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Corn Wet Mill Liquid Holding Capacity of Corn Stover

R. P. K. Ambrose, K. E. Ileleji, P. Doane, M. Cecava


Published in Applied Engineering in Agriculture 32(6): 909-914 (doi: 10.13031/aea.32.11694). Copyright 2016 American Society of Agricultural and Biological Engineers.


Submitted for review in December 2015 as manuscript number PRS 11694; approved for publication by the Processing SystemsCommunityof ASABE in July 2016.

The authors are R. P. Kingsly Ambrose, ASABE Member, Assistant Professor, Klein Ileleji, ASABE Member, Associate Professor, Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana, and Perry Doane, Research Scientist, Mike Cecava, Director, ADM J.R. Randall Research Center, Decatur, Illinois. Corresponding author: Klein E. Ileleji, 225 S University St., West Lafayette, IN 47907, phone: 765-494-1198; e-mail: ileleji@purdue.edu.

Abstract. Despite its low nutritive value, corn stover is considered an important feed material because of its dry matter and roughage content. Alkali treatment of stover improves its digestibility and increases the dry matter content. Mixing stover with liquid coproducts from industrial corn wet mill processing gives ample opportunity to improve the feed value and to develop feed with higher nutritive value. The coproduct liquid (corn steep liquor and corn molasses) holding capacity of corn stover (treated and untreated) was investigated in this study. Increase in particle size reduced the liquid holding capacity of untreated stover. In general, one gram of untreated stover held approximately 5 to 7 g of corn steep liquor and 5 to 8 g of corn molasses. Alkali treatment of corn stover reduced the liquid holding capacity. For treated stover, moisture content had an inverse relationship, while particle size had a direct relationship with the liquid holding capacity. The liquid holding capacity depended on the physical structure, moisture content, and particle size of corn stover, and the physical properties of coproduct liquids. This study was conducted with the aim of quantifying the amount of coproduct liquids that can be mixed with corn stover and the results indicate that corn stover can act as a good carrier for the corn coproduct liquids.

Keywords.Corn molasses, Corn steep liquor, Corn stover, Liquid holding capacity, Particle size.

Corn stover is being utilized as a ruminant feed for its fiber and dry matter content. The lack of nutritive content, especially low nitrogen content and high fiber content (Manyuchi et al., 1994), and the physical characters (Kadam and McMillan, 2003) limits its extensive use as a feed. Trials on size reduction (Morris and Mowat, 1980), anhydrous ammonia treatment (Saenger et al., 1982), and NaOH and Ca(OH)2 treatment (Oji et al., 1977) indicated that the nitrogen content of stover and its digestibility can be improved. Other than increasing digestibility, these pretreatments could also help in bioconversion processes (Iroba et al., 2013). Russell et al. (2011) indicated that CaO treated stover (5% on dry matter basis) can be included up to 20% (calculated based on dry matter), in feed lot diets. Alkali treatment dissolves lignin, silica, and hemicellulose present in the roughages (Jackson, 1977), which helps in increasing the dry matter and digestible organic matter (Sriskandarajah and Kellaway, 1984). Corn stover is comprised mostly of fibrous and needle-shaped particles with rough surfaces and high aspect ratio. Due to this fibrous nature, corn stover is capable of absorbing and holding liquids in its polymer matrix and pore structure. This absorption is highly influenced by the chemical composition and the method of pretreatment (Viamajala et al., 2009).

Corn steep liquor (CSL) and corn molasses (CM), two liquid coproducts from corn wet milling processes, are high in energy content and can be utilized in ruminant feed rations by blending with corn stover. Feed for ruminant livestock species developed using coproducts (Gonzalez-Valadez et al., 2008) are considered as alternatives of feeding corn grain, also known as corn replacement feed. Mixing CSL or CM to other common ingredients such as corn fiber is a common practice during feed preparation for a balanced diet and also to increase digestibility. High protein content, low fat and fiber content makes these coproducts a good complimentary feed substitute. Feed ration with CSL increases the digestibility (Eckerle et al., 2011) and CM increases animal performance and production efficiency (Pate, 1983). Mixing CSL and CM with ground corn stover is a way of improving the nutrient value and digestibility of the roughage and has the capability to reduce the amount of corn in feed ration.

In CSL or CM feed evaluation studies, various inclusion/addition rates of these coproducts are based on nutritive balance or on trial and error eventually evaluated by animal growth. There is a lack of studies to find the maximum liquid holding limit for adding these coproduct liquids with other feed materials. High suspended solids content and the viscous nature of these coproduct liquids, especially CSL, can limit their addition and uniform blending with other dry feed materials. Due to the fibrous nature of corn stover, these coproduct liquids may be able to bind/adhere to the surface due to high solids content along with absorption of moisture inside the pores. This study was undertaken to find the maximum holding capacity of liquid corn wet mill coproducts (CSL and CM) by ground corn stover. The specific objectives of the study were: (i) to determine the effect of particle size on the liquid holding capacity of untreated and treated corn stover, and (ii) to study the effect of moisture content on the liquid holding capacity of treated stover.

Materials and Methods

Corn stover (untreated) at 10.36% moisture content (wet basis) was procured by ADM Research (Decatur, Ill.). The corn stover contained leaves, stalk, stalk pith, and stalk shell. Calcium hydroxide treated stover was prepared at 38% moisture content (wet basis) from the procured stover using the Readco Kurimoto, LLC Continuous Processor® (Readco Kurimoto, LLC, York, Pa.) at the University of Illinois, Urbana-Champaign Beef Feedlot Facility (Urbana, Ill.). The collected samples were transported in sealed plastic bags to Purdue University (West Lafayette, Ind.), and the high moisture treated samples were stored at 5°C until time for the experiments. Before the experiments, the samples were equilibrated to ambient temperature. During alkali treatment, moisture is added to the stover and samples have a moisture content in the range of 30-50% (w.b.). To account for the moisture variability, liquid holding capacity of treated stover was tested at different moisture contents. The stover samples contained 38% moisture, and at different moisture contents (8%, 31%, and 52% w.b.) were prepared based on the method suggested by Kingsly and Ileleji (2009).

To evaluate the effect of particle size on liquid holding capacity, ground corn stover passing through 3.2, 6.4, and 12.8 mm hammer mill screen sizes were used in this study. A hammer mill (10HMBD, Glenn Mills Inc., Clifton, N.J.) was used for grinding the untreated and treated corn stover samples using the screen sizes as mentioned above.

Coproduct Liquids and their Properties

Solids Content

The corn wet mill coproducts of CSL and CM were evaluated for their holding capacity with corn stover. These coproducts were collected at the ADM J. R. Randall Research Center in Decatur, Illinois, and transported to Purdue University in sealed containers. The samples were stored at 5°C until the experiments. The solids content of these coproducts were measured using the following method. About 5 g of sample was dried in a hot air oven at 105±1°C for 24 h (Sluiter et al., 2008) and the total solids content was calculated using the following relationship:

       (1)

Measurement of Viscosity

During preliminary trials (results not reported), it was found that the miscibility of CSL and CM with stover was very poor at ambient temperature conditions due to their high viscosity. To optimize the property of liquids during mixing, the viscosity of CSL and CM were studied at different temperatures (22.2°C, 30°C, 40°C, 50°C, and 60°C) using a Brookfield DV-II+ programmable viscometer (Brookfield Engineering Laboratories, Inc., Middleboro, Mass.). The spindle and speed combination should be used that gives a value of 10-100 on the instrument torque scale (http://www.brookfieldengineering.com/support/documentation/operator-manuals.asp). With this in consideration, the spindle and speed combination was selected based on trial and error for the samples at ambient temperature condition. For CSL, spindle number 5 (19.05 mm diameter) at 20 rpm and for CM, spindle number 2 (24.00 mm diameter) at 60 rpm was used for viscosity measurement.

For measuring the apparent viscosity 400 mL of liquid coproduct was sampled from the bulk in a 500 mL glass beaker based on the standard provided by the instrument manufacturer. Before measuring the viscosity, the sample was manually mixed thoroughly (for about 1 min.), using a stirrer, to avoid settling of solids. For measuring viscosity at elevated temperatures, the samples were heated in a hot plate and once the sample reached the target temperature, the viscosity was measured. The experiments were conducted in triplicate.

Liquid Holding Capacity Measurement

The method suggested by Robertson et al. (2000) and Raghavendra et al. (2004) for finding the water holding capacity of fiber products was modified to adapt to this present study. Lack of even miscibility of stover with coproduct liquids during the preliminary trials (results not reported) due to the viscous nature of CSL and CM and also due to the complex aspect ratio of particles present in corn stover necessitated the modification of the method. Weighed ground corn stover (about 1 g, W1) was transferred to a 50 mL test tube. Then 30 mL of coproduct liquid was added to the corn stover in the test tube. After mixing, the test tubes were placed in a plastic drum and rotated in a bench-top tumbling system at low speed (40 rpm) for 2 h. After rotation, the corn stover particles were drained and separated from the liquid (that was not absorbed by the particles) using a sieve (sieve size: 53 microns). After draining, the excess moisture was removed by slightly pressing (dabbing) the sample in paper towels. After recording the hydrated weight (W2) of corn stover, the samples were then dried in a hot air oven for 3 hours at 105°C to measure the residue dry weight (W3).

From the hydrated weight and residue dry weight, liquid holding capacity (LHC, eq. 2), and liquid holding capacity (LHCD) (after drying, eq. 3) was calculated. As control, the liquid holding capacity of corn stover samples in distilled water was studied and compared with the coproduct liquids.

   Liquid holding capacity (LHC)

   (g/g stover) = (W2-W1)/W1    (2)

   Liquid holding capacity, after drying (LHCD)

   (g/g of stover) = (W3-W1)/W1    (3)

Statistical Analysis

The experiments were conducted in triplicate. Statistical analysis was conducted using SAS v 9.1 (SAS Institute, Cary, N.C.). The effect of particle size on the LHC and LHCD of corn stover with CSL and CM was evaluated by subjecting the data to one-way ANOVA (a=0.05) using PROC GLM. Using two-way ANOVA, the effect of moisture content and particle size on the LHC and LHCD of treated stover was analyzed. Tukey’s Honestly Significant Difference test was conducted to compare the means between untreated and treated corn stover. PROC CORR was used to find the Pearson’s correlation between the particle size and LHC, LHCD using SAS.

Results

Properties of Coproduct Liquids

The CSL had higher solids content and also was very viscous, but the density was low compared to CM (table 1). The suspended solids in corn molasses tend to settle down at the bottom so extra care has to be taken while sampling from bulk and also before mixing with the stover. The viscosity of CSL was approximately 30 times higher than that of CM. This indicates the difference in flowability of both of these liquids with CM being easily flowable. For both the CSL and CM, viscosity reduced with increase in temperature. The increase in kinetic energy of the molecules in the liquid due to increase in temperature makes it to easily overcome the attractive forces that tend to hold the molecules together. The results (table 1) indicate that heating the coproduct liquids before adding with stover aid in uniformity during mixing. However, no extra energy will need to be expended to heat either coproduct prior to blending because CSL and CM are obtained from the plant hot (above 80°C).

Table 1. Solids content, density and viscosity of liquid coproducts.[a]
CoproductSolids Content
(%)
Density
(g/mL)
Viscosity (mPas) at Different Temperatures °C
22.230405060
CSL53.66 (0.05)1.20 (0.03)5096.7 (51.25)a4513.3 (167.73)b3033.3 (11.54)c2726.7 (161.66)d2400.0 (87.18)e
CM48.31 (0.07)1.31 (0.01)162.7 (0.40)a122.7 (2.31)b104.7 (4.16)c82.9 (5.73)d74.7 (3.35)d

    [a]    Values in parenthesis are standard deviation; same superscript letter within the same row indicates no significant difference (P = 0.05).

Figure 1. Liquid holding capacity (before drying) of untreated corn stover [same superscript letter within the same coproduct liquid indicates no significant difference (P = 0.05)].

Liquid Holding Capacity of Untreated Corn Stover

Corn stover can carry about 5 to 7 g of CSL and 5 to 8 g of CM per g at 10.36% moisture content (fig. 1). The holding capacity of CM was higher than that of CSL. The stickiness and the low suspended solids content in CM aided greater retention of this liquid with untreated corn stover. Also, the suspended solids in both CSL and CM assisted in higher holding than the stover could absorb moisture which is evident from the comparative low moisture absorption. This could also be due to the higher density of CSL and CM, in comparison with moisture, that resulted in a larger difference in weight. Increase in particle size reduced the LHC of untreated corn stover. Reducing particle size increased the surface area and in turn the area exposed to the suspended solids present in CSL and CM for binding.

The data presented in figure 1 indicates that particles with higher surface area absorbed more liquids (CSL and CM). By size reduction, the macro-pores within the stover tissue and the overall porosity gets reduced which may have limited the absorption of water by the fibrous corn stover. Auffret et al. (1994) also found that grinding fibers from wheat bran, sugar-beet, and citrus reduced the water binding due to the collapse of the fiber matrix.

Figure 2. Liquid holding capacity (after drying) of untreated corn stover [same superscript letter within the same coproduct liquid indicates no significant difference (P = 0.05)].

For ease of handling, transportation, and storage, the stover can be mixed with these coproducts liquids and dried to a safe moisture content level.The results from LHCD will give a quantified value for the amount of dry coproduct liquids that can be carried by the dry stover. Though the suspended solids content in CSL was higher than CM, the LHCD was higher for stover particles mixed with CM irrespective of particle size (fig. 2). But the difference was not significant at 12.8 mm particle size (P = 0.05). The result was similar to LHC before drying with higher holding of CM with stover. Untreated dry stover particles can hold about 3 g of dry CSL and about 3 to 4 g of dry CM. The particle size of untreated stover had high correlation with LHC and LHCD (table 2). The negative correlation indicated that increase in particle size reduces the liquid holding capacity of untreated stover.

Table 2. Correlation between particle size and liquid holding capacity of untreated corn stover.
Corn Steep LiquorCorn Molasses
LHCLHCDLHCLHCD
Particle size-0.63-0.64-0.87-0.79

Liquid Holding Capacity of Alkali-Treated Corn Stover

As the moisture content of alkali treated stover increased, the liquid holding capacity decreased (fig. 3) for any particle size and for both CSL and CM. At higher moisture contents, the presence of moisture in stover pore spaces limits the absorption/binding of liquids. Similar to untreated stover, the adsorption of CM was higher than CSL. In comparison, the coproduct liquids holding with treated stover was lower (fig. 3) than the untreated stover (fig. 1). An interesting trend was the increase in coproduct liquids binding with increase in particle size of the treated stover, which was statistically significant for most particles except for corn molasses at 31% and 52% moisture contents. Increase in larger-sized pores and also the modification of the micro-pores in the cell wall due to pretreatment most likely increased the liquid adsorption at larger particle size. According to Shinoj et al. (2010), alkali treatment of fibers reduces the water absorption at high fiber loadings. The presence of hydroxyl groups in cellulose makes the fibers more hydrophilic. However, the disruption of hydrogen bonds in these hydroxyl groups during alkali treatment, affects the liquid absorption characteristics of the fiber particles (Li et al., 2007).

There was no specific trend in holding capacity when treated stover was mixed with CM (fig. 3). Also, in most cases, the water absorption was lower for the control than for the coproduct liquids. Therefore, in alkali treated stover, the holding capacity depended more on the adhering capability of suspended solids in the coproduct liquids rather than on the moisture absorption. The Tukey’s means comparison analysis also indicated a difference (P = 0.05) in untreated and treated corn stover LHC. The structural change during the alkali treatment process might have limited the LHC of treated stover.

Figure 3. Liquid holding capacity of alkali treated corn stover at different moisture contents [same superscript letter within the same coproduct liquid at the same moisture content indicates no significant difference (P = 0.05).
Figure 4. Liquid holding capacity of alkali treated corn stover (after drying) [same superscript letter within the same coproduct liquid at the same moisture content indicates no significant difference (P = 0.05)].

The LHCD was also low for the alkali treated corn stover (fig. 4). Only the reduced moisture content sample (8% and 31% w.b.) had significant dry liquid holding for CSL. The CM had a higher dry liquid holding capacity, compared to CSL, except at 52% moisture content. This might be due to the sticky nature and the higher mixing ability of CM due to lower viscosity. Unlike untreated stover (table 2), particle size had a positive correlation with liquid holding capacity (LHC and LHCD) for treated stover (table 3). In comparison, the Pearson correlation coefficient value for particle size was higher for CSL than CM. Moisture content had a negative correlation indicating higher moisture content results in lower LHC and LHCD.

Conclusions

The effect of pretreatment, moisture content, and particle size of corn stover on liquid holding capacity of two coproducts of corn wet milling was studied. Two coproducts from the corn wet milling industry, namely corn steep liquor (CSL) and corn molasses (CM), were used in this study. The viscosity of both these liquids decreased with increasing temperature. This indicated that mixing these liquids hot with stover increases the miscibility and can reduce the energy required for mixing. Reduction in stover particle size increased the liquid holding capacity with untreated stover. The alkali treatment significantly reduced the liquid holding capacity of corn stover. Surface and structural modification during this pretreatment might have caused this lower liquid holding capacity. The liquid holding capacity of treated corn stover significantly decreased with increasing moisture content. The treatment Process increased the moisture content of corn stover after treatment. For both untreated and treated stover, the binding of CM was higher than CSL due to sticky nature, percent suspended solids and lower viscosity. The liquid holding capacity after drying indicated that the suspended solids from the coproduct liquids adheres to the surface of stover particles, and will not be lost during the drying process. The results from this study indicate that corn stover (both treated and untreated) can act as a carrier for these coproduct liquids.

Table 3. Correlation between particle size, moisture content and liquid holding capacity of alkali treated corn stover.
Corn Steep LiquorCorn Molasses
LHCLHCDLHCLHCD
Particle size0.4700.4670.1920.179
Moisture content-0.835-0.836-0.906-0.883

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