cotton straw: composition, variability and effect of...

Biomass 9 (1986) 101-112

Cotton Straw: Composition, Variability and Effect of Anaerobic Preservation

Nissim Silanikove

MIGAL -- Galilee Technological Centre, Kiryat Shmona, 10200, Israel

and

Dan Levanon

Volcani Institute, Bet Dagan, Israel

(Received: 18 July, 1985)

A B S T R A C T

The chemical composition of cotton straw (CS) was studied in order to evaluate its biotechnological potential.

The main components in CS, as in other straws, are the cell wall carbohydrates. In comparison with cereal straws, CS contains considerably more cell solubles but the level of soluble carbohydrate is low, and the amounts of secondary metabolites high. The CS cell wall contains less hemicellulose and more pectic substance than wheat straw. It is possible to preserve CS under anaerobic conditions. However, an increase in level of lignin and a small loss of soluble organic compounds during aerobic fermentation was observed.

Key words: Cotton, straw, composition, preservation.

INTRODUCTION

Cotton is the main field crop in Israel (60 000 ha) and generates the largest portion of local organic agricultural wastes. 1 Cotton straw (CS) is that part of the plant which remains in the field after the cotton yield is picked. This straws consists of main stalk, side branches, leaves, boll and seeds with adhering cotton lints. The amount of cotton straw left is 5000-7000 kg ha -1. Cotton straw is not utilized for any purpose and the usual treatment in the field is to cut the straw to small pieces and to

101 Biomass 0144-4565/86/S03.50- © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

102 Chemical characteristics o f cotton straw

plough it under the soil surface. This treatment has a few disadvan- tages: (i), a potential agricultural product is not collected and treated as wastes; (ii), monoculture ploughing every year enhances the danger of potential cotton diseases; (iii), the cotton stalks need a long period to decompose in the soil and this leads to difficulties in the cultivation needed for the next year's crop.

The main obstacles preventing economical use of CS are: high moisture content, which requires drying or special storage; high lignin content, which results in poor nutritional quality for ruminants; and high levels of residual toxic chemicals used during the growing period. Cotton straw, like other collectable plant resources, may be con- sidered for the production of chemicals, fuels, energy and animal feed. It is obvious that knowledge of the chemical composition of CS will be crucial for the realization of the full biotechnological potential of the waste. We therefore first assessed the variability of the chemical composition of CS. Growth conditions may affect cotton straw's chemical composition, soil water content during the crop's growth may govern the structure of the cotton plant and other cultivation procedures (fertilization, cultivation, etc.) may also affect the amount and composition of the CS.

Before utilization of CS for any purpose, a proper storage system must be developed. The comparatively high moisture content (70-40%) necessitates drying or any other treatment that will keep the cotton straw from aerobic biodegradation. Anaerobic preservation seems to be one of the most economical ways to conserve CS. Hence, the effectiveness of anaerobic preservation and its influence on the CS's chemical composition were assessed.

MATERIALS AND METHODS

Soils, cultivation procedures, cotton plants

The Hula Valley is located in the northern part of Israel. There are three main types of soil in the valley: peat soils of the reclaimed Hula swamp, the soils of the drained Hula lake, and alluvial mineral soils. The main properties of these soils are presented in Table 1. Acala (var. SJ1) cotton is grown in the Hula Valley according to the usual local practices. In the reclaimed swamp and lake soils the water table is kept 30-40 cm below the soil surface, in order to minimize aerobic biodegradation of the organic matter.

N. Silanikove, D. Levanon

TABLE 1 The Main Properties of the Hula Valley Soils

103

Mineral soil Reclaimed Peat soil lake soil

pH 7-0-7"5 7.1-7"8 5-0-7-6 Electrical conductivity (millimhos cm -~ ) 0-8 2.5-5.0 2.5-4.6 CaCO 3 (%) 25-28 60-80 0-28 Organic matter (%) 1-1 5-7 20-60 Clay (%) 59 20 60-70 Silt (%) 37 70 20-25

Samples and their preparation

About 20 plants were sampled from three fields from each type of soil. The plants were cut 10 cm above the ground. A quarter of the samples were divided into main stalk, small branches, leaves, boils and seeds adhered with cotton. The separated plant parts were dried at 105°C, weighed and the contribution of each part to the total plant weight was calculated.

The result of the samples were dried in air to about 80% dry matter, chopped, dried at 60°C overnight, and ground in a knife-mill to pass through a 1-mm screen. The latter material was used for chemical analysis.

Anaerobic preservation

About 2 tons cot ton straw material containing 50% water was cut and chopped into particles of 2 -3 cm. Water was added quantitatively to a level of 65% to ensure optimal ensiling conditions. The material was then pressed and covered to obtain anaerobic conditions. Samples were taken at the beginning and henceforth at intervals of I month for 6 months. The samples were dried at 60°C and prepared for analysis as described.

Chemical methods

Nitrogen was determined by the macro Kjeldhal procedure in a Tecator apparatus (model 1030); dry matter, pH, crude fat, ash, calcium, and phosphorous concentrat ions were determined according to

104 N. Silanikove, D. Levanon

s tandard A O A C 2 procedures. Neutral detergent fiber, acid detergent fiber, sulphuric acid lignin, and acid detergent insoluble nitrogen were determined according to Goering and van Soest, 3 except that sodium sul- fite was omitted from the neutral detergent fiber solution, and preheated f ibre-glass- instead of a sbes to s - was added before treatment with 72% H E S O 4.

Hemicellulose was determined as the organic difference between neutral-detergent fiber and acid detergent fiber; cellulose, was calculated as organic acid detergent fiber minus lignin. Total cell solubles were determined by the MCF extraction procedure. 4 Soluble carbohydrates in defatted cell extractives were determined by the Anthron method. 5 Pectic substance in the cell wall was quantified approximately as the difference between the organic yield of MCF and NDF extrac- tions, since the latter contained Ca-EDTA salt, which depectinates cell walls. Identification of pectic substance in the cell wall was confirmed histochemically by staining CS with Ruthernium Red according to Johansen. 6

RESULTS AND DISCUSSION

The chemical characteristics of cotton straw

The main components in CS, as in other straws, are the cell wall carbohydrates (Table 2). However, CS is unique in several important characteristics in comparison with most abundant utilizable straws such as cereal straws and especially, wheat straw. The water content in CS varied between 50 and 60% (Table 2) in comparison with about 10% in

TABLE 2 The Main Chemical Components in Cotton Straw Sampled from Three Types of Soil

(as % of Dry Matter)

Soil type Dry Organic Ether Crude Carbo- Lignin Residual matter matter extract protein hydrates unidentified

organic matter

Mineral soil 50.41 92.53 0.72 3.50 65.15 17"60 5"56 Reclaimed lake soil 65.72 90.41 2.40 9.63 56.39 15.49 6.50 Peat 67.22 91-80 2.80 8.52 57.62 14.11 8.75

Chemical characteristics of cotton straw 105

TABLE 3 Cell Solubles, Soluble Carbohydrates and the Composition of Carbohydrates in the Cell Wall of Cotton Straw Sampled From Three Types of Soil (as % of Dry Matter)

Soil type Cell solubles Soluble carbohydrates

Cell wall carbohydrates

Pectin Hemicellulose Cellulose

Mineral soil 19-90 2"1 4.21 13.60 45"53 Reclaimed lake soil 31-59 3.2 5.20 11.24 36-75 Peat soil 29.10 3.0 4-71 13.33 38.58

e-

.9

0

C I

0

0.30 '

0.36,

0 .32

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G24 '

0 .20

0.16

0.12

0.08 ,

0.04

0

0.4

i i i J

0.44 0.48 0 .52 0 .56 0 .60 0.64 0.68 0 7 2 0 7 6 0.80

moisture

Fig. 1. Cell solubles' organic extraction content as a function of moisture content in straw, y -- - 0.16 + 0-66x, r = 0"63, n = 12, p < 0"05. y = cell solubles as a fraction of dry

matter, x = fractional moisture content.

wheat. 7,8 CS contains a cons iderable amoun t of cell solubles (Table 3, Fig. 1), in compar i son with 15 -20% typical ly found in wheat. 7,8 T h e amoun t of soluble ca rbohydra te s in CS (2 -4%) is relatively small in p ropor t ion to all cell solubles, but still h igher than in wheat (0.4-1.4%). 8 T h e low level of soluble ca rbohydra te s is most p robably a result of their extensive uti l izat ion as s t ructural componen t s and energy resources dur ing the p roduc t ion of the co t ton lints.


Cotton straw contains between 3 and 12% crude protein (Table 2), the typical value being about 8% (Fig. 2). The concentration of crude protein in CS is considerably higher than in wheat, which typically contains 2-40/0. 7,8 Specific examination of the distribution of the pro- teins in CS showed that about 25% of the total N is bound to the lignocellulose fraction. CS has been found to contain 0.7-2% ether extractives (lipids and wax), in comparison with the 1.1-3"9% reported for wheat. 8 Ash content is at about the same level in CS (6"3-10%, Table 4) and wheat. 7,8 However, while the majority of the ash content in CS (99-6%, Table 4) is soluble, wheat contains a relatively high amount of insoluble ash (0.5-3.4%). 7

Calcium concentration in CS varies between 0.7 and 1% (Table 4), in comparison with a typical value of around 0.2% in wheat. 7 Phosphorus concentration in CS varies between 0-12 and 0.21% (Table 4), in comparison with a typical value of around 0" 1% in wheat. 7

Cotton straw contains a considerable amount (5.5-8.8%) of organic soluble compounds which were not identified (Table 2). In comparison, the amount of organic extractives not identified by the methods used here in wheat is small. 8 The unidentified organic matter in CS is most probably produced by the secondary metabolites of the cotton plant; the organic composition of cotton plant is quite com- plex. 9 Many of these compounds have biological activity 9,1° and have economic importance. The cotton plant is especially rich in phenolic compounds, which include simple phenols (coumarins, cinnamic acids, Shikimate-derived compounds), polyphenols (terpenoids, steroids) and polymers (tannins, lignin). It is concluded that CS extractives may be of economic importance as raw material for biotechnological and chemical industries.

The cell wall content in CS is lower than in wheat, in an inverse relationship to the differences in cell solubles' content. In addition, CS contains considerable amounts of pectic substances (4-7%, Table 3)

TABLE 4 Ash, Insoluble Ash, Calcium and Phosphorus Contents in Cotton Straw Sampled from

Three Types of Soil (as % of Dry Matter)

Soil type Total ash Insoluble ash Calcium Phosphorus

Mineral soil 7.50 0.03 0-67 0"12 Reclaimed lake soil 9-63 0.02 1-02 0.21 Peat 8.25 0-02 0.96 0.15

Chemical characteristics of cotton straw

TABLE 5 The Distribution of Canopy Dry Matter (as % of Total) in Cotton Straw

Sampled from Three Types of Soils

107

Mineral soil Reclaimed Peat soil lake soil

Main stem 55"9 48.2 50" 1 Side and small branches 28"3 32"1 31"5 Boll 10.5 10.2 10-6 Leaves 4"2 8.5 6.5 Fibres and seeds 1.1 1.0 1'3

whereas there are only negligible amounts in wheat. 8 The proportions of the main components of the cell wall are different in CS and wheat. The hemicellulose content in CS (13-15%, Table 3) is much lower than in wheat (25%). 8 The content of lignin in CSin the present work varied between 14 and 18% in comparison with about 7-10% charac- teristically found in wheat. 7,8 In the southern, dry parts of Israel, lignin concentrat ion in excess of 20% has been measured in CS. 11 CS was also found to be much more resistant than wheat to treatment with basic agents such as N a O H and NH4OH. 11,12

Effect of moisture content on CS composition

In general, the chemical composit ion of CS is more variable than that of wheat. Its composit ion seems to depend largely on its moisture content.

The water content in peat and reclaimed lake soils is greater than in other mineral soils. The moisture content in CS taken from peat and lake soils is consequently greater than that in CS from mineral soils. As a result, on peat and lake soil there is considerable regrowth of new green leaves in the cotton plants in spite of defoliation treatments. The contribution of the leaves to the entire dry matter content is larger than in CS taken from mineral soils (Table 5).

The relationship between moisture content in CS (as an indepen- dent variable) and the main constituents (as a dependent variable) is shown in Figs 1-6. There is a significant linear relationship between moisture content and total cell solubles, lipid and mineral content. The relationship with crude protein is not significant; two possible reasons exist for the lack of relationship between moisture content and crude

108 No Silanikove, D. Levanon

0.12

0.10

0.08

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E

0 .04 .

0 .02 , . . . . . .

0.4 0 .44 0 .48 0.52 0.56 0.60 0 .64 0.68 0.72 0.76 0.80

m o i s t u r e

Fig. 2. Protein content as a funct ion of mois ture content in co t ton straw, y = 0 .09 + 0 .26x , r- - 0"45, n = 12, p, NS . y - - prote in content as a fract ion of dry matter.

x = fract ional mois ture content .

Fig . 3. 0-14x,

0.12 -

0.11 •

0.10 "

0 .09 -

0.08 -

0 .07 -

0 .06 - -

0.4 0.44 0.48 i i

0.52 0.56 0.60 0.64 0.68 0 .72 0.76 0.80

m o i s t u r e

A s h content as a funct ion of mois ture content in co t ton straw, y = - 0 . 0 0 9 + r = 0 '77 , n = 12, p < 0 " 0 1 , y = a s h content as a fraction of dry matter.



0,18

0.16

0.14

0.12 o

0

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0 . 4 0 . 4 4 0 .44 0 . 5 2 0 . 5 6 0 .60 0 .64 0 . 6 8 0 .72 0 .76 0 . 8 0

m o i s t u r e

Fig. 4. H e m i c e l l u l o s e c o n t e n t as a f u n c t i o n of m o i s t u r e c o n t e n t in c o t t o n s t raw. y = 0.18 +O-09x , r = 0"33, n = 12, p , NS. y = h e m i c e l l u l o s e c o n t e n t as a f r ac t i on of

d r y ma t t e r , x = f r ac t i ona l m o i s t u r e con t en t .

0.50

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0.46

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'~ 0.40

0.38

0.36

0.34

0.32

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0 . 40 0.44 0.48 0.52 0.56 0 6 0 0.64 0.68 0 7 2 0 7 6 0.80

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Fig. 5. Ce l l u lo se c o n t e n t as a f u n c t i o n of m o i s t u r e c o n t e n t in c o t t o n straw. y = 0 . 8 4 - 0 .70x , r = 0"72, n = 12, p < O ' O 1 , y = ce l lu lose c o n t e n t as a f r ac t i on of d ry

ma t t e r , x = f r ac t iona l m o i s t u r e con t en t .


0 . 2 0 -

0 . 1 8 "

0 . 1 6 "

C;

C: 014

O I 2

0.10 , , , ,

0 , 4 0 0 . 4 4 0 . 4 8 0 . 5 2 0-.56 0 .6 0 . 6 4 0 . 6 8 072 0 , 7 6 0 8 0

m o i s t u r e

Fig. 6. Lignin con ten t as a func t ion of mois ture content in co t ton straw, y = 0-25 - 0.16x, r = 0-62, n = 12, p < 0 " 0 5 , y = lignin con ten t as a f ract ion of dry matter .


TABLE 6 Effect of Anaerobic Preservation on the Chemical Composi t ion of Cot ton Straw

Harvested from Mineral Soil (as % of Dry Matter)

Item Dry Cellulose Hemi- Lignin Pro- Ash Calcium Phos- pH matter cellulose tein phorus

Orig ina lmat te r 44.6 44-35 11-2 16"15 6"19 7"30 0.74 0"09 6"3 2 months ' 43.9 45.20 10.35 19-10 4.81 7"50 -- - - 7-2

preservation 6 m o n t h s ' 23-45 49-96 8.81 23-32 5-25 7.80 0-77 0.11 7-1

preservation

protein: (i) as noted above, significant portions of the nitrogen in CS are bound to the lignocellulose; and (ii) local differences in nitrogen fertilization regimes between cotton fields may affect the nitrogen content in the plant. The lack of a linear correlation between calcium and phosphorus concentration and total ash content is also probably related to local differences in fertilization regimes.


There is an inverse linear relationship between moisture content and cell wall components' content (Figs 4-6). The relationship between moisture content, lignin and cellulose has the same trend as with hemicellulose. However, while the relationship with cellulose and lignin is statistically highly significant, that between moisture content and hemicellulose is not significant (Fig. 4). This is because the range in hemicellulose content relative to cellulose and lignin is very small. The correlation between moisture content and cellulose is more significant than that between moisture content and lignin. This is surprising, since lignification is a known mechanism for protection against dehydration in plants. 13 However, cotton is a woody plant and contains a significant amount of lignin even when the plant is green and irrigated (10-12%, authors' unpublished results). The final lignin concentration in CS is a result of two factors; lignin content during the growing period and response to dehydration. The ability to partially control CS composition through control of its moisture content may be of importance in relation to utilization of its organic content or in relation to delignification procedures.

Effect of anaerobic preservation on CS composition

Cotton straw was not ensiled during anaerobic preservation, even after water content was adjusted to the optimal level in the silage. This could be attributed mainly to the low soluble carbohydrate content, which was much lower than needed (2% vs 10%) to initiate efficient ensilage. Based on the increase in pH (Table 6), it seems that slow aerobic fermentation has prevailed during storage. Since CS particles are fibrous and resist compressing, the stack contains trapped air which could supply the oxygen for the fermentation. Lignin concentration increased by 44% (Table 6), indicating further lignification during storage. The large increase in moisture content in 6 months of preservation is undoubtedly related to penetration of rain into the stack through unprotected sides.

In conclusion, anaerobic treatment of CS preserves the cell wall constituents of CS. However, it produces two negative side effects: (i) increase in lignin content and (ii) loss and conversion of some of the organic components (presumably soluble carbohydrates) to fermentation products, further reducing its already low nutritional value. Improved anaerobic conditions may reduce this effect and minimize losses of available nutrients.


R E F E R E N C E S

1. Marchaim, U. & Criden, J. ( 1981 ). Research and development in the utilization of agricultural wastes in Israel for energy, feedstock fodder, and industrial products. In: Fuel gas production from biomass, Vol.1, D. L. Wise (ed.), CRC Press, Boca Raton, FL, pp. 95-120.

2. AOAC (1980). Association of official agricultural chemists. Official methods of analysis, AOAC, Washington, DC.

3. Goering, K. H. & Van Soest, P. J. (1970). Forage fiber analysis, Handbk US Dept. Agric. 379.

4. Redgwell, R. J. (1980). Fractionation of plant extracts using ion- exchange sephadex. Anal. Biochem., 107, 44-50.

5. Morris, D. L. (1948). Quantitative determination of carbohydrates with Drey woods anthrone reagent. Science, 1tl7, 254-5.

6. Johansen, R. A. (1940). Plant microtechnic, McGraw-Hill, London. 7. National Research Council (1895). Nutrient requirements of domestic

animals, No.5, Nat. Acad. Sci., Washington, DC. 8. Theander, O. & Aman, P. (1978). Chemical composition of some

Swedish cereal straws. Swedish J. Agric. Res., 8, 189-94. 9. Hedin, P. H., Thompson, A. C. & Gueldner, R. C. (1975). A survey of the

volatile constituents of cotton lint and waste with regard to byssinosis. J. Agric. Food Chem., 23, 698-703.

10. Bell, A. A. & Stipanovic, R. O. (1983). Biologically active compounds in cotton: An overview proceeding of the seventh cotton dust research con- ference. P. J. Wakelyn (ed.).

11. Ben-Ghedalia, D., Shefet, G., Miron, J. & Dror, Y. (1982). Effect of ozone and ammonium hydroxide treatments on the composition and in vitro digestibility of cotton straw. J. Sci. Food. Agric., 31, 1337-42.

12. Ben-Ghedalia, D., Shefet, G., Miron, J. & Dror, Y. (1982). Effect of ozone and sodium hydroxide treatments on some chemical characteristics of cotton straw. J. Sci. Food. Agric., 33, 1213-18.

13. Janshekar, H. & Fiechter, A. (1985). Lignin: Biosynthesis, application and biodegradation. In: Advances in biochemical engineering bio- technology, 27,119-178.

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