Elsevier

Bioresource Technology

Volume 98, Issue 16, November 2007, Pages 3106-3111
Bioresource Technology

Enzyme enhanced solid-state fermentation of kenaf core fiber for storage and pretreatment

https://doi.org/10.1016/j.biortech.2006.10.032Get rights and content

Abstract

Kenaf is an annual fiber crop adaptable to a wide range of climates and soil types. This study investigated the use of kenaf core fiber as a feedstock for enzyme-enhanced fermentation. Triplicate kenaf core fiber samples were treated with enzymes having cellulase:hemicellulase activity ratios of 0:1, 0.015:1, 0.45:1, and 2.54:1 at a rate of 5010 IU/kg dry matter hemicellulase activity, vacuum-sealed, and incubated at 37 °C for 21 d. Samples were analyzed for pH, water soluble carbohydrates, organic acids, and hemicellulose and cellulose concentrations. All treatments produced a pH less than 4.0, which is sufficient for stable storage. Treatments with 2.54:1 and 0.45:1 produced the highest water soluble carbohydrate and lactic acid concentrations. Enzymes with no or low cellulase activity produced results similar to the control. Utilizing enzyme mixtures with high cellulase activity is an effective pretreatment method for ensiled kenaf core fiber.

Introduction

Kenaf (Hibiscus cannabinus L.) is a warm-season annual fiber crop mainly adapted for cultivation in tropical and sub-tropical areas. It is native to parts of Africa and is an important cordage crop in many developing countries. Closely related to cotton, okra, and hibiscus, kenaf is similar in appearance to hemp. Kenaf can be grown in a wider range of climates and soil types than any other commercial fiber crop (Dempsey, 1975). Most production in the United States has been limited to Mississippi, Texas, Florida, and California, but it has been grown for fiber production as far north as Iowa. It requires 150 days to complete maturity, so seed production is limited to frost-free areas in Texas, California, and Florida (Sullivan, 2003).

Kenaf is harvested for its stalk, which can reach heights of 2.5–6 m, depending on environmental conditions. The stalk is composed of two distinct fibers, bast and core, which comprise approximately 35% and 65% of the stalk mass, respectively (Columbus and Fuller, 1999). Bast is characterized as a bark, containing long fibers, with the core being physically similar to balsawood, containing soft, short fibers.

Since the early 1990s the area planted to kenaf in the United States has increased significantly and during the period from 1992 to 1997, planted area increased from 1600 to 3200 ha (USDA-ERS, 1997). More recent estimates place planted area levels at 6000 ha (NCIS, 2002). Accurate acreage and production values are difficult to establish because most kenaf is grown under contract and kenaf is not part of any current government farm program.

Kenaf is typically allowed to dry in the field, with a killing frost needed to initiate the drying process. This is necessary because moisture concentrations above 20% moisture content on a wet basis (w.b.) greatly reduce separation efficiency of the bast and core (Columbus and Fuller, 1999). In areas with a mild winter, desiccants or defoliants may be used to hasten drying (Bowyer, 1999). Currently, whole-plant kenaf is harvested with unmodified or slightly modified sugarcane harvesters, forage harvesters and baling equipment, but in cotton production areas, cotton modulation equipment is also used (Webber et al., 2002).

When kenaf is not used for whole plant applications, such as paper pulping or composite building materials, bast and core are mechanically separated after harvest. Each fiber has its own distinct uses, so the greatest economic returns are produced when fibers are separated (Columbus and Fuller, 1999). Bast fiber is considered to be a high quality papermaking material, with chemically refined bast suitable for replacement of softwood pulp in products like newsprint and multiple tissue paper grades (Bowyer, 1999). Core fiber has much lower quality uses, such as oil absorbent, livestock bedding and as a replacement for vermiculite and Styrofoam packaging material (Young, 1992).

Because of the limited range and low value of products produced from core material and its high lignocellulose content, core fiber has potential as a biomass conversion feedstock. Fermentation and enzymatic processes have been thought to hold the most potential for conversion of biomass to industrial products, such as acetic acid, acetone, butanol, and lactic acid (NRC, 2000), and fuels, such as ethanol and biodiesel (NRC, 1999). Richard et al. (2001) found the use of solid-state fermentation to be an effective method for pretreatment and limited bioconversion of corn stover in order to improve biomass characteristics for downstream conversion processes.

Solid-state fermentation, when used for biomass storage and pretreatment, is similar to the ensilage technology traditionally utilized by ruminant producers to preserve high fiber feedstuffs for year-round use. Ensilage is referred to as solid-state fermentation because materials are handled as solids as opposed to a fermentation system for ethanol, which is in a liquid phase. Ensilage is characterized as primarily a lactic acid fermentation process. During the initial stage of fermentation, excess oxygen is consumed producing an anaerobic environment. During the second stage, from one day to three weeks, soluble carbohydrates are converted to lactic and acetic acid, ethanol, mannitol, acetaldehyde, and carbon dioxide by anaerobic bacteria (Roberts, 1995). This period is characterized by a significant decrease in pH. After three weeks, significant acetic and lactic acid accumulation result in pH declining to a level which inhibits further microbiological growth (pH < 4.5) and the ensilage is considered to be stable. If lactic and acetic acid levels are not sufficient, secondary fermentation will occur. During this process, lactic acid is converted to secondary fermentation products, such as butyrate and iso-butyrate, by clostridia. This process is characterized by an increase in pH and the presence of propionate, butyrate, and iso-butyrate. Secondary fermentation is considered to be detrimental to the ensilage process because butyric acid is a weak acid for preserving silage and its formation causes significant dry matter losses from the silage (Jaster, 1995).

Enzyme additions can be used to enhance the ensilage produced by hydrolyzing structural carbohydrates (hemicellulose and cellulose) into additional fermentable sugars (Jaster, 1995, McDonald et al., 1991). Ren et al., 2006, Richard et al., 2002 found that enzyme-enhanced fermentation is a suitable pretreatment method for corn stover to be used for further processing. Enzymes have also been demonstrated to improve fermentation in other high lignocellulose materials, like whole-plant wheat (Adogla-Bessa and Owen, 1995), orchardgrass, and alfalfa (Nadeau et al., 2000), but this approach has not been applied to kenaf core fiber.

The purpose of this project was to determine concentrations of fermentation products in enzyme-treated kenaf core fiber preserved using solid-state fermentation, and possible strategies for use of enzymes in a kenaf core fiber ensilage system for storage and pretreatment.

Section snippets

Experimental design

Whole kenaf plants were harvested from a plot near Brooklyn, Iowa in early December 2003, after a killing frost and subsequent period of field drying. Plants were allowed to dry an additional week indoors to improve bast and core fiber separation. Separation and fractionation of the fibers was done with a portable shredder (MTD Products, Inc.). The resulting material was then sifted and the large bast particles removed, producing a lower quality core fiber, containing small amounts of bast

Results and discussion

Acidification occurred with all treatments, with the largest pH declines occurring in the 0.45:1 and 2.54:1 enzyme treatments (Fig. 1). The 0:1 and 0.015:1 ratios produced values that were not different from the ensiled control. All treatments progressed below the pH 4.0 level by day 21, which is considered adequate for stable storage and to inhibit clostridium growth. Differences in pH between treatments can be attributed to the relative levels of organic acids produced. Ren et al. (2006) also

Conclusions

Results indicate solid-state fermentation, enhanced with enzymes, to be a suitable method for storage and pretreatment of kenaf core fiber. All treatments produced pH values below pH 4.5, which is sufficient for proper storage. Treatments with 2.54:1 and 0.45:1 cellulase:hemicellulase ratios produced the highest water soluble carbohydrate and lactic acid concentrations. Treatments with no or low cellulase activity produced results that were similar to the control treatment. Treatments with

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