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Recommended Maximum Moisture for Storing Baled Cotton

Recommended Maximum Moisture Content for Storing Baled Cotton

Table of Contents

graphic bulletRecommendation

graphic bulletIssue
graphic bulletARS Bale Moisture Studies
         – Summary
         – Results from Individual Studies

graphic bulletMicrobial Contamination
graphic bulletFuture ARS Studies
graphic bulletARS Cotton Research Program Links
graphic bulletAdditional Links of Interest

Recommendation
Unginned and ginned cottonAgricultural 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

Cotton harvest in Texas 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
Cotton harvesting
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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Cotton fiber productionResults 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
Cotton fiber spooling 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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