Recommended
Maximum Moisture Content for Storing Baled Cotton
Table
of Contents
Recommendation
Issue
ARS
Bale Moisture Studies
– Summary
Results
from Individual Studies
Microbial
Contamination
Future
ARS Studies
ARS
Cotton Research Program Links
Additional
Links of Interest
Recommendation
Agricultural
Research Service studies currently indicate that moisture contents above
7.5-percent in stored cotton bales cause fiber quality deterioration.
After a review of ARS and other research by their Quality Task Force,
the National Cotton Council recommends 7.5 percent moisture content (wet
basis) as the maximum moisture level for stored cotton bales, until such
time as more research refines this number further.
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Issue
The United States is a major world supplier of cotton. The desirability
of U.S. cotton depends heavily on its reputation for consistent, high-quality
fiber. Maintaining proper moisture levels in cotton bales from unloading
through bale packaging and during storage is critical for preserving fiber
quality for textile processing.
Moisture management is critical to cotton cleaning, handling, and fiber
quality preservation at the gin. Cotton with too high a moisture content
will not easily separate into single locks but will form wads that may
choke and damage gin machinery or entirely stop the ginning process. Cotton
with too low a moisture content may stick to metal surfaces as a result
of static electricity generated on the fibers and cause machinery to choke
and stop. Fiber dried and processed at low moisture contents is more brittle
and easily damaged by the mechanical action required for cleaning and
ginning. When pressing and baling low-moisture cotton, hydraulic pressure
dramatically increases causing excessive equipment wear, and problems
with bale tie breakage escalate.
The effort required to measure and control moisture pays dividends in
gin operation efficiency and market value of the cotton. Research has
shown fiber moisture content for seed cotton cleaning and ginning cotton
is best at 6 to 7 percent, which allows for sufficient cleaning with minimal
fiber damage. Packaging bales at these moisture contents reduces press
force, static electrity, and bale tie breakage. Bale storage at moisture
contents greater than 7.5 percent can cause degradation to the fiber color
during long-term storage.
Reports from textile plants and marketing firms as well as results from
ARS research studies have shown that there is potential for a distinct
loss of fiber quality when bales are stored with a moisture content greater
than 7.5 percent. Two methods are generally used to add moisture to cotton:
humidified air pushed or pulled through the cotton and liquid water droplets
directly sprayed on the top of a batt of cotton as it moves down the lint
slide into the bale press.
Adding water poses two important problems: applying just the right amount
of water to reach the most desirable moisture content and applying the
water in an even and consistent manner. Humidified air, which contains
water in vapor phase that is readily absorbed by fiber, pulled or blown
through the cotton, tends to be absorbed in a relatively uniform manner.
Directly spraying liquid water on the top of a several inch-thick-batt
of cotton, however, can uniformly apply the water only to the top surface
of the batt of cotton. It is generally believed that transfer of the liquid
water to the remainder of the 4- to 12-inch thickness of cotton is difficult
because 1) raw cotton fiber with all its original surface waxes does not
readily absorb liquid water and 2) the cotton bale is immediately packaged
at high densities, which greatly retards any further movement of water
vapor or liquid.
The exact "best" moisture level for cotton bales during storage
has been difficult to precisely determine. This is due in part to the
complicated interactions of the water application method, bale covering,
bale density, and storage climate.But ARS studies have found a definite
pattern of fiber quality loss when the moisture content of bales is raised
beyond 7.5 percent.
The main problem is that, when enough water is sprayed directly on the
outer layer of cotton to achieve the 7.5-percent moisture level, it often
results in fiber color changes as well as creating an environment more
conducive for microbial growth, due to the presence of liquid water on
the fibers.
Moisture applied to cotton directly influences the High Volume Instrument
color grade of cotton, which is based on a combination reading of the
grayness (Rd) and yellowness (+b) of the cotton sample. The color grade
is used as a basic market factor and, as color loss occurs, cotton value
decreases.
Several recent ARS studies conducted at public and private gins clearly
demonstrated that too much water sprayed directly on cotton fiber as it
moves down the lint slide has negative effects on grayness and yellowness.
Although the initial color grade at the Classing Office was not changed
because the water quickly evaporated from the loose samples taken from
the outside of the bale, the color degraded in the bales during extended
storage at high densities. The color reduction was as much as several
grades and depended on the amount of moisture added. In fact, some bales
decreased in value as much as $35 per bale due to color change and $9
per bale due to evaporative weight loss. The bales also arrive at the
textile mill at a lower color grade than anticipated and adversely affect
mill plans to integrate the bales with many others to produce the desired
final product.
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ARS
Bale Moisture Studies
Summary
Based on several studies under a number of conditions, ARS research has
found that water sprayed on cotton fiber can adversely impact grayness
and yellowness, and thus color grade, at moisture levels as low as 7.3
percent (wet basis). The degree of impact increases as moisture content
increases and may cause a shift in color grade. The amount of water that
can be safely applied to cotton fiber depends on the bale covering, the
grayness and yellowness of the fiber before storage, and storage climate.
Regardless of bale covering, color problems were seen at moisture levels
greater than 8 percent.
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Results
from Individual Studies
In one ARS study, water was sprayed on several bales of cotton fiber
as they came down the lint slide in 1- to 2-inch thick batts. The bales
were covered in 3 layers of polyethylene and stored at initial moisture
contents of 6.0 (control), 7.3, 8.9, 13.9, and 15.4 percent (wet basis)
for more than 4 months. Bales were then opened and 100 samples taken from
each for grade, fiber, and moisture analyses. The grayness and yellowness
of all the bales that had water sprayed on them were negatively affected,
including the bale at 7.3% moisture content. Color was reduced from 31
(Middling) to 41, 42, or 43, depending on the amount of water added. Bales
with water sprayed on them lost from 3.5 to 12.5 pounds per bale.
For
more information on this study click here
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In a second ARS study, water was sprayed on cotton fiber as it came down
the lint slide in 1- to 2-inch thick batts. The bales were packaged in
strip-coated, woven polypropylene, and stored at initial moisture contents
of 4.8, 7.4, 8.5, 10.4, 12.0, and 12.7 percent (wet basis) for five months.
Bales were then opened and 100 samples were taken from each one for grade,
fiber and moisture analyses. The grayness and yellowness of all of the
bales packaged above 7.4 percent were negatively affected. Color was reduced
from 31 (Middling) to 32 (Middling light spot). It is also important to
note that bales with water applied to an initial bale moisture level of
more than 7.4 percent lost 10.5-22.5 pounds per bale while in storage,
at a cost of about 70 cents per pound, despite being packaged in strip-laminated,
woven polypropylene, which is used on 52 percent of U.S. cotton bales.
For
more information on this study click here
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In a third ARS study, water was sprayed on cotton fiber as it came down
the lint slide in 1- to 2-inch thick batts. The bales were packaged in
fully coated, woven polypropylene, and stored at moisture contents of
7.8, 8.0, 9.0, 9.6, and 10.7 percent (wet basis) for 6 months. (Note that
the fully coated, woven polypropylene is a promising new bagging planned
for use on U.S. bales.) Bales were then opened and 100 samples were taken
from each bale for grade, fiber and moisture analyses. The grayness and
yellowness of all the bales packaged above 7.4 percent moisture content
were negatively affected. Color was reduced from 31 (Middling) to 32 (Middling
light spot). Bales given an initial moisture level of more than 7.8 percent
lost 4-11 pounds per bale while in storage, despite being packaged in
fully coated, woven polypropylene.
For
more information on this study click here
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In a fourth ARS study, 36 bales of cotton were packaged in non-ventilated
polyethylene film after water was sprayed on the top of a 4- to 6-inch-thick
batt as it came down the lint slide at the bale press of a commercial
gin. Polyethylene film is used on 39 percent of U.S. bales. Four target
moisture content levels were 5.7 (control no water added), 7.4,
9.1, and 10.7 percent (wet basis). Liquid water application rates, in
total pounds water per bale, to reach each level were determined for a
500-pound bale at ambient conditions. The liquid water application rates
for the 4 target levels were 0, 10, 20, and 30 pounds per bale. Nine bales
were processed at each of the 4 target levels, which were then put into
storage in Clemson, SC. Three bales from each of the target moisture levels
were randomly selected, opened, and tested at intervals of 1, 2, and 6
months. Six-month average moisture contents (wet basis) for the bales
at each of the 4 moisture levels were 5.7, 6.7, 8.4, and 9.5 percent.
In general, the bales stored at final wet basis moisture levels of 5.7
and 6.7 percent did not change in any of the fiber properties measured
at any time interval. However, after 6 months, both the 8.4- and 9.5-percent
average moisture bales had significant changes in both reflectance (Rd)
and yellowness (+b). The reflectance for the 9.5-percent bales after 6
months storage had dropped from 82 to 77, and yellowness increased from
about 7 to over 9. The fiber from these bales were then processed into
yarn. There was a marked deterioration of quality in yarns spun from the
two higher moisture level cottons. In addition, the bales stored at the
two lowest moisture levels had gained weight and the bales stored at the
two highest levels had lost weight after 6 months.
For
more information on this study click here
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Moisture restoration systems at 18-20 gins in Mississippi and Arkansas
were also examined in 2002 and 2003. Gins used several different types
of moisture restoration systems such as lint slide grid, humidified air,
and direct water spray. More than half of the gins packaged bales at moisture
contents above 7.5 percent, and most of those gins used the direct-spray
method. In 2003, gins using the direct spray method to add moisture packaged
22.9 percent of their bales at moisture contents above 7.5 percent, as
compared with 0.6 percent for those using the humidified air method. One
gin using the direct spray method packaged 48 percent of its bales at
moisture contents above 7.5 percent.
For
more information on this study click here for Article 4a and Here
for Article 4b
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Microbial
Contamination
Previous ARS long-term studies have shown that very little microbial activity
occurs in bales during storage. But these bales had not been augmented
with additional moisture to restore moisture loss or to add weight to
the bales. During moisture restoration studies, the level of microbial
activity was influenced by the amount of moisture added to the cotton
and the temperature and length of storage.
In a study where very moderate amounts of moisture were added, increases
in microbial populations began to be observed at an 8-percent moisture
level (initial moisture level of 8.0
percent with a level of 6.8 percent after 1 year storage).
In another study with a more extreme moisture range (15.4 percent moisture
at baling), fungal activity was visibly and odorously noticeable after
less then 4 months of storage.
For
more information on this study click here
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Future
ARS Studies
To
further refine knowledge of the best moisture percent for stored bales,
ARS is pursuing additional studies to verify the maximum safe storage
moisture content more precisely as well as the quantity of water that
can be safely added to cotton fiber. These studies will be evaluated across
the various industry segments to meet the needs of the entire industry
rather than specific segments.
ARS has two studies in progress. One study has replicated bales treated
and in storage at moisture levels from 6.1 to 7.8 percent in increments
of 0.5 percent. These bales are wrapped in non-ventilated polyethylene
for a 6-month storage period. A second study involves a replicated test
of bales wrapped in non-ventilated polyethylene and stored at 7.4, 8.3,
and 9.9 percent moisture target levels. These bales will be opened after
10 months to better quantify the long term effects of bale storage at
elevated moisture levels.
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ARS
Cotton Research Program Links
Cotton research is part of ARS National Program 306, Quality and Utilization
of Agricultural Products
http://www.ars.usda.gov/research/programs/programs.htm?NP_CODE=306
For more information, contact John W. Radin, National Program Leader,
Plant Physiology & Cotton jwr@ars.usda.gov
Cotton Quality Research Unit, Clemson, SC
http://www.cqrs.saa.ars.usda.gov/
Cotton Ginning Research Unit, Stoneville, MS http://msa.ars.usda.gov/ms/stoneville/uscgl/index.html
South Western Cotton Ginning Research Laboratory, Mesilla Park, NM http://www.swcgrl.ars.usda.gov/
Office of Cotton Technology Transfer & Education
http://msa.ars.usda.gov/gintech
Thomas Valco
Cotton Technology Transfer Coordinator
cttec@ars.usda.gov
Additional
Links of Interest
National Cotton Council of America Research Service (USDA) Briefing Room
- Cotton
http://www.ers.usda.gov/Briefing/Cotton/
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