Mohair Biology and Characteristics

Christopher John Lupton

Texas A&M University, San Angelo, Texas, U.S.A.

INTRODUCTION

Mohair is the white, lustrous fiber produced by the Angora goat (capra hircus aegagrus). Most goat breeds have double coats (to highly variable degrees), consisting of an outer coat of coarse guard hairs and a relatively short undercoat of fine down that sheds annually. The practice from early times of selecting for white, wavy, lustrous, single-coated (i.e., no coarse guard hair) fleeces that do not shed eventually resulted in the stabilization of this type of fleece and an animal that has both high priority for and high efficiency of fiber production. Only recently have nonwhite (e.g., brown and black) Angora goats become popular with specialty breeders. In spite of mohair's specialty status and historically low production, it has been the subject of many studies and research reports. Some of these were in association with wool, and much of the research added to our understanding of human hair growth, a topic that remains economically and sociologically important.

SKIN BIOLOGY

Mohair fibers are produced by cell division in primary (P) and secondary (S) follicles in the skin of Angora goats (Fig. 1). The two types of follicles are distinguished by their accessory structures. The P follicles each have a sebaceous gland, a sudoriferous (sweat) gland, and an arrector pili muscle. The S follicles have only a sebaceous gland. Some S follicles produce more than one fiber. The central P follicles are first observed on the fetal head about 40 days into pregnancy and spread across the body over the next 20 days. During this time, two more P follicles (laterals) appear on either side of the central P follicle, thus forming a trio group. After 80 days of pregnancy, S follicles associated with each trio group begin to emerge, forming a follicle group. At birth (day 149), all P follicles are fully formed and are actively producing fiber, whereas only a small but quite variable proportion of the S follicles are producing fibers. Twelve weeks after birth, most of the secondary follicles are producing fibers. The ratio of S follicles to P follicles in mature Angora goats ranges from 6 to 12:1 (compare merino sheep at 15 to 25:1). Prenatal and early postnatal nutrition affects the rate of maturation and the ultimate number of active S follicles. This number has a direct influence on the lifetime production of mohair.

Fibers produced by P follicles are coarser than those produced by S follicles. In some cases, the fiber produced in a P follicle is medullated (hollow) to varying degrees. When medullation exceeds 60% of the fiber diameter, the fiber is termed a kemp, which is a chalky white, objectionable (from a textile viewpoint) fiber that appears not to accept dyestuff. Centuries of visual selection against kemp have resulted in low levels in most commercial animals. When the degree of medullation is less than 60% of the fiber diameter, this fiber is termed a med or heterotype. Angora goats containing excessive amounts of these fibers are also discriminated against, because med and kemp production appear to be inseparable genetic traits.[2]

FIBER MORPHOLOGY

Fine mohair fibers are round in cross-section. As the fibers become coarser, the cross-section becomes more elliptical. Some kemp fibers are collapsed and have the appearance of a flattened straw. The microstructure of mohair fibers is composed predominantly of cortex, a collection of long, cigar-shaped cortical cells 8 x 100 microns) lying parallel to the fiber axis that are embedded in a matrix sometimes referred to as intercellular cement (composed of lipids and sulfur-rich proteins). The cigar-shaped cortical cells are themselves composed of macro-fibrils, which in turn are made up of microfibrils (low sulfur-content proteins). It has been suggested that microfibrils consist of protofibrils, which in turn are composed of three polypeptide molecules arranged in an a-helix.

Overlapping scales (about 0.4 microns in thickness) composed of cuticle cells surround the cortex, each having a free edge that points toward the tip of the fiber (Fig. 2). The scales consist of three distinct layers the epi-, exo-, and endocuticle and form a protective coating around the cortex. Though similar in appearance to the scales on coarse wools, scale thickness in mohair is invariably less than that of wool (0.7 to 1.0 micron, Fig. 3). This difference is the basis of a scanning electron

Fig. 1 Simplified drawing of adult Angora goat skin. (From Ref. 1 with permission from Elsevier.)
Fig. 2 Magnified image of mohair fibers obtained with a scanning electron microscope. (Courtesy of the International Wool Textile Organization.)

microscope test method for quantifying mohair and wool in textile blends.

A third component of the fiber, the medulla, is contained in some of the fibers produced by P follicles and consists of a continuous or fragmented central core composed of air-filled cell residues (Fig. 1). A bilateral arrangement of two types of cell has been reported in the cortex of wool: orthocortical and paracortical cells. Both types also exist (but randomly mixed) in mohair, but orthocortical cells predominate, especially in mohair grown by young animals. Paracortical cells have about twice the sulfur content of ortho-cells. This sulfur is contained in amino acids that cross-link adjacent peptide chains in the protein structure. Consequently, mohair has lower

Fig. 3 Scanning electron microscope images of mohair and wool scale edges showing the difference in thickness. (Courtesy of the International Wool Textile Organization.)

resistance to chemicals (e.g., acids and alkalis) but can absorb dye molecules faster than wool.

cystine. In the raw fleece, natural nonprotein impurities also are present. These include suint (water-soluble dried sweat secreted by the sudoriferous glands), wax (secreted by the sebaceous glands), inorganic dust and dirt, and organic vegetable matter (twigs, seeds, burrs, etc.). One objective of textile scouring and processing is to eliminate these contaminants. Moisture (up to 30%) is also associated with mohair. On a dry basis, cleansed mohair contains approximately 50% carbon, 22 25% oxygen, 16 17% nitrogen, 7% hydrogen, and 3 to 4% sulfur. The basic building blocks for keratin are 18 amino acids. Differences among the amino acid contents of adult, kid, and kemp mohair fibers have been documented.

CHEMICAL COMPOSITION

Chemically, mohair, cashmere, and wool are very similar. The parent protein of the fibers is keratin, a complex mixture of proteins containing the sulfur-amino acid

FIBER GROWTH

Mature females (called does, ewes, or nannies) produce 2 to 2.5 kg of greasy mohair every six months. This production is under genetic and nutritional control to varying degrees. In contrast, kids (6 months and 12

Table 1 Ranges for important mohair characteristics

Clean yield (U.S. ranges;

Kids (6 12 mo)

70 76

South African values are

reported to be higher), %

Young goats (18 mo)

72 78

Mature does (> 2 yr)

74 80

Grease content, %

1 10

Suint content, %

1.5 5.0

Average fiber diameter, mm

Kids (6 12 mo)

21 29

Young goats (18 mo)

30 34

Mature does (> 2 yr)

32 38

Mature bucks (> 2 yr)

32 45

Secondary/primary follicle ratio

(mature animal)

6 12:1

(at birth)

2.5:1

Follicles per unit area, mm2

10 20

Lock length (6 month), cm

7.5 15

Lock type

Ringlet, flat, intermediate, sheepy, unclassifiable

Waves per unit length, cm"1

27

Medullation, %

08

Color

White (yellow, brown, stained); brown; black

Specific gravity

1.297 1.320

Regain at 65% rh and 21 °C, %

14.5 17.5

Tenacity, cN/tex

13 17

Elongation at break, %

40 43

Work at rupture, cN/tex

2.65

Initial modulus, cN/tex

350 410

months of age) typically yield less than 1 kg at their first shearing and between 1 and 1.5 kg at the second shearing. In a six-month period, the fiber grows to a length of 10 to 15 cm, this being a requirement for worsted (long-staple) processing.

Age, nutrition, season, sex, and reproductive status each have important effects on mohair production and properties. From birth to about four years of age, the mohair fibers become progressively coarser, after which time the average fiber diameter remains fairly constant. After four years of age, Angora does begin to produce less mohair, and some follicles stop producing altogether. Mohair production by kids and mature animals is influenced greatly by the plane of nutrition. Inadequate nutrition causes fewer, shorter, and finer fibers to be produced. Unless goats are on a very high plane of nutrition, mohair production and average fiber diameter will usually respond positively to protein supplementation. Mohair shorn in February (grown in autumn and winter) is usually shorter, finer, and less kempy than that shorn in August (grown predominantly in spring and summer). Males tend to produce more, coarser, and longer mohair than females of similar age maintained under similar conditions. Pregnant and lactating Angora does tend to produce less and finer mohair than comparable nonpreg-nant does.

PHYSICAL PROPERTIES

Physical properties important for marketing, textile processing, and product performance are listed in Table 1. The great versatility of mohair is due in part to its availability in a wide range of fiber diameters. The more valuable 26-micron mohair can be manufactured into fine, lightweight suitings, whereas 40-micron mohair might be used to make hard-wearing rugs, carpets, or upholstery. Several of mohair's unique or superior properties may be attributed to its relatively thin scale structure. These include high luster, smooth feel (handle), good resistance to soiling and physical abrasion, excellent soil release, and low propensity for felting. Similarly, the physical and chemical structure of the mohair cortex is responsible for superior durability, tenacity, strength, and elasticity compared to wool of similar dimensions. The predominance of orthocortical cells in mohair explains mohair's superior ability to be set in a predetermined shape (e.g., a crease in a pair of trousers) and to be dyed to deeper shades than wool, whereas the perceived brilliance of such dyeings is likely more a function of mohair's exceptional whiteness and luster. In common with wool and other animal fibers, mohair has low flammability, is subject to attack by moths, bacteria, and mildew, has excellent moisture absorption and release (important for comfort), has very good insulation properties, and blends well with other fibers.

CONCLUSION

Growth and structure of mohair have been the subjects of numerous studies and reports.[4-6] Many scientists would agree on the gross aspects of structure and chemical composition. However, the absolute physical structure of mohair fibers and the chemical composition of the many individual protein fractions and associated lipids that constitute mohair's keratin are either unknown or are the subjects of much conjecture. With declining national budgets for animal fiber research (and agricultural and textile research in general), it is doubtful these unknowns will be determined in the foreseeable future. Nevertheless, this unique and versatile fiber continues to be grown and used, and it is still quite an important commodity in specific areas of South Africa, the United States, and Turkey. With the increasing movement toward natural and organic products by (relatively) affluent consumers in developed nations, and perhaps with a favorable change in fashion, it is possible that the downward trend in mohair production could be reversed.

ARTICLES OF FURTHER INTEREST

Angora Goats: Production and Management, p. 13 Mohair: Production and Marketing, p. 649

REFERENCES

1. Ryder, M.L.; Stephenson, S.K. Wool Growth; Elsevier, 1968.

2. Lupton, C.J.; Pfeiffer, F.A.; Blakeman, N.E. Medullation in mohair. Small Rumin. Res. 1991, 5, 357 365.

3. Van der Westhuysen, J.M.; Wentzel, D.; Grobler, M.C. Angora Goats and Mohair in South Africa, 3rd Ed.; Mohair Board: Port Elizabeth, 1988; 258 pp.

4. Hunter, L. Mohair: A Review of Its Properties, Processing and Applications; CSIR Division of Textile Technology, Port Elizabeth, South Africa and International Mohair Association, Ilkley, U.K., 1993; 278 pp.

5. Hunter, L.; Hunter, E.L. Mohair. In Silk, Mohair, Cashmere and Other Luxury Fibres; Franck, R.R., Ed.; Woodhead Publishing Ltd.: Cambridge, England, 2001; 68 132.

6. Shelton, M. Angora Goat and Mohair Production; Mohair Council of America: San Angelo, 1993; 233 pp.

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