As indicated previously, moisture was measured by oven-drying at 103°C for 24 h (8). Three samples from each of the five silages were taken for moisture measurement. Mean particle length (MPL) was measured by the standard separator method using five screens and a pan (11). Three samples of about 2 kg each were taken to measure MPL for each of the five silages.

Statistical analyses were done using analysis of variance (ANOVA) with a single factor. The single factor in the first experiment was corn component at five levels: grain, stalk, leaf, husk, and cob. The single factor in the second experiment was silage source at four levels: Binversie Farm (unprocessed, short), Prairie-du-Sac Farm (unprocessed, long), Ziegler Farm (processed, short), and Arlington Farm (processed, long). The single factor in the third experiment was treatment at six levels: fresh silage, partially dried to lose 10 percentage units of moisture, partially dried to lose 20 percentage units of moisture, completely dried in the oven, sieved and fresh, sieved and partially dried to lose 10 percentage units of mois­ture. ANOVA was used to determine significant differences. The least significant difference method was used to rank results (12).

Chemical Analyses

The following components were selected for chemical analyses: five corn components from the first experiment (grain, stalk, leaf, husk, and cob), five whole-plant corn silages from the second experiment, four efflu­ent DMs obtained from the four corn silages in the second experiment, four components from the third experiment (sunk grain, sunk stover, suspended stover, and floating stover). Three replications of each component were analyzed by the UW Soil and Plant Analysis Lab in Marshfield, WI, using wet chemistry for acid detergent fiber (ADF), neutral detergent fiber (NDF), crude protein (CP), minerals (P, Ca, K, Mg), and starch.

When considering the supply and price of feedstocks, it is necessary to consider how much of each feedstock would be required to produce 40 million gal of ethanol annually, and how much of each feedstock is avail­able throughout the year. It is not necessary that the plant use a single feedstock. In fact, further analysis could be conducted to determine opti­mal combinations of feedstocks based on price, seasonality, and yield. The estimated feedstock requirements and the amount of each feedstock that

should be available in California, given current production levels, are shown in Table 1.

Corn is the only feedstock that would be available in sufficient supply to support production of 40 million gal of ethanol, given current produc­tion levels. The current production of grain corn in California is 920,000 t, while the estimated requirement is for 449,438 t (Table 1). The proportions of supply that would be available for other feedstocks range from 5% for other tree fruit to 25% for raisins. The production, availability, and prices of feedstocks would change with farm-level and industry responses to public policies and market developments that influence the demand for ethanol production.

Field thistle is a robust annual and member of the sunflower family. It grows to monster proportions (easily 2 m) with good soil and proper moisture. Flowering heads bear elongated, purple to lavender disk flowers that bloom in July and August. Leaves emerge individually along the entire stem. Leaf margins, the tips of leaf lobes, and parts of the stem all bear spines. The plant dies off in early September and dries down to low mois­ture content within four weeks.

One plant has the potential to produce up to 5200 seeds in a season, but the average seed production is about 1500 seeds per plant. Seeds are dis­persed primarily by wind.

Thistle grows in a wide variety of soils, including sand dunes, but it is most abundant in clay soils. It can tolerate saline soils and wet or dry soils, but it grows best in dry soils. There are many other varieties, including Canadian and Bull.

In large thistle plants, the stem can be 3 cm in diameter. Although this stem size is admirable from the prospect of bulk, it may make cutting by conventional cutter-baler equipment difficult. The stalk consists of a large, spongy pith that does not rot or dry up for up to 1 yr. Although the stem diameter is impressive, the density of burnable mass is only moderate and the expected yield/acre is low.

Although thistle does thrive in sandy soil, most varieties of thistle are highly competitive and in some areas are classified as a noxious weed. It is also referred to as a "highly disruptive exotic plant."

Conidia were harvested from 30-d-old stock cultures, by adding 5 mL of sterile distilled water to the agar slant and then resuspending the conidia.

The spore concentration in the conidial suspension was determined by counting with a Burker Counting Chamber. To prepare the inoculum, 1 mL of spore suspension (approx 4 x 107 spores/mL) was inoculated into a 300­mL Erlenmeyer flask containing 75 mL of growth medium similar to the basic nutrient medium of Mandels and Weber (13) with the exception that urea was omitted, a double amount of (NH4)2SO4 was included, and the peptone content was elevated by 20%. This medium (later referred to as medium with a single set of nutrients) contained 10 g/L of glucose as the sole carbon source, which was fed separately in the form of a thick solution to the salt medium after sterilization. The initial pH of the sterilized medium was adjusted to 5.0 by adding sterile 2 M H2SO4 before inoculation, and no pH control was applied during the cultivation run. The inoculum was incubated on a rotary shaker with an agitation rate of 200 rpm at 30°C for 3 d and then was used to inoculate the fermentor.

The quantity of residue that is produced and can potentially be removed is directly related to the production yields of crops in the rotation. County-level harvested acres, yield, and total production for 1997-2001 were obtained from USDA-NASS, and 5-yr averages were determined from these data for all counties in the 10 states. These average yields were then converted to gross residue estimates using ratios of fresh grain weight to bushel factors and ratios of dry weight residues to fresh grain weight. For the three major crops considered, these factors were as follows: for corn, 25.1 kg of dry stover/bu of grain (56 lb/bu) and a 1-to-1 ratio of dry stover to fresh grain mass; for spring wheat, 35.4 kg of dry residue/bu (60 lb/bu)

Field Operations Associated with Corn Rotations for Conventional,
Reduced/Mulch, and No-till Field-Management Practices

and a 1.3-to 1-ratio of dry residue (chaff and straw) to grain; and for winter wheat, 46.3 kg/bu of grain (60 lb/bu) and a 1.7-to-1 ratio of dry residue to grain. For soybeans, these factors were 40.8 kg of dry residue/bu (60 lb/bu) and a 1.5-to-1 ratio of dry residue to beans (6).

Field Operations Associated with Wheat Rotations
for Conventional, Reduced/Mulch, and No-till Field-Management Practices

To quantify the amount of residue that can be sustainably removed, quantities of residues that must be left on the field to maintain rain and/or wind erosion at or below tolerable soil-loss levels (T) must first be esti­mated. The revised universal soil loss equation (RUSLE) and the wind erosion equation (WEQ) are used to estimate these residue quantities (7,8).

RUSLE and WEQ are designed primarily to estimate long-term, average annual soil erosion on a site-specific field characterized by a particular soil type, slope and runoff length, field length, cropping and management prac­tices used, and localized climate conditions. Residues that must be left on the field, with respect to rainfall and wind erosion, are estimated for each soil type, each crop rotation, and each tillage combination considered in this analysis, with the higher of the two estimates being the quantity needed to remain on the field.

Higher conversions of xylan and glucan were seen with increases in both moisture content and inoculum size (Table 4), but no correlation was observed between the conversions and the relative amounts of inoculum and moisture (ratio of inoculum to moisture content; not shown). Thus, it is unlikely that these two parameters comprise an interaction effect that is important to the operation of the system. Lower moisture contents gave lower overall amounts of degradation, but seemingly better selectivities for xylan degradation although coefficients of variation for conversions were higher at low moisture contents owing to the smaller changes in overall composition. Higher moisture gave better overall degradation but poorer selectivity for xylan degradation. Selective xylan degradation may not have as great an effect on the properties of straw-thermoplastic composites as may overall degradation. In the present study, selectivity for xylan removal was a convenient proxy measure of relative activity of the inoculated fun­gus to indigenous microbes. However, there are other uses for treated straw feedstock, such as for production of fermentable sugars for fuels and chemi­cals, in which selective xylan removal would be useful. If achieving high selectivity for xylan degradation is important to the final use of the feed­stock, lower moisture levels would be preferred. Finally, higher inoculum was found to give faster overall degradation, which was expected.

Experiment 1: Specific Gravity of Corn Components

Table 1 presents the specific gravity of corn components as measured by the gas pycnometer. All components were oven-dried prior to measure­ments. Because of ambient rehydration, the MC of components varied between 2 and 8% at the time of measurements. The data presented in Table 1 were corrected on a DM basis using Eq. 1. Intact grain was signifi­cantly denser (1305 kg of DM/m3) than chopped stalk and leaf (average of 635 kg of DM/m3) or chopped husk and cob (average of 826 kg of DM/m3). However, when all material was ground through a 1-mm screen, there was no significant difference among the five components (average of 1546 kg of DM/m3). The corn used to measure specific gravity was very mature, being harvested in December, and had DM fractions of grain, stalk, husk, cob, and leaf of 65, 18, 4, 10, and 3%, respectively. Measures might be different for earlier maturity corn. However, the data show a remarkable homogeneity in specific gravity when material becomes very fine.

Experiment 2: Sequential Water Separation

Table 2 presents the physical characteristics of the four silages used for the sequential water separation experiment. The two processed silages (Ziegler Farm and Arlington Farm) had very similar MPL (13 and 14 mm, respectively). They also had a relatively low MC; the Arlington silage had the highest DM content (36% DM). The Prairie-du-Sac Farm silage was unproc­essed and had a long particle size (17 mm), whereas the Binversie Farm silage was unprocessed and had a short particle size (8 mm). The moisture reported for grain and stover in Table 2 may slightly underestimate the actual values because components were exposed to natural air-drying for about 1 h during manual sorting prior to oven-drying.

Table 3 shows the proportion of grain and stover in the sunk material from the four silages. After the first separation, the grain concentration in the sunk material was 75% and highest for the Arlington silage, which also was the driest. The grain concentration was only 41% and lowest for the Prairie — du-Sac silage, which was the wettest. The Binversie silage was different from the other three silages because it produced a higher amount of sunk grain (31% of total DM) than the three other silages (19% of total DM). This might be the result of a later maturity harvest; a greater presence of fully formed kernels; and no used of a processor, thereby leaving more intact grain.

After the eighth separation, grain concentrations ranged from 27 to 46% and were lower than after the first separation. At each separation, more stover sank and mixed with the corn grain. Only the Arlington silage released more than 1.5% of total DM as grain beyond the first separation. The actual concentration of DM in the effluent ranged from 0.71 to 1.22%, with an average of 1.01%. DM in the effluent reported in Table 3 represents the DM as a proportion of the original DM in the silage.

Figures 1 and 2 illustrate the curves of sunk grain, sunk stover, DM in the effluent, and floating material over the course of the eight water sepa­rations for two contrasting cases: the Arlington Farm silage with a low MC and the Prairie-du-Sac Farm silage with a high MC. The sunk grain and sunk stover reported in Figs. 1 and 2 were measured at each separation.

Table 3 Grain and Stover Proportions After the First and Eighth Separations of Fresh Silage in Water in Experiment 3 Silage source After first separation After eighth separation DM (%) Grain concentration in sunk material (%) DM (%) G ain concentration in sunk material (%) Sunk grain Sunk stover Sunk grain Sunk stover Floating stover DM in effluent Ziegler Farm 18.5 b 11.4 bc 62.0 b 20.0 c 36.3 b 17.6 b 26.0 a 35.6 b Prairie-du-Sac Farm 19.1 b 27.1 a 41.3 c 19.5 c 53.2 a 6.7 c 20.7 b 26.8 c Arlington Farm 19.3 b 6.6 c 75.2 a 25.2 b 29.6 c 23.8 a 21.6 b 45.9 a Binversie Farm 30.7 a 14.7 b 67.7 ab 31.6 a 37.2 b 10.2 c 21.0 b 45.9 a SEM 1.7 2.3 4.9 2.1 2.1 1.7 0.4 3.5 LSD 3.9 5.3 11.2 4.9 4.9 3.9 1.2 7.5 a Average of three replications. Values with the same superscript letter in a given column indicate no significant difference (p < 0.05). SEM, standard error of means; LSD, least significant difference.

The floating material and the effluent DM were measured only after the eighth separation. The curve for DM in the effluent was inferred by assum­ing that 70% of DM in the effluent was released after the first separation (see experiment 3 for a justification) and by assuming that the release followed a logarithmic curve. The curve for floating material was obtained by mass balance. The suspended stover recovered after the first separation was considered to be part of the floating material.

We base our ethanol facility and production assumptions on a study conducted earlier by the California Energy Commission (8). That study examined the potential for using traditional biomass sources as feedstocks for producing ethanol in California. We extend that analysis by considering nontraditional feedstock alternatives, such as California-grown corn, sur­plus grapes and raisins, and culled oranges and other tree fruit produced in the San Joaquin Valley. We also consider almond hulls and whey, and we use updated estimates of energy prices in our analysis.

Some of the data we use are taken from the California Energy Commission’s 2001 report (8). Other data sources include the California Department of Food and Agriculture; the Raisin Administrative Commit­tee; the Renewable Fuels Association; and interviews with individuals in the tree fruit, citrus, almond, raisin, and grape industries.

We consider a new, 40 million-gal ethanol facility built in the San Joaquin Valley. Feedstocks for the facility include corn and surplus fruit products. Coproducts include dried distiller’s grain (DDG), and pomace, another animal feedstock. We assume that the facility operates throughout the year, using selected combinations of feedstock materials. The seasonal­ity of biomass availability is demonstrated in Table 2. Corn and raisins are available throughout the year, because both crops can be stored after har­vest (Table 2). Oranges also are available throughout the year, because we consider two varieties that are harvested at different times of the year.

Table 2 Seasonality of Biomass Availability Feedstock Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Culled oranges Other tree fruit • — • • • — • — • Grapes Raisins California corn • — • Midwestern corn • — •

Other tree fruit and grapes are considered to be available only from May through October.

Fixed Costs

The estimated cost of constructing a 40 million-gal ethanol facility in the San Joaquin Valley is $55 million (9). Amortizing that investment over an expected useful life of 20 yr at a discount rate of 5% generates an amortized expense of $4.41 million/yr. Dividing that cost by the expected annual production of 40 million gal generates an average amortized cost of $0.11/gal of ethanol.

Dames Rocket is a showy, spring perennial wildflower with large, loose clusters of fragrant white, pink, or purple flowers that bloom in April and May on flowering stalks about 1 m high. This member of the mustard family has flowers with four petals. Many seeds are produced in long, narrow fruits. The leaves are oblong, sharply toothed, and alter­nately arranged. Leaves decrease in size as they ascend the stem.

This plant usually grows in moist soil and does best in sun condi­tions. The seed is commercially available since it is often planted orna­mentally and included in many wildflower seed mixes.

Dames Rocket dies off and dries out by early August. It is one of the earliest of the sclerified stalked plants that is ready to harvest. The plant grows to over 1 m, and the stem diameter is about 1.1 cm. The XPR for this plant is a lowly 0.22, with a correspondingly low expected yield.

Goldenrod (Asteraceae Solidago) (Fig. 4)

Goldenrod is a perennial wildflower with a multitude of varieties. It is the state flower of Alabama, Nebraska, and Kentucky. Most species have feathery, rich sprays of florets atop sturdy stems. These small clus­ters of yellow flowers are prominent features of the landscape in Septem­ber and October, and signal the end of summer. Goldenrod blooms late and dries down slowly, probably owing to the protective waxy epidermis of its stem.

Goldenrod is an erect perennial with simple, alternate, toothed or smooth-margined leaves. Its dried leaves have been used for a tealike bev­erage by the Indians.

There are many varieties, including early and Canadian. All enjoy full sun and a variety of soil conditions. In general, they present no diffi­culties in growing. The plant propagates itself by both a spreading root system and seed.

It can grow to 2 m with a stalk that is about 0.8 cm in diameter. In the stem cylinder, the XPR is 0.78 and it has a density that rates well.

Some varieties of late goldenrod (Canadian) are late blooming and even later drying down, so they may not be ready for harvest until almost December. There is a misperception that goldenrod causes hay fever; it is actually the pollen of ragweed and grasses that causes this. Goldenrod’s pollen grains are relatively large, heavier than air, and therefore are carried off by flies, bees, butterflies, even ants or birds, but not by the wind.

Annual Sunflower (Asteraceae Helianthus) (Fig. 5)

To most people, sunflower conjures an image of a domestic plant with a large stalk crowned by a single large flower. The wild sunflower, or annual sunflower, however, exhibits a branched growing form with numerous smaller flowers at each branch tip. The average diameter of wild sunflower is about 1 cm, unlike cultivated forms, which commonly reach 30 cm.

The stem is erect, columnar at the base and branched at the top. The leaves are alternate, simple, rough, hairy, and ovate or heart-shaped with toothed edges. The heads are showy, with yellow to orange-yellow ray flowers and brown or dark reddish-brown disk flowers. Sunflowers begin to grow in early June, flower in August and September, and mature seed and die in late September.

Wild sunflower, or common sunflower, is an annual, reproducing by seed. The seeds are shaped like the commercial sunflower seeds bought in stores, but much smaller and spread by the wind.

These plants can grow to 3 m with a stalk that is about 1.1 cm in diameter. Stem construction consists of a moderately waxy, dark red epi­dermis. The xylem cylinder has an XPR of 0.46. Xylem material has a density of 540 kg/m3, which is about half that of pine wood. The pith is a solid spongy inner core that appears to be resilient to rot for many months.

Methods

Table 1 is important to the analysis of SSPs because it establishes a basis for future evaluation of the fuel source. Yet, much of the information is subject to growing conditions such as soil, water, and amount of compe­tition with other plants. Because there are no cultivated acres of these plants, the projected yields listed in Table 1 are based on stalk weights, which were measured, and growth density (stalks/m2), which were estimated. The estimations came from grid layouts made in productive parcels of wild — grown fields. They represent what I feel to be a conservative estimate of what densities can be achieved in cultivation.

The material density was determined from stalk weight and XPR based on the assumption that the stalk is a composite material of hard biomass and soft pith. The weight, but not the volume, of the pith was neglected:

(avg stalk height x cross-sectional area

Density = Avg stalk weight/

of stalk — cross-sectional area of pith)

Projected yield (PY) was determined using the following formula:

PY = (density x biomass stalk volume x no. of stalks/acre)

PY (t/acre) = Mat density x Avg Stalk Height x n x diam2 (1 — 1/XPR2)/4 /2.2/2000

Analysis

It is not the objective of this article to analyze the plants in the set of SSPs to determine which would be the most acceptable as an energy crop. One reason for this is that SSPs, as a fuel source, are a variety of plants rather than just one. As a variety, they have much more flexibility. It is expected that power plants fueled by SSPs will actually have farm contracts for sev­eral of the species within the set. Some species will be harvested and deliv­ered early in the season and others late. This facilitates the storage of the fuel since the window of harvest can be up to 6 mo.

In addition, some SSPs dry down better than others. Having a variety of moisture conditions may be of benefit to the burner. Burning some green material with fully dried material is often the best solution for determining burner feed rates and utilizing the full combustion chamber.

Another reason for keeping all the varieties in the SSP set is that some grow better in certain soils and climates. This is not to say that the nine species identified here will all be viable energy crops, or that other species will not be added. For example, it is hard to imagine any farmer wanting to work with field thistle, no matter how much protective clothing the farmer has. Yet, thistle shows some signs of having the capability of delivering high yield, and, therefore, it should stay a member of the list awaiting further research.

Cocklebur may not be an acceptable member because it is a Class C noxious weed in many parts of the United States. Couple this with the fact that it does not have good physical characteristics for a biofuel and cockleburris place on the SSP list is precarious.

Some may argue that Evening Primrose is not a true candidate owing to its biannual nature. However, its SSP harvestable material is the densest of all the members of the list, as if the additional year that it takes to mature may have been well spent. More research is needed to determine the den­sity at which it can be planted, and what effect the first-yr plant will have in occupying space in the field.

A further argument for keeping active all members of the SSP list is that there is some interest in targeting acreage that is currently in govern­ment "set-aside" programs as sources of the biomass material. In this case, without preparation of the field, the herbaceous stalked material may sim­ply grow wild on the acreage. If this is to be the case, it may not be possible to select the actual plants that will be harvested, and they may include all members of the list along with grasses and more.

It is also not the objective of this article to compare SSPs as a biofuel crop against other energy crops that are currently under research. The primary reason for this is that there is no competition among energy crops. If biofuel is to become significant in the mix of energy sources, all biofuels will need to be collected. In fact, it can be imagined that if a power plant is built to burn biofuels, it may well prefer a proper mix of woody and herba­ceous materials along with quantities of animal waste.

However, to demonstrate that SSPs deserve their place in the mix of energy crops, one comparison with other biofuel crops should be made. Table 1 gives the physical characteristics of the nine plants in the SSP set. Of particular importance in Table 1 is the last column, which represents the projected yield of each (in t/acre).

With this caveat, Table 1 indicates that the highest yields are >10 dry t/acre. This compares to corn stover at 2.8 t/acre (8), switchgrass at

2.5 t/acre (9), and willow seedling at 6 t/acre/annum (10). Figure 6 puts Table 1 to life, showing the cross sections of each of the SSPs discussed.

Conclusion

SSPs have been introduced as a group of herbaceous plants that should be given recognition as a biofuel feedstock. They should be given their

A

B

C

place in the development of renewable biomass material for direct energy conversion ("directly burnable crop" [11]).

In this article, this new energy crop has been introduced and its ben­efits extolled, in particular, the yield data of Table 1. However, the specifics were done within the economic and time constraints of the grant and are left open for more detailed scrutiny. In particular, the botany and plant physiology of the SSPs need clarification by experts, as do the physical characteristics of Table 1. Further studies are needed to establish heating values for the SSPs and to determine combustion characteristics. Knowl­edge of the cultivation and harvest of these plants must be expanded. The hope is, however, that the set of plants discussed here makes direct burn­able biomass an economically feasible alternative on a broad scale.

Acknowledgments

I wish to thank Gary Haase of The Nature Conservancy at Kitty Todd Nature Preserve in the Oak Openings Region of Lucas Country, OH, for his great insight into the identification and management of the indigenous plants of northwest Ohio. This research was funded by the Ohio Biomass Energy Program.

References

1. OHIO Biobased Fuels, Power and Products Fact Sheet (2003), access to state facts sheet from Website: www. bioproducts-bioenergy. gov/default. asp.

2. Shakya, B. (2000), Directory of Wood Manufacturing Industry of Ohio, Ohio Biomass Energy Program, The Public Utilities Commission of Ohio, Cols, OH.

3. DOE. Department of Energy, Biopower—Renewable Electricity from Plant Material, Website: www. eren. doe. gov/biopower/feedstocks/fe_energy. htm.

4. Teel, A. (1998), Paper presented at BioEnergy ’98: Expanding Bioenergy Partnerships, Madison, WI, October 4-8, 1998 (unpublished).

5. Neuhauser, A., White, R., and Peterson, B. (1996), Paper presented at the 1st Conference of the Short Rotation Woody Crops Operations Working Group, Paducah, KY, September 23-25, 1996 (unpublished).

6. Salisbury, F. and Ross, C. (1992), Plant Physiology, Wadsworth Publishing, Beverly, MA.

7. Rickett, H. (1975), Wildflowers of the United States, McGraw-Hill, New York, NY.

8. Farm Progress Companies. Farm Progress: What Are Corn Stalks Worth?, Farm Progress Companies, Website: www. farmprogress. com/frmp/articleDetail/1,1494,11451+ 19,00.html.

9. Center for Integrated Agricultural Systems. Switchgrass Production for Biomass, Research Brief #51, Center for Integrated Agricultural Systems, University of Wiscon — sin-Madison, Website: www. wisc. edu/cias/pubs/briefs/051.html.

10. Tharakan, A., Isebrands, R. (1998), Paper presented at BioEnergy ’98: Expanding BioEnergy Partnerships, Madison, WI (unpublished).

11. Kamm, J. (1993), Small Farm Today 10(3), 16,17.

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All rights of any nature whatsoever reserved. 0273-2289/04/113/0071-0093/$25.00

Fermentations were performed in a 3-L stirred-tank laboratory fer- mentor (Biostat A-DCU300; B. Braun Biotech International GmbH, Ger­many) with a working volume of 2 L. The bioreactor was equipped with a pH electrode and a polarographic oxygen electrode (Mettler-Toledo). To 1950 mL of sterilized (121°C, 20 min) growth medium containing a double set of nutrients, 50 mL of inoculum was added; thus, the volume of the inoculum was made up to 2.5% (v/v) of the total broth volume. Fermenta­tions were performed at 28°C with an agitation rate of 600 rpm and an aeration rate of 500 mL/min (0.25 vvm) at atmospheric pressure. A rather low airflow was used to avoid excessive foaming; however, the dissolved oxygen tension was always above 15% of the saturation value. The initial pH was adjusted to and further controlled at 5.0 by the automatic addition of 2 M H2SO4 and 2 M NaOH. Foaming was controlled by the manual addition of filter-sterilized antifoam agent (Sigma Antifoam 289). Fermen­tation was continued until the glucose was completely depleted, and then pulse addition of Solka-floc (10 g/L) was applied by adding a thick suspen­sion of 20 g of Solka-floc in a calculated volume of distilled water, thus filling the fermentor to the original volume of 2 L. Nutrients required to support the growth on the second batch of carbon source (i. e., Solka-floc) were supplied by including a double set of nutrients in the original batch in order to avoid any coincidences of response signals that may occur from adding any component along the Solka-floc. The outlet gas composition was continuously monitored using a gas analyzer (Tandem dual gas sen­sor; Adaptive Biosystems, Leagrave, England). Measurement values from the gas analyzer as well as electrode signals were logged to a computer every 10 min.