T he level of moisture in seed-cotton at harvest through to the level in baled lint can have signiicant effects on the quality of ibre sold to the spinning mill. There are optimum moisture levels for seed-cotton and lint for each harvest and ginning process that allow good and efi- cient harvesting, cleaning, ginning, baling and safe storage. To optimise processing and ibre quality the amount of moisture taken up or lost by cotton needs to be balanced with an appropriate amount of drying or moisture application during the pre-cleaning, gin- ning and baling processes. It is well known that cotton with an ac- ceptable moisture level has improved i- bre tensile properties resulting in greater strength, extensibility and work-to-break values. These effects make cotton ibre more resilient during ginning and lint cleaning, and less prone to damage. Dry cotton is easier to clean but will be damaged during ginning and lint cleaning. On the other hand, cotton with excessive moisture is dificult to gin and clean, and will degrade during storage. This article is the irst in a series of re- views on the management of moisture and ibre quality in cotton from harvesting through to bale storage in warehouses. Its aim is to provide growers, ginners and merchants with an up-to-date and concise collection of information on the subject of measuring and managing ibre quality dur- ing early stage processing and shipment. In this irst article the fundamental re- lationships that determine the extent and rate of moisture absorption (or more cor- rectly moisture adsorption) and drying in cotton are discussed. The interaction of water at a molecular level and the effects on ibre properties are also discussed and will be used in subsequent reviews on the addition of moisture or humidiication of cotton during ginning and baling. Cotton ibre cellulose and water As living plant cells, cotton ibres within a growing boll are literally full of water. Wa- ter is critical for ibre growth and the avail- ability of water during early boll growth is related to the inal length achieved by the ibre. When the boll matures and opens, ibres lose water and equilibrate with the ambient humidity. Both constituents of seed-cotton – ibre and seed – are hygroscopic, for ex- ample sensitive to moisture, but to differ- ent degrees. For example, if seed-cotton is placed in air of 50 per cent relative humid- ity (rh) and 21°C: • The ibres will tend to reach a moisture content of approximately six per cent; • The seed will tend to reach a moisture content of about nine per cent; and, • The combined moisture content will be around eight per cent. In its mature dried form, nearly 90 per cent by weight of the cotton ibre is cel- lulose. In fact the cellulose found in cotton ibres is the purest form of cellulose found in all plants. The cellulose in cotton ibres is mostly (88–96.5 per cent) α-cellulose1. The non-cellulose components (4–12 per cent) are located either on the outer lay- ers of the cotton ibre in the cuticle and primary cell wall or inside the residual pro- toplasm called the lumen. The secondary wall of mature ibres is primarily cellulose in its most highly crystalline and oriented form (see Figure 1). Figure 2 shows the structure of the cel- lulose molecules in cotton. From a physi- cal viewpoint the molecule is a ribbon-like structure of linked six-membered rings each with three hydroxyl groups (OH) on the C2, C3 and C6 atoms projecting out of the plane of the ribbon. As well as providing structural stability the hydroxyl groups allow extensive inter- molecular hydrogen bonding with many molecules, including water. The acces- sibility of water to these hydroxyl groups depends on the spacing between crystal lattice planes. From a completely dry state, water molecules will form hydrogen bonds with hydroxyl groups that are not already linked within crystalline regions. 32 — THE AUSTRALIAN COTTONGROWER DECEMBER 2009–JANUARY 2010 Moisture in cotton – the fundamentals By Stuart Gordon 1 , Susan Horne 1 and Marinus van der Sluijs 1 AT A GLANCE… The level of moisture in seed-cotton at harvest through to the level in baled lint can have signiicant effects on the quality of ibre sold to the spinning mill. Dry cotton is easier to clean but will be damaged during ginning and lint cleaning. On the other hand, cotton with excessive moisture is dificult to gin and clean, and will degrade during storage. Both constituents of seed-cotton – ibre and seed – are hygroscopic, for example sensitive to moisture, but to different degrees. In this article the fundamental relationships that determine the extent and rate of moisture absorption and drying in cotton are discussed. FIGURE 1: Representation of the structure of a cotton ibre FIGURE 2: Assembly of cellulose molecules in a sheet. Hydrogen bonds are shown by dotted lines. Circled carbon atoms; C2, C3 and C6, show location of hydroxyl (-OH) groups. Ginning & Fibre Quality Series