleurone attack developing and mature grain in the field
and embryo are the tissues involved and, like and those that arise during storage. Field fungi
other organisms present, their rate of respiration thrive in a relative humidity (RH) of 90-100%
increases with moisture content and temperature. while storage fungi require 70-90% RH. Several
Respiration is a means of releasing energy from investigations have shown that a RH of 75%
stored nutrients (mainly carbohydrates) and a is required for germination of fungal spores
consequence of long storage is a loss of weight. (Pomeranz, 1974).
Under conditions unfavourable to respiration this Storage fungi are predominately of the genera
may, however, be insignificant and under any Aspergillus, of which there are five or six groups,
circumstances it is likely to be of little consequence and Penicillium, the species of which are more
in relation to other storage hazards. Respiration clearly defined. Some of the more common storage
can be reduced by artificially depleting the oxygen fungi and the minimum relative humidity in
in the atmosphere. which they can thrive are listed in Table 5.2.
As with other spoilage agents dependent upon
a minimum moisture content, fungi may be a
problem even when the overall moisture content
Germination
Germination of grain is an essential and natural in the store is below the safe level. This can
phase in the development of a new generation of result from air movements leading to moisture
plant. It involves the initiation of growth of the migration. Warm air moving to a cooler area will
embryo into a plantlet. Roots develop from the give up moisture to grains, thus remaining
106 TECHNOLOGY OF CEREALS
TABLE 5.2
Approximate Minimum Equilibruim Relative Humidity for
Growth of Common Storage Fungz
Thermophilic fungi die at 60°C and the process
thermophilic yeasts up to 70°C.
Mould RH(%) limit In recent years attention has been given to the
Aspergillus halophiticus
68* toxic products of fungi such as Aspergillus flavus
A. restictus group 70* and Fusarium miliforme which produce aflatoxin
A. glaucus group 73* and zearalenone (see Ch. 14).
A. chevalieri 71t
A. repens 71t
Insects and arachnids
A. candidus group 80*
A. candidus 75t
Insects that infest stored grains belong to the
A. ochraseus group 80*
A. jlavus group 85*
A. jlavus 78t beetle or moth orders: they include those capable
A. nidulans
78t of attacking whole grain (primary pests) and those
80-90* 82t that feed on grain already attacked by other pests
A. fumigatus
Penicillium spp.
P. cyclopium 82t (secondary pests). All arachnid pests belong to
P. martensii 79t the order Acarina (mites) and include primary
83t and secondary pests. Most of the common insects
P. islandicum
and mites are cosmopolitan species found through-
OUt the world where grain is harvested and stored
(Storey, 1987). Insects and mites can be easily
distinguished as arachnids have eight legs and
insects, in their most conspicuous form, have six.
Reference to the most conspicuous form is neces-
sary as some insects (including those that infest
grain) develop through a series of metamorphic
forms. There are four stages: the egg, the larva,
the pupa and the adult or imago. Although some
female insects lay eggs without mating having
occurred, this is less usual than true sexual
reproduction, and this and dispersion are the two
principal functions of the adult. Large numbers
of eggs are produced and these are very small.
Those of primary pests may be deposited by the
female imago inside grains, in holes bored for the
purpose prior to the egg laying. Under suitable
conditions eggs hatch and from each a larva
is kept going by spore forming bacteria and
* Christensen, C.M. and Kaufmann, H.H. (1974) Micro-
flora. In: Storage of Cereal Grain and thezr Products. Christensen,
C. M., (Ed.) Amer. Assoc. of Cereal Chemists Inc. St. Paul,
MN. U.S.A.
t Ayerst, G. (1969). The effects of moisture and temperature
on growth and spore germination in some fungi. J. Stored
Prod. Res. 5: 127-141.
Table adapted from Bothast, R. J. (1978).
in equilibrium with them. Unless temperature
gradients are extreme the exchanges occur in
the vapour phase; nevertheless, variations in
moisture content up to 10% within a store are
possible.
If mould growth continues in the presence of
oxygen, fungal respiration increases, producing
more heat and water. If the moisture content is
allowed to rise to 30% a succession of progressively
heat-tolerant micro-organisms arises. Above 40°C
mesophilic organisms give way to thermophiles.
TABLE 5.3
Alphabetical List of Primary Insect and Arachnid Pests
Systematic name Common name Family
Acarus siro. L Grain (or flour) mite Acarid a e
Cryptolestes ferrugineus Stephens Rust red grain beetle Cucujidae
Rhyzopertha dominica F. Lesser grain borer Bostrichidae
Sitophilus granarius L. Grain weevil Curculionidae
Stiophilus oryzea L. Rice weevil
Sitophilus zeamais Motschulsky Maize weevil
Sitotroga cerealella Olivier Angoumois grain moth Gelechiidae
107STORAGE AND PRE-PROCESSING
FIG 5.4 Acarus siro, the flour mite (top), Sitophilus granarius, the grain weevil, and Gryptolestes
ferrugineus, the rust-red grain beetle (bottom). Crown Copyright Central Science Laboratory 1993.
108 TECHNOLOGY OF CEREALS
TABLE 5.4
Alphabetical List of Most Important Secondary Insect Pests
Systematic Name Common Name Family
Anagasta kuehniella Zella* Mediterranean flour moth Phycitidae
Gadra cautella Walker Almond moth "
Cyptolestes pusillus Schonherr Flat grain beetle Cucu jidae
Cyptolestes turacus Grouv Flour mill beetle "
Ephestia elutella Hubner Tobacco moth Phycitidae
Oyzaephilus surinamensis L. Saw-toothed grain beetle Cucujidea
Oryzaephilus mercator Fauv.
Plodia interpunctella Hubner Indian meal moth Phycitidae
Tenebriodes mauritanicus L. Cadelle Ostomatidae
Tribolium castaneum Herbst. Red flour beetle Tenebrionidae
Tribolium confusum Duval
Trogodenna granarium Everto Khapra beetle Dermestidae
Merchant grain beetle "
Confused flour beetle "
* Formerly Ephestia kuehniella.
FIG 5.5 Oryzaephilus
Laboratory 1993.
108 TECHNOLOGY OF CEREALS
TABLE 5.4
Alphabetical List of Most Important Secondary Insect Pests
emerges. The larva is the form most damaging
to the stored crop as it feeds voraciously. In
consequence it grows rapidly, passing through a
series of moults during which its soft cuticle is
shed, thus facilitating further growth. Finally
pupation occurs; the pupa, chrysalis or cocoon
does not eat and appears inactive. However ,
changes continue and the final metamorphosis
leads to the emergence of the adult form. The
life cycle of mites is simpler as eggs hatch into
nymphs which resemble the adult form, although
there are only six legs present at this stage. By a
series of four moults the adult form is achieved.
The time taken for development of both insects
and mites is influenced by temperature, the
greater the temperature the more rapid the deve-
lopment up to the maximum tolerated by the
species.
The primary pests -those attacking whole
grains- are given in Table 5.3.
The three most damaging of these pests in the
U.K. are shown in Fig. 5.4.
The most important secondary insect pests ~
those feeding only on damaged or previously
attacked grains -are given in Table 5.4.
The saw-toothed grain beetle is shown in Fig. 5.5.
FiG 5.5 Oryzaephilus surinamensis. Saw-toothed grain beetle. Crown Copyright. Central Science
Laboratory 1993.
STORAGE AND PRE-PROCESSING
109
FiG 5.6 X-ray photograph of wheat grains, two uninfested (bottom left), the others showing cavities
caused by insect infestation. An insect is visible within one of the cavities. (Part of a picture in J .
Photograph. Sci., 1954, 2: 113; reproduced by counesy of Prof. G. A. G. Mitchell and the Editor of
Journal of Photographic Science.)
even though only a hollow bran coat may remain.
Detection by means of soft X-rays is possible
(Fig. 5.6).
Those insects listed in the tables are considered
major pests. They are particularly well adapted
to life in the grain bin and are responsible for
most of the insect damage to stored grain and
cereal products. Minor pests occur mainly in
stores in which grain has started to deteriorate
due to other causes, while incidental pests include
those that arrive by chance and need not even be
able to feed on grains. For further information
on minor and incidental insect pests specialist
works such as Christensen (1974) should be
consulted.
Among the major primary pests five species
develop inside grains. Weevils (grain, rice and
maize) lay eggs inside while lesser grain borers
and Angoumois grain moths deposit eggs outside
but their newly hatched larvae promptly tunnel
into grains. The presence of the insect and the
damage it causes may not be evident from outside
Damage caused by insects and mites
Serious grain losses due to consumption of
grain by insects and mites occurs only after
prolonged storage under suitably warm conditions.
They are most serious in hot climates. Other
problems caused by insects include creation ofhot-
spots around insect populations where metabolic
activity leads to local heating. Moisture move-
ments and condensation in cooler areas results in
caking, and encourages fungal infestation (see
Fig. 5.1).
Introduction of insects and mites from wheat
stores to flour mills can cause serious deteriora-
tion in the products. Mite excreta taints flour with
110 TECHNOLOGY OF CEREALS
a minty smell and hairs from the animals’ bodies however, have several advantageous features.
can cause skin and lung disorders in workers Thus underground stores provide protection from
handling infected flour. Silk from the larvae of temperature fluctuations, the most successful
the Mediterranean flour moth webs together simple ones being found in hot dry regions. They
causing agglomeration of grains and blockages in are filled, to leave little air space, and sealed, to
handling and processing equipment. In tropical approach the concept of hermetic storage under
countries termites can weaken the structure of a which insects and moulds rapidly use up oxygen,
store, leading to its collapse. giving rise to high C02 content of the intergrain
atmosphere. In more humid regions ventilation
is desirable as the crop may have to be stored
before reaching a safe moisture level. Such a
Vertebrate pests
The principal vertebrate pests in cereal stores system is suited to cob maize rather than threshed
are rodents and birds. In many countries the three grains, as adequate space for air movement within
main rodent species involved are: the store is essential. Clearly the requirements
of ventilation and exclusion of insects are not
immediately compatible and hence careful design
Rattus nomegzcus - the Norway, common or
brown rat;
is essential.
Rattus rattus -the roof, ships or black rat;
Storage of maize as cobs is practised now
largely by small scale growers producing for the
Mus musculus - the house mouse.
Apart from consuming grains, particularly the requirements of the local community. It was at
embryo of maize, rodents cause spoilage through one time adopted more widely even in highly
their excretions which contain micro-organisms commercial practice, much as small grain cereals
pathogenic to man. These include salmonellosis, were stored unthrashed in ricks.
murine typhus, rat-bite fever and Weil’s disease. In the commercial context stores are needed
Rodents also damage stores’ structural elements, for three purposes:
1. Holding stocks on the farm prior to sale.
containers, water pipes and electric cables.
2. Centralization before distribution or processing
In well-managed stores access by rodents is
denied and good housekeeping practice, such
during the year following harvest.
as removal of grain spillages, maintenance of
3. Storage of annual surpluses over a longer
uncluttered surroundings and regular inspec-
period.
tions, prevent problems. The same is true of
birds. These are serious pests only when access Farm stores may consist of any available space
is easy, as for example in hot countries where that will keep out the elements. The facilities for
grain may be left to dry in the sun. Damage to protection against mould and pests are very
drains and blockage of pipes by nests can variable. Stores range from small wooden enclos-
give rise to secondary storage problems through ures in the barn, to round steel bins holding 25-
promoting local dampness in some stores. 80 tonnes, to silos of larger capacities. Good on-
farm storage facilities allow farmers to choose the
time to sell, to receive the best prices.
The degree of centralization depends upon the
Design of storage facilities
The requirements of long term safe storage are marketing regime within the country of produc-
protection against dampness caused by weather tion. In North America, Country elevators and
or other sources, micro-organisms, destructively Terminal elevators with storage capacity up to
high temperature, insects, rodents and birds, 500,000 tonnes exist. The country elevators provide
objectionable odours and contaminants and un- a local staging en route to terminal elevators which
authorized disturbance. Clearly the simplest stores include high-capacity equipment for cleaning,
such as piles on the ground, unprotected, are suit- drying and conditioning of grain. The term
able for short periods only. Other simple stores, ‘elevator’ is applied to the entire facility although
STORAGE AND PRE-PROCESSING 111
Settling is a continuous process arising in part
from the collapse of hulls, brush hairs, embryo
tips etc.
Control of pests and spoilage of grains
in store
Deterioration in store is less likely if care is
taken to ensure that the grain is in a suitable
condition for storing. Criteria for the latter include
a suitably low moisture content, a low mould
count and freedom from insects. Wheat contain-
ing live insects can be sterilized by passage
through an entoleter (Fig. 5.7), run at about 1450
rev/min. (BP 965267 recommends speeds of 3500
rev/min for conditioned wheat, 1700 rev/min for
dry wheat.) Hollow grains and insects may be
broken up and can be removed by subsequent
it refers literally to the mechanism (normally belt
and bucket) by which grains are raised to a
level from which they can be deposited into the
large capacity silos invariably found on the sites.
Elevators are associated with good transport
facilities by road, rail, water or all three. Many
are capable of loading grain into vessels at a rate
of 2,750 t/h.
It is sometimes necessary to provide storage
for grain beyond the normal capacity of an
elevator facility or elsewhere. In such conditions
a relatively inexpensive expedient is the flat store.
This is little more than a cover for a pile of dry
grain adopting its natural form as poured. Such
a form is described by the angle of repose. In the
case of wheat the angle is 27" to the horizontal,
hence flat stores have roofs close to this angle.
Very temporary stores may make use of inflatable
covers. aspiration.
Flat stores are easy to fill but, as they have flat
floors, removal of stocks is more difficult, usually
requiring the use of mechanical shovels. In con-
trast, silos usually have a floor formed like a
conical hopper whose walls make an angle greater
than 27" to the horizontal. Piles created by grains
falling freely from a central spout are not uniform
as whole grains tend to roll from the apex down
the sloping surfaces. Small impurities and broken
grains roll less readily and thus become trapped
in the central core of the pile. Such a core is
described as the spoutline. As the interstices can
amount to 30% of the occupied space, fines in
the spoutline can reach that level. Because air
circulation and hence heat loss is prevented, the
spoutline can be associated with early deterioration
through overheating. The diameter of the spoutline
is proportional to the width of the bin.
Also in contrast to tall tower-like stores, flat
stores require little strength in the side walls. In
a silo much of the pressure of the column of grain
is borne not by the floor but by the side walls.
This is because each grain rests on several grains
below it so that some of the weight is distributed
laterally until it reaches the walls and, by friction,
rests on them. In all stores some settling occurs
~~~~~~~~~~~~~~~ff~~~~~~~~~~. R+M
is re1ative1y dense and sett1ing may be Only 6oh
of volume but oats may pack as much as 28%.
FIG 5.7 Diagrammatic section through an Entoleter Aspirator.
1, fixh ide~; 2, 3, e; 4< swkg mm; -5< .ph
discharge over 6, cone; 7, valve controlling air flow. Arrows
indicate path taken by air. (Reproduced from Milling, 1969,
Oct 10, by courtesy of the Editor.)
112 TECHNOLOGY OF CEREALS
The store itself should provide protection from Pesticides used to control insects, during storage
weather (particularly wet) and intrusion by insects of cereal grains, are of two types. Those that are
and rodents. High temperatures are undesirable designed as a respiratory poison, and are hence
and variation should be reduced to a minimum applied as gas or volatile liquid, are described as
as this can lead to local accumulation of moisture. fumigants. Those designed to kill by contact or
All spoilage agents depend upon respiration ingestion are described as insecticides. They may
and hence a depletion of oxygen inhibits their be applied in liquid or solid form.
proliferation and activity. To achieve this it is Of the gaseous fumigants, methyl bromide and
necessary to provide a seal around the grain and phosphine (PH3) are the main examples. Examples
a minimal headspace. In a sealed store oxygen of ‘liquid’ fumigants are mixtures of 1,2 dichloro-
depletion can be achieved by natural or artificial ethane and tetrachloromethane: although the
means. Natural depletion results from respiration most effective fumigant is methyl bromide, this
which in most organisms consumes oxygen and gas does not penetrate bulk grain well and -the
produces carbon dioxide. Artificial atmosphere use of a carrier gas such as tetrachloromethane
control comes about by flushing of interstitial and is an alternative to the fan-assisted circulatory
head spaces with a gas other than oxygen, usually systems required if methyl bromide is used alone.
nitrogen or COz as these are relatively inexpensive. Few stores have the necessary fans.
Complete removal of oxygen is not possible. The period of treatment required depends
Experiments carried out in artificial conditions in upon the susceptibility of the species of insects
the U.K. showed that baking properties of wheats present to the fumigant. For example a three day
were maintained for eighteen years in low oxygen exposure to phosphine may eliminate the saw-
conditions. At ambient temperatures germinative toothed grain beetle but six days at low temperature
energy was seriously reduced and although this may be needed to kill the grain weevil.
reduction was prevented by storage at 5°C this ‘Liquid’ fumigants penetrate bulks well. The
was the only advantage of low temperature in proportions need to be adjusted to suit the depth
addition to oxygen depletion recorded (Pixton, of the grain stored. Up to three metres deep a
1980). 3: 1 mixture of 1,2 dich1oroethane:tetrachloro-
Sealed conditions are unusual and prevention methane is suitable but for penetration to a depth
of spoilage in many cases depends upon careful of 50 m equal proportions are needed. Fumigation
maintaining of the stored grains’ condition, and requires the stores to be sealed to prevent escape
prophylactic treatments with chemicals. Fortun- of the toxic fumes.
ately, nearly all threats to grain quality cause
Pesticide residues
temperature rises and monitoring of temperature,
through incorporation of thermocouples, can
reveal a great deal about condition. Some of the various types of pesticide (herbi-
Forced ventilation can reduce temperatures cides, fungicides, insecticides and rodenticides)
but it may be necessary to remove the cause by used in the field or in storage, may persist in
use of chemical treatments. Such treatments are grains being processed or indeed into foods as
relevant primarily when the problem is caused consumed. In the U. K. the maximum residue limits
by insects. Because of the possible persistence of (MRLs) permitted are mostly defined in EC
pesticides on cereals, their use in stores is increas- Directive 86/232 and amendment 88/298, which
ingly becoming regarded as a last resort. In most came into force on 29 July 1988. Additional
countries strict codes of practice apply to their MRLs came into force in December 1988 refer-
use. In the U.K. the legislation is contained in ring to pesticides that have been refused approval
the Food and Environment Protection Act 1985, in the U.K. but are used elsewhere; or those that
the Control of Pesticides Regulations 1986 and have been consistently found in U.K. monitoring,
HSE Guidance Notes (E440/85) on Occupational where the limits provide a check that good agri-
Exposure Limits. cultural practice is being observed (see Table 5.5).
STORAGE AND PRE-PROCESSING 113
TABLE 5.5 in residue level was also noted. Of the 1340
samples tested between ~~~il 1990 and March
1992, none approached the MRL for organophos-
The Maximum Residue Limits for Pesticides (mglkg) in Cereals
Excluding Rice (UK)
Aldrin and Dieldrin 0.01 Captafol 0.05 phorus or other classes of pesticide.
Carbaryl 0.5 Carbendazim 0.5
Carbon disulphide 0.1 Carbon tetrachloride 0.1
Chlordane 0.02 Chlorpyrifos methyl 10
DDT (total) 0.05 Diazinon 0.05
Before processing of cereals it is necessary to
Dichlorvos 2 Endosulfan 0. I*
Endrin 0.01 Ethylene dibromide 0.05
Entrimfos 10 Fenitrothion 10 carry out certain treatments on them. In cases
Hexachlorobenzene 0.01 a and
where grains undergo a period of storage before
Hydrogen cyanide 15 Heptachlor 0.01
Mercury compounds 0.02 Malathion 8 The treatments include drying, cleaning, grading,
0.1 Methacrafos lo conditioning and blending. Of these, drying is
Trichlorphon 0.1 Pyrethrins 3 likely to be carried out on the farm but the
remainder are more likely to be performed at the
the marketing system in use.
Drying
Monitoring of residues of fumigants applied at Purchasing-contracts typically stipulate an
acceptable range of moisture contents correspond-
ing to that required for safe storage. These may
be considerably lower than the moisture content
at harvest and some means of drying must be
used.
At its simplest, and in suitable climates, drying
can consist of spreading the grain to dry in the
sun assisted by frequent turning to expose all
grains. Such treatment is unlikely to cause damage
to grain beyond that which any handling imposes.
More sophisticated drying methods employing
artificially heated air or surfaces have inherent
dangers that must be avoided if grain quality is
to be maintained. In this context quality may be
defined in several ways but the most fundamental
is retention of the ability to germinate. This test
is particularly relevant to malting barley as the
malting process requires grain to be viable. In
other cereals viability provides an indication that
other quality factors have not diminished during
drying. In milling-wheat, the most damaging
change is denaturation of gluten and the consequent
deterioration in baking quality. Aside from a
germination test other, more rapid, tests include
those using 2,3,5-tri-phenyltetrazolium chloride.
A 0.2% aqueous solution applied to longitudinally
Preprocessing treatments
Y-He~cHorocYclohe~e 0.1 B-HexacHor~Yclohex~e 0.02
use, Some or all the treatments may take place
Inorganic bromide 50 Hydrogen phosphide 0.1 before entering the store-
Methyl bromide
Pirimiphos-methyl 10 Phosphamidon 0.05
* 0.2 for maize.
Italic type indicates limits are set by Good Agricultural
mi11 Or the e1evator Or receiva1 as appropriate to
Practice. Others are set by detection limit.
Source: Osborne et al. (1988).
levels specified by manufacturers in an experiment,
revealed that only traces remained in cooked
products. They were associated mostly with the
bran fractions.
Insecticide residues of laboratory-milled wheat
grains, treated at manufacturers' recommended
rates, were four times as concentrated in bran
and fine wheatfeed as in white flour. In commer-
cia1 samples the germ contained five times as
much as in the white flour. The milling process
removed only about 10% of the residue on the
whole grain, however only 50-70% of that in
white flour survived bread baking.
Overall, the results suggested that insecticides
applied at recommended rates are unlikely to
lead to residues above the MRLs of Codex
Alimentarius (Osborne et al., 1988).
In the U.K. less than one quarter of stored
grain is treated with pesticides (MAFF, 1991,
H-GCA, 1991). As part of a surveillance pro-
gramme, surveys have been carried out, since
1987, by the Flour Milling and Baking Research
Association, in association with the National
Association of British and Irish Millers, on samples
representing those purchased by all flour millers.
Residues have been low throughout but a decline
116
108
v 100-
e! 92-
a
e, 84-
+,
f 60-
52
44
a
$ 76-
0 68
remote grains and the introduction of a mixing
mechanism minimizes the effects of this gradient.
The diagrams in Fig. 5.9, show the differences.
Many types of hot air drier exist, they are
frequently classified as batch or continuous but
they may also be distinguished on the basis of
the direction of airflow in relation of that of the
grain (Nellist, 1978). Thus in the Crossflow type,
air flows across the path of the grains. The layer
of grain adjacent to the air inlet is soon dried and
- A its temperature rapidly approaches that of the
-
inlet air. The grain on the exhaust side remains
-
-
-
D
C
11111111
STORAGE AND PRE-PROCESSING 115
Receiving bin Receiving bin
Grain (Movement
Grain (Movement
Adjustable speed
FIG 5.9 Non-mixing (left) and mixing (right) type columnar driers. (Wasserman and Calderwood,
1972).
grain, but the heat and moisture transfer that
occurs ensures that grain temperature does not
rise to the inlet air temperature and that the air
temperature falls rapidly. For the final phase of
drying, the air and grain are almost at the same
temperature. Advantages of this design are that
high drying-air temperatures can be used to give
high initial drying rates without overheating the
grain and the period during which the grain and
air are in temperature equilibrium is thought to
help temper the grains and relieve stress cracks.
In Counterflow driers air travels in the opposite
direction to the grain and the dried grain temper-
ature approaches that of the inlet air. The air
tends to exhaust near to saturation, and drying
is therefore efficient. The temperature of the hot
air at inlet and the final grain temperature are
almost the same (Nellist, 1978).
Separations mites.
Cleaning may occur before storage, or shortly
before processing, or both. Cleaning before storage
has the advantage that it helps to minimize
deterioration in store which is aggravated by the
parts. Receival at harvest time however is often
accompanied by pressure to deal with grains
rapidly and hence time for cleaning may not be
available. Even grains cleaned before storage
may require cleaning again before processing to
remove undesirable elements resulting from the
storage itself. Impurities, together with damaged
and shrunken and broken grains, are collectively
known as ‘screenings’ in the U.K., ‘Bezatz’ on
the continent of Europe or ‘dockage’ in the
U.S.A. Use of one term in the following includes
all.
,mpurities
Frequently encountered components of screen-
ings may be classified according to their composi-
tion thus:
Vegetable matter - weed seeds, grains of other
cereals, plant residues such
as straw, chaff and sticks.
- fungal impurities - bunt
balls, ergot.
- rodent excreta and hairs,
insects and insect frass,
Mineral matter - mud, dust, stones, sand,
metal objects, nails, nuts
etc.
- string and twine. Miscel-
laneous rubbish.
Purity of cereal samples can be improved by
cleaning and by separating. Cleaning involves the
removal of material adhering to the surface of
grains, while separation involves the removal of
freely assorting material. Considerable ingenuity
has been demonstrated in the design of devices
Animal matter
Other
,-ss&ense cf: .l>s9ken gr2ins, &sr and.-gXxa p!azs..
116 TECHNOLOGY OF CEREALS
for eliminating impurities by both methods and
as a result a wide range of machines is produced
by a number of manufacturers. Wide though the
range is, the principles involved are few. Thus
cleaning depends upon abrasion, amhim or impact,
and separation upon discrimination by size, shape,
specific gravity, composition and texture. No single
machine can perform all the necessary operations
and it is customary for parcels of cereal to pass
through a series of operations based on the above
principles.
Metals
Because of the potential danger to be caused
to machinery by hard objects, these are usually
among the first to be removed. Thus metals are
removed by devices capable of detecting their
composition, installed in spouting through which
the grains are directed early in the process.
Ferrous metals are attracted by strong magnets.
Their removal from the magnetic surface by grain
flow cannot be avoided so magnets are designed
to revolve allowing removed metal objects to fall
into a reservoir out of the grain stream. Non-
ferrous metals are detected by the interruption
which they cause to an electric field when passing
through a metal detector. The interruption rapidly
activates a mechanism which temporarily diverts
the stream containing the offending item.
Destoners
Stones, sand and string also constitute potentially
damaging impurities and hence these are also
removed early. A single separator is capable of
removing these and other less hazardous materials
such as straw and some seeds of other cereals.
Size and shape are the distinguishing criteria
here, as the process is essentially one of sieving
through a screen coarser than the grain (to remove
string, straw and larger objects such as stones and
large grains of different species) and over a screen
finer than the required grains, through which
broken grains, small seeds and sand can pass.
Specific gravity separations may be made on
the selected material leaving the separator. The
grains leave the machine as a curtain and this is
subjected to an upward stream of forced air which
lifts light material such as chaff, while the denser
particles fall under gravity. The aspirated air
can flow to an expansion chamber where solid
particles are deposited; air can then be recycled.
For more rigorous removal of stones special-
ized dry stoning machines may be used. Stoners
have an inclined vibrating deck through which a
uniform current of air flows upward. Feed is
directed on to the lower end of the deck and
spreads and mixes in response to the vibration.
On reaching the region of the bed where the
fluidizing effect of air is experienced, the grains
are lifted beyond the range of influence of the
vibration. They fall under gravity and are dis-
charged at the lower end of the deck. Stones are
too dense to be lifted by the air stream and
continue to the top of the deck where they are
discharged.
Unwanted species
Elimination of grains of other cereals and weed
seeds surviving the above treatments is achieved
by disc separators (see Fig. 5.10). Discrimination
is on the basis of length, the width being of no
consequence to this operation. A series of discs
are mounted on a single horizontal axle in a trough
partly filled with grain. The axle is driven causing
the discs to rotate through the bed of grain. Each
disc has, on both surfaces, a series of indentations
arranged concentrically. The indentations behave
as pockets into which grains of the required depth
or smaller can fit. They are thus lifted out of the
grain mass. As pockets pass the high point of the
rotation they fall out of the pockets into channels
adjacent to the disc faces by which they are
conducted into a discharge in common with
channels from the other discs. Material failing to
be picked up in pockets is driven by worm from
the feed end to the discharge end of the machine.
A gate at the discharge end can be adjusted to
control the amount of time spent in the trough.
The more impurities the shorter the time needed
for sorting.
By use of several machines, each with different
sized pockets in its discs, a number of separations
may be made in sequence, with the required
117STORAGE AND PRE-PROCESSING
ledium clean grain
,eed
A 8 I-I --"
I
/ I I
r Oats and barley
I ~maLl seeds
G- Small clean grain
FiG 5.10 Carter-Day Disc Separator for length separation of impurities from wheat, rye and other
cereals grains. Exterior view of machines, right; diagram of operation, left. (by courtesy of the
Carter-Day Company, Minneapolis.)
to the destoner. Stocks are stratified according to
their specific gravity by the oscillating action of
the sloping deck, the lighter stocks are conveyed
on a current of air while the heavier material is
directed by friction on the oscillating deck. The
routes of the two fractions can thus be separately
ordered. Dust is removed by the air flow that
provides the air cushion.
Aspiration
The rate at which a particle falls in still air is
the resultant of the speed imparted to it by the
force of gravity, balanced by the resistance to free
fall offered by the air .The rate of fall, or 'terminal
velocity' , depends on the weight of the particle
and its surface-area:volume ratio. Compact
spherical or cubical particles thus have a higher
terminal velocity than diffuse or flake-like particles.
Instead of allowing the particles to fall in still air ,
it is more usual to employ an ascending air current
into which the stock is introduced. The velocity
of the air current can be regulated so that particles
of high terminal velocity fall, while those of low
terminal velocity are lifted. Utilizing this principle,
cereal behaving as the selected material when
eliminating larger impurities, or the rejected
material when eliminating small impurities. In
some machines several separations can be made
within a single unit.
The same principle of separation in pockets on
a rotating device is used in trieur cylinders. In these
the interior surface of the cylinder has the pockets
in it. The capacity of disc machines is greater
than that of cylinders of the same diameter but
the selectivity of cylinders is said to be better .
The loading of discs or cylinders can be reduced
if concentrators are used upstream of them.
Concentrators, otherwise known as gravity selectors
or combinators (Fig. 5.11), serve to effect a prelim-
inary streaming of stocks so that the individual
streams can be subjected to treatment appropriate
to the impurities they are most likely to contain.
Thus a light fraction would be treated to remove
small seeds and grains of unwanted small cereals
by use of discs or cylinders. The denser stream
would not need to pass through these machines
but may be routed to the more appropriate
destoner .
The concentrator operates on a similar principle
118 TECHNOLOGY OF CEREALS
-*
2
__- --
I
-I
*IC;
FIG 5.11 Gravity selector. (Reproduced by courtesy of Satake UK Ltd, Stockport.)
STORAGE AND PRE-PROCESSING 119
slots. The recess upends undersize grains directing
them through the hole.
Where size and shape are similar and only small
differences in specific gravity may exist, sophisti-
cated gravity separators are required. An example
of this type of machine is the Paddy separator
(Fig. 5.12). In this a table mounted on rubber
tyres is made to pivot around its horizontal axis.
Compartments on the table are formed by a series
of zig-zags arranged at right angles to the direc-
tion of motion. Grains accumulate and stratify
within the channels and the motion of the table
causes those light grains which rise to the top, to
travel towards the upper side of the channel
flanks, while the heavier stocks pass down the
table. As might be expected with a separation
based on small differences careful adjustment of
variables is necessary.
Gravity separations have recently been found
capable of separating satisfactory grains of wheat
from sprouting grains. The sprouting, having led
to some mobilization of starch, has rendered the
affected grains lighter while not changing their
particles of chaff, straw, small seeds, etc., having
terminal velocity lower than that of wheat, can
be separated from wheat in an aspirator, in which
an air current is directed through a thin falling
curtain of stock. In a duo-separator the stock is
aspirated twice, permitting a more critical separa-
tion to be made. In this type of machine, the
lifted particles are separated from the air current
by a type of cyclone, and the cleaned air is re-
cycled to the intake side of the machine. Such
closed-circuit aspirators save energy and minimize
effluent problems.
The separations described thus far are related
to the separation of the required species from
other cereal species, weed seeds and other materials
quite unlike the grains of the required species.
There are occasions when it is required to make
a separation of two types of grain of the required
species. This usually means shrivelled grains
or those that have been hollowed as a result
of insect infestation. They are removed mainly
by aspiration or other methods depending on
specific gravity. In oats the following types are
present: overall size.
1. Double (bosom) oats.
2. Pin oats. Cleaning
Cleaning, in contrast to separating, is performed
3. Light oats.
4. Other types of oats.
on scourers. As their name suggests, these subject
In double oats the hull of the primary groat grains to an abrasive treatment designed to remove
envelopes a normal groat, plus a second complete dirt adhering to the surface of grains. Surface
grain. Both groats are usually of poor size. Pin layers such as beeswing (outer epidermis of the
oats are thin, short and contain little or no groats, pericarp of wheat) may also be removed. Scourers
light oats consist of a husk which encloses only propel grain within a chamber by means of rotors
a rudimentary grain. Other types are twins, to which are fixed beaters or pins. The axis of
discoloured green and hull-less. (Deane and the rotor may be horizontal or vertical according
Commers, 1986). to type. In the horizontal type, beaters rotate
While indented discs and/or cylinders separate within a cylindrical wire mesh. Grains enter at
on the basis of length, it is sometimes required, one end and are cast against the mesh by the
particularly with oats, to eliminate narrow grains inclined rotor beaters; cleaning is achieved by the
by a separation on the basis of width. This friction of grains against each other or against the
involves the use of screens with elongated but mesh. Dust removed passes through the screen
narrow slots, through which only reject grains and falls into hoppers by which it is discharged.
can pass. The screens are commonly in the form Aspiration following scouring removes particles
of a perforated rotating cylinder through which the loosened but not removed from the grains. Some
bulk flows horizontally. Undersize grains drop vertical machines work on a screen and beater
through into a hopper. Small round holes recessed system but others combine a scouring action with
into the cylinder walls are used as an alternative to aspiration.
120
TECHNOLOGY OF CEREALS
FIG 5.12 Paddy separator. (Reproduced by courtesy of Satake UK Ltd. Stockport.)
FIG 5.13 Schematic diagram of a possible flourmill screen room.
STORAGE AND PRE-PROCESSING 121
ings removed stage by stage. As the total quantity
of screenings removed frequently amounts to only
1-1.5% of the feed by wt, every machine in the
screenroom flow must have a capacity (i.e. be
able to deal with a rate of feed) practically equal
to that of the first machine.
Durum wheats are subjected to particularly
vigorous treatment with beaters. In addition to
cleaning, the beaters also eliminate much of
the embryos, considerably facilitating the clean-
ing of semolina produced and reducing its ash
yield.
Scourers have replaced washers in the cleaning
programmes of wheat in most countries on account
Of
1. Problems of pollution control concerning the
2. Problems with microbiological control (mainly
3. High costs of operating machinery and of
Loop system
The feed rate to many of the machines can
discharge of effluent. be reduced, and the efficiency of separation
increased, by an arrangement known as the loop
bacterial). or by-pass system.
In the loop system, the first machine is set to
water. reject a large cut-off (say 10% of the feed)
containing all the separable impurities, together
Washing may still be practised in the former with a proportion of clean grain. The remaining
Soviet Union, where ultrasonic vibrators were 90% or so is accepted as clean, requiring no
used to assist the cleaning process (Demidov and further treatment. The cut-off is retreated to
Kochetova, 1966). Wet cleaning is also in use in recover clean wheat. As the cut-off amounts to
the United States for removing surface dirt from only a fraction of the total feed, the feed rate
maize before milling (Alexander, 1987). in the retreatment machines can be much re-
duced, and the efficiency of separation improved.
The loop system is frequently applied in the
operation of disc separators and trieur cylinders,
and is a feature of the Twin-Lift Aspirator, which
embodies main aspiration of the whole feed, and
re-aspiration of a substantial cut-off to recover
clean grain.
The Buhler ‘concentrator’ operates on the loop
system. A cut-off of about 25% by wt. is lifted
by aspiration and specific gravity stratification:
this contains various impurities but is stone-free,
and goes to a gravity table or scourer for separa-
tion of clean wheat from the impurities. The
remaining 75% is free of impurities other than
stones and needs only to be de-stoned on a dry
stoner or gravity table.
Individual components of a mixed grist to a
flour mill (cf. p. 192) are cleaned separately before
blending. Wheat types differ in grain size and
shape, and in the types of impurities contained,
and require individual treatment in the screen-
room, particularly as regards choice of sieve sizes
for the milling separator, and of indent sizes for
the cylinders and discs.
Screenroom operation
In practice, the grain passes in succession
through a series of machines working on the
various principles described. No single machine
can remove all the impurities, but all the machines,
considered as a unit, remove practically all the
impurities for the loss of very little of the required
grain.
A possible cleaning flow for wheat is shown in
Fig. 5.13.
The efficiency of operation of screenroom
machinery depends on machine design, feed rate
and proportion of cut-off (reject fraction). As
the feed rate is reduced, interference between
particles decreases, and efficiency of separation
increases. As the proportion of cut-off increases,
the rejection of separable impurities becomes
more certain.
In the conventional screenroom arrangement,
it is customary to feed to each machine in
succession, the entire feed, except for the screen-
122 TECHNOLOGY OF CEREALS
Dust explosions
Dust is released whenever grain is moved: the
atmosphere in grain silos and mills therefore
tends to be dusty, leading to conditions under
which dust explosions may occur. A suspension
of dust in air, within certain limiting concentra-
tions, may explode if a source of ignition above
a certain limiting temperature is present. A series
of dust explosions in U.S. grain elevators in 1978
caused the death of at least fifty persons, and led
to the initiation of extensive programmes of
research into the causes and avoidance of dust
explosions.
Avoidance of dust explosions is directed to:
washing and, as the water is absorbed by the
grains, there is no effluent problem. Neither is
bacterial infestation usually a problem if lying
times are short. If problems do arise they can be
minimized by chlorination to levels above that of
the water supplied.
The study of the effects of water on the physical
properties of cereals, and hence their milling
properties, is complex. The cells of the grain have
several components, the physical properties of
which are altered, each in its own way. Water
also affects the adhesion between the components
of the cells, the cells themselves and the various
tissues of the grain.
Although applied to almost all cereals, condition-
ing is not a single process appropriate as a
preparation for all milling treatments. Even
different types of the same cereal undergoing the
same treatment may receive different treatments
or different degrees of the same treatment. Con-
sequently each cereal type will be considered
separately here.
Common or bread wheat
The effects of moisture content and distribu-
tion have received more attention in relation to
wheat than any other cereal. Variation in endo-
sperm texture demands different treatments; in
general the amount of water added and the length
of lying time increase in proportion to endosperm
hardness.
The reasons for conditioning are:
1. to toughen the outer, non-starchy endosperm
components of the grain so that large pieces
survive the milling, and powdering of bran is
avoided;
2. to mellow the endosperm to provide the
required degree of fragmentation.
The term ‘mellow’ is much used to describe a
desirable state of endosperm but a useful definition
of its meaning is difficult to find. The changes
induced by water penetration have been better
described by Glenn et al. (1991) who found that
endosperm strength decreased. ‘Strength’ as used
by these authors has its strict materials-science
meaning, i.e. a measure of the stress required to
1. suppression of dust and
2. avoidance of sources of ignition.
Devices that minimize dust formation include
light damping with water (about 1% by wt), dust-
free intake nozzles, dust suppressors, and the
‘Simporter’ system - a method of mechanical
grain handling in which the grain is conveyed
‘sandwich fashion’ between two belts held together
by air pressure. By the use of inclined belts
operating on this principle, grain can even be
conveyed vertically.
Sources of ignition most frequently responsible
for dust explosions in flour mills are welding and
hand lamps. Other potential sources are flames,
hot surfaces and bearings, spontaneous heating,
electrical appliances, friction sparks, static elec-
tricity, magnets, bins and bucket elevators.
Damping
Damping, as a pretreatment for milling grists,
is a long established practice. It probably origin-
ated as part of the cleaning process when washing
featured as an important treatment. It was adopted
as a treatment in its own right when the beneficial
effects of washing followed by a period of lying
were noted. In current practice the washing of
most cereals for cleaning purposes has now been
abandoned but conditioning - or tempering
(both terms mean the controlled addition of
moisture) remains an important stage of proces-
sing. Conditioning requires much less water than
STORAGE AND PRE-PROCESSING 123
bring about fracture. The toughness (senso stricto) Losses of between 1 and 2.5% have been
of the endosperm (manifested as the energy recorded due to evaporation of water from stocks.
needed to bring about complete fracture) was
found by the same authors to decrease with
Water penetration
increasing moisture content, although this was
less consistently observed than the increase Water is absorbed rapidly by capillarity into
in strength. The hardness of the endosperm is the empty pericarp cells. They are capable of
not the only factor determining the level of taking up 80% of their weight, equivalent to about
water addition. Clearly the grain moisture has 8% (w/w) of total grain weight. Once wetted
an effect on the fractions separated during mill- therefore, the bran can behave as a reservoir from
ing. It influences their surface properties which which water can pass into the endosperm. The
are particularly significant in relation to their rate of passage is restricted by the presence of
behaviour on sieves if the covers are made from impermeable components of the testa and possibly
absorbent fibres such as nylon or (particularly) the nucellar epidermis. Since these are thinnest in
silk. Moisture can also be transferred to the fabric the non-adherent layers overlying the embryo this
of the sieves themselves, possibly altering the is the area through which most water gains access
frictional properties and even changing the to the inner components of the grain. It advances
tension and the effective aperture size. The towards the distal end of the endosperm, leading
ease with which starchy endosperm may be to near-uniform distribution within the whole
separated from bran is also influenced by mois- grain. Whereas saturation of the pericarp takes
ture content and distribution. Both increase only minutes, equilibration throughout the grain
and decrease of difficulty of this separation takes hours.
with increasing moisture content have been The exact time taken depends on several factors
imputed (although an increase is the more including:
frequently claimed) but direct evidence is scarce
and measurements must be difficult to make. It
is generally accepted that, for the milling of high
extraction rate flour, conditioning to a lower than
normal moisture level is appropriate. Another
factor influencing the degree of damping and
post-damping lying is the target moisture content
of the final products. Storage life of flour declines
with increasing moisture content and most speci-
fications impose a maximum moisture content -
mainly to avoid buying water at flour prices.
The relative costs of transport and the require-
ment to absorb adequate water in dough making
also exert an influence however. In calculating
the amount of water required to achieve the target
moisture content of stocks, characteristics of the
mill and its location have to be considered. These
include:
1. the conveying system in use (pneumatic sys-
tems incur more moisture loss than elevator
systems);
1. the initial moisture content of the grain;
damper wheats absorb more rapidly (Moss,
1977) as well as requiring less damping.
2. The degree of permeability of the testa -
particularly in the embryo region. There is
little information on this from conditioning
studies, but studies on imbibition in relation
to germination show this to vary (Wellington
and Durham, 1961; King, 1984).
3. The mealiness of the endosperm - water
permeates through mealy endosperm more
quickly than through vitreous endosperm.
The essential feature appears to be the greater
proportion of air spaces in mealy tissues.
As soft wheats are more mealy than hard,
equilibration is achieved more rapidly in softer
wheats (Stenvert and Kingswood, 1976). As
the continuity of the protein matrix is influ-
enced by total nitrogen content of the endo-
sperm, moisture movement is slower in high
nitrogen wheats than in low.
4. The temperature of the water - a 3-fold
increase in penetration rate was recorded for
2. ambient temperature and humidity;
3. roll temperatures.
124 TECHNOLOGY OF CEREALS
each 12°C rise, by Campbell and Jones (1955)
within the range 20"-43"C, above 43°C smaller
rate increases applied up to 60°C.
5. The uniformity of the water distribution among
grains. Where less than 8% water (w/w) is
added it is preferentially absorbed into the
pericarp of the grains which it contacts first,
leaving little for distribution to more remote
grains. Equilibration among grains takes even
longer than within a grain.
It is possible to exploit the above to achieve
more rapid conditioning. Thus:
to achieve the optimum moisture content of a
particular type of wheat clearly depends upon the
initial moisture content of the wheat. Wheat that
travels long distances by sea tends to be traded
at lower moisture content than wheat bought
locally (water is expensive to transport!). If more
than 3-5% needs to be added, it must be done
in more than one stage. The amount capable of
addition in one stage depends on the method of
addition.
In preparing wheats for milling as a mixed
grist, in wfich components are introduced into the
mill at different moisture contents, the individual
higher starting moisture content for each blended after lying. Although the time taken for
water to penetrate into the grain is reduced
addition;
impermeable layers;
conditioning at elevated temperatures has declined
3. damping cauSeS expansion of the endosperm, as a result of prohibitive costs and the introduction
but this is reversed as equilibrium is achieved of alternative practices that allow reduced lying
or if drying occurs. such changes induce
times. Details of warm and hot conditioning
increase effective mealiness;
Lockwood, 1960; Smith, 1944). They will not be
4. warm (above ambient to 46"C), or hot (above described here*
46°C) conditioning may be practised; Principles underlying the design of modern
5. precision placement of water in a uniform
conditioning systems are:
fashion andor VigOrOUS mixing of grains during 1. addition of accurately metered quantities of
and immediately following damping may be water;
employed.
2. uniform distribution of moisture.
Much research has been carried out on the length Accurate water addition is achieved by control-
of time required to achieve equilibrium moisture ling the water addition and flow rate of wheat,
conditions. With so many factors affecting the rate possibly using feed-forward and feed-back control
of penetration it is not surprising that results are systems. Uniform distribution is ensured by
variable. Estimates range from several hours to careful placement of water followed by rapid
several days, with more recent work tending to mixing. Most milling engineers now produce
favour the shorter periods. Such revision of ideas conditioning equipment with sophisticated control
has influenced the manner in which conditioning systems. The H20-Kay (Satake UK Ltd) system,
of wheat is carried out (Hook et al., 1984). consisting of the Kay-Ray controller and a Techno-
vator mixer, employs moisture measurement by
microwave attenuation meters. This is the only
method by which the moisture of freshly damped
Conditioning practice
Both phases of conditioning (i.e. addition of wheat can be monitored, allowing corrections to
water, and lying for a period of equilibration) be made to the levels of water addition on a feed-
require specially designed equipment, the design back basis. The system has the additional facility
of which has evolved to suit changing circum- for adding steam. A sampling stream is diverted
stances and changing ideas about conditioning. from the main flow, through a cell in which its
The amount of water required to be added moisture content is measured. Additionally, flow
1. multistage damping provides a progressively
components are best conditioned separately and
2. abrasion of the grain surface can remove the
as a function of temperature, the practice of
stresses which cause minute Stress cracks that
systems are provided in older textbooks (e*g*
Capacitance moisture
- measurement
-- - - - - - - - - -
I
c- - -------
Microprocessor
- Temperature
measurement
I
I
I
Throughput
measurement
I1
I
yjjy+------A I
. *. ...
.e....
. . . . . .
126 TECHNOLOGY OF CEREALS
Rice
Rice is milled at 14% moisture content with no
damping immediately preceding dehulling. For
rice subjected to parboiling however an elaborate
heat/water treatment is involved. Paddy rice has
a hull of adherent pales (Fig. 2.23, p. 43) and
water penetrates very slowly. Parboiling breaks the
tight seal between the two pales, thus removing
the main obstacle to entry. This was the original
purpose of parboiling, which is an ancient tradi-
tion, originating in India and Pakistan. Other
changes occur however, including an improve-
ment in nutritional quality of the milled product.
The hot water involved in treatment dissolves
vitamins and minerals present in the hull and
bran coats, and carries them into the endosperm.
Conversely, rice oil migrates outwards during
parboiling, reducing oil content of the endosperm
and increasing it in the bran. Starch present in
the endosperm becomes gelatinized, toughening
the grain and reducing the amount of breakage
during milling. Aleurone and scutellum adhere
more closely so that more of each remains in the
milled rice. Some discoloration of grains occurs
but susceptibility to insect attack is reduced.
.-.--------.-/--
Barley
FIG 5.15 Manifold discharge from a First-in-first-out type
conditioning bin. From: Screenroom Operations 1. Flour
Milling Industry Correspondence Course. By courtesy of
Incorporated National Association of British and Irish Millers
Ltd.
ing . It is particularly important for conditioned
grain. It is usual for a bin to be filled by deposition
from a central spout: grains delivered to the
centre tumble towards the edges but the pile
remains deepest at a point below the delivery
spout and a smooth conical profile with an ‘angle
of repose’ characteristic of the particular sample
remains at the top. As the bin empties, the profile
changes because the grains in the centre fall more
quickly than those at the edges. This leads to a
reversal of the original surface profile, the centre
becoming the lowest point. Such a means of
emptying is disorderly, it leads to a mixture of
grains and a sequence of removal different from
that of filling. The sequence of emptying may not
Conditioning for pearling consists of adjusting
the moismre cOntent Of&e grhs to 15vo, followed
by a rest of 24 h.
Maize
The details of the conditioning protocol in
preparation for degerming of maize depend upon
the process in use: for the Beall degerminator,
the moisture content is raised to 20-22%, while
for entoleter or rolling processes an addition of
only 2-3% to stored grain is appropriate. Follow-
ing damping a rest of 1-2 h is usual but up to
24 h is possible.
Conditioning bins
In all storage bins used for cereal grains, sound
design is important with regard to efficient empty-
STORAGE AND PRE-PROCESSING 127
F. H. (Ed.) Amer. Assoc of Cereal Chemists Inc., St.
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DEMIDOV, P. G. and KOCHETOVA, A. A. (1966) Effect of
ultrasonic waves on the technological properties of grain.
Izv. Vyssh. Ucheb. Zaved., Pishch Tekhnol. 1: 13.
EVERY, D. (1987) A simple four-minute protein-solubility
GLENN, G. M., YOUNCE, F. L. and PITTS, M. J. (1991)
Fundamental physical properties characterizing hardness
of wheat endosperm. J. Cereal sei. 13: 179-194.
HOOK, S. C. W., BONE, G. T. and FEARN, T. (1984) The
conditioning of wheat. A comparison of U.K. wheats
milled at natural moisture content and after drying and
conditioning to the same moisture content. 3. Sci. Food
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MOSS, R. (1977) The influence of endosperm structure,
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NELLIST, M. E. (1978) Safe Temperatures for Drying Grain.
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OSBORNE, B. G., FISHWICK, F. B., SCUDAMORE, K. A. and
ROWLANDS, D. G. (1988) The Occurrence and Detection of
Pesticide Residues in UK Grain. H-GCA Research Review
No. 12. Home-Grown Cereals Authority, London.
PIXTON, S. W. (1980) Changes in quality of wheat during
18 years storage. In: Controlled Atmosphere Storage of
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POMERANZ, Y. (1974) Biochemical, functional, and nutritive
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be important in general storage considerations
but in conditioned grain, where lying time is
controlledy it is importan' that the grains intra-
duced into the bin first are also the first to leave.
The difficulties in achieving this are compounded
the tendency to non-sequentid flow is exaggerated*
Also the shortening of lying times associated with
contemporary practice demands greater precision
in controlling the lying period, emphasizing the
need for sequential flow.
The expedient by which improvements have
been made is the deconcentration of outlets. In
new bins a matrix of funnel+haped hoppers may
that ducts to a common discharge hopper via a
manifold (see Fig. 5.15).
~~i~~i~~ bins may be converted by imposing a
cone-shaped baffle near the top of the discharge
hopper. The cone may be point-up, surrounded
by an annular space, directing flow to the periphery
Of the biny Or point-down with an annu1ar space
and an orifice in the centre also. In bins of square
section pyramidal baffles replace the conical ones
described for cylindrical bins.
as damped grain flows less readily than dry and
test for heat-damage in wheat. J. Cereal Sci. 6: 225-236.
~gn~. 35: 591-596.
be provided at the base> each leading to spouting
LOCKWOOD, J. F. (1960) Flour Milling. 4th edn. Henry
References
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128 TECHNOLOGY OF CEREALS
Further Reading
ANON. (1987) Pests of Stored Wheat and Flour. Module 2 in
Workbook Series, National Association of British and Irish
Millers. H-GCA London.
BAKKER-ARKEMA F. W., BROOK, R. C. and LEREW, L. E.
(1978) Cereal grain drying. Adv. Cereal Sci. Technol. 2: 1-77.
BHATTACHARYA, K. R. and ALI, S. Z. (1985) Changes in
rice during parboiling, and properties of parboiled rice.
Adv. Cereal Sci. Technol. 7: 105-159.
CORNWELL, P. B. Pest Control in Buildings. A Guide to the
Meaning of Terms. Rentokill Ltd. E. Grinstead.
GORHAM, J. R. (Ed) (1991) Ecology and Management of Food-
Industry Pests. FDA Technical Bulletin 4. Arlington VA.
Assoc. of Official Analytical Chemists.
HOME-GROWN CEREALS AUTHORITY (1992) Storage Crops
SAUER, D. B. (Ed.) (1992) Storage of Cereal Grains and their
Products. Amer. Assoc. of Cereal Chemists Inc. St. Paul
MN. U.S.A.
TKATCHUK, R., DEXTER, J. E. and TIPPLES, K. H. (1991)
Removal of sprouted kernels from hard red spring wheat
with a specific gravity table. Cereal Chem. 68: 390-395.